Analytical Seminar (Candidacy Examination)
Kaho Kwok
April 16, 2004
Preliminary Studies on UV Raman Spectrometry with Acousto-Optic Tunable Filters
Raman spectroscopy has long been recognized as a versatile tool in the studies of
molecular structures because of its many advantages. For instance, Raman often requires
minimal sample preparation, and water can be used as solvent without causing significant
interference. These advantages are especially important for the study of biological
samples. During the past decade, acousto-optic tunable filters (AOTF) have been applied
as wavelength selective devices in Raman instrumentation1,2,3,4. These studies utilized
visible lasers. However, applications of UV-AOTF Raman have not been demonstrated.
Possible reasons include lack of appropriate UV lasers and AOTFs in the past. Recent
successes with a quartz AOTF having an output wavelength down to 200 nm5, and the
availability of compact deep UV lasers make this goal possible. The use of a compact
pulsed deep UV laser (224.3 nm) with a quartz AOTF for Raman spectrometry is
proposed. This spectrometer has several potential advantages. First, the technique should
have high sensitivity because Raman intensity is directly proportional to the fourth power
of the excitation frequency. Second, high spectral resolution will be obtained because the
spectral bandpass of an AOTF is directly proportional to the square of the excitation
wavelength. Third, a larger wavenumber region can be covered with the same wavelength
shift. Finally, power consumption of the AOTF can be reduced by triggering the AOTF
with the pulsed laser. Preliminary experiments with a Nd:YAG laser (532 nm) to
demonstrate aspects of this research will be shown along with examinations with the UV
laser.
References:
1)
E. N. Lewis, P. J. Treado, and I. W. Levin, Appl. Spectrosc. 47, 539 (1993).
2)
N. Gupta, and N. F. Fell Jr., Talanta 45, 279 (1997).
3)
N. Gupta, and R. Dahmani, Spectrochim. Acta A 56, 1453 (2000).
4)
B. M. Cullum, J. Mobley, Z. Chi, D. L. Stokes, G. H. Miller, and T. Vo-Dinh,
Rev. Sci. Instrum. 71, 1602 (2000).
5)
S. R. Gillespie, and J. W. Carnahan, Appl. Spectrosc. 55, 730 (2001).