Abstract

Frequency-modulation-enhanced magnetic rotation spectroscopy has been shown to be a sensitive and selective technique for detecting local concentrations of gaseous NO2. Detection levels of 20 parts in 109 have been achieved in the laboratory (in the presence of larger ambient atmospheric levels). Simple signal-processing algorithms, demonstrated here, promise detection levels at the parts-in-1012 level. Improvements are suggested for further optimization of the technique, and the effects of background atmospheric NO2 levels are discussed.

© 1995 Optical Society of America

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References

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  1. J. M. Supplee, E. A. Wittaker, and W. Lenth, "Theoretical description of frequency modulation and wavelength modulation spectroscopy," Appl. Opt. 33, 6294–6302 (1994), and references therein; see also U.S. Patent 4,297,035 (October 22, 1981).
    [CrossRef] [PubMed]
  2. A. Righi, "Sur l' absorption de la lumière produite par un corps placé dans un champ magnetique," C. R. Acad. Sci. Paris 127, 216–219 (1898); T. Carroll, "Magnetic rotation spectra of diatomic molecules," Phys. Rev. 52, 882–835 (1937).
    [CrossRef]
  3. G. C. Bjorklund, "Frequency-modulation spectroscopy: a new method for measuring weak absorptions and dispersions," Opt. Lett. 5, 15–17 (1980); G. C. Bjorklund, M. D. Levenson, W. Lenth, and C. Ortiz, "Frequency-modulation (FM) spectroscopy—theory of lineshapes and signal-to-noise analysis," Appl. Phys. B 32, 145–152 (1987).
    [CrossRef] [PubMed]
  4. M. Gehrtz, G. C. Bjorklund, and E. A. Whittaker, "Quantum-limited laser frequency-modulation spectroscopy," J. Opt. Soc. Am. B 2, 1510–1525 (1985).
    [CrossRef]
  5. M. D. Levenson and S. S. Kano, Introduction to Nonlinear Laser Spectroscopy (Academic, San Diego, Calif., 1988), pp. 72–75.
  6. J. A. Silver, D. S. Bomse, and C. A. Stanton, in Optical Methods for Ultrasensitive Detection and Analysis: Techniques and Applications, B. L. Fearey, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1435, 64–71 (1991).
    [CrossRef]
  7. D. E. Cooper and R. E. Warren, "Frequency-modulation spectroscopy with lead-salt diode lasers: a comparison of single-tone and two-tone techniques," Appl. Opt. 26, 3726–3732 (1987); "Two-tone optical heterodyne spectroscopy with diode lasers: theory of line shapes and experimental results," J. Opt. Soc. Am. B 4, 470–480 (1987); D. E. Cooper and C. B. Carlisle, "High-sensitivity FM spectroscopy with a lead-salt diode laser," Opt. Lett. 13, 719–721 (1988).
    [CrossRef] [PubMed]
  8. M. C. McCarthy, J. C. Bloch, and R. W. Field, "Frequency-modulation enhanced magnetic rotation spectroscopy: a sensitive and selective absorption scheme for paramagnetic molecules," J. Chem. Phys. 100, 6331–6346 (1994).
    [CrossRef]
  9. W. B. Grant, R. H. Kagmin, and W. A. McClenny, "Optical remote measurement of toxic gases," J. Air Waste Manage. Assoc. 42, 18–29 (1992).
    [CrossRef] [PubMed]
  10. S. M. Khan, ed., in Proceedings of the First International Symposium on Explosive Detection Technology (Federal Aviation Administration Technical Center, Atlantic City, N.J., 1992).
  11. X. Zhao, E. J. Hintsa, and Y. T. Lee, "Infrared multiphoton of RDX in a molecular beam," J. Chem. Phys. 88, 801–810 (1988).
    [CrossRef]
  12. C. G. Stevens and R. N. Zare, "Rotational analysis of the 5933 Å band of NO2," J. Mol. Spectrosc. 56, 167–187 (1975).
    [CrossRef]
  13. R. E. Smalley, L. Wharton, and D. H. Levy, "The fluorescence excitation spectrum of rotationally cooled NO2," J. Chem. Phys. 63, 4977–4989 (1975).
    [CrossRef]
  14. T. Tanaka, R. W. Field, and D. O. Harris, "Microwave optical double resonance and continuous wave dye laser excitation spectra of NO2: rotational assignment of the K = 0–4 subbands of the 593 nm band," J. Mol. Spectrosc. 56, 188–199 (1975).
    [CrossRef]
  15. G. Liftin, C. R. Pollock, R. F. Curl, Jr., and F. K. Tittel, "Sensitivity enhancement of laser absorption spectroscopy by magnetic rotation effect," J. Chem. Phys. 72, 6602–6605 (1980).
    [CrossRef]
  16. The shot-noise limit (50 mW, modulation depth = 0.83, 1 Hz) is 1.7 × 10−7 fractional absorbance. Typical FM spectroscopies achieve only of the order of 1 × 10−5 because of accidental étalons and residual amplitude modulation. The crossed-polarizer scheme of FM-MRS suppresses this excess noise by a factor (relative to the signal) of 1/θuuncrossing = 20.
  17. The signal strengths of FM, MRS, and FM-MRS are expected to be comparable (see Ref. 8). We allow a signal-reduction factor of 0.1 because neither ωrf nor the magnetic field was optimized.
  18. D. L. Massart, B. G. M. Vandeginste, S. N. Deming, Y. Michotte, and L. Kaufman, Chemometrics: A Textbook. Data Handling in Science and Technology, B. G. M. Vandeginste and L. Kaufman, eds. (Elsevier, Amsterdam, 1988), Vol. 2; M. A. Sharaf, D. L. Illman, B. R. Kowalski, Chemometrics. Monographs on Chemical Analysis, Monograph 82, P. J. Elving and J. D. Winefordner, eds. (Wiley, New York, 1986).
  19. S. C. Bloch, SSP: The Spreadsheet Signal Processor (Prentice-Hall, Englewood Cliffs, N.J., 1992), and references therein.
  20. T. A. Blake, C. Chackerian, Jr., and J. R. Podolske, "Prognosis for a mid-infrared magnetic rotation spectrometer for the in situ detection of atmospheric free radicals," submitted to Appl. Opt.
  21. W. Dillenschnider and R. F. Curl, Jr., "Color center laser spectroscopy of v1 + v2 + v3 of NO2 using magnetic rotation," J. Mol. Spectrosc. 99, 87–97 (1983).
    [CrossRef]
  22. National Research Council Commission on Geosciences, Environment, and Resources, Board on Atmospheric Sciences and Climate, Committee on Tropospheric Ozone Formation and Measurement, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences—National Research Council, Washington, D.C., 1991), pp. 215–221. We thank G. McRae for bringing this reference to our attention.
  23. H. Adams, D. Reinert, P. Kalkert, and W. Urban, "A differential detection scheme for Faraday rotation spectroscopy with a color center laser," Appl. Phys. B 34, 179–185 (1984).
    [CrossRef]

1994

M. C. McCarthy, J. C. Bloch, and R. W. Field, "Frequency-modulation enhanced magnetic rotation spectroscopy: a sensitive and selective absorption scheme for paramagnetic molecules," J. Chem. Phys. 100, 6331–6346 (1994).
[CrossRef]

J. M. Supplee, E. A. Wittaker, and W. Lenth, "Theoretical description of frequency modulation and wavelength modulation spectroscopy," Appl. Opt. 33, 6294–6302 (1994), and references therein; see also U.S. Patent 4,297,035 (October 22, 1981).
[CrossRef] [PubMed]

1992

W. B. Grant, R. H. Kagmin, and W. A. McClenny, "Optical remote measurement of toxic gases," J. Air Waste Manage. Assoc. 42, 18–29 (1992).
[CrossRef] [PubMed]

1988

X. Zhao, E. J. Hintsa, and Y. T. Lee, "Infrared multiphoton of RDX in a molecular beam," J. Chem. Phys. 88, 801–810 (1988).
[CrossRef]

1987

1985

1984

H. Adams, D. Reinert, P. Kalkert, and W. Urban, "A differential detection scheme for Faraday rotation spectroscopy with a color center laser," Appl. Phys. B 34, 179–185 (1984).
[CrossRef]

1983

W. Dillenschnider and R. F. Curl, Jr., "Color center laser spectroscopy of v1 + v2 + v3 of NO2 using magnetic rotation," J. Mol. Spectrosc. 99, 87–97 (1983).
[CrossRef]

1980

1975

C. G. Stevens and R. N. Zare, "Rotational analysis of the 5933 Å band of NO2," J. Mol. Spectrosc. 56, 167–187 (1975).
[CrossRef]

R. E. Smalley, L. Wharton, and D. H. Levy, "The fluorescence excitation spectrum of rotationally cooled NO2," J. Chem. Phys. 63, 4977–4989 (1975).
[CrossRef]

T. Tanaka, R. W. Field, and D. O. Harris, "Microwave optical double resonance and continuous wave dye laser excitation spectra of NO2: rotational assignment of the K = 0–4 subbands of the 593 nm band," J. Mol. Spectrosc. 56, 188–199 (1975).
[CrossRef]

1898

A. Righi, "Sur l' absorption de la lumière produite par un corps placé dans un champ magnetique," C. R. Acad. Sci. Paris 127, 216–219 (1898); T. Carroll, "Magnetic rotation spectra of diatomic molecules," Phys. Rev. 52, 882–835 (1937).
[CrossRef]

Adams, H.

H. Adams, D. Reinert, P. Kalkert, and W. Urban, "A differential detection scheme for Faraday rotation spectroscopy with a color center laser," Appl. Phys. B 34, 179–185 (1984).
[CrossRef]

Bjorklund, G. C.

Blake, T. A.

T. A. Blake, C. Chackerian, Jr., and J. R. Podolske, "Prognosis for a mid-infrared magnetic rotation spectrometer for the in situ detection of atmospheric free radicals," submitted to Appl. Opt.

Bloch, J. C.

M. C. McCarthy, J. C. Bloch, and R. W. Field, "Frequency-modulation enhanced magnetic rotation spectroscopy: a sensitive and selective absorption scheme for paramagnetic molecules," J. Chem. Phys. 100, 6331–6346 (1994).
[CrossRef]

Bloch, S. C.

S. C. Bloch, SSP: The Spreadsheet Signal Processor (Prentice-Hall, Englewood Cliffs, N.J., 1992), and references therein.

Bomse, D. S.

J. A. Silver, D. S. Bomse, and C. A. Stanton, in Optical Methods for Ultrasensitive Detection and Analysis: Techniques and Applications, B. L. Fearey, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1435, 64–71 (1991).
[CrossRef]

Chackerian, C.

T. A. Blake, C. Chackerian, Jr., and J. R. Podolske, "Prognosis for a mid-infrared magnetic rotation spectrometer for the in situ detection of atmospheric free radicals," submitted to Appl. Opt.

Cooper, D. E.

Curl, R. F.

W. Dillenschnider and R. F. Curl, Jr., "Color center laser spectroscopy of v1 + v2 + v3 of NO2 using magnetic rotation," J. Mol. Spectrosc. 99, 87–97 (1983).
[CrossRef]

G. Liftin, C. R. Pollock, R. F. Curl, Jr., and F. K. Tittel, "Sensitivity enhancement of laser absorption spectroscopy by magnetic rotation effect," J. Chem. Phys. 72, 6602–6605 (1980).
[CrossRef]

Deming, S. N.

D. L. Massart, B. G. M. Vandeginste, S. N. Deming, Y. Michotte, and L. Kaufman, Chemometrics: A Textbook. Data Handling in Science and Technology, B. G. M. Vandeginste and L. Kaufman, eds. (Elsevier, Amsterdam, 1988), Vol. 2; M. A. Sharaf, D. L. Illman, B. R. Kowalski, Chemometrics. Monographs on Chemical Analysis, Monograph 82, P. J. Elving and J. D. Winefordner, eds. (Wiley, New York, 1986).

Dillenschnider, W.

W. Dillenschnider and R. F. Curl, Jr., "Color center laser spectroscopy of v1 + v2 + v3 of NO2 using magnetic rotation," J. Mol. Spectrosc. 99, 87–97 (1983).
[CrossRef]

Field, R. W.

M. C. McCarthy, J. C. Bloch, and R. W. Field, "Frequency-modulation enhanced magnetic rotation spectroscopy: a sensitive and selective absorption scheme for paramagnetic molecules," J. Chem. Phys. 100, 6331–6346 (1994).
[CrossRef]

T. Tanaka, R. W. Field, and D. O. Harris, "Microwave optical double resonance and continuous wave dye laser excitation spectra of NO2: rotational assignment of the K = 0–4 subbands of the 593 nm band," J. Mol. Spectrosc. 56, 188–199 (1975).
[CrossRef]

Gehrtz, M.

Grant, W. B.

W. B. Grant, R. H. Kagmin, and W. A. McClenny, "Optical remote measurement of toxic gases," J. Air Waste Manage. Assoc. 42, 18–29 (1992).
[CrossRef] [PubMed]

Harris, D. O.

T. Tanaka, R. W. Field, and D. O. Harris, "Microwave optical double resonance and continuous wave dye laser excitation spectra of NO2: rotational assignment of the K = 0–4 subbands of the 593 nm band," J. Mol. Spectrosc. 56, 188–199 (1975).
[CrossRef]

Hintsa, E. J.

X. Zhao, E. J. Hintsa, and Y. T. Lee, "Infrared multiphoton of RDX in a molecular beam," J. Chem. Phys. 88, 801–810 (1988).
[CrossRef]

Kagmin, R. H.

W. B. Grant, R. H. Kagmin, and W. A. McClenny, "Optical remote measurement of toxic gases," J. Air Waste Manage. Assoc. 42, 18–29 (1992).
[CrossRef] [PubMed]

Kalkert, P.

H. Adams, D. Reinert, P. Kalkert, and W. Urban, "A differential detection scheme for Faraday rotation spectroscopy with a color center laser," Appl. Phys. B 34, 179–185 (1984).
[CrossRef]

Kano, S. S.

M. D. Levenson and S. S. Kano, Introduction to Nonlinear Laser Spectroscopy (Academic, San Diego, Calif., 1988), pp. 72–75.

Kaufman, L.

D. L. Massart, B. G. M. Vandeginste, S. N. Deming, Y. Michotte, and L. Kaufman, Chemometrics: A Textbook. Data Handling in Science and Technology, B. G. M. Vandeginste and L. Kaufman, eds. (Elsevier, Amsterdam, 1988), Vol. 2; M. A. Sharaf, D. L. Illman, B. R. Kowalski, Chemometrics. Monographs on Chemical Analysis, Monograph 82, P. J. Elving and J. D. Winefordner, eds. (Wiley, New York, 1986).

Lee, Y. T.

X. Zhao, E. J. Hintsa, and Y. T. Lee, "Infrared multiphoton of RDX in a molecular beam," J. Chem. Phys. 88, 801–810 (1988).
[CrossRef]

Lenth, W.

Levenson, M. D.

M. D. Levenson and S. S. Kano, Introduction to Nonlinear Laser Spectroscopy (Academic, San Diego, Calif., 1988), pp. 72–75.

Levy, D. H.

R. E. Smalley, L. Wharton, and D. H. Levy, "The fluorescence excitation spectrum of rotationally cooled NO2," J. Chem. Phys. 63, 4977–4989 (1975).
[CrossRef]

Liftin, G.

G. Liftin, C. R. Pollock, R. F. Curl, Jr., and F. K. Tittel, "Sensitivity enhancement of laser absorption spectroscopy by magnetic rotation effect," J. Chem. Phys. 72, 6602–6605 (1980).
[CrossRef]

Massart, D. L.

D. L. Massart, B. G. M. Vandeginste, S. N. Deming, Y. Michotte, and L. Kaufman, Chemometrics: A Textbook. Data Handling in Science and Technology, B. G. M. Vandeginste and L. Kaufman, eds. (Elsevier, Amsterdam, 1988), Vol. 2; M. A. Sharaf, D. L. Illman, B. R. Kowalski, Chemometrics. Monographs on Chemical Analysis, Monograph 82, P. J. Elving and J. D. Winefordner, eds. (Wiley, New York, 1986).

McCarthy, M. C.

M. C. McCarthy, J. C. Bloch, and R. W. Field, "Frequency-modulation enhanced magnetic rotation spectroscopy: a sensitive and selective absorption scheme for paramagnetic molecules," J. Chem. Phys. 100, 6331–6346 (1994).
[CrossRef]

McClenny, W. A.

W. B. Grant, R. H. Kagmin, and W. A. McClenny, "Optical remote measurement of toxic gases," J. Air Waste Manage. Assoc. 42, 18–29 (1992).
[CrossRef] [PubMed]

Michotte, Y.

D. L. Massart, B. G. M. Vandeginste, S. N. Deming, Y. Michotte, and L. Kaufman, Chemometrics: A Textbook. Data Handling in Science and Technology, B. G. M. Vandeginste and L. Kaufman, eds. (Elsevier, Amsterdam, 1988), Vol. 2; M. A. Sharaf, D. L. Illman, B. R. Kowalski, Chemometrics. Monographs on Chemical Analysis, Monograph 82, P. J. Elving and J. D. Winefordner, eds. (Wiley, New York, 1986).

Podolske, J. R.

T. A. Blake, C. Chackerian, Jr., and J. R. Podolske, "Prognosis for a mid-infrared magnetic rotation spectrometer for the in situ detection of atmospheric free radicals," submitted to Appl. Opt.

Pollock, C. R.

G. Liftin, C. R. Pollock, R. F. Curl, Jr., and F. K. Tittel, "Sensitivity enhancement of laser absorption spectroscopy by magnetic rotation effect," J. Chem. Phys. 72, 6602–6605 (1980).
[CrossRef]

Reinert, D.

H. Adams, D. Reinert, P. Kalkert, and W. Urban, "A differential detection scheme for Faraday rotation spectroscopy with a color center laser," Appl. Phys. B 34, 179–185 (1984).
[CrossRef]

Righi, A.

A. Righi, "Sur l' absorption de la lumière produite par un corps placé dans un champ magnetique," C. R. Acad. Sci. Paris 127, 216–219 (1898); T. Carroll, "Magnetic rotation spectra of diatomic molecules," Phys. Rev. 52, 882–835 (1937).
[CrossRef]

Silver, J. A.

J. A. Silver, D. S. Bomse, and C. A. Stanton, in Optical Methods for Ultrasensitive Detection and Analysis: Techniques and Applications, B. L. Fearey, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1435, 64–71 (1991).
[CrossRef]

Smalley, R. E.

R. E. Smalley, L. Wharton, and D. H. Levy, "The fluorescence excitation spectrum of rotationally cooled NO2," J. Chem. Phys. 63, 4977–4989 (1975).
[CrossRef]

Stanton, C. A.

J. A. Silver, D. S. Bomse, and C. A. Stanton, in Optical Methods for Ultrasensitive Detection and Analysis: Techniques and Applications, B. L. Fearey, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1435, 64–71 (1991).
[CrossRef]

Stevens, C. G.

C. G. Stevens and R. N. Zare, "Rotational analysis of the 5933 Å band of NO2," J. Mol. Spectrosc. 56, 167–187 (1975).
[CrossRef]

Supplee, J. M.

Tanaka, T.

T. Tanaka, R. W. Field, and D. O. Harris, "Microwave optical double resonance and continuous wave dye laser excitation spectra of NO2: rotational assignment of the K = 0–4 subbands of the 593 nm band," J. Mol. Spectrosc. 56, 188–199 (1975).
[CrossRef]

Tittel, F. K.

G. Liftin, C. R. Pollock, R. F. Curl, Jr., and F. K. Tittel, "Sensitivity enhancement of laser absorption spectroscopy by magnetic rotation effect," J. Chem. Phys. 72, 6602–6605 (1980).
[CrossRef]

Urban, W.

H. Adams, D. Reinert, P. Kalkert, and W. Urban, "A differential detection scheme for Faraday rotation spectroscopy with a color center laser," Appl. Phys. B 34, 179–185 (1984).
[CrossRef]

Vandeginste, B. G. M.

D. L. Massart, B. G. M. Vandeginste, S. N. Deming, Y. Michotte, and L. Kaufman, Chemometrics: A Textbook. Data Handling in Science and Technology, B. G. M. Vandeginste and L. Kaufman, eds. (Elsevier, Amsterdam, 1988), Vol. 2; M. A. Sharaf, D. L. Illman, B. R. Kowalski, Chemometrics. Monographs on Chemical Analysis, Monograph 82, P. J. Elving and J. D. Winefordner, eds. (Wiley, New York, 1986).

Warren, R. E.

Wharton, L.

R. E. Smalley, L. Wharton, and D. H. Levy, "The fluorescence excitation spectrum of rotationally cooled NO2," J. Chem. Phys. 63, 4977–4989 (1975).
[CrossRef]

Whittaker, E. A.

Wittaker, E. A.

Zare, R. N.

C. G. Stevens and R. N. Zare, "Rotational analysis of the 5933 Å band of NO2," J. Mol. Spectrosc. 56, 167–187 (1975).
[CrossRef]

Zhao, X.

X. Zhao, E. J. Hintsa, and Y. T. Lee, "Infrared multiphoton of RDX in a molecular beam," J. Chem. Phys. 88, 801–810 (1988).
[CrossRef]

Appl. Opt.

Appl. Phys. B

H. Adams, D. Reinert, P. Kalkert, and W. Urban, "A differential detection scheme for Faraday rotation spectroscopy with a color center laser," Appl. Phys. B 34, 179–185 (1984).
[CrossRef]

C. R. Acad. Sci. Paris

A. Righi, "Sur l' absorption de la lumière produite par un corps placé dans un champ magnetique," C. R. Acad. Sci. Paris 127, 216–219 (1898); T. Carroll, "Magnetic rotation spectra of diatomic molecules," Phys. Rev. 52, 882–835 (1937).
[CrossRef]

J. Air Waste Manage. Assoc.

W. B. Grant, R. H. Kagmin, and W. A. McClenny, "Optical remote measurement of toxic gases," J. Air Waste Manage. Assoc. 42, 18–29 (1992).
[CrossRef] [PubMed]

J. Chem. Phys.

X. Zhao, E. J. Hintsa, and Y. T. Lee, "Infrared multiphoton of RDX in a molecular beam," J. Chem. Phys. 88, 801–810 (1988).
[CrossRef]

G. Liftin, C. R. Pollock, R. F. Curl, Jr., and F. K. Tittel, "Sensitivity enhancement of laser absorption spectroscopy by magnetic rotation effect," J. Chem. Phys. 72, 6602–6605 (1980).
[CrossRef]

R. E. Smalley, L. Wharton, and D. H. Levy, "The fluorescence excitation spectrum of rotationally cooled NO2," J. Chem. Phys. 63, 4977–4989 (1975).
[CrossRef]

M. C. McCarthy, J. C. Bloch, and R. W. Field, "Frequency-modulation enhanced magnetic rotation spectroscopy: a sensitive and selective absorption scheme for paramagnetic molecules," J. Chem. Phys. 100, 6331–6346 (1994).
[CrossRef]

J. Mol. Spectrosc.

T. Tanaka, R. W. Field, and D. O. Harris, "Microwave optical double resonance and continuous wave dye laser excitation spectra of NO2: rotational assignment of the K = 0–4 subbands of the 593 nm band," J. Mol. Spectrosc. 56, 188–199 (1975).
[CrossRef]

W. Dillenschnider and R. F. Curl, Jr., "Color center laser spectroscopy of v1 + v2 + v3 of NO2 using magnetic rotation," J. Mol. Spectrosc. 99, 87–97 (1983).
[CrossRef]

C. G. Stevens and R. N. Zare, "Rotational analysis of the 5933 Å band of NO2," J. Mol. Spectrosc. 56, 167–187 (1975).
[CrossRef]

J. Opt. Soc. Am. B

Opt. Lett.

Other

S. M. Khan, ed., in Proceedings of the First International Symposium on Explosive Detection Technology (Federal Aviation Administration Technical Center, Atlantic City, N.J., 1992).

The shot-noise limit (50 mW, modulation depth = 0.83, 1 Hz) is 1.7 × 10−7 fractional absorbance. Typical FM spectroscopies achieve only of the order of 1 × 10−5 because of accidental étalons and residual amplitude modulation. The crossed-polarizer scheme of FM-MRS suppresses this excess noise by a factor (relative to the signal) of 1/θuuncrossing = 20.

The signal strengths of FM, MRS, and FM-MRS are expected to be comparable (see Ref. 8). We allow a signal-reduction factor of 0.1 because neither ωrf nor the magnetic field was optimized.

D. L. Massart, B. G. M. Vandeginste, S. N. Deming, Y. Michotte, and L. Kaufman, Chemometrics: A Textbook. Data Handling in Science and Technology, B. G. M. Vandeginste and L. Kaufman, eds. (Elsevier, Amsterdam, 1988), Vol. 2; M. A. Sharaf, D. L. Illman, B. R. Kowalski, Chemometrics. Monographs on Chemical Analysis, Monograph 82, P. J. Elving and J. D. Winefordner, eds. (Wiley, New York, 1986).

S. C. Bloch, SSP: The Spreadsheet Signal Processor (Prentice-Hall, Englewood Cliffs, N.J., 1992), and references therein.

T. A. Blake, C. Chackerian, Jr., and J. R. Podolske, "Prognosis for a mid-infrared magnetic rotation spectrometer for the in situ detection of atmospheric free radicals," submitted to Appl. Opt.

National Research Council Commission on Geosciences, Environment, and Resources, Board on Atmospheric Sciences and Climate, Committee on Tropospheric Ozone Formation and Measurement, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences—National Research Council, Washington, D.C., 1991), pp. 215–221. We thank G. McRae for bringing this reference to our attention.

M. D. Levenson and S. S. Kano, Introduction to Nonlinear Laser Spectroscopy (Academic, San Diego, Calif., 1988), pp. 72–75.

J. A. Silver, D. S. Bomse, and C. A. Stanton, in Optical Methods for Ultrasensitive Detection and Analysis: Techniques and Applications, B. L. Fearey, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1435, 64–71 (1991).
[CrossRef]

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Figures (5)

Fig. 1
Fig. 1

Simplified block diagram of the experimental apparatus for FM-MRS. P1–P3, polarizers; EOM, electro-optic phase modulator; AOM, acousto-optic modulator. A more complete description of the setup can be found in Ref. 8.

Fig. 2
Fig. 2

Typical direct absorption and FM-MRS spectra recorded under similar conditions (10 Torr). The vertical scale for the absorption scan (transmission) is an estimate (±10%), reflecting the congestion of the spectra. Tentative assignments are shown on the lower, FM-MRS, trace. The features labeled 1, 2, and 3 are assigned as R(1)Ka = 2, R(2)Ka = 2, and R(2)Ka = 0, respectively.

Fig. 3
Fig. 3

FM-MRS signal (peak to peak for strongest Q line) versus NO2 pressure.

Fig. 4
Fig. 4

FM-MRS spectra as a function of NO2 partial pressure. From top to bottom, the NO2 pressure was 10 Torr, 1.5 × 10−3 Torr, 1.5 × 10−3 Torr, and 2 × 10−5 Torr (corresponding to 20 parts in 109). The appearance of the spectrum at 10 Torr reflects the influence of self-broadening; the spectrum at 2 × 10−5 Torr shows broadening from background atmospheric NO2.

Fig. 5
Fig. 5

Results of a spectral cross correlation are shown to demonstrate the potential for pattern-recognition algorithms in the processing of FM-MRS data. Left-hand panel: each of the input spectra for the correlations, vertically offset [with a new horizontal (relative) axis for clarity]. Trace (a), a high-pressure FM-MRS spectrum; trace (b), the highlighted portion of trace (a); trace (c), a FM-MRS spectrum contaminated by accidental étalons; trace (d), a simulated spectrum, constructed from one tenth of trace (a) and Gaussian noise (amplitude 200); trace (e), just the noise. Right-hand panel, correlations. Each of the correlations is normalized with the autocorrelation of trace (b) and thus contains not only information on the location of trace (b) but also the amplitude of trace (b) in the other traces. Clearly, the use of a normalized cross correlation quantitatively identifies the FM-MRS NO2 signature even in the presence of both strong baseline modulation (étalons) and noise.

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