Fluorescence, types stepwise

Nonresonance Fluorescence. Nonresonance fluorescence occurs when the exciting wavelength and the wavelength of the emitted fluorescence line are different. There are two basic types direct-line fluorescence and stepwise-line fluorescence. 

Atomic fluorescence transitions may be divided into three types a) resonance fluorescence (the excitation and fluorescence lines have the same upper and lower energy levels), b) direct line fluorescence (the excitation line and fluorescence line have the same upper energy level, but different lower energy levels), and (c) stepwise line fluorescence (the excitation line and the fluorescence line have different upper energy levels) (Figure 141). 

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