Photodissociation of formic acid

Abstract

The photodissociation of formic acid has been studied experimentally and theoretically. Ab initio calculations were performed to study the dissociative profiles of five reaction channels on the S 0 , S 1 , and T 1 potential energy surfaces. The vibrationally excited nascent products were detected using a time-resolved Fourier transform infrared spectrometer after laser photolysis at 248 or 193 nm. In the 248 nm photolysis, the HCOOH molecule was first 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 vibrationally excited CO, CO 2 (v 3) and H 2 O(v 1) were detected. During the dissociation process the vibrationally energized molecule is geometrically memorized and dynamically controlled, with the yield preference of CO and H 2 O over that of CO 2 and H 2. The ratio of CO(vу1)/CO 2 (v у1) is estimated as Ͻ7.5. Vibrationally excited CO (v) and CO 2 (v 3) are also found in the 193 nm photolysis but the CO/CO 2 ratio increases to 11. Most of the dissociation is thought to occur on the S 0 state. At this wavelength another dissociation channel which produces 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.

Theoretical investigations on valence vibronic transitions

“…At this wavelength, dissociation on the S 1 surface, intersystem crossing, or funneling through a S 0 /S 1 conical intersections to the ground state are all energetically inaccessible [64]. Therefore, it was put forward recently [62,64,65] that formic acid vibrationally relaxes from the S 1 to the S 0 state via internal conversion and/or fluorescence. Vibronic coupling can enhance the fluorescence probability (i.e, its optical oscillator strength), as we have shown above for other cases, and is responsible for the internal conversion process [2,31].…”

“…It was recently suggested both experimentally [62,70] and theoretically [64] that the CH bendings (specially the CH out of plane wagging) and CO stretchings (specially the C=O mode) would be the most important normal modes involved in the vibronic coupling responsible for the internal conversion/fluorescence processes. Although the branching ratio of the reactions (1) and (2) is essentially dynamically independent of the way the molecule at the transition state is initially excited (i.e, random distribution of vibrational states or selective excitation of vibrational modes), as we have shown with our classical trajectory calculations [66], the present results shed light on the details of the internal conversion/fluorescence processes driven by vibronic effects.…”

“…Formic acid, HCOOH, is an important intermediate in the oxidation of unsaturated hydrocarbons in combustion, one of the most abundant pollutants in the atmosphere, and was identified in interstellar clouds [61,62]. Absorption of a photon of 248 nm (4.99 eV) excites HCOOH from the ground (S 0 ) to the first excited (S 1 ) electronic state (A ← A ).…”

Transient HDO rovibrational satellite peaks in solid parahydrogen: Evidence of hydrogen atoms or vacancies?

“…Even when the sample is maintained at 1.9 K after photolysis the intensity of these satellite peaks decay slowly with a time constant of 121(7) min [1]. Photoexcitation of HCOOH in the gas phase at 193 nm leads to direct dissociation to give HCO + OH as the dominant photochannel [2,3]. In the preliminary studies we used the lack of satellite peaks for D 2 O to assign the observed satellite peaks to H···HDO or H···H 2 O radical clusters that form as by-products of the 193 nm photochemistry via reactions of the OD or OH photofragments with the pH 2 host [1].…”

“…Photoexcitation of HCOOH in the gas phase at 193 nm leads to direct dissociation to give HCO + OH as the dominant photochannel [2,3]. In the preliminary studies we used the lack of satellite peaks for D 2 O to assign the observed satellite peaks to H···HDO or H···H 2 O radical clusters that form as by-products of the 193 nm photochemistry via reactions of the OD or OH photofragments with the pH 2 host [1]. However, at that time we were unable to definitively explain why we did not observe the analogous satellite peaks associated with the b-type R(0) rovibrational transitions of either HDO or H 2 O.…”