Abstract

A far ir spectrometer has been constructed with an Ebert-Fastie monochromator of 1.8-m focal length plane gratings 19 cm × 13 cm, and slit height of 10 cm. The source optics of the evacuable instrument are double beam, with an electronic system of recording the ratio of the radiant powers in each beam. The instrument has been used thus far over the range 20–300 cm−1 and the resolution achieved with the four gratings of 2 lines/mm, 4 lines/mm, 10 lines/mm, and 30 lines/mm varies from 0.1 cm−1 to about 0.4 cm−1, depending on the grating angle. The combinations of reflection and transmission filters used to eliminate higher orders reduce unwanted radiation to 1% or less in most ranges, though at a few wave-numbers it may be as large as 5%. A commercial bolometer of gallium-doped germanium cooled with liquid helium serves as the detector. Details of the operation of the instrument are given, and the results obtained are illustrated with the spectra of gaseous polyatomic molecules.

© 1968 Optical Society of America

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References

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  1. W. G. Fastie, J. Opt. Soc. Amer. 42, 647 (1952); J. Opt. Soc. Amer. 48, 106 (1958).
    [CrossRef]
  2. See, for example, H. Yoshinaga, Jap. J. Appl. Phys. 4, Suppl. 1, 420 (1965).
  3. F. J. Low, J. Opt. Soc. Amer. 51, 1300 (1961).
    [CrossRef]
  4. J. U. White, J. Opt. Soc. Amer. 32, 285 (1942).
    [CrossRef]
  5. R. G. Schmitt, R. K. Brehm, Appl. Opt. 5, 1111 (1966).
    [CrossRef] [PubMed]
  6. J. Strong, J. Opt. Soc. Amer. 39, 360 (1949).
  7. H. W. Randall, Rev. Mod. Phys. 10, 78 (1938).
    [CrossRef]
  8. R. C. Lord, T. K. McCubbin, J. Opt. Soc. Amer. 47, 689 (1957).
    [CrossRef]
  9. G. R. Harrison, MIT Wavelength Tables (Technology Press, Cambridge, Massachusetts, 1939). The refractive index of air was taken as 1.0002772 at 5461 Å.
  10. H. M. Mould, W. C. Price, G. R. Wilkinson, unpublished communication to Commission on Molecular Structure and Spectroscopy, IUPAC. Compare Spectrochim. Acta16, 479 (1960).
    [CrossRef]
  11. K. N. Rao, R. V. deVore, E. K. Plyler, J. Res. Nat. Bur. Stand. 67A, 351 (1963).
    [CrossRef]
  12. R. T. Hall, J. M. Dowling, J. Chem. Phys. 47, 2454 (1967).
    [CrossRef]

1967

R. T. Hall, J. M. Dowling, J. Chem. Phys. 47, 2454 (1967).
[CrossRef]

1966

1965

See, for example, H. Yoshinaga, Jap. J. Appl. Phys. 4, Suppl. 1, 420 (1965).

1963

K. N. Rao, R. V. deVore, E. K. Plyler, J. Res. Nat. Bur. Stand. 67A, 351 (1963).
[CrossRef]

1961

F. J. Low, J. Opt. Soc. Amer. 51, 1300 (1961).
[CrossRef]

1957

R. C. Lord, T. K. McCubbin, J. Opt. Soc. Amer. 47, 689 (1957).
[CrossRef]

1952

W. G. Fastie, J. Opt. Soc. Amer. 42, 647 (1952); J. Opt. Soc. Amer. 48, 106 (1958).
[CrossRef]

1949

J. Strong, J. Opt. Soc. Amer. 39, 360 (1949).

1942

J. U. White, J. Opt. Soc. Amer. 32, 285 (1942).
[CrossRef]

1938

H. W. Randall, Rev. Mod. Phys. 10, 78 (1938).
[CrossRef]

Brehm, R. K.

deVore, R. V.

K. N. Rao, R. V. deVore, E. K. Plyler, J. Res. Nat. Bur. Stand. 67A, 351 (1963).
[CrossRef]

Dowling, J. M.

R. T. Hall, J. M. Dowling, J. Chem. Phys. 47, 2454 (1967).
[CrossRef]

Fastie, W. G.

W. G. Fastie, J. Opt. Soc. Amer. 42, 647 (1952); J. Opt. Soc. Amer. 48, 106 (1958).
[CrossRef]

Hall, R. T.

R. T. Hall, J. M. Dowling, J. Chem. Phys. 47, 2454 (1967).
[CrossRef]

Harrison, G. R.

G. R. Harrison, MIT Wavelength Tables (Technology Press, Cambridge, Massachusetts, 1939). The refractive index of air was taken as 1.0002772 at 5461 Å.

Lord, R. C.

R. C. Lord, T. K. McCubbin, J. Opt. Soc. Amer. 47, 689 (1957).
[CrossRef]

Low, F. J.

F. J. Low, J. Opt. Soc. Amer. 51, 1300 (1961).
[CrossRef]

McCubbin, T. K.

R. C. Lord, T. K. McCubbin, J. Opt. Soc. Amer. 47, 689 (1957).
[CrossRef]

Mould, H. M.

H. M. Mould, W. C. Price, G. R. Wilkinson, unpublished communication to Commission on Molecular Structure and Spectroscopy, IUPAC. Compare Spectrochim. Acta16, 479 (1960).
[CrossRef]

Plyler, E. K.

K. N. Rao, R. V. deVore, E. K. Plyler, J. Res. Nat. Bur. Stand. 67A, 351 (1963).
[CrossRef]

Price, W. C.

H. M. Mould, W. C. Price, G. R. Wilkinson, unpublished communication to Commission on Molecular Structure and Spectroscopy, IUPAC. Compare Spectrochim. Acta16, 479 (1960).
[CrossRef]

Randall, H. W.

H. W. Randall, Rev. Mod. Phys. 10, 78 (1938).
[CrossRef]

Rao, K. N.

K. N. Rao, R. V. deVore, E. K. Plyler, J. Res. Nat. Bur. Stand. 67A, 351 (1963).
[CrossRef]

Schmitt, R. G.

Strong, J.

J. Strong, J. Opt. Soc. Amer. 39, 360 (1949).

White, J. U.

J. U. White, J. Opt. Soc. Amer. 32, 285 (1942).
[CrossRef]

Wilkinson, G. R.

H. M. Mould, W. C. Price, G. R. Wilkinson, unpublished communication to Commission on Molecular Structure and Spectroscopy, IUPAC. Compare Spectrochim. Acta16, 479 (1960).
[CrossRef]

Yoshinaga, H.

See, for example, H. Yoshinaga, Jap. J. Appl. Phys. 4, Suppl. 1, 420 (1965).

Appl. Opt.

J. Chem. Phys.

R. T. Hall, J. M. Dowling, J. Chem. Phys. 47, 2454 (1967).
[CrossRef]

J. Opt. Soc. Amer.

J. Strong, J. Opt. Soc. Amer. 39, 360 (1949).

R. C. Lord, T. K. McCubbin, J. Opt. Soc. Amer. 47, 689 (1957).
[CrossRef]

W. G. Fastie, J. Opt. Soc. Amer. 42, 647 (1952); J. Opt. Soc. Amer. 48, 106 (1958).
[CrossRef]

F. J. Low, J. Opt. Soc. Amer. 51, 1300 (1961).
[CrossRef]

J. U. White, J. Opt. Soc. Amer. 32, 285 (1942).
[CrossRef]

J. Res. Nat. Bur. Stand.

K. N. Rao, R. V. deVore, E. K. Plyler, J. Res. Nat. Bur. Stand. 67A, 351 (1963).
[CrossRef]

Jap. J. Appl. Phys.

See, for example, H. Yoshinaga, Jap. J. Appl. Phys. 4, Suppl. 1, 420 (1965).

Rev. Mod. Phys.

H. W. Randall, Rev. Mod. Phys. 10, 78 (1938).
[CrossRef]

Other

G. R. Harrison, MIT Wavelength Tables (Technology Press, Cambridge, Massachusetts, 1939). The refractive index of air was taken as 1.0002772 at 5461 Å.

H. M. Mould, W. C. Price, G. R. Wilkinson, unpublished communication to Commission on Molecular Structure and Spectroscopy, IUPAC. Compare Spectrochim. Acta16, 479 (1960).
[CrossRef]

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Figures (11)

Fig. 1
Fig. 1

Optical diagram of the monochromator.

Fig. 2
Fig. 2

Optical diagram for the double beam system. M = spherical Mirror, EM = ellipsoidal mirror, F = reflection filter, FM = field mirror, and PM = plane mirror.

Fig. 3
Fig. 3

Optical diagram for the image slicing system. Plane mirror PM–11 is vertically above PM–12, and M–8 above M–9. The field mirrors FM–7 and FM–8 are of the same height and on the same vertical level as the gap between PM–11 and PM–12.

Fig. 4
Fig. 4

Far ir spectrometer as installed in the MIT Spectroscopy Laboratory.

Fig. 5
Fig. 5

Power operated door at source end of spectrometer opened to show part of source optics.

Fig. 6
Fig. 6

Plot of d ν ¯/dx, in cm−1/mm, against ν ¯ in cm−1. Left-hand scale applies for a monochromator of 1800-mm focal length, and can be used directly to estimate Δ ν ¯ resulting from a change in grating angle Δθ [Eq. (8)]. For calculation of spectral slit widths, the ordinates on left and right must be divided by 2 [Eq. (5)].

Fig. 7
Fig. 7

Schematic diagram of efficiency of a grating monochromator at constant spectral slit width in wavenumber as a function of wavenumber for orders n = 1, 2, and 100.

Fig. 8
Fig. 8

Doublet in the pure rotational spectrum of water vapor at 59.9 cm−1, separation of the components 0.07 cm−1. Instrument operated single beam with power from both channels added; grating, 10-lines/mm; filtering as shown in Table I for region 78–54 cm−1 of 4-lines/mm grating; Ge bolometer; σ = 0.033 cm−1, r = 5 × 10−4 cm−1/sec, τ = 17 sec (see text).

Fig. 9
Fig. 9

(a) Pure rotational absorption spectrum of isocyanic acid vapor, 175–240 cm−1. 45-torr pressure, 15-cm path length, 30-lines/mm grating, filters as in Table I; σ = 0.2–0.4 cm−1, r = 0.005 cm−1/sec, τ = 17 sec. (b) Pure rotational absorption spectrum of isocyanic acid vapor, 20–37 cm−1. 80-torr pressure, 15-cm path length, 2-lines/mm grating, filters as in Table I; σ = 0.1–0.4 cm−1, r = 0.005 cm−1/sec, τ = 17 sec.

Fig. 10
Fig. 10

Vibrational progression in absorption spectrum of cyclobutanone vapor, 65–100 cm−1. 0.5-torr pressure (upper curve), (lower curve) 1 torr, 20-m path length, 10-lines/mm grating, filters as in Table I; σ = 0.2–0.4 cm−1, r = 0.005 cm−1/sec, τ = 17 sec.

Fig. 11
Fig. 11

Vibrational transitions in absorption spectrum of cyclopentene, 65–95 cm−1. The fine structure from 65 cm−1 to 76 cm−1 is chiefly due to the rotational P branches of the Q branches at 76.61 cm−1 and 83.06 cm−1. 115-torr pressure, 4-m path length; 10-lines/mm grating, filters as in Table I; σ = 0.2–0.4 cm−1, r = 0.01 cm−1/sec, τ = 17 sec.

Tables (1)

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Table I Operating Conditions of Instrumenta

Equations (8)

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ν ¯ = ( n / 2 d ) csc θ sec δ / 2 ,
ν ¯ = n κ csc θ .
d ν ¯ / d θ = n κ csc θ cot θ = ν ¯ cot θ ,
d ν ¯ / d θ = ν ¯ [ ( ν ¯ / n κ ) 2 - 1 ] 1 2 .
( d ν ¯ / d x ) = ( d ν ¯ / d θ ) / 2 F = ( ν ¯ / 2 F ) [ ( ν ¯ / n κ ) 2 - 1 ] 1 2 .
P ν ¯ = T ν ¯ B ν ¯ A ( d θ / d ν ¯ ) α ( Δ ν ¯ ) 2 .
P ν ¯ ~ ν ¯ 2 ( n κ / ν ¯ 2 ) Δ ν ¯ 2 ~ n κ Δ ν ¯ 2 .
Δ ν ¯ = ν ¯ [ ( ν ¯ / n κ ) 2 - 1 ] 1 2 Δ θ .

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