We combine experimental and theoretical approaches to explore excited rotational states of molecules embedded in helium nanodroplets using ${\mathrm{CS}}_2$ and ${\mathrm{I}}_2$ as examples. Laser-induced nonadiabatic molecular alignment is employed to measure spectral lines for rotational states extending beyond those initially populated at the 0.37 K droplet temperature. We construct a simple quantum-mechanical model, based on a linear rotor coupled to a single-mode bosonic bath, to determine the rotational energy structure in its entirety. The calculated and measured spectral lines are in good agreement. We show that the effect of the surrounding superfluid on molecular rotation can be rationalized by a single quantity, the angular momentum, transferred from the molecule to the droplet.

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• Atomic & molecular clusters
• Molecular spectra
• Quasiparticles & collective excitations
• Strong electromagnetic field effects
• Superfluidity
• Ultrafast phenomena
• Bose gases
• Bose-Einstein condensates
• Quantum fluids & solids