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.
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