Vinu V. Namboodiri

• Laser spectroscopy in the frequency domain has, very early, developed into a powerful tool for the analysis of structural properties of molecules. The development of ultrafast lasers added a new dimension to conventional spectroscopy by rendering time resolved measurements possible. The high time resolution offered by picosecond and femtosecond laser pulses enabled the real time observation of extremely fast processes, such as vibrations and rotations of molecules. With pulses of duration about 100 fs, it is now possible to monitor processes such as internal conversion, vibrational relaxation, and many other processes occurring in the excited electronic states which leads to reactive or energy transfer pathways. The work presented in this thesis focuses on the study of molecular dynamics in these excited electronic states using timeresolved four-wave mixing (FWM) techniques. It is demonstrated that, by combining the FWM process with an excitation pulse, it is possible to study molecular dynamics in the excited states of gaseous and condensed phase samples. The advantages of the four-wave mixing technique over the commonly used time-resolved fluorescence and pump-probe techniques are also discussed. The pump-FWM method is applied to simple molecules in the gas phase as well as to complex molecular systems where internal conversion processes dominate the ultrafast dynamics. The thesis also presents preliminary studies on the effect of the surface enhancement effect of the nonlinear optical process coherent anti-Stokes Raman scattering (CARS) in presence of colloidal metal particles.