The AVPD model

1. Modeling slow sequential dissociation dynamics processes with wave packet methods: Application to Cl2-He2 vibrational predissociation • A. García-Vela • Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas, • C/ Serrano 123, 28006 Madrid, Spain Introduction: The vibrational predissociation dynamics of Cl2(B,v’)-He2 is studied using a full dimensional wave packet method (including six fully coupled degrees of freedom) for the initial vibrational excitations v’=8,10-13. The aim is to investigate the effect of increasing both the grid size in the dissociative coordinates and the propagation time, on the convergence of observable magnitudes like predissociation lifetimes and Cl2 product vibrational and rotational distributions. In particular, previous full dimensional simulations [1] showed that the calculated Cl2(B,vf=v’-1) product vibrational populations were substantially overestimated while the Cl2(B,vf=v’-2, v’-3) populations were correspondingly underestimated, due to an artifact caused by the absorbing boundary conditions imposed on the wave packet at the edges of the grid. Comparison with available experimental data [2,3] is carried out in order to assess convergence of the calculated magnitudes. Calculated lifetimes, linewidths and vibrational populations using different grid sizes. Experimental linewidths and lifetimes are also reported in Table 1 for comparison. The calculated lifetimes [7] do not exhibit large variations when increasing the grid size and the propagation time. This means that lifetimes can be obtained reasonably converged with a relatively short-time propagation and a relatively reduced grid size. A trend of convergence is found in the vibrational populations when the grid size is increased and the absorbing region is placed increasingly farther away in the asymptotic region, in the sense that the Cl2(B,v’-1) vibrational population gradually decreases and the Cl2(B,v’-2) population increases correspondingly. However, convergence is found to be very slow, making the calculation of accurate vibrational distributions extremely costly, even imposible. Decay curves of the initial resonance state Time evolution of the normalized vibrational populations of the Cl2(B,vf) fragment (vf=v’-1, v’-2, v’-3) for four initial Cl2 vibrational excitations v’ calculated with the AVPD model. The simulations were carried out using a 120 point grid. The AVPD model The model redistributes the absorbed wave packet intensity by means of three assumptions based on the nature of vibrational predissociation in X2-Rg2 and X2-Rg vdW complexes [7]. 1) Only the most likely dissociation channel is allowed for the intermediate X2-Rg complex absorbed, namely Δv’=0 or Δv’=-1. 2) More specifically, for the v’ and v’-1 manifolds the Δv’=-1 channel is considered, while for the manifolds v<v’-1 the Δv’=0 dissociation channel is allowed. 3) Overlapping and interference terms between the redistributed wave packet amplitude are modeled and included in the model. The vibrational distributions estimated with the AVPD model are in good agreement with the experimental data (see TABLE 3). Rotational distributions of the Cl2(B,vf) fragment for the vf=v’-1 channel (upper figure) and for the vf=v’-2 dissociation channel (lower figure). The labels calc.1 and calc. 2 indicate the use of the 96 and 120 point grids, in the simulations, respectively. The calculated vf=v’-2 distributions are also compared with the experimental ones of Ref. [2]. The vf=v’-2 distributions do not change much with increasing grid size, while the vf=v’-1 distributions display somewhat larger variations. Calculated Cl2(B,vf) fragment vibrational populations (in percentage) after predissociation through the channels vf=v'-1, v'-2, and v'-3 using the AVPD model with 70, 96, and 120 point grid simulations. The experimental distribution for v'=8 is also shown for comparison. Conclusions: The good agreement found with experiment for lifetimes and Cl2(B) rotational distributions indicates that such properties can be calculated with reasonable accuracy with a relatively short-time propagation and a relatively reduced grid size. By contrast, convergence of the vibrational populations with increasing grid size by placing the absorbing region increasingly farther away in the asymptotic region is very slow, making the calculation of converged vibrational populations out of reach in practice. An approximate model to redistribute the absorbed vibrational probability is proposed based on the nature of vibrational predissociation of vdW complexes. The vibrational distributions estimated with this model agree very well with the experimental data available. References [1] A. García-Vela, J. Chem. Phys. 122, 014312 (2005). [2] W.D. Sands,C.R. Bieler, and K.C. Janda, J. Chem. Phys. 95, 729 (1991). [3] M.I. Hernández, N. Halberstadt, W.D. Sands, and K.C. Janda, J. Chem. Phys. 113, 7252 (2000). [4] A. García-Vela, J.Chem. Phys. 119, 5583 (2003). [5] J.I. Cline, D.D. Evard, F. Thommen, and K.C. Janda, J. Chem. Phys. 84, 1165 (1985). [6] J. Williams, A. Rohrbacher, J. Seong, N. Marianayagam, K.C. Janda, R. Burcl, M.M. Szczesniak, and G. Chalasinski, J. Chem. Phys. 111, 997 (1999). [7] A. García-Vela, Phys. Chem. Chem. Phys.,(2011), DOI: 10.1039/C0CP02367A. Acknowledgements: Ministerio de Ciencia e Innovación, Spain, Grant No. FIS2010-18132, the Consolider program, Grant No. CSD2007-00013 and the Centro de Supercomputación de Galicia (CESGA).