UV Raman Excitation Profiles of Imidazole, Imidazolium, and Water

ination of periods of low magnetic field strength, during which the radial Lorentz force is insufficient to retard the radial diffusion of ions and electrons in the plasma. In addition to increasing analyte line-to-background intensity ratios, the hotter plasma core associated with the unidirectional discharge should be useful in the analysis of refractory solid powder samples. Preliminary studies t4 have indicated that the graphite vapor theta-pinch plasma may be useful for the direct analysis of solid powders. The principal problem associated with the present unidirectional discharge system is the limitation to operation at 6 kV or less. This situation should be corrected by the use of fast-recovery diodes. Alternatively, a discharge circuit based on a distributed-element LC transmission line should be capable of producing unidirectional, nearly square-wave current pulses without the use of a diode shunt. A low-voltage prototype circuit has produced very encouraging results. 1. S. R. Goode and D. T. Pipes, Spectrochim. Acta 36B, 925 (1981). 2. G. J. Kamla and A. Scheeline, Anal. Chem. 58, 923 (1986). 3. G. J. Kamla and A. Scheeline, Anal. Chem. 58, 932 (1986). 4. R. J. Klueppel and J. P. Waiters, Spectrochim. Acta 35B, 431 (1980). 5. V. Majidi and D. M. Coleman, Appl. Spectrosc. 41, 200 (1987 Received 6 June 1987. * Author to whom correspondence should be sent. UV Raman Excitation Profiles of I N T R O D U C T I O N The recent extension of R a m a n spectral measurements into the UV spectral region has resulted in a number of R a m a n excitation profile studies of small molecules such as acetamide, N-methylacetamide, 1,2 acetonitrile, sulfate, nitrate, and perchlorate 3 and larger aromatic molecules such as benzene, 4-7 substituted benezene derivatives, 8'9 imidazole, imidazolium, 1° and aromatic amino acids. 1°-15 Other studies examined nucleic acids, 16-19 proteins, ~,2°-22 and polycyclic aromatic hydrocarbons such as pyrene. 23-2~ The motivations for these studies include establishing the principles of resonance enhancement, ~-~° exploring excited states of these species, and demonstrating the analytical utility of resonance Raman scattering for studying aromatics in complex systems such as aromatic amino acids in proteins and polycyclic aromatic hydrocarbons in coal liquid samples. 1°-~5,23-26 Often resonance R a m a n enhancements of six to seven orders of magnitude are observed with UV excitation, in comparison to results from visible wavelength excitation. The vibrational modes enhanced are generally totally symmetric and distort the molecule along directions of electron density changes between the ground and the resonant electronic excited state. H Preresonance excitation of derivatives such as sulfate, acetone, and ace