Quenching, electronic energy transfer, and rotational relaxation of S1 formaldehyde

Three types of fluorescence decay measurement are reported in order to clarify the collisional decay mechanisms of S1 formaldehyde. From the fluorescence decay of an H2CO/D2CO mixture after selective excitation of 40 H2CO, the rate constant for electronic energy transfer from S1 H2CO to S0 D2CO is derived to be less than 2% of the gas kinetic collision rate. The fluorescence decay after excitation of single rotational levels of pure H2CO in the 41 band was measured in the pressure range 10−3–5 Torr. A simple model shows that rotational and vibrational relaxation to S1 levels of different lifetime from that of the initially prepared state makes a major contribution to the observed curvature of Stern–Volmer plots. Intrinsic curvature of the electronic relaxation rate definitely occurs for pure D2CO and may occur for pure H2CO. Quenching of 40/41 H2CO and D2CO by up to 700 Torr of Ar, CO2, CH3F, O2, and NO was studied. The Stern–Volmer plots for Ar, CO2, and CH3F curve gradually, again due to intrinsic curvature of the electronic relaxation rate. Quenching by O2 and NO is more linear and more efficient. Surprisingly, Ar, CO2, and CH3F quench D2CO more efficiently than H2CO, while the opposite is true for O2 and NO. The results are discussed in terms of mixed state theories of electronic relaxation. Inferences concerning the collisional photochemical decay of S1 formaldehyde are suggested.