Direct-line fluorescence

AFS quantifies the discrete radiation emitted by excited state atoms that have been excited by radiation from a spectral source. There are a number of mechanisms that are responsible for the atomic fluorescence signal resonance fluorescence, step-wise fluorescence, direct-line fluorescence, and sensitized fluorescence. Generally, the lowest resonance transition (l->0) is used for AFS. If a line source is used for excitation and if the atomic vapor is dilute, then the radiant power of the atomic... 

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. 

In direct-line fluorescence, an atom is excited (usually from the ground state) by a radiation source, and then undergoes a direct radiational transition to a metastable level above the ground state. An example is absorption at the 283.31 nm line by ground-state lead atoms, with subsequent emission at 405.78 nm. Xs with resonance fluorescence, direct-line fluorescence may be excited by absorption of a nonresonance line (e.g., tin fluorescence at 333.06 nm). 

All types of nonresonance fluorescence, particularly direct-line fluorescence, can be analytically useful sometimes it is more intense than resonance fluorescence, and it offers the advantage that scattering of the exciting radiation can be eliminated from the fluorescence spectrum by removing it with a filter or a monochromator. Self-absorption problems (absorption of the emitted radiation by the sample atoms) can also be avoided by measuring fluorescence at a nonresonance line that is not also absorbed. 

Figure 7.57 Example of atomic fluorescence transitions (a) resonance fluorescence, (b) Stokes direct-line fluorescence, and (c) anti-Stokes direct-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). 

If the wavelength of the fluorescence line is greater than that of the excitation line, the effect is called Stokes direct line fluorescence. In the... 

Fluorescence of the type shown in Figure 2-1 Ic is called direct line fluorescence. Excitation raises the electron to an excited state above the... 

In a nonresonance fluorescence transition, the photons involved in absorption and fluorescence processes have different wavelengths (Figure IB). The particular transition shown in Figure IB is called Stokes direct-line fluorescence, which is frequently used for AFS with laser excitation. Nonresonance transitions have the advantage that a wavelength selection device can be used to distinguish between fluorescence and scattered source radiation. 

Figure 2. Classification of LEAF schemes A) Resonance fluorescence B) Direct-line fluorescence C) Collisionally assisted direct-line fluorescence D) Anti-Stokes direct-line fluorescence...

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