Further evidence for the OCN- assignment to the XCN band in astrophysical ice analogs

The well-known ''XCN'' feature near 2167 cm(-1) observed towards various low- and high-mass embedded protostars can be well matched with a feature produced by photolysis of astrophysical ice analogs containing CO and NH3. An assignment of this laboratory XCN band to OCN- was earlier proposed based on the correspondence in position and in the C-13 and N-15 isotopic shifts, We report here a further experimental study of this identification. First, the isotope shift of the XCN band obtained by irradiating O-18 labelled ice samples equals 8.7 +/- 0.6 cm(-1), in good agreement with OCN-. Second, highly sensitive spectroscopy reveals three additional weak features which can be assigned to the OCN- ion. Together with the earlier work, these results leave little doubt to the correctness of the identification of the laboratory XCN feature with OCN-.