Sodium molecule on the surface of helium-4 droplets: optical transitions probe collective excitations

Alkali molecules trapped at the surface of a liquid helium-4 droplets offer a unique tool to explore their surface excitations. An optical transition between the triplet states in a sodium dimer leads to a contraction of the molecule and a conversion of the helium-droplet collective excitations localized around the sodium dimer into much softer, delocalized excitations of the liquid droplet surface similar to tidal waves. The localized phonon zero-point energy released in this process yields a peculiar asymmetric triangular shape of the surface-phonon sideband of the molecular vibronic lines, with the sup-pression of the zero-phonon line. An optical transition between the singlet states of a sodium molecule changes the forces between the molecule and the neighbor helium-4 atoms. As a result, the singlet-singlet transition in a sodium molecule leads, in addition to the zero-phonon transition, to transitions accompanied by the excitation of bulk phonons of the quantum liquid droplet and its surface phonons. In the low-frequency limit surface phonons contribute more strongly to the vibration of the neighbor helium-4 atoms than bulk phonons. This results in a dominant contribution of surface phonons in the low-energy wing of the zero-phonon line in the spectrum