Photofragment imaging differentiates between one- and two-photon dissociation pathways in MgI

The bond strength and photodissociation dynamics of MgI⁺ are determined by a combination of theory, photodissociation spectroscopy, and photofragment velocity map imaging. From 17 000 to 21 500 cm^-1, the photodissociation spectrum of MgI⁺ is broad and unstructured; photofragment images in this region show perpendicular anisotropy, which is consistent with absorption to the repulsive wall of the (1) Ω = 1 or (2) Ω = 1 states followed by direct dissociation to ground state products Mg⁺ (²S) + I (²P_3/2). Analysis of photofragment images taken at photon energies near the threshold gives a bond dissociation energy D₀(Mg⁺-I) = 203.0 ± 1.8 kJ/mol (2.10 ± 0.02 eV; 17 000 ± 150 cm^-1). At photon energies of 33 000-41 000 cm^-1, exclusively I⁺ fragments are formed. Over most of this region, the formation of I⁺ is not energetically allowed via one-photon absorption from the ground state of MgI⁺. Images show the observed product is due to resonance enhanced two-photon dissociation. The photodissociation spectrum from 33 000 to 38 500 cm^-1 shows vibrational structure, giving an average excited state vibrational spacing of 227 cm^-1. This is consistent with absorption to the (3) Ω = 0⁺ state from ν = 0, 1 of the (1) Ω = 0⁺ ground state; from the (3) Ω = 0⁺ state, absorption of a second photon results in dissociation to Mg* (³P^° _J) + I⁺ (³P_J). From 38 500 to 41 000 cm^-1, the spectrum is broad and unstructured. We attribute this region of the spectrum to one-photon dissociation of vibrationally hot MgI⁺ at low energy and ground state MgI⁺ at higher energy to form Mg (¹S) + I⁺ (³P_J) products.