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 12m 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, ∼ 2.8 × 10¹³ cm^-2, corresponding to a fractional abundance relative to H₂ of f ∼ 3 × 10^-9. This abundance is at least one order of magnitude higher than the predictions of ionmolecule chemistry. Detection of the hyperfine structure clearly establishes the presence of HCNH⁺ in interstellar space.