DYNAMICS OF PHOTODISSOCIATION OF XeF2 IN ORGANIC SOLVENTS

Transient electronic absorption measurements with 1 ps time resolution follow XeF2 photoproducts in acetonitrile and chlorinated solvents. Ultraviolet light near 266 nm promptly breaks one Xe-F bond, and probe light covering 320-700 nm monitors the products. Some of the cleaved F atoms remain in close proximity to an XeF fragment and perturb the electronic states of XeF. The time evolution of a perturbed spectral feature is used to monitor the FXe-F complex population, which decays in less than 5 ps. Decay can occur through geminate recombination, diffusive separation or reaction of the complex with the solvent.


TABLE OF CONTENTS
This article explores photodissociation of XeF2 in solution, using femtosecond pump probe spectroscopy to follow the fates of photoproducts following cleavage of one Xe-F bond.
like XeF2 that have known spectroscopic features 9,10 and relatively few reaction pathways, 4,5,11 we can build on our chemical intuition of condensed-phase chemistry.
Our current effort uses time-resolved transient absorption spectroscopy to determine the condensed-phase dynamics and products of XeF2 from 1 ps to 1.5 ns after photodissociation.
This work provides ready analysis of XeF2 as a source of F atoms for use in our on-going studies of liquid-phase bimolecular reaction dynamics. 12 Following absorption of a 266 nm photon by XeF2, homolytic fission of one Xe-F bond is predicted to dominate the gas-phase dynamics. 6 Facile migration of photolytically released F atoms occurs in rare-gas matrices, 13 but reaction with the solvent may limit the F-atom mobility in organic solvents.
Application of XeF in excimer lasers provided some original impetus for studying the ground and electronically excited XeF states. 10,[14][15][16][17] Figure 1 shows the electronic states of XeF that are important in 266-nm photolysis of XeF2. One-photon absorption by XeF2 leads to groundstate XeF through Xe-F bond cleavage, and its strong absorption to the B state near 345 nm provides a probe of the XeF photoproduct. 2 The ground state is bound by about 1200 cm -1 in the gas phase, 10 and a large fraction of the XeF products remain bound well beyond the timescale of our measurement. 2 The electronically excited states of XeF are ionic in character, and an F atom in the vicinity of XeF perturbs the optically bright B state. 18 We use this known shift of the XeF spectrum as a marker for incomplete separation of the condensedphase products, allowing us to follow with detail the condensed-phase dynamics that occur within the first few ps after photolysis of XeF2.  18 The ion-pair character of the B state makes it susceptible to electronic perturbations. The red curve shows the shift of the B state caused by a neighbouring F atom, first identified by Apkarian and co-workers. 18

A. Transient Absorption Spectroscopy
The ULTRA Facility at the Central Laser Facility of the Rutherford Appleton Laboratory houses the apparatus we used to photolyse XeF2 and to measure electronic-absorption spectra with 1-ps time resolution. The instrumentation used is unchanged from our recent study of chlorine-atom reactions 19 and is described in detail elsewhere. 20 A Ti:sapphire laser operating  at 10 kHz generates both a 266-nm photolysis pulse and a white light continuum (WLC) probe pulse covering 320-700 nm, and an optical delay stage controls the relative timing, t, of their interaction with the liquid sample. A 512-element Si photodiode array measures the intensity of the spectrally dispersed broadband probe pulses. An optical chopper wheel modulates the photolysis pulse at 5 kHz for realtime background subtraction and normalisation.
Imperfect alignment of the beam along the optical delay line that controls the relative timing of photolysis and probe pulses can affect signal intensities at t ≥ 100 ps. To compensate for this we use the known excited-state lifetime of phenol in cyclohexane 21 to apply a linear correction factor to the transient XeF2 photolysis data we present. This correction yields longtime kinetics in the XeF2 systems that are consistent with previous measurements. 2 The spectra are not corrected for the temporal chirp in the WLC: instead, data for time delays <1 ps are excluded from analysis.
The solute XeF2 is fragile, and keeping samples colder than 290 K before and during measurements helps ensure its stability. Dried glassware, solvents, and sample holders prevent unwanted side reactions between XeF2 and water. Rapid thermal reaction of the possible contaminant XeF4 with solvent 5 ameliorates its effect on our measurements.
Acquiring a pre-determined set of time delays between photolysis and probe pulses in random order further reduces the effects of longer timescale changes in the sample on our measurement. Dilute solutions of 0.25 M XeF2 (>99.9 % trace metal basis, Sigma Aldrich) in dried CH3CN, CCl4, CDCl3, or CD2Cl2 pass through a sealed flow system. A rastering sample cell, consisting of CaF2 windows separated by a 0.2 mm PTFE spacer, ensures that each laser pulse interrogates a fresh sample volume.

B. Computational Methods
photophysics. Excited-state CASMPT calculations followed by CASPT2 correction, of the XeF2 potential energy surface use the aug-cc-pVDZ 22 basis set for F atoms and the ECP46MWB effective core potential for Xe. Calculations were implemented with use of the MOLPRO program. 23 This treatment is an effective quasi relativistic model for the large Xe core and has proven successful in modelling the stability of Au + -Xe complexes. 24 We also replicate the XeF X and B states using known constants 10 to calculate the vibrational wavefunctions through an RKR analysis. 25 The electronic absorption spectrum of vibrationally excited XeF is then simulated through a Frank-Condon analysis with use of the Level programme. 26

A. XeF from XeF2
The steady state absorption spectrum of gas phase XeF2, is shown in Figure 2, and is fraught with multiple features and possible dissociation products. 27 The lowest energy electronic absorption, which has a maximum near 240 nm, populates the u state and promptly produces XeF(X) + F. 2,6 Photolysis at 266 nm, about 4.6 eV of excitation, also makes Xe + F2 and Xe + 2F energetically available, but previous studies have not directly observed these more complicated dissociation pathways. 27 Absorbing two photons, which we show below to be unlikely in our experiment, could produce XeF in electronically excited states.  27 One-and two-photon absorption (indicated by the horizontal dashed lines) by XeF2 give many energetically accessible photoproducts, but a linear power dependence of the XeF(B─X) feature has confirmed that only one photon processes occur within our experimental setup Previous work has identified the condensed-phase XeF absorption spectrum in both steady-state cryogenic matrices 16,28 and in liquid samples of photolysed XeF2. 2 We use these prior assignments to guide the interpretation of our measurements, which are the first to follow this process on the ps to ns timescale. Ground-state XeF isolated in cryogenic rare gas matrices shows a broad absorption feature near 320 nm. This feature is the B←X transition, which lies approximately 4000 cm -1 lower in energy than in the gas phase. 16,28 Photolysis of XeF2 at 266 nm in CD3CN produces XeF with a sharp feature near 345 nm. 2 XeF has a 25 s lifetime in acetonitrile, and our transient absorption spectrum at t = 500 ps matches the previously measured spectrum at t = 100 ns. 2 Because the spectrum is largely unchanged between our longest time delay and ns to s intervals, we take our long-time spectrum to represent the equilibrated XeF photoproduct.
It is necessary to verify that one-photon absorption dominates our condensed-phase study.
Photolysis with ns laser pulses at 266 nm and intensities greater than 9×10 12 W/cm 2 saturates the transition to the u state and ensuing absorption of a second photon produces electronically excited XeF(B) and an F atom. 29 Our photolysis intensity of about 1×10 12 W/cm 2 is ten times smaller than the onset of two-photon absorption. 29 The integrated area of the transient absorption feature at t = 500 ps, which measures XeF from photolysis of XeF2, as a function of photolysis pulse energy, 2 produces a linear dependence within the range 0.25 -2.5 × 10 12 W/cm 2 , spanning more than an order of magnitude of attenuation below the onset of two-photon absorption. 29 We therefore conclude that transient signals measured and shown here result from one-photon absorption leading to the XeF(X) + F photodissociation pathway.
Decomposition to Xe+F2 is a lower energy channel than XeF + F production and so might compete following UV excitation of XeF2. We discount contributions to the transient spectra from this pathway because F2 absorbs only weakly in the observable, 320-700 nm region 30 and the strong Xe--F2  Xe + --F2charge transfer band lies to much shorter wavelength. In liquid xenon, for example, the band is observed below 185 nm. 31,32

B. Electronic States of XeF and FXe-F
The XeF BX absorption was also observed in CD2Cl2, CDCl3 and CCl4 solvents and the maximum of each spectrum shifts monotonically with the dielectric constant of the solvent, r. The polar solvents preferentially solvate the ion-pair B state of XeF relative to its X state, 18 decreasing the energy of the transition. Apkarian and co-workers 18 showed that the Onsager reaction-field description of the stabilization of a point dipole in a dielectric cavity 33,34 captures the essence of the energy shift of the B state. 18 In this model, the change in energy of the solvated B state relative to the gas phase, E, is with the dipole moment of the B state, , and radius, , of the host cavity in the solvent as the only molecular parameters. The XeF(BX) absorption spectra from the current work fit the general trend of Equation 1, as shown in Figure 3, providing a two-fold lesson that we carry forward to our interpretation of the transient electronic absorption spectra in the next section.  One factor perturbing the XeF spectrum is the electronegative F-atom photoproduct of XeF2.
Because some photolysis events lead to incomplete separation of the XeF and F fragments, we observe the spectral signatures of the metastable FXe-F complex. This complex is stable in rare-gas matrices at 20 K and indeed influences the B state. 18 Apkarian and co-workers first described how the F atom decreases the energy gap between the XeF(X) and XeF(B) states and labelled this altered state XeF(B*), 18 shown by the red line in Figure 1. Figure 4 shows the results of our calculations of the isolated FXe-F complex, as detailed in the Experimental Approach, and they provide an extension of the original description of the electronic states in the FXe-F complex. 18 We label the states of the complex as they correlate calculations with linear FA-Xe-FB geometries and a fixed FA-Xe bond length at its calculated minimum of 2.3 Å, which is in agreement with previous work. 14,15 These calculations show no barrier to association back to bound XeF2, but the FXe-F complex is known to be stable in Ar and Ne matrices. 18 Stabilisation of the B-state by approach of an F atom collinear to the diatomic axis, and a reduction in the gap between the B and X states are consistent with the red shifted B*-X absorption band produce purely repulsive potentials over all states. These repulsive states show that the complex has a preferred linear geometry and that the stabilization is likely to occur through the overlap of the XeF 2  + state with the F pz orbital.

C. Photolysis of XeF2 in Acetonitrile
We have established that condensed-phase photolysis of XeF2 leads to FXe-F bond cleavage, and Figure   Absorption to the A state of XeF may be discounted as a source of the longer wavelength absorption, because it has a gas-phase band origin near 1400 nm. 16 Other candidates for the prompt absorption that we rule out include vibrationally excited XeF, charge-transfer transitions between XeF or F and the solvent or between Xe atoms and F2 (see earlier), and excited-state absorptions of XeF or XeF2. A spectral simulation of the XeF X and B states using previously derived spectroscopic constants 10 in red, faithfully represents the kinetics of the raw spectra of Figure 5. The rapid rise of the XeF(B*─X) feature shows that the FXe-F complex forms within our time resolution, which is about 1 ps. It is also possible for the liberated F atom to escape the vicinity of the XeF co-fragment directly, similar to the facile migration reported for F atoms in rare gas matrices. 13 The kinetic scheme used to model the data is described below, with fits to the model shown as solid lines in Figure 6.
The kinetic model described allows for initial photodissociation of XeF2 to produce either the FXe-F complex (with rate coefficient k1) or XeF(X) and an escaped F atom (k2). The complex  36 ). The model is fitted to the time-dependent relative yields of XeF(X) and FXe-F by numerical integration.
The above scheme neglects some plausible competing loss processes for the FXe-F complex.
For example, work in Ar and Ne liquids observed XeF2 growth from FXe-F decay, although this pathway was cut off upon matrix isolation. This recombination of the FXe-F complex is not included explicitly in our analysis because we have no spectral signature for XeF2 recovery. Similarly, our experiments are insensitive to reactive decomposition of FXe-F to Xe + F2 because the charge-transfer band is outside our spectral window, while the near-UV band of F2 is weak and we see no evidence for build-up of this stable molecule in our spectra.
However, we recognise that these unobserved pathways could contribute to the magnitude of k3 as additional loss pathways for FXe-F. Fits of the experimental absorption band intensities to the kinetic model produce the rate coefficients reported in Table 1.

Dynamics of Photoproducts
The rate coefficients in Table 1  First, if very few F atoms escape the solvent cage ballistically, and nearly all remain trapped within the cage, most of the prompt photoproduct will appear as the FXe-F complex. Small atoms and fragments such as H and CN are known to find their way directly through the solvent cage, 7 37,38 and F atoms show high mobility in inert matrices, 13 Figure 5 hint that a significant proportion of the FXe-F survives recombination and instead decays by other routes.
The delayed cage escape and reactive pathways would be expected to have individual rates that depend upon the complex stability and any barrier to reaction. Time-resolved infra-red absorption spectra, reported elsewhere, 36 show build-up of DF product on a timescale that is consistent with the decay of the FXe-F band and rise of the XeF(X) band intensities. We conclude that the reaction of the F atom with the solvent is a substantial loss pathway for the of the complex and its recombination to XeF2.

E. Photolysis of XeF2 in Chlorinated Solvents
The transient absorption spectra in Figure 7 result from XeF2 photolysis in CD2Cl2, CDCl3, and CCl4 solvents. Photolysis of these chlorinated solvents at 266 nm also produces transient photoproducts with absorption features in the UV/Vis region. 19,41,42 Subtracting those features from our transient spectra with XeF2, acquired under identical conditions, produces signals resulting only from solute-dependent photo-chemistry. 19,42 In all these chlorinated solvents, it is evident that the majority of the photolysis of XeF2 occurs in less than a picosecond. We fit the transient spectra in CD2Cl2, but not the other two solvents, to Equation 2 because there is definite separation between the two transient features. The integrated intensities are then simultaneously fitted to the kinetic model described above, as shown in Figure 6. Unlike in acetonitrile, the XeF(X) feature near 350 nm appears promptly after photolysis in these solvents and the FXe-F complex is not as evident in CDCl3 or CCl4. Less clearcut signatures of the complex in these two solvents may be explained by shifts in the FXe-F band to shorter wavelength, a decrease in the branching ratio of complex formation, or an increase in the rate of the loss of the complex.
The rate coefficients extracted from the fit of the integrated band intensities following photolysis of XeF2 in CD2Cl2 are displayed in Any difference in the observed FXe-F complex yields between the chlorinated solvents and acetonitrile may result from two distinct mechanisms: 1) The photolytically produced F atom immediately escapes from the solvent cage. Ballistic escape can be affected by the ability of the solvent to quench the excess energy from the system, the mobility of F atoms in the solvent, and the strength of the solvent cage.
2) The probability of prompt recombination to XeF2 increases within the chlorinated solvents.
However, the origins of any barrier to recombination are uncertain, so we draw no conclusion about this trend.
The XeF molecule is observed to have a nanosecond or longer stability in CD3CN and CCl4 solvents but there is a substantial loss channel in CDCl3 and CD2Cl2. We suggest that the XeF reacts with the solvent to produce DF, and our transient IR absorption studies support this interpretation. 36 Time constants for the loss of the XeF molecule are found to be 800 ± 200 ps in CD2Cl2 and 350 ± 50 ps within CDCl3.
The XeF B─X absorption in CCl4 solvent is observed to shift to shorter wavelength with a 4.1 ± 0.3 ps time constant, which may be indicative of vibrational cooling of the XeF. The XeF(X) ground state potential supports ~8 vibrational levels. However, in CD3CN and CD2Cl2 solvents the FXe-F complex feature obscures any spectral signatures of vibrational relaxation dynamics.

IV. CONCLUSIONS
Photodissociation of XeF2 in organic solvents produces XeF(X) and F with a time constant of less than 1 ps. If confined within the same solvent cage, the nearby photolytically produced F atom perturbs the ionic B state of the XeF molecule, which allows the formation of these FXe-F complexes to be observed spectroscopically. Such complexes have previously been observed in Ar and Ne matrices. 18 The FXe-F is expected to be only weakly bound, and the decay of its spectral band provides information on the reactive removal of F atoms. The The XeF molecule is stable in CD3CN and CCl4 solvents for nano-to microseconds.
Substantial loss channels of XeF in CDCl3 and CD2Cl2 solvents are suggestive of reaction with the solvent.