Characterization of RbSr molecules: spectral analysis on helium droplets

We report an experimental investigation of RbSr molecules attached to helium nanodroplets. The molecules are prepared on the surface of helium droplets by utilizing a sequential pickup scheme. We provide a detailed analysis of the excitation spectrum in the wavelength range 11 600–23 000 cm−1. The spectrum has been recorded by resonance enhanced multi-photon ionization time-of-flight spectroscopy. The inherent mass sensitivity of the method allows for an unraveling of the RbSr spectrum, which is influenced by Rb and Sr dimer contributions, because of the proximity of their respective isotopologues. In addition, the vibrationally resolved 42Σ+ band was investigated using laser induced fluorescence spectroscopy. The vibronic transitions exhibit a lambda-shaped peak form, which is characteristic of excitations on helium droplets and indicative of strong coupling of the molecule to the superfluid helium environment. Furthermore, the vibrationally resolved 42Σ+ state enables the determination of molecular parameters, which are in excellent agreement with previously measured dispersed fluorescence spectra, originating from bare RbSr molecules. The assignment of recorded transitions is based on calculated transition dipole moments and potential energy curves. The theoretical results allow for the identification of transitions from the vibronic X2Σ+ ground state to the 22Π, 32Σ+, 42Σ+, 32Π, 42Π and 62Σ+ states. The detailed investigation of RbSr on helium droplets provides a solid basis for further high resolution gas phase studies of this diatomic molecule that holds promise in the area of cold molecular physics.