Femtosecond pulses and dynamics of molecular photoexcitation: RbCs example

TL;DR

This study explores femtosecond laser pulse-driven molecular photoexcitation in RbCs, comparing modeling approaches to understand dynamics and optimize ground state transfer, highlighting the role of far-from-resonance levels and excitation blockade.

Contribution

It introduces a comparative analysis of global and restricted modeling approaches for femtosecond pulse excitation in molecules, revealing insights into high-field dynamics and transfer limitations.

Findings

Selective transfer feasible only in low-field regime.

Far-from-resonance levels significantly influence excitation dynamics.

Excitation blockade occurs due to quasi-degenerate levels.

Abstract

We investigate the dynamics of molecular photoexcitation by unchirped femtosecond laser pulses using RbCs as a model system. This study is motivated by a goal of optimizing a two-color scheme of transferring vibrationally-excited ultracold molecules to their absolute ground state. In this scheme the molecules are initially produced by photoassociation or magnetoassociation in bound vibrational levels close to the first dissociation threshold. We analyze here the first step of the two-color path as a function of pulse intensity from the low-field to the high-field regime. We use two different approaches, a global one, the 'Wavepacket' method, and a restricted one, the 'Level by Level' method where the number of vibrational levels is limited to a small subset. The comparison between the results of the two approaches allows one to gain qualitative insights into the complex dynamics of the high-field regime. In particular, we emphasize the non-trivial and important role of far-from-resonance levels which are adiabatically excited through 'vertical' transitions with a large Franck-Condon factor. We also point out spectacular excitation blockade due to the presence of a quasi-degenerate level in the lower electronic state. We conclude that selective transfer with femtosecond pulses is possible in the low-field regime only. Finally, we extend our single-pulse analysis and examine population transfer induced by coherent trains of low-intensity femtosecond pulses.