Intrinsic phasing of heterodyne-detected multidimensional infrared spectra

We show that it is possible to phase multidimensional infrared spectra generated by a boxcars geometry four-wave mixing spectrometer directly from the signal generated by the molecular vibration of interest, without the need for auxiliary phasing measurements. For isolated vibrations, the phase profile of the 2D response smoothly varies between fixed phase limits, allowing for a general target for phasing independent of the degree of anharmonicity exhibited between the ground and excited state. As a proof of principle, the 2D response of the ∼2155 cm^-1 thiocyanate stretch vibration of MeSCN, a system exhibiting anharmonicity such that the 0-1 and 1-2 transitions are spectrally isolated, is successfuly phased directly from the experimental spectra. The methodology is also applied to correctly phase extremely weak signals of the unnatural amino acid azidohomoalanine following background subtraction.