Development and application of laser methods for electronic spectroscopy of radicals

Abstract

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.