|
Description:
|
The application of electronic -resonance enhanced (ERE ) coherent anti -Stokes
Raman scattering (CARS ) for the detection of nitric oxide (NO ) and acetylene (C2H2 ) is
experimentally demonstrated and the effects of various parameters on the ERE CARS
signal investigated . In addition , the detection of dipicolinic acid (DPA ) using â  normalâ Â
CARS is demonstrated .
For NO detection , the frequency difference between a visible Raman pump beam
and Stokes beam is tuned to a vibrational Q -branch Raman resonance of the No
molecule to create a Raman polarization in the medium . The second pump beam is tuned
into resonance with the rotational transitions in the (1 ,0 ) band of the A2à £+ -X2à Â
electronic transition at 236 nm , and the CARS signal is thus resonant with transitions in
the (0 ,0 ) band . A NO gas cell was used for the experiment to detect NO at various
pressure levels . A significant resonant enhancement of the NO CARS signal was
observed and good agreement between calculated and experimental data was obtained .
For C2H2 detection , ERE CARS experiments were performed in a roomtemperature
gas cell using mixtures of 5000 ppm C2H2 in N2 . Visible pump and Stokes beams were used , with the frequency difference between the pump and Stokes tuned to
the 1974 cm -1 Ã Â 2 Raman transition of C2H2 . An ultraviolet probe beam with the
wavelengths ranging from 232 nm to 242 nm is scattered from the induced Raman
polarization to generate the ERE CARS signal . The effects of probe wavelength and
pressure on signal generation are discussed .
CARS was used to detect the 998 cm -1 vibrational Raman transition from a
sample of polycrystalline DPA . The transition is the breathing ring vibration in the
pyridine ring structure in the DPA molecule . The DPA 998 cm -1 transition is detected
with excellent signal -to -noise ratio and the full -width -at -half -maximum is very narrow ,
approximately 4 cm -1 . |