A He-Ne gas laser (2950 cm⁻¹) has been used to excite the asymmetric stretch (\νₐ) of methane. Vibrational fluorescence has been observed from the \ν₄ mode around 1300 cm⁻¹. Detailed study of the phase lag between excitation and fluorescence has allowed us to determine not only the relaxation time of the \ν₄ mode (pr=1.7 \μ \. atm) but also the time for relaxation of vibrational energy from \ν₄ to \ν₄ (pr = 6\± 2 nsec. atm). We believe this to be the first direct measurement of a vibration-vibration relaxation time in a polyatomic molecule. This work supported from grants from the National Science Foundation and from ARPA.
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Yardley et al. (1966) studied this question.
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