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

Abstract

The bond strength and photodissociation dynamics of MgI+ are determined by a combination of theory, photodissociation spectroscopy and photofragment velocity map imaging. From 17000 to 21500 cm-1 the photodissociation spectrum of MgI+ is broad and unstructured; photofragment images in this region show perpendicular anisotropy, 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 (2P3/2). Analysis of photofragment images taken at photon energies near threshold gives a bond dissociation energy D0(MgI+) = 203.0 {plus minus} 1.8 kJ/mol (2.10 {plus minus} 0.02 eV; 17000 {plus minus} 150 cm-1). At photon energies of 33000 to 41000 cm-1, exclusively I+ fragments are formed. Over most of this region, formation of I+ is not energetically allowed via one-photon absorption from the ground state of MgI+. Images show the observed products result from resonance enhanced two-photon dissociation. The photodissociation spectrum from 33000 to 38500 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 v=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{degree sign}J ) + I+ (3PJ). From 38500 to 41000 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+ (3PJ) products.