Laser-photolysis/time-resolved Fourier-transform absorption spectroscopy: Formation and quenching of HCl(v) in the chain reaction Cl/Cl₂/H₂

A system to measure time-resolved Fourier-transform infrared absorption spectra of gaseous samples using a commercial step-scan spectrometer is described. To increase the signal intensity, the incident infrared light is multipassed within a White cell. Light from a photolysis laser passes through the reaction cell to initiate the reaction in the flowing gaseous sample. The variation of absorbance is obtained from the ac-coupled signal whereas phase information and a reference spectrum are from the dc-coupled signal. The system is tested by probing the temporal evolution of HCl(v) in the chain reaction of H₂ and Cl₂ initiated by photolysis at 355 nm. Time-resolved absorption spectra of HCl(v=0-2) were obtained with spectral resolution 0.75 cm^-1 and intervals down to 5 μs. Kinetic modeling of deduced temporal profiles of HCl(v=0-2) yields rate coefficients of (1.38+0.04) × 10^-14 and (5.8±0.4) × 10^-15 cm³ molecule^-1 s^-1 (in which error limits represent only the uncertainty of the fit) for reactions Cl+H₂→HCl(v=0)+H and Cl+H₂→HCl(v=1)+H, respectively; the total rate coefficient is in agreement with previous kinetic measurements.