Dispersed Fluorescence Spectrum of AgAu

Emission spectra axe usually much more complicated than Direct Absorption spectra, because an absorption spectrum contains all possible upward transitions from a single initial vibration-electronic state, typically the ground state, whereas an emission spectrum contains all possible downward transitions from 

Although Dispersed Fluorescence (DF) spectroscopy is probably better classified as a form of double resonance spectroscopy, DF is discussed here because it is a form of emission spectroscopy where all of the emission originates from a single, laser-populated, upper electronic-vibrational-rotational level, (e, v, J ). A DF spectrum typically contains two [R J = J — 1), P(J = J + 1)] or three [i ( J — 1), Q(J ), P J +1)] rotational transitions per electronic-vibrational e ,v level. Often there is a progression of vibrational bands, [ v, v = n), (v, v = n + 1),. .. (v, v = n + to)] where v = n is the lowest vibrational level (band farthest to the blue) and v = n + m is the highest vibrational level observable (limited either by the detector response or Franck-Condon factors) in the DF spectrum (see Fig. 1.8 and Fig. 1.15). 

Sometimes the DF spectrum will contain transitions terminating on several different lower electronic states, each with its own progression of R, P doublets or R,Q,P triplets. DF spectra virtually assign themselves If J is initially known, then 

The relative R Q P intensities often reveal the identity of the lower electronic state. The spacings between members of the progression of R, P doublets are vibrational intervals 

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Figure 1.15 Dispersed Fluorescence Spectrum of AgAu. The AgAu A — X1 E+ DF spectrum contains long v —> v" vibrational progressions because the bond length in the A-state is much longer than in the X-state. The nodal structures of the v = 0, 2, and 3 vibrational states are displayed as intensity minima in the DF spectra. Since the vibrational quantum number is equal to the number of internal nodes in the wavefunction, a vibrational progression in an absorption or DF spectrum often reveals the absolute assignment of the initial vibrational state (from Fabbi, el al., 2001).

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