CO and H2 quantum yields in the photodecomposition of formaldehyde in air

Abstract

Absolute quantum yields have been measured at 25° C for the products CO and H2 resulting from the photolysis of formaldehyde admixed to air at ppm level concentrations. The dependence on wavelength and pressure was established for the region 277–355 nm. In the 290–330 nm wavelength range the quantum yields showed no pressure dependence between 380 and 800 Torr of air, at 355 nm the quantum yields increase with decreasing pressure. This effect is interpreted as being caused by collisional quenching. Data from other investigations are used to show that quenching occurs at all wavelengths greater than 335 nm, and quenching efficiencies for air, CO2, NO, and formaldehyde are estimated for formaldehyde excited at 340 and 355 nm. From a discussion of the various excited states involved in H2CO photolysis it is concluded that the CO and H2 product quenching is due to electronic quenching of vibronic levels of the S 1 state. The measured quantum yields provide the first realistic data for the photochemical behavior of formaldehyde in the atmosphere. Quantum yields for H2CO loss, CO and H2 production, and radical formation are given and discussed with respect to previous data.