Dynamics of H Atom Production from Photodissociation of Formic Acid at 205 nm

Radiation-induced transformations of matrix-isolated formic acid: evidence for the HCOOH → HOCO + H channel

“…It is worth noting that the production of H atoms was observed in the gas-phase studies on UV-photodissociation of formic acid and it was attributed to the involvement of reaction (4) occurring at the T 1 potential surface of HCOOH. 7,8 Although the yield of channel (4) in our studies is not large (as compared to the CO 2 formation), the HOCO radical is clearly seen in all the matrices. Furthermore, in the cases of argon and krypton, the intensity of the major HOCO absorption at low doses is comparable with that of CO, or even higher.…”

“…An indication of the C-H bond rupture leading to HOCO was found under photodissociation of HCOOH in the gas phase. 7,8 However, to our knowledge, it was not reported previously in any study on the HCOOH photochemistry in solid matrices.…”

“…It can be reasonably explained by larger probability of population of the T 2 triplet excited states (similar to that produced by intersystem crossing under HCOOH photolysis in xenon). In the case of radiolysis, such states may result from both triplet exciton transfer (7) and ion-electron recombination (8). On the other hand, we cannot exclude the additional formation of CO 2 from higher singlet states, which are not populated under a 193 nm photolysis, or vibrationally hot species.…”

“…As stated above, in the case of matrix isolation under high dilution, the primary matrix hole should typically migrate over a long distance (more than ten lattice periods) to find a guest molecule, and the electron also travels a long distance before thermalization. Thus, quite likely, the recombination process (8) involves non-correlated ion-electron pairs, which should lead to significant or predominating formation of triplet excited states. Furthermore, there should be a significant contribution of high-energy triplet states with E B IP, which cannot be produced by intersystem crossing from the S 1 state populated in photolysis.…”

“…The photoinduced transformations of formic acid were extensively investigated for several decades, both in the gas phase and in solid media. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Regarding photodissociation channels, there is a remarkable difference between the processes observed in the gas phase and in low-temperature matrices. In the former case, UV photolysis (193-225 nm) leads to predominant dissociation of OH and HCO radicals: [3][4][5][6] HCOOH -HCO + OH (1) On the other hand, UV irradiation of the matrix-isolated formic acid results in dissociation via two channels: 10,11,13 HCOOH -…”