Two-dimensional infrared photon echo spectroscopy has been used to describe the anharmonic nuclear potential of two coupled molecular vibrations. The two-dimensional spectrum shows diagonal and off-diagonal features, each composed of two peaks. The splitting between these peaks is directly related to the anharmonicity, while the relative amplitude of the diagonal and off-diagonal features describes the projection angle between interacting dipoles.

Two-dimensional vibrational spectroscopy has been used to characterize transient molecular structure by measuring the couplings and projection angles between two strongly coupled anharmonic vibrations. Two-dimensional Fourier-transform infrared spectra of the coupled carbonyl stretches of Rh(CO)2(C5H7O2) in hexane have been obtained from femtosecond vibrational echo signals detected with spectral interferometry. The eight resonances in the two-dimensional spectrum can be interpreted as two diagonal peaks and two cross peaks, each split into a pair. The splitting between the peak pairs is directly related to the diagonal and off-diagonal anharmonicity of the symmetric and asymmetric carbonyl stretches. The ratio of the amplitude of the cross peaks for two different polarization geometries determines the projection angle between the coupled transition dipoles. The experimental characterization of the vibrational eigenstates allows the local carbonyl structure to be modeled as bilinearly coupled cubic anharmonic oscillators. The interaction between the carbonyl stretches arises from the mutual bonding with the rhodium metal center. This two-dimensional infrared experiment characterizes the structure with a time window of roughly 20 ps, suggesting a general method for capturing transient molecular structure in solution.

The orientational contribution to the third-order nonlinear response of coupled vibrational or electronic states is evaluated considering the dipole orientations of these states and molecular orientational relaxation. A general formalism is developed for calculating the third-order orientational response function for processes involving up to four distinct transition dipole moments, which are fixed in a molecular frame that is free to diffusively reorient. In particular, all tensor components of the third-order orientational response function for two coupled vibronic states were calculated as a function of the projection angle between the transition dipole moments of the fundamental transitions. The results are discussed in conjunction with specific third-order infrared nonlinear experiments on coupled vibrational systems: pump–probe, dispersed two-dimensional pump–probe, and two-dimensional photon-echo experiments. The anisotropy of the nonlinear signal and ratios of independent tensor components of the response, are shown to depend on the projection angle between the transition dipole moments and therefore can be used to access structural information about molecular systems.

Cascaded third-order Raman signals can be separated from direct two-dimensional fifth-order Raman signals using heterodyne detection with a local oscillator that is cycled from in-phase to out-of-phase with the direct signal. The difference of these scans gives the direct signal, while the sum gives the in-quadrature cascaded signal.

A phase-sensitive detection method that uses two diffractive optics for femtosecond nonresonant Raman spectroscopy is demonstrated. One diffractive optic is used for generating the three input pulses for the thirdorder nonlinear experiment, and the second is used for recombining the signal with a passively phase-locked local oscillator derived from the probe pulse. This approach allows for phase-sensitive detection, direct phase calibration, control over all field polarizations, and removal of unwanted two-pulse signal contributions. Experiments on the intermolecular dynamics of CS 2 and CH 3 CN demonstrate that the birefringent (in-quadrature) signal amplitude is significantly greater than the dichroic (in-phase) contribution. Polarizationselective measurements are used to project the isotropic birefringent response for CS 2 , which suppresses reorientational dynamics and allows interaction-induced effects to be observed.