Continuous-scan time-resolved FTIR spectroscopy measurements of high energy molecular collisions

We have studied the dynamics of gas phase collisions between high energy radicals and several small molecules under low pressure conditions, which enable us to resolve single collision events. The experiments are carried out in a fast-flow, low pressure chemiluminescence emission apparatus. Using nanosecond laser photofragmentation, we create pulses of high energy radicals in a flowing stream of a reagent molecule, then record a series of broad-band IR spectra of the emission from the products of the resulting reaction, using time-resolved FTIR spectroscopy (TRFTS). Our TRFTS instrument uses a continuous scan Michelson interferometer to which we have added a data acquisition system of our own design. The latter is based on VME architecture, and permits time resolutions between 1 μs and 3 ms at the full spectral bandwidth and maximum spectral resolution of the interferometer. Slower time resolutions, down to a few tens of milliseconds, can be obtained at reduced spectral bandwidths. The work to be reported uses a time resolution of a few tens of nanoseconds for the first spectrum, then from 1-3 μs for the second and subsequent spectra; typically, we record in excess of 20 spectra. The principal observable is the infrared emission from the vibrationally and rotationally excited products of the reaction. This is a very low-background experiment; the sensitivity is on the order of 10-15 moles/litre/quantum state. Where more than one reaction is possible, the relative intensities of the various spectra give the branching ratios for the different channels, while the time dependence of the emission gives the reaction kinetics