Quantum-State-Sensitive Detection of Alkali Dimers on Helium Nanodroplets by Laser-Induced Coulomb Explosion

TL;DR

This study introduces a laser-based Coulomb explosion technique to distinguish alkali dimers in different quantum states on helium nanodroplets, enabling detailed analysis of their formation and potential for ultrafast structural dynamics studies.

Contribution

The paper presents a novel method for quantum-state-sensitive detection of alkali dimers on helium nanodroplets using laser-induced Coulomb explosion, applicable to multiple alkali species.

Findings

Identified dimers in different quantum states via fragment ion kinetic energy.

Estimated dimer abundance ratios as a function of droplet size.

Demonstrated applicability to Li2, Na2, and K2 dimers.

Abstract

Rubidium dimers residing on the surface of He nanodroplets are doubly ionized by an intense fs laser pulse leading to fragmentation into a pair of $\mathrm{Rb^+}$ ions. We show that the kinetic energy of the $\mathrm{Rb^+}$ fragment ions can be used to identify dimers formed in either the X $^1\Sigma_{\mathrm{g}}^+$ ground state or in the lowest-lying triplet state, a $^3\Sigma_{\mathrm{u}}^+$. From the experiment, we estimate the abundance ratio of dimers in the a and X states as a function of the mean droplet size and find values between 4:1 and 5:1. Our technique applies generally to dimers and trimers of alkali atoms, here also demonstrated for $\mathrm{Li_2}$, $\mathrm{Na_2}$, and $\mathrm{K_2}$, and will enable fs time-resolved measurements of their rotational and vibrational dynamics, possibly with atomic structural resolution.