Identification and Location of Metastable States by Perturbation Effects

8 Identification and Location of Metastable States by Perturbation Effects 

The location of metastable states is often made possible by observation of intercombination systems. For example, the intensity of the Cameron system in CO (a3II — X1E+) is borrowed from singlet transitions through spin-orbit mixing (see Section 6.4). When intercombination transitions are not observed, it is often possible to locate the states of different multiplicity from that of the ground state by observation of spin-orbit AS 0 perturbations. 

The example of the C2 molecule given in the previous section is a very important one. A similar result has been demonstrated in the case of the MgO molecule (Ikeda, et al., 1977). Thanks to perturbations in the v 3 levels of the X1E+ state by the a3II state, the electronic ground state has been definitely proven to be X1E+ and the a3II state has been found to lie above the X1E+ state at Te = 2623 cm-1. 

The NO spectrum is one of the most completely known among all diatomic molecules. However, it was not until 1980 (Miescher, 1980) that it became possible to locate the quartet states precisely with respect to the well-known doublet states. Small perturbations between the B2n state (v = 1) and the b4E- state (v = 0) had been predicted by Miescher on the basis of ab initio calculations (reported by Field, et al., 1975) to be characterized by interaction matrix elements on the order of 5 cm-1. Such perturbations have been observed (Miescher, 1980) and, based on direct b4E — X2n (Dyer, et al., 1993) and a4n — X2n (Copeland, et al., 1995 Drabbels, et al., 1995) excitation spectra, b4E — a4n emission spectra (Huber and Vervloet, 1988), and a4n — X2n excitation detected by B2n — X2n emission due to perturbation-facilitated a4n B2n collisional transfer (Cosby, et al., 1997), a complete and accurate picture of the NO quartet states and the locations of all b B and a B perturbations have been assembled. 

Heavy molecules are, in principle, more favorable for detecting spin-orbit perturbations and, in this way, locating metastable states. In the NS molecule, which is isovalent with NO, a perturbation matrix element between b4E and B2n of 8 cm-1 has allowed the 4E state to be located (Jenouvrier and Pascat, 1980). In the NSe molecule, the 4n state has been detected by its interaction 

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