Jeremy M. Merritt, Vladimir E. Bondybey,and Michael C. Heaven

Ionization Energy Measurements and Spectroscopy of HfO and HfO+

DoE

Motivation

Previous studies of ThO ionization revealed unexpected results. Ionization of the molecule weakened the bond (D0-D0

+=0.3 eV), but the vibrational frequency for the ThO+ ion was higher (e

+=955 vs. e=896 cm-1) and the bond length was shorter (Re

+=1.807 vs. Re=1.840 Å). Franck-Condon principle violations were observed in the photoelectron spectrum.

Is this unusual behavior of ThO simply a consequence of the electronic configurations involved or are relativistic effects playing a significant role?

A comparison of Hf2+(6s2)O2- with Th2+(7s2)O2- can be used to probe this question. Ionization energy (IE) measurements by Rauh & Ackerman indicate that D0(HfO)>D0(HfO+). In the present work we obtained an accurate IE and the first spectroscopic constants for HfO+.

Th+ + O

Th + O

D0+

D0

IE(Th)

IE(ThO)

Ionization makes the Th-O bond weaker but stiffer

IE(Th)=6.3067 eV

IE(ThO)=6.6027 eV

Hence, the ThO+

bond is weaker

D0-D0+=0.296 eV

but its vibrationalfrequency is higher

e /cm-1

ThO 895.77ThO+ 954.97

LIFREMPI& ZEKE

Experimental Techniques

Pulsed laser vaporization of metal samples.

Laser induced fluorescence spectroscopy of neutral species.

Two color resonance enhanced multi-photon ionization spectroscopy with mass selection.

Pulsed-field ionization zero kinetic energy photoelectron spectroscopy

Multi-Photon Ionization Processes

HfO

HfO*

HfO+ + e-

hv1

hv2

hv1

hv2

REMPI ZEKEPIE MATI

Pulsedelectricfield

6 8 10 12 14 16 18

0

200

400

12.8 13.0 13.2 13.4 13.6

HfO+

Ion

Sig

nal

(arb

. uni

ts)

Time of Flight (s)

Hf+

gas pulse onIon

Sig

na

l (a

rb.

un

its)

Time of Flight (s)

1

2

3

45 6

gas pulse off

HfO

Time-of-flight mass spectrum showing the products from pulsed laser ablation of Hf

Hf %174 0.2176 5.3177 18.6178 27.3179 13.6180 35.1

Isotopes

(Hf)n+ clusters up to n=6 are observed, as well as signals due to (Hf)nO+, upon non-resonant ionization

with 193 nm light. The inset shows a higher resolution mass spectrum recorded with a longer flight tube to aid in separating the 6 naturally occurring isotopes of Hf. Resonant excitation of the G-X band of HfO has been used in this case. The peaks observed when the gas pulse is turned off are due to background impurity molecules.

27000 27500 28000 28500 29000

27000 27500 28000 28500 29000 I

nteg

rate

d F

luor

esce

nce

(au)

F-X(1-0) (4-2) (2-0)(3-2) (1-0)(2-2) (0-0)

(4-0)(4-1) (3-0)

‡

‡

‡‡

‡‡

*

*

*

* *

wavenumber (cm-1)

*

†

‡

(2-0)E-X

Laser induced fluorescence spectrumfor jet-cooled HfO

Survey scan in the region of the E-X and F-X bands systems of HfO. The peak marked with a † is assigned to the D X 4-0 transition. Sequence band transitions marked with a ‡ have been tentatively assigned as originating from a metastable triplet state. Peaks marked with an asterisk are due to atomic Hf.

HfO

30030 30040 30050 30060

0.0

0.3

0.6

Sig

nal (

arb.

uni

ts)

wavenumber (cm-1)

LIF

Simulation

RE

MP

I

G1(v=0) – X 1(v=0)

Hf %174 0.2176 5.3177 18.6178 27.3179 13.6180 35.1

Comparison of LIF and REMPI spectra

63500 64000 64500 65000 65500

0

6

12

18

63940 63950 63960 63970

HfO

+ S

igna

l (a.

u.)

Total Energy (cm-1)

HfO

+ S

ign

al (

a.u

.)

Total Energy (cm-1)

Photoionization efficiency (PIE) spectra for HfO recorded with the first laser tuned to the P(3) line of the F(0+) (v’=0) X 1(v”=0) band at 27353 cm-1. The insert shows part of the spectrum recorded under higher resolution illustrating the sharp resonance structure above the ionization threshold. The different traces in the inset correspond to gating on the different isotopomers of HfO+.

IE=

637

60 c

m-1

Two-color photoionization of HfO

64000 65000 66000 67000

0

10

20

30

40

P

FI-

ZE

KE

Sig

na

l (a

rb. u

nits

)

Total Energy (cm-1)

v+ = 0

1

2

3*

Tv+ = Te + e(v+1/2) – exe(v+1/2)2

e = 1017.7 cm-1 exe = 3.2 cm-1

No evidence forexcited electronicstates

Vibrationally resolved photoelectron spectrum for HfO

Excitation via F1 (v=2) X1 (v=0)

63840 63860 63880 63900 63920 63940 63960

0

via J'=13

via J'=11

via J'=9

via J'=6

1615141312111098765

PF

I-Z

EK

E S

igna

l (a.

u.)

Total Energy (cm-1)

N+ 4

via J'=4

...

0

2

0 2 BRRe

Rotational constant for the ion is B0

+ = 0.403(5) cm-1

1.687(3) Å

Rotationally resolved photoelectron spectrum for HfO

Rotational structure confirms that the electronic ground state for HfO+ is X2+.

Electron impact PFI-ZEKE

6.1(1) U 6.194

5.6(1) UO 6.031

5.6(1) Th 6.307

6.1(1) ThO 6.604

6.65(10) Hf 6.825

7.55(10) HfO 7.917

IE for HfO is greater than the literature value

Electronic structure calculations for HfO and HfO+

1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.30

10000

20000

60000

70000

80000

90000

CC

SD(T

) en

ergy

(cm

-1)

R (Angstrom)

HfO+

HfO

Hf = ECP60MWB (8s7p6d2f1g)/[6s5p3d2f1g]

Hf: 5s2 5p6 5d2 6s2

O = aug-cc-pVTZ

Molpro

HF MP2 CISD CCSD CCSD(T) B3LYP Expa

B0 .39171 .38566 .39350 .39017 .38475.38775b

.38180c

.38242 0.386537(7)

B0+ .40959 .39473 .40807 .40422 .39722

.40061b

.39432c

.39601 0.403(5)

G1/2 1036.69 989.67 1043.11 1024.93 988.62989.56b

976.76c

969.75 974.09

G1/2+ 1112.78 1004.54 1099.09 1077.03 1027.35

1030.19b

1015.81c

1019.79 1013(1)

IE 6.7810 7.5466 7.30377.361e

7.374c,e

7.6687 7.73657.7371b

7.7485c

7.753c,e

7.7286 7.91687(10)

CASSCFd MRCISDd MRCISD(Q) CCSDTIE 6.480c,e 7.357 c,e 7.631 c,e 7.755 c,e 7.91687(10)

Computed properties of HfO and HfO+ All constants are given in cm-1 units except for the

ionization energy which is in eV.

HfO/HfO+ ThO/ThO+

IE(eV) 7.91687 6.6026

D0-D0+ (eV) 1.092 0.2957

e+(cm-1) 1017.7 954.97

e 974.09 859.77

Be+ 0.403 0.3451

Be 0.3865 0.3326

Comparison of measured properties for HfO and ThO

Both HfO and ThO exhibit MO+ bonds that are weaker and stifferthan those of the neutral molecules. This behavior is associated with the ns2 (n-1)d2 configuration of the metal atom.

Conclusions

The HfO IE measured by PFI-ZEKE is 0.37 eV higher than previous estimates from electron impact measurements.

The Hf-O bond is weakened by ionization. However, the bond length contracts and the vibrational frequency increases. This is the same as the anomalous behavior observed for ThO.

Franck-Condon violations observed in the photoelectron spectrum of ThO were not present in the spectra for HfO. This difference is attributed to a mixing of ionic and neutral states of ThO which is not possible for the excited levels of HfO+.