Absolute excited state molecular geometries revealed by resonance Raman signals

Abstract

Ultrafast reactions activated by light absorption are governed by multidimensional excited-state (ES) potential energy surfaces (PESs), which describe how the molecular potential varies with the nuclear coordinates. ES PESs ad-hoc displaced with respect to the ground state can drive subtle structural rearrangements, accompanying molecular biological activity and regulating physical/chemical properties. Such displacements are encoded in the Franck-Condon overlap integrals, which in turn determine the resonant Raman response. Conventional spectroscopic approaches only access their absolute value, and hence cannot determine the sense of ES displacements. Here, we introduce a two-color broadband impulsive Raman experimental scheme, to directly measure complex Raman excitation profiles along desired normal modes. The key to achieve this task is in the signal linear dependence on the Frank-Condon overlaps, brought about by non-degenerate resonant probe and off-resonant pump pulses, which ultimately enables time-domain sensitivity to the phase of the stimulated vibrational coherences. Our results provide the tool to determine the magnitude and the sensed direction of ES displacements, unambiguously relating them to the ground state eigenvectors reference frame.

The Impact of Pulse Shaping on Coherent Dynamics near a Conical Intersection

“…Furthermore, the pulse chirp can represent a convenient parameter to tune the beam temporal profile from an experimental point-of-view. By applying ultrashort chirped pulses, it can be found that the timescale of excitation is comparable to that of the electronic dephasing while the following adiabatic process relies on the chirp of the pulse inverting the population on a timescale much shorter than the dephasing time [40][41][42].…”

Continuum vs. atomistic approaches to computational spectroscopy of solvated systems

“…6 In fact, thanks to the high level of accuracy that has been reached in the reproduction of experimental data in the condensed phase, it often provides a theoretical rationalization of many experimental findings, yielding truly synergistic investigations. [7][8][9] In fact, although many algorithms of increasing accuracy have been proposed and tested for systems in the gas phase, [10][11][12] the large majority of molecular spectra are routinely measured in the condensed phase, i.e. when chemical systems are dissolved in a solvent or other kinds of external environments (biological matrix, polymeric materials, crystal phase etc.).…”

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

“…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.…”

“…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. Further, in broadband ISRS, spectral association of coherent oscillations with population kinetics pertaining to ground-state bleach (GSB), stimulated emission (SE) and excited-state absorption (ESA) aids in isolation of signals since the GSB band projects vibrational dynamics in the ground electronic state while SE/ESA band maps dynamics in excited state.…”