Laboratory measurement of the pure rotational spectrum of vibrationally excited HCO^+ (v_2 = 1) by far-infrared laser sideband spectroscopy

Abstract

Laboratory observations of the pure rotational spectrum of HCO^+ in its lowest excited bending state (v_1, v^l_2 v_3)_= (0,1^1,0) are reported. Because of their severe excitation requirements, such vibrational satellites and the high-J ground-state lines also measured here sample only hot, dense regions of matter in active molecular cloud cores and circumstellar envelopes. As the HCO^+ abundance is tied directly to the gas fractional ionization, it is probable that the vibrationally excited formyl ion transitions will provide high-contrast observations of shocked molecular material, rather than the more quiescent, radiatively heated gas surrounding stellar sources detected with the few vibrationally excited neutral species observed to date.