DETECTION OF NONPOLAR IONS IN {sup 2}{Pi}{sub 3/2} STATES BY RADIOASTRONOMY VIA MAGNETIC DIPOLE TRANSITIONS
The possibility of magnetic dipole-induced pure rotational transitions in the interstellar medium is investi- gated for symmetric Hund's case (a) linear molecules, such as H-C{identical_to}C-H{sup +} (X-tilde {sup 2}{Pi}{sub 3/2u}), CO{sub 2} {sup +} (X-tilde {sup 2}{Pi}{sub 3/2g}), H-C{identical_to}C-C{identical_to}C-H{sup +} (X-tilde {sup 2}{Pi}{sub 3/2g}), and N{sub 3} (X-tilde {sup 2}{Pi}{sub 3/2g}). These species lack an electric dipole moment and therefore cannot undergo pure rotational electric dipole transitions. These species can undergo pure rotational transitions via the parallel component of the magnetic dipole operator, however. The transition moments and Einstein A coefficients for the allowed pure rotational transitions are derived for a general Hund's case (a) linear molecule, and tabulated for the examples of H-C{identical_to}C-H{sup +} ({sup 2}{Pi}{sub 3/2u}) and H-C{identical_to}C-C{identical_to}C-H{sup +} ({sup 2}{Pi}{sub 3/2g}). It is found that the rates of emission are comparable to collision rates in interstellar clouds, suggesting that this decay mechanism may be important in simulating rotational population distributions in diffuse clouds and for detecting these molecules by radioastronomy. Expected line positions for the magnetic dipole-allowed R{sub ef} (J) and R{sub fe} (J) transitions of H-C{identical_to}C-H{sup +} ({sup 2}{Pi}{sub 3/2u}), H-C{identical_to}C-C{identical_to}C-H{sup +} ({sup 2}{Pi}{sub 3/2g}), CO{sub 2}{sup +} ({sup 2}{Pi}{sub 3/2g}), and N{sub 3} ({sup 2}{Pi}{sub 3/2g}) are tabulated to assist in their observation by radioastronomy or in the laboratory.