Gas-phase detection of c-C_3_H_3_^+^

An astronomical identification has been prohibited so far, because of a lack of gas-phase data. Here we report the first high resolution infrared laboratory gas-phase spectrum of the {nu}_4_(C-H asymmetric stretching) fundamental band of c-C_3_H_3_^+^.

The c-C_3_H_3_^+^ cations are generated in supersonically expanding planar plasma by discharging a propyne/helium gas pulse, yielding a rotational temperature of ~35 K. The absorption spectrum is recorded in the 3.19 {mu}m region using sensitive continuous-wave cavity ring-down spectroscopy. The analysis of about 130 ro-vibrational transitions results in precise spectroscopic parameters. These constants allow for an accurate comparison with high-level theoretical predictions, and provide the relevant information needed to search for this astrochemically relevant carbo-cation in space.