Observation of discrete energy states of weakly confined vibron in solid parahydrogen

The first overtone pure vibrational Q2(0) [v=2←0, J=0←0] transition of solid parahydrogen was studied using high-resolution laser spectroscopy. The absorption spectrum exhibits extremely narrow linewidths (20 MHz full-width-at-half-maximum), which allowed us to observe rich spectral structure in the transition. Part of the structure is attributed to the M sublevel splitting of orthohydrogen that induces dipole moments on surrounding parahydrogen molecules, but the finer spectral structure is due to vibron hopping of the v=2 excited state of parahydrogen molecules. The Stark field of the orthohydrogen localizes the vibron on a finite number of parahydrogen molecules, which results in the discrete vibron states. From the analysis of the spectral structure, the vibron hopping frequency of the v=2 state is determined to be −114 MHz. It is also found that the spectral structure cannot be explained completely unless the quantum nature of solid hydrogen is taken into account for the analysis.