In solution, the environment around the solute is determined solely by the solvent molecules, which are present closer to the solute. This interaction between solute and solvent shell is very crucial for equilibrium structure and reactivity of the solute. In the thesis, first we have investigated control of solvent polarity on the excited structure of 2,2,2 trifluroacetophenone and later effect of electronic excitation on the solvent shell organization has been described. It has been reported in literature that the lowest energy triplet configuration of 2,2,2 trifluroacetophenone corresponds to n,π* state. There are some other reports in favor of the probable existence of 2,2,2 trifluroacetophenone in n,π* lowest triplet state. Thus, transient absorption and time resolved resonance Raman spectroscopic methods have been used along with theoretical calculations to investigate the discrepancy in the assignment of the lowest triplet state configuration It has been observed that the lowest triplet state of 2,2,2 trifluroacetophenone is indeed nπ* and there is a solvent polarity induced change in triplet state energy ordering and structure changes. The absorption spectrum of hydrated electron is broad and structureless. So, it was though that the broadening feature could be attributed to homogeneous and inhomogeneous broadening. Transient resonance Raman spectrum of the water bending mode in presence of hydrated electron has been recorded at different excitation wavelengths. Interestingly, it has been observed that, peak position of water bending mode in presence of hydrated electron alters with change of excitation wavelength. A model has been proposed based on the experimental data.

The subject of this thesis is the design and development of a unified set up for femtosecond transient absorption and ultrafast Raman loss spectroscopy and demonstrate its potential in capturing the ultrafast photophysical and photochemical processes with excellent time and frequency resolution. Ultrafast spectroscopy has been serving as a powerful tool for understanding the structural dynamical properties of molecules in the condensed and gas phase. The advent of ultrashort pulses with their high peak power enables the laser spectroscopic community to study molecular reaction dynamics and photophysics that happen at extremely short timescales, ranging from picosecond to femtosecond. These processes can be measured with extremely high time resolution, which helps to resolve the under-lying molecular process. But in order to understand the global mechanism of the underlying molecular processes, we have to resolve the nuclear dynamics with the proper frequency resolution. However, achieving both, time and frequency resolutions simultaneously is not possible according to the Heisenberg uncertainty principle. Later, this limitation was overcome by femtosecond stimulated Raman spectroscopy (FSRS), a third order non-linear Raman spectroscopy. In this thesis we introduced the ultrafast Raman loss spectroscopic (URLS) technique which is analogous to FSRS, offering the modern ultrafast community to resolve molecular processes with better signal-to-noise ratio along with proper time and frequency resolution. We demonstrate the experimental procedure including the single shot detection scheme to measure whitelight background, ground state Ra-man, transient absorption and transient Raman in shot-to-shot detection fashion. URLS has been applied to understand the excited state planarization dynamics of 1,4-bis(phenylethynyl)benzene (BPEB) in different solvents. In addition, excitation wavelength dependent conformational reorganization dynamics of different sub-sets of thermally activated ground state population of BPEB are also discussed. Using the same techniques along with femtosecond transient absorption, we demonstrate the ultrafast vibrational energy transfer and the role of coherent oscillations of low frequency vibrations on the solution phase photo-isomerization of trans-stilbene from an optically excited state. The effects of solvents on the coherent nuclear motion are also discussed in the context of reaction rates. 2