Chemical Physics Letters 476 (2009) 15–18

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Chemical Physics Letters

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Photochemistry and solvation of HI(H2O)n clusters: Evidence of biradical formation

Darren P. Hydutsky, Nicholas J. Bianco, A.W. Castleman Jr. *

Departments of Chemistry and Physics, Pennsylvania State University, 104 Chemistry Building, University Park, PA 16802, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 March 2008In final form 4 May 2009Available online 7 May 2009

0009-2614/$ - see front matter � 2009 Published bydoi:10.1016/j.cplett.2009.05.005

* Corresponding author. Fax: +1 814 865 5235.E-mail addresses: awc@psu.edu, cmf11@psu.edu (A

During the course of investigating the solvation and photochemical dynamics of HI(H2O)n clustersemploying a pump–probe technique, we observed unusual behavior associated with iodine atoms disso-ciating from the HI(H2O)n clusters, displaying different time responses than for the case of the HI mono-mer or HI clusters. Further experimentation provided experimental evidence for a theoretically predictedbiradical species which consists of a halo-acid clustered to water molecules. In addition, as a side issue,we discuss our results in conjunction with others that point to a possible alternative explanation com-pared to the traditionally accepted one for the phenomena known as the solvated electron.

� 2009 Published by Elsevier B.V.

1. Introduction tron solvation which we then touch on as a side issue in the discus-

Understanding the mechanism of acid dissolution and concom-itant ion-pair formation is a subject of considerable current inter-est. Study of the mechanisms associated with proton transfer andionization dynamics has been the subject of several theoreticaland experimental studies in recent years, with particular focus inour laboratory on the minimum number of water molecules re-quired for ion-pair formation [1–7]. Recent interest has turned tothe photophysics of hydrated complexes, where mechanisms arefar less well understood [6–8]. Among the considered operativeprocesses, the role of a biradical species has been proposed bySobolewski and Domcke as a likely mechanism which will ensueupon the photoexcitation of the HCl(H2O)4 cluster, for example [9].

The solvation of halo-acids is a process of long standing interest,bearing on fundamental aspects of ion-pair formation, as well asother related issues pertaining to atmospheric science [3]. Ourmotivation for investigating simple acids solvated in water clustersstems from our interest in solvation dynamics, as well as under-standing heterogeneous atmospheric processes involving the re-moval of halogens from sea-salt aerosols [10–16]. Prior studies inour laboratory of HBr [6,7] and HI [8] solvated by water clus-ters have focused on interrogating the dynamics of solvent reorga-nization around newly formed ion-pairs. Recent exploration ofHI(H2O)n clusters (with n in the range 0–6) revealed unexpecteddynamics associated with the iodine atom evolving from waterclusters in contrast to phenomena observed with the sole presenceof the HI monomer. Upon further experimentation, we came to theconclusion that we were obtaining strong evidence for a theoreti-cally predicted species called a biradical [9].

Sobolewski and Domcke, whose work pointed to a mechanisminvolving this species, also suggested that finding this mechanismmight have a bearing on interpretations of the mechanism of elec-

Elsevier B.V.

.W. Castleman Jr).

sion section. Though the espoused ideas remain controversial, overthe years others have proposed alternative structures for thehydrated electron [17–19]. The theory of Sobolewski and Domckediffers from the cavity model for the solvated electron in that itaccounts for the observed phenomena via a solvated hydroniummoiety.

2. Experiment

In our experiments, a femtosecond pump–probe technique [20]was employed to track the dynamics of HI(H2O)n, and (HI)n clus-ters, as well as neat HI. The technique consists of a femtosecondpump pulse to initiate photoexcitation and a delayed probe pulseto monitor ensuing photochemical reactions. Thus, we are able tofollow a photochemical reaction on a femtosecond time scale,but the focus here is on the picosecond timescale. Further detailsabout the experimental setup are presented in previous publica-tions (see for example: [8]). For the purposes of this work, thepump energy is 6.2 eV and the probe energy is 3.1 eV.

Depending on the cluster, different amounts of energy and thusdifferent numbers of photons of the probe are necessary to ionize aphoto-excited cluster for detection via time-of-flight mass spec-trometry. Probe laser fluence studies were performed by adjustingthe power of the probe laser while leaving the power of the pumplaser constant. The logarithm of the probe laser power is plottedverse the log of the signal intensity of a cluster or atom in the massspectrum. The slope of the first order line that fits the data is takento be the number of probe photons needed to ionize the cluster oratom.

3. Results and discussion

During the course of our experiments on HI(H2O)n clusters, weobserved a long growth in the I+ transient (see Fig. 1A). Since the

16 D.P. Hydutsky et al. / Chemical Physics Letters 476 (2009) 15–18

observed dynamics of the I atom could have originated from any ofthe species in the cluster distribution, we performed several con-trol experiments to confirm that the dynamics seen were the resultof I atoms that dissociate from an HI(H2O)n cluster. Specifically, HImonomer and HI cluster experiments were also performed. Signif-icantly, only a small plateau (Fig. 1B) was seen in the case of the HImonomer, while a decay (Fig. 1C) was observed in the case of (HI)n

clusters in the absence of water clusters. We therefore concludethat the dynamics seen in the I atom with water present originatefrom the HI(H2O)n clusters.

Subsequently, laser fluence studies of the I atom were per-formed under varying probe powers to determine the number ofprobe photons needed to ionize the I atom, allowing us to inferthe internal energy of the evolving I atom. This information, com-bined with the known ionization potential (IP) of the iodine atom(10.45 eV) [21], allows us to gauge the possible internal energy of acluster or atom. Our fluence studies reveal that the I atom from theHI(H2O)n cluster requires the same number of photons to ionize asthe I atoms that dissociate from HI monomer, and thus are either intheir ground or first excited spin state [22]. The HI monomer is wellknown [23] to predissociate when excited at these energies. There-fore, the potential influence of caging [24,25] needs to be consid-ered as a possible mechanism.

At first glance a caging mechanism might appear to fit our data.However, after considering the probe photon power dependencedata (Table 1), it is seen that this explanation for the trend in our

-1 9 19 29 39 49

-1 4 9 14 19

I+

Arb

itra

ry I

nten

sity

Time in Picoseconds

A

B

A

B

I+

-1 19 39 59 79

CI+

Fig. 1. The dynamic response of atomic I that originated on an HI(H2O)n cluster isshown in A. In B, the response of the I atom when only HI monomer is present. Theresponse of the (HI)n clusters in C shows a decay.

data does not fit our observations. In a caging mechanism, wewould expect smaller clusters with fewer solvent molecules toblock dissociation poorly and larger ones with many solvent mol-ecules to block dissociation more effectively. However, powerstudies employed at a 200 ps delay (Table 1) demonstrate thatthe smaller clusters require more probe photons for ionization(closer to the power dependence of the I), suggesting that the Iatom is still bound to the cluster. This trend of the clusters havinga larger number of solvent molecules requiring less probe photonsfor ionization is opposite to the solvation trend that would be ex-pected to occur in a caging-dominated mechanism.

As an example, we consider the case of HI(H2O). If this were acaging-dominated mechanism, where one solvent molecule isinsufficient to block dissociation, we would expect to see a photonorder of one (HI ? H + I and H + H2O ? H3O):

HIðH2OÞ þ kpump ! HIðH2OÞ� 6:20 eVHIðH2OÞ� ! H3Oþ I � �3:20 eVH3O! H3Oþ þ e� �4:30 eVI! I� �0:94 eV

HIðH2OÞ þ kpump ! H3Oþ þ I� þ e� � 2:24 eV

Here, the IP of D3O is known to be 4.3 eV [26], and we can as-sume the IP of H3O to be the same. Given the pump energy(6.2 eV), the dissociation energy of HI (3.20 eV, solvation lowersthis number) [27], and the maximum internal energy with whichthe iodine can leave the cluster (0.94 eV); we calculate that the en-ergy needed to ionize an H3O radical produced by this mechanismis at most 2.24 eV [22]. This is easily accomplished with one pho-ton of the probe (3.1 eV), and hence we would expect a photon or-der of one. However, the power studies (Table 1) reveal that thiscluster requires four probe photons for ionization, not one as thecaging model suggests, and hence we can discount the caging argu-ment. Additionally, the trend of decreasing photon order in the flu-ence study could be associated with ion-pair formation in theground state. As this Letter is concerned only with the biradical,this topic will be covered in another work.

In order to explain the implications of the findings of our work,a brief discussion of the proposed cluster structures is fitting. Thebiradical consists of a hydronium radical solvated by water [28]while in the presence of another loosely bound radical. The identityof the other radical depends on the specific system, a hydroxyl rad-ical for pure water [9], or a halogen for a halo-acid [29]. Fig. 2Ashows the proposed biradical structure for an HI(H2O)4 cluster,adapted from the analogous HCl–H2O system studied by Sobolew-ski and Domcke [29]. The other structures shown are a covalently-bound species where the H–I bond is still intact (Fig. 2B), and anion-pair structure where the H–I bond has been broken formingH+ and I� moieties in the cluster (Fig. 2C).

Table 1The results of our probe power study at a 200 picosecond (ps) delay from the pump.The number of photons needed to ionize the cluster reflects whether or not thecluster has dissociated an iodine atom. The solvated hydronium clusters that wouldform from the neutral dissociation of an iodine atom would require one probe photonto ionize. All energies shown in eV; Ep is the energy of the pump, while the probeenergy is 3.1 eV.

Species Probe photon order @ 200 ps

Neutral Detected

H2O H20+ 4.0 ± 0.1HI(H2O) H+(H2O) 3.9 ± 0.1HI(H2O)2 H+(H2O)2 2.9 ± 0.1HI(H2O)3 H+(H2O)3 2.5 ± 0.1HI(H2O)4 H+(H2O)4 2.2 ± 0.1HI(H2O)5 H+(H2O)5 1.8 ± 0.1HI(H2O)6 H+(H2O)6 1.5 ± 0.2I I+ 4.0 ± 0.1

Fig. 2. Three possible structures of the neutral species in an experiment in which abiradical species is believed to be generated upon excitation: A represents thebiradical structure, B represents an HI(H2O)n where ion-pair formation of the halo-acid (HI) has not occurred and the H–X molecule is still intact, and C represents theion-pair structure. Note that in the biradical structure the iodine has only one bond.This bond is longer, and thus weaker, than in the ion-pair case, where there arethree shorter bonds. Figure adapted and reprinted from Ref. [27].

Fig. 3. The energetics and dynamics of an HI(H2O)4 cluster as it excited anddissociates an I atom in what we believe to be a biradical-like excited state areshown in the figure. All clusters begin in the ground state as either a covalentlybound HI solvated by water molecules or dissociated into an ion-pair. The pumplaser excites the clusters with 6.2 eV of energy and a photochemical reactionensues. The clusters are then thought to reorganize to a biradical-like structure witha poorly bound I atom (some clusters evaporate the iodine).

D.P. Hydutsky et al. / Chemical Physics Letters 476 (2009) 15–18 17

The dynamics of the I atom are not from caging, and the behav-ior must be accounted for in terms of another event. In van derWaals bound clusters, evaporation of single molecules or atomstakes place when the internal energy in the cluster redistributesinto ro-vibrational modes of the van der Waals bonds and exceedsthe binding energy of the most weakly bound molecule or atom.The slow rise seen in the I+ transient suggests such a mechanism,and also suggests that the I atom has the weakest bonding to theclusters. In the case of the HI(H2O)n clusters that we studied, an Iatom could evaporate if the species involved is a biradical, like thatseen in Fig. 2A. Evaporation of an I atom likely occurs in theHI(H2O)6 or larger clusters because these clusters will have thelargest amount of internal energy from solvation effects, such asthe breaking of the H–I bond in the ground state to form H+ andI�; this process has a one-photon power dependence. Evaporationof an iodine atom from the HI(H2O)6 cluster suggests that the Iatom is the most weakly bound species. Computational work[29] predicts just such a structure for the excited state biradicalfor another hydrogen halide, consisting of an HCl(H2O)4 cluster.Moreover, recent work by Kim and coworkers [30] predicts the lossof iodine atoms from clusters excited by similar energies andinvolving a similar mechanism. Since the iodine atom leaves thecluster through an evaporative process, and all of the clusters arelong-lived, we conclude that we have experimentally observed a

bound excited state with a loosely bound I, which displays behav-ior expected for the predicted biradical. A summary of the interpre-tation of our experiments is given in Fig. 3.

4. Implications for the solvated electron

The solvated electron is known to manifest in many solventswhere an electron is released from a solute or from a solvent mol-ecule by radiation, which is captured by the surrounding solvent.Cavity models of the solvated electron assert that the electron iscaptured in a void space in the solvent. The model was originallyproposed for ammonia, with alkaline metals as the solute, andwas later used to describe the radiolysis of water and other sol-vents. The assignment of a cavity is largely due to the similarityof the solvated electron to an f-center defect in solids because ofits optical absorption, and, in the case of ammonia and alkalinemetals, the increase in molar volume.

Recent work on the hydrated electron aims at understanding,through the use of gas phase and computational methods, theamount of water necessary to capture an electron in a cavity. Bind-ing motifs have been studied extensively, and unique structureshave come from this work [31–33], as well as from studies of theultrafast dynamics of the hydrated electron [34]. The excitinginterplay of experiment [35–38] and theory [39] has scrutinizedthe results, and though large clusters of water have been experi-mentally investigated up to ðH2OÞ�200 [40], debate continues overwhether or not the electron resides on the surface or the interiorof the cluster [41].

With strong evidence for the existence of a biradical-like spe-cies, a discussion of the biradical as the moiety for the solvatedelectron is fitting in light of the conjecture by Domcke and Sobo-lewski of the involvement of similar species. The electron in a cav-ity model for solutions was originally used to describe thephenomena of dissolving an alkali metal in liquid ammonia. Theresult is an f-center-like species, and an increase in molar volume.The phenomenon that occurs in water upon radiation of the solu-tion produces a spectral signature similar to that seen in theammonia case. Additionally, ion mobility studies of the hydratedelectron in water [42,43] have shown the mobility to be similarto that of an OH� moiety.

The biradical has been shown computationally to fit the mea-surable phenomena of the hydrated electron. Additional experi-ments [44] on HCl and HBr adsorbed to large water clusters andexcited by similar energies (6.4 eV), have also found evidence ofbiradical formation. Also, the computational work by Kim et al.mentioned previously [30] that predicts the loss of I atoms fromclusters excited by similar energies, also predicts the loss of ahydrogen atom as measured in the work by Buck et al. [44]. Whilethese cluster experiments do not show explicitly that the biradicalis the moiety associated with the hydrated electron, they do seemto support the idea of the biradical as the lowest energy confirma-tion of all excited states for water-solvated acid halide systems,and specifically HI(H2O)n, and point to a similar mechanism analo-gous to the loss of the iodine in the case of the hydrated electron.

5. Conclusion

Upon excitation by 6.2 eV of energy, some of the HI(H2O)n clus-ters reorganize to a structure similar to the biradical. According toSobolewski and Domcke [29], an ionic or covalent HCl(H2O)4 clus-ter will reorganize into a biradical when excited with these ener-gies, and we expect similar behavior from our HI(H2O)n clusters.This reorganization will increase the internal energy of the clusterand an evaporation of an I atom seams likely. An evaporative pro-cess explains the long growth for the I atom dissociation and the

18 D.P. Hydutsky et al. / Chemical Physics Letters 476 (2009) 15–18

electronic state appears to be bound on the time scale of our exper-iment. As the iodine atom evaporates from the cluster, the iodineatom appears to be poorly bound, and suggests a structure similarto Fig. 2A, the biradical. We thus conclude that the species we areobserving in our molecular beam is similar to the proposed birad-ical, as the excitation energies are similar to those predicted [29],the excited state is long-lived, and the iodine atom is only weaklybound.

In summary, we conclude from the above data and discussionthat our experiments are indeed producing a biradical-like species,as predicted theoretically. Establishing the existence of a biradicalspecies in the gas phase suggest that perhaps a liquid phase analogcan be found. However, further experimental and computationalwork is necessary. Future experiments will seek to observe similarbehaviors in NaI(H2O)n clusters and gauge the atmospheric signif-icance of the biradical, where the wavelengths needed to excitethis species could be atmospherically relevant. Additionally, wemight be able to use excited state biradical formation to distin-guish between ground state ion-pair formation and covalently-bound species in simple acids. If further experimental and compu-tational work can confirm the observations of this work, thesebiradical species might better explain many phenomena that arecurrently thought to be the result of cavity bound hydratedelectrons.

Acknowledgements

Financial support by the Atmospheric Sciences Division and theExperimental Physical Chemistry Division of the United States Na-tional Science Foundation, Grant No. ATM-0715014, is gratefullyacknowledged. We also acknowledge helpful conversations withW. Domcke.

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