The interplay of nonlinear multi-photon processes and vibrational population effect on vibration-modulated fluorescence

Vibrationally promoted electronic resonance offers a powerful strategy for modulating the electronic properties of molecular systems through vibrational-electronic (vibronic) coupling. A detailed understanding of the underlying coupling dynamics is essential for elucidating excited-state relaxation pathways. In this study, we investigate the influence of vibrational excitation on the electronic response of coumarin 6 using mode-selective mid-infrared (IR) pre-excitation fluorescence spectroscopy, complemented by IR/visible (IR/Vis) and IR/IR transient absorption techniques. Significant enhancements in both absorption and fluorescence are observed, particularly on the red edge of the electronic transition. Two distinct mechanisms contribute to these enhancements: (1) a nonlinear multi-photon absorption process, which dominates at low visible photon energies and when IR and visible pulses are temporally overlapped; and (2) a vibrationally excited population effect, which prevails at shorter visible wavelengths and persists over picosecond timescales, strongly influencing electronic excitation efficiency. Notably, the maximum enhancement in visible photon absorption arising from the vibrational population effect is orders of magnitude greater than that produced by nonlinear optical contributions. The interplay of these mechanisms yields several key observations: (1) a delay-dependent peak shift in the visible absorption enhancement; (2) a maximum fluorescence enhancement of approximately 10 times at a visible excitation wavelength of 515 nm; and (3) the appearance of two temporally distinct enhancement peaks in both absorption and fluorescence upon IR excitation at 1620 cm^-1. The close correlation between IR-induced changes in visible absorption and fluorescence indicates that fluorescence can serve as a sensitive proxy for transient absorption dynamics. This work provides fundamental insight into vibrationally mediated modulation of electronic transitions and demonstrates the potential for controlling fluorescence through dual-mode excitation. These findings advance our understanding of vibronic coupling dynamics and open new avenues for applications in molecular sensing, photochemical control, and bond-selective fluorescence imaging.