toluene high temps

toluene high temps - UV absorption spectra of vibrationally...

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UV absorption spectra of vibrationally highly excited toluene molecules H. Hippler, J. Tree, and H. J. Wendelken Institutfiir Physikalische Chemie, Universitiit GOttingen, Tammannstrasse 6, D 3400 Giittingen, Germany (Received 8 November 1982; accepted 19 January 1983) Vibrationally highly excited toluene molecules with an energy of 52 000 cm -1 have been prepared by UV laser excitation and subsequent isomerization of the isomer cycloheptatriene. The UV absorption spectrum of the toluene, whose vibrational energy would correspond to the average energy of thermal molecules at 2800 K, has been measured over the range 210-270 nm. It is compared with the UV spectrum of thermal molecules heated in shock waves up to 1800 K. All spectra, corresponding either to microcanonical or canonical energy distributions, can be represented by a modified Sulzer-Wieland equation. is concluded that the absorption coefficients can be expressed to a first approximation by the average energy of the excited molecules independent of their energy distribution. I. INTRODUCTION Molecular electronic spectra often show pronounced changes with increasing temperature. Line structures of band systems disappear, absorption continua broaden, and different electronic transitions overlap. Although these changes in principle depend on the full complexity of the potential surfaces involved, often a surprisingly Simple picture arises at high temperatures: the bands become continuous and temperature effects can be at- tributed to only one or at most very few modes.' is the aim of the present article to investigate this phe- nomenon for the poly atomic molecule toluene over a very wide temperature range. We compare room tem- perature spectra with spectra from shock wave experi- ments up to 1800 K where the dissociative lifetime of toluene approaches 1 J1.S. 2 ,3 In addition to these ther- mal, "canonically hot" UV spectra, one can study even higher excitations in laser experiments. We take ad- vantage of the fact that vibrationally highly excited elec- tronic ground state molecules of toluene can be pre- pared, energy selected with high concentrations, by la- ser excitation and subsequent isomerization of cyclo- heptatriene molecules. 4-8. The UV spectra of such molecules high up in the vibrational quasicontinuum at level densities of the order 1~O/cm-1 have been de- tected. 7 ,8 This "microcanonically hot" UV spectrum can be related directly to the canonically hot thermal UV spectra, since the single (or few) modes indUCing marked spectral changes with increasing excitation, in a polyatomic molecule at high total vibrational energies, are essentially coupled to an intramolecular heat bath (see, e. g., Ref. 9). In this sense laser-excited toluene molecules with dissociative lifetimes of the order of 100 ns correspond closely to thermal molecules near 2800 K.
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toluene high temps - UV absorption spectra of vibrationally...

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