Dynamic Raman Line Shapes on an Evolving Excited-State Landscape: Insights from Tunable Femtosecond Stimulated Raman Spectroscopy

Abstract

Tracking molecular motions in real time remains a formidable challenge in science and engineering fields because the experimental methodology requires simultaneously high spatial and temporal resolutions. Building on early successes and future potential of femtosecond stimulated Raman spectroscopy (FSRS) as a structural dynamics technique, we present a comprehensive study of stimulated Raman line shapes of a photosensitive molecule in solution with tunable Raman pump and probe pulses. Following femtosecond 400 nm electronic excitation, the model photoacid pyranine exhibits dynamic and mode-dependent Raman line shapes when the Raman pump is tuned from the red side toward and across the excited-state absorption (ESA) band (e.g., from S1) with varying resonance conditions. On the anti-Stokes FSRS side, low-frequency modes below ∼1000 cm–1 exhibit a line shape change from gain to dispersive to loss, whereas the dispersive intermediate is much less notable for high-frequency modes. The characteristic mode frequency blue shift involving vibrationally hot states in S1 with time constants of ∼9.6 and 58.6 ps reveals the sensitivity of anti-Stokes FSRS to vibrational cooling and solvation. This work lays the foundation for expanding tunable FSRS technology on both the Stokes and anti-Stokes sides to investigate a variety of photoinduced processes in solution with sufficient resolution to expose functional motions and increased sensitivity to monitor vibrational cooling.

Femtosecond stimulated Raman line shapes: Dependence on resonance conditions of pump and probe pulses

“…The anti-Stokes stimulated Raman can be attributed to an IRS term where probe acts as the excitatory pulse. Unlike spontaneous Raman, the anti-Stokes stimulated Ra-man signal is not necessarily weaker than its Stokes counterpart in spite of the inversely mirrored vibrational modes [39,40,48,55,56]. This is because: (i) R pr initiates the Raman process from the most populated vibrational ground state (v=0) in IRS, (ii) the resonance of both Raman pump and probe pulses are important for the signal strength.…”

“…In both cases, the vibrational coherence is created in the same electronic ground state (S 0 ) and major contribution comes from the ground vibrational state (i.e., quantum number v=0). In contrast to conventional spontaneous Raman wherein the typically weaker anti-Stokes peak intensity is subject to Boltzmann distribution, the anti-Stokes FSRS peak intensity could be larger than the Stokes side when the Raman probe wavelength is closer to an electronic transition peak [39,41]. When Raman pulses overlap with a resonant band, the stimulated Raman line shapes become nontrivial due to additional contributions from the other SRS and IRS terms [47].…”

“…However, dispersive features are often generated near or on resonance conditions, which could obscure the spectral analysis after data collection. This effect is more dramatic on the anti-Stokes side of the FSRS spectrum when compared to the Stokes side [35,38,39]. Therefore, it becomes important to gain a deeper understanding of the molecular origin of dispersive line shapes so they can be interpreted and made use of when necessary to streamline the FSRS data analysis.…”

“…In an electronic excited state, transient vibrational population complicates the Raman line shapes to a greater extent. The resonance with excited-state absorption (ESA) and stimulated emission (SE) bands provides more channels for "dynamic" resonance Raman during the study of a broad range of photoactive systems [20,35,39,[50][51][52][53][54]. Some theoretical work has been performed for the ES Raman line shapes [18,20,46,50].…”

“…By employing different Feynman diagrams, the ES line shape can be explained based on the third-order polarization in Raman scattering after electronic excitation. Our recent work systemat-ically explored the Stokes and anti-Stokes FSRS line shapes on resonance with the ESA band of a photoacid pyranine, 8-hydroxyperene-1,3,6-trisulfonic acid (HPTS) [39]. The Raman pump wavelength was tuned from the red side to the blue side of the ESA band.…”