Mid-infrared quantum cascade laser spectroscopy probing of the kinetics of an atmospherically significant radical reaction, $$\hbox {CH}_{3}\hbox {O}_{2}+\hbox {NO}_{2}+\hbox {M}\rightarrow \hbox {CH}_{3}\hbox {O}_{2}\hbox {NO}_{2}+\hbox {M}$$ CH 3

Abstract

The kinetic parameters of an important atmospheric reaction, CH 3 O 2 +NO 2 +M → CH 3 O 2 NO 2 + M, have been recorded by monitoring directly the changes in concentrations of methylperoxy radicals (CH 3 O 2) in the gas phase employing a new mid-infrared quantum cascade laser (QCL)-based apparatus. CH 3 O 2 radicals in our apparatus have been generated by pulsed UV laser (266 nm) photolysis of CH 3 I in a gaseous mixture with oxygen. The absorption band corresponding to the mid-infrared O-O stretching fundamental of the peroxy radical, within a narrow spectral range, 1070-1120 cm −1 , has been recorded by tuning the wavelength of the QCL operated in CW mode. The kinetics of the aforementioned reaction of CH 3 O 2 with NO 2 has been followed by analyzing the changes of the infrared (QCL) decay profile of CH 3 O 2 at 9.1 μm (1098.9 cm −1) maintaining a pseudo first order reaction condition. We noticed that the rate constant of the reaction at 298 K varies in the range of (1.21-3.08) ×10 −12 cm 3 molecule −1 s −1 for changing the total pressure in the range of 75-730 mbar. The absorption cross-section of CH 3 O 2 at the probe wavelength (1098.9 cm −1), has been estimated for the first time to be 8.3 ± 0.4 × 10 −20 cm 2 .