Absolute rate parameters for infrared photochemistry: CF3I→CF3+I

Abstract

IR-photolysis yields are reported for CF3I irradiated with the R14 line of the (001→0200) transition of a pulsed TEA-CO2 laser (at 1074.65 cm−1) with unstable resonator optics providing a smooth and well defined fluence profile in the far field. The reaction products (CF3+I) of the primary reaction are scavenged by reactions with added NO, Br2, or ClNO in order to reliably establish the primary yields. A data treatment valid for arbitrary, smooth spatial fluence profiles is proposed for the evaluation of true yields and rate coefficients kI(st). The influence of collisions with buffer gas molecules on the evaluation of kI(st) is investigated in experiments and by model calculations. A steady state rate coefficient for the unimolecular decomposition of CF3I under irradiation with constant intensity I is obtained to be k(st) = 10(6.2±0.3) (I/MW cm−2)s−1 valid for intensities near and above 100 MW cm−2. This is consistent with a low a priori estimate of rate coefficients from a simple theoretical model involving only easily measurable molecular properties. The physical significance of this absolute rate parameter is discussed.