Nuclear spin symmetry state relaxation in formaldehyde

New measurements of the rate constant of the ortho-para conversion in monomeric gaseous formaldehyde (H2CO) are presented. Separation of the nuclear spin isomers of formaldehyde was obtained by selective UV laser photolysis of ortho-formaldehyde in the natural ortho-para mixture. A first group of experiments is devoted to the question of surface relaxation. Measurements in low pressure formaldehyde gas show fast relaxation at very low pressures, falling to a minimum at pressures of about 1 mbar and then rising linearly with pressure. The fast relaxation at very low pressures is interpreted as surface relaxation and this interpretation is confirmed by the fact that different wall materials of the fluorescence cells yield different relaxation constants. A second group of experiments extended the pressure range of the nuclear spin symmetry state relaxation measurements in formaldehyde by adding H2 or SF6 to it up to pressures of 1 bar. The experimental data follow qualitatively the behaviour that is predicted by theory [R.F. Curl Jr, J.V.V. Kasper, K.S. Pitzer, J. Chem. Phys. 46 (1967) 3220]. Quantitative agreement with theory could be reached by taking calculated values of the spin-rotation coupling constants [P.L. Chapovsky, J. Mol. Struct. 599 (2001) 337] about 35% higher and using surprisingly large nuclear spin symmetry state relaxation cross-sections.