Nuclear spin state relaxation in formaldehyde: dependence of the rate constant on pressure

Measurements of ortho–para conversion rates in pure formaldehyde are described. Separation of the nuclear spin isomers was obtained by selective UV laser photolysis of ortho-formaldehyde. Observed total rate constants are of the order of (7±2)×10−4 s−1 at 1.3 mbar and 300 K. The dependence of the rate constant on the pressure of pure formaldehyde in the range from 0.1 to 13 mbar was determined. At low pressures the interconversion because of wall collisions was observed. After correcting the measurements for the wall collisions, the pressure dependence of the rate constant for the ortho to para conversion in formaldehyde–formaldehyde collisions k1 could be determined to be k1=(2.1±0.5)×10−4p/p0 [s−1], with p0=1 mbar.