Phasing problem of heterodyne-detected two-dimensional infrared spectroscopy

A rigorous method is presented to measure and adjust the phase difference between the two pulse pairs used in heterodyne-detected 2D-IR spectroscopy with an accuracy better than |Deltaphi|=0.1 rad. The method, which can easily be automated, avoids the otherwise tedious measurement of the much weaker pump-probe spectrum as a reference, which is the commonly used approach to phase 2D-IR spectra in a postprocessing step.