We have carried out photodissociation spectroscopy studies of the bimolecular complex Mg~+ (H_2CO) in the visible and near-uv regions. The work is supported by electronic structure calculations of the ground and low-lying excited states of the complex. Mg~+-formaldehyde is bound in a C_(2v) Mg~+-O=CH_2 geometry with a theoretical bond energy of D"_e(Mg-OCH_2)=1.35 eV. The complex shows absorption bands that correlate with Mg~+-base and formaldehyde-based radiative transitions. The lowest-energy band is assigned as A ~2A' (~2B_1) <-X~2A_1, to an excited state of mixed Mg~+ (3p pi) and H_2CO(pi~*) orbital character. The band exhibits complex vibrational structure with considerable excitation of the CH_2 out-of-plane wag and C=O stretch modes; the vibrational frequencies are shifted dramatically from their values in the ground state, showing the effect of a significant weakening of the C=O bond and out-of-plane distortion of the complex. Excitation in the Mg~+-based B~2A' (~2B_2)<-X~2A_1 bans shows predominantly low-frequency vibrational motions assigned to the intermolecular in-plane wag and Mg-O stretch modes. Birge-Sponer analysis gives the Mg-O bond energy in the ground state as D"_e = 1.29 eV. Partially resolved rotational substructure clearly demosntrates a change in geometry from a linear or near linear Mg-O-C (C_(2v)) ground state to a bent (C_s) excited state [zeta(Mg-O-C) = 139 deg +- 3 deg]. Spectroscopic rotational constants are in very good agreement with ab initio predictions for this band. The Mg~+-based D~1A_1<-A_1 band also exhibits pronounced vibrational streucture including strong Mg-O and C=O stretch signals, consistent with formation of a partial a partial Mg-O sigma bond in this state. Mg~+ is the major dissociation product through through the uv-visible region. However, in the B<-X, C <-X, and D<-X absorption bands, we also observe a substantial branching to the reactive dissociation product MgH~+. The reactive branching ratio increases with photon energy through the absorption bands, reaching a reactive quantum yield of approx 1/3 in the D<-X band. Our results suggest that there is no significant activation barrier to reaction above the endothermicity.
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