POPULATION ANTI-INVERSION IN THE 2(0)-]3(-1)E TRANSITION OF CH3OH

To investigate the observed absorption of the 2(0)-->3(-1)E transition of CH3OH at 12.2 GHz against the 2.7-K microwave background, statistical equilibrium calculations involving radiative transitions, collisional transitions, and photon trapping have been carried out for the lowest 69 rotational levels of E-species CH3OH. The results show that population anti-inversion of the transition is a combined effect of spontaneous transitions and non-dominating collisional excitation. For molecular clouds not associated with continuum emission, stronger absorption at the transition is expected from warmer clouds (kinetic temperature greater-than-or-equal-to 30 K). An E-species CH3OH abundance of 10(-8)-10(-9) is derived. The model also suggests that the transition can occur as a maser emission in the presence of a bright continuum background with a temperature higher than the kinetic temperature of the cloud. The results are used to re-assess the CH3OH properties of the giant molecular cloud G1.6-0.025.