J . Phys. Chem. 1984, 88, 3936-3938
3936
Infrared Vibrational Predissociation of van der Waals Clusters: Appllcations to Isotope Separation J.-M. Philippoz, J.-M. Zellweger, H. van den Bergh,* Institut de Chimie Physique, Ecole Polytechnique FPdPrale de Lausanne, Lausanne, Switzerland CH- 101 5
and R. Monot Institut de Physique ExpPrimentale, EPFL, CH-1015, Lausanne. Switzerland (Received: May 21, 1984)
Isotope separation is demonstrated following the selective infrared laser-induced photodissociation of van der Waals clusters in a free jet. Irradiation of a natural abundance mixture of SF6 isotopomers diluted in argon with a 20-W cw C 0 2 laser gives overall enrichment factors in excess of 1.1. By adjusting the wavelength one can either enrich or deplete the center of the free jet in any one of the sulfur isotopes. Furthermore, unselectiue photodissociation of clusters can be used to enhance the separation of isotopes in a recently reported selective condensation method.
Weakly bound van der Waals clusters can undergo vibrational predissociation following the absorption of infrared radiation. This property has been used extensively to study the infrared spectra of such clusters.’ An essential feature of many of these experiments is the recoil of the photodissociated cluster fragments away from the center of the cluster beam, and hence out of the field of view of the mass spectrometric detector. In such experiments the absorption of an infrared photon, and the subsequent vibrational predissociation, can be detected as a decrease in signal intensity at a given mass which is characteristic of the cluster being studied. Clusters containing different isotopomers, like for instance 32SF6Arand 34SF6Ar,will exhibit different infrared vibrational predissociation (IRVP) spectra and may thus be photodissociated selectively. Hence selective infrared excitation of 32SF6Arin a mixture of 32SF6Arand 34SF6Ar,followed by recoil of the 32SF6 fragment away from the beam centroid, will result in a relative enrichment near the center of the beam in 34SF6.A similar scheme has been proposed by Lee.2 In this Letter we report preliminary results on the separation of sulfur isotopes by infrared laser-induced vibrational predissociation at different wavelengths and stagnation conditions. A tentative interpretation is given for the observed wavelength dependence of the isotopic enrichment factor, and a method is proposed in which IRVP is combined with isotopically selective condensation3 to improve the efficiency of the latter. The apparatus used has been described previously in some detail!ss Some of the essential features are briefly reviewed here. The gas mixture of SF, diluted in argon is expanded through a 0.1-mm sonic nozzle into the first chamber of a molecular beam apparatus. This chamber is pumped to torr by a Roots blower; it contains two NaCl windows through which any part of the free-jet expansion between the nozzle and the skimmer can be irradiated at right angles with the focussed radiation of a 20-W line tunable cw C 0 2laser. The laser beam is focussed on the free jet 4-mm downstream from the nozzle, and the beam waist is 0.4-mm fwhm at the focal point. The skimmer through which the central part of the free jet enters the second chamber is placed 10-mm downstream from the nozzle and has a diameter of 1 mm. The molecular beam leaves the second chamber, which is pumped by an oil diffusion pump to torr, through a collimator of 5-mm diameter 120-mm downstream from the nozzle. The beam then (1) K. C. Janda, Adv. Chem. Phys., in press. (2) Y. T. Lee, private communication. (3) J.-M. Zellweger,, J.-M. Philippoz, P. Melinon, R, Monot, and H. van den Bergh, Phys. Rev. Lett., 52, 522 (1984). (4) P. Melinon, J.-M. Zellweger, R. Monot, and H. van den Bergh, Chem. Phis., 84, 345 (1984). (5) J.-M. Zellweger, Ph.D. Thesis, EPFL, Lausanne, 1984.
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enters the third chamber which contains the quadrupole mass spectrometer (QMS), in which the pressure is maintained at 2 X IO-’ torr by two turbomolecular pumps and a liquid nitrogen trap. Values are reported for the overall enrichment factor a which is defined by a =
( X E ( 1 - XD))/{XD( 1 - XE))
(1)
where X is the average mole fraction of the desired component, which is 34SF6in our case. Subscripts E and D refer to the enriched and depleted fractions, respectively. To obtain efficient isotope separation by IRVP of van der Waals clusters in a free jet, one must chose stagnation conditions so as to yield a large proportion of clusters in the beam. In principle, this implies a high stagnation pressure (Po)and a low stagnation temperature ( T o ) , However, such a large fraction of clusters in the beam goes hand in hand with a wide distribution of species, in our case SF6(m)Ar(n).All these clusters have different IR spectra.’S5-* This leads to a complicated dependence of the isotopic enrichment on wavelength, as at one particular wavelength many clusters 1SF6(,)Ar(n)may photodissociate and contribute to the enrichment and/or depletion of a particular isotopomer. We have shown that in the limiting case, where too many different species 1SF6(m)Ar(n) are present in the beam, there is a tendency toward cancellation of isotopic enrichment and depletion at all wavelengths tested. In the experiments described below we have chosen To,Po, and the percentage of SF6 in Ar (51%), so that the beam contains SF,(,)Ar(,) with m predominantly I 2. Under these conditions we can expect efficient isotope enrichment or depletion. Furthermore, the reduced number of different species in the beam permits a tentative identification of the features in the effective isotope enrichment spectrum spectrum with the photodissociation of particular species (SF6),Arn, of which the IRVP spectra have been reported.’s5-* Parts a and b of Figure 1 show the dependence of a on the wavenumber at different stagnation conditions. Clearly the changes in the effective isotope enrichment spectra with Toand Po indicate changes in the populations of the clusters in the beam. In the following section the spectral features of Figure l b are assigned from the known IRVP spectra of (SF6),Arn. The (6) R. Rechsteiner, R. Monot, L. Woste, H. van den Bergh, and J.-M. Zellweger, Helv. Phys. Acta, 54, 282 (1981). (7) J. Geraedts, S. Stolte, and J. Reuss, Z. Phys. A, 304, 167 (1982): J. Geraedts, M. Waayer, S. Stolte, and J. Reuss, Faraday Discuss. Chem. Soc., 73, 375 (1982). (8) J.-M. Zellweger, J.-M. Philippoz, P. Melinon, R. Monot, and H. van den Bergh, “Laser Spectroscopy VI”, Springer Series in Optical Sciences 40, H . P. Weber and W. Luthy Eds., Springer, Berlin, 1983.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 3937
Letters
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Figure 1. (a) The dependence of the relative isotope enrichment factor 01 measured directly in the molecular beam upon the wavenumber of the IR laser radiation: To = 237 K, Po = 1.5 bar. The dilution of SF6 in Ar is 0.5%. (b) the same experiment as l a but with To= 225 K, Po = 1.4 bar. (c) The relative total attenuation measured at the mass of 32SF5c as a function of the wavenumber; the conditions are identical with lb.
measured decrease in 32SF6in the beam induced by the laser at each wavelength (as indicated by the SFS+signal) shown in Figure I C is used to corroborate this assignment. Different spectral regions in Figure 1b are labeled A-E. A: These two regions are at approximately 933-936 cm-' and 953-956 cm-'. The enrichment in 34SF6observed in both regions may be ascribed to the photodissociation of 32SF6dimers with few or no Ar atoms a t t a ~ h e d . The ~ . ~ sulfur hexafluoride dimer bands are separated by 20 cm-' and do not display the red shift that has been reported upon attachment of many Ar atoms to such a m o l e c ~ l e . ' ~Figure ~ * ~ IC shows that at these wavelengths there is a significant loss of 32SF6,which is consistent with (32SF6)2Ar, photodissociation. B: These two regions which are adjacent to the regions labeled A are at 929-933 cm-I and 948-953 cm-'. These regions are not separated from the regions labeled A by a well-defined border. Here the enrichment in 34SF6is assigned to the photodissociation of 32SF6 dimers with at least several Ar atoms attached. The latter causes a further red shift of the SF6dimer spectrum. It has also been reported that the red shift between consecutive species in the homologous series SF6Ar, decreases with increasing n,',' Le., the red shift between SF6and SF6Ar is larger than the red shift between sF6A1-g and S F ~ A I ' which ~ is negligible. Hence the photodissociation spectra of several different clusters (32SF6)2Ar, are expected to be essentially superimposed for large n. As a large number of species decompose at these wavelengths one observes rather high values of the effective enrichment factors. Again the large losses of (32SF6)Ar,are corroborated by the total loss of 32SF6 a t the same wavelengths as can be seen in Figure IC. C: In this spectral region near 946 cm-' a significant depletion in 34SF6is observed which is probably due to the photodissociation of the mixed dimers 34SF632SF6Ar(,).s37 Simultaneous elimination of 34SF6and 32SF6in a ratio of 1:l from the beam leads to a relative depletion in 34SF6due to the small natural abundance of this isotope (4.22%). This interpretation is substantiated by the fact that the strong depletion in 34SF6in region C goes hand in
V
Figure 2. The dependence of the absolute isotope enrichment factor a measured with the special collimator upon the wavenumber of the IR radiation. The experimental conditions are identical with those of Figure lb.
hand with a rather weak depletion in overall 32SF6,as shown in Figure IC. D: In this spectral region near 942 cm-' a moderate enrichment in 34SF6 is apparently caused by the photodissociation of 32SF6Ar(!). The spectral red shift relative to the monomer spectrum indicates that several Ar atoms are present in the cluster. There is probably some spectral overlap between the regions C and D, the SF6Ar, (with few Ar atoms attached) absorbing somewhat to the blue of region D. This overlap implies cancellation of the enrichment and depletion in 34SF6.Such a cancellation might be responsible for region D being nearly absent in Figure la. E: This spectral region is at 922-927 cm-l. Here the strong depletion in 34SF6is probably caused by the photodissociation of 34SF6Ar(nl.As expected region E is shifted by about 17 cm-' (the isotope shift in the v3 mode between 32SF6and 34SF6)from region D. At these wavelengths there is essentially no overall depletion in 32SF6in the beam as indicated in Figure IC, which supports the evidence that only 34SF6Ar, clusters absorb. The tentative interpretation of Figure 1b given above will be substantiated in future measurements. For instance, a higher dilution of SF6 in Ar will substantially reduce contributions from (SF6)2Ar,, which should lead to a strong decrease in the importance of regions A, B, and C relative to regions D and E. Furthermore, isotopically selective condensation can be used to eliminate nearly all clusters containing one isotopomer, thus also causing predictable spectral changes. The results shown in Figure 1 are based on the ratio 34SFS+/32SFS+ measured by the QMS directly in the molecular beam. Factors such as the degree of internal excitation of the species to be ionized and the degree of clustering can influence the observed 34SFs+/32SFS+ ratio. Hence the enrichment factors a reported in Figure 1 can at best be taken to be ~emiquantitative.~ To avoid this problem we have constructed a special collimator in which the clusters in the beam are broken up and all species in the beam are thermalized prior to ionization by multiple wall collisions. We have previously shown3 that QMS measurements of the 34SF6/32SF6 ratio made with this special collimator are essentially identical with the true values. The data of Figure l b remeasured at the same conditions with this special collimator are shown in Figure 2. The same spectral trends are conserved, but, as found in ref 3, the absolute values of a are smaller, at best slightly in excess of 1.1. The data presented above demonstrate the feasibility of isotope separation by selective IRVP in a cluster beam. This demonstration is limited to the case of SF6 excited in the v3 mode, where separation between the spectra of the isotopomers is larger than the width of the IRVP spectra. A second application of IRVP to the separation of isotopes is the enhancement of the selectivity in the selective condensation m e t h ~ d .Briefly, ~ in this method a free jet is created in which one isotopomer is "heated" and does not condense, whereas the other isotope condenses to 34SF6(m)Ar(,).The large relative mass differences between 32SF6molecules and 34SF6(m$k(n)clusters leads to an aerodynamic separation in which the heavier particles concentrate near the centroid of the free jet. Enrichment factors of nearly 2.0 are observed. If, however, we irradiate between
J . Phys. Chem. 1984,88, 3938-3939
3938 skimmer
tttt
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ttlt
ISt laser induces
pnd laser induces
selective condensation
(un)selective photodissociation
Figure 3. A schematic representation in which separation of isotopes by selective condensation is significantlyenhanced by (un)selectivevibrational predissociation. The first IR laser selectively excites %F6 in the v3 vibrational mode, thus inhibiting formation of van der Waals clusters containing '*SF6. The 34 sulfur isotope contained in 34SF6Arnclusters which are now significantly heavier than the %F6 molecules concentrate at the center of the free jet. This selective condensation takes place in the collisional part of the gas expansion. A second IR laser then removes the clusters containing 34SF6efficiently from the collision free part of the molecular beam by IR vibrational predissociation, which may or may not be isotopically selective. An encircled 32 represents a %F6 molecule, represents a 34SF6Arzcluster, and a solid circle is an Ar atom.
skimmer and collimator to cause IRVP of 34SF6(m)Ar(n) and the consequent recoil of 34SF6out of the beam, the gas pumped in this region should contain a very high fraction of 34SF6indeed. This separation method in which selective condensation is com-
bined with IRVP is shown schematically in Figure 3. A preliminary experiment along these lines using two cw C 0 2 lasers, one for the selective condensation in the collisional part of the free jet and the second one for IRVP in the collision free region, showed the expected effect. It should be stressed that for the isotopomers of the heavier elements, with very small optical shifts, even unselective IRVP will efficiently separate isotopes in such a multiple laser experiment, i.e., unselective irradiation of a mixture of 'MF, and JMF6(,,&) at wavelengths where all (or almost all) species containing isotopes i and j absorb will cause selective recoil of jMF, out of the beam. The 'MF6 molecules that do absorb cannot undergo IRVP and hence do not recoil strongly out of the beam. I is an inert gas atom. It is not clear at this point whether the combination of selective condensation and IRVP which we have applied successfully to the separation of the isotopomers of sulfur hexafluoride can be applied effectively to the separation of isotopomers of the heavier elements. This would imply that the collisional region of the free jet would have to be used for both (1) cooling of the 'MF6 and JMF, molecules to separate their spectra and (2) selective IR excitation followed by selective condensation.
Acknowledgment. The authors are grateful to the Swiss Fonds National for financial support. Registry No. SF6, 2551-62-4; 32SF6,211 10-12-3;34SF6,31719-72-9; Ar, 7440-37-1;%, 13981-57-2;34S, 13965-97-4.
Sequential Dichromatic-Biphotonic Photochemistry: A Simple Relationship of the Intermediate Lifetime with the Delay between Synthesizing and Photolyzing Pulses G . Ferraudi Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: April 27, 1984)
Sequential biphotonic processes in phthalocyanines, pc,have been investigated in irradiations with two trains of monochromatic pulses. The pulses of one trans, X = 640 nm, were used for the generation of (3m*)Al(pc)C1and the pulses of the other, X = 490 nm and delayed by 0 C 6 d 2 ms with respect to the 640-nm pulses, were used for the photolysis of this state. A direct relationship between the yield of the biphotonic process and the delay time 6 was found to be in agreement with a proposed mechanism for the dichromatic-biphotonic photochemistry.
Introduction
That sequential biphotonic processes take place when metallophthalocyanines are irradiated with high-intensity monochromatic light has been previously e~tab1ished.l~~ In these biphotonic reactions, the lowest-lying 3ir7r*was enough long lived, a condition that allowed it to be the acceptor of the second photon for the formation of the reactive state, eq 1. Similar biphotonic processes
induced by irradiating a t 640 nm with a train of pulses, and at 490 nm with another train of pulses which are delayed by 6 with (1) The abbreviated name, pc, has been used for the phthalocyanine ligand throughout this work. (2) Prasad, D. R.; Ferraudi, G. Inorg. Chem. 1982, 21, 2967. (3) Muralidharan, S.; Ferraudi, G. J . Phys. Chem. 1982, 87, 4877.
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respect to the first one, were investigated in this work. Such a dichromatic irradiation permitted the finding a direct relationship between the lifetime of the acceptor of the second photon and the delay between pulses. Experimental Section
For the dichromatic-biphotonic irradiations, two flash-pumped dye lasers, Candelas SLL-66A and SLL-200, were synchronously fired in order to obtain a constant delay between the light pulses. The time delay between pulses from each laser, Le., 6 , could then be adjusted up to a maximum of 2 ms with an error, (6 f 0.01) f i s , much smaller than any of the pulse widths (Figure 1). A third beam of monochromatic light traveled along the optical axis and was used for the monitoring of optical transients generated in the laser irradiation. The light of the lasers was directed at sharp angles, namely less than 3 O , with respect to the axis defined by the beam of monitoring light. This optical arrangement ensured a good overlap of the light beams on the sample. A pair of beam splitters and photodiodes allowed the determination of the relative 0 1984 American Chemical Society
