High-resolution spectroscopy of [H,C,N] ⁺ : III. Infrared à ² Σ ⁺ ← X ² Π electronic transition of HCN ⁺

The òΣ⁺ ← X̃²Π electronic transition of the fundamental HCN⁺ ion is reported. The spectrum exhibits a rotational, fine, and hyperfine structure that provides insights into the geometries and wavefunctions of the òΣ⁺ and X̃²Π states. The extracted spectroscopic constants for the à state include (in cm^-1) <i>T</i>_e = 3238.8224(2), <i>B</i> = 1.39568(3), <i>D</i> = 2.27(9) × 10^-5, <i>γ</i> = -0.07332(7), <i>γ</i>_D = 1.87(3) × 10^-4, and the Fermi contact hyperfine constant for the HCN⁺ nitrogen atom <i>b</i>_F = 0.0059(1). Based on this value, the òΣ_1/2⁺ state electronic wavefunction is estimated to have 11.5 ± 0.2% s orbital character. The spin-orbit coupling constant for the X̃ state was also determined with high precision <i>A</i>_SO = -49.3122(4) cm^-1. This work illustrates that leak-out spectroscopy can be applied to measure high-resolution spectra of low energy electronic transitions.