Photodissociation spectroscopy and dynamics of Mg+-formaldehyde

Abstract

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 C2v Mg+–O=CH2 geometry with a theoretical bond energy of De″(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 à 2A′(2B1)←X̃ 2A1, to an excited state of mixed Mg+(3pπ) and H2CO(π*) 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 B̃ 2A′(2B2)←X̃ 2A1 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 De″=1.29 eV. Partially resolved rotational substructure clearly demonstrates a change in geometry from a linear or near linear Mg-O-C (C2v) ground state to a bent (Cs) excited state [θ(Mg-O-C)=139°±3°]. Spectroscopic rotational constants are in very good agreement with ab initio predictions for this band. The Mg+-based D̃ 2A1←X̃ 2A1 band also exhibits pronounced vibrational structure including strong Mg–O and C=O stretch signals, consistent with formation of a partial Mg–O σ bond in this state. Mg+ is the major dissociation product through the uv–visible region. However, in the B̃←X̃, C̃←X̃, and D̃←X̃ 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 ∼13 in the D̃←X̃ band. Our results suggest that there is no significant activation barrier to reaction above the endothermicity.