Abstract

<p>The paper describes an experiment on the absorption of the 3<i>S</i><sub>2</sub>-3<i>P</i><sub>4</sub> helium—neon laser emission at 2947.903 cm<sup>-1</sup> (3.39µ) by methane. The emission frequency coincides closely to one of the components of the <i>P</i>(<i>F</i><sup>+</sup>) branch of the ν<sub>3</sub> band of methane. Methane and nitrogen in different mixing ratios were introduced into an absorption cell and the transmittance as a function of pressure was determined. By relating the measured absorption coefficient with the known interaction of collision and Doppler effects on the broadening of the absorption line, the separation of the emission line and the nearest absorption line was deduced to be 0.003±0.002 cm<sup>-1</sup>.</p><p>The collision broadened full-width at half-maximum of the absorption line was determined to be 0.13±0.04 cm<sup>-1</sup> at atmospheric pressure. At 1 atm in the earth’s atmosphere, the transmittance can be calculated to be <i>T</i> = exp (-1.1 <i>L</i>) by using the published value of the concentration of methane where <i>L</i> is the path length in kilometers. The effects of the laser emission in several possible cavity modes and of the several absorption lines in the methane group which overlap each other at high pressures are discussed.</p>

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  1. W. R. Bennett, Jr., Appl. Opt. Suppl. 1, 24 (1962).
  2. C. E. Moore, Natl. Bur. Std. (U. S.), Circ. No. 467 (1949).
  3. E. K. Plyler, E. D. Tidwell, and L. R. Blaine, J. Res. Natl. Bur. Std. 64A, 201 (1960).
  4. K. T. Hecht, J. Mol. Spectry. 5, 390 (1960).
  5. E. K. Plyler (private communication).
  6. Bennett1 gives the full Doppler width at half maximum at 1.15 µ to be 800 Mc/sec where ΔνD = 2ν0[(2kT/Mc2) (ln2)]½. This corresponds to 270 Mc/sec at 3.39 µ (0.009 cm-1 at 2947 cm-1).
  7. A. C. G. Mitchell and M. W. Zemansky, Resonance Radiation and Excited Atoms (Cambridge University Press, London, 1961).
  8. F. Hjerting, Astrophys. J. 88, 505 (1938).
  9. All widths are "full width at half maximum."
  10. Computed from formula for Doppler width with T =300°K. The Doppler widths of the neon transition and the methane absorption line are, by coincidence, approximately equal.
  11. U. Fink, D. H. Rank, and T. A. Wiggins, J. Opt. Soc. Am. 54, 472 (1964) give the amount of methane in a vertical path from sea level to be 1.11 atm cm (NTP). Since the total air content is 8×105 atm cm (NTP), the fractional concentration by volume of methane is 1.4×10-6 They find their results to be in good agreement with those of other workers for different altitudes and at different geographical locations.

Bennett, Jr., W. R.

W. R. Bennett, Jr., Appl. Opt. Suppl. 1, 24 (1962).

Blaine, L. R.

E. K. Plyler, E. D. Tidwell, and L. R. Blaine, J. Res. Natl. Bur. Std. 64A, 201 (1960).

Fink, U.

U. Fink, D. H. Rank, and T. A. Wiggins, J. Opt. Soc. Am. 54, 472 (1964) give the amount of methane in a vertical path from sea level to be 1.11 atm cm (NTP). Since the total air content is 8×105 atm cm (NTP), the fractional concentration by volume of methane is 1.4×10-6 They find their results to be in good agreement with those of other workers for different altitudes and at different geographical locations.

Hecht, K. T.

K. T. Hecht, J. Mol. Spectry. 5, 390 (1960).

Hjerting, F.

F. Hjerting, Astrophys. J. 88, 505 (1938).

Mitchell, A. C. G.

A. C. G. Mitchell and M. W. Zemansky, Resonance Radiation and Excited Atoms (Cambridge University Press, London, 1961).

Moore, C. E.

C. E. Moore, Natl. Bur. Std. (U. S.), Circ. No. 467 (1949).

Plyler, E. K.

E. K. Plyler, E. D. Tidwell, and L. R. Blaine, J. Res. Natl. Bur. Std. 64A, 201 (1960).

E. K. Plyler (private communication).

Rank, D. H.

U. Fink, D. H. Rank, and T. A. Wiggins, J. Opt. Soc. Am. 54, 472 (1964) give the amount of methane in a vertical path from sea level to be 1.11 atm cm (NTP). Since the total air content is 8×105 atm cm (NTP), the fractional concentration by volume of methane is 1.4×10-6 They find their results to be in good agreement with those of other workers for different altitudes and at different geographical locations.

Tidwell, E. D.

E. K. Plyler, E. D. Tidwell, and L. R. Blaine, J. Res. Natl. Bur. Std. 64A, 201 (1960).

Wiggins, T. A.

U. Fink, D. H. Rank, and T. A. Wiggins, J. Opt. Soc. Am. 54, 472 (1964) give the amount of methane in a vertical path from sea level to be 1.11 atm cm (NTP). Since the total air content is 8×105 atm cm (NTP), the fractional concentration by volume of methane is 1.4×10-6 They find their results to be in good agreement with those of other workers for different altitudes and at different geographical locations.

Zemansky, M. W.

A. C. G. Mitchell and M. W. Zemansky, Resonance Radiation and Excited Atoms (Cambridge University Press, London, 1961).

Other (11)

W. R. Bennett, Jr., Appl. Opt. Suppl. 1, 24 (1962).

C. E. Moore, Natl. Bur. Std. (U. S.), Circ. No. 467 (1949).

E. K. Plyler, E. D. Tidwell, and L. R. Blaine, J. Res. Natl. Bur. Std. 64A, 201 (1960).

K. T. Hecht, J. Mol. Spectry. 5, 390 (1960).

E. K. Plyler (private communication).

Bennett1 gives the full Doppler width at half maximum at 1.15 µ to be 800 Mc/sec where ΔνD = 2ν0[(2kT/Mc2) (ln2)]½. This corresponds to 270 Mc/sec at 3.39 µ (0.009 cm-1 at 2947 cm-1).

A. C. G. Mitchell and M. W. Zemansky, Resonance Radiation and Excited Atoms (Cambridge University Press, London, 1961).

F. Hjerting, Astrophys. J. 88, 505 (1938).

All widths are "full width at half maximum."

Computed from formula for Doppler width with T =300°K. The Doppler widths of the neon transition and the methane absorption line are, by coincidence, approximately equal.

U. Fink, D. H. Rank, and T. A. Wiggins, J. Opt. Soc. Am. 54, 472 (1964) give the amount of methane in a vertical path from sea level to be 1.11 atm cm (NTP). Since the total air content is 8×105 atm cm (NTP), the fractional concentration by volume of methane is 1.4×10-6 They find their results to be in good agreement with those of other workers for different altitudes and at different geographical locations.

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