ROVIBRATIONAL ENERGY TRANSFER IN HIGHLY VIBRATIONALLY EXCITED $CH_{4}$
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Date
1994
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Publisher
Ohio State University
Abstract
We have developed a new form of PUMP-PROBE double resonance applicable to non-polar non-fluorescing species like $CH_{4}$ for use In collisional energy transfer measurements and spectroscopic studies. $CH_{4}$ has been the subject of few double-resonance studies because of a lack of pump sources and of low-noise probes. We use a pulsed, Raman-shifted Ti-S source as the pump, and MBE, $LN_{2}$-cooled lead salt diode lasers as a probe. The STI (now continuum) HRL-100 Ti-S laser produces Fourier-transform-limited 2-3 nsec pulses from 720-950 nm with midband energies of 40-50 mJ. Using a 2.5 m $H_{2}$ Raman cell with a single refocusing lens, under weakly-focussing conditions we achieve outputs of 2-3 mJ at $4300 cm^{-1} (CH_{4} \nu_{3}+\nu_{4}$ band) with minimum rotational Raman. The MBE diode Is lower in noise by a factor of 10 than He-cooled types and covers the $2970 cm^{-1}$ region, allowing probes on the $\nu_{3}\leftarrow 0$ and $\nu_{3}+\nu_{4} \leftarrow \nu_{4}$ bands. Most of the data obtained has been on single-state-resolved rovibrational relaxation in the $\nu_{3}+\nu_{4}$ manifold, and in the ground state. Since both $\nu_{3}$ and $\nu_{4}$ are $F_{2}$ vibrations with i = 1, 0, -1, resulting in no nuclear-spin-forbidden levels, energy transfer is complex. At high pressures, when populating the $2 -A_{1}, \nu_{3}+\nu_{4}$ level via $\nu_{3}+\nu_{4} \leftarrow 0$ and probing the same level using, $\nu_{3}+\nu_{4}\leftarrow \nu_{4}$ the several rates visible in the decay rule out the simplest models and indicates that the effective rotational partition function is larger in $\nu_{3}+\nu_{4}$ than in $\nu_{4}$ (Fig. 1). We will present the vibrational energy transfer analysis and discuss the current stare of rotational energy analysis. [FIGURE].
Description
Author Institution: Dept. of Chemistry, M.I.T; Dept. of Chemistry, U. of Gottingen