A full-dimensional coupled-surface study of the photodissociation dynamics of ammonia using the multiconfiguration time-dependent Hartree method

Abstract

Full-dimensional quantum mechanical computations are carried out to investigate the photodissociation dynamics of à state NH(3) and ND(3) using the multiconfiguration time-dependent Hartree (MCTDH) method with recently developed coupled ab initio potential energy surfaces (PESs) [Z. H. Li, R. Valero, and D. G. Truhlar, Theor. Chim. Acc. 118, 9 (2007)]. To use the MCTDH method efficiently the PESs are represented as based on the high-dimensional model representation. The à ← X̃ absorption spectra for both isotopomers were calculated for the zeroth vibrational state of the ground electronic state. With a view to treating larger systems, Jacobi coordinates are used. Computations on the coupled PES are carried out for two-, three-, five-, and six-dimensional model systems to understand the validity of reduced-dimensional calculations. In addition to the fully coupled calculations, the effect of nonadiabatic coupling on absorption spectra is shown by propagating the initial wavepacket only in the à electronic state. The calculated absorption spectra are shown to be in good agreement with available theoretical and experimental observations. Comparisons with calculations using Radau and valence coordinates show the effect of including the symmetry of the system explicitly. Finally, branching ratios for loss of a hydrogen atom via the two available channels are calculated. These predict that the nonadiabatic product increases with the dimension of the calculations and confirm the importance of the full-dimensional calculations.

First principles determination of the NH2/ND2($\skew4\tilde A, \skew3\tilde X$Ã,X̃) branching ratios for photodissociation of NH3/ND3 via full-dimensional quantum dynamics based on a new quasi-diabatic representation of coupled ab initio potential energy surfaces

“…48,49 While reduced-dimensional models help to shed light on the dynamics, 50-55 a complete understanding requires a fulldimensional quantum treatment. 56,57 We have recently reported full-dimensional quantum calculations of the ν 2 progression in the A-band absorption spectra for NH 3 and ND 3 based on the new PESs, 44 which significantly improved the agreement with known experimental data including spectral positions, widths, and intensities. However, the absorption spectrum calculations did not involve non-adiabatic dynamics, as the absorption spectrum is completely determined by the short-time evolution of the wave packet in the Franck-Condon region.…”

“…3. It is currently unclear if the underestimation is due to the PESs or to the reduced dimensionality of the dynamic model, but the recent work of Giri et al 57 seems to sug-gest the latter. From the figure, it is evident that theà 2 A 1 state of NH 2 cannot be accessed for the first three members of the 2 n progression, due to the energy constraint.…”

A Practical Diabatisation Scheme for Use with the Direct-Dynamics Variational Multi-Configuration Gaussian Method

“…12 (15) where dR is the nuclear displacement from the point of degeneracy. The original method, as applied to predetermined sets of adiabatic energies, uses molecular symmetry to uniquely define the elements of the matrix in eq 14, 71 the diagonal elements only containing components from totally symmetric molecular modes, and the off-diagonal elements consisting of elements that transform according to the direct product of the irreducible representations of the interacting states.…”

Direct grid-based quantum dynamics on propagated diabatic potential energy surfaces

“…[6,7] One can model these non-adiabatic transitions using classical mechanics, as in the trajectory surface hopping (TSH) algorithm, [2,[8][9][10] but as the dynamics are inherently quantum mechanical, it is better to use a quantum mechani-25 cal method such as the multi-configuration timedependent Hartree (MCTDH) approach [1, 11,12] where possible.…”