H atom formation dynamics in the dissociation of CH3–CF2Cl (HCFC-142b) after UV and VUV laser photoexcitation

Abstract

Using the laser photolysis/laser-induced fluorescence (LIF) “pump-and-probe” technique, the dynamics of H atom formation in the photodissociation of CH3–CF2Cl (HCFC-142b) after excitation at 193 nm and the Lyman-α wavelength were studied under collision-free conditions in the gas-phase at room temperature. The H atoms produced were detected by (2p2P←1s2S)-LIF using tunable narrow-band Lyman-α laser radiation (λLα≈121.6 nm) generated by resonant third-order sum-difference frequency conversion of pulsed dye laser radiation. In the VUV photodissociation experiments the Lyman-α laser radiation was used both to photodissociate the parent molecules and to detect the produced nascent H atoms via laser induced fluorescence. The following quantum yields ΦH for H atom formation were determined by a photolytic calibration method: ΦH(193 nm)=(0.06±0.02) and ΦH(Lα)=(0.53±0.12). From the measured H atom Doppler profiles the average H atom kinetic energy was determined to be ET(193 nm)=(51±10) kJ/mol and ET(Lα)=(72±4) kJ/mol, respectively.