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

The first overtone pure vibrational Q(2)(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. (C) 2002 American Institute of Physics.