DETECTION OF THE QUADRUPOLE HYPERFINE-STRUCTURE IN HCNH

We report the first measurement of the electric quadrupole hyperfine structure of HCNH+. The J = 1 --> 0 transition of this interstellar molecular ion was observed toward the cold, dark cloud TMC-1, using the NRAO 12 m telescope at 74 GHz. The three hyperfine components of this transition were clearly detected and resolved, enabling the first experimental determination of the quadrupole coupling constant eqQ of HCNH+. The value of this constant is calculated to be eqQ = -0.49 +/- 0.07 MHz. The column density of HCNH+ toward TMC-1 was found to be N(tot) approximately 2.8 x 10(13) cm-2 corresponding to a fractional abundance relative to H-2 of f approximately 3 x 10(-9). This abundance is at least one order of magnitude higher than the predictions of ion-molecule chemistry. Detection of the hyperfine structure clearly establishes the presence of HCNH+ in interstellar space.