Intra- and Intermode Vibrational Energy Flow in CH3F Excited by Irradiation With an Intense CO2 Laser: A Non-Linear Vibrational Relaxation

Abstract

The vibrational relaxation mechanism of CH3F whose ν3 mode is excited by a transversely excited atmospheric (TEA)CO2 laser-pulse has been discussed on the basis of observation of the laser-induced fluorescence (LIF) of the ν3 overtones and of the C–H stretching modes and kinetic analyses. The time-evolved LIF in the 3-μm region was found to be dependent significantly on the laser fluence; at a low fluence (<0.01 J cm−2), the emission intensity increased almost exponentially with a rate similar to the one determined in the experiment with a Q-switched laser, while at a higher fluence (>0.1 J cm−2) the emission rose much faster to form a peak which decayed quickly before a second broad peak appeared. In the wavelength-resolved fluorescence measurement, it was found that the kinetics of population in the ν4 level were similar to those in the 3ν3 level, while those in the ν1, 2ν5, ν2 + ν5 and 2ν2 levels behaved in a way different from those in the ν4 level. These observations lead to the conclusion that the laser energy poured into the ν3 mode flows to the ν4 level directly through 3ν3 as well as by the successive inter- and intramode V–V energy transfers, i.e., ν3→ν6→ν2, ν5→2(ν2, ν5)→ν1→ν4. A model calculation of the relaxation kinetics, including the direct V–V energy transfer between two 3ν3 and ν4 levels, could reproduce the 3-μm emission data. Another significant finding is a non-exponential depopulation in the 2ν3 level, the ν4, ν1 and other vibrational levels in the 3-μm region in the final stage of the relaxation. The deactivation rate is larger for a higher level and is a decreasing function of time and is much larger than the V–T/R energy transfer rate. determined in the experiment of weak laser excitation. This may be attributed primarily to the successive intermode V–V energy transfers from the 3ν3 to the ν4, ν1, 2ν5, . . . levels which lie between the 2ν3 and 3ν3 levels. This process may be repeated if the intramode V–V energy transfer from the 2ν3 to the 3ν3 level occurs. The deactivation mechanism found in this experiment is called ‘catastrophic cyclic path’ which is effective until the relative population distribution in relevant levels between 2ν3 and 3ν3 becomes that in equilibrium at the translational temperature of the CH3F gas.