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.