Nonequilibrated energy distribution in polyatomic molecules: The infrared MPD of cyclobutanone

The TEA CO₂ infrared MPD of cyclobutanone, on irradiation of the ?₂₄ fundamental mode, has been investigated as a function of incident fluence F_i, wavelength and pressure. Dissociation proceeds via two molecular routes; the first yields ethylene plus ketene CH ₂CO, and the second cyclopropane and CO plus minor amounts of propylene. The branching ratio R=[C₂H₄]/[C ₃H₆] increases dramatically at pressures less than ~3 Torr from the high pressure thermal limit. This nonthermal feature is most pronounced at low F_i. The average number of quanta absorbed per molecule , measured by pyroelectric and optoacoustic detection, is independent of cyclobutanone pressure between 0.1 and 10.0 Torr. varies linearly with F_i in the range investigated 0.2-2.0 J cm ^-2. An explicit formulation of the infrared MPD process is presented based on a truncated s-fold degenerate harmonic oscillator approach. Vibrational energy transfer is incorporated via v-v bimolecular collisions. The predicted dependence of R on pressure is in good agreement with the experimental data presented herein. Moreover, the observed linear dependence of with F_i, and its independence with respect of pressure, is also borne out by this theory. © 1983 American Institute of Physics.