Chemical Physics Letters 506 (2011) 156–160

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Chemical Physics Letters

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The kinetic energy release in the photodissociation of anilineðwaterÞþn (n = 1–10)clusters at photon energies from 0.43 to 4.66 eV

Md. Alauddin, Jae Kyu Song, Seung Min Park ⇑Department of Chemistry and Research Center for New Nano Bio Fusion Technology, Kyung Hee University, Seoul 130-701, Republic of Korea

a r t i c l e i n f o

Article history:Received 19 February 2011In final form 8 March 2011Available online 11 March 2011

0009-2614/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.cplett.2011.03.026

⇑ Corresponding author.E-mail address: smpark@khu.ac.kr (S.M. Park).

a b s t r a c t

Photodissociation dynamics of hydrated aniline cluster cations, anilineðH2OÞþn (n = 1–10) was investi-gated in the infrared and UV–visible region focusing on the kinetic energy release (KER). In case of infra-red photodissociation at 0.43 eV, the KER was independent of the cluster size while it was clearly size-dependent at UV–visible energies. The excitation of different vibrational modes had no effect on theKER despite that the internal energies of the parent ions were different. The KER increased from 53 to160 meV for anilineðH2OÞþ1 cluster with increase in the photon energy from 0.43 to 4.66 eV.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

The photodissociation of size-selected cluster ions in gas phasehas been a common approach in cluster-ion spectroscopy. In par-ticular, partitioning of the available energy during photofragmen-tation is of unique interest and many researchers havecapitalized on the manifold advantages by photofragment spec-troscopy to investigate the dynamics of chemical reactions andpartitioning of the available energy occurring in the confines ofweakly bound clusters [1–11].

The general applicability of the photofragment spectroscopyand its ability to accurately determine the kinetic energy release(KER) make it useful in cluster-based reaction where differentproduct fragments in different final states are produced. The resul-tant spectra from the product fragments are found to be extremelysensitive to numerous aspects of the cluster environment such asthe cluster geometry and the details of the intermolecular poten-tials involved.

For the study of small cluster ions of rare-gas atoms, measure-ments of the kinetic energy and angular distributions of the prod-ucts have provided detailed information on the dynamics of thedissociation processes [12–14]. On the other hand, for the largercluster ions of rare-gases, metals, semiconductor, and so forth,the fragmentation mass spectra have been analyzed to determinewhether the fragmentation is simultaneous or sequential [15–22].

Photodissociation processes of cluster ions consisting of mole-cules are more challenging compare to those of atoms in that thereexist intramolecular vibrational modes in addition to the intermo-lecular vibrational modes. Excitation of intramolecular modes playan important role in the photodissociation kinetics and dynamics.Bowers and co-workers [23–27] have studied photodissociation of

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dimer and trimer ions of small molecules such as NOþ2 , ðN2Þþ2 ,ðSO2Þþ2 , ðNOÞþ3 , and ðCO2Þþ3 and reported on the kinetic energiesand angular distributions of the fragment ions to shed light onthe nature of the excited state, partitioning of the excess energy,and dissociation pathways.

In case of larger clusters, there are so many factors such asnumerous degrees of freedom and electronic excitation which of-ten relaxes into vibrations faster than any other processes. Re-cently, photofragmentation of hydrated cluster ions with an ioncore has been a topic of utmost research interest to explore the en-ergy transfer process via hydrogen bond networks [21,28–33].Nam et al. have reported on the photodissociation dynamics of hy-drated adenine cluster ions and observed that adenine monomerions are produced due to fragmentation of the solvent water mol-ecules via three-photon process, where the fragmentation takesplace in the vibrationally excited electronic ground state and inthe vibrationally excited ionic states [33].

Here, we discuss the photodissociation dynamics of anilinewater cluster cation, AnWþ

n (An = aniline, W = H2O, n = 1–10) inthe mid-infrared, visible, and ultraviolet region mainly focusingon the KER. We measured the kinetic energy carried by a neutralfragment, H2O, ejected via photodissociation in the mid-infraredregion at 3430 nm (0.43 eV), in the visible, 415–691 nm (2.99–1.79 eV), and in the ultraviolet, 266–355 nm (4.66–3.49 eV) to ver-ify its dependence on the cluster size, energy, and, furthermore,vibrational modes.

2. Experimental

Major features of the experimental setup have been describedpreviously [21]. Briefly, the experimental vacuum system consistsof two chambers: a source chamber and a main chamber with atandem mass spectrometer. Neutral aniline–water cluster beam

Md. Alauddin et al. / Chemical Physics Letters 506 (2011) 156–160 157

was generated by injecting the gas mixture of aniline, water, andHe (27 psi) into a high vacuum chamber through a pulse nozzle(General Valve, Series 9). Aniline–water clusters were ionized byabsorption of two 266 nm photons (the fourth harmonic of aNd:YAG laser, Continuum, SL III-10) in the first stage of the tandemmass spectrometer and accelerated into the field-free region.AnWþ

n cluster ions were mass selected using a mass gate and en-tered into the second stage, where photodissociation took place.For vibrational excitation, infrared radiation in the range of2600–4000 cm�1 was generated using an infrared optical paramet-ric oscillator (OPO) system. The spectral bandwidth of the OPO sys-tem was �1 cm�1. Another OPO laser (Continuum, Panther EXPLUS OPO) was employed to create photons in 410–450 nm wave-length range for electronic excitation of hydrated aniline clustercations. Dye lasers (Quantel, TDL90 and Lumonics, Spectrum Mas-ter) were also adopted to generate photons ranging from 600 to700 nm.

3. Result and discussion

3.1. Photodissociation mass spectra

Typical photodissociation mass spectrum for AnWþ7 which

shows the positive daughter ion signal and negative parent ion in

Figure 1. Photodissociation mass spectra (laser on–laser off) of AnWþ7 obtained by

exciting bonded OH mode showing (a) daughter ions and (b) neutral fragment(water).

Figure 1a was obtained by subtracting the mass spectrum withthe IR laser off from on at 3462 cm�1 corresponding to the bondedOH mode of water. To increase the signal intensity of neutral waterfragments impinging on the detector, the voltages applied to theelectrodes in the photodissociation region were changed fromthose for daughter ions, and the neutral water fragments were de-tected as shown in Figure 1b. The peaks of daughter ions and neu-tral water fragments were assigned based on the simulation ofmass spectrum using the SIMION software.

As shown in Figure 1a, mostly single water solvent moleculewas ejected by infrared absorption as detected for the time win-dow in our experiment, which was measured to be �300 ns. Dueto the limited time window, the ejection of two or more water mol-ecules was negligible although it is ultimately allowed because theaverage total internal energy of the parent ions prior to photodis-sociation is estimated to be as large as �0.8 eV [34]. The bindingenergies of water molecules range from 0.4 to 0.8 eV and the totalinternal energy of parent ions after absorption of an infrared pho-ton of �0.4 eV may surpass the energy required to kick out twowater molecules. For larger clusters, in particular, the bindingenergies are relatively smaller since the water molecules awayfrom the aniline ion core are more loosely bound due to the re-duced ion–dipole interaction. The single-photon absorption wasconfirmed by power-dependence experiments as illustrated inFigure 2.

3.2. Derivation of the kinetic energy release

The time-of-flight (TOF) distributions of neutral fragments to-gether with those of parent ions as shown in the insets of Figure 3were measured to determine the KER in the photofragmentationprocess. Neutral fragment, H2O arising from the metastable decayprocesses can appear at the same arrival time as the photofrag-ments and inevitably contributes to the signal [35,36]. Henceforth,the background TOF spectrum recorded without infrared irradia-tion was subtracted from that with infrared on in order to removeinterfering metastable ion signals. In the photodissociation pro-cess, the neutral fragment, H2O is produced as a molecule ratherthan neutral clusters including dimers as indicated by a single neu-tral mass peak in Figure 1b. Namely, the H2O molecule with thelowest binding energy, presumably the one bound farthest awayfrom the ion core, is considered to be released. The width of thedistribution of the neutral fragments, H2O is obviously broader

Figure 2. The intensities of daughter ion as a function of laser intensity at3430 cm�1 (NH mode) for AnWþ

1 , which indicates that the infrared photodissoci-ation is a single-photon process.

Figure 3. The kinetic energy release by photodissociation of AnþWn (n = 1–10) at 0.43 eV as a function of cluster size. The insets show typical normalized distributions ofarrival times of the parent ions AnþWn (n = 1,4) and the neutral fragment (water) used to obtain the kinetic energy release as described in the Section 3.2. Each distribution isfitted with a GAUSSIAN function (solid line) to determine the peak width.

158 Md. Alauddin et al. / Chemical Physics Letters 506 (2011) 156–160

than that of the parent ions due to the KER during the fragmenta-tion process. The observed distributions of the neutral fragment,H2O as well as the parent ion for all clusters size are well fit by asingle GAUSSIAN function, which indicates that the velocity of neutralfragment, H2O has a Boltzmann distribution.

The kinetic energy of the neutral fragment can be determinedfrom the broadening width of the profile of the TOF distributions.Baldwin and co-workers [37] reported that the broadening widthWt, resulting from the KER can be obtained from the relationshipWt

2 = Wf2 �Wp

2, where Wf and Wp are the widths at 22% of themaximum of the TOF profiles for neutral fragment and the parention, respectively. It is of note that the formula Wt

2 = Wf �Wp2 is

applicable only if both parent ion and neutral fragment peakshapes are GAUSSIAN. This method is similar to that employed inthe study of metastable decomposition of ammonia cluster ionsby Castleman and co-workers [38] who calculated the kinetic en-ergy of the daughter ions during the decomposition of parent ion.

In the photodissociation process, the average kinetic energy, eav

carried by one neutral molecule can be expressed as

eav � m½WtV2=ð2LÞ�2=2

where m is the mass of a water molecule, L (= 0.867 m) is the driftlength of the neutral fragment, H2O to the detector, V is the velocityof the parent ion in the laboratory frame, and Wt is the broadeningwidth. In case of infrared photodissociation of AnWþ

1 at 0.43 eV, theTOF profiles displayed in the inset of Figure 3 show that the widthsof Wf = 63.34 ns and Wp = 18.5 ns yield the broadening width ofWt = 60.58 ns and subsequently V was calculated to be1.16 � 105 m/s. Then, the kinetic energy release of 53.58 meV is ob-tained from the excitation of AnWþ

1 at 0.43 eV. The KERs in the pho-todissociation of larger cluster ions were obtained similarly.

3.3. Kinetic energy release: The cluster size dependence

Figure 3 shows that the KER during the photofragmentation ofsingle neutral H2O from the parent ions, AnWþ

n (n = 1–10) at0.43 eV. At first glance, the KER looks just independent on thenumber of solvent molecules but the details need to be understood

with a relation between the temperature of the dissociating ionsand the resultant KER.

According to the quasiequilibrium theory on the unimoleculardecomposition introduced by Klots [39] in 1973 and further devel-oped by Stace [40] later, the average kinetic energy released, EKER,due to photoabsorption can be assumed to be

EKER ¼ kBðT� þ DTÞ

where kB is Boltzmann’s constant, T⁄ is the temperature of the reac-tion transition state (namely, for metastable decay) without photo-excitation, and DT denotes an increase in temperature byabsorption of an infrared photon. The apparent independence ofEKER on the cluster size in Figure 3 thus reflects the revere behaviorsof T⁄ and DT on the size, or, number of vibrational and rotational de-grees of freedom in the cluster; T⁄ increases with the cluster sizewhile DT decreases [40]. Nakai et al. also reported that EKER is inde-pendent on the clusters size in the photodissociation of benzenecluster cations [41].

The kinetic energy release in the photofragmentation of neutralfragment H2O from the parent ion, AnWþ

n (n = 1–4) at differentphoton energies in the UV–visible region is shown in Figure 4.The dominant dissociation channel during the photofragmentationof AnWþ

n (n = 2–4) clusters at different photon energies in the UV–visible region are the formation of An+ and nH2O:

½AnWþn �� ! ½AnWn�1�� þW

½AnWþn�1�� ! ½AnWn�2�� þW

½AnWþ1 �� ! Anþ þ W

where the asterisk denotes the cluster ion with sufficient internalenergy for the next evaporation. The ejection of the neutral frag-ment, H2O continues until the internal energy of the cluster be-comes low enough unable to overcome the dissociation barrier.The kinetic energy carried by all the sequentially ejected H2O mol-ecules contributes to the observed broadening width of the TOFprofiles, although we cannot measure the kinetic energy releasefor each step independently.

Figure 4. The kinetic energy release in the photodissociation of AnþWn (n = 1–4), at2.33, 3.49, and 4.66 eV.

Figure 5. The kinetic energy release in the photodissociation of AnþW1 as afunction of photon energy ranging from infrared to ultraviolet. The inset shows thatthe kinetic energy release is mode-independent.

Md. Alauddin et al. / Chemical Physics Letters 506 (2011) 156–160 159

For n = 4 clusters, we found that all four water molecules areejected at 2.33, 3.49, and 4.66 eV: the total amount of calculatedbinding energies for n = 4 ? n = 3, n = 3 ? n = 2, n = 2 ? n = 1, andn = 1 ? n = 0 is 2.70 eV: 0.5636 eV for n = 4 ? n = 3, 0.5875 eV forn = 3 ? n = 2, 0.7274 eV for n = 2 ? n = 1, and 0.8193 eV forn = 1 ? n = 0 [21]. The overall binding energy so obtained is0.37 eV greater than that of the lowest photon energy, 2.33 eV.To understand such discrepancy, it is important to analyze theinternal energy of the parent cluster ions. Upon ionization of ani-line molecule by two-photon absorption at 266 nm, the internalenergy of aniline ion has been estimated to be 0.8 eV [34]. More-over, according to Kim et al. the ionization energies for the solvatedclusters decrease with the increase in the number of solvent mol-ecules [42] and, accordingly, larger cluster ions have in generalmore internal energy. For n = 4 clusters, it is thus energetically pos-sible to lose all four water molecule at 2.33 eV with an aid of theinitial internal energy of the parent ion. For the photodissociationof AnWþ

4 clusters at 2.33 eV, the KER is determined to be103 meV from the arrival-time distribution of the neutral frag-ment. This corresponds to 3.3% of the total internal energy if it isloosely assumed that the initial internal energy of the parent ionbefore photoexcitation is 0.8 eV.

Ohashi et al. [43] reported that only a small fraction (maximum10%) of the available energy is partitioned into the kinetic energyduring the photodissociation of benzene cluster ions, ðC6H6Þþn(n = 2–8) in the photon energy ranging from 1.17 to 1.62 eV. TheKER for n = 4 clusters at 3.49 eV was 134 meV, 3.1% of the totalinternal energy, which is certainly larger than that at 2.33 eV. At4.66 eV, 160 meV was the kinetic energy release. This amounts to2.9% of the total internal energy. For the photodissociation ofAnWþ

n (n = 1–4) clusters at the photon energy range of 2.33–4.66 eV, we also observed similar behaviors which is shown in Fig-ure 4. This leads us to conclude that the ejection of water moleculein the hydrated cluster ions is an evaporation process. On the con-trary, the photofragmentation of cluster ions of main-group ele-ments (C, Si, Ge, Sb, Bi and so forth) is to proceed via the ejectionof neutral clusters, not as a sequential evaporation of neutral mol-ecules [17,44]. For instance, the loss of C3 from Cþn is reported to beindependent of photon energy [18].

3.4. Kinetic energy release: The photon energy dependence

As shown in Figure 3, the KER turned out not to depend on clus-ter size apparently. Now, it would be intriguing to find out whether

it would depend on vibrational modes or not. In this regard, wecarried out experiment for the infrared photodissociation ofAnWþ

1 at different vibrational modes. Figure 5 illustrates thedependence of the KER in the photodissociation of AnWþ

1 on thephoton energy ranging from infrared to ultraviolet. In particular,the inset shows the vibrational energy dependence, varying infra-red energy over four different vibrational modes: bonded NH modeof aniline ion at 3099 cm�1, free NH mode of aniline ion at3430 cm�1, sym. free OH mode of H2O at 3626 cm�1, and asym.free OH mode of H2O at 3711 cm�1. Interestingly, the KER for allthe different vibrational modes were nearly identical. This mani-fests that the excitation of a particular vibrational modes and thesubsequent change in the internal energy do not play a significantrole in determining the kinetic energy release although the infra-red predissociation is known to be quiet fast and just partially sta-tistical [32]. However, the energy dependence of the kinetic energyrelease was apparent as the internal increases up to few eV, whichshows a similar behavior compared with the experimental resultsfit by a phase space theory calculation for (C6H6)2

+ [9].In summary, we measured the KER during the photodissocia-

tion of AnþWn (n = 1–10) clusters at photon energies ranging fromIR to UV. The KER was in essence independent of the number of thesolvent molecules at 0.43 eV. The dependence of the KER on thephoton energy was clear: it increased from 53 meV to 160 meVfor AnþW1 cluster with the increase in photon energy from0.43 eV to 4.66 eV. This indicates that, after ejection of a watermolecule, the available energy is rapidly converted into kinetic en-ergy of fragments. At higher photon energies, the portion of theKER among the whole internal energy was relatively small. Thesmall fraction of the available energy partitioned into the kineticenergy of the fragments indicates that the upper state in the photo-absorption process is a bound state.

Acknowledgements

This research was supported by the Basic Science Research Pro-gram through the National Research Foundation of Korea (NRF)funded by the Ministry of Education, Science, and Technology(2010-0 26 00).

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