Letter | Published:

Effect of Collisions on the Intensities of Nebular Lines

Nature volume 142, page 644 (08 October 1938) | Download Citation



THERE appears to be a widespread misconception concerning the effect of electron collisions on the intensities of forbidden lines. The prevailing view appears to be that, at high densities, collisions of the second kind operate to de-excite atoms from the metastable levels before the atoms have a chance to radiate, and that only at low densities, as in the gaseous nebulæ, can a sufficiently high population of atoms be obtained to give appreciable intensity to the forbidden lines. The mathematical reasoning advanced to support this argument1 is as follows. Let N1b12be the number of atoms excited per second from the ground to the metastable level by inelastic electron impact, which process is ordinarily assumed to be the predominant source of excitation. Let N2b21 be the number of super-elastic collisions per second. Let A21 be the Einstein probability of spontaneous emission. Then the intensity of the line may be written: The customary argument is that the increase of b21 with density causes the value of I to decrease.

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  1. 1.

    cf., , Rev. Mod. Phys., 8, 55 (1936).

  2. 2.

    cf., , "Statistical Mechanics" (2nd ed.), 677–684.

  3. 3.

    , Mon. Not. Roy. Astro. Soc., 88, 134 (1927).

  4. 4.

    , Publ. Astro. Soc. Pacific, 50, 228 (1938).

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  1. Harvard Observatory, Cambridge, Mass. Aug. 19.



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