Abstract

We have developed an instrument for carrying out submicrosecond time-resolved Fourier transform spectroscopic measurements on transient infrared spectra and have applied this to measurements of the spectra of intermediates and products from laser-initiated gas phase processes. We carry out the experiments in a low pressure flow reactor. Typically, we admit a molecular reagent and a precursor at the entrance of the reactor and photodissociate the precursor with an excimer laser, thereby creating a reactive fragment. The latter reacts rapidly with the molecular reagent, giving vibrationally and rotationally excited products. The excimer laser traverses the flowing gases at the centre of an observation region defined by a multipass Welsh optical cell which has its axis at right angles to the direction of the laser. The optical cell focuses the infrared chemiluminescence emitted by the reaction products into the entrance optics of a Michelson interferometer. We record the interferogram produced by the infrared detector of this interferometer at accurately known time intervals following the initiation of the reaction, using a data acquisition system which we have developed for fast time-resolved Fourier transform spectroscopy (TRFTS). Our implementation of TRFTS places no limit on the spectral resolution of the measurement, while providing a one microsecond time resolution. Typically, we use a spectral resolution which is adequate to resolve the vibrational - rotational spectra of hydrides and small molecules such as CO, NO, CO₂, etc. Larger molecules are treated by using spectral simulation and deconvolution techniques to estimate the band intensities. In either case, the intensities of the transitions are used to calculate the populations of the vibrational-rotational states as a function of time after the initiation of the reaction. The dynamics of the process which forms the observed products are deduced from these data. The sum of the emission intensities gives a measure of the total amount of product formed, and the time dependence of the latter gives the overall kinetics of the process.

© 1995 Optical Society of America