NANOSECOND TIME-RESOLVED FTIR EMISSION-SPECTROSCOPY - MONITORING THE ENERGY-DISTRIBUTION OF HIGHLY VIBRATIONALLY EXCITED MOLECULES DURING COLLISIONAL DEACTIVATION

The 10(-8) second time resolution in infrared emission spectroscopy ha s been demonstrated using a Fourier Transform spectrometer paired with a fast HECdTe detector. The rapid time response of this system has en abled us to measure, with subcollisional period time resolution, the e mission spectrum of highly vibrationally excited NO2 molecules during collisional deactivation by room temperature NO2. The greatly improved time resolution of the spectra allows the determination of N(E,t), th e instantaneous energy distribution of the ensemble of excited molecul es, with virtually no distortion due to collisional averaging. In addi tion, an improved procedure for extracting optimized N(E,t) from the s pectral data makes no prior assumptions about the shape of the energy distribution. It is found that the distribution is well approximated a s the sum of a Gaussian function at high vibrational energies and a po pulation at low energies resulting from V-V transfer to bath NO;? mole cules. The observation of a Gaussian-like function for the highly exci ted molecules is consistent with the widely invoked assumption that th e step-size function of energy transfer per collision is exponential. (C) 1998 American Institute of Physics. [S0021-9606(98)02404-0].