Abstract

The paper describes an experiment on the absorption of the 3s2–3p4 helium–neon laser emission at 2947.903 cm−1 (3.39 μ) by methane. The emission frequency coincides closely to one of the components of the P(F+) branch of the ν3 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−1.

The collision broadened full-width at half-maximum of the absorption line was determined to be 0.13±0.04 cm−1 at atmospheric pressure. At 1 atm in the earth’s atmosphere, the transmittance can be calculated to be T=exp(−1.1 L) by using the published value of the concentration of methane where L 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.

© 1965 Optical Society of America

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References

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  1. W. R. Bennett, Appl. Opt. Suppl. 1, 24 (1962).
    [Crossref]
  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).
    [Crossref]
  4. K. T. Hecht, J. Mol. Spectry. 5, 390 (1960).
    [Crossref]
  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)]12. 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).
    [Crossref]
  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.
    [Crossref]

1964 (1)

1962 (1)

W. R. Bennett, Appl. Opt. Suppl. 1, 24 (1962).
[Crossref]

1960 (2)

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

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

1949 (1)

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

1938 (1)

F. Hjerting, Astrophys. J. 88, 505 (1938).
[Crossref]

Bennett, W. R.

W. R. Bennett, Appl. Opt. Suppl. 1, 24 (1962).
[Crossref]

Blaine, L. R.

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

Fink, U.

Hecht, K. T.

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

Hjerting, F.

F. Hjerting, Astrophys. J. 88, 505 (1938).
[Crossref]

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).
[Crossref]

E. K. Plyler (private communication).

Rank, D. H.

Tidwell, E. D.

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

Wiggins, T. A.

Zemansky, M. W.

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

Appl. Opt. Suppl. (1)

W. R. Bennett, Appl. Opt. Suppl. 1, 24 (1962).
[Crossref]

Astrophys. J. (1)

F. Hjerting, Astrophys. J. 88, 505 (1938).
[Crossref]

J. Mol. Spectry. (1)

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

J. Opt. Soc. Am. (1)

J. Res. Natl. Bur. Std. (1)

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

Natl. Bur. Std. (U. S.), Circ. No. 467 (1)

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

Other (5)

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.

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)]12. 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).

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Figures (3)

Fig. 1
Fig. 1

Absorption spectrum of methane, taken from Plyler et al.3 The laser emission is shown by the arrow.

Fig. 2
Fig. 2

Relative absorption cofficient at ν of a single absorption line as a function of the collision, natural, and Doppler widths, and the separation of ν from the center of the absorption line ν0. Since, for the conditions of this experiment, the natural broadened width ΔνN is much smaller than the collision-broadened width ΔνC and since ΔνC is proportional to pressure, these curves represent the relative absorption coefficient as a function of pressure. For example, the ω=1 curve gives the relative absorption coefficient as a function of pressure at a frequency displaced approximately one-half a Doppler width from the center of the absorption line where ω = [ 2 ( ν - ν 0 ) / Δ ν D ] ( ln 2 ) 1 2.

Fig. 3
Fig. 3

Experimentally determined effective absorption coefficient as a function of the total pressure of the methane and nitrogen, an inert broadening gas. ○, 1/300 mixture. ×, 1/100 mixture. At low pressures where only one of the lines in the methane group contributes to the absorption, the ω=0.6 curve of Fig. 2 offers a good fit to the data points, permitting estimation of the frequency separation of the laser emission from the methane line. At higher pressures, because of collision broadening, all the lines in the methane group contribute significantly and the measured effective absorption coefficient is larger than would be expected on the basis of the existence of a single isolated methane line. By noting the correspondence of the a axis of Fig. 2 to the pressure axis of this figure for the ω=0.6 curve, the collision-broadened width of the methane line nearest the laser emission can be calculated as a function of pressure.

Tables (1)

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Table I α and k as functions of P, using the measured values of k from Fig. 3.

Equations (11)

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T ( ν ) = e - k ( ν ) u .
ω = [ 2 ( ν - ν 0 ) / Δ ν D ] ( ln 2 ) 1 2 ,
a = [ ( Δ ν C + Δ ν N ) / Δ ν D ] ( ln 2 ) 1 2
k ( ν ) = C P / [ ( ν - ν 0 ) 2 + C P 2 ] ( C / C P ) .
T = n A n exp [ - k n ( ν ) u ] .
T = e - k u             or             k = - ln T / u ,
Δ ν = ν - ν 0 = ω Δ ν D / 2 ( ln 2 ) 1 2 0.003 ± 0.002 cm - 1 .
2947.903             cm - 1 ( laser emission 1 , 2 ) 2947.888 ± 0.015 cm - 1 _ ( methane line 3 - 5 ) 0.015 ± 0.015 cm - 1 ( difference ) .
Δ ν C = a Δ ν D / ( ln 2 ) 1 2 = P Δ ν D / 63 ( ln 2 ) 1 2 ( cm - 1 Torr - 1 ) ,
Δ ν C = 0.13 ± 0.04 cm - 1 .
k ( ν ) = const / { 1 + [ 2 ( ν - ν 0 ) / Δ ν C ] 2 } .