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Coherent anti-Stokes Raman spectroscopy of gases
Volume 18, n u m b e r 1
()I"IICS COMMUNICATIONS
~t,i)' I97t,
References [ 1 ] D. Heiman, R. Hellwarth, M.D. Levenson and Graham Martin, Phys. Rev. Lett. 36 (19"76) 189 12l M. Levenson and N. Bloembergen, Phys. Rev. BI0 (1974) 4 4 4 7 - 4 4 6 3 .
Q2 Q,
V:O
10 cm"1 L----J
Q~
03
C O H E R E N T ANTI-STOKES RAMAN SPECTROSCOPY OF GASES J.W. N1BLER *, J.R. McDONALD and A.B. ftARVEY
.-2 _=
Naval Research Laboratory, Code 6110, Washington, D.C. 20375, USA Although the generation of coherent anti-Stokes R a m a n emission has been k n o w n for several years, the application of the technique, which is called Coherent Anti-Stokes R a m a n Spectroscopy or CARS, to general spectroscopy is relatively new. Part of the problem with the development of the technique is the lack of convenient, high power, narrow linewidth, tunable. laser sources and the costs of the lasers. In our laboratory we have examined several dye laser configurations, including a grating/telescope, etalon/grating/telescope, echelle grating/ telescope and pressure tuned systems, etc. Conclusions regarding these configurations for CARS applications will be discussed. Experimentally, our apparatus for CARS in part consists of a high power (5 MW at 532 nm) Q-switched Nd: YAG laser operating single m o d e on its second harmonic, The laser has a linewidth of ~ 0 . 0 5 cm - 1 , a pulse width of ~ 20 ns, and a m a x i m u m repetition rate of 10 pps. A portion of the YAG laser ( ~ 0 . 8 MW) is used to p u m p a dye laser which is either pressure (etalon/grating) tuned or angle tuned with only grating or echelle and an "inch w o r m " driver. A n o t h e r portion of the YAG laser ( ~ 1.5 MW) p u m p s a dye laser amplifier. The resulting dye laser beam (0.6 MW) and remainder of the YAG ( ~ 2 . 5 MW) are combined and focused ( f = 10 cm) in a sample gas cell, CARS signals ( 2 ~ 1 = w2 = 033, where e l , ~ 2 , and t~ 3, are the YAG laser frequency, dye laser frequency, and CARS frequency, respectively) generated in the sample cell are separated by a beam splitter which reflects the CARS beam but transmits the two exciting laser beams, which may then be used to create anti-Stokes signals in a reference gas. The reference signal m a y be ratioed against the sample signal to minimize the jitter in the CARS signal: Additional filters are used to further reduce stray light. With regard to low level detection, in some early e x p e r i m e n t s we detected signals from D 2 below 100 m t o r r total pressure (without reference cell and with rather broad laser line w i d t h s - 0 . 1 5 to 0.3 c m - 1 ) . With refinements, the lower limit of detection for species like D 2 should be on the order of 1 mtorr. By scanning the dye laser, it is possible to record spectra continuously over at least 100 c m - 1 . Sample spectra of gases * On sabbatical leave from Oregon State University, Dept. of Chemistry, Corvallis, Oregon 97331, USA.
134
Od
V--1
1
I
O2
c
.<
~d
Q5
r~ Dye Laser Wavelength
. . . .
Fig. 1. CARS spectrum of discharged gas (1) 2, 48 torr). are u I and v 3 of methane, and the Q(J) structure of D 2. In the case of D 2 (at 48 torr), CARS spectra were recorded from the center of a DC electrical discharge region (see fig. 1). The Q-branch structure o f both V = 0 and V = 1 were readily recorded with little interference from the intense emission of the discharge. The relative intensities of the Q (J) lines indicated a rotational/translational temperature near ambient, whereas the vibrational temperature, as measured from the relative intensities of V = 0 and V = 1, is estimated to be slightly above 1000 K. Our m o s t recent experimental results will be presented and discussed, including the use o f CARS as a diagnostic to detect and record spectra of excited state and transient species in plasmas, flames and during photolysis experiments.
04
THE IRREDUCIBLE SPHERICAL T E N S O R TECHNIQUE, APPLIED TO N O N L I N E A R PROCESSES IN GASES: APPLICATION TO C O H E R E N T ANTISTOKES R A M A N SCATTERING (CARS) M.A. YURATICH and D.C. ttANNA
Department of Electronics, University of Southampton, Southampton S09 5NH, UK The calculation of nonlinear susceptibilities for atoms and molecules is of great interest b o t h for device applications (e.g. harmonic generation and frequency conversion) and for spectroscopy. An example of the latter is the recent interest in CARS, which has experimental advantages over R a m a n Scattering. These calculations can be carried out using cartesian tensors alone, b u t for gases, where individual molecules are considered and then orientation averaged, such a procedure would be
()I"IICS COMMUNICATIONS
~t,i)' I97t,
References [ 1 ] D. Heiman, R. Hellwarth, M.D. Levenson and Graham Martin, Phys. Rev. Lett. 36 (19"76) 189 12l M. Levenson and N. Bloembergen, Phys. Rev. BI0 (1974) 4 4 4 7 - 4 4 6 3 .
Q2 Q,
V:O
10 cm"1 L----J
Q~
03
C O H E R E N T ANTI-STOKES RAMAN SPECTROSCOPY OF GASES J.W. N1BLER *, J.R. McDONALD and A.B. ftARVEY
.-2 _=
Naval Research Laboratory, Code 6110, Washington, D.C. 20375, USA Although the generation of coherent anti-Stokes R a m a n emission has been k n o w n for several years, the application of the technique, which is called Coherent Anti-Stokes R a m a n Spectroscopy or CARS, to general spectroscopy is relatively new. Part of the problem with the development of the technique is the lack of convenient, high power, narrow linewidth, tunable. laser sources and the costs of the lasers. In our laboratory we have examined several dye laser configurations, including a grating/telescope, etalon/grating/telescope, echelle grating/ telescope and pressure tuned systems, etc. Conclusions regarding these configurations for CARS applications will be discussed. Experimentally, our apparatus for CARS in part consists of a high power (5 MW at 532 nm) Q-switched Nd: YAG laser operating single m o d e on its second harmonic, The laser has a linewidth of ~ 0 . 0 5 cm - 1 , a pulse width of ~ 20 ns, and a m a x i m u m repetition rate of 10 pps. A portion of the YAG laser ( ~ 0 . 8 MW) is used to p u m p a dye laser which is either pressure (etalon/grating) tuned or angle tuned with only grating or echelle and an "inch w o r m " driver. A n o t h e r portion of the YAG laser ( ~ 1.5 MW) p u m p s a dye laser amplifier. The resulting dye laser beam (0.6 MW) and remainder of the YAG ( ~ 2 . 5 MW) are combined and focused ( f = 10 cm) in a sample gas cell, CARS signals ( 2 ~ 1 = w2 = 033, where e l , ~ 2 , and t~ 3, are the YAG laser frequency, dye laser frequency, and CARS frequency, respectively) generated in the sample cell are separated by a beam splitter which reflects the CARS beam but transmits the two exciting laser beams, which may then be used to create anti-Stokes signals in a reference gas. The reference signal m a y be ratioed against the sample signal to minimize the jitter in the CARS signal: Additional filters are used to further reduce stray light. With regard to low level detection, in some early e x p e r i m e n t s we detected signals from D 2 below 100 m t o r r total pressure (without reference cell and with rather broad laser line w i d t h s - 0 . 1 5 to 0.3 c m - 1 ) . With refinements, the lower limit of detection for species like D 2 should be on the order of 1 mtorr. By scanning the dye laser, it is possible to record spectra continuously over at least 100 c m - 1 . Sample spectra of gases * On sabbatical leave from Oregon State University, Dept. of Chemistry, Corvallis, Oregon 97331, USA.
134
Od
V--1
1
I
O2
c
.<
~d
Q5
r~ Dye Laser Wavelength
. . . .
Fig. 1. CARS spectrum of discharged gas (1) 2, 48 torr). are u I and v 3 of methane, and the Q(J) structure of D 2. In the case of D 2 (at 48 torr), CARS spectra were recorded from the center of a DC electrical discharge region (see fig. 1). The Q-branch structure o f both V = 0 and V = 1 were readily recorded with little interference from the intense emission of the discharge. The relative intensities of the Q (J) lines indicated a rotational/translational temperature near ambient, whereas the vibrational temperature, as measured from the relative intensities of V = 0 and V = 1, is estimated to be slightly above 1000 K. Our m o s t recent experimental results will be presented and discussed, including the use o f CARS as a diagnostic to detect and record spectra of excited state and transient species in plasmas, flames and during photolysis experiments.
04
THE IRREDUCIBLE SPHERICAL T E N S O R TECHNIQUE, APPLIED TO N O N L I N E A R PROCESSES IN GASES: APPLICATION TO C O H E R E N T ANTISTOKES R A M A N SCATTERING (CARS) M.A. YURATICH and D.C. ttANNA
Department of Electronics, University of Southampton, Southampton S09 5NH, UK The calculation of nonlinear susceptibilities for atoms and molecules is of great interest b o t h for device applications (e.g. harmonic generation and frequency conversion) and for spectroscopy. An example of the latter is the recent interest in CARS, which has experimental advantages over R a m a n Scattering. These calculations can be carried out using cartesian tensors alone, b u t for gases, where individual molecules are considered and then orientation averaged, such a procedure would be