A comparative study of methods for describing non-adiabatic coupling: diabatic representation of the 1Sigma +/1Pi HOH and HHO conical intersections

State-to-state photodissociation dynamics of H(2)O in its B band has been investigated quantum mechanically on a new set of non-adiabatically coupled potential energy surfaces for the lowest two (1)A' states of H(2)O, which are developed at the internally contracted multi-reference configuration interaction level with the aug-cc-pVQZ basis set. Quantum dynamical calculations carried out using the Chebyshev propagator yield absorption spectra, product state distributions, branching ratios, and differential cross sections, which are in reasonably good agreement with the latest experimental results. Particular focus is placed here on the dependence of various dynamical observables on the photon energy. Detailed analyses of the dynamics have assigned the diffuse structure in absorption spectrum to short-time recurring dynamics near the HOH conical intersection. The non-adiabatic dissociation to the ground state OH product via the HOH conical intersection is facile, direct, fast, and produces rotationally hot OH(X̃) products. On the other hand, the adiabatic channel on the excited state leading to the OH(Ã) product is dominated by long-lived resonances, which depend sensitively on the potential energy surfaces.

Section: Introductionmentioning

confidence: 99%

State-to-state photodissociation dynamics of triatomic molecules: H2O in the B band

Jiang

Xie

Guo

2012

“…12 In this work, calculations are restricted to the ground 1 1 AЈ DK PES. 39 at E c = 56 meV, which correspond to the collision energy of the experiments of Yang and coworkers 12 on the ground As mentioned above, this PES is barrierless for most of the skewed configurations. Only for collinear approach there is a small early barrier of Ϸ10 meV ͑R H 2 = 0.76 Å, R OH = 1.7 Å͒.…”

Section: O" 1 D… +H 2 Reactionmentioning

confidence: 99%

A comparison of quantum and quasiclassical statistical models for reactions of electronically excited atoms with molecular hydrogen

Aoiz

González‐Lezana

Rábanos

2008

A detailed comparison of statistical models based on the quasiclassical trajectory ͑SQCT͒ and quantum mechanical ͑SQM͒ methods is presented in this work for the C͑ 1 D͒ +H 2 , S͑ 1 D͒ +H 2 , O͑ 1 D͒ +H 2 and N͑ 2 D͒ +H 2 insertion reactions. Reaction probabilities, integral ͑ICS͒ and differential ͑DCS͒ cross sections at different levels of product's state resolution are shown and discussed for these reactions. The agreement is in most cases excellent and indicates that the effect of tunneling through the centrifugal barrier is negligible. However, if there exists a dynamical barrier, as in the case of the N͑ 2 D͒ +H 2 reaction, some of the SQM results can be slightly different than those calculated with the SQCT model. The rationale of the observed similarities and discrepancies can be traced back to the specific topologies of the potential energy surfaces for each of the reactions examined. The SQCT model is sensitive enough to show the relatively small inaccuracies resulting from the decoupling inherent to the centrifugal sudden approximation when used in the SQM calculations. In addition, the effect of ignoring the parity conservation is also examined. This effect is in general minor except in particular cases such as the DCS from initial rotational state j =0, which requires, in order to reproduce the sharp forward and backward peaks, the explicit conservation of parity.

“…(1) The third term, V X 2 O * , represents the intramolecular interactions in the excited X 2 O molecule, and is given by the gas-phase H 2 O Dobbyn-Knowles (DK) PES. 15,33,34 For the dynamics of D 2 O and HOD photodissociation the same PESs were used, but with appropriate masses in each case. All the potentials and the corresponding switching functions used in this study are the same as the ones used for H 2 O and D 2 O ice photodissociation calculations.…”

Section: A Potentialsmentioning

confidence: 99%

Isotope effects on the photodesorption processes of X2O (X = H,D) and HOD ice

Koning

Kroes

Arasa

2013

To investigate the isotope effects on the photodesorption processes of X 2 O (X = H,D) ice, molecular dynamics calculations have been performed on the ultraviolet photodissociation of an H 2 O or a D 2 O molecule in an H 2 O or a D 2 O amorphous ice surface, and on HOD photodissociation in an H 2 O amorphous ice surface, where the photodissociated molecules were located in the top four or five monolayers at ice temperatures of 10, 20, 30, 60, and 90 K. Three photodesorption processes can occur upon X 2 O photodissociation: X atom photodesorption, OX radical photodesorption, and X 2 O (or HOD) molecule photodesorption. X 2 O (or HOD) photodesorption can occur after recombination of X and OX, or after an energetic X atom photofragment kicks a surrounding X 2 O molecule from the ice surface. Isotope effects are observed for the X atom and the OX radical photodesorption as well as for the kick-out photodesorption. However, no isotope effects were noticeable for the photodesorption of recombined X 2 O molecules. The average D atom photodesorption probabilities are about a factor 0.9 smaller than those for the H atom, regardless of the isotope of the surrounding ice system. Also, the kick-out mechanism is more likely to occur if a D photofragment is created upon dissociation than if an H atom is created. These observations can be explained by more efficient energy transfer from the D atom to water molecules than from the H atom. Reasoning based on the X 2 O phonon frequencies associated with the librational modes and energy transfer efficiencies explain why the OX radical photodesorption probabilities are noticeably larger if the OX radical desorbs from a D 2 O ice system than from an H 2 O ice system. Also, the OX radical photodesorption is more probable upon dissociation of DOX (X = H,D) than upon dissociation of HOX (X = H,D), because the initial kinetic energy of the OX radical is larger if the dissociation products are D + OX than H + OX. The branching ratio of OD OH desorption following photodissociation of an HOD molecule in ice (about 1.0) is much lower than the OD OH branching ratio in gas-phase HOD photodissociation. This may lead to differences in isotope fractionation in OH(g) formation in dense and diffuse clouds in the interstellar medium.