Hydroxyl, interstellar

Maser transitions have been observed in many important molecules and have been used to carry out surveys of the entire sky. The 22.235 GHz water maser transition is the strongest transition in the radio universe and represents an interesting candidate for an interstellar broadcast frequency. If extraterrestrial intelligence is trying to communicate with us, the choice of the broadcast frequency is an important one and would be known to all intelligent life. Of course it would have a different label, 22.235 GHz being a distinctly Earthly label, but it is a fundamental transition frequency and is observed everywhere. Other maser transitions include the 6.7 and 12.2 GHz methanol maser, the SiO maser v = 1, J = 7-6, 301.8 GHz, which occurs between levels in the first vibration state of the SiO molecule, and finally the OH maser first discovered in 1963 and buried deep in the 2n3/2 electronic state of the hydroxyl radical near 18 cm. This is actually four transitions at 1612, 1665, 1667 and 1720 MHz, all of which must be seen as a group but not necessarily of the same intensity. 

The hydroxyl radical, OH, occupies an extremely important position in spectroscopy, in free radical laboratory chemistry, and in atmospheric, cometary and interstellar chemistry. Its ultraviolet electronic spectrum has been described in many papers published over the past seventy years. It was the first short lived gaseous free radical to be studied by microwave spectroscopy, described in a classic paper by Dousmanis, Sanders and Townes [121] in 1955. The details of this work are presented in chapter 10. It was the first free radical to be studied by microwave magnetic resonance, in pioneering work by Radford [141] the microwave and far-infrared laser magnetic resonance studies are... 

Deloule E, Robert F, Doukhan JC (1998) Interstellar hydroxyl in meteoritic ehondrales Implieations for the origin of water in the iimer solar system. Geochim Cosmochim Acta 62 3367-3378 DesMarais DJ (1978) Carbon, nitrogen and sulfur in Apollo 15, 16 and 17 roeks. Proe 9th Lunar Sei Conf, p 2451-2467... 

The hydroxyl radical OH was the first interstellar molecule detected at radio frequencies (Weinreb et ah, 1963 [356]). 

Water signatures in protostars have been observed already 30 years ago. Hydroxyl and Water Masers in Protostars were discussed e.g. by Litvak, 1969 [204]. The first detection of interstellar water molecules was made by Cheung et al., 1969 [67]. They analyzed the 1.35 cm microwave emission in the sources Sgr B 2, Orion and W 49. For numerical calculations, de Jong, 1973 [100] used the abundance of water relative to hydrogen as 10 that means for 10 molecules or atoms of hydrogen there is one H2O molecule. 

Deloule, E., Robert, E, Doukhan, J.C. Interstellar hydroxyl in meteoritic chondiules implications for the origin of water in the inner solar system. Geochim. Cosmoch. Acta 62, 3367-3378(1998)... 

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