Resonance Raman Spectroscopy in the Ultraviolet Using a Tunable Laser
Document ID: 241
Doctoral Dissertation
The Pennsylvania State University
The Graduate School
Department of Electrical Engineering
Abstract
The characterization of minor atmospheric constituents, such as pollutants, bio-hazards, and other rogue materials has been a difficult task for the research community. Raman scattering techniques have been employed to detect and identify materials in both a laboratory and field settings; however, Raman scattering processes have an extremely small scattering cross section; however, which limits its utility. Through the use of the resonance enhancement it is possible to extend the utility of the Raman scattering technique.
This dissertation presents a novel Raman scattering experiment that was designed and developed in collaboration with the research group of Professor Hans Hallen and performed at The North Carolina State University Optics Lab. Measurements of resonance-enhanced Raman scattering were made on multiple samples. In particular, this dissertation reports an investigation of the phenomenon of resonance-enhanced Raman scattering in two aromatic hydrocarbons: benzene and toluene.
Algorithms were developed for the analysis of the Raman data that included removing the noise background and normalizing data to remove fluctuations in laser power and detector efficiencies. An algorithm for calculating the resonance gain was developed and applied to both benzene and toluene. The resonance-enhanced Raman scatter of benzene and toluene was found to be 100-10,000× larger than their normal Raman scattering spectrum when the molecules are excited by radiation at peaks in their respective electronic resonance excitation bands. Further analysis of the resonant Raman spectra of benzene was performed, and the Raman scattered lines observed were assigned to the modes of vibration. The resultant spectra are in good agreement with the earlier work performed by Zeigler and Hudson[1981] and Asher [1984].
The original goal of the investigation of the resonance enhancement was to seek seek improved capability for lidar based on the experiments. It can be argued that the absorption of energy in the energetic double bonds of carbon which form the benzene ring can be used for resonance Raman enhanced scatter, and future investigations should result in demonstrations of our ability to remotely detect a number of interesting species.
Several achievements from my research described in this dissertation include:
- A new experiment has investigated resonance-enhanced Raman scattering for the first time using small steps in excitation wavelength [~ 0.12 nm].
- Excitation at wavelengths in the ultraviolet spectrum resulting in resonance-enhanced Raman scattering in both benzene and toluene occur over extremely narrow wavelength bands.
- A high resolution (fine-tuning) of the ultraviolet excitation has been used to investigate the enhancement, and to isolate the gain region for resonance-Raman scattering in benzene.
- The peak resonance enhancement is found to correlate with the vapor phase absorption peaks of both benzene and toluene, when the experiments were performed in liquid phase.
- The resonance Raman spectra of benzene has been analyzed and vibrational modes associated with the scattering have been assigned.
- An algorithm is described for use in the analysis of the Raman spectra, including noise removal and normalization corrections for source and detector wavelength dependence, and it is applied in analyzing the data collected.
- An algorithm was developed for the calculation of the resonance enhancement to determine the gain factor for the resonance Raman scattered signals.
Keywords: resonance Raman, ultraviolet, Raman scatter
| Citation: | A. H. Willitsford, "Resonance Raman Spectroscopy in the Ultraviolet Using a Tunable Laser", The Pennsylvania State University, Doctoral Dissertation, August 2008, 201 pages |