Real-time tracking of the molecular structural dynamics of photochemical pathways using vibrational spectroscopy techniques

During a chemical reaction, direct observation of transition state structure merits its place in the Holy Grails of chemistry. The transition state represents a local energy maximum along the reaction coordinate. Upon photoexcitation of molecules, the initial Franck–Condon active state also corresponds to the energy maximum in the excited state potential energy surface, which soon relaxes to the minimum energy configuration. The energy relaxation can occur as fast as a few femtoseconds to picoseconds. Therefore, capturing the molecular structure transiting from the reactant to the product state through the transition state entails ultrafast spectroscopy with bond-structural specificity. The initial structural dynamics after the photoexcitation dictates subsequent photochemical or photophysical phenomena. Hence, comprehending the structural dynamics can provide insights into the photochemical or photophysical processes. This chapter presents a strong background and introduces the principles and methods of the ultrafast vibrational spectroscopy techniques. We also discuss some recent examples, including selective preparation of the Franck–Condon states using ultrafast laser pulses, and elucidate its role in subsequent photophysics or photochemistry using ultrafast vibrational spectroscopy techniques. Finally, we will discuss the future scope and impact of ultrafast spectroscopy techniques in various emerging fields like coherence spectroscopy, and so forth.