Quantum-State-Sensitive Detection of Alkali Dimers on Helium Nanodroplets by Laser-Induced Coulomb Explosion
Henrik H. Kristensen, Lorenz Kranabetter, Constant A. Schouder, Christoph Stapper, Jacqueline Arlt, Marcel Mudrich, Henrik Stapelfeldt
Abstract
Rubidium dimers residing on the surface of He nanodroplets are doubly ionized by an intense femtosecond 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 $X$ $^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ ground state or in the lowest-lying triplet state, $a\text{ }^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$. 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 $4\ensuremath{\mathbin:}1$ and $5\ensuremath{\mathbin:}1$. 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 femtosecond time-resolved measurements of their rotational and vibrational dynamics, possibly with atomic structural resolution.