Genuine Dynamics vs Cross Phase Modulation Artifacts in Femtosecond Stimulated Raman Spectroscopy

Abstract

Femtosecond stimulated Raman scattering is a time-resolved vibrational spectroscopic technique able to access sub-picosecond dynamics and providing accurate structural information. Thanks to an appropriate combination of three laser pulses, triggering vibrational coherences delayed with respect to the photoinduced event of interest, it is capable of uncompromised temporal precision (down to 50 fs) and spectral resolution (a few wavenumbers), better than spontaneous Raman. Reaching such extreme time scales requires significant temporal overlap of pulses, which gives rise to undesired nonlinear artifacts, often hampering an immediate interpretation of the Raman spectra. Building on a perturbative expansion of the density matrix, we identify the origin of such artifacts in cross phase modulation effects and show how they can be theoretically evaluated and factored out from the signals. We experimentally benchmark the theoretical predictions in a nonreactive model system, namely cyclohexane.

Two-Dimensional Impulsively Stimulated Resonant Raman Spectroscopy of Molecular Excited States

“…Briefly, for each time point along T 2 , we record three wavelength-resolved transient absorption maps as a function of T 2 , corresponding to the Ramanpulse-induced differential absorbance in the presence and absence of the actinic pulse as well as the actinic-pulseinduced differential absorbance in the absence of the Raman pulse. For all three transient absorption maps, we discard time delays prior to T 2 ¼ 100 fs, due to cross-phase modulation [75,76], and describe the electronic background by a global fit consisting of a sum of three exponential decay functions to isolate the underlying vibrational coherence. For each probe wavelength, we subsequently construct the two-dimensional signal SðT 1 ; T 2 Þ and apply a 2D mono-exponentially modified Gaussian window function [23-fs rise time (sigma), 1526-fs decay constant in T 2 , and 6-fs rise time (sigma), 422-fs decay constant in T 1 ] prior to zero padding and 2D fast Fourier transformation.…”

Broadband Impulsive Stimulated Raman Scattering Based on a Chirped Detection

“…Sample PP g e g' (3) S A (3) S B RP Impulsive Stimulated Raman Scattering (ISRS) is a powerful technique able to monitor in time-domain vibrational fingerprints of solid state or molecular compounds using femtosecond broadband pulses. Within the ISRS experimental scheme, two temporally separated laser fields, conventionally referred to as Raman pulse (RP) and probe pulse (PP), are exploited to stimulate and read out vibrational signatures in the system of interest.…”

“…Moreover, at odds with nonlinear Raman approaches in the frequency domain, such as coherent anti-Stokes Raman scattering or femtosecond stimulated Raman scattering, vibrational information retrieved in ISRS is not hampered by background signals generated by the temporal overlap of multiple pulses. 2,3 Adding an ultrashort actinic pump or tuning the Raman pulse in resonance with an electronic transition enables to trigger photochemical processes, which can then be probed with femtosecond time precision and with the structural sensitivity peculiar of Raman spectroscopy. 4,5 During the last decade, the great potential of ISRS has been exploited for studying a broad range of phenomena, including photo-isomerization processes in retinal, 6,7 intermolecular vibrational motions in liquids, 8 excited-state proton transfer in fluorescent protein, 9 nuclear motions in photoreceptor proteins, 10-12 singlet exciton fission in organic semiconductors 13 and polaron formation in hybrid organic-inorganic perovskites.…”

The Potential of EuPRAXIA@SPARC_LAB for Radiation Based Techniques

“…Notably, the probed Raman features are engraved onto the highly directional PP, and, hence, SRS provides an efficient suppression of the incoherent fluorescence background. Moreover, thanks to the different temporal and spectral properties of the pulses, femtosecond SRS represents an ideal tool to study structural changes in ultrafast photophysical and photochemical processes, providing both femtosecond time precision and high spectral resolution [45,[50][51][52]. The narrowband RP can be generated from a two-stage optical parametric amplifier that produces tunable infrared-visible pulses, followed by spectral compression via frequency doubling in a 25 mm beta-barium borate crystal [53].…”