H, C, N, and O Isotopic Substitution Studies of the 2165 Wavenumber (4.62 Micron) “XCN” Feature Produced by Ultraviolet Photolysis of Mixed Molecular Ices

To better understand the chemical species that gives rise to the 2165 cm -1 (4.62 μm) "XCN" absorption feature seen toward embedded protostars such as W33A, we have performed laboratory studies using deuterium ( 2 H) isotopic labeling. We report the observation of a small but significant deuterium isotope shift for the "XCN" peak which demonstrates that the atomic motion(s) causing the "XCN" band in the laboratory samples must involve hydrogen. We also report the results of 13 C, 15 N, and 18 O labeling experiments that are consistent with previously reported values.