Parkes observations for project P990 semester 2021OCTS_06

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The only published instance of natural, fast-switching, stimulated OH emission was observed toward PSR B1641-45. Without having to move the beam, the natural ON and OFF signals in the time-domain from a pulsar were successfully utilized as a unique tool for probing microphysics as well as ISM structures. The amplification of pulsar photons by intervening gas, presumably with non-LTE excitation, also demonstrate a feasible principle for interstellar communication by extraterrestrial intelligence. Recently, we have carried out systematic studies of OH and CH ?-doubling transitions, showing that they both well trace the so-called “dark-molecular-gas”. The similarity in their astrochemical conditions, distribution, and energy level structures, motivate us to try to detect stimulated emission in CH, toward a small sample of sources showing OH absorption against pulsars. If successful, this will be the 2nd such detection as well as the first in a species other than OH. We have successfully processed UWL baseband data to confirm Weisberg et al. (2005) results with potential new results, and thus confident of the feasibility of the proposed observations. We thus propose to obtain emission, absorption, and stimulated emission of HI, OH, and CH, simultaneously. A combined analysis of stimulated emission (or non-detection) with multiple other emission/absorption components in the Parkes beam, will significant improve our understanding of ISM.