Chemical Kinetics of Triplet Methylene from Infrared Diode Laser Flash Kinetic Spectroscopy.

A flash kinetic spectrometer based on a tunable infrared diode laser has been constructed. This spectrometer can measure Doppler limited spectra in the frequency region from 300-3000 cm^{-1}, allowing for the detection of nearly any molecular species. The spectrometer can be used in a spectroscopy mode to obtain the high resolution (<=q10^ {-3} cm^{-1})^ectrum of the species, or in a kinetic mode to detect time resolved behavior of events as fast as 20 ns. Using this spectrometer, room temperature, triplet methylene removal rate constants have been measured for NO_2, NO, SO_2, O_2, and H_2S (in order of decreasing reaction rate). For less reactive species, upper bounds to the removal rate constants were established. When no reactive gas is present, triplet methylene is removed from the system by self reaction. The triplet methylene self reaction rate constant was found to be faster than gas kinetic. To further study the self reaction of triplet methylene, the technique of product appearance kinetic spectroscopy (PAKS) was developed. Using PAKS, time resolved appearance of ground state acetylene was monitored with the diode laser spectrometer. The appearance rate of acetylene from triplet methylene self reaction was found to be equal to the removal rate of triplet methylene. This establishes that acetylene is a direct product of triplet methylene self reaction. Over the range of 2 to 10 torr ketene, a falloff in the quantum yield of acetylene produced by triplet methylene self reaction is observed. Rate constants for collision-induced intersystem crossing of singlet methylene to triplet methylene with the collision partners He and Ar were measured using flash kinetic spectroscopy with a visible probe laser. The rate constants for individual quantum states show small but definite variations. These variations are due to different degrees of singlet-triplet coupling for different levels. One singlet methylene state which is highly coupled to the triplet methylene manifold, 4_{31} (0,0,0), shows a significantly faster intersystem crossing rate with He. This observation leads to the conclusion that the 4_{31} rotational level is a gate state for intersystem crossing in methylene.