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

The vibrational relaxation mechanism of CH 3 F whose ν 3 mode is excited by a transverselyexcited atmospheric (TEA)CO 2 laser-pulse has been discussed on the basis ofobservation of the laser-induced fluorescence (LIF) of the ν 3 overtones and of the C–Hstretching modes and kinetic analyses. The time-evolved LIF in the 3-μm region wasfound 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 onedetermined 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 quicklybefore a second broad peak appeared. In the wavelength-resolved fluorescencemeasurement, it was found that the kinetics of population in the ν 4 level were similar tothose in the 3 ν 3 level, while those in the ν 1 , 2 ν 5 , ν 2 + ν 5 and 2 ν 2 levels behaved in a waydifferent from those in the ν 4 level. These observations lead to the conclusion that thelaser energy poured into the ν 3 mode flows to the ν 4 level directly through 3 ν 3 as well asby the successive inter- and intramode V–V energy transfers, i.e., ν 3 → ν 6 → ν 2 , ν 5 →2( ν 2 , ν 5 )→ ν 1 → ν 4 . A model calculation of the relaxation kinetics, including thedirect V–V energy transfer between two 3 ν 3 and ν 4 levels, could reproduce the 3-μmemission data. Another significant finding is a non-exponential depopulation in the 2 ν 3 level, the ν 4 , ν 1 and other vibrational levels in the 3-μm region in the final stage of therelaxation. The deactivation rate is larger for a higher level and is a decreasing functionof 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 successiveintermode V–V energy transfers from the 3 ν 3 to the ν 4 , ν 1 , 2 ν 5 , . . . levels which liebetween the 2 ν 3 and 3 ν 3 levels. This process may be repeated if the intramode V–Venergy transfer from the 2 ν 3 to the 3 ν 3 level occurs. The deactivation mechanism foundin this experiment is called ‘catastrophic cyclic path’ which is effective until the relativepopulation distribution in relevant levels between 2 ν 3 and 3 ν 3 becomes that in equilibriumat the translational temperature of the CH 3 F gas.