Dissociation dynamics of HNCO and DNCO after laser photoexcitation in the vacuum ultraviolet

Full-dimensional potential energy surface for the photodissociation of HNCO via its S1 band

A full-dimensional potential energy surface (PES) for the first excited state S1(1A'') of HNCO has been built up by the neural network method based on more than 36 000 ab initio points, which were calculated at the multireference configuration interaction level with Davidson correction using the augmented correlation consistent polarized valence triple zeta basis set. It was found that two minima, namely, trans and cis isomers of HNCO, and another seven stationary points exist on the S1 PES for the two dissociation pathways: HNCO(S1) → H + NCO/NH + CO. Particularly, a new out-of-plane transition state between the two minima was found in this work, thanks to including all the degree of freedoms for this system. The adiabatic excitation energy of the S1(1A'') ← S0(1A') transition and dissociation energies D0(HNCO → H + NCO) and D0((HNCO →NH(a1Δ) + CO) calculated on the PES are in good agreement with experimental results. In addition, based on the newly constructed S1 PES, the percentage of products H + NCO in the photodissociation of HNCO(S1) was obtained by a quasi-classical trajectory method at the photon wavelengths ranging from 190 to 225 nm, which is in reasonably good agreement with earlier theoretical and experimental results. For the dissociation lifetimes of the trajectories, they were calculated to be less than 5 ps, which is also consistent with experimental observations.

Vacuum ultraviolet photodissociation dynamics of isocyanic acid: the hydrogen elimination channel

Photodissociation dynamics of the H-atom channel from HNCO photolysis between 124 and 137 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determined. Two dissociation channels, H + NCO (X(2)Π) and H + NCO(A(2)Σ(+)), have been observed. The former channel involves two different dissociation pathways; one is a slow predissociation pathway through internal conversion from the excited state to the S0 state, and the other is a fast predissociation pathway through internal conversion from the excited state to the S1 state. The latter channel dominates by a prompt dissociation via coupling to the S2 state. As the photon energy increases, dissociation on the ground state S0 becomes dominant. Vibrational structures are observed in both the NCO(X) and NCO(A) channels, which can be assigned to the bending mode excitation with some stretching vibrational excitation.