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Gas-Phase Emission Spectra of Supercooled Organic Ions Terry A. Miller,“ 8. R. Zegarskl, Trevor J. Sears, and V. E. Bondybey Bell Laboratories, Murray Hill, New Jersey 07974 (Received: September 19, 1980)
The emission spectra of “supercooled” fluorobenzene radical cations have been obtained from the electron impact ionization of a free jet expansion of He seeded with the neutral fluorobenzenes. The spectra are considerably simpler and more highly resolved than corresponding spectra taken from an ambient thermal gas. The jet spectra of the isolated ions resemble very closely fluorobenzene cation spectra obtained in a solid Ne matrix. It is shown that the true origin of the electronic transition in C6F6+is shifted very considerably from the values previously reported based upon ambient temperature spectra. Introduction In this Letter, we report the merging of two research areas, recently both extremely active and productive. Of late much spectroscopic work has been done in seeded supersonic beams.l One important outcome of this work has been the great simplification of the spectra of gas-phase organic molecules. The “freezing out” by the supersonic expansion of these molecules to their lowest vibrational and rotational levels has produced sharp vibrational, and in some cases rotational, structure from molecules whose spectra are nearly hopelessly overlapped and congested at ambient temperatures. Molecules examined by this technique include tetrazine,2a number of alkylben~enes,~ and recently the very large molecules, tetracene, pentacene, and ~ v a l e n e . ~ The only apparent limitation of this technique for obtaining sharp, analyzable spectra of large molecules is that they have a nonzero quantum yield for emission and that they be chemically sufficiently stable so that they can be seeded into a free jet expansion. In this Letter we demonstrate that the last requirement is not necessary. We have observed the emission spectra of several fluoro-substituted benzene ions, C6F6+,C6HF5+,1,3,5-C6H3F3+, etc., by ionizing the parent molecules in a supersonic jet. These spectra retain the sharpness and simplicity characteristic of the stable molecules previously studied in supersonic expansions. We believe that the parent neutrals are cooled both vibrationally and rotationally to a few degrees Kelvin by the supersonic expansion. The ionization process probably alters the rotational distribution by relatively little. Vibrational excitation upon ionization is probably governed by Franck-Condon factors. In the present study we find that most of the ions are produced in the excited electronic state in their vibrationless level, with only a small amount of vibrational excitation. Organic molecular ions, while extremely important chemically, have long been elusive species for spectroscopy. Recently, however, considerably progress has been made in this field and electronic spectra of numerous moderately large organic cations have been reported.“ A particularly interesting example of such a class of ions is the halosubstituted benzene cations, with C6F6+being the prototype. These ions have been studied8t9 by conventional “high-resolution” spectroscopy using a discharge source. Some vibrational information was obtained but overall the spectra were badly overlapped and highly congested. Early laser-induced fluorescence experiments performed on these ions in the gas phase at about the same tirnel0J1 showed somewhat better resolution than the “discharge” spectra because the ions had been cooled to room temperature before being interrogated by the laser, but the spectrum of C6F6+,for example,was still badly overlapped. A detailed and meaningful analysis of the vibrational 0022-3654/80/2084-3154$01 .OO/O
structure (and Jahn-Teller effects) in these ions required recording their spectra under conditions of even lower temperature and higher resolution. Recently we reported the cooling of these ions by liquid N2 in a gas-phase flow ~ y s t e m . l ~ However, -~~ perhaps the most progress in spectral interpretation and understanding has resulted from our observations of these same ions at =5 K isolated in a solid Ne matrix.14-18 These latter two techniques for cooling the ions have lead to an essentially complete understanding of the vibrational structure and, where applicable, of the JahnTeller effects in these and demonstrated the critical importance of “cold” ion spectra. However, in some sense, neither is the ideal method. Even near liquid N2 temperature, spectral congestion can be considerable. The matrix spectra, while sufficiently cold, are open to the objection that they may be perturbed by the matrix, although there is now an increasing body of evidence arguing that such perturbations are absent in Ne matrices. The present free jet experiment combines, in principle, the best aspects of the previous experiments: ultralow temperatures and an isolated gas-phasemolecule free of perturbing effects. Experimental Section The present apparatus is relatively simple. A 12-in. cube vacuum chamber is pumped by a Leybold-Heraeus WS700/DK180 Roots blower system. The expansion takes place through a =75-ym diameter nozzle to form a free jet, The nozzle is backed by 5-25 atm of He passing through liquid fluorobenzene contained in a metal “bubbler”. When operating, the background pressure in the chamber is -100 mtorr. The free jet expansion is crossed by an electron beam from a gun of our own design. It consists of a cathode, one accelerating grid, and a collector beyond the jet. The cathode is a directly heated thoria-coated iridium filament. Under typical operating conditions, we have an electron beam of -1 mm diameter, 150 V, and -0.1 mA collected current. The beam is collimated by an external permanent magnet which can be used to “steer” the beam to optimize its interaction with the free jet. The emission is collected in a direction perpendicular to the plane defined by the nozzle axis and the electron beam. The light is focussed into a l-m Spex double pass monochromator operating in second order. Typical spectral resolution was 0.16 nm under our operating conditions. Results The principal result of the experiment for hexafluorobenzene is shown in Figure 1. The lowest trace (C) shows a C6F6+emission spectrum from a pure C6F6+gas at am0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 24, 1980 3155
Letters
(El FREE JET
simple. While trace C is dominated by a large plateau in the baseline, the free jet spectrum shows an almost flat baseline with very sharp features. (The slight rise in the baseline in the free jet spectrum is likely due to background ions, nclt in the cold part of the expansion, and could probably be eliminated entirely by observing a skimmed beam,j The three very strong lines in trace B are He atom emissions; the riemaining features_of the spectrum we assign to transitions from B 2A2u X 2E1 in C6F6+.Looking to the right of thLe origin, the similarity getween the matrix and the free jet emission is remarkable. For every assigned matrix transitioln, there is a corresponding emission line in the free jet spectrum. Except for some lines which are clearly satellites of the main transitions (to be discussed below) there are no distinctive emission lines in the free jet which do not have counterparts in the matrix. These observations give rise to two extremely important conclusions. (i) The bulk of the free jet emission is from the upper electronic state's vibrationless level. (ii) The identity of the vibrational intervals and even the intensities of the transitions between the free jet and matrix spectrum remove any doubts about the perturbations of the latter. To the left of the origin in trace B, three identifiable lines exist before the strong He emissions. Their positions seem to be within experimental error consistent11J3J7with the following transitions from excited vibrational levels in the B 2A2ustate, l8lO,1710,210. Some excitation of, and emission from, excited state vibrational levels would be expected upon electron impact ionization of vibrationless, neutral C6F6based upon Franck-Condon arguments. We expect that this explanation also accounts for the satellites observed to the long wavelength side of the origin and the other stronger bands. Presumably these are sequence transition! invohing equally excited vibrational levels in both the B and X states. Another possible explanation is that these satellite emission lines result from an ion complexed with one or more He atoms. We have attempted to vary the expansion pressure to verify this hypothesis; however, over the limited range presently available, the relative satellite to main peak intensities appear invariant. Upon initially observing the free jet spectrum, we made a relatively precise determination of the wavelength of the origin transition using the He atom lines to calibrate the spectrometer. VVe obtained a value of 21 617 f 3 cm-', quite surprising since our gas-phase laser excitation spectrall~~~ yielded a value of 21 606 f 5 cm-l and the origin of the discharge emission spectrum was reported8 a t 21 600.6 f 0.1 cnn-l. Particularly disturbing was the apparent shift between the free jet and discharge spectrum which was well over 100 times the reported uncertainty in the latter. The matrix spectrum offers no help in this problem, became the entire electronic transitions of fluorobenzene ions are known16to be typically shifted in a Ne matrix by 50-100 cm-l. The problem was resolved by some further laser-induced fluorescence experiments in the gas phase. When we first cooled C6F6' with liquid N1, we observed obvious, probably sequence, structure in the origin band which was broad and completely unresolved at room temperature. The strongest band was centered a t -21 606 cm-l but another weaker, clearly resolved band appeared at higher energy. By improving our experiments, so that the ion's temperature became closer and closer to 77 K, we found the subband at the high-frequency end of the structure to become increasingly more intense relative to other subbands. This band is almost certainly the true -+
440
480
4 60 nM
Figure 1. Experimental traces of the B 'A, R emission spectrum of C6F6'. Trace A results from laser excitation of c~F6' in a solid Ne matrix. Trace E3 results from electron impact ionization and excitation of C,F6 seeded in a supersonic He expansion. Trace C results from electron impact ionization and excitation of an ambient, thermal, pure CBFBgas. The monochromator resolution (fwhm) in traces B and C is the same and -0.16 nm, while for trace A it is -0.08 nm. In both traces A and E,, but not C, the spectral resolution is limited by the monochromator. Trace A has been shifted so that the origin bands line up, thereby eliminating the -6O-cm-' matrix red shift. -+
bient temperature, excited by electron impact from the source previtously described. It is very similar to the "high-resolution" C6F6+spectrum reported in ref 8 from a discharge source. It can be seen to be badly overlapped and congested even though some vibrational structure is apparent. The top trace (A) in Figure 1is the wavelength-resolved emission, excited by laser pumping, of C6F6+isolated in a solid Ne matrix. It is remarkably simple compared to trace C. We have recently a s ~ i g n e d ' ~the J ~ various lines labeledjn Figure 1,t,o emission from the vibrationless level of the 13 2A2uexcited state to various alg and e2g(ahowed by Jahr-Teller interactions) vibrational levels of the X 2Elg ground electronic state. We have not continued trace A to the left of' the origin as, under these conditions, no transitions are observed in the matrix to the short wavelength side of the origin. The extreme simplicity of the matrix spectrum results from the absence of rotation and from the low 5 K vibrational temperature. While, in principle, the spectra can be complicated by the presence of phonon side bands, these are in most ions studied in a Ne matrix ralher weak, as seen in Figure la. However, phonon side bands can exist and are evident in trace A to the right of the origin and other strong bands. Because of very rapid vibrational relaxation in the matrix the bulk of the fluorescence always occurs from the vibrationless level of the B state, regardless of which level was initially excited by the laser. (Only by special effort21can unrelaxed emission be observed.) The matrix spectrum thus, in many ways, represents the simplest possible spectrum. The middle trace (R) in Figure 1 is the most interesting one for the present. It shows the emission spectrum excited by electron impact in a free expansion jet of He seeded with hexafluorobenzene. Compared to the thermal gas spectrum, the free jet expansion spectrum is extremely
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origin and we measure it, in the laser-induced fluorescence experiment, to be 21 614 f 3 cm-l, in good agreement with the free jet experiment. Overall, this experiment has demonstrated that it is possible to observe the spectra of organic molecular ions cooled to a few degrees Kelvin by a supersonic expansion and to obtain significant new information. The spectra of parent ions produced by electron impact are very simple and well resolved. It is likely that similar ions produced by other means, e.g., photoionization, would exhibit similar spectra. Such experiments will be particularly attractive in conjuction with synchrotron radiation sources. Other transient molecular species such as free radicals might well be studied by similar techniques. However, in processes involving the breaking of ci chemical bond, significant vibrational and rotational excitation might occur. It may, in turn, be possible to probe just such dynamical processes themselves by the presently described techniques.
References and Notes (1) See, for example, D. H. Levy, L. Wharton, and R. E. Smalley in "Chemical and Biochemical Application of Lasers", Vol. 2, Academic Press. New York. 1977. D 1: Acc. Cbem. Res.. 10. 139 11977). (2) R. E. Smalley, L. Wharton: D:H. Levy, and D. W. Chandler, j . Moi. Spectrosc., 66, 375 (1977).
J. B. Hopkins, D. E. Powers, and R. E. Smalley, J. Chem. Pbys., 72, 5039 (1980). A. Amirav, U. Even, and J. Jortner, Opt. Commun., 32, 266 (1980). J. H. Callomon, Can. J. Pbys., 34, 1046 (1956). M. Albn, J. P. Maier, and 0. Marthaler, Cbem. phys., 26, 131 (1977). J. P. Maier, Chimia, 34, 219 (1980), and references therein. C. Cossart-Magos, D. Cossart, and S . Leach, Mol. Pbys., 37, 793 (1979). C. Cossart-Magos, D. Cossart, and S.Leach, Cbem. phys., 41,345, 363 (1979). T. A. Miller and V. E. Bondybey, Cbem. Pbys. Left.,58, 454 (1978). V. E. Bondybey and T. A. Miller, J. Chem. Pbys., 70, 138 (1979). T. J. Sears, T. A. Miller, and V. E. Bondybey, J . Am. Cbem. Soc., 102, 4864 (1980). T. J. Sears, T. A. Miller, and V. E. Bondybey, J. Am. Cbem. Soc., in Dress. V. 'E. Bondybey, T. J. Sears, J. H. English, and T. A. Mlller, J. Chem. Pbys., 73, 2063 (1980). V. E. Bondybey, T. A. Miller, and J. H. English, J. Cbem. Phys., 71, 1088 (1979). V. E. Bondybey, J. H. English, and T. A. Miller, J . Mol. Spectrosc., 81, 455 (1980). V. E. Bondybey and T. A. Miller, J . Chem. Pbys., 73,3053 (1980). T. A. Miller, V. E. Bondybey, and J. H. English, J . Cbem. Pbys., 70, 2919 (1979). T. J. Sears,'T. A. Miller, and V. E. Bondybey, J. Chem. Phys., 72, 6070 (1980). T. J. Sears, T. A. Miller, and V. E. Bondybey, J. Cbem. Phys., in Dress. V. E. Bondybey, T. A. Miller, and J. H. English, Pbys. Rev. Lett., 44, 1344 (1980).
EPR Evidence for the Formation of the Hexamethylethane Radical Cation by Charge Transfer in a Freon Matrix Jih Tzong Wang and Ffrancon Williams" Department of Chemistry, University of Tennessee, Knoxville, Tennessee 379 16 (Received: September 8, 1960)
The hexamethylethaneradical cation [Me3CCMe3]+has been generated through positive charge transfer by y irradiation of both glassy and polycrystalline Freon solutions, its EPR spectrum consisting of seven lines with binomial intensities and having the parameters 'A(6) = 29.0 f 0.2 G and g = 2.0031 f 0.0003. In contrast, only neutral alkyl radicals are produced from the hydrocarbon by y irradiation of the hexamethylethane matrix, even when powerful electron scavengers are present. It is suggested that the formation of the radical cation by positive charge transfer in a Freon matrix is a more relaxed process than that of vertical ionization by electron impact in a hexamethylethanematrix.
Introduction The first electron-deficient ~7radical to be derived from nontransition elements and characterized by EPR spectroscopy was the boron-centered species, [ (MeO),B-B(0Me)J.l This radical resembles the prototype species Hzt and the ethane positive ion CzH6+in possessing a formal one-electron bond between two atoms or groups which are i d e n t i d 2 Another potential radical in this class with the required symmetry is the radical cation of 2,2,3,3-tetramethylbutane or hexamethylethane (HME), [Me3C.CMe3]+,so the recent report3 which attributed a multiplet EPR spectrum in y-irradiated HME to this species attracted our interest. Unfortunately, this assignment? was soon disproved, a thorough reinvestigation showing that the spectral analysis which had been proposed in terms of an expected binomial set of 19 lines3was erroneous,* and that the EPR spectrum of y-irradiated HME can be interpreted solely on the basis of the line components originating from neutral radical^.^,^ While the original claim3 was withdrawn: it was then reported that a new seven-line spectrum tentatively identified as that of the same radical cation was produced on y irradiation of the parent compound containing an
excess of electron scavengers such as CC14and CBr4,6athe conclusion being that the prevention of electron return allowed the radical cation to be detected. In a parallel development, we have shown that HME is a suitable matrix for the observation of narrow-line isotropic EPR spectra from fluorocarbon radical anions ~~~ produced by the y irradiation of solid ~ o l u t i o n s .However, the routine examination of the central features in these spectra produced no obvious evidence for any unusual matrix radicals, in apparent conflict with the above interpretation6 which would suggest that the radical cation of HME should be produced in these experiments. We decided, therefore, to carry out further work in an attempt to resolve this discrepancy. In particular, we have sought to establish whether the radical cation of HME can be formed by a mechanism of positive charge transferQ?l0 from a halogen-containingcompound when the latter functions as the matrix and HME is present as the solute. Accordingly, we now report the results of two independent sets of y-irradiation and EPR experiments at 77 K: first, HME has been used as a matrix with various electron scavengers as solutes and, secondly, dilute solutions of HME in a Freon mixturell have been studied, the latter
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