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

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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.