Excimers in the Lowest Rotational Quantum State in Liquid Helium.

Evidence for helium excimers (He2*) in the lowest allowed rotational quantum state in liquid helium is presented. He2* was generated by a corona discharge in the gas and normal liquid phases. Fluorescence spectra recorded in the visible region between 3.8 and 5.0 K and 0.2 and 5.6 bar showed the rotationally resolved d3Σu+ → b3Πg transition of He2*. Analysis of the pressure and temperature dependence of lineshifts and line intensities showed features of solvated He2* superimposed on its gas-phase spectrum and, in the liquid phase only, pressure-induced rotational cooling. These findings suggest that He2* can be used to investigate bulk helium in different phases at the nanoscale.