Multimode vibrational dynamics and orientational effects in fluorescence-encoded infrared spectroscopy. II. Analysis of early-time signals.

Developing fluorescence-encoded infrared (FEIR) vibrational spectroscopy for single-molecule applications requires a detailed understanding of how the molecular response and external experimental parameters manifest in the detected signals. In Paper I [L. Whaley-Mayda, A. Guha, and A. Tokmakoff, J. Chem. Phys. 159, 194201 (2023)] we introduced a nonlinear response function theory to describe vibrational dynamics, vibronic coupling, and transition dipole orientation in FEIR experiments with ultrashort pulses. In this second paper, we apply the theory to investigate the role of intermode vibrational coherence, the orientation of vibrational and electronic transition dipoles, and the effects of finite pulse durations in experimental measurements. We focus on measurements at early encoding delays-where signal sizes are largest and therefore of most value for single-molecule experiments, but where many of these phenomena are most pronounced and can complicate the appearance of data. We compare experiments on coumarin dyes with finite-pulse response function simulations to explain the time-dependent behavior of FEIR spectra. The role of the orientational response is explored by analyzing polarization-dependent experiments and their ability to resolve relative dipole angles in the molecular frame. This work serves to demonstrate the molecular information content of FEIR experiments, and develop insight and guidelines for their interpretation.