Photo-excitation photon energy

In photo CVD, the chemical reaction is activated by the action of photons, specifically ultraviolet (UV) radiation, which have sufficient energy to break the chemical bonds in the reactant molecules. In many cases, these molecules have a broad electronic absorption band and they are readily excited by UV radiation. Although UV lamps have been used, more energy can be obtained from UV lasers, such as the excimer lasers, which have photon energy ranging from 3.4 eV (XeF laser) to 6.4 eV (ArF laser). A typical photo-laser CVD system is shown schematically in Fig. 5.14.117]... 

It is easy to get burned by the sun while out sunbathing, because the second law of photochemistry shows how each UV photon from the sun releases its energy as it impinges on the skin. This energy is not readily dissipated because skin is an insulator, so the energy remains in the skin, causes photo-excitation, which is experienced as damage in the form of sunburn. 

We used the word simplistic in the previous paragraph because we described the vibrations getting more and more violent, as though there was no alternative to eventual bond cleavage. In fact, there is a very straightforward alternative absorption of a photon (i.e. energy) to X2 will also cause an electron to photo-excite, as follows. 

The multiplicity of excitations possible are shown more clearly in Figure 9.16, in which the Morse curves have been omitted for clarity. Initially, the electron resides in a (quantized) vibrational energy level on the ground-state Morse curve. This is the case for electrons on the far left of Figure 9.16, where the initial vibrational level is v" = 0. When the electron is photo-excited, it is excited vertically (because of the Franck-Condon principle) and enters one of the vibrational levels in the first excited state. The only vibrational level it cannot enter is the one with the same vibrational quantum number, so the electron cannot photo-excite from v" = 0 to v = 0, but must go to v = 1 or, if the energy of the photon is sufficient, to v = 1, v = 2, or an even higher vibrational state. 

The energies of rotation are quite small, so we require photons of relatively low energy to photo-excite between rotational quantum levels. For this reason, the spacings between rotational energy levels correspond to transitions in the far infrared and microwave regions of the electromagnetic spectrum. 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

Hitherto the discussion of Fig. 5.2 has neglected the possibility of non-radiative decay following 4d shell excitation/ionization. These processes are explained with the help of Fig. 5.2(h) which also reproduces the photoelectron emission discussed above, because both photo- and autoionization/Auger electrons will finally yield the observed pattern of electron emission. (In this context it should be noted that in general such direct photoionization and non-radiative decay processes will interfere (see below).) As can be inferred from Fig. 5.2(h), two distinct features arise from non-radiative decay of 4d excitation/ionization. First, 4d -> n/ resonance excitation, indicated on the photon energy scale on the left-hand side, populates certain outer-shell satellites, the so-called resonance Auger transitions (see below), via autoionization decay. An example of special interest in the present context is given by... 

Other excited electronic states can be located by direct and induced absorption and by two-photon spectroscopy. Photo-excitation and triplet energy transfer will produce populations mainly in the Si and 7 states, respectively, and thereafter induced-absorption proceeds by transitions from these to higher S and T states. But photo-excitation will also populate other excited states and... 

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