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 + ( 2 S) + I ( 2 P 3/2 ). Analysis of photofragment images taken at photon energies near the threshold gives a bond dissociation energy D 0 (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* ( 3 P ° J ) + I + ( 3 P 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 ( 1 S) + I + ( 3 P J ) products.