Schematic illustration of intensity interference effects

Hi2 = 10 cm-1). The inner edges and widths of the solid curves depict, respectively, the energies of the perturbed levels, E+ and E- [reduced by BJ(J+ 1)], and the intensities of the transitions into E+ and E-. The top frame illustrates simple intensity borrowing, fi°0 = 0. The middle frame shows the vanishing of intensity at the crossing point (J = 15), E = E, when (i 0 — fi20. The bottom frame exemplifies the general case, fi°0 0 (here 

2/rJo = M2o) where the location of the intensity vanishing point (J 17.3) is given by Eq. (6.3.15) or Eq. (6.3.16). See also Fig. 5.8 for other properties of the same homogeneous perturbation illustrated here. [Rrom Dressier (1970).] 

The intensity interference pattern illustrated by Fig. 6.8 for bound-bound transitions is very similar to the Beutler-Fano lineshape for bound-free transitions discussed in Sections 7.9 and 8.9. The 101,000-106,000 cm-1 region of Fig. 6.8 is a band-by-band rather than a continuous representation of a Fano profile with q 0 [see Fig. 7.26 and compare Eq. (7.9.6) to Eq. (6.3.17)]. 

There are two classes of transitions between pure case (a) basis functions. Parallel and perpendicular transitions are readily distinguished by their characteristic rotational branch intensity patterns. AA = 0 transitions are called parallel bands. They have R and P branches of comparable intensity and a weak or absent Q branch. A A = 1 transitions are perpendicular bands and have a strong Q branch and R and P branches of approximately half the intensity of the Q branch. 

When a AA = 1 perturbation occurs, the PQR intensity pattern for transitions terminating on the mixed levels is not necessarily intermediate between the parallel and perpendicular patterns. For example, consider a perturbation in the upper electronic state which affects the J level. The intensity ratio for the P(J + 1) and R(J — 1) lines can depart from the approximate 1 1 ratio typical of both parallel and perpendicular unperturbed transitions. In fact, one or the other line can vanish. Furthermore, in a fluorescence progression from a mixed upper level, v J, the P(J + l) -line can be anomalously weak relative to R(J — 1) in some (v, v ) bands and anomalously strong in others. The sign and magnitude of the effect can vary with v even though the perturbation is in the v level  

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Figure 6.7 Schematic illustration of intensity interference effects. The straight lines are the basis function (i.e., deperturbed) reduced term values, E — BJ J + 1), where B — (B + Sj)/ 2. The level crossing occurs at J = 15 = 0.9 cm-1, B = 1.1 cm-1,...

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