Role of Pulse Phase and Direction in Two-Dimensional Optical Spectroscopy

Abstract

This paper examines the parallels between magnetic resonance and optical spectroscopy, with the goal of determining to what extent the benefits of 2DNMR might be extended into the optical regime. Precise optical analogues of the simplest 2DNMR sequences (collinear pulse sequences with phased laser pulse generation, phase sensitive detection, and phase cycling) are now feasible, and we demonstrate that they do generate cross-peaks which reveal common energy levels, even when averaged over the distribution of pulse flip angles expected in most optical experiments. One enormous difference between laser and NMR experiments the use of pulses in different directions in optics can be exploited to eliminate much of the phase cycling required in NMR. Phase control does permit rotating-frame detection, which is likely to be a substantial practical advantage. Finally, we point out optical analogues of the simplest 2DNMR sequences (COSY and NOESY) will likely add little to our understanding of ultrafast dynamics. Optical analogues of more complex 2D sequences, combining phase control for selective refocusing with noncollinear pulse generation for coherence pathway selection, show more promise.