Photofragmentation Spectroscopy of Magnesium Cluster Ion-Molecule Complexes.

The photofragmentation spectroscopy and dynamics of various magnesium cluster ion-molecule complexes are studied in this work. A laser vaporization cluster source (LVCS) and a reflectron time-of-flight mass spectrometer (RTOFMS) have been constructed for these studies. We have studied the photofragmentation of MgD _sp{2}{+} and observed the reactive fragment (MgD^+) only to the red of the atomic resonance. The reactive action spectrum shows vibrational- and rotational-resolved structures superimposed on a broad continuum, indicating two pathways (direct and indirect) leading to the reaction. Based on the theoretical calculations, we assign the continuum as a 1 ^2A_1 to 1 ^2B_2 transition and the resonance structure as a 1 ^2A_1 to 1 ^2B_1 transition in C _{2v}>=ometry. The photodissociation of the molecular ion Mg _sp{2}{+} via the lowest excited state (1 ^2Sigma _sp{g}{+}) has been studied. The photodissociation spectrum shows a broad, structureless continuum. A spectral simulation based on theoretical potential curve points with a minor adjustment agrees very well with the experimentally observed spectrum. A triple -vector ( E-mu - v) correlation is observed in the TOF distribution of the fragment ions. Analysis of this correlation allows us to determine the anisotropy parameter (beta) and to estimate the binding energy in the ground state (1 ^2 Sigma_sp{u}{+}) of D_sp{0}{'' } = 10,200 +/- 300 cm ^{-1}, which agrees very well with the theoretical predictions. We have also investigated the photofragmentation of Mg_2(CO_2)_sp{1,2 }{+}. Three distinct molecular absorption bands in the red, green, and the near ultraviolet (UV) spectral regions are observed in the Mg_2(CO_2)^+ complex. Based on the theoretical calculations, we assigned the red band as the perpendicular transition 1 ^2Sigma ^+ to 1^2Pi, the green band as the parallel transition 1 ^2 Sigma^+to 2^2Sigma^+ , and the near UV band as a 1 ^2 Sigma^+to 2 ^2Pi transition in linear Mg_2(CO_2) ^+ complexes. In addition to the Mg ^+ and Mg_sp{2} {+} products, we have also observed the chemical product Mg_2O^+ with a {~}10% branching in the near UV band. Photofragmentation of the complex Mg_2(CO_2)_sp {2}{+} occurs over a broad range in the visible spectral region. We have found that the addition of the second solvent molecule dramatically enhances the solvent cage effect, as evidenced by a large increase in the Mg_sp{2}{+}Mg ^+ branching ratio.