Infraredndash;infrared double resonance measurements of the SF6ngr;3mode spectral region for excitation levels of up to 1.2 J/cm2are reported. Immediately following the pump laser excitation, the induced absorption spectrum of the ngr;3mode shows a pronounced red shift. The spectrum then shifts to shorter wavelengths on a time scale of several microseconds, showing a cooling of the ngr;3mode. At an excitation level of 3 photons/molecule the measured spectra approach the thermal spectrum expected for complete vibrational energy thermalization among all of the vibrational degrees of freedom on a time scale of 3plusmn;2 mgr;sec. At higher excitation levels (sim;10 photons/molecule) the induced spectrum still shifts on a comparable time scale but is broader than a thermal spectrum. At the lower fluence levels (sim;3 photons/molecule), a ngr;3mode temperature can be assigned to the spectra. The time evolution of this temperature exhibits only a weak pressure dependence. A much broader induced absorption band, approximately a factor of 100 weaker than this rsquo;rsquo;hotrsquo;rsquo; ngr;3mode absorption, has been observed extending more than 100 cmminus;1to the high frequency side of the ngr;3mode absorption band. The spectral dependence of this absorption band is in qualitative agreement with the multiphoton excitation spectrum found in twohyphen;frequency dissociation experiments.
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