HCO production, vibrational relaxation, chemical kinetics, and spectroscopy following laser photolysis of formaldehyde

Abstract

Formaldehyde vapor was photolyzed with a tunable pulsed uv laser. Flash kinetic absorption spectra of the HCO produced were recorded by intracavity dye laser spectroscopy with a time resolution of 1 μs. The energy threshold for radical production was confirmed to be at 86±1 kcal/mole. Photolysis at 294.1 nm produced HCO in its ground vibronic state (∼2/3) and with one quantum of vibrational excitation in either the bending (∼1/3) or CO stretching (10⁻¹–10⁻²) vibrations. Observation of the CO stretching hot band absorptions allowed that frequency to be determined as 1868.4±1 cm⁻¹. Quantitative, state‐resolved measurements of concentration vs time were made in pure H₂CO and in mixtures with O₂, NO, or Ar. The vibrational relaxation rate for the bending vibration of HCO in collisions with H₂CO was (4.3±1) ×10⁻¹² cm³ molecule⁻¹ s⁻¹. Reaction rates for HCO+NO→HNO+CO and HCO+O₂→HO₂+CO were measured as (1.4±0.2) ×10⁻¹¹ and (4.0±0.8) ×10⁻¹² cm³ molecule⁻¹ s⁻¹, respectively. Approximate rates were determined for the radical–radical reactions H+HCO→H₂+CO and 2HCO→H₂CO+CO as 10^−9.26±0.3 and 10^−10.2±0.5, respectively.