Real-time dissociation dynamics of the Ne2Br2 van der Waals complex

Abstract

We have characterized the vibrational predissociation (VP) of the Ne(2)Br(2) van der Waals complex using time- and frequency-resolved pump-probe spectroscopy. After exciting Br(2) within the complex to a vibrational level 16<or=nu(')<or=23 in the B state, we follow the flow of halogen vibrational energy to the van der Waals modes in real time by recording the time-dependent behavior of Ne(2)Br(2) (nu(')), the NeBr(2) (nu(')-m) intermediates, and the Br(2) (nu(')-n) products. For Ne(2)Br(2) (nu(')=16-18), the only intermediate observed is NeBr(2) (nu(')-1), and the majority of the final product is Br(2) (nu(')-2), indicating the dissociation happens via two sequential direct VP steps. We fit the time-dependent behavior of these species to a sequential mechanism and extracted time constants for each step. For higher nu(') levels, the results show that the dissociation occurs via multiple pathways. Product Br(2) from levels lower than (nu(')-2) becomes much more important, with products as low as (nu(')-5) being observed. For nu(')=21, we observe both NeBr(2) (nu(')-1) and (nu(')-2) intermediates. The intermediates have significantly different kinetics, with the decay rate of the (nu(')-1) transient being nearly twice that of the (nu(')-2) transient. Similarly, both Br(2) (nu(')-2) and (nu(')-3) are formed in almost equal amounts, but the (nu(')-2) product formation rate is faster than the (nu(')-3) rate. The broad vibrational product state distributions and multiple dissociation pathways indicate that intramolecular vibrational energy redistribution becomes increasingly important for nu(')>19. We also report vibrational product state distributions for direct excitation to NeBr(2) 16<or=nu(')<or=23. For NeBr(2), the dominant product channel is Br(2) (nu(')-1) for all initial nu(') studied, consistent with this complex dissociating primarily via direct VP.

Quasi-classical trajectories study of Ne2Br2(B) vibrational predissociation: Kinetics and product distributions

“…7 and 8 in Ref. 47). This is consistent with several results from our QCT calculations: (1) once the Ne 2 Br 2 complex decayed into NeBr 2 , the only process observed was the fragmentation of the resulting triatomic into Ne and Br 2 products; (2) less than about 0.05% of all trajectories dissociated via the concerted mechanism (subset of all trajectories for which both Ne atoms dissociated within one Br 2 vibrational period, excluding those predicted by the sequential mechanism to dissociate within that time interval 73 ); and (3) no significant statistical correlation was found between the variables defining the intermediate complex and t 2 − t 1 .…”

“…We compare both theoretical methods, as well as their ability to reproduce the experimental observations 47 and previous MDQT predictions 11 . Our main goal is to build upon the current understanding of the capabilities of QCT in the simulation of VP processes involving larger vdW clusters, and to distinguish whenever possible the results which are intrinsic to the methodology from those that are characteristic to the systems under consideration.…”

“…1. Choosing J to be zero is a well-justified constraint when studying the photodissociation of rotationally cold species 58 as produced in the experiment through a supersonic free jet expansion 47 . The complete set of variables in classical phase space Γ is finally obtained by adjusting the respective conjugate momenta so that the Hamilton function reads…”

“…Schemes (1)- (4) 20,47,75 . However, QCT will yield the same lifetimes no matter what scheme is used (though numerical fitting and convergence will in practice result in small differences).…”

“…They therefore inferred the evolution of intermediates by fitting the product dihalogen formation to a sequential first-order kinetic mechanism. In 2010, Pio et al 47 reported the characterization of the VP of Ne 2 Br 2 (B) at an unprecedented level of detail. Using time-and frequency-resolved pump-probe spectroscopy they were able to record the real time evolution of all complexes involved, propose a kinetic mechanism, and determine time constants and product vibrational state distributions.…”