Vibrational relaxation of highly excited molecules: Mode specific vibrational energy transfer from SF6 to N2O.

Vibrational energy transfer from SF6 to N2O was studied by using a pulsed CO2 laser for multiphoton excitation of SF6 and monitoring the 4.5 μm emission from N2O(v3=1). The initial average energy of SF6 was selected by varying the fluence of the laser. The vibrational temperature of the ν2 mode of N2O was determined by measuring the attenuation of the fluorescence with a cold gas filter. The vibrational temperature of the ν3 mode was estimated by equating the SF6 and N2O(ν3) vibrational temperatures at the fluorescence maximum. The principal finding of this study is that N2O(ν2) is preferentially excited. While the mode specificity of the VV transfer declines with energy, it is greater than expected statistically. We also observed that the relaxation of ν2 is an order of magnitude slower than the relaxation of ν3. A kinetic model which accounts for most of the observations shows that VV transfer from SF6 to N2O occurs from high lying states of SF6 while the decay of N2O occurs by back transfer to the discrete levels of SF6. The slow relaxation of the bending mode of N2O indicates the existence of a bottleneck in VV transfer between N2O(ν2) and the discrete levels of SF6, which may be caused by a mechanism involving long-range attractive forces. [ABSTRACT FROM AUTHOR]