Formaldehyde photochemistry: Appearance rate, vibrational relaxation, and energy distribution of the CO product

Abstract

The mechanism of formaldehyde photochemistry has been investigated by monitoring the appearance rate, relative yield, and vibrational distribution of the CO photochemical product detected either by its infrared fluorescence or by its absorption of a cw CO laser. In the limit of low formaldehyde pressures, the CO product appears with a rate more than 100 times slower than the decay rate of the formaldehyde first excited singlet state. This fact indicates the presence of a long‐lived intermediate state between S₁ and the molecular products. Collision‐induced CO production following 337.1 nm formaldehyde excitation occurs with appearance rates of 2.7×10⁻¹¹ cm³ molecule⁻¹⋅sec⁻¹ for D₂CO and 4.7×10⁻¹¹ cm³ molecule⁻¹⋅sec⁻¹ for H₂CO. After its production, CO(v=1) relaxes to the ground vibrational state in collisions with D₂CO at a rate of 3.3×10⁻¹² cm³ molecule⁻¹⋅sec⁻¹ and in collisions with H₂CO at a rate of 3.7×10⁻¹⁴ cm³ molecule⁻¹⋅sec⁻¹. These rates have been confirmed by a separate measurement which monitors CO(v=1) fluorescence following excitation with a Q‐switched CO laser. The CO photolysis yield decreases with the addition of argon, but increases dramatically for 305.5 nm photolysis upon the addition of NO or O₂. Vibrational distributions of the CO product have been measured as a function of energy and vibrational level of the formaldehyde singlet state. Although the amount of energy appearing in CO vibrational modes increases with increasing excitation energy, the CO vibrational energy accounts for only between 0.7% and 4.5% of the energy available to the products at the measured dissociation wavelengths.