Laser temperature-jump spectrophotometer using stimulated Raman effect in H2 gas for the study of nanosecond fast chemical relaxation times

Abstract

A laser temperature-jump spectrophotometer for the study of chemical relaxation times with a heating time of about 18 nsec is described. The stimulated Raman effect in hydrogen gas at 80-atm pressure produces a frequency shift of a neodymium-glass laser from 1.06 to 1.89 μ, at which the absorbance of water is very high. The use of H2 as a Raman active material for the frequency shifting, instead of liquid N2 previously used, is emphasized, which eliminates the need for low-temperature storage facilities and creates the possibilities for generation of other wavelengths, by using a mixture of gases and varying the pressures to obtain other absorption frequencies for the heating of the solvent molecules. For aqueous solutions heated by 1.89-μ radiation pulses, a special spacer cell is described, which allows maximum coaxial overlap between the laser pulse and the analyzing beam. For other nonaqueous solvents a multireflection cell is constructed. The H2 Raman-shifted pulse is narrowed from a 28-nsec half-width of the original 1.06-μ Nd+3-glass laser to 15-nsec half-width at 1.89 μ. The conversion efficiency of usually about 20% is increased to about 40% using a multistage H2 gas Raman cell device, in which the scattering focal volume is increased.