DETECTION OF NONPOLAR IONS IN²Π_3/2STATES BY RADIOASTRONOMY VIA MAGNETIC DIPOLE TRANSITIONS

Abstract

The possibility of magnetic dipole-induced pure rotational transitions in the interstellar medium is investigated for symmetric Hund's case (a) linear molecules, such as H-C≡C-H + ( X 2 Π 3/2u ), CO 2 + ( X 2 Π 3/2g ), H-C≡C-C≡C-H + ( X 2 Π 3/2g ), and N 3 ( X 2 Π 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≡C-H + ( 2 Π 3/2u ) and H-C≡C-C≡C-H + ( 2 Π 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 ef (J) and R fe (J) transitions of H-C≡C-H + ( 2 Π 3/2u ), H-C≡C-C≡C-H + ( 2 Π 3/2g ), CO 2 + ( 2 Π 3/2g ), and N 3 ( 2 Π 3/2g ) are tabulated to assist in their observation by radioastronomy or in the laboratory.

The abundance of C3H2 and other small hydrocarbons in the diffuse interstellar medium

“…Species as complicated as naphthalene are likely to prove as elusive at radio wavelengths as in the optical/uv domain, but the sort of work performed here may be straightforwardly extended to other polar species such as C 3 H and CH 2 CN, especially considering the modest observing times required and the increasing availability of wider instantaneous bandwidths at the GBT and VLA. Searches may also be possible for non-polar species that are observable in their magnetic dipole transitions as recently discussed by Morse & Maier (2011).…”

Electronic Absorption Spectrum of Triacetylene Cation for Astronomical Considerations

“…The radiative rates were calculated with the PGOPHER program for a magnetic dipole transition of the X 2 Π u ground state of HC 6 H + . The expression for the transition moment operator was from ref .…”

“…The population in the ISM is non-Boltzmann because the 2 Π 1/2 spin–orbit component will be depopulated by magnetic dipole transitions ( 2 Π 1/2 → 2 Π 3/2 ). The theory of this has been worked out . The population and rotational profile are modeled considering the magnetic dipole transitions, 2.7 K blackbody radiation, and collisional rates.…”

Laboratory Electronic Spectra of Carbon Chains and Rings

“…the X 2 Π 1/2 → X 2 Π 3/2 transition. This effect has been considered in detail for several species, including the acetylene and diacetylene cations (Morse & Maier 2011). In diffuse clouds the rates of magnetic dipole transitions can exceed the collisional rate with hydrogen.…”