Development and application of laser methods for electronic spectroscopy of radicals

In the past few decades, progress has been made in the electronic spectroscopy of unsaturated
carbon-chain radicals both in gas phase. Highly-unsaturated carbon chains have
been of interest as reactive intermediates in interstellar hydrocarbon chemistry, combustion,
and discharge processes. High-resolution experimental techniques such as cavity
ring-down spectroscopy (CRDS) allow the detection of rotationally resolved electronic
absorption spectra of hydrocarbon radicals. By using a single-mode continuous wave
(cw) laser combined with a pulsed slit nozzle discharge, such high resolution electronic
transitions of linear and nonlinear carbon chains have been measured. This method led
to the finding of a hydrocarbon cation CCCCH+
3 , accidentally. The rotationally resolved
spectrum of the cation thus obtained and its analysis has been reported in the second
chapter.
There are the two essential elements of gas phase electronic spectroscopy,
• A probe beam.
cw lasers or Fourier-Transform (FT)-limited pulsed lasers provide narrow bandwidth
laser beam. Laser sources in the infrared to vacuum ultraviolet range have
found many applications in atomic and molecular physics and physical chemistry.
The near and far UV region is of special interest for the detection of electronic transitions
of various radicals. Moreover, it is always desirable to have a single laser spanning a long wavelength range than switching to different lasers for different
wavelength regions. A cw laser in Ti:Sapphire configuration has a limited operating
range from mid visible to IR region of the spectrum. This demands the application
of non-linear optical processes such as sum- and difference- frequency mixing and
second-harmonic generation for frequency extension. Such processes need higher
peak power than provided by cw lasers. In addition, high resolution spectroscopy is
only possible with narrow-bandwidth lasers. The best bet, which meets the opposing
requirements is with high peak power. This motivated to build a near FT-limited
pulsed amplification system with a cw ring laser. The third chapter gives a description
of this work.
• Source of the radicals/molecules to be studied
Discharge plasmas and laser ablation are commonly used to produce transient species.
These are then probed by the laser to obtain the spectrum. The fourth chapter illustrates
the building and demonstration of a laser ablation source which was accomplished
with the aim of producing clusters.
There are different methods employed to carry out the gas phase electronic spectroscopy,
each with their own specific advantages and disadvantages. Four wave mixing (FWM)
and CRD are two such methods. The fifth chapter presents the observation of a cation,
first time in the history by FWM. It, then, describes the versatility of the technique proved
by the study of ground state vibrations of HC2S radical.