Diode laser probing of I*(2P1/2) Doppler profiles: Time evolution of a fast, anisotropic velocity distribution in a thermal bath

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Abstract
The relaxation of a nonthermal translational population distribution of fast I*(2P1/2) atoms dilutely dispersed in a gaseous bath at thermal equilibrium is studied by time‐resolved Doppler spectroscopy. The fast, anisotropic velocity distribution of I* atoms is produced by pulsed laser photolysis of n‐perfluoropropyl iodide (n‐C3F7I) at 266 nm. A frequency‐narrowed, GaAsInP diode laser is tuned across the iodine (2P1/2,F=3←2P3/2,F=4) transition at 1315 nm to measure the Doppler gain profile of the I* photofragments. The velocity distribution is expressed as a separable product of a radial speed function and an angular function describing the anisotropy. The collision‐induced time evolution of both the speed and anisotropy components of the nascent velocity population distribution relaxing to form a 300 K Maxwellian equilibrium distribution is determined. The thermalization dynamics of I* are studied for a heavy bath gas (n‐C3F7I) and a light (He) bath gas. In the case of the heavy bath gas the anisotropy is removed by collisions 2.5 times faster than the speed is thermalized, while for the light bath gas the anisotropy and speed relaxation occur on the same time scale. The velocity and angular distributions of the I* photofragment from the 266 nm photolysis of n‐C3F7I are also reported.