Infrared Laser Absorption Measurements of the Kinetics of Propargyl Radical Self-Reaction and the 193 nm Photolysis of Propyne

Abstract

Infrared laser absorption spectroscopy is used to investigate recent discrepancies in measurements of the propargyl radical self-reaction rate coefficient and product formation in 193 nm photolysis of propyne. The cross section of the propargyl radical is measured relative to that of HCl using the Cl + propyne reaction, yielding a peak absorption cross section (assuming Doppler-limited line shapes) of (1.9 ′ 0.4) x 10 - 1 8 cm 2 for the P(12) line of the υ 1 fundamental at 296 K. The rate coefficient for the propargyl radical (CH 2 CCH) self-reaction is determined by modeling the infrared absorption of the propargyl radical formed in the 193 nm photolysis of propargyl chloride (HCCCH 2 Cl) and propargyl bromide (HCCCH 2 Br), using a more precise literature determination of the pressure-broadened absorption cross section. The propargyl self-reaction rate coefficient so obtained, (3.9 ′ 0.6) x 10 - 1 1 cm 3 molecule - 1 s - 1 , is consistent with several recent measurements but disagrees significantly with the one previous infrared absorption determination. Both the propargyl radical and acetylene (HCCH) are observed in the 193 nm photolysis of propyne (CH 3 CCH). The propargyl is formed promptly following the UV photolysis pulse, and the magnitude of the signal is unaffected by the addition of O 2 . The observed propargyl signal is consistent with direct CH 2 CCH formation in the 193 nm photolysis of propyne and appears inconsistent with formation by secondary reactions of the 1-propynyl radical (CH 3 CC). The observed CH 2 CCH yield per 193 nm photon absorbed is 0.49 ′ 0.10.