Fifth-order Raman spectroscopy: Liquid carbon disulfide and liquid benzene

Fifth-order Raman spectroscopy has the potential to enable chemists to directly probe the intermolecular potential between molecules within the liquid state. Understanding the liquid state has been a long standing goal in physical chemistry as most chemical and biological processes occur in solution. This new spectroscopy has the capacity to examine these low-frequency intermolecular modes and gives direct access to the many body potential of liquids. Effectively this spectroscopy provides a direct window on the enharmonic motions of molecules that distinguish the dynamics of the liquid state. The observable in this experiment provides that most rigid test of potentials used to model liquids and as such provides an important new tool for arriving at a first-principles treatment of liquids. The experiment is complicated by the extremely small signals associated with the Raman processes probing the intermolecular frequency correlations. A new approach based on diffractive optics has solved the last remaining obstacles to the successful implementation of this spectroscopy. The heterodyned fifth-order Raman response of liquid CS2 and liquid benzene have been measured and characterized by specifically exploiting the passive-phase stabilization provided through the use of diffractive optics. The two liquids are compared and contrasted with each other and various recent theoretical results. The measurement of the low-frequency Raman two-time delay correlation function indicates the intermolecular modes of both liquids to be primarily homogeneously broadened and that the liquid loses its nuclear rephasing ability very rapidly. This rapid loss of nuclear correlations indicates a lack of modal character in the low-frequency motions of liquid CS2 and benzene.