Rotational Analysis of a Spectrum

When a spectrum is recorded, there are no molecule or quantum number labels attached to it. Sometimes the spectrum contains a previously unobserved band of a known species. More often it contains well known bands of well known species, but the experimentalist usually does not initially know to which of several plausible or implausible species the band belongs. The best way to answer this band identity question quickly and unambiguously is to perform a rotational analysis. It is often easier and always more conclusive to identify a band by the molecular constants (e.g., B, B ) derived from it than from the frequency location of the maximum intensity feature of the band. (In fact, incompletely resolved band intensity maxima are notoriously unreliable, because the position of the maximum can depend strongly on resolution, temperature, and pressure.) 

A dispersed fluorescence (DF) spectrum isolates individual pairs of P, R, lines from which A2F values may be directly measured. The change in A2F values observed via consecutive (but unknown) J -levels immediately gives a value for B because A2F (n + 1) - A2F (n) = 4B . 

Other methods for dealing with a complex spectrum include simplifying it by cooling the molecule to 5 K in a supersonic jet (Section 1.2.3) or selectively enhancing the relative intensities of certain low-7 rotational lines (Magnetic Rotation Spectroscopy, Section 1.2.5). 

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