Spontaneous Raman and coherent anti-Stokes Raman spectroscopy of infrared multiphoton excited molecules

Abstract:

This thesis is a study of infrared multiphoton excitation using spontaneous and coherent anti-Stokes Raman spectroscopy. The spontaneous Raman measurements provide information on the intramolecular vibrational energy distribution over the different modes. This information is complemented by the CARS measurements which make it possible to perform state-specific studies of the vibrational and rotational distribution. For SF6, the time-resolved spontaneous Raman measurements show complete equilibration of energy from the pump mode to other vibrational modes. In contrast, for smaller molecules such as CF2Cl2, a nonthermal energy distribution is observed after excitation. These measurements therefore disprove the general belief that the intramolecular energy distribution in infrared multiphoton molecules is always in equilibrium. The CARS measurements on bulk OCS provide values for the anharmonicities and for the energy transfer rates between modes. In addition the spectra show a very fast relaxation of the vibrational energy within the n2 mode. For SO2, the CARS measurements show that it is the n1 symmetric stretching mode and not the overtone excitation of the n2 bending mode that is pumped by the CO2 laser. Moreover, it is shown that the hot bands of SO2 have been incorrectly assigned up to now. Corrected values for the anharmonicities are given. In the second half of the thesis, a pulsed supersonic molecular beam is added to the infrared multiphoton excitation study. Combined with the state- specific CARS technique, the collisionless and internally cooled molecules in the beam open the door to a more detailed study of the excitation process. Pure rotational CARS is used to study the change in rotational distribution of ethylene due to infrared excitation in the beam. The appearance of rotational holes reveal which rotational states are pumped by the CO2 laser. For OCS the evolution of the overtone population into a thermal distribution is studied, providing a value for the intramode relaxation rate. Finally, the study of SF6 in the supersonic beam sheds new light on the energy distribution in SF6 after infrared multiphoton excitation. It is shown that the two-ensemble population distribution observed by other investigators after infrared multiphoton excitation involves a considerable amount of collisional relaxation.