Photodissociation of formic acid

The photodissociation of formic acid has been studied experimentally and th eoretically. Ab initio calculations were performed to study the dissociativ e profiles of five reaction channels on the S-0, S-1, and T-1 potential ene rgy surfaces. The vibrationally excited nascent products were detected usin g a time-resolved Fourier transform infrared spectrometer after laser photo lysis at 248 or 193 nm. In the 248 nm photolysis, the HCOOH molecule was fi rst excited to the S-1 state, but it was found that the dissociation takes place on the S-0 surface after internal conversion. The products of the vib rationally excited CO, CO2(v(3)) and H2O(v(1)) were detected. During the di ssociation process the vibrationally energized molecule is geometrically me morized and dynamically controlled, with the yield preference of CO and H2O over that of CO2 and H-2. The ratio of CO(v greater than or equal to 1)/CO 2(v greater than or equal to 1) is estimated as < 7.5. Vibrationally excite d CO (v) and CO2(v(3)) are also found in the 193 nm photolysis but the CO/C O2 ratio increases to 11. Most of the dissociation is thought to occur on t he S-0 state. At this wavelength another dissociation channel which produce s OH and HCO radicals on S-1 surface has been identified. The dissociation is unlikely to occur on the T-1 surface, as the energy barriers are fairly high. (C) 2000 American Institute of Physics. [S0021-9606(00)00620-6].