Photodissociation spectroscopy and dynamics of Mg+-formaldehyde

We have carried out photodissociation spectroscopy studies of the bimolecular complex Mg+(H2CO) 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=CH2 geometry with a theoretical bond energy of D-e(')(Mg-OCH2)=1.35 eV. The complex shows absorption bands that correlate with Mg+-based and formaldehyde-based radiative transitions. The lowest-energy band is assigned as (A) over tilde (2)A(')(B-2(1))<--(X) over tilde (2)A(1), to an excited state of mixed Mg+(3ppi) and H2CO(pi(*)) orbital character. The band exhibits complex vibrational structure with considerable excitation of the CH2 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) over tilde (2)A(')(B-2(2))<--(X) over tilde (2)A(1) band 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 demonstrates a change in geometry from a linear or near linear Mg-O-C (C-2v) ground state to a bent (C-s) excited state [theta(Mg-O-C)=139degrees+/-3degrees]. Spectroscopic rotational constants are in very good agreement with ab initio predictions for this band. The Mg+-based (D) over tilde (2)A(1)<--(X) over tilde (2)A(1) band also exhibits pronounced vibrational structure including strong Mg-O and C=O stretch signals, consistent with formation of a partial Mg-O sigma bond in this state. Mg+ is the major dissociation product through the uv-visible region. However, in the (B) over tilde <--(X) over tilde, (C) over tilde <--(X) over tilde, and (D) over tilde <--(X) over tilde 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 similar to1/3 in the (D) over tilde <--(X) over tilde band. Our results suggest that there is no significant activation barrier to reaction above the endothermicity. (C) 2002 American Institute of Physics.