Multiphoton dissociation dynamics of BrCl and the BrCl cation · Multiphoton dissociation dynamics...

Multiphoton dissociation dynamics of BrCl and the BrCl1cation

Olivier P. J. Vieuxmaire,w N. Hendrik Nahler,z Richard N. Dixon and

Michael N. R. Ashfold*

Received 19th June 2007, Accepted 27th July 2007

First published as an Advance Article on the web 30th August 2007

DOI: 10.1039/b709222a

Ion imaging methods have enabled identification of three mechanisms by which 79Br1 and 35Cl1

fragment ions are formed following one-color multiphoton excitation of BrCl molecules in the

wavelength range 324.6 4 l 4 311.7 nm. Two-photon excitation within this range populates

selected vibrational levels (v0 ¼ 0–5) of the [X 2P1/2]5ss Rydberg state. Absorption of a third

photon results in branching between (i) photoionization (i.e. removal of the Rydberg electron—

a traditional 2 þ 1 REMPI process) and (ii) p* ’ p excitation within the core, resulting in

formation of one or more super-excited states with O ¼ 1 and configuration [A 2P1/2]5ss. Thefate of the latter states involves a further branching. They can autoionize (yielding BrCl1(X 2P)

ions in a wider range of v1 states than formed by direct 2 þ 1 REMPI). Further, one-photon

absorption by the parent ions resulting from direct ionization or autoionization leads to

formation of Br1 and (energy permitting) Cl1 fragment ions. Alternatively, the super-excited

molecules can fragment to neutral atoms, one of which is in a Rydberg state. Complementary ab

initio calculations lead to the conclusion that the observed [Cl**[3PJ]4s þ Br/Br*] products result

from direct dissociation of the photo-prepared super-excited states, whereas [Br**[3PJ]5p þCl/Cl*] product formation involves interaction between the [A 2P1/2]5ss and [X 2P1/2]5psRydberg potentials at extended Br–Cl bond lengths. Absorption of one further photon by the

resulting Br** and Cl** Rydberg atoms leads to their ionization, and thus their appearance in the

Br1 and Cl1 fragment ion images.

1. Introduction

The potential of velocity map imaging methods for high

resolution studies of the photodissociation dynamics of neu-

tral molecules is becoming ever more widely recognized,1–4 but

the application of these methods to studies of molecular

cations remains relatively rare. Examples include our own

detailed studies of the photofragmentation of vibrational

and spin–orbit state-selected Brþ2 ,5,6 BrCl1,7,8 and DCl19

cations, investigations of OCS1 photofragmentation by Liu

and coworkers,10 of the photolysis of C2H41 by Suits’ group11

and of CF3I1 by Aguirre and Pratt.12

The present paper describes detailed investigations of the

speed and angular distributions of the Br1 and Cl1 products

formed following one-color multiphoton excitation of BrCl;

the ensuing analysis will serve to highlight the rich variety of

fragment ion formation mechanisms. The parent excitation

wavelengths were chosen so as to be two photon resonant with

successive vibrational levels of the first (n ¼ 5) member of the

nss ’ 6p Rydberg series of BrCl. This Rydberg excitation

takes the form of two short vibronic progressions, separated

by the spin–orbit splitting of the 2P ion core (B2070 cm�1). It

proves convenient to persist with the previously introduced

labelling scheme for these progressions, viz. a5(v0, 0)

(i.e. [2P3/2]5ss, v0 ’ X1S1, v00 ¼ 0) and b5(v0, 0) ([2P1/2]5ss,

v0 ’ X 1S1, v00 ¼ 0).13 In the case of the b5(v0) levels, the

requisite photon energies are such that absorption of just one

further photon enables parent ion formation via a process

that conserves both the vibrational (v) and core spin–orbit

(O) quantum numbers of the resonance enhancing level.

Only in the case of the b5(v0 ¼ 0) level (populated via the

b5(0, 0) transition), however, does such a 2 þ 1 resonance

enhanced multiphoton ionization (REMPI) process give a

significant parent ion yield.7,8 Br1 and/or Cl1 fragment ions

dominate the ion yields measured at all other wavelengths.

One of the goals of the current paper is to rationalize this

observation.

Such a dominance of fragment (rather than parent) ions

following one-color REMPI of diatomic molecules is not

without precedent. Related observations have been reported

for the cases of H2,14,15 NO,16,17 O2

18 and Cl2,19,20 and a range

of fragment ion formation mechanisms proposed. These in-

clude: (i) dissociation of super-excited states of the parent

molecule (formed, in the present case, at the energy of three

absorbed photons), followed by photoionization of one of the

resulting neutral fragments; (ii) photodissociation of the par-

ent ions that are formed by autoionization of such super-

excited states; and (iii) (generally unintended) photodissocia-

tion of parent ions formed by the direct REMPI process.

These various routes to forming fragment ions are illustrated

in Fig. 1, to aid the ensuing discussion. [Note that in our

School of Chemistry, University of Bristol, Bristol, UK BS8 1TS.E-mail: [email protected]; Fax: (117)-9250612; Tel: (117)-9288312/3w Lehrstuhl fur Theoretische Chemie, Technische UniversitatMunchen, Lichtenbergstraße 4, D-85747 Garching, Germany.z Department of Chemistry and Biochemistry, University of Califor-nia Santa Barbara, CA 93106, USA.

This journal is �c the Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 5531–5541 | 5531

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

previous imaging studies of the fragmentation of state-selected

BrCl1 cations,7,8 a second (independently tunable) laser pulse

was introduced, deliberately, in order to enable detailed

investigation of process (iii).] Fig. 1 also serves to illustrate

the rich variety of possible product combinations. Four-

photon absorption, resonance enhanced at the two photon

energy by the various b5(v0) levels of BrCl, provides sufficient

energy to form Br1 fragment ions in all of their 3PJ spin–orbit

states and in the 1D2 excited state. Four-photon excitation via

the b5(v0 ¼ 0) level provides only enough energy to form

ground state Cl1(3P2) fragment ions, but all three 3PJ states of

Cl1 are energetically allowed when exciting via the higher

b5(v0) levels. Additional richness is provided by the fact that

the neutral co-fragments (Cl or Br) each have 2P ground states.

In most cases, the energy provided by absorption of four

photons will be sufficient that these can also be formed in

either spin–orbit state.

The present imaging studies reveal that each of pathways

(i)–(iii) contribute to the fragment ion images observed

following multiphoton excitation of BrCl, but to differing

extents in the Cl1 and Br1 forming channels. Analogies are

noted with previously reported multiphoton imaging studies

of Cl2, resonance enhanced at the two photon energy by low

v0 vibrational levels of its first ([2Pg]4ss) Rydberg state.20

We also see that certain of the velocity sub-groups evident

within the Cl1 and Br1 fragment ion images obtained follow-

ing excitation via the b5(v0) levels of BrCl exhibit unusually

modulated angular distributions. Such recoil anisotropies

are shown to be characteristic of three-photon excitation

to, and dissociation of, super-excited states, with subsequent

one-photon ionization enabling detection of the Rydberg

atoms that result.21 Given the ground state symmetry

(1S1), and that of the predissociated state providing re-

sonance enhancement at the two photon energy (1P), the

observed angular anisotropy allows determination of the

symmetry of the super-excited state (O ¼ 1, most probably1P) populated en route to these particular products. This,

in turn, provides clues as to the electronic promotion(s)

leading to formation of these super-excited state(s), and the

mechanisms of their subsequent fragmentation. The study

serves to provide a particularly clear illustration of both the

advantages, and of the potential complications, associated

with the use of REMPI as a means of preparing state-selected

molecular ions.

2. Experimental method and ab initio calculations

The velocity map ion imaging experiment in Bristol has been

described previously.7,8,22 A skimmed, pulsed molecular beam

containing BrCl (in a Br2/Cl2 mixture) seeded in Ar was directed

at the centre of the detector (i.e. along the z axis). The skimmed

beam was crossed at right angles by the frequency doubled

output of a pulsed dye laser (Spectra-Physics GCR-250 Nd-

YAG laser plus Sirah Cobra Stretch dye laser, yielding an

output bandwidth o0.1 cm�1 in the visible) which propagated

along the x axis and was focused into the interaction volume

with a 20 cm focal length plano-convex lens. Ultraviolet (UV)

excitation frequencies were chosen so as to be resonant, at the

two-photon energy, with the b5(v0 ¼ 0–5) levels of BrCl;

wavelengths (in air) and the corresponding vacuum wavenum-

bers were measured using a wavemeter (Coherent, Wave-

Master). The polarisation vector, e, of the UV radiation was

usually arranged to be perpendicular to the molecular beam axis

and parallel to the front face of the detector (i.e. e // y), but the

radiation could also be circularly polarized by application of an

appropriate voltage to a Pockel’s cell inserted in the UV beam

path. Excitation via the b5(v0 ¼ 3) level was investigated using

both linearly and circularly polarized radiation. The resulting

ions were extracted, along z, under velocity map imaging

conditions. The ion cloud impinged on the front face of a

position sensitive detector (a pair of microchannel plates and a

phosphor screen) mounted 860 mm downstream from the

interaction volume. This was read out by a CCD camera

equipped with a fast intensifier (Photonic Science) that was

gated to the time-of-flight (TOF) appropriate for 79Br1 or35Cl1 ions. Each ion image resulting from a single laser shot

was processed with an event counting, centroiding algorithm

provided with the commercial camera software DaVis

(LaVision) running on a PC, and the resulting counts

accumulated for, typically, 104 laser shots. The accumulated

fragment ion images were analysed by reconstructing the

3-D velocity distribution as described previously23 using

an algorithm based on the filtered back projection method of

Sato et al.24

High-level ab initio calculations for the various singlet

valence and Rydberg states of BrCl deemed most relevant to

the current investigation were performed using the MOLPRO

2002 electronic structure package.25 As in our previous studies

on BrCl1,8 these calculations were carried out in the abelian

C2n sub-group of the full CNn point group at the

CASSCF/MRCI level. The extended molecular orbitals of

the Rydberg states required use of the aug-cc-pVTZ basis

set.26,27 The active space consisted of 14 electrons occupying

the usual valence orbitals, (10s)(11s)(12s)(5p)(6p)(13s), towhich 2 extra s orbitals were added in order to describe the

lowest Rydberg orbitals of interest.

Fig. 1 Illustration of the various fragmentation pathways for form-

ing atomic ions following one-color multiphoton excitation of jet-

cooled BrCl molecules, resonance enhanced at the two-photon energy

by the b5(v0 ¼ 0–5) levels.

5532 | Phys. Chem. Chem. Phys., 2007, 9, 5531–5541 This journal is �c the Owner Societies 2007

3. Results

3.135Cl

1fragment imaging

35Cl1 ion images were recorded at six different wavelengths,

chosen to be resonant (at the two photon energy) with the

b5(v0 ¼ 0–5, v00 ¼ 0) transitions. Fig. 2 shows an illustrative

image, obtained following excitation via the b5(3, 0) transition

(~n ¼ 31 568.4 cm�1). Cl1 velocity distributions were obtained

by integrating the reconstructed 3-D speed distribution over

all angles. These were then converted into total kinetic energy

release (TKER) distributions, assuming an (isotope averaged)

mass mBr ¼ 79.904 amu for the neutral partner fragment. Also

shown in Fig. 2 are the TKER distributions derived from such

images for the Cl1 þ Br products formed when exciting at

~n ¼ 30 807.0, 31 066.4, 31 318.5, 31 568.4, 31 818.3 cm�1 and

32 075.3 cm�1, i.e. via the b5(v0 ¼ 0–5) levels. All of the major

features evident in these spectra can be assigned in terms of

dissociation mechanisms (i) and (iii). These are now consid-

ered in turn.

3.1.1 Dissociation of super-excited states. Super-excited

states are here defined as electronically excited states of the

neutral molecule lying at energies above the ionization poten-

tial of BrCl. As discussed more fully below, generation of the

super-excited states relevant to the present work is best

envisaged in terms of two-photon excitation to the

[2P1/2]5ss Rydberg state, followed by a further one-photon

p* ’ p excitation within the 2P1/2 core.

In the case of 35Cl1 images formed by absorbing three

photons of wavenumber ~n, the TKER (in cm�1) associated

with any given products from dissociation of a super-excited

state will be given by the energy conserving relation:

TKER ¼ 3~n �D0ðBr�ClÞ � EðCl��Þ � EðBr=Br�Þ; ð1Þ

where D0(Br�Cl) is the dissociation energy of BrCl [18 027

cm�1 (ref. 28)]. Table 1 lists TKERs (in cm�1) calculated for

all combinations of [Br þ Cl**] and [Br* þ Cl**] products for

which TKER 4 0 at the various 3~n energies used here, i.e.

combinations for which

3~n �D0ðBr�ClÞ4EðCl��Þ þ EðBr=Br�Þ: ð2Þ

Cl** denotes a Rydberg state of atomic chlorine, with term

value E(Cl**),29 that can be ionized by absorption of one

further photon with wavenumber ~n. All Cl** states identified

in the present study have electronic configuration

[Ne]3s23p4(3P)4s1. Energy conservation requires that the part-

ner bromine atom is formed in its ground (2P3/2, henceforth

Br) or spin–orbit excited (2P1/2, Br*) state. The TKER spectra

in Fig. 2 have been arranged so that a given [Br/Br* þ Cl**]

product channel aligns vertically; assignments of the various

active channels are indicated above the uppermost TKER

spectrum.

3.1.2 Dissociation of BrCl1 molecular ions. The hetero-

nuclear BrCl1 cation can dissociate via either channel (3) or

(4), yielding distinguishable cation products:

BrCl þ þ hn ! Br þ þ Cl ð3Þ

! Brþ Cl þ: ð4Þ

Our previous multiphoton imaging studies of BrCl focused on

Br1 product formation.7,8 Since the ionization potential (IP)

of Cl (104 591 cm�1) is greater than that of Br (95 284.8

cm�1),29 process (4) requires use of shorter photon wave-

lengths than are needed for products (3). The TKERs

(in cm�1) associated with Cl1 ion formation following

one-photon dissociation of BrCl1 at an excitation wavenum-

ber ~n are given by:

TKER ¼ ~n �DiðBr þ � ClÞ � EðCl þÞ � EðBr=Br�Þ � 9306:2

ð5Þ

Fig. 2 Cl1 ion image obtained following excitation via the b5(3, 0)

transition (~n ¼ 31 568.4 cm�1) of BrCl with e aligned vertically (double

headed white arrow), together with TKER distributions of the Cl1 þBr/Br* fragments derived from images such as that shown in (a)

following excitation via b5(v0) levels with v0 ¼ 0 (~n ¼ 30 807.0 cm�1),

1 (~n ¼ 31 066.4 cm�1), 2 (~n ¼ 31 318.5 cm�1), 3 (~n ¼ 31 568.4 cm�1), 4

(~n ¼ 31 818.3 cm�1) and 5 (~n ¼ 32 075.3 cm�1). The TKER spectra are

plotted on a common dispersion and positioned so that a given

product channel aligns vertically, but the o2000 cm�1 part of the

v0 ¼ 4 spectrum is plotted on a �3 expanded vertical scale. Assign-

ments of the various [Cl** þ Br/Br*] product channels are indicated

by the comb festooned above the spectra. Selected peaks at low TKER

due to one photon dissociation of BrCl1(2P1/2, v1) parent ions

yielding [Cl1(3PJ) þ Br(2P3/2)] fragments are indicated by their asso-

ciated (v1: J) values on the relevant spectra.


where Di(Br1–Cl) is the dissociation energy for a BrCl1

molecular ion with internal (spin–orbit and/or vibrational)

energy Ei dissociating via channel (3), E(Cl1) and E(Br) are the

term values of the Cl1 and Br fragments, respectively, and

9306.2 cm�1 is the difference in the threshold energies for

fragmentations (3) and (4). To form Cl1 ions via process (4),

the photon wavenumber must thus satisfy the condition:

~n � DiðBrþ�ClÞ þ 9306:2: ð6Þ

The wavenumber of the photon used to induce BrCl1 dis-

sociation in the present one-color experiments is dictated by

the energy of the state of neutral BrCl resonant at the two

photon energy. Given the propensity for preserving both the

vibrational and core spin–orbit quantum numbers during the

third photon ionization step, it follows that ~n in eqn (6) will be

correlated with Ei and thus Di. From the six excitation

wavenumbers used here, the lowest ~n for which we should

expect 35Cl1 fragment ion formation via dissociation of the

Table 1 List of energetically accessible dissociation channels from super-excited states of BrCl following absorption of three photons withwavenumber ~n. Br/Br* and Cl/Cl* represent atoms with ground state electronic configurations in, respectively, their ground (2P3/2) and spin–orbitexcited (2P1/2) states. Br** and Cl** indicate Rydberg states, with configurations � � �4s24p4[2S11LJ]5s

1 or � � �4s24p4[2S11LJ]5p1, and

� � �3s23p4[2S11LJ]4s1, respectively. For compactness, only the term symbol of the core and the outer electron are listed in the first two columns.

Columns 3–5 give the term values of the respective atomic products, and their sum. Bold characters indicate all those product combinations thatare accessible, energetically, following absorption of three ~n photons, i.e. which satisfy the inequality shown in eqn (2) using the particularresonance enhancing b5(v

0, 0) transition indicated in column 6. Fragmentation channels observed as being open in the present work are shown inbold font

Br term Cl term E(Br)/cm�1 E(Cl)/cm�1 E(total)/cm�1 b5(v0, 0) 3~n � D0(Br � Cl)/cm�1

Br Cl 0 0 0Br Cl* 0 882.4 882.4Br* Cl 3685.2 0 3685.2Br* Cl* 3685.2 882.4 4567.6[3P2]5s;

4P5/2 Cl 63 436.5 0 63 436.5[3P2]5s;

4P5/2 Cl* 63 436.5 882.4 64 318.8[3P2]5s;

4P3/2 Cl 64 907.2 0 64 907.2[3P2]5s;

4P3/2 Cl* 64 907.2 882.4 65 789.5Br1(3P2) Cl�(1S0) 95 284.2 �29 151.9 66 132.3[3P1]5s;

4P1/2 Cl 66 883.9 0 66 883.9[3P1]5s;

4P1/2 Cl* 66 883.9 882.4 67 766.2[3P1]5s;

2P3/2 Cl 67 183.6 0 67 183.6[3P1]5s;

2P3/2 Cl* 67 183.6 882.4 68 065.9(3P0)5s;

2P1/2 Cl 68 970.2 0 68 970.2Br1(3P1) Cl�(1S0) 98 420.6 �29 151.9 69 268.7(3P0)5s;

2P1/2 Cl* 68 970.2 882.4 69 852.6Br1(3P0) Cl�(1S0) 99 121.7 �29 151.9 69 969.8Br [3P]4s; 4P5/2 0 71 958.4 71 958.4Br [3P]4s; 4P3/2 0 72 488.6 72 488.6Br [3P]4s; 4P1/2 0 72 827.0 72 827.0Br [3P]4s; 2P3/2 0 74 225.9 74 225.8 (0, 0) 74 394.0[3P2]5p;

4P5/2 Cl 74 672.3 0 74 672.3Br [3P]4s; 2P1/2 0 74 865.7 74 865.7[3P2]5p;

4P3/2 Cl 75 009.1 0 75 009.1 (1, 0) 75 169.5

[3P2]5p;

4D7/2 Cl 75 521.5 0 75 521.5

[3P2]5p;

4P5/2 Cl* 74 672.3 882.4 75 554.7

Br* [3P]4s; 4P5/2 3685.2 71 958.4 75 643.6[3P2]5p;

4D5/2 Cl 75 697.1 0 75 697.1[3P2]5p;

4P1/2 Cl 75 814.0 0 75 814.0[1D]5s; 2D5/2 Cl 75 890.3 0 75 890.3[3P2]5p;

4P3/2 Cl* 75 009.1 882.4 75 891.5 (2, 0) 75 907.2

[1D]5s; 2D3/2 Cl 75 908.5 0 75 908.5Br* [3P]4s; 4P3/2 3685.2 72 488.6 76 173.8[3P2]5p;

4D7/2 Cl* 75 521.5 882.4 76 403.9Br* [3P]4s; 4P1/2 3685.2 72 827.0 76 512.3[3P2]5p;

4D5/2 Cl* 75 697.1 882.4 76 579.4 (3, 0) 76 680.6[3P2]5p;

4P1/2 Cl* 75 814.0 882.4 76 696.4

[3P2]5p;

4D3/2 Cl 76 743.1 0 76 743.1

[1D]5s; 2D5/2 Cl* 75 890.3 882.4 76 772.7[1D]5s; 2D3/2 Cl* 75 908.5 882.4 76 790.9 (4, 0) 77 398.2Br�(1S0) Cl1(3P2) �27 129.0 104 590.2 77 461.2[3P2]5p;

4D3/2 Cl* 76 743.1 882.4 77 625.4Br* [

3P]4s;

2P3/2 3685.2 74 225.8 77 911.1

[3P1]5p;

2S1/2 Cl 78 076.0 0 78 076.0

Br�(1S0) Cl1(3P1) �27 129.0 105 286.2 78 157.2 (5, 0) 78 202.2Br1(1D2) Cl�(1S0) 107 373.3 �29 151.9 78 221.4Br�(1S0) Cl1(3P1) �27 129.0 105 586.7 78 457.7Br* [3P]4s; 2P1/2 3685.2 74 865.7 78 550.9[3P1]5p;

2S1/2 Cl* 78 076.0 882.4 78 958.4Br1(3P2) Cl(2P3/2) 95 284.2 0 95 284.2Br(2P3/2) Cl1(3P2) 0 104 590.2 104 590.2


REMPI-generated BrCl1 molecular ions is ~n ¼ 31 318.5 cm�1,

exciting the b5(2, 0) band and thus favoring formation of

BrCl1(2P1/2, v1 ¼ 2) parent ions.

Consistent with such expectations, the TKER distribution

measured at ~n ¼ 31 318.5 cm�1 shows two slow peaks at

TKER values that match none of those predicted for the

dissociation of the super-excited BrCl** molecules. These

peaks are assignable to Cl1(3P2) ions formed via dissociation

process (4), from BrCl1 cations in their 2P1/2, v1 ¼ 2 and v1 ¼

3 states, respectively, and are labeled as such in Fig. 2 using the

convention (v1: J). The peak-to-peak separation (485 cm�1) is

reasonable for successive v1 levels of BrCl1(X), matching well

with the reported spacing between v0 ¼ 2 and v0 ¼ 3 levels of

the resonance enhancing b5 Rydberg state (B500 cm�1, ref.

13). Four slow peaks are evident in the TKER spectrum

measured at ~n ¼ 31 568.4 cm�1 (b5(3, 0) transition). The more

intense peaks derive from ions with v1 ¼ 3 (as expected on

Franck–Condon grounds), and reveal formation of Cl1 ions in

both the ground (J ¼ 2) and first excited spin–orbit (J ¼ 1)

states, while the two weaker features are attributable to

formation of ground state products following dissociation

from 2P1/2 parent cations with v1 ¼ 4 and 2. Weak peaks

attributable to dissociation of 2P1/2 parent cations with v1 ¼ 4

and 3 were observed following excitation at ~n ¼ 31 818.3 cm�1

(b5(4, 0) transition), but no features attributable to parent ion

photolysis were identifiable in images recorded at ~n ¼ 32 075.3

cm�1 (b5(5, 0) transition).

No features attributable to Cl1 products from the photo-

dissociation of BrCl1(2P3/2) cations are identified, nor do we

see any peaks consistent with Cl1 þ Br� ion-pair formation.

3.1.3 Recoil angular anisotropy. As the illustrative image in

Fig. 2 shows, the various Cl1 product velocity sub-groups

exhibit anisotropic angular distributions, some of which show

an unusual degree of modulation. The angular distribution of

each well-resolved product channel in this and all other images

recorded with linearly polarized laser radiation were fitted in

terms of expression (7)

PðyÞ ¼ 1

4p1þ

Xn¼2;4;6

bnPnðcosyÞ" #

; ð7Þ

where y is the angle between the laser polarization and the

fragment recoil velocity vector, bn are anisotropy parameters

and Pn are nth order Legendre polynomials. Fig. 3 shows

angular distributions of 35Cl1 products arising in the multi-

photon excitation and dissociation of BrCl at ~n ¼ 31 568.3

cm�1 (b5(3, 0) transition). The distributions shown in Fig. 3a

and 3b are for Cl1 ions arising as a result of one photon

ionization of Cl**(2P3/2) fragments formed by three-photon

excitation to, and dissociation of, a super-excited state using

(a) linearly (i.e. as in the image shown in Fig. 2) and (b)

circularly polarized laser radiation. By energy conservation

arguments, the partner fragment is known to be a ground

(2P3/2) state Br atom. The former image, in particular, is

noteworthy, showing maximum intensity at y B 33 and 1471.

The product recoil anisotropy will be determined by three-

photon excitation to the super-excited state. In the language of

ref. 21, the b5(3, 0) transition is a two photon P ’’ S

excitation. The super-excited state(s) of BrCl populated at the

three-photon energy could thus have S,P or D symmetry. The

general forms of the recoil anisotropy observed under both

linear and circular excitations follow curves B in Fig. 4 and 5

of ref. 21, showing that the dissociating super-excited state(s)

populated at the three photon energy have P symmetry

(i.e. O ¼ 1). P(y) in such a case varies as sin2y cos4y (for

linearly polarized excitation), or sin4y (1 þ cos2y) when using

circularly polarized light. The best-fit values of the anisotropy

parameters (eqn (7)) used to fit the angular distributions in

Fig. 3a and 3b are, respectively: b2 ¼ 1.77 � 0.01, b4 ¼ �0.82� 0.01 and b6 ¼ �0.87 � 0.01, and b2 ¼ �1.15 � 0.07, b4 ¼

Fig. 3 Angular distributions of 35Cl1 products formed by multi-

photon excitation and dissociation of BrCl at ~n ¼ 31 568.3 cm�1

(b5(3, 0) transition). (a) and (b) are for Cl1 ions created by one photon

ionization of Cl**(2P3/2) atoms formed (along with Br(2P3/2) co-

fragments) by three-photon excitation to, and dissociation of, a

super-excited state using (a) linearly and (b) circularly polarized laser

radiation. (c) is for the Cl1(3P2) ions formed (together with Br(2P3/2)

co-products) by linearly polarized, one photon dissociation of

BrCl1(2P1/2, v1 ¼ 3) parent cations. The smooth curve through each

data set is the line of best-fit using eqn (7) and the bn values listed in

the text.


0.13 � 0.09 and b6 ¼ 0.05 � 0.10, where the quoted un-

certainties are 1s values from fitting the 101 r y r 1701 data

in terms of eqn (7) with n r 6. Similar recoil anisotropies,

characterized by a b2 4 þ1 and negative b4 and b6 values arefound for all Cl** þ Br(2PJ) products formed following

linearly polarized excitation via b5(v0, 0) transitions with

1 r v0 r 4. [Cl** þ Br(2PJ)] products formed when exciting

at the lowest and highest wavenumbers (resonant with the

b5(0, 0) and b5(5, 0) transitions) display recoil anisotropies

with b4 and b6 values closer to zero, however, and b2 values

close to þ1 and þ2, respectively.30 All show some deviation

from the limiting values calculated21 for such aP’P’’ Sexcitation; these almost certainly reflect the breakdown of one

or more of the assumptions implicit in deriving the limiting

anisotropy values. For example, the resonance enhancing

b5(v0) levels are predissociated (i.e. have a finite lifetime).

The b5(v0, 0) bands thus have a rotational contour and, as

shown by Dixon,21 the choice of ~n will determine the mix of J0

in the coherent superposition state created by the initial two-

photon absorption and the degree to which rotation degrades

their spatial anisotropy prior to absorption of the third

photon.

The angular distribution shown in Fig. 3c is for the

[Cl1(3P2) þ Br(2P3/2)] products from linearly polarized, one

photon dissociation of BrCl1(2P1/2, v1 ¼ 3) parent cations at

~n ¼ 31 568.4 cm�1 (the innermost bar one of the more intense

rings evident in the image shown in Fig. 2). The best-fit to this

distribution in terms of eqn (7) yields: b2 ¼ 1.65 � 0.01, b4 ¼�0.20 � 0.01 and b6 ¼ �0.11 � 0.01. Similar anisotropy

parameter values were measured for the [Cl1(3P1) þ Br(2P3/2)]

spin–orbit excited products arising in this same dissociation,

and for the [Cl1(3P2) þ Br(2P3/2)] products formed from

BrCl1(2P1/2, v1 ¼ 3) parent cations formed when exciting at

~n ¼ 31 318.5 cm�1. These anisotropy parameter values are

broadly reminiscent of those we have reported previously for

the case of Br1 fragments arising from photolysis of BrCl

(X 2P1/2, v1 ¼ 0) cations8 and, as in that case, we attribute the

non-limiting value of b2 and the non-zero best-fit values of b4and b6 to the effects of (i) some residual alignment of the

parent cation introduced by the multiphoton preparation step

at the instant of the final A 2P ’ X2P excitation, and (ii)

partial saturation of this latter transition at the intensities

required to drive the initial multiphoton excitation.

3.279Br

1fragment imaging

79Br1 ion images were also recorded at wavenumbers appro-

priate for two-photon excitation via each of the b5(v0, 0) (0 r

v0 r 5) bands. Fig. 4 shows the 79Br1 image measured at ~n ¼31 568.4 cm�1 (b5(3, 0) transition). Image analysis reveals

contributions from all three of the fragment ion formation

mechanisms depicted in Fig. 1. As for the Cl1 products,

velocity distributions obtained from the reconstructed 3-D79Br1 recoil distributions were converted into TKER distribu-

tions (assuming mCl ¼ 35.453 amu). Fig. 4 also provides an

overview of the TKER distributions obtained when exciting

via each of b5(v0) (0 r v0 r 5) levels. It proves convenient to

consider the various Br1 forming channels in a different order

to those yielding Cl1 (above).

3.2.1 Dissociation of BrCl1 molecular ions. The TKERs (in

cm�1) of products arising through channel (3) are given by:

TKER ¼ ~n �DiðBrþ�ClÞ � EðBr þÞ � EðCl=Cl�Þ; ð8Þ

where E(Br1) and E(Cl/Cl*) are the respective term values of

the Br1 and Cl fragments. Analysis of the image obtained by

imaging 79Br1 fragments following excitation at ~n ¼ 30 807

cm�1 (b5(0, 0) transition) has been reported previously.7 Most

of the resolved structure was rationalized in terms of one

photon dissociation of BrCl1(X 2PO, v1 ¼ 0) cations to a

range of [Br1(3PJ) þ Cl(2PJ)] limits. In contrast to the Cl1

image analysis above, some Br1 products clearly derive from

photolysis of parent ions with O ¼ 3/2. Measurement and

analysis of the complementary photoelectron image recorded

at this same wavenumber served to reinforce the conclusion

that a fraction (B16%) of the parent ions are formed in the

X 2P3/2 spin–orbit state.7

The earlier analysis failed to account fully for the signal at

higher velocities, however. As the lower panels in Fig. 4 show,

the velocity distributions derived from images recorded when

exciting via higher b5(v0, 0) transitions are less well-resolved,

but have a qualitatively similar appearance. The peaks of the

respective distributions shift to higher TKER, and maximize

at TKER values appropriate for [Br1(3P2) þ Cl(2P3/2)] pro-

duct formation following dissociation of BrCl1(X 2P1/2) ca-

tions with v1 ¼ v0. However, the detailed form of the

distributions, especially at high TKER, requires that BrCl1

cations with v1 a v0 contribute as well. Analysis of the Cl1

images provided term values for the v1 ¼ 0–3 levels of BrCl1

with sufficient accuracy to allow estimation of the term values

for higher v1 levels of the parent ion. The combs superposed

above the various spectra in Fig. 4 indicate predicted TKERs

for the [Br1(3P2) þ Cl/Cl*] products arising from dissociation

of BrCl1(X 2P) cations with O ¼ 1/2 and 3/2. Peaks appearing

at the low TKER end of the main feature (e.g. the peaks at

B3300 and 4900 cm�1 in the case of excitation via the b5(1)

level) are attributable to dissociations with v1 ¼ v0 yielding

Br1(3PJ) products with J ¼ 1 or 0.

3.2.2 Dissociation of super-excited states. As Fig. 4

showed, TKER spectra obtained by monitoring 79Br1 ions

formed by excitation via all b5(v0 4 0) levels show additional

sharp features at low TKER. Three of these, with 1 r v0 r 3,

are illustrated in greater detail in Fig. 5. As in the case of Cl1,

many of these can be unambiguously associated with one

photon ionization of Rydberg atoms (Br** in this case)

formed by dissociation of super-excited states at the energy

of three absorbed photons. As before, to appear in the TKER

spectrum a given Br** product state must satisfy the relations

3~n �D0ðBr�ClÞ4EðBr��Þ � EðCl=Cl�Þ ð9Þ

Table 1 also lists term values for the various possible [Br** þCl/Cl*] product limits, ordered in terms of increasing wave-

number. The dashed vertical lines in Fig. 5 indicate the active

product channels. The populated Br** Rydberg states all have

the (dominant) configuration [Ar]4s23d104p4(3PJ)5p. Most

converge to the ground (3P2) state of Br1, but excitation at

~n ¼ 32 076.4 cm�1 (via the b5(5) level) provides just sufficient

energy to form, and to observe, a 5p Rydberg state converging


to the first spin–orbit excited core [Ar]4s23d104p4(3P1).

Dissociation channels leading to Br** states with configura-

tion [Ar]4s23d104p4(3PJ)5s are open at all 3~n wavenumbers

investigated, as are one or more states with configuration

[Ar]4s23d104p4(1D)5s when exciting via b5 levels with

v0 Z 2, but no such products have been identified in any of

the measured images. Neither are any peaks consistent with

Br1 þ Cl� ion-pair formation.

3.2.3 Recoil angular anisotropy.Anisotropy parameters for

the most intense part of the outer ring (arising via process (3))

in 79Br1 images obtained when exciting via the b5(0) level have

been reported previously.8 As in the corresponding one

photon dissociation yielding Cl1 ions (process (4)), b2 4 1

and b4 and b6 are close to zero. Attempts to extract meaningful

anisotropy parameters from 79Br1 images recorded following

excitation via b5(v0 4 0) levels were thwarted by the density

of active product channels and the limited available image

resolution.

4. Discussion

The resonance enhancing b5 levels of BrCl have dominant

configuration � � �p4p*3[2P]5ss.13 Direct 2 þ 1 REMPI via this

state leads to loss of the Rydberg electron and formation of

ground (X 2P) state BrCl1 cations. The present data reveal

significant probability for an alternative electron excitation,

Fig. 479Br1 image measured at ~n ¼ 31 568.4 cm�1 (b5(3, 0)

transition) with e aligned vertically (as shown by the double headed

white arrow), along with TKER distributions of the [Br1 þ Cl/Cl*]

fragments derived by analyzing 79Br1 ion images obtained by exciting

via b5(v0) levels with v0 ¼ 0 (~n ¼ 30 807.0 cm�1), 1 (~n ¼ 31 065.5 cm�1),

2 (~n ¼ 31 311.4 cm�1), 3 (~n ¼ 31 569.2 cm�1), 4 (~n ¼ 31 808.4 cm�1) and

5 (~n ¼ 32 076.4 cm�1). As in Fig. 2, the TKER spectra are plotted on a

common dispersion and positioned so that a given product channel

aligns vertically. Combs festooned above the spectra indicate the

TKERs expected for [Br1(3P2) þ Cl/Cl*] products arising from one

photon induced photodissociation of BrCl1(2PO, v1) parent ions at

the relevant ~n wavenumber. Each comb is labeled (O: J) to enable

identification of the spin–orbit states of the cation and the Cl

fragment. The long (- - - -) and short (� � �� ) dashed vertical lines

distinguish products from dissociation of parent ions with O¼ 3/2 and

1/2, respectively.

Fig. 5 Detailed view of low energy part of the TKER spectra

obtained by monitoring 79Br1 ions formed by excitation via b5(v0)

levels with 1 r v0 r 3, plotted on a common dispersion and arranged

so that a given [Br** þ Cl/Cl*] product channel aligns vertically. Br**

products formed in association with Cl and Cl* products are distin-

guished by long (- - - -) and short (� � �� ) dashed vertical lines,

respectively.


forming one (or more) super-excited states of BrCl at the

energy of three absorbed photons. The most likely candidate

for this excitation is a p*’ p promotion within the core, with

the outer electron remaining in the same Rydberg orbital. This

particular core excitation was found to be dominant in transi-

tion dipole moment calculations performed from the ground

state to various excited states of the BrCl1 molecular ion,8 and

the discussion that follows is based on such an assignment. We

note that the analogous one-photon pg* ’ pu core promotion

has been invoked to explain the formation of core-excited

Rydberg states in the multiphoton excitation of Cl2 molecules

via the corresponding two-photon resonant intermediate state

21Pg(� � �pu4pg*34ssg).19 We now consider the various frag-

mentation mechanisms in turn.

4.1 Dissociation of super-excited states

The super-excited state(s) populated at the three photon

energy are deduced to have O ¼ 1 (from the measured recoil

anisotropies) and assumed to have dominant electronic con-

figurations 1,3P(� � �p3p45ss) and to be members of Rydberg

series converging to the A 2P(� � �p3p4) state of BrCl1. Ab initio

calculations show that the A 2P state of the ion correlates with

the asymptotic products [Br(2P) þ Cl1(3P)].8 In a diabatic

picture, the parent - product correlation accompanying

Rydberg state dissociation should follow that of the corre-

sponding ion. On this basis, we assume that the1,3P(� � �p3p45ss) super-excited state(s) correlate diabatically

to the products [Br(2P) þ Cl**(3P)4s]. Readers should be

aware of the potential ambiguity introduced by the Rydberg

numbering scheme; the orbital labelled as 5ss in BrCl, the 5s

orbital in atomic Br and the 4s atomic orbital in atomic Cl are

the lowest s Rydberg orbitals in the respective species. [Br(2PJ)

þ Cl**((3P)4s; 2PJ)] and [Br(2PJ) þ Cl**((3P)4s; 4PJ)] atomic

products are indeed identified following excitation via all b5(v0)

levels (see Fig. 2), consistent with three photon induced

dissociation following excitation to the repulsive inner limb

of the 1,3P(� � �p3p45ss) super-excited state potential(s).

The Br1 images (Fig. 3) reveal selective formation of

Br**(3P)5p Rydberg atoms. Ab initio calculations for the

singlet Rydberg states of BrCl provide a rationale for this

observation as well. Specifically, as Fig. 6a shows, the poten-

tial curves for the 1,3P(� � �p3p45ss) super-excited states are

crossed, near their minima, by the long range part of Rydberg

state potentials with configuration � � �p4p35ps—that converge

to the X 2P ground state ion. BrCl1(X 2P) correlates with the

ground state atomic products [Br1(3P) þ Cl(2P)]. Using the

same logic as previously, Rydberg states built on this core (e.g.

states with configuration � � �p4p35ps) should correlate diaba-

tically with the corresponding Br** Rydberg atom products

(i.e. [Br**(3P)5p þ Cl(2P)] in this case). It should be noted that

the curve shown in Fig. 6a for the (X 2P1/2)5ps Rydberg state

is simply the ab initio potential for the (X 2P1/2)5ss state,

uplifted in energy to match the experimentally reported 13

origin for the 1P(� � �p4p35ps) state. Nonetheless, we can be

confident that the (A 2P1/2)5ss and (X 2P1/2)5ps potentials

cross, given that the asymptotic limits associated with the

former lie below those of the latter (Table 1). The Br**

fragments observed in the Br1 images can thus be understood

in terms of two photon resonance enhanced three photon

excitations to the same 1,3P(� � �p3p45ss) super-excited state(s)

as implicated in Cl** atom formation, followed by Rydberg–

Rydberg interaction in the crossing region between the

(A 2P1/2)5ss and (X 2P1/2)5ps potentials.

4.2 Dissociation of molecular ions

Our previous photodissociation studies of Brþ25,6 and BrCl1

cations7,8 relied on direct ionization of the resonance enhan-

cing Rydberg state level to generate the required state-selected

parent molecular ions. However, as the previous sub-section

shows, two photon resonance enhanced, three photon excita-

tion of BrCl also results in population of super-excited1,3P(� � �p3p45ss) Rydberg states. These states lie at energies

above the threshold for forming ground state BrCl1 cations.

Autoionization of these super-excited states thus offers

Fig. 6 (a) Ab initio potential energy curves (spin–orbit averaged) relevant to the present one-color multiphoton excitation studies of BrCl,

resonance enhanced at the two photon energy by levels of the b5 [(X 2P1/2)5ss] state. (The curve shown for the (X 2P1/2)5ps Rydberg state is

simply the (X 2P1/2)5ss potential, raised in energy to match the experimentally reported origin for the 1P(� � �p4p35ps) state.) The asymptotic limits

are as follows: (a) Br þ Cl; (b) Br**(5s) þ Cl; (c) Br**(5p)þ Cl; (d) Br þ Cl**(4s) and (e) Br1 þ Cl, and the shaded bar illustrates the energy range

sampled by three-photon absorption in the present work. (b) Schematic illustration of the competition between dissociation and electronic

autoionization following excitation to one or both of the 1,3P(� � �p3p45ss) super-excited state(s).


another route for generating parent cations in their ground

(2P) electronic state.

The TKERs of the [79Br1 þ Cl] and [Br þ 35Cl1] fragments

resulting from one photon dissociation of BrCl1 (eqns (5) and

(8)) depend not just on the respective product term values but

also on the vibrational energy content of the parent ion

(through the value of Di(Br1–Cl)). Image analysis allows

estimation of the relative contributions from different parent

v1 levels; the distribution of v1 state populations so derived

can give insights into the parent ion formation mechanism(s).

The ionization step in a direct 2 þ 1 REMPI process will

normally show a strong propensity for Dv ¼ v1 � v0 ¼ 0

transitions, because of the very similar equilibrium geometries

of the intermediate Rydberg state and the corresponding ionic

state.31 The excitation wavenumbers used in this work have

been chosen to enable resonance enhanced excitation via

successive b5(v0) levels. If all BrCl1 cations were formed by

direct 2 þ 1 REMPI, the choice of v0 should dictate their v1

quantum number, and thus Di(Br1–Cl). One photon dissocia-

tion should then yield a comparatively simple TKER spec-

trum, with just a single peak for each active [Br1/Cl1(3PJ) þCl/Br(2PJ)] product channel. Such expectations are reasonably

satisfied in TKER spectra obtained by monitoring Br1 ions

when exciting via the b5(0) level but, as Fig. 4 showed, TKER

spectra recorded at higher wavenumbers (i.e. via b5(v’ 4 0)

levels) indicate population of (and dissociation from) a wider

spread of v1 levels, stretching up to the highest v1 quantum

numbers permitted by energy conservation (assuming three

photon absorption).

Electronic autoionization32 offers a route to forming BrCl1

cations in a range of v1 levels. It can occur when, as here, a

Rydberg state belonging to a series converging to an excited

state of the ion (i.e. the 1,3P super-excited states) couples with

the continuum of a lower electronic state of the same ion (i.e.

one, or other, or both spin–orbit components of the X 2P state

in this case). The case of a molecule autoionizing along the

repulsive part of a potential curve during bond extension has

been investigated, both experimentally and theoretically, for

the cases of H2 and NO.33–36 A key signature of the process is

that autoionization can occur over a time interval that is long

enough to allow significant movement of the nuclei—giving

rise to a non-Franck–Condon vibrational population distribu-

tion within the ground state cation. Weak features attributable

to such Dv a 0 transitions are identifiable in the previously

reported 2 þ1 REMPI-photoelectron image of BrCl recorded

at 30 807.6 cm�1 (i.e. when exciting via the b5(0, 0) transition).7

Fig. 6b illustrates the possible competition between disso-

ciation and autoionization following three photon excitation

to the 1,3P(� � �p3p45ss) super-excited states. We assume

that autoionization of this state can occur at any internuclear

separation up to the stabilization point (i.e. the point at

which the potential curves for the super-excited state and

the ground state ion cross). The classical model of Hazi33

then predicts that the resulting BrCl1(X) ions should

be formed with a smoothly varying vibrational state

population distribution, ranging from v1 ¼ v0 up to

the maximum value of v1 allowed energetically. Such

predictions are wholly consistent with the experimental

observations, whereby we deduce maximal population of

parent ion levels with v1 ¼ v0 (from direct REMPI), supple-

mented by a broader v1 population distribution from auto-

ionization.35Cl1 fragments from dissociation of BrCl1(2P1/2) cations

are observed when exciting via b5 levels with 2 r v0 r 4, but

not v0 ¼ 5. The v0 Z 2 onset is understandable in terms of

energy and Franck–Condon arguments (see section 3.1.2), as

is the non-observation of 35Cl1 fragments from dissociation of

BrCl1(2P3/2) cations. Neither do we observe any obvious

indication of 35Cl1 fragments attributable to dissociation from

a spread of parent v1 states, such as might be formed by

autoionization of the super-excited state, at any of the excita-

tion energies for which the [Br(2P) þ Cl1(3P2)] dissociation

channel is energetically open. The previously reported ab initio

potential curves for BrCl1 provide a possible explanation

for these observations.8 Within the energy range investigated,

the dominant transition moment from low v1 levels of the

ground state ion is to the A 2P state. Diabatically, this state

correlates with [Cl1(3P) þ Br(2P)] products, but the calcula-

tions predict strong avoided crossings, for both spin–orbit

components, with a repulsive 2P state. Adiabatically, there-

fore, the calculations predict that much of the flux excited to

the A 2P state will dissociate to the [Br1(3P1)þ Cl(2P3/2)] limit.

We note, however, that wavepacket calculations using these

same potentials and couplings failed to replicate the observed

[Br1(3PJ) þ Cl(2PJ)] spin–orbit branching fractions,8 so there

must be some remaining uncertainty as to the validity of this

rationale.

A few peaks remain unidentified in the lower energy part of

some of the TKER spectra measured monitoring Br1 ions.

These might arise via one photon excitation of BrCl1(X)

cations to an excited state [e.g. the 2S1(� � �s1p4p*4)state] which the ab initio calculations suggest should

correlate with the dissociation limit [Br1(1D2) þ Cl(2P)].8 This

limit lies 12530.3 cm�1 above that for the ground state

products, and could only be reached by higher v1 parent ions

such as are formed via the autoionization mechanism. Un-

fortunately, the term values of such higher v1 levels are not

known to sufficient accuracy to be able to validate (or refute)

this suggestion.

5. Conclusions

Analysis of the TKER and angular distributions of the 79Br1

and 35Cl1 fragments resulting from one-color multiphoton

excitation of BrCl molecules, resonance enhanced at the two

photon energies by the v0 ¼ 0–5 vibrational levels of the

[X 2P1/2]5ss Rydberg state, has enabled identification of three

distinct fragment ion formation mechanisms. Absorption of a

third photon by [X 2P1/2]5ss Rydberg molecules is shown to

yield two distinct outcomes: (i) photoionization (i.e. removal

of the Rydberg electron—a traditional 2 þ 1 REMPI process)

and (ii) p* ’ p excitation within the core, resulting in

formation of one or more super-excited states with O ¼ 1

and configuration [A 2P1/2]5ss. These latter state(s) lie above

the first ionization limit and can autoionize (yielding

BrCl1(X 2P) ions in a wider range of v1 states than would

be expected by direct 2 þ 1 REMPI), and/or fragment to


neutral atoms. Further one-photon absorption by the parent

ions formed by 2 þ 1 REMPI or by autoionization results in

formation of Br1 and (energy permitting) Cl1 fragment ions.

One or other atom formed when the super-excited molecule

fragments will itself be in a Rydberg state. Guided by ab initio

potentials, we deduce that [Cl**[3PJ]4s þ Br/Br*] products

arise from direct dissociation of the super-excited state(s),

whereas [Br**[3PJ]5p þ Cl/Cl*] product formation occurs as

a result of interaction between the [A 2P1/2]5ss and

[X 2P1/2]5ps Rydberg potentials at extended Br–Cl bond

lengths. Absorption of one further photon by the resulting

Br** and Cl** Rydberg atoms leads to their ionization, and

thus their appearance in the resulting Br1 and Cl1 fragment

ion images.

The present results, taken together with those reported

previously from similar multiphoton excitation and ionization

studies of Br25,6 and Cl2,

19,20 serve to illustrate both the

advantages, and the potential complications, associated with

REMPI as a means of preparing state-selected molecular ions.

In favorable cases, n þ 1 REMPI via an appropriate

Rydberg state can serve as a clean and efficient route to

forming quantum-state selected molecular ions for subsequent

use in, for example, photodissociation or reaction dynamics

experiments. Such will apply in those cases where the cross-

section for ionizing the Rydberg electron, by one-photon

absorption at the chosen excitation wavelength, far exceeds

that for any alternative photo-excitation process. Multiphoton

excitations can drive multi-electron promotions, however—as

illustrated by the present results. Thus, particularly in cases

where the parent ion shows strong absorption at the chosen

excitation wavelength, the corresponding core excitation may

have a greater cross-section than that for promoting the

Rydberg electron into the continuum. The super-excited states

that result can decay by autoionizing and/or by (pre)disso-

ciating. The (pre)dissociation products may include Rydberg

species, lying at energies within one photon of their ionization

limit. These will be very susceptible to ionization at the

intensities prevailing in most REMPI experiments. Such a

multiplicity of possible ion (and electron) formation channels

is unlikely to be revealed through the traditional parent

REMPI spectrum (in which some selected (or the total)

ion yield is measured as a function of excitation wavelength)

but can, at least in the case of small molecules, be unraveled

by kinetic energy resolving the resulting ions and/or

photoelectrons.

Acknowledgements

The authors acknowledge financial support from the

EPSRC (via the pilot portfolio partnership LASER) and the

Commission of the European Communities [IHP contract

no. HPRN-CT-2002-00183 (PICNIC), and MEIF-CT-2003-

500999 (PHOSPHOR) to NHN]. We are also most grateful

to colleagues J. R. Jones, K. N. Rosser and A. D. Webb,

Drs J. N. Harvey, F. R. Manby and Prof. A. J. Orr-Ewing for

their many and varied contributions to the work described

herein.

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Multiphoton dissociation dynamics of BrCl and the BrCl cation · Multiphoton dissociation dynamics...

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