A typical vibrational band of an electronic transition

The second differences may be measured directly from the spectrum without any knowledge whether J increases or decreases as one follows a branch toward higher frequency. Thus one immediately obtains a good estimate for B — B neglecting contributions from D and D . The line intensities usually vary smoothly along a rotational branch because collisions cause rotational populations to approach Boltzmann equilibrium rapidly and rotational linestrength factors (see Section 6.1 and Whiting, et ai, 1980) are approximately linear in J. 

The bandhead conveys useful qualitative and quantitative information, but it may also conceal the low-J regions of the R, Q, and P branches. The qualitative information comes directly from the crude shape of a band. If a band has its head in the R branch, the band will appear to terminate abruptly on the high-frequency (blue) side and the rotational branches will extend, with increasing line spacings toward low-frequency (red). Such a band is said to be red 

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Figure 1.2 A typical vibrational band of an electronic transition, the (1,0) band of a 102Ru12C 1I1 <— X1 "1" transition. The assignments of rotational lines are distinguished by vertical tie lines. The lines in each R, Q, P rotational branch connect with a different horizontal tie line. The bandhead occurs in the R branch near i" = 9 (from Langenberg, et al., 1998).

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