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, 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 (S) + I (P). Analysis of photofragment images taken at photon energies near the threshold gives a bond dissociation energy D(Mg-I) = 203.0 ± 1.8 kJ/mol (2.10 ± 0.02 eV; 17 000 ± 150 cm). At photon energies of 33 000-41 000 cm, 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 shows vibrational structure, giving an average excited state vibrational spacing of 227 cm. 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* (P ) + I (P). From 38 500 to 41 000 cm, 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 (S) + I (P) products.