Laser-probe beam, schematic depiction

Figure 2. Schematic depiction of physical processes affecting the laser probe beam for an opaque homogeneous sample including thermoelastic deformation of the air-sample interface and thermal lens effects in the air above the sample.

Typically 60-150 ns after the photodissociation pump pulse, nascent OH product radicals were probed by a second copropagating UV probe laser beam by LIF in the A Z" - X H system as schematically depicted in Fig. 1. The probe beam with a bandwidth of 0.2 cm was provided by a frequency-doubled dye laser, pumped by a XeCl excimer laser and was tunable in the wavelength region 305-320 nm in order to scan OH LIF excitation spectra. Line positions for the different OH fine-structure transitions were taken from Ref. 30. Both lasers were operated at 20 Hz and each data point in a LIF excitation spectrum was obtained by averaging the collected fluorescence signal over 40-80 laser shots. The measured LIF spectra were stored on a computer for further data processing. OH vibrational and rotational fine-structure state distributions were determined by numerical integration of... 

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