A Revisit on Impulsive Stimulated Raman Spectroscopy: Importance of Spectral Dispersion of Chirped Broadband Probe

Abstract

The usefulness of a chirped broadband probe and spectral dispersion to obtain Raman spectra under nonresonant/resonant impulsive excitation is revisited. A general methodology is presented that inherently takes care of phasing the time-domain low-frequency oscillations without probe pulse compression and retrieves the absolute phase of the oscillations. As test beds, neat solvents (CCl4, CHCl3, and CH2Cl2) are used. Observation of periodic intensity modulation along detection wavelengths for particular modes is explained using a simple electric field interaction picture. This method is extended to diatomic molecule (iodine) and polyatomic molecules (Nile blue and methylene blue) to assign vibrational frequencies in ground/excited electronic state that are supported by density functional theory calculations. A comparison between frequency-domain and time-domain counterparts, i.e., stimulated Raman scattering and impulsive stimulated Raman scattering using degenerate pump–probe pairs is presented, and most importantly, it is shown how impulsive stimulated Raman scattering using chirped broadband probe retains unique advantages offered by both.

Separating Vibrational Coherences in Ground/Excited Electronic States of Solvent/Solute Following Non‐Resonant/Resonant Impulsive Excitation

“…A detailed discussion of the set‐up is provided in the Supporting information (hereafter SI) Section SI‐I. A systematic methodology to obtain Raman spectra from such transient absorption maps (for neat solvents, for iodine in CCl 4 under partially resonant excitation centered on 650 nm and for polyatomic molecules) is presented in detail elsewhere [31] . Briefly, the data processing involves picking the maxima/minima of the signal and skipping the data points corresponding to 100 femtoseconds to avoid contributions from the ‘coherent artifact’.…”

“…Using iodine in carbon tetrachloride (CCl 4 ), in this article we show how ultrafast vibrational wavepacket dynamics in condensed phase, arising from solvent versus solute as well as from ground versus excited electronic states of the solute, can be disentangled. We show how spectral dispersion of chirped broadband probe [31] manifests vibrational features specific to vibrational modes of the solvent (periodic amplitude patterns along detection wavelength) or the solute (nodal line around absorption maximum) as well as furnishes information on vibrational dephasing timescales in different electronic states. More specifically, we demonstrate how vibrational spectra of different origin are obtained as islands in the contour plot of Raman shift at different detection wavelengths and a sum over intensities for a select island yields spectrum of a particular vibrational mode of specific species (solute or solvent) and origin (ground or excited electronic state), thereby preserves true lineshapes of vibrational modes that are spectrally overlapping, and extricates their individual contributions.…”

“…In broadband ISRS, [17–33] the third electric field ($${{E}_{3}}$$ ) interaction involves a range of frequencies in broadband ISRS. Thus, several virtual levels are simultaneously accessed which may have different strength of coupling, leading to a variation of intensities of the signal with detection wavelengths which is manifested as islands in the contour plot of Raman shift at different detection wavelengths [23,31] . Broadband ISRS has been put to use to map the dynamics, [30] and quite recently the absolute geometries of molecules [33] in the electronic excited states.…”

“…Introduction of broadband (white-light) probe and using an array detector following spectral dispersion offers certain advantages in broadband ISRS over spontaneous Raman scattering and SRS as well as degenerate ISRS. [16] In broadband ISRS, [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] the third electric field (E 3 ) interaction involves a range of frequencies in broadband ISRS. Thus, several virtual levels are simultaneously accessed which may have different strength of coupling, leading to a variation of intensities of the signal with detection wavelengths which is manifested as islands in the contour plot of Raman shift at different detection wavelengths.…”

Absolute excited state molecular geometries revealed by resonance Raman signals

“…To evaluate the ISRS dependence on the PP chirp 52,55,56,66,67 , the probe spectral field can be expressed as E P ðωÞ = E 0 P ðωÞe iC 2 ωÀω P ð Þ…”