4186
J. Phys. Chem. 1991,95,4186-4189 IC'Z')
Cplpu'2+)
+ C,(su'Z+) + Calda'E+)
(1)
Then, there will be an interference among the contributions of the three orbital characters to the transition moment from this state to the II- component. As a result, it will be highly possible that a particular rotational branch disappears by the interference. In Figure 4 is shown the Fortrat diagram of the observed band and an example of the simulated spectra using a particular set of the coefficients. As is seen in the figure, not only does the OQo branch disappear but also the relative intensities agree very well with the observed spectra. The admixture of several orbital characters in the C state explains the appearance of the four Rydberg series in the transition from this state. This is also the case for the 3suB1Z+state, from which the four Rydberg series appeared with comparable intensities. Though the coefficients of the individual orbital characters in the B and C states cannot be uniquely determined, a recent ab initio calculation by Sekine and Iwata supports this admixture of the B and C state.*4 They found that the B(3su) has a pa (24) Sekine, S.;Iwata, S. Private communication.
character of about 15%and the C(3pu) has a su character of 13%. The high Rydberg series of CO generally exhibit a complicated pattern. For example, Figure 5 shows the rotational structure of the s and p-Rydberg series for n = 8-13. The rotational bandwidth of the 9s-and 10s-Rydberg states is 6 cm-'and is much broader than other Rydberg states. In the case of the pRydberg states, the rotational bandwidth changes depending on the principal quantum number. As can be seen in the figure, the rotational bandwidth of the 9pRydberg state is broad and its width is around 6 cm-I, while those of 1Op and 12p are sharp. On the other hand, 1 l p is missing. This peculiar behavior suggests the existence of some complicated nonradiative processes besides the vibrational autoionization in this energy region. The systematic analysis of np, m,nd, and nf is now in progress and will be published in the future.
Acknowledgment. We thank Dr. Shigeyuki Sekine for sending his ab initio calculation. We are grateful to Dr. Klaus P. Huber for sending of copy of vacuum-UV absorption spectrum of CO and his helpful suggestions, and we also thank Prof. Marshall L. Ginter for helpful discussions.
Infrared Spectroscopy of SF6 In and on Argon Clusters in an Extended Range of Cluster Sires: FinltcSize Particles Attaining Bulkllke Properties S.Goyal, G. N.R o b i n , + D.L.%butt, and G.Stoles* Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (Received: March 11, 1991)
The spectroscopy of the u3 mode of sF6 seeded in or deposited on argon clusters has been extended to larger cluster sizes using a new thermal detection photodissociation spectrometer. A spectral feature characteristic of an SF6 molecule in a tightly bound monomeric site has been observed at 937.9 cm-'for clusters consisting of approximately 1000 atoms that are prepared by coexpanding a mixture of 0.025% sF6 in argon. This "matrixn feature, which is red-shifted by 10.1 cm-I with respect to the SF6gas-phase band center, disappears for larger clusters where an absorption characteristic of a chromophore residing on the surface of the cluster appears. However, on producing even larger clusters, Le., above approximately 2000 atoms, a different absorption appears which is accurately located at the same position as the main SF6 absorption in a well-annealed matrix. This feature is blueshifted by 0.7 cm-l with respect to the earlier matrixlike feature, which itself corresponds closely to the absorption produced by SF, in an unannealed (or moderately annealed) argon matrix. This behavior may be related to the transition of the clusters from a polyicosahedral structure, which has been shown to be more stable for smaller clusters, to a face-centered cubic (fcc) structure which is observed in the bulk phase. Finally, for SF6 molecules located at the surface of the cluster, we have resolved a line splitting of 1.5 cm-I. In addition, the intensity ratio of these two absorptions strongly suggests that, in this site, the chromophore is more than half buried in the cluster's surface.
One of the principal motivations for the study of clusters has been to determine the range of cluster sizes in which the transition to bulk-phase behavior takes place. The systems for which such a transition can be studied most readily are the noble gases, due to both experimental and theoretical convenience. The size dependence of the structural configuration in these systems has been investigated mainly by electron diffraction and computational methods such as molecular dynamics and Monte-Carlo simulations. It has been observed that small clusters are structurally more stable as polyicosahedra, medium-sized clusters as Mackay icosahedra, while larger clusters eventually must evolve to the fcc structure observed in the bulk solid. The electron diffraction studies of Farges, Torchet, and collaborat~rs~-~ have suggested an argon cluster size of 750 atoms for the transition to the fcc solid phase. Similar studies performed by Lee and Stein,' at a colder temperature, have reported a structural crmover point from icosahedral to f a crystal at a cluster size between 1500 and 3500 'Present address: Aerodyne Rgearch, Inc., 45 Manning Road, Billerica, MA 01821.
0022-365419112095-4186$02.50/0
atoms. These authors have also performed energy minimization calculations for Lennard-Jones systems and predicted a crossover at a size of roughly 3000 atoms. On the other hand, molecular dynamics simulations by Anderson et ala5have predicted a transition size of approximately 5000 atoms for argon. While these studies have focused mainly on pure argon clusters, laser spectrosoopy of noble gas clusters seeded with infrared active chromophores has been shown to provide information about both the solvation effects on the guest molecule and the structural properties of the host cluster itself.&* In these earlier studies (1) Fargcs, J.; deFeraudy, M. F.;Raoult. B.; Torchet, G. J . Chem. Phys. 1983. 78. 5067. (2) Farges, J.; deFeraudy, M. F.;Raoult, B.; Torchet, G. J . Chem. Phys. 1986.81. 3491. ( 3 ) Fargcs, J.; deFeraudy, M. F.;Raoult, B.; Torchet, G. In Lmge Finire Sysrems, Proceedings of rhe 20rh Jerusalem Symposium on Quantum Chemistry and Biochemistry; Jortner, J., Pullman, B., Eds.;Reidel: Dor-
drecht, 1987. (4) Lee,J. W.; Stein, G. D. Surf. Sci. 1985, 156, 112. (5) Honeycutt, J. D.; Anderscn, H. C. J. Phys. Chem. 1987, 91, 4950. (6) Levandier, D. J.; Mengel, M.; Purscl, R.; McCombie, J.; Scoles, G. Z. Phys. D 1988, 10, 337.
0 1991 American Chemical Society
Letters carried out in our laboratory, cluster sizes ranged from a few tens to several hundreds of argon atoms, as generated by a room temperature 30 pm diameter nozzle and source stagnation pressures of 30 atm. These results were interpreted with the help of molecular dynamics simulation carried out by Perera and Amar9 for clusters of up to 100 atoms. These results established a correlation between the size and well depth of the mixed interaction (measured in units of the pure solvent parameters) and the interior versus surface location of the guest chromophore in the In order to study the quantum mechanical properties of the clusters made of light atoms and molecules (e.&, He and H2) we have recently built a new molecular beam photodissociation spectrometer. Because of the large pumping speed available in the source chamber (32000 L s-I), we are now capable of producing CW beams of argon clusters ranging in size up to several thousand atoms. Using this apparatus we have remeasured the SF6/Ar system and extended its study to include larger cluster sizes. While in the lower cluster size range we confirm the previous results, the data obtained in the larger cluster regime are sufficiently different to deserve reporting in the present paper.
Experimental Section The principle of our experimental technique has been published before6*'and is described here only briefly. A full description of the new apparatus will be provided at a later time. Clusters are produced by expanding the gas through a 50 pm diameter nozzle into a primary chamber pumped by a Varian/NRC 32 OOO L s-I diffusion pump. Stagnation pressures of up to 100 atmospheres can be sustained. This corresponds to gas fluxes about 10 times larger than in the previous apparatus. This not only expands the accessible cluster size range but also increases the sensitivity of the apparatus which is now capable of operating at lower chromophore concentrations. The infrared-active molecule is introduced in the cluster either by coexpanding a very dilute (typically 0.025%)mixture of SF6 in argon or, alternatively, producing neat argon clusters and attaching the chromophore by collisions with SF, contained in a pick-up cell located in the experimental chamber. This chamber, which is connected to the source chamber by a 390 pm diameter skimmer, also contains a liquid helium cryostat that supports the detection system. Downstream of the pick-up cell, the beam is crossed at right angles by a line tunable C 0 2 laser. After the absorption of a photon, the chromophore relaxes, transferring energy to the cluster and leading to its fragmentation via vibrational predissociation. After the interaction with the laser, the core of the cluster beam is allowed to pass through the center hole of an annularly shaped silicon bolometer operated near 1 K. A sizable fraction of the fragments, which are released at a small angle (around 1.4O) with respect to the primary beam, impinge on the bolometer and are hence detected. The size of the cone of scattered fragments is optimized by changing the distance and the diameter of a circular aperture located between the laser crossing point and the detector. To compensate for the fact that C 0 2 lasers are not continuously tunable, we increase the number of points at which spectral intensities are measured by employing three C 0 2 isotopic combinations as lasing media, namely, I2C1Q2,IpCL602, and 12C1*0 2. The infrared laser beams are made to cross the molecular beam at a carefully controlled point, so that, after normalization to laser power, no scaling of the data is necessary. This was demonstrated by measuring the inhomogeneously broadened spectrum of the SF6 dimer, produced by expanding a 5% mixture of SF6 in He. The recovery of a spectrum, which is in perfect agreement with the spectra obtained both at Waterloolo and Nijmegen" for the same system, was taken as evidence that no scaling between the (7) Levandier, D. J.; Goyal, S.;McCombie, J.; Pate, B.; Scoles, G. J. Chem. Soc., Faraday Trans. 1990.86,2361. (8) Gu, X.J.; Levandim, D.J.; Zhang, B.;&des, G.; Zhuang, D. J. Chem. Phys. 1990, 93,4898. (9) Perera, L.; Amar, F. J . Chem. Phys. 1990, 93, 4884. (IO) Gough, T. E.;Knight, D. G.; Rowntrce, P. A.; Scoles, G. J . Phys. Chem. 1986, 90,4026. (1 1) Heymen, B.;Bizzari, A,; Stoke, S.; Reus, J. Chem. Phys. 19B9.132, 331.
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4187
(b) 13.6 atm
b
(9) 68.0 otm
.(c) 20.4 atm
~:J-.-fh.::., , I I
h
(h) 81.6 atm
4I
l;i7,Jk;;*,, l;JL;:;.,
932
936 940 944 Frequency (cm-1)
948932
936 940 944 Frequency ("1)
948
Figure 1. Spectra obtained by the coexpansion of 0.025% SF6/Ar at (a) 6.8 atm; (b) 13.6 atm; (c) 20.4 atm; (d) 27.2 atm; (e) 40.8 atm; (f) 54.4 atm; (9) 68.0 atm; and (h) 81.6 atm. Gaussian line shapes are fitted to the spectra. The gas-phase absorption of SF, is at 948.0 cm-I.
data obtained with different lasers was needed. The signal-to-noise ratio for most of the data was above 1O00. However, at high beam fluxes the quality of the signal degrades due to a partial overloading of the bolometer and an increase in background pressure around the detector. In addition, the signal intensity was found to decrease with increasing cluster size. This is consistent with the fact that the unimolecular lifetimes of photoexcited clusters are expected to increase with cluster size. From laser scan to laser scan, the intensities could be reproduced within 5%. Gaussian curves have been fitted to the data for a more precise determination of the spectral features.
Results and Discussion Spectra of SF6 in argon clusters obtained with our earlier apparatus, using a 30-pm nozzle and stagnation pressures of up to 30 atm (corresponding to a pressure of 20 atm when scaled to a 50-pm nozzle using the scaling laws first suggested by Hagenai2), have been published previously.6.' We have now extended the range of pressures to above 80 atm (equivalent to 122 atm in the old apparatus). Using the size estimates made by Farges et al.'-3 determined by electron diffraction and the same scaling law, these clusters correspond to an average cluster size of 9000 atoms. As in the past, most of our conclusions are based on comparisons with the matrix isolation spectra of SF6in argon published by Swanson and Jones.I3 A series of spectra taken by coexpanding a mixture of 0.025% SF6/Ar is shown in Figure la-h. The spectrum of clusters produced by a 6.8-atm beam (Figure la) shows two absorptions at 937.5 and 938.4 cm-' that bracket a feature recorded at 937.9 cm-' in the spectrum of SF6in an argon matrix by Swanson et al." This comsponds to the largeat matrix red shift of the u3 mode of an SF6 molecule due to its interaction with an unannealed or moderately annealed (at 30 K) argon environment. However, the peak with the largest shift at 937.5 cm-' is definitely to the red of the matrix feature. This suggests that the argon atoms in the clusters formed at this pressure can pack around the chromophore more densely than in the bulk matrix. Red shifts that are greater (12) Hagena, 0. F.; Obert, W . J. Chem. Phys. 1973.56, 1793. (13) Swanson, B. I.; Jones, L. H. J. Chem. Phys. 1981, 74, 3205.
Letters
4188 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991
for small clusters than for larger clusters have been observed before. For example, Millerihi5and cuworkers, in their LIF study of the C6F6+ion in argon clusters, have observed a shift of 492 cm-I in the electronic energy level of the chromophore when bonded to six argon atoms. However, for larger clusters, the shift was reduced to 249 cm-’, which is very near to the 244-cm-I value of the matrix isolation spectra. The spectrum at 13.6 atm (Figure 1 b) shows a principal absorption at 937.9 cm-I and two other broader features from which peaks emerge at higher pressures. This absorption is also very prominent in the spectra taken at 20.4 atm but has faded away when the stagnation pressure reaches 27.2 atm. We conclude that in the pressure region up to 20 atm (which corresponds to the full pressure range covered by the previous apparatus) the matrixlike features of the SF6/Ar cluster spectra are very close to those found in the unannealed bulk matrix. However, in general agreement with the previous work, at 20.4 atm three additional peaks located at 940.0,941.1, and 942.6 cm-I are resolved. The absorption at 941.1 cm-l (comparable in intensity to the matrixlike absorption) and the one at 942.6 cm-I are, as explained below, due to the chromophore residing in the surface of the cluster. The identity of these features was previously established by comparing spectra obtained by coexpansion with those obtained by using the pick-up technique, since in the latter the peak at 941.1 cm-I was greatly enhanced (see below). What is new in the present results is that, as the pressure is increased above 20 atm, the surface features gain intensity at the expense of the unannealed matrix peak which almost totally disappears by 40.8 atm. At this pressure (Figure le) a previously unobserved peak is resolved at 938.6 cm-l. This peak gains prominence at the still higher pressure of 54.5 atm and becomes the dominant peak at 68.0 and 81.6 atm. We believe that this observation is important for several reasons. Swanson and Jones have reported two principal absorptions in SF6/Ar matrix spectra, annealed at temperatures below 30 K. These are a doublet at 938.6 and 938.45 cm-I caused by a splitting of the three u3 modes of SF6 residing at a single site and the absorption at 937.9 cm-I discussed above. Upon annealing the matrix a t higher temperatures (39 and 42 K), the peak at 937.9 cm-I loses prominence with respect to the doublet because, according to the authors, of the disappearance of unstable matrix sites. This indicates that the 938.6 and 938.45-cm-l doublet can be used as an infrared signature of bulk-solid argon. In light of these observations, it is natural to conclude that the appearance of the peak at 938.6 cm-I, first observed in clusters produced at a stagnation pressure of 40.8 atm, signifies the point at which the transition to the bulk fcc solid argon structure occurs. Computer simulation^^^^ and electron diffraction datal-’ have shown that, for smaller cluster sizes, icosahedral configurations are more stable than the fcc structure found for larger cluster sizes. The icosahedral clusters are locally packed more densely than the fcc solid. Assuming that the increase in density also occurs for the smaller clusters containing SFs, this could explain the larger red shift found in the present case. It should be noted that, because of lack of spectral resolution, we do not resolve the two components of the annealed matrix doublet in the larger clusters. For argon clusters produced a t 40.8 atm through a 50-pm nozzle, the estimated cluster size is about 2000 atoms. It is important to realize that these scaled cluster size estimates become less precise as the cluster size increases. Within the 20%error limits of the cluster diameter, we can state that our observations are in agreement with the theoretical predictions and the electron diffraction data of Lee and Stein (who found the icosahedral to fcc crossover to occur around 2500 atoms), but not with those of Farges and collaboratorsi-’ (for whom the crossover occurs before 1000 atoms). We note that for clusters produced by stagnation pressures between 27.2 (-1000 atoms) and 40.8 atm (-2000
(14) Heaven, M.; Miller, T. A.; Bondeybey, V. E. J . Chem. Phys. 1982, 76, 3831. (15) Dimauro, L. F.; Heaven, M.; Miller, T. A. Chem. Phys. Lett. 1984, 104. 526.
(e) 40.8 atm
,(g) 68.0 atm
(h) 81.6 atm
(d) 27.2 atm
h
h
LJh;:;.,LJh;::-, ,
932
936
940
944
,
948932
Frequency (em-1)
936
940
944 Frequency (cm-1)
948
Figure 2. Spectra obtained with the pick-up technique at (a) 6.8 atm; (b) 13.6 atm; (c) 20.4 atm; (d) 27.2 atm; (e) 40.8 atm; (f) 54.4 atm; (8) 68.0 atm; and (h) 81.6 atm. Gaussian line shapes are fitted to the spectra. The gas-phase absorption of SF, is at 948.0 cm-I.
932
934
936
938
940
942
944
946
940
Frequency (cm-1)
Figure 3. The Gaussian fits of the spectra obtained by the coexpansion (solid line) and pick-up (dashed line) method are superimposed at (a) 6.8 atm; and (b) 81.6 atm.
atoms) the chromophore does not prefer to reside at either of the ‘matrix”4ike sites, and is pushed out toward the cluster surface. In view of the fact that for still higher pressures this trend is reversed and the cluster again solvates the chromophore we feel a complete explanation of our results will have to be based on a detailed description of the condensation kinetics. The situation is complicated by the fact that spectra taken at even lower SF6 concentrations than those presented in this work show a narrowing of the peaks. This in turn makes the determination of peak intensities even more uncertain than at present. We are presently investigating this aspect, but we want to stress that the qualitative conclusions reach here (Le., the achieving of the annealed matrix
Letters condition and the resolution of the surface doublet described below) are not affected by these uncertainties. The above assignments made on the basii of axpansion spectra are further supported by contrasting them with the pick-up spectra taken at the same stagnation pressures (Figure 2a-h). At the lower pressures (6.8-27.2 atm) the differences in the spectra produced by the two techniques are minor (see Figure 3a) and similar features are seen. However, starting below 27.2 atm, the chromophore has a higher probability of residing at the surface of the cluster when it is deposited there by pick-up as compared to coexpansion. Apparently diffusion of SF6from the surface to the interior of the neat argon clusters occurs up to about 27.2 atm, which may indicate that, at the cluster's temperature, which is believed to be between 30 and 40 K,the system can rearrange within the time scale of our experiment (approximately 100 ps). On comparing the higher pressure results (40.8-81.6 atm), this picture changes dramatically. The matrix peak at 938.6 cm-' is almost entirely absent in the pick-up spectra at all these pressures. Figure 3b superimposes the spectra at 81.6 atm obtained from the two techniques to highlight this difference. At these pressures, because of the presence of .bulklike solid argon clusters, the chromophore deposited by the pick-up method is unable to diffuse into the cluster and is forced to stay on the surface, giving rise to the dominant doublet at 941.1 and 942.6 cm-'. We note that the polyicosahedral configuration, while more tightly packed around the SF,, is bound to be more rich in defects and therefore is more likely to allow for diffusion than the more periodic bulklike fcc structures. SwansonI3and co-workers recorded no signs of diffusion of the chromophore in an argon matrix even at a temperature of 42 K. The clusters produced in the molecular beam are probably at an even lower temperature, leading us to believe that no diffusion is possible in these finite-sized systems once they have attained a bulk solid character. The occurrence of two resolved surface peaks at 941.1 and 942.6 cm-', in both the pick-up and the axpansion spectra at pressures above 20.4 atm, deserves further analysis. Except in the lowpressure pick-up spectra, these peaks maintain an intensity ratio of 2:l Over the entire range of pressures, leading to the suggestion that they might be due to a site asymmetry splitting of the triply degenerate u3 mode. Eichenauer and LeRoy16 have calculated (16) Eichenauer. D.; LeRoy, R. L. J . Chem. Phys. 1988,88, 2998.
The Journal of Physical Chemistry, Vol. 95, No. 1I, I991 4189 the magnitude of such a splitting for SF6interacting with a bulk argon surface modeled as a polarizable continuum with the same density as the solid. For an SF6 molecule more than half embedded in the surface of the cluster, their model predicts (neglecting effects due to the curvature of the surface) that the two v3 modes which are perpendicular to the normal to the cluster surface experience a greater shift than the parallel mode, the difference attaining a maximum value of approximately 1.5 cm-'. Since the peak with the larger red shift has the larger intensity, our results are consistent with the assumption that these "surface" chromophores are indeed in such a buried location. The peak observed near 940.0 cm-', which is well resolved in the pressure range between 13.6 and 27.2 atm, has been previously assigned7 to liquidlike clusters, or amorphous solid clusters with the same density and local structure as that of the liquid. This assignment was made on the basis of spectra recorded by Turnert7 and co-workers for SF6in liquid argon. They observed a single absorption line at 939.4 cm-' with a half-width of 0.5 cm-I. We have nothing to add to what was said before except that this feature has been seen again with a different apparatus. Further study on this system and others is presently in progress in our laboratory. Since at present pick-up occurs in a scattering cell positioned after the skimmer, as opposed to occurring in the jet expansion region, measurements of pick-up cross sections and signal attenuation by scattering from other gases are possible and indeed in progress. Furthermore, because of the very high sensitivity of the present apparatus, it is possible that we will be able to obtain the spectrum of H F embedded in argon clusters using a low power continuously tunable color center laser. Such librationally or rotationally resolved spectra may provide information on the very elusive temperature of the cluster. Acknowledgment. This work was made possible by the generosity of Professor John B. Fenn, who gave us the 32 000 L s-' diffusion pump and the large stainless steel chamber in which our source chamber was located. We also thank the machine shop staff at the Princeton Chemistry department for providing us with the necessary technical support in the building of the new apparatus. This work was supported by the NSF grant CHE 9016491 and by Princeton University. (17) Turner, J. J.; Poliakoff, M.; Howdle, S. M.; Jackson, S. A,; McLaughlin, J. G. Faraday Discuss. Chem. Soc. 1!J88,86, 271.
