REFRIGERATION OF THE 18.3 GHZ C3H2 TRANSITION IN DARK CLOUD G1.6-0.25

We have observed 1(10)-1(01) (18.3 GHz) transition of ortho-cycloprope nylidene, C3H2, at 24 positions in the unusual dense cloud G1.6-0.025. Except for one position, the transition is refrigerated, a phenomenon which has not been seen in this transition before and was not predict ed to occur. In general, the absorption correlates with the absorption seen in the 2(0)-3(-1) (12.2 GHz) transition of methanol, but there a re significant differences. We suspect that collisional excitation at relatively high kinetic temperature (50 K) enhanced by photon trapping is responsible for the refrigeration.