A-type doubling

Fig. 4.16 A-type doubling in a 2ni/(2 electronic level. Half the rotational levels belong to the 2Hi /2 electronic state, half to the 2n, 2 state. The doubling is exaggerated in practice, it is small and often not resolved.

In linear molecules, the electronic-rotation interaction terms in H cause the A-type doubling of electronic states, whereas the vibration-rotation interaction terms in H cause the /-type doubling of vibrational states. In addition, the perturbation H can cause interactions between vibration-rotation levels of different electronic states. If it happens that two vibration rotation levels of different electronic states of a molecule have... 

In nitric oxide, which is an exception among stable diatomic molecules, each level has a multiplicity of two (A-type doubling), so that the electronic partition function is actually 4.0. 

The two tetraorganodistannoxanes 77 and 78, containing phenyl-derived spacers, are listed in entries 39 and 40, respectively. Both compounds adopt A-type double ladder structures in the solid state (Figure 2.10.13) and for 78 this structure is retained in solution. 

By the argument given above we know that four of these are 2 states, with A = 0, and four are n states. The n states are those formed from px and p (which are the linear combinations of the complex exponential functions p+1 and p i). The two II states uA + uB are separated widely by the exchange integrals from the two uA — uB, and the A-type doubling will cause a further small separation of the nuclear-symmetric and nuclear-antisymmetric levels. The exchange terms similarly separate the Ua + uB s and p, functions from the uA — uB functions. The best approximate wave functions would then be certain linear combinations of the two nuclear-symmetric functions and also of the two nuclear-antisymmetric functions. 

Each energy level of a multiplet with A 0 is doubly degenerate, corresponding to the two values for M. Thus a A term has six different wave functions [Eqs. (13.87), (13.89), (11.57) to (11.59)] and therefore six different molecular electronic states. Spin-orbit interaction splits the A term into three levels, each doubly degenerate. The double degeneracy of the levels is removed by the A-type doubling mentioned previously. 

Fine structure splittings have been observed in the far-IR [1 to 3] and mid-IR [4] LMR spectra, in the IR emission [6] and absorption [7] spectra, and in the near-UV emission [8 to 12] and absorption [13] spectra of PH and PD. The fine-structure constants for the electronic ground state X i.e., X for the spin-spin coupling, y for the spin-rotation coupling, and Xq, Yd for their respective centrifugal distortions, and constants for the excited state A rij, i.e.. A, Ad for the spin-orbit coupling and p, q, Po or C (i = 0 to 3) for the A-type doubling, have been derived for definitions of the constants and relations with reference to earlier notations, see [8, table 3]. 

Fig. 5.34 Experimental arrangement for OODR A-type double resonance...

Moreover, for a vibrationally degenerate excited state, the rotational A-type doubling has to be corrsidered. Its matrix elements are ofif-diagonal in the quantum nttmbers A and k ... 

The high-resolution Fourier transform emission spectrum of the c 0- >0, 0- > 1 transitions [23] and the high-resolution LIF spectrum of the c n transition [22] gave the following A-type doubling constants (in cm" ) ... 

---