Ultraviolet fluorescent pumping of OH 18-centimeter radiation in comets

In the absence of collisions, optical pumping of cometary OH by solar ultraviolet radiation determines the relative population of OH molecules in the A-doubled levels of the ll2i2 ground state. The distribution is extremely sensitive to the heliocentric radial velocity of the comet, and at appropriate velocities the level populations will be either inverted or anti-inverted. Thus the OH t8-cm spectrum of a comet, measured with respect to the 100 K galactic background at 1665 and 1667 MHz, may be observed either in emission or in absorption at different periods during the trajectory. Optical pumping also will produce an alignment of the magnetic sublevels along the axis of the incident solar radiation, and variable degrees of linear polarization are expected. Subject headings: comets - line formation - polarization radio lines - spectra, solar