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J . Phys. Chem. 1985, 89, 4608-4612
of aromatics in both pyrolysis and oxidation environments are being actively pursued. Acknowledgment. I am grateful to P. R. Westmoreland for many stimulating discussions, particularly regarding the isomerization of the energized complexes; he also suggested use of the
temperature-dependent collisional deactivation efficiency model of Troe. A critical analysis of the manuscript by W. N. Olmstead and R. L. Woodin is greatly appreciated. Registry No. CH,., 2229-07-4; HC=C., 85-1: HC=CH, 74-86-2.
21 22-48-7; H,C=CH,,
74-
Measurement of Internal Energies by Hot Ultraviolet Absorption Spectroscopy: Spectra of Excited Azulene Molecules L. Brouwer, H. Hippler, L. Lindemann, and J. Troe* Institut f u r Physikalische Chemie der Universitat Gottingen, 0-3400 Gottingen, West Germany (Received: April 8, 1985)
The gas-phase absorption spectrum of azulene in the wavelength range X = 220-300 nm has been measured in shock waves between 600 and 1750 K. Combined with the room-temperature spectrum, a dependence of the absorption coefficients €(A) on the average thermal energy ( E ) over the range 10-420 kJ mol-’ is established. The “canonically hot” UV spectra are compared with “microcanonicallyhot” UV spectra obtained by laser excitation of azulene at 337 and 308 nm, corresponding to excitation energies of 367 and 400 M mol-’. ”Microcanonicallyhot” and “canonicallyhot” absorption spectra agree perfectly when the average internal energies ( E ) of the ensembles are the same. This experimental observation is rationalized. Various applications of hot UV absorption spectroscopy in chemical kinetics are reviewed.
Introduction Vibrationally highly excited molecular states play an important role in a vast class of chemical reactions. For this reason, a spectroscopic detection of these states is of great interest. There are various spectroscopic techniques which can be used for a time-resolved observation of excited-state populations. Fluorescence or multiphoton ionization is particularly sensitive. Stateresolved absorption, fluorescence excitation, or stimulated emission is useful whenever selected individual states can be separated. There exist, however, situations where the density of states is so large that one cannot realize a resolution of single states or where neither fluorescence nor multiphoton ionization signals can be detected. Such situations are frequently met with vibrationally highly excited polyatomic molecules in the electronic ground state. In this case, “hot UV absorption spectroscopy” can be used for the time-resolved observation of excited-state populations. In our laboratory we have frequently used this type of spectroscopy for the observation of the dynamics of intramolecular processes, such as isomerization and bond fission under isolated molecule conditions, as well as of collisional intermolecular energy transfer. Spectroscopists have not too intensely looked into “hot UV absorption” because of the lack of state resolution. However, this type of spectroscopy may provide a direct access to the average internal energy of the excited molecules, more or less independent of the energy distributions. For this reason, apart from just indicating the presence of excited species, this spectroscopy can serve as an “internal energy thermometer”. In order to prove this statement, in the present work we report a key experiment with excited azulene molecules. Before doing this, we briefly describe some applications of hot UV absorption spectroscopy. Well-known effects of temperature on absorption spectra are the broadening of continuous spectra, like in C1, near 330 nm,l or the increasing number and overlap of hot bands in spectra with rovibrational structure, like in CS, near 200 nm.233 The phenomenon is well documented from the low temperatures of jetcooled molecules up to high temperatures realized in shock waves. (1) G. Herzberg, “Molecular Spectra and Molecular Structure”, Vol. I, Van Nostrand, Princeton, NJ, 1950. (2) R. J . Hemley, D. G. Leopld, J . L. Roebber, and V. Vaida, J . Chem. Phys., 79, 5219 (1983). (3) J. E. Dove, H. Hippler, H. J. Plach, and J. Troe, J . Chem. Phys., 81, 1209 (1984).
0022-3654/85/2089-4608$01.50/0
Apart from C12 and CS,, one may cite the examples N20,403,5 CF31,6 benzene,’ t ~ l u e n e , cycloheptatrienes,8 ~~* and others. Often there are wavelengths where the absorption coefficient 6 depends strongly on the temperature, such that it can serve as a spectroscopic thermometer (see e.g. ref 9-1 1). If the absolute value of 6 is not sufficient, its temperature-dependent wavelength dependence can also be evaluated. The response of this thermometer is very fast such as demonstrated recently in picosecond experiments by Kaiser and co-workers.” Theoretical predictions of the temperature dependence of UV absorption spectra over large temperature ranges are still very limited. The Franck-Condon principle, applied to suitable potential energy curves, has allowed for an accurate analysis of the temperature dependence of continuous spectra of halogens in the visible or near-UV.1s12 With increasing temperature, the absorption decreases at the center and increases at the wings of the nearly Gaussian profiles of the continuous absorption spectrum. By employing several approximations, the Sulzer-Wieland treatment1) satisfactorily describes this behavior by a simple formula. It may appear somewhat surprising that the continua of polyatomic molecules, in spite of their more complex excitedstate geometries, often show nearly the same behavior8J4and, to (4) W. Jost, K. W. Michel, J. Troe, and H. Gg. Wagner, Z . Naturforsch., A , 19A,59 (1964); M. G . Holliday and B. G. Reuben, Trans. Faraday SOC., 64, 1735 (1968). (5) D. C. Astholz, A. E. Croce, and J. Troe, J . Phys. Chem., 86, 696 (1982); J. W. Simons, R. J. Paur, H. A. Webster, and E. J. Bair, J . Chem. Phys., 59, 1203 (1973). (6) L. Brouwer and J. Troe, Chem. Phys. Lett., 82, 1 (1981). (7) W. R. Richardson, S.H. Lin, and D. L. Evans, J. Chem. SOC., Faraday Trans. 2, 78, 1 (1982). ( 8 ) D. C. Astholz, L. Brouwer, and J. Troe, Ber. Bunsenges. Phys. Chem., 85, 559 (1981). (9) M. Heymann, H. Hippler, and J. Troe J . Chem. Phys., 80, 1853 (1983). (10) N. Selamoglu and C. Steel, J . Phys. Chem., 87, 1133 (1983); P. M. Mui and E. Grunwald, J . A m . Chem. SOC.,104, 6562 (1982); E. Grunwald, J . Phys. Chem, 85, 3409 (1981). (11) W. Kaiser, Ber. Bunsenges. Phys. Chem., 89, 213 (1985); A. Seilmeier, P. 0. J. Scherer, and W. Kaiser, Chem. Phys. Lerr., 105, 140 (1984). (12) J. Le Roy, R. G. Macdonald, and G. Burns, J . Chem. Phys., 65, 1485 (1976). (13) P. Sulzer and K. Wieland, Helu. Phys. Acta, 25, 653 (1952). (14) G. Herzberg, “Molecular Spectra and Molecular Structure”, Vol. 111, Van Nostrand, Princeton, NJ, 1966.
0 1985 American Chemical Society
Excited Azulene Molecules a first approximation, can be represented by the Sulzer-Wieland equation as well. Among the few attempts to provide a more quantitative theoretical analysis of the temperature-dependent absorption continua of polyatomic molecules, the O3system has attracted some a t t e n t i ~ n . ~ ,From ’ ~ the difficulties even with this system, one realizes how far one is from a detailed understanding of the spectra of larger polyatomic systems. While the success of a Sulzer-Wieland type representation for the continua of polyatomic molecules is already surprising, it is even less expected that very structured spectra of polyatomic molecules like in CS22 at high temperatures often again approach a Sulzer-Wieland shape.3 Nevertheless, in the absence of sufficiently detailed excited-state potential energy surface calculations for polyatomic molecules, one has to calibrate the “Sulzer-Wieland parameters” of the spectrum by empirical measurements in static systems or shock waves. It should be emphasized that minor (sometimes major) deviations from the Sulzer-Wieland equation require an empirical calibration of the hot UV absorption spectrum anyway. As is discussed above, UV absorption spectra provide a means of measuring internal temperatures in thermal systems. In the following we describe observations of hot UV absorption spectra in non-thermal systems. By UV laser excitation of excited electronic states and subsequent internal conversion, we have created so high concentrations of vibrationally highly excited cycloheptatriene, substituted cycloheptatrienes,I6 and cyclooctatetraene molecule^'^ that transient absorption of theses species could be detected. Unimolecular isomerizations subsequently produced highly excited aromatic molecules like toluene for which again hot UV absorption ~ p e c t r a ’ * could ~ ’ ~ be observed. Hot toluene molecules finally underwent simple bond fission, forming vibrationally excited benzyl radicals20 with similar absorption properties as thermally hot benzyL8 This series of unimolecular processes of energy-selected species could directly be visualized by the transient absorption signals. As long as one is sure about the nature of the species involved, an absolute calibration of the absorption coefficients in these processes is not required. However, it becomes necessary when the transient species have to be identified and their yields are of interest. If collisions with other molecules take place prior to unimolecular reactions, the transient hot spectra disappear by “cooling processes”. At this place, a calibration of absorption coefficients vs. internal excitation allows to analyze the cooling by a sequence of deactivating collisions. Studies of collisional energy transfer in this way have been performed with toluene2] substituted cycloheptatrienes,2zCS2,23and S02.24325A summary of the results of these studies is given in ref 34. The basis for this analysis was the assumption that “microcanonically hot spectra” from laser excitation and “canonically hot spectra” from thermal excitation in shock waves are the same if the average energies ( E ) of the ensembles are equal. Although ( E ) was larger in the laser experiments than accessible with thermal excitation, the laser spectra followed by smooth extrapolation from thermal spectra using (15) S. E. Adler-Golden, J . Quant. Spectrosc. Radiat. Transfer 30, 175 (1983); J. A. Joens and E. J. Bair, J . Chem. Phys., 79, 5780 (1983); M. G. Sheppard and R. B. Walker, J . Chem. Phys., 78, 7191 (1983). (16) H. Hippler, K. Luther, J. Troe, and R. Walsh, J . Chem. Phys., 68, 323 (1978); H. Hippler, K. Luther, and J. Troe, Faraday Discuss. Chem. SOC., 67, 173 (1979). (17) D. Dudek, K. Gliinzer, and J. Troe, Eer. Bunsenges. Phys. Chem., 83, 788 (1979). (18) H, Hippler, J. Troe, and H. J. Wendelken, J. Chem. Phys., 78, 5351 (1983). (19) H, Hippler, K. Luther, J. Troe, and H.J. Wendelken, J . Chem. Phys., 79, 239 (1983). (20) H, Hippler, V. Schubert, J. Troe, and H. J. Wendelken, Chem. Phys. Lett., 84, 253-(1981). (21) H, Hippler, J. Troe, and H. J. Wendelken, Chem. Phys. Lett., 84,257 (1981); J . Chem. Phys., 78, 6709 (1983). (22) H, Hippler, J. Troe, and H. J. Wendelken, J . Chem. Phys., 78, 6718 (19831. . (23) J. E. Dove, H. Hippler, and J. Troe, J. Chem. Phys., 82, 1907 (1985). (24) M. Heymann, H. Hippler, D. Nahr, and J. Troe, to be submitted for
publication. (25) H. J. PIach and J. Troe, Int. J . Chem. Kinef., 16, 1531 (1984); C. J. C o b s , H. Hippler, and J. Troe, J . Phys. Chem., 89, 1778 (1985).
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 4609
Sulzer-Wieland type expressions. For CS2 and SO2, large inaccessible energy gaps had to be bridged, whereas for the larger molecules the energy gaps where smaller. Similarly as in our own observations, for benzenez6and hexafluorobenzeneZ7in the “channel three range” hot UV absorption signals were observed which were attributed to vibrationally highly excited electronic ground-state molecules. Photolysis of benzyl chloride at 193 nm28produced hot benzyl absorption spectra which subsequently cooled down by collisions. In these systems, however, no quantitative analysis of the spectra in terms of internal energies of the excited species could be made. The similarity of “microcanonically hot” and “canonically hot” UV absorption spectra in ref 3 and 18 was rationalized by considering the energy distributions of individual oscillators in the molecule. For equal average energies ( E ) of the two ensembles, nearly the same internal distributions are present. Since these internal energy distributions govern the hot spectra, the similarity of microcanonically and canonically hot spectra becomes plausible. Nevertheless, at least in one example the validity of this hypothesis should be demonstrated experimentally. With triatomic molecules the gap between the accessible excitations is very large. For example, laser-excited CS2molecules in ref 3 had internal energies corresponding to average energies at about 14000 K whereas thermally excited molecules could only be studied up to 4000 K. In order to produce the same average internal energies with laser and thermal excitation, one should choose a sufficiently large and stable polyatomic molecule. We found that azulene is a particularly suitable molecule for this purpose. Apart from the demonstration of the identity of canonical and microcanonical spectra, this system is also attractive for collisional energy-transfer studies. By using hot IR emission signals, Barker and c o - ~ o r k e r ins~~ vestigated this system in detail. Although most of their results some discrepancies agreed with our studies on other molecules,z1~22 about the energy dependence of energy transfer remained. By the use of the calibration curves between 6 and ( E ) determined in the present work, azulene energy transfer can be directly investigated by hot UV absorption spectroscopy as well.30 Before describing our azulene work, we finally refer to a series of other hot UV absorption spectra recorded with various type of excitation. There have been a series of “ground state recovery” absorption spectra observed after one UV photon excitation. Such signals were detected, e.g., in the photoisomerization of electronically excited t r a n ~ s t i l b e n e on , ~ ~a picosecond time scale or in the collisional deactivation of pyridine.32 For a quantitative analysis of these signals, however, a calibration of the energydependent absorption coefficients should be made. Vibrationally highly excited molecules are easily also generated by IR multiphoton excitation. Hot UV absorption spectra of these species have also been detected, e.g., for CF316333and O3.I5 Experimental Techniques Azulene on the one hand is sufficiently stable such that its UV absorption can be measured in shock waves up to temperatures near 2000 K. On the other hand, excitation of the molecule into the SI and Szstates by laser flashes, via fast internal c o n v e r s i ~ n , ~ ~ (26) N. Nakashima and K. Yoshihara, J . Chem. Phys., 79,2727 (1983). (27) T. Ichimura, Y. Mori, N. Nakashima, and K. Yoshihara, Chem. Phys. Lett., 104, 533 (1984). (28) N. Ikeda, N. Nakashima, and K. Yoshihara, J . Chem. Phys., 88,5803 (1984). (29) G. P. Smith and J. R. Barker, Chem. Phys. Lett., 78,253 (1981); J. R. Barker, M. J. Rossi, and J. R. Pladziewicz, Chem. Phys. Lett., 90, 99 (1982); M. J. Ross, J. R. Pladziewicz, and. J. R. Barker, J . Chem. Phys., 78, 6695 (1983). (30) H. Hippler, L. Lindemann, and J. Troe, J . Chem. Phys., in press. (31) V. Sundstrom and T. Gillbro, Eer. Bunsenges. Phys. Chem., 89,222 (1985). (32) J. I. Selco, P. L. Holt, and R. B. Weisman, Chem. Phys. Lett., 106, 437 (1984). (33) Yu. A. Kudriavtsev and V. S . Letokhov, Chem. Phys., 50,353 (1980). (34) H. Hippler, Eer. Bunsenges. Phys. Chem., 89, 303 (1985). (35) I. C. McDade and W. D. McGrath, Chem. Phys. Lett., 72, 432 (1980); 73,413 (1980); S . M. Adler-Golden and J. I. Steinfeld, Chem. Phys. Lett., 76, 2201 (1982); S . M. Adler-Golden, E. L. Schweitzer, and J . I. Steinfeld, J . Chem. Phys., 76, 2201 (1982).
4610 The Journal of Physical Chemistry, Vol. 89. No. 21. 1985
populates vibrationally highly excited states in the ground electronic state SD Because of the large number of oscillators of the molecule, average thermal energies under shock wave excitation ex& the energies of the laser photons. In this way, absorption spectra of canonical and (nearly) microcanonical ensembles of the same average energies can be compared. Shock Waue Experiments. Azulene highly diluted with Ar was heated in incident and reflected shock waves up to temperatures in the range 6(xt1900 K. Its absorption coeffcient was measured over the wavelength range 22W300 nm. i.e. in the region of the S,, S,, and S, transitim."l At the highest temperatures, azulene reacts with a half-life of a few microseconds. In this case, absorption signals directly behind the shock wave were used. At temperatures below 1200 K the lifetime of azulene is longer than the observation time of 2 ms of our experiment. Details of our shock tube setup have k e n described earlie6 and are only briefly summarized here. The light source for our absorption measurements was a Xe high-pressure arc lamp (either Varian VIX I50 or Hanovia 910-C-I). The light passed the tube through q u a m windows close to the end plate of the shock tube. The transmitted light was divided into I H O detection beams in order to simultaneously record the absorption at two wavelengths. It was dispersed b) grating monochromators (Oriel 7241. grating with 2400 lines/mm. used a spectral resolution of 2.5 nm) and detected with photomultipliers (RCA IP28A or Hamamatsu R 166).
mbar." At 300 K. azulcne has a vapor pressureof 1.52 X Based on this. we determined a gas-phase decadic absorption axfficient at the absorption maximum A = 264 nm of 4 2 6 4 nm) = 5.9 X 10' L mol-'cn-' (measured in a Cary 17 D spectrometer with capacity manometry). One obtains sufficient absorption signals with a fraction of the vapor pressure in the reaction mixtures used in the shock tube. We prepared azulene-argon mixtures in a 100-L glass bulb containing crystalline azulene and 200-1000 mbar of Ar. The mixtures were introduced into the tube, and the azulene concentration. which decreases after filling of the tube bccause of wall adsorption. was continuously recorded via the light absorption at 264 nm. The concentration of azulene in the reaction mixtures in this way was determined directly before arrival of the shock wave. Lambert-Beer's law was found to hold, and the decadic a b sorption coefficient = (c0-I log (lo/l)was determined (c = azulene concentration: I = IO cm = shock tube diameter; I = intensity of light). We employed argon concentrations between about 2 X IOd and 2 X IO4 mol cm-', azulene concentrations alwayr below 0.1% in Ar, and temperatures between 600 and 1900 K. The azulene disappearance, bccause of thermal isomerization to naphthalene, followed first-order time laws with rate constants rising from 4.8 X IO' s-I at I305 K ((Ar] = 3.7 X IOd mol cm-') Minor 103.3 X 10'sdat 1900K([Ar] = 1.2 X 10-"olcm-'). pressure dependences due to falloff behavior were noticed. More details about the thermal isomerization will bc published later. Laser Exrirarion Experiments. Azulene in the prexnce of high excess of various inert bath gases was irradiated by flashes from a N, laser or a XeCl excimer laser in a cylindrical irradiation cell. The light from a continuous light source for absorption measurements was guided beam in beam with the excitation light through the cell along the cylindrical axis. The expcrimcntal setup and the applied technique were very similar to our recent experiments on highly excited CS,'and arc only briefly summarized in the following. The excitation laser was a multigas-excimer laser (Lambda Physik EMC 102). used alternatively as a N 2laser at 337 nm (36) C. J. Haehsnsdel. J. A. Ghormlcy.and J. W. Boyle. J. Chrm. Phyi., 48. 2416 (1968): J. F. Riley and R. W. Cahill. 1. Chrm. Phy,.. 51. 3291 (1970);P.L.T. Bcvsn and G. R. A. Johnmn. J Chem. Soc., Faraday Tram. 1.69.216 (1973). (37) T. Klcindimrt. J. B. Burkholdn. and E. J. Bair. Chrm. Phyr. Loll.. I 1 1 (1980): J A. J a n s . J. 6. Burkholder. and E. J. Bair. J. C h m . Phys.. 76, 5902 (1982) (38) P. Gillapic and E. C. Lim, 1. C h m . Phys.. 66.4578 (1978). (39) R. Pariur. J. Chcm. Phys.. 25. 1112 (1956). (40) A. Bauder and Hr. H. GOnthard. Hdu. Chim. Aero. 45, 1698 (1962).
m.
Brouwer et al.
Figure 1. Absorption steps of azulene behind incident and reflected shock waves (A = 255 nm, [azulene]/[Ar] = 4.1 X IOd, Tim= 655 K, T,
