Communications: A model study on the electronic predissociation of the NeBr[sub 2] van der Waals complex (original) (raw)
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A theoretical study on electronic predissociation in the NeBr2 van der Waals molecule
Chemical Physics, 2012
We present the first comprehensive ab initio study of the Ne-Br 2 potential energy surfaces and the nonadiabatic couplings between the valence excited electronic states. These ab initio results are used to obtain 3-D approximate potentials for each electronic state, and these potentials are used in a wave packet calculation of the competing electronic predissociation and vibrational predissociation dynamics. The results of this calculation are in excellent agreement with both experimental results and a previous empirical fit to the experiments. The calculations allow us to observe not only the competition between vibrational and electronic dynamics for the dimer, but also the competition between two different electronic channels. Coupling to the 2 g state dominates for the levels studied here, but coupling to the C state is progressively more important for low vibrational levels, and may dominate at levels below which the current results pertain. The ability of ab initio surfaces and couplings to so accurately reproduce experimental data raises the hope of a complete understanding of the VP and EP dynamics for other Rg-halogen dimers. Success in the case presented here is largely due to the fact that the VP dynamics for the vibrational levels in this study are in the simple, direct regime. Understanding the simple case so thoroughly provides new hope that the more complicated examples, such as ArI 2 and NeCl 2 , for which experiment and theory are not currently in accord, may yet yield to analysis.
Quasi-classical trajectories study of Ne79Br2(B) vibrational predissociation
Physical Chemistry Chemical Physics, 2006
A full-dimensional quasi-classical trajectories study on the vibrational predissociation (VP) of the Ne 79 Br 2 (B) complex is presented. Following the most recent experiments, the Br 2 (B) vibrational levels v 0 = 16-29 were explored. The total angular momentum, J, was taken to be zero, and a semiclassical Franck-Condon model to compute initial conditions from quantum distributions was employed. Predissociation lifetimes were extracted from Ne 79 Br 2 population decay by using two different exponential laws. Predicted lifetimes are in excellent agreement with the last experimental results [J. A. Cabrera, C. R. Bieler, B. C. Olbricht, W. E. van der Veer and K. C. Janda, J. Chem. Phys., 2005, 123, 054311]. The Br 2 fragment ro-vibrational distributions resulting from the VP of the molecule were obtained from the statistics of classical magnitudes using the standard binning procedure. Computed rotational distributions (for the Dv 0 = À1, À2 channels) are also in very good agreement with the experimental results [M. Nejad-Sattari and T. A.
Chemical Physics Letters, 2002
Techniques of quantum and nonlinear classical mechanics are employed to study the photodissociation dynamics of NeBr 2 (B) near the Br 2 dissociation threshold. A time-dependent treatment is used to obtain the absorption spectrum for the B X transition. As energy increases up to 3600 cm À1 , the spectrum shows a progressive congestion but narrow peaks appear again close to the Br 2 dissociation limit. These peaks are associated with long lived resonances that develop nodes along the minimum energy path of the linearization. Periodic orbit (PO) analysis in two dimensions is carried out for the van der Waals NeBr 2 system and POs that emerge from saddle-node bifurcations are found to be related to the structure of the above resonances. Ó
Photodissociation of NeBrsub 2 below and above the dissociation limit of Brsub 2
The Journal of Chemical Physics, 2001
The photodissociation dynamics of the NeBr 2 complex in the B electronic state is studied, for the first time, near the Br 2 ͑B͒ dissociation limit, below and above, when the complex is promoted from the ground T-shaped level in the X electronic state. A time-dependent treatment is used in which the initial wave packet is divided in two portions, one describing the slow predissociation dynamics below the Br 2 dissociation threshold, and the second one, the fast complete dissociation in NeϩBrϩBr fragments. Below that threshold, the absorption spectrum shows an increasing congestion as the vibrational energy content of Br 2 increases, but narrow peaks appear again for the highest energy region of the spectrum. These peaks correspond to long lived resonances associated with ''horseshoe'' type states, as demonstrated by two-dimensional calculations. These resonances have a significant probability density for the linear geometry in which the Ne atom is inserted between the two bromine atoms. At this configuration the exchange of vibrational energy is rather inefficient which explains both why the spectrum is so sparse and resonances are so narrow. Above the Br 2 dissociation threshold, the recombination of Br 2 is found to be very inefficient, except for very low kinetic energies. The small recombination probabilities are due to vibrational couplings and not to any collisional caging effect. Since the complex remains essentially T-shaped during dissociation, extensive two-dimensional calculations are performed for longer times to better determine final vibrational distributions at low kinetic energies.
Photodissociation of NeBr2(B) below and above the dissociation limit of Br2(B)
Journal of Chemical Physics, 2001
The photodissociation dynamics of the NeBr 2 complex in the B electronic state is studied, for the first time, near the Br 2 ͑B͒ dissociation limit, below and above, when the complex is promoted from the ground T-shaped level in the X electronic state. A time-dependent treatment is used in which the initial wave packet is divided in two portions, one describing the slow predissociation dynamics below the Br 2 dissociation threshold, and the second one, the fast complete dissociation in NeϩBrϩBr fragments. Below that threshold, the absorption spectrum shows an increasing congestion as the vibrational energy content of Br 2 increases, but narrow peaks appear again for the highest energy region of the spectrum. These peaks correspond to long lived resonances associated with ''horseshoe'' type states, as demonstrated by two-dimensional calculations. These resonances have a significant probability density for the linear geometry in which the Ne atom is inserted between the two bromine atoms. At this configuration the exchange of vibrational energy is rather inefficient which explains both why the spectrum is so sparse and resonances are so narrow. Above the Br 2 dissociation threshold, the recombination of Br 2 is found to be very inefficient, except for very low kinetic energies. The small recombination probabilities are due to vibrational couplings and not to any collisional caging effect. Since the complex remains essentially T-shaped during dissociation, extensive two-dimensional calculations are performed for longer times to better determine final vibrational distributions at low kinetic energies.
Mathematics
The vibrational predissociation of NeBr2 has been studied using a variety of theoretical and experimental methods, producing a large number of results. It is therefore a useful system for comparing different theoretical methods. Here, we apply the trajectory surface hopping (TSH) method that consists of propagating the dynamics of the system on a potential energy surface (PES) corresponding to quantum molecular vibrational states with possibility of hopping towards other surfaces until the van der Waals bond dissociates. This allows quantum vibrational effects to be added to a classical dynamics approach. We have also incorporated the kinetic mechanism for a better compression of the evolution of the complex. The novelty of this work is that it allows us to incorporate all the surfaces for (v=16,17,…,29) into the dynamics of the system. The calculated lifetimes are similar to those previously reported experimentally and theoretically. The rotational distribution, the rotational ener...
Chemical Physics Letters, 2008
A detailed application of the Gaussian-weighted trajectory method to the photodissociation of the Rg� � �Br 2 (Rg = He, Ne, Ar) van der Waals triatomics is presented. In agreement with previous applications on molecular collisions, the approach significantly enhances the quasi-classical predictions of product state distributions with respect to those obtained with the Standard Binning procedure, especially near a vibrational channel closing. The different molecules studied shed light on the sort of improvement to expect for various densities of vibrational quantum-states involved in the fragmentation process. Extension to larger polyatomic molecules, its possible difficulties and solutions are briefly sketched.
The Journal of Chemical Physics, 1999
We report two independent sets of experimental spectroscopic data which both contain information about the vibrational dynamics occurring in the aniline-neon van der Waals complex in its S 1 electronically excited state. The high resolution excitation spectra of the three vibronic bands, 6a 0 1 , I 0 2 , and 1 0 1 , of the S 1 ←S 0 transition, exhibit lifetime broadening with respect to transitions to the corresponding states in the aniline monomer. The dispersed emission spectra taken under excitation of the same three vibronic bands give access to both the distribution of aniline monomer states produced by vibrational predissociation of the complex and to the rates at which this dynamics proceeds. The overall results are discussed in a consistent way, with emphasis being given to the role of the coupling between the intramolecular and the intermolecular vibrational states. In the case of I 0 2 excitation, it is shown that this coupling is reflected in the shape of the van der Waals wavefunction, as accessed through the analysis of the high resolution spectra ͓M. Becucci, G. Pietraperzia, N. M. Lakin, E. Castellucci, Ph. Bréchignac, Chem. Phys. Lett. 260, 87 ͑1996͒.͔.
The Journal of Chemical Physics, 2002
The B←X rovibronic excitation spectrum of the HeBr 2 van der Waals complex is calculated using an ab initio potential energy surface for the ground electronic state. The coupled-cluster single double triple calculations predict double-minimum topology ͑linear and T-shaped wells͒ for the X-state potential with a low isomerization barrier. The two lowest vibrational levels, assigned to T-shaped and linear isomers using the localization patterns of the corresponding wave functions, are almost degenerated and lie slightly above the isomerization barrier. This indicates that T-shaped and linear isomers can coexist even at low temperatures and give rise to two separated bands in the excitation spectrum. The main band of the B←X excitation spectrum is assigned to transitions from the T-shaped isomer, whereas the very good agreement between the observed and calculated spectrum, using the ab initio X-state potential, demonstrates that the unassigned secondary band corresponds to excitation of the linear isomer of the HeBr 2 (X) complex. The complete assignment of the spectrum in terms of individual rovibronic transitions is presented.