NATURALLY OCCURING MICROWAVE LASERS

Javan (1958) predicted that negative absorption may eventually be discovered radioastronomy. Seven years later the Radio Astronomy Group led by Professor H Weaver (1965, 1968) at Berkeley California discovered radio emissions at 1670 MHz coming from OH molecules near stars which can excite a population inversion by radiation or other energy mechanisms. Today hundreds of maser transitions are known in more than 36 molecules.
OH Energy levels
Energy level diagram depicting the 18 cm microwave transition and its hyperfine structure.

To demonstrate amplified spontaneous emission, the astronomers came up with the following evidence :

LINE RATIO EVIDENCE
The emission from the Hydroxyl molecule at 18 cm wavelength is composed of four hyperfine lines at 1612, 1665, 1667 and 1720 MHz (see diagram). Normally the line ratio for a gas in thermodynamic equilibrium is 1:5:9:1, however the observed intensity ratios were different and even varied over time.
LINE WIDTH and LINE INTENSITY EVIDENCE
A paradoxical situation occurs if the gas was assumed in thermodynamic equilibrium : The emission profiles of the OH lines as a function of wavelength indicated a Doppler temperature of 50 K, however the emission intensity was so strong that they would correspond to a source temperature of several thousand trillion degrees Kelvin !
POINT SOURCE EMISSION EVIDENCE
The emission originates from extremely narrow point sources or highly collimated beams.
POLARIZATION EVIDENCE
The emission lines were polarized in a particular way, which depended on time and which could not possibly have been produced by spontaneous emission.

CONCLUSION

Based on these facts the radio astronomers conclude that the microwaves are produced from amplification by stimulated emission of radiation from an inverted population.

There is another review of the history of masers written in a popular magazine for a more general audience.

REFERENCES

  1. Megamasers or megawrong ?
  2. Cook,A.H.: 1966, Nature, 210, 611.
  3. Javan,A. : 1958, Bull.Am.Phys.Soc., Ser. 2 3, 213.
  4. Litvak,M.M., McWhorter,A.L., Meeks,M.L., Zieger,H.J.: 1966, Phys.Rev.Lett. 17, 821.
  5. Perkins,F., Gold,T., Salpeter,E.E.: 1966, Astrophys.J. 145, 361.
  6. Weaver,H., Williams,D.R., Nannilou Dieter,H., Lum,T.W.: 1965, Nature 208, 29.
  7. Weaver,H., Nannilou Dieter,H., Williams,D.R.: 1968, Astrophys.J.Supp. 16, 10, 146, 219.
  8. Gaume,R.A., Wilson,T.L., Johnston,K.J.: 1996, ApJ., 457, L47. Non-LTE Effects in Ammonia (maser)
  9. Torrelles, et al. 1996, ApJ., 457, L107. Cepheus A HW2 jet and H2O masers
  10. Sjouwerman,L.O.: 1996, ApJ., 461, L41. OH Counterparts for HO Masers
  11. Tsuboi,M., Ohta,E., Kasuga,T., Murata,Y., Handa,T.: 1996, ApJ., 461, L107. SiO Maser Emission from Orion-KL IRc2
  12. Maoz,E.: 1995, ApJ., 455, L131. Masers in NGC 4258 ?
  13. Zhang,Q., Ho,P.T.P.:1995, ApJ., 450, L63. Ammonia Maser in Molecular Outflow toward W51 (VLA GIF)
  14. Neufeld,D.A., Maloney,P.R.: 1995, ApJ., 447, L17. Mass Accretion Rate through Disk in NGC 4258 (masers discovered)
  15. Maotz,E.: 1995, ApJ., 447, L91. Alternatives to Massive Black Hole in NGC 4258 (based on masers)
  16. Dust Grains and Water Maser Luminosity

Maser Catalogs

Maser abstracts

Other maser related web site

Diagram from p.58 of 'Physics of The Galaxy and Interstellar Medium'
Laser History

Laser Stars