Photodissociation and photoionization of highly excited HI molecules

Abstract

The spectroscopy and photochemistry of HI have been studied at excitation energies in the vicinity of H-atom Lyman–α (82259 cm⁻¹) using the techniques of 1-photon vacuum UV photodissociation, resonance-enhanced multiphoton ionization (REMPI) spectroscopy and photofragment ion imaging. REMPI spectra, recorded using UV photons in the energy range 27350–27750 cm⁻¹, exhibit complicated structure in channels corresponding to H⁺, I⁺ and HI⁺ ion formation. The former two channels require absorption of a minimum of 5 UV photons, whereas HI⁺ X ²Π formation requires 4 UV photons. The spectral features are assigned to resonances at the 3-photon energy involving six states with Ω = 0 and one state with Ω = 3. Ion images recorded in the same range of UV frequencies exhibit multiple velocity components for the H⁺ ionic fragment, and the excitation energy dependences of the ring radii reveal two mechanisms for H⁺ formation. The first route is 3 + 1 photon excitation to super-excited Rydberg states of neutral HI with [A ²Σ⁺] cores that autoionize to high vibrational levels (⁺15) of HI⁺ (X ²Π); subsequent 1-photon photolysis of these vibrationally excited ions makes H⁺. The second route is dissociation of 4-photon excited HI to H(n  = 2) + I(²P_3/2), followed by 1-photon ionization of the H(n  = 2) atoms. Vacuum UV photolysis of HI at 121.6 nm combined with H-Rydberg atom photofragment translational spectroscopy reveals dissociation ia four different channels: H(n  = 1) atoms are formed in association with I(5p⁵, ²P_3/2), I(5p⁵, ²P_1/2), I(5p⁴ 6s¹, ⁴P_5/2) and I(5p⁴ 6s¹, ²P_3/2). Anisotropy parameters for all these channels are positive but non-limiting, and photolysis propensities show a marked dependence on the initial rotational state of the HI. These observations are discussed in terms of excitation to predissociated Rydberg or ion-pair states identified in the REMPI spectra. A previously unreported Ω = 3 Rydberg state is identified ia HI⁺ formation following multiphoton absorption resonant at the 2 UV + 1 visible photon energy. A band with origin at 68326.18(73) cm⁻¹, is assigned to excitation to the ′ = 0 level of a ³Δ₃ state composed of a Rydberg dπ electron orbiting an HI⁺ X ²Π_3/2 core.