Free Radical Studies By Excimer Laser Ultraviolet Photolysis

A promising new technique for studying vibrational and low-lying electronic states of radicals has been developed. Infrared emission from vibrationally excited radicals is observed following ultraviolet photolysis of several small polyatomics with a rare gas-halide excimer laser. Strong emission from the C-H stretches of the CH2I radical is directly detected following photolysis of CH2I2 at 248 nm. Photolysis of CH2I2 at 308 nm and CH3I at 248 nm does not produce excitation of the C-H stretches in the radicals generated. In all three cases, strong emission from excited I*(2P1/2) atoms is observed. With the techniques described, infrared fluorescence is monitored as a function of both wavelength and time, allowing information to be obtained about the initial vibrational state distribution of radicals following photolysis. Subsequent vibrational deactivation, energy transfer, and reaction kinetics may also be studied. By using low resolution, continuously variable interference filters, measurements of the gas phase vibrational frequencies of the radical can be obtained, since the temporal resolution allows radical emission to be readily distinguished from that of the parent molecules.