Ultrafast collisional dissipation of symmetric-top molecules probed by rotational alignment echoes

Abstract

We experimentally and theoretically investigate the ultrafast collisional dynamics of a symmetric-top molecule (${\mathrm{C}}_{2}{\mathrm{H}}_{6}$) in pure gas and mixtures with He at high density by employing the rotational alignment echo created by a pair of time-delayed intense laser kicks. The decrease of the amplitude of the echo when increasing the delay between the two laser pulses, reflecting the collisional relaxation of the system, is measured by probing the transient birefringence induced in the medium. The theoretical predictions, carried using purely classical molecular dynamics simulations, reproduce well the observed features, as demonstrated previously for a linear molecule. The analysis shows that the dissipation of the ethane alignment, despite the fact that this species has an extra rotational degree of freedom as compared to a linear molecule, barely involves more complex collisional relaxation channels due to characteristics of the ${\mathrm{C}}_{2}{\mathrm{H}}_{6}\ensuremath{-}{\mathrm{C}}_{2}{\mathrm{H}}_{6}$ and ${\mathrm{C}}_{2}{\mathrm{H}}_{6}\ensuremath{-}\mathrm{He}$ interactions. However, our findings reveal that the dissipative dynamics of a symmetric-top molecule can be properly approached using the recently discovered rotational alignment echoes, which, so far, have been only tested for probing rotational decoherence of simpler (linear) molecules.