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

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 Rb+ ions. We show that the kinetic energy of the Rb+ fragment ions can be used to identify dimers formed in either the X (1)Sigma(+)(g) ground state or in the lowest-lying triplet state, a (3)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 4:1 and 5:1. Our technique applies generally to dimers and trimers of alkali atoms, here also demonstrated for Li-2, Na-2, and K-2, and will enable femtosecond time-resolved measurements of their rotational and vibrational dynamics, possibly with atomic structural resolution.

Automated summary

In this study, rubidium dimers on helium nanodroplets were doubly ionized by an intense femtosecond laser pulse, resulting in fragmentation into a pair of Rb+ ions. We demonstrate that the kinetic energy of the Rb+ fragment ions can be used to identify dimers in either the X or a state. The abundance ratio of dimers in the a and X states was found to be between 4:1 and 5:1, depending on the mean droplet size. This technique can be applied to alkali atom dimers and trimers and enable femtosecond time-resolved measurements of their rotational and vibrational dynamics.