Excimers in the Lowest Rotational Quantum State in Liquid Helium

Evidence for helium excimers (He∗_2) in the lowest allowed rotational quantum state in liquid helium is presented. He∗_2 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 d3 Σ+ u → b3 Πg transition of He∗_2. Analysis of the pressure and temperature dependence of lineshifts and line intensities showed features of solvated He∗_2 superimposed on its gas phase spectrum and, in the liquid phase only, pressure-induced rotational cooling. These findings suggest that He∗_2 can be used to investigate bulk helium in different phases at the nanoscale.