Density behavior of the double rotational transition in liquid parahydrogen

We report a detailed observation and theoretical analysis of the double rotational transition $S_0\mathrm{}\left(0\right)+S_0\mathrm{}\left(0\right)\mathrm{}$ of a molecular pair in liquid hydrogen. The experiment has been performed in a wide density range at constant temperature $T=28\mathrm{}$ K. The density behavior of the integrated intensity has been compared with the results of a theoretical model assuming pairwise additivity of intermolecular potential and polarizability. The calculations have been carried out using quantum-mechanical path-integral Monte Carlo simulations to determine the microscopic structure. The importance of the short-range form of the induced anisotropy is pointed out. A satisfactory agreement is found between theory and experiment.