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Real-time tracking of coherent nuclear motions on ground and excited (electronic) state
potential energy surfaces lies at the heart of femtochemistry and femtobilogy. It is crucial to
identifying the key vibrations leading to physical and chemical transformations, for example,
trans to cis isomerisation in photo-proteins, twisted intra-molecular charge transfer, phase
transitions in crystals, etc. In ‘pump-probe’ experiments employing a pair of pulses (having
pulse-width few tens to few hundreds of femtoseconds), these motions are manifested by
observation of oscillatory signatures arising from molecular vibrations. In the context of
impulsive stimulated Raman scattering (ISRS), such oscillations were interpreted as time-
dependent modulations in the refractive index of the sample by the (usually) non-resonant
Raman pump pulse (shorter than the time period of vibrations to be observed) which are
subsequently interrogated by the time-delayed probe pulse. On the other hand, under resonant
excitation, coherent vibrational motions are exemplified by oscillatory features superposed on
population kinetics (absorptive signal) in transient absorption experiments. Thus, although
discussed contextually, the vibrational coherences emerging under non-resonant and resonant
excitations are nothing but two sides of the same coin. Despite the huge amount of work over
three decades on this topic, unambiguous assignment of origin of such vibrations remains a
challenge till date. In this presentation, I will discuss how broadband impulsive stimulated
Raman spectroscopy (broadband-ISRS) can be employed to track time evolution of
vibrational coherences in ground as well as excited states, distinctly, under non-
resonant/resonant impulsive excitation.
Reference: Shaina Dhamija § , Garima Bhutani § , Ajay Jayachandran and Arijit K. De, A Revisit
on Impulsive Stimulated Raman Spectroscopy: Importance of Spectral Dispersion of Chirped
Broadband Probe, The Journal of Physical Chemistry A, 1019-1032, 126 (7), 2022. ( §
Equally contributed to this work.) https://doi.org/10.1021/acs.jpca.1c10566 |