We have generated MgNC in supersonic free jet expansions and observed the laser　induced fluorescence(LIF) of the A˜2 – X˜2Σ+ transition. We measured the LIF　dispersed spectra from the single vibronic levels of the A˜ 2Π electronic state of MgNC, following excitation of each ν2 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 ν2 bending mode in the X˜2Σ+ 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 ν2 bending mode in the X˜2Σ+ 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+(d2/2)) 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 X˜2Σ+ 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 X˜2Σ+ state are discussed by comparing the potential derived in this study with the surface obtained by quantum chemical calculation.

This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C), 16510078, 2004 and by Hiroshima City University Grant for Special Academic Research (General Studies), 1102.