The ν2 bending vibrational structure of the X̃2Σ+ state of MgNC.

We have generated MgNC in supersonic free jet expansions and observed the laser induced fluorescence (LIF) of the Ã(2)Π-X̃(2)Σ(+) transition. We measured the LIF dispersed spectra from the single vibronic levels of the Ã(2)Π electronic state of MgNC, following excitation of each ν(2) bending vibronic band observed, i.e., the κ series of the (0,v(2)('),0)-(0,0,0), v(2)(') = 0, 1, 2, 4, and 6 vibronic bands. In the vibrational structure in the dispersed fluorescence spectra measured, the long progression of the ν(2) bending mode in the X̃(2)Σ(+) state is identified, e.g., up to v(2)('')=14 in the (0,6,0)-(0,v(2)(''),0) spectrum. This enables us to derive the potential curve of the ν(2) bending mode in the X̃(2)Σ(+) state. We used two kinds of models to obtain the potential curve; (I) the customary formula expressed in the polynomial series of the (v(2)('')+(d(2)/2)) term and (II) the internal rotation model. The potential curve derived from model (I) indicates the convergence of the bending vibrational levels at about 800 cm(-1) from the vibrationless level of MgNC, which may correspond to the barrier height of the isomerization reaction, MgNC ⇌ MgCN, in the X̃(2)Σ(+) state. Model (II) gives a simple picture for the isomerization reaction pathway with a barrier height of about 630 cm(-1) from the vibrationless level of the more stable species, MgNC. This shows that the v(2)('')=8 bending vibrational level of MgNC is already contaminated by the v(2)('')=2 bending vibrational level of the isomer, MgCN, and implies that the isomerization reaction begins at the v(2) ('')=8 level. The bending potential surface and the isomerization reaction pathway, MgNC ⇌ MgCN, in the X̃(2)Σ(+) state are discussed by comparing the potential derived in this study with the surface obtained by quantum chemical calculation.