Nanosecond time-resolved FTIR emission spectroscopy: Monitoring the energy distribution of highly vibrationally excited molecules during collisional deactivation

Abstract

The 10−8 second time resolution in infrared emission spectroscopy has been demonstrated using a Fourier Transform spectrometer paired with a fast HgCdTe detector. The rapid time response of this system has enabled us to measure, with subcollisional period time resolution, the emission 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), the instantaneous energy distribution of the ensemble of excited molecules, with virtually no distortion due to collisional averaging. In addition, an improved procedure for extracting optimized N(E,t) from the spectral data makes no prior assumptions about the shape of the energy distribution. It is found that the distribution is well approximated as the sum of a Gaussian function at high vibrational energies and a population at low energies resulting from V–V transfer to bath NO2 molecules. The observation of a Gaussian-like function for the highly excited molecules is consistent with the widely invoked assumption that the step-size function of energy transfer per collision is exponential.