Photodissociation of vibrationally excited ammonia: Rotational excitation in the NH2 product

In two previous papers we have introduced a method to generate coupled quasi-diabatic Hamiltonians (H(d)) that are capable of representing adiabatic energies, energy gradients, and derivative couplings over a wide range of geometries including seams of conical intersection. In this work, two new synergistic features are introduced. Firstly, the functional form of H(d) is generalized. Rather than requiring there to be a low energy point of high symmetry to serve as the unique origin, functions centered on points distributed in nuclear coordinate space are used in the polynomials that comprise the matrix elements in H(d). The use of functions with distributed origins, allows reproduction of the ab initio data with lower order expansions, and offers the possibility of describing multichannel dissociation. The fitting algorithm is combined with a three-step procedure in which the domain of H(d) is extended from a core set of nuclear configurations to a region of nuclear coordinate space appropriate for nuclear dynamics, with a prescribed accuracy. This significant extension of the domain of definition compared to our original work, which is facilitated by the distributed origin approach, is achieved largely through the use of surface hopping trajectories. The 1,2(1)A states of NH(3), which provide an archetypical example of nonadiabatic dynamics, are used to demonstrate the utility of this approach. The representation describes 21 points on the 1(1)A-2(1)A seam of conical intersection and their local topography flawlessly and on the entire domain, the electronic structure data is represented to an accuracy of 77.00 (46.90) cm(-1), as measured by the root mean square (mean unsigned) error for energies lower than 50 000 cm(-1). This error is a factor of 10 lower than that of the most accurate representation of high quality ab initio data, on a comparable domain, previously reported for this system.

“…[35][36][37] For this reason we have used, 29,30 and use here, the 1,2 1 A states of NH 3 as our test case.…”

Section: Application To the 12 1 A States Of Nhmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

On the representation of coupled adiabatic potential energy surfaces using quasi-diabatic Hamiltonians: A distributed origins expansion approach

Zhu

Yarkony

2012

“…Detailed measurements of the absorption spectra, 4-12 product internal state distributions, [13][14][15][16][17][18][19][20][21][22][23][24][25] and dissociation lifetimes 26,27 have been reported. Particularly relevant to the a) Authors to whom correspondence should be addressed.…”

Section: Introductionmentioning

confidence: 99%

“…28,29 Strong mode and bond selectivity has been found. 24,[30][31][32][33][34][35][36][37] One of the most intriguing findings in the vibrationally mediated ammonia photodissociation is the strong mode selectivity of the NH 2 (Ã 2 A 1 )/NH 2 (X 2 B 1 ) branching ratio. 34,37 Unlike the commonly observed dominance of the ground state NH 2 (X 2 B 1 ) channel, excitation in the antisymmetric stretching mode of NH 3 prior to photolysis resulted in mostly adiabatic dissociation to NH 2 (Ã 2 A 1 ).…”

Section: Introductionmentioning

confidence: 99%

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

Zhu

Guo

et al. 2012

The A-band photodissociation of ammonia is an archetypical and long studied example of nonadiabatic dynamics in a polyatomic system. Due to a well-known conical intersection seam, electronically excited NH 3 can produce either the ground (X 2 B 1 ) state or the excited (Ã 2 A 1 ) state of the NH 2 fragment. In this work, the non-adiabatic dynamics is investigated using a six-dimensional wave packet method and an improved version of a newly developed diabatic Hamiltonian based on high quality ab initio data. TheÃ 2 A 1 /X 2 B 1 branching ratios are in excellent agreement with experimental estimates, thus validating the non-adiabatically coupled Hamiltonian.

“…The dynamics of photoexcited ammonia has been experimentally investigated by several research groups, [30][31][32][33][34][35][36][37][38][39][40][41][42] of which we single out two. [30][31][32]43,44 Biesner et al 30 studied the effect of excitation of the umbrella mode and found that NH 2 fragments were highly rotationally excited when n 2 = 0, where n 2 is the number of umbrella-mode quanta initially in vibration, and nonadiabatic dynamics, presumed to be due to passage through or near the conical intersection, was favored. For larger n 2 values the situation became harder to interpret due to the increase in electronically and vibrationally excited products.…”

Section: Introductionmentioning

confidence: 99%

Mixed quantum/classical investigation of the photodissociation of NH3(Ã) and a practical method for maintaining zero-point energy in classical trajectories

Bonhommeau

Truhlar

2008

The photodissociation dynamics of ammonia upon excitation of the out-of-plane bending mode (mode nu(2) with n(2)=0,[ellipsis (horizontal)],6 quanta of vibration) in the A electronic state is investigated by means of several mixed quantum/classical methods, and the calculated final-state properties are compared to experiments. Five mixed quantum/classical methods are tested: one mean-field approach (the coherent switching with decay of mixing method), two surface-hopping methods [the fewest switches with time uncertainty (FSTU) and FSTU with stochastic decay (FSTU/SD) methods], and two surface-hopping methods with zero-point energy (ZPE) maintenance [the FSTUSD+trajectory projection onto ZPE orbit (TRAPZ) and FSTUSD+minimal TRAPZ (mTRAPZ) methods]. We found a qualitative difference between final NH(2) internal energy distributions obtained for n(2)=0 and n(2)>1, as observed in experiments. Distributions obtained for n(2)=1 present an intermediate behavior between distributions obtained for smaller and larger n(2) values. The dynamics is found to be highly electronically nonadiabatic with all these methods. NH(2) internal energy distributions may have a negative energy tail when the ZPE is not maintained throughout the dynamics. The original TRAPZ method was designed to maintain ZPE in classical trajectories, but we find that it leads to unphysically high internal vibrational energies. The mTRAPZ method, which is new in this work and provides a general method for maintaining ZPE in either single-surface or multisurface trajectories, does not lead to unphysical results and is much less time consuming. The effect of maintaining ZPE in mixed quantum/classical dynamics is discussed in terms of agreement with experimental findings. The dynamics for n(2)=0 and n(2)=6 are also analyzed to reveal details not available from experiment, in particular, the time required for quenching of electronic excitation and the adiabatic energy gap and geometry at the time of quenching.