Rotational dissociation of impulsively aligned van der Waals complexes

Abstract

The nonadiabatic alignment dynamics of weakly bound molecule-atom complexes, induced by a moderately intense 300 fs nonresonant laser pulse, is calculated by direct numerical solution of the time-dependent Schrödinger equation. Our method propagates the wave function according to the coupled channel equations for the complex, which can be done in a very efficient and stable manner out to large times. We present results for two van der Waal complexes, CS-He and HCCH-He, as respective examples of linear molecules with large and small moments of inertia. Our main result is that at intensities typical of nonadiabatic alignment experiments, these complexes rapidly dissociate. In the case of the CS-He complex, the ensuing rotational dynamics resembles that of isolated molecules, whereas for the HCCH-He complex, the detachment of the He atom severely perturbs and essentially quenches the subsequent rotational motion. At intensities of the laser pulse ≲2.0 × 10 W/cm, it is shown that the molecule-He complex can rotate and align without breaking apart. We discuss the implications of our findings for recent experiments on iodine molecules solvated in helium nanodroplets.

Impulsive alignment of 4He–CH3I: A theoretical study

“…Our results on the 4 He-CH 3 I complex presented in this work can also be compared to the theoretical study of Søndergaard et al 31 on the nonadiabatic alignment dynamics of the acetylene and of the CS 2 molecule complexed to a He atom. When one aims at comparing to an experimentally measured signal, an average over the spatial laser beam profile should be performed, as has been done in their work.…”

“…Our main finding is that for laser pulse intensities needed to induce alignment in gas phase, the 4 He-CH 3 I complex mostly dissociates as observed in Ref. 31. The initial rotational dynamics coincides with that of an isolated molecule, and the observed revival pattern is due to the alignment of CH 3 I coming from the dissociated complex, superimposed to a broadly oscillating background coming from the part of the wave packet that remains bound.…”

“…The proportion of the wave packet that dissociates depends on the laser pulse intensity since with the lower intensity used in this work we observe almost no dissociation. On the other hand, Søndergaard et al 31 have found that He-CS 2 or He-HCCH does not reach 100% dissociation when further increasing the intensity. We have not attempted to check if a limit lower than 100% also existed for He-CH 3 I at higher intensities.…”

“…Thus, the probability current at a defined distance R I smaller than R max is integrated over time as in Ref. 31. R I should be chosen large enough that the interaction potential V (R, θ BF ) is negligible compared to the CH 3 I rotational constant for any angle θ BF .…”

“…Recent theoretical work has studied the non-adiabatic alignment of a van der Waals complex of linear molecules and one single He atom by numerically solving the time dependent Schrödinger equation. 31 Two different regimes were found depending on the laser intensity. At intensities typical of non-adiabatic alignment, the rotational dynamics resembles that of isolated molecules; however, the complexes rapidly dissociate.…”