3E states

Flash photolysis experiments95, 97 have revealed the presence of S2(3E ) iQ its ground vibrational state. There is good reason to believe, however, that the product of reaction between S(1D2) and COS is S2(XA9) or S2(XS4) which is collisionally deactivated slowly to the ground state. Vibrational relaxation in the 3E state is very rapid. S2(3 ) disappears in a second-order reaction, presumably... 

The validity of Johnston s interpretation of the experimental facts in terms of the simple unimolecular dissociation (1) has been questioned by Lindars and Hinshelwood120 and by Reuben and Linnett121. These workers maintain that isothermal plots of k versus p are not smooth curves, but consist of a number of straight lines linked by markedly curved portions. To explain such behaviour they incorporate into their mechanism a collision-induced crossover of vibrationally excited N20 (XS) to repulsive 3II and 3E states. While we incline towards the simpler view held by Johnston105 and others106-116, we feel that this feature of the decomposition kinetics merits further investigation. 

Applying S. to this 3E state with Ms=l produces the 3E state with Ms = 0 ... 

The only other state that can have Ml=0 and Ms=0 is the E state, which must itself be a combination of the two determinants, Itc i Ptt iOcI and l7tiOC7t iPl, with Ml=0 and Ms=0. Because the E state has to be orthogonal to the 3E state, the combination must be... 

Consequently both the temperature broadening and the main part of the shift of the ZPL of the N-V centers at 637 nm ZPL are well described by presented theory supposing that strong softening of the elastic springs in the excited state takes place (see Figs 2a and b). Only relatively small blue shift at T < 40 K most probably has another origin it can be explained by repopulation between strain-induced sublevels of the excited 3E state [32]. 

The most important examples of 2S states to be described in this book are CO+, where there is no nuclear hyperfine coupling in the main isotopomer, CN, which has 14N hyperfine interaction, and the Hj ion. A number of different 3E states are described, with and without hyperfine coupling. A particularly important and interesting example is N2 in its A 3ZU excited state, studied by De Santis, Lurio, Miller and Freund [19] using molecular beam magnetic resonance. The details are described in chapter 8 the only aspect to be mentioned here is that in a homonuclear molecule like N2, the individual nuclear spins (1 = 1 for 14N) are coupled to form a total spin, It, which in this case takes the values 2, 1 and 0. The hyperfine Hamiltonian terms are then written in terms of the appropriate value of h As we have already mentioned, the presence of one or more quadrupolar nuclei will give rise to electric quadrupole hyperfine interaction the theory is essentially the same as that already presented for1 + states. 

Figure 8.22. Schematic energy level diagram for the first four rotational levels of N2 in its A 3E+ state, showing the nuclear hyperfine states which are allowed to combine with each N level. Relative vertical spacings are not drawn to scale [43].

Thus, an approximate value for the unknown 3E 1E+ interaction can be obtained from an observable diagonal spin-orbit constant. As will be discussed later (Sections 3.4.4 and 5.3.3), second-order spin-orbit effects of this type contribute significantly to the effective spin-spin interaction in 3E states. 

The splitting of E states with 5 > is due to the spin-spin effect. For example, for 3E states,... 

Expressed in the form of Eq. (3.4.30), the calculation of matrix elements of the spin-spin operator is not trivial. For 3E states, only the following terms in Hss give rise to nonzero matrix elements ... 

For example, the direct spin-spin interaction of a n2 3E state may be derived as follows (see Kayama, 1965). From Eq. (3.4.31b),... 

For valence states of homonuclear molecules, the it orbital is the antibonding counterpart, ng, of the bonding nu orbital. One then finds that aWu a g (aw aw ). Thus the interaction between the 3E+ and 3E states will dominate the Xs ° values for the 7t37t configuration and is given in Table 3.8. For example, in the P2 molecule, it has been observed that A (a3E+) = —3.26 cm-1 and A (b3E ) = +3.20 cm-1 (Brion, et al., 1974 Brion and Malicet, 1976). For this heavy (third-row) molecule, the direct spin-spin parameter is negligible. 

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