J. Phys. Chem. 1995,99, 6333-6339
6333
Laser-Induced Infrared Photofragmentation of SFs*(C6H6),+ Cluster Ions A. B. Jones, R. Lopez-Martens, and A. J. Stace* School of Molecular Sciences, University of Sussex, Falmer, Brighton BNI 9QJ, U.K. Received: October 24, 1994; In Final Form: February 14, 1 9 9 9
Size-selected SF6*(C6H6),+ cluster ions have been photoexcited in the infrared using radiation from a linetuneable COZlaser. In contrast to metastable (unimolecular) decay where loss of C6H6 is the dominant decay channel, photoexcitation only promotes the loss of SF6. This observation is rationalized in terms of a structure where SF6 resides on the surface of the cluster and there is no statistical partitioning of the photon’s energy prior to dissociation. Further support for a surface-bound SF6 is provided by infrared absorption profiles recorded as a function of laser wavelength. These latter measurements also show that SFs*(CsH6)14+may adopt a structure that is more symmetric than those of its immediate neighbors.
Introduction By their very nature, van der Waals clusters consist of fragile collections of atoms andor molecules that are very susceptible to the mildest form of perturbation. Therefore, even infrared excitation is frequently sufficient to promote dissociation, and as a consequence, IR photofragmentation has proved to be a very powerful technique for studying the vibrational absorption spectra of neutral clusters.’-4 Although the presence of a positive charge in a cluster ion can increase the short-range binding energy at the core to a value beyond the energy (hv)of a single IR photon, two factors can contribute to the continued presence of a photofragmentation signal? (i) cluster ions can accommodate a distribution of intemal energies, and some ions will have energies that are within hv of the dissociation limit; (ii) beyond the first solvation shell, the binding energies of single molecules will rapidly revert to values that are more typical of neutral van der Waals systems. The attraction of single-photon IR photofragmentation is that it provides a gentle route to the interrogation of cluster structures, because hv is frequently comparable to the binding energy of a single molecule. Therefore, both the absorption frequency and the photofragment yield can act as sensitive probes of chromophore environment.”* Disadvantages of the technique are the following: (i) a molecule may absorb a photon but not respond by dissociating on the time scale of the experiment; this could happen, for example, with a large cluster where the many degrees of freedom constitute a substantial heat sink; (2) absorption features, such as vibrational structure, represent a response from the cluster to excitation at that wavelength and, as such, the intensity of any single feature may not necessarily equate to an absorption cross section. Through the careful selection of clusterlchromophore combinations, we have recently shown that it is possible to study the infrared photofragmentation spectroscopy of a single neutral chromophore in association with a mass-selected cluster The advantage offered by these experiments is that the evolution of spectral features can be correlated with other experimental observables, such as mass spectra and the decay dynamics of photoexcited cluster ions. Thus, changes in absorption frequency can be equated with such physical quantities as the following: the presence of “magic numbers”; the introduction of new fragmentation routes as identified in a mass spectrometer; andor the progressive solvation of the chromophore. The key @
Abstract published in Advance ACS Absrracts, April 1, 1995.
0022-365419512099-6333$09.0010
feature of these experiments is that, in heterogeneous clusters of the form XY,, the chromophore X should have an ionization potential which is larger than that of the host cluster Y,. Under these circumstances, the cluster can be ionized in the knowledge that it is Y, which carries the positive ~ h a r g e . ~The - ~ same philosophy applies to negatively charged heterogeneous clusters where differential electron affinities exist.9 A disadvantage of the approach discussed here is that only limited control is available over the internal energies of the cluster ions. Electron impact ionization is a comparatively violent process capable of creating a broad range of excited states; however, because the observation time scale is s, many of these states will decay to leave a residual vibrational energy in the ground state. This residual energy can then promote the breakup of a cluster. Since unimolecular decay is still observed after s,Io it is known that a certain fraction of cluster ions possess an internal energy which is, at least, equivalent to the minimum dissociation energy, E ! . Gas cluster ions of the type discussed here are, for the most part, weaklybound; therefore, their internal energy cannot be very much higher than e, or else extensive fragmentation would occur before the ions reach the point of laser excitation. Hence, the energy deposited from an IR photon ( ~ 0 . 1 eV) 1 is a significant fraction of the total internal energy, and as a consequence, photofragmentation signals are frequently found to be very much stronger than those recorded for metastable d e ~ a y . ~An -~ estimate of the average internal energy can be obtained by measuring the kinetic energy release associated with unimolecular decay.” Reported here are the results of a series of measurements undertaken on the IR photodissociation spectroscopy of SF*(C&&+ clusters for n in the range 1-30. Of particular interest is the fact that loss of sF6 is the only photofragmentation channel for clusters of all sizes, a pattern which is quite different from that observed p r e v i ~ u s l y . ~ -From ~ a summation of fragment ion intensities, it has been possible to derive vibrational absorption profiles for the sF6 as a function of cluster size, and these we used to discuss the structures of the ions. Only one size of benzene cluster exhibits the presence of structure in the IR absorption profile, and this we attribute to a lifting of the degeneracy of the vibrational mode of the resident SF6 molecule.
Experimental Section Mixed neutral clusters of the form SF6*(C6H& were prepared using the “pick-up” technique on an apparatus that consists of 0 1995 American Chemical Society
Jones et al.
6334 J. Phys. Chem., Vol. 99, No. 17, 1995 Mass Spectrum
IR-Detector
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a supersonic nozzle coupled to a modified double-focusing "reverse geometry" VG ZAB-E mass spectrometer. Figure 1 shows a schematic diagram of the apparatus, and details of the exact construction have been given previously;I0 the only additional feature is a line-tuneable C02 infrared laser (Edinburgh Instruments PL4). Neutral clusters of benzene were generated via the adiabatic expansion of a mixture consisting of approximately 1% C6H6 in argon through a 200 pm diameter pulsed conical nozzle. Stable mixtures of the gas and vapor were achieved by passing argon over a reservoir containing benzene held at approximately -50 "C. Following collimation through a skimmer, the cluster beam entered a flight tube approximately 70 cm long, in which the background pressure had been increased to mbar through the introduction of SFg via a needle valve. During their passage through this region, some of the benzene clusters formed collision complexes with SF6 molecules and optimum SF6*(C6H6),+ ion signals were observed when the nozzle stagnation pressure was approximately 60 psi. This pressure is considerably higher than that necessary for the generation of intense beams of pure benzene clusters, and we attribute this difference in behavior to the fact that success of the pick-up experiment appears to require the initial formation of (C6H6),*Arm clusters.* Attempts to repeat the experiments using helium as the carrier gas have not been as successful as those where argon is used.12 Ionization to SF6*(C6H6),+ was achieved using 70 eV electrons, and use of the pick-up procedure proved very successful in generating stable ion currents with almost uniform intensity for clusters consisting of a single chromophore and up to 30 host molecules (see below). Ions were extracted from the source with a potential of +6 kV and were size-selected using a magnetic sector. These ions then entered a field-free region (frequently referred to as the second field-free region) where they were irradiated with photons from a line-tuneable carbon dioxide laser (frequency range 920- 1100 cm-', resolution 1.5 cm-I), which was aligned coaxially with the ion flight path; the overlap between the laser and ion beams covered a distance of approximately 150 cm. The masses of any ionic photofragments were determined using the electrostatic analyser in the MIKES (mass-analyzed ion kinetic energy spectra) mode.13 To eliminate possible interference from fragment ions due either to metastable decay (see below) or collision-induced dissociation (CID), the laser beam was modulated at half the nozzle frequency and foreground - background subtraction performed on photofragment signals. Two data collection techniques were employed: photodissociation and unimolecular decay signals at a single kinetic energy were recorded using
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Figure 2. Relative intensities of the sF6*(c6H6),+ ions as recorded from a typical mass spectrum and plotted as a function of cluster size (n).
gated photon counting via a scintillation (Daly) ion detector; mass spectra were recorded using phase-sensitive detection in conjunction with analogue output from the ion detection system. To keep CID processes to a minimum, the background pressure in the flight tube was maintained at 6 x mbar. Both metastable and photodissociation signals were between lo4 and lo5 counts min-I, and a typical error bound on the photofragmentation signals is f5%. In addition to laser-induced decay, the metastable or unimolecular decay patterns of the cluster ions were also recorded. Ions that have retained a residual vibrational energy from the electron impact ionization process may undergo unimolecular decay during their passage through the laser interaction region. By monitoring the latter behavior, we are able to gauge how effective infrared photons are at increasing the fragment yield. Power-dependence studies showed that most photodissociation steps were single-photon events; however, in some instances, multiple dissociations were ob~erved.~ In these processes, ions underwent a sequence of single-photon absorption/fragmentation steps as they traveled along the flight tube between the magnet and the electric sector.
Results and Discussion Neutral SF6 has an infrared active, triply degenerate (TI,) vibrational mode (213) which gives a very strong absorption feature in the gas phase at 948 cm-I and falls within the 10 P branch of a line-tuneable C02 laser. Frequency shifts in the v3 mode associated with the clustering of neutral SF6 in argon have been the subject of a very detailed investigation by Scoles and co-workers. l4-I6 Of interest in these latter experiments has been use of the pick-up technique in order to differentiate between solvated and surface-bound SF6 molecules. Benzene and benzene clusters also have an allowed infrared transition that falls within the range of the C02 laser;4 however, since this transition lies in the region 1020- 1040 cm-l (approximately 100 cm-l above that recorded for SF6), there is no interference. The relative intensities of SF6*(C6H6),+ cluster ions as recorded from a typical mass spectrum are shown in Figure 2. Although certain ions are comparatively intense, i.e., SF6.(C&I6)5+, the distribution is quite uniform which we consider one of the prime advantages of using the pick-up technique to prepare mixed clusters. The alternative is coexpansion of a gas/ vapor mixture which is inclined to favor formation of a narrow range of the required species. None of the intensity fluctuations shown in Figure 2 follow the pattern identified previously for pure benzene cluster ions.17 Following the mass selection of a
Photofragmentation of SF6*(C6H6),+ Cluster Ions
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J. Phys. Chem., Vol. 99, No. 17, 1995 6335 Photo-induced loss of SFe from (C&),+sF6 clusters
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Figure 3. Relative fragment ion intensities recorded for the metastable loss of (a) C6H6 and (b) SF6 from SF6-(C6H&+ ions (reactions 1 and 2, respectively) and plotted as a function of cluster size (n).
single size of cluster ion, two metastable fragmentation channels are observed:
The intensities of the two product ions as a function of cluster size (n)are shown in Figure 3, where it can be seen that reaction 1 is a factor of between 4 and 6 times more probable than reaction 2. The gradual increase shown in both sets of data as a function of size is typical of cluster ionsI8 and is due to the larger ions having a higher probability of decomposing in the second field-free region between the magnetic and electric sectors. Part of the large difference between the fragment ion intensities for the two reaction channels could be attributed to reaction path degeneracy.g Following infrared excitation of the v3 vibrational mode of the sF6 molecule, the only observed photofragmentation channel is
(3) and the signal intensities recorded for this reaction are plotted in Figure 4. The observed fluctuations are quite different from those seen previously in Figure 3, with pronounced photofragment yields from SFs*(C&),+ clusters being recorded for n = 9, 13, and 19. However, there is some correspondence between Figure 4 and the higher than average intensities of the SF6.(C&)13+ and SF6-(C6H6),9+ions in the mass spectrum shown in Figure 2. Clearly, the most interesting observation is that of a single photodissociation channel which is itself only a minor compo-
Figure 4. Relative fragment ion intensity recorded for the photoinduced loss of SF6 from SF6*(C6H&+ ions (reaction 3) and plotted as a function of cluster size (n). nent of the metastable decay signal. Previous results for SF6.(c02),+,8 SF6*(N0),+,9and SF,j*(NO),- have been characterized by loss of SF6 being the dominant dissociation route in small clusters, to be gradually replaced by the loss of X or 2X (X = C02 or NO) as the clusters increase in size. In addition, photodissociation almost always mirrors the metastable fragmentation pattem. An RRKh4 model of the competition between loss of SF6 and CO;! in SF6*(C02),+clustersgsuggested that the decay processes were preceded by a statistical partitioning of the energy of the photon to the cluster as a whole. Reaction path degeneracy was also shown to make an important contribution to the switch in fragmentation pathway from loss of SF6 to the loss of co2.8 In the case of SF6-(NO),+ and SFs(NO),- clusters? comparisons between the metastable and photo-induced decay pattems for the loss of 2 N 0 indicated that the addition of a photon increased the total energy (temperature) of each cluster. Taking these earlier examples into consideration, it is evident that, for SF&&),+ clusters, the energy of the IR photon (-0.1 1 eV) is not partitioned to all the available degrees of freedom prior to dissociation. What appears more likely is that the energy remains localized within the SF6 molecule and eventually contributes to breaking the SF6(C&,),+ intermolecular bond. This localization of the photon energy would account for the absence of any photoinduced loss of benzene. The sharp contrast between metastable decay and photofragmentation arises because of differences in the observation time window and origins of the ions involved. For metastable decay, ions that reach the second field-free region before fragmenting are approximately 5 x s old, which is more than sufficient time for complete energy randomization; in addition, the initial excitation step (electron impact) is much less specific than a photon. Finally, any nonstatistical decay processes will have taken place long before the ions reach the region where measurements on metastable decay are made. From a summation of photofragment yields as a function of laser wavelength, it is possible to determine infrared absorption profiles for the SF&6H6),+ clusters as a function of size. A selection of the profiles are shown in Figures 5-9, where in each case the data points have been fitted to a curve using a procedure which minimizes the least-squares difference between calculated and experimental values. With the experimental data normalized to 1, the residual least-squares error for fitted lorentzian profiles is 5 in contrast, attempts to fit Gaussian profiles resulted in errors >> For the most part, the shapes of the profiles follow a trend similar to that observed previo ~ s l y ,with ~ , ~the line widths becoming progressively narrower as the clusters increase in size. For small clusters, we attribute the broad absorption profiles to rotationalkranslational motion of the SF6 in close proximity to a positive charge which, in this
Jones et al.
6336 J. Phys. Chem., Vol. 99, No. 17, 1995 SFs.(CsHs)jLOSSof SFs I
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Figure 5. Infrared absorption profiles for the ions SF6*(cd-I6)3+ and SF6*(C&)s+ determined by summing all fragment ion intensities as a function of laser wavelength. The points are the experimental data, and the solid line is a fitted Lorentzian profile. The error limits on the experimental data points are f5%.
case, is assumed to be located on a benzene dimer ion.I7 As the benzene clusters increase in size, it is thought that the SF6 becomes progressively displaced away from the site of the charge, to take up a position on the surface of the cluster in contact with (almost) neutral molecules. This latter assumption is based on the observed photofragmentation pattern; if the SF6 were in part solvated, then it would not be so easy to displace from c!usters of all sizes without imparting some fraction of the photon's energy to the benzene component. Indeed, for those systems where the SF6 is assumed to be at least partially solvated, Le., SF6*(CO&+ and SF6-(N0),+,8*9no photoinduced loss of SF6 is observed for positively charged ions containing more than eight molecules of the host cluster. From the set of profiles given in Figures 5-9, those shown in Figure 8 are of particular interest because, within the available resolution (-1.5 cm-I), they show the presence of structure. Similar features have been observed p r e v i o u ~ l yand ~ ~ ~have almost always been associated with a change in the laser-induced fragmentation pattern of the mixed clusters. Localization (solvation) of the SF6 within a cluster leads to a marked reduction in its contribution to the photofragment signal and, at the same time, splits the triply degenerate v3 vibrational mode into two or more component^.^.^ The latter effect arises from asymmetric polarization of the SF6 by a fixed point charge a n d or a nonuniformity in the packing of solvent molecules. However, in view of the discussion presented above, arguments concerning solvent packing would not appear to account for the sF6m(C&&f system, where it is assumed that SF6 is located on the surface. It is quite possible that many of the other SF6.(C6H&+ profiles also contain structure, but this is not resolved
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Figure 7. AS for Figure 5 , but for S F ~ . ( C ~ H ~and ) I ~ S+ F ~ ' ( C ~ H ~ ) I ~ + .
because of the finite spacings between laser lines. One such example is the result shown in Figure 6a for SFs'(CsH&+, which has similar shape to those shown in Figure 8, but with the
Photofragmentation of SF6*(C6H&+ Cluster Ions
J. Phys. Chem., Vol. 99, No. 17, 1995 6337 Frequency Maximum for (C&)f.s&
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Figure 8. AS for Figure 5 , but for SF~'(C&)I~+and S F ~ ' ( C ~ H ~ ) I ~ + .Figure 10. (a) Line centers for the SF6*(C&),+ absorption profiles plotted as a function of cluster size (n).In each case, the position of the center was determined by fitting a Lorentzian profile to the experimental infrared absorption data. The separate points shown for SF~'(C&)I~+and SF6'(CsH6)ls+ correspond to the additional features exhibited by these ions. (b) Line widths for the s&-(c&&+ absorption profiles plotted as a function of cluster size (n).In each case the full width at half-maximum was determined by fitting a Lorentzian profile to the experimental infrared absorption data.
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Figure 9. As for Figure 5 , but for SF6-(C6H6)19+.
available data it did not appear appropriate to attempt to resolve a second feature. In an earlier paper on pure benzene cluster ions, Schriver et al. l 7 noted that (C&&+ exhibited behavior characteristic of a stable "magic number" cluster. The structure they proposed to account for their observationi7 was that of an icosahedron consisting of a strongly bound benzene dimer ion core surrounded by 12 nearly equivalent nearest neighbors. If the mixed clusters were to behave in a similar manner, then the result in Figure 8 would suggest that the SF6 infrared absorption profile is sensitive to the presence of an underlying stable (C&)14+ structure. Since the 12 nearest-neighbor benzene molecules all occupy approximately equivalent sites,I7 the two features in Figure 8a are unlikely to be due to isomers. Hole-buming experiments on other SFdsolvent systems show that resolved spectral splittings,I2 Le., as seen with the COz laser, in smaller clusters are due to a lifting of the v3 degeneracy and that isomers do not make a significant contribution to the spectra until the
clusters contain 30-40 solvent atoms or molecules.'* In addition, isomers might be expected to become more prevalent beyond SF6*(C&)14+, which does not appear to be the case. What is more difficult to understand is why the stability of SF6.(CsH6)14+ is not reflected in either the mass spectrum or the metastable decay pattem. Given that the splitting in Figure 8a is due to a lifting of the v3 degeneracy, then the pattem would suggest that one of the F-S-F bonds is polarized toward the positive charge to give the feature at -945 cm-' and that the peak at -942 cm-' is associated with the two F-S-F bonds which form a plane perpendicular to the direction of polarization. This configuration would give an intensity ratio of 2:1, and similar experiments on SFdAr,+ clustersi2 indicate that one or more of the vg components are blue-shifted in the presence of a positive charge (see below). Figure 10 presents a summary of the peak centers and widths (full width at half-maximum, FWHM) as a function of benzene cluster size. The red shift exhibited by the peak centers for the v3 mode takes the form of a gradual change away from the gas phase value (948 cm-') exhibited by the smaller clusters toward a fixed frequency of -941.5 cm-I as the cluster size increases. However, contained within the graph are some abrupt changes in slope, and these, for reasons as yet unexplained, coincide with particularly intense features in the mass spectrum (Figure 2, SFs'(CsH6)4+, sF6'(C6H6)13+, SF6'(C6H6) 19+, and SF6'(C6&)23+) and two of the more intense photofragment signals (Figure 4, S F ~ ' ( C ~ H ~and ) I ~S+F ~ ' ( C ~ H ~ ) IThe ~ + )two . isolated points in Figure 10a represent the second component to the
Jones et al.
6338 J. Phys. Chem., Vol. 99, No. 17,I995 splittings identified for SF~*(C,&)I~+and SF&&)ls+ (Very weak). As can be seen, the peak centers for the latter have shifted back toward the blue and are now similar in magnitude to those values recorded for the smaller clusters where the SF6 is closer to the positive charge. A plot of the peak widths as a function of cluster size (Figure lob) exhibits a number of irregularities, the single most noticeable single feature coinciding with S F ~ * ( C ~ H (taking ~)I~+ just the more intense peak; if the two peaks are treated as a whole then a substantial increase in width appears in the plot at n = 14). SF&6H&+ is also seen to go against the trend; this behavior does not correlate with any other experimental observations made on this ion and, as discussed above, could be due to poor resolution of a split v3 vibration. The decline in width seen between n = 1 and 5 is consistent with a gradual displacement of the SFg away from the positive charge; however, the molecule should still be held in position by a strong ioninduced-dipole interaction. With the addition of further benzene molecules, the interaction between the charge and the SF6 molecule will decline in strength, with the result that the latter may become more mobile. One of the consequences of this increase in mobility could be an increase in spectral line width; however, the line broadening seen in Figure lob for n > 5 could also be attributed to a lifting of the v3 degeneracy. Either way, SFs(C6H6)14+ stands out as a species that is different from its immediate neighbors. There exists a qualitative similarity between the experimental profiles and frequency shifts seen in Figures 5-10 and a series of recent calculations by Eichenauer and LeRoyIg and Kmetic and LeRoyZ0on the infrared spectra of SF6 molecules trapped in and on argon clusters. Since the calculations were concerned with neutral clusters, the intermolecular interactions will be different from those seen in small cluster ions; however, the calculations do reveal splittings associated with a lifting of the v3 degeneracy in symmetric or near-symmetric division, Le., similar to those discussed here for S F ~ ' ( C ~ H ~ ) IAssuming ~+. that in SF6*(C6H6),+ clusters the SF6 molecule does reside on the surface of the cluster and that the latter will be shielded from the charge by a layer of benzene molecules, then use can be made of a model presented by Eichenauer and LeRoyIg in an attempt to account for frequency shifts observed in the larger clusters. Eichenauer and LeRoyIg used a continuum model to represent the perturbation between a large cluster and a spherical SFg molecule. Frequency shifts can then be attributed to an interaction between the instantaneous dipole of the vibrating molecule and an induced dipole on a semi-infinite flat surface. For a vibrational mode lying parallel to a normal extending out from the surface, the corresponding frequency shift is given byI9 Av3 = -nqAc6f2Z3
(4)
and for the two SFg modes perpendicular to the normal, the frequency shifts are given byi9 AVl = AVI = -nr]AC6f4Z3
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where z is the distance between the molecule and the surface. hC6 is given by
where a is the polarizability of the atoms or molecules which make up the surface, w/&c = 948 cm-I is the angular frequency of the v3 mode for an isolated SF6 and &/8&3 is the transition
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Displacement z(%L) Figure 11. Calculated frequency shifts for the v3 mode using the continuum model proposed by Eichenauer and LeRoy and which is outlined in the text. The results shown were calculated using a, for benzene; the use of a, and a,, for the perpendicular modes makes the parallel and perpendicular shifts approximately equal. dipole moment of the v3 mode. Apart from a and the density, r ] , all the parameters necessary to evaluate eqs 4-6 can be taken from Eichenauer and LeRoy.19 For a benzene molecule, the principal components of the polarizability tensor are a, = 12.31 A3; a, = 12.31 A3; and a, = 6.53 A3;where the z axis of the polarizability ellipsoid is taken to coincide with the principal symmetry axis. A value of 0.0086 molecules A-3 was taken for the density of solid benzene as determined from crystallographic measurements;21a similar value can be estimated from the volumes of small benzene clusters.22 Figure 11 shows the frequency shifts calculated using a,, where it can be seen that the parallel mode exhibits the largest shift which, assuming a radius of 3 8, for the SF6 molecule, will achieve a maximum value of approximately -2.5 cm-'. However, modes operating perpendicular to the surface normal are more likely to polarize benzene molecules in the x and y directions, in which case a, and a, become the required quantities in eq 5. Under those circumstances, both the parallel and perpendicular frequency shifts are approximately -2.5 cm-I for an interaction distance of 3 A. For clusters of a size that is most appropriate for the model, the predicted shifts are clearly much smaller than the experimental values given in figure lob. Since the experimental results appear to converge to a shift of approximately -6.5 cm-I, it is unlikely that the presence of a positive charge continues to have a strong influence. In fact, a shift of -6 cm-' is comparable to that recorded by Goyal et aLZ3in an experiment which involved SF6 being "picked-up" by an argon cluster. In their case, Goyal et aLZ3attributed the shift to a surface-bound molecule. If it is assumed that the SF6 molecule lies below the surface of the cluster, then the model of Eichenauer and J X R O ~ ' ~ is capable of predicting red shifts which are larger than those shown in Figure 11; however, such configurations would not equate with the continued loss of SF6 as a consequence of photoexcitation.
Conclusion Photofragmentation patterns and infrared absorption profiles have been determined for SFs(C&),+ cluster ions. The results appear sensitive both to the location of the SF6 molecule and to the nature of the interaction between it and the host cluster. Data on SF~*(C&)I~+ is interpreted in terms of the benzene molecules forming an underlying stable icosahedron.
J. Phys. Chem., Vol. 99, No. 17, 1995 6339
Photofragmentation of SF6*(C&),+ Cluster Ions
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