High-performance transient absorption spectrometry on the nanosecond scale

Abstract

A flexible apparatus has been constructed to measure the spectra and time dependence of weak optically induced absorption changes in gas phase samples. The instrument uses a pulsed Nd:YAG-pumped dye laser for excitation and a second, independent YAG/dye laser for probing in a coaxial counter-propagating geometry. The excitation wavelength, probe wavelength, and time delay between the two pulses may be separately varied. An on-line laboratory computer acquires induced absorbance data as a function of probe wavelength and delay, and helps to suppress ratiometric noise to the point that induced probe absorptions as low as 2×10−4 absorbance units may be routinely detected in a path length of 75 cm. The sample cell forms part of a gas handling system that permits a wide range of sample and buffer gas mixtures to be prepared. The width of the instrumental time response is approximately 4 ns; it may be deconvoluted from kinetic data to provide an effective resolution of ∼1 ns. This combination of sensitivity and time resolution allows the study of collision-free as well as collision-induced effects in many molecular systems.