Nuclear spin state relaxation of formaldehyde in mixtures with inert gases

Abstract

AbstractMeasurements of the rate constant of the ortho‐para conversion of monomeric gaseous formaldehyde H2CO in mixtures with He, Ar, H2, N2, CO, O2, CH3F and SF6 are described. Separation of the nuclear spin isomers of formaldehyde was obtained by selective UV laser photolysis of ortho‐formaldehyde in the natural ortho‐para mixture. Inert gas pressure was varied up to atmospheric pressure, formaldehyde pressure was 2.8 mbar in all experiments. At low pressures a linear increase of the rate constant with the inert gas pressure was found in all gases. With the noble gases, this increase was very small. In mixture with H2, N2, O2 and SF6 the rate constant rises up to a maximum and then shows a steep decrease as a function of the inert gas pressure. With the noble gases, CO and CH3F only the increase of the rate constant with pressure could be measured. The results are compared to the theory of Curl et al. [37]for the pressure dependence of the nuclear spin state relaxation of formaldehyde.