Absorption event

Fig. 6.1. Jablonski diagram, representing electron energy levels of fluorophores and transitions after photon excitation. S = electronic state, different lines within each state represent different vibrational levels. Blue arrows represent absorption events, green arrows depict internal conversion or heat dissipation, and orange arrows indicate fluorescence emission. Intersystem crossing into triplet states has been omitted for simplicity (see also Chaps. 1 and 12).

We start with an atom containing N electrons, with a total energy E in the initial state, denoted with the superscript i. The atom absorbs a photon of energy hv, the absorption event taking less than 10 17 s. Some 10 14 s later, the atom has emitted the photoelectron with kinetic energy Ek and is itself in the final state with one electron less and a hole in one of the core levels. The energy balance of the event is... 

A spectrometer with rapid response electronics should be used for electrothermal atomization, as it must follow the transient absorption event in the tube. Automatic simultaneous background correction (see Section 2.2.5.2) is virtually essential, as non-specific absorption problems are very severe. It is important that the continuum light follows exactly the same path through the furnace as the radiation from the line source (assuming a deuterium lamp is being used rather than Smith-Hieftje or Zeeman effect). The time interval between the two source pulses should be as short as possible (a chopping frequency of at least 50 Hz) because of the transient nature of the signal. 

Both the initial absorption event, and the subsequent fluorescence emission are directional processes. The strength of a particular fluorescence process will then be propor-... 

It was suggested by Megill and Hasted [169] in relation to polar-cap absorption events, and by Kummler and Bortner [170] with respect to... 

The Heisenberg uncertainty principle [Eq. (1.6)] defines the time scale of radiation absorption event as inversely proportional to the radiation s frequency. Processes that occur faster than the spectroscopic time scale are time averaged during the absorption process. 

The primary processes of photochemistry involve the light absorption event, which we have already discussed, together with the subsequent deexcitation reactions. We can portray such transitions on an energy level diagram, as in Figure 4-9 for chlorophyll. In this section we discuss the various deexcitation processes, including a consideration of their rate constants and lifetimes. 

The first step in photosynthesis is light absorption by one of the pigments. The absorption event (discussed in Chapter 4, e.g., Section 4.2E) for the various types of photosynthetic pigments described in this chapter can be represented as follows ... 

In all cases, the aim is to determine the position of the centroid of the induced signal with the highest resolution possible. The various types of position sensitive detectors differ only in the way in which the localisation of a photon absorption event is achieved. 

A position sensitive detector (PSD) employs the principle of a gas proportional counter, with an added capability to detect the location of a photon absorption event. Hence, unlike the conventional gas proportional counter, the PSD is a line detector that can measure the intensity of the diffracted beam in multiple (usually thousands) points simultaneously. As a result, a powder diffraction experiment becomes much faster, while its quality generally remains nearly identical to that obtained using a standard gas proportional counter. ... 

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