Development of Infrared Diode Laser Kinetic Spectroscopy and Its Applications to Chemical Reactions

Abstract

Infrared diode laser spectroscopy is versatile; it allows us to observe vibration-rotation transitions of transient molecules, free radicals, and molecular ions in nearly entire infrared region. We need 10¹⁰ to 10¹² molecules to observe their spectra in most favorable cases, provided that the pathlength can be extended as long as 20 m or so. This method has been providing us with very detailed spectroscopic data on such short-lived molecules as reaction intermediates; the number is approaching one hundred [1]. It is a natural extension to try to make this versatile method time-resolved, in order to get more dynamical information in real time on systems where chemical reactions are taking place. We have achieved this by simply introducing a transient digitizer in our signal detection circuitry and by controlling it with a personal computer [2]. Using this system we collect the signal for a pair of time gates, one after and the other before a trigger to induce a chemical process, and take the difference between the two signals, while we scan the wavelength of the source diode slowly. We normally set three pairs of such gates to record time-resolved spectra. We may employ the system also as a signal averager to follow the time evolution of a spectral line. The time resolution of our system is 300 ns, which is determined mainly by the infrared detector and its peripherals. We have applied this system to a few photochemical reactions and also photochemically induced reactions.