Facile Background Discrimination in Femtosecond Stimulated Raman Spectroscopy Using a Dual-Frequency Raman Pump Technique

Abstract

Femtosecond stimulated Raman spectroscopy (FSRS) is a useful technique for uncovering chemical reaction dynamics by acquiring high-resolution Raman spectra with ultrafast time resolution. However, in FSRS, it can be challenging to discern Raman features from signals arising from transient absorption and other four-wave mixing pathways. To overcome this difficulty, we combine the principles of shifted excitation Raman difference spectroscopy with a simple fixed frequency comb to perform dualfrequency Raman pump FSRS. Through the addition of only two mirrors and a slit to the standard FSRS setup, this method provides Raman spectra at two different excitation wavelengths that can be processed by an automated algorithm to reconstruct the Raman spectrum. Here, we demonstrate the utility of dual-frequency Raman pump FSRS to easily identify Raman signatures by visual inspection for excited-state and ground-state spectra, both on-and off-resonance. We show that previously assigned short-lived vibrations of photoexcited β-carotene are actually not vibrational in nature. We also use crystalline betaine-30 as a challenging test case for this method, as the FSRS spectra contain a number of narrow transient vibronic and non-SRS features. By reliably reducing interference from background signals, the interpretation is substantially more quantitative and enables the future study of new systems, particularly those with small Raman cross-sections or solid-state samples with narrow vibronic features.

Tracking Ultrafast Structural Dynamics by Time-Domain Raman Spectroscopy

“…Because femtosecond time-resolved Raman spectra are usually measured under the electronically resonant condition, the Raman signals appear with a broad background that arises from the transient electronic signals (bleaching of transient absorption and/or stimulated emission) induced by the Raman pump pulse. Because the shape of these background signals is neither simple nor predictable, the background subtraction becomes somewhat arbitrary, leaving uncertainty in the obtained spectra, although some attempts at reliable background subtraction have been reported. − Second, because the stimulated Raman signal is automatically heterodyned by the Raman probe pulse in FSRS, the obtained frequency-domain spectrum corresponds to the imaginary part of the relevant third-order non-linear susceptibility (Imχ (3) ). This automatic heterodyning is convenient, but it can also make the bandshape of the Raman signal complicated when the Raman pump and probe pulses are in resonance with electronic transitions: The electronic resonances may be realized not only with the upper electronic excited state but also with the lower electronic ground state, which introduces additional imaginary factors to χ (3) .…”

Femtosecond stimulated Raman spectroscopy – guided library mining leads to efficient singlet fission in rubrene derivatives

“…We measured time-resolved FSRS experiments on our homebuilt optical setup, described elsewhere. 48,[58][59][60][61] In short, we used the fundamental output of 4.6 W at 800 nm from a 1 kHz repetition rate Ti:sapphire regenerative amplier (Coherent model Libra-F-1K-HE-110) to generate the Raman pump, Raman probe and actinic pulses for the FSRS experiments. We focused 2.5 mW of the 800 nm fundamental output through a 2 mm sapphire crystal to generate a white light continuum and then compressed it with a fused silica prism pair to generate the femtosecond broadband Raman probe.…”

Multidimensional Harmonic Generation Determines Halide Perovskite Crystal Symmetry: Disentangling Second Harmonic Generation from Multiphoton Photoluminescence

“…Note that our methodology is similar to the shifted excitation difference technique used in the resonance and stimulated Raman community to remove fluorescence interference. 49,50 Our main spectroscopic results are shown in Figure 3, in which we measure the output intensity for six different samples as a function of both excitation OPA frequency, ω, and emission frequency, ω m . The most consistent and prominent feature in all six spectra (right hand side) has an excitation/emission frequency dependence of ω m = 3 ω.…”