Photo-excitation free electron

The photolysis of diazoalkanes both in the gas phase and in solution is a carbenoid reaction. Moreover, the results of EPR-spectroscopic investigations (Section IIB) demonstrate that triplet carbenes can be generated by irradiation of diazoalkanes. That the reactive intermediates in carbenoid reactions are free carbenes is usually taken as self-evident. While such an assumption is probably wholly justified in most cases, it is worth remembering that both in the gas phase and in solvents such as n-hexane, the electronic absorption spectra of simple diazoalkanes show definite fine structure (Bradley etal., 1964a). This implies that the photo-excited state is bonding (Hoffmann, 1966) and consequently may have a life-time long enough to enable it to react directly with another molecule... 

In a supramolecular approach to fullerene-porphyrin hybrids, the assembly of a rigidly connected dyad, in which a zinc tetraphenylporphyrin, Zn(TPP), is noncovalently linked to a C60 derivative via axial pyridine coordination to the metal, was reported [219-222]. Photo excitation of the dyad Zn-complex led to electron transfer with very long lifetimes of the charge-separated pairs, as revealed by optical spectroscopy and confirmed by time-resolved electron paramagnetic resonance spectroscopy. Accordingly, two different solvent-dependent pathways can be considered for the electron-transfer processes. Either the excitation of the porphyrin chromophore is followed by fast intramolecular electron transfer inside the complex, or alternatively the free porphyrin is excited undergoing intermolecular electron transfer when the acceptor molecules ap-... 

Normally the term radical (or free radical ) is confined to molecules or ions with one unpaired electron (called doublet states because the electron has two magnetic quantum numbers +1/2). In the non-metal field the most common paramagnetic species other than radicals are those with two unpaired electrons, called triplet states (magnetic quantum numbers 0, 1). Quite the most important triplet-state molecule is dioxygen and it is a great pity that ESR spectroscopy can, for various reasons, only be used to detect O2 in the gas-phase or certain crystalline solids. Other important triplet-states are sometimes obtained on photo-excitation of ordinary (singlet-state) molecules or ions, and these have reactions in some ways typical of di-radicals (i.e.,... 

When light of the appropriate wavelength is allowed to shine on a compound containing a 77 bond, an electron from the 77 orbital is promoted to the 77 orbital. The product can be considered to be a 1,2-diradical, and it undergoes reactions typical of free radicals. The C=0, C=S, and C=C 77 bonds can all be photoexcited in this manner. The weaker the 77 bond, the easier it is to photo-excite. 

Figure 27.3 Relationship between the distribution of nascent photon-excited electrons and the DOS of a metal. The excited electrons are classified as sub-vacuum electrons Ek < OeV) and photoelectrons, or free electrons > OeV). Adapted from Zhou et al, in Laser Spectroscopy and Photo-Chemistry on Metal Surfaces, II, 1995, with permission of World Scientific Publishing Co...

Figure 7-11 and its caption (Crutzen, 1983) depict the most important of the gas phase and photochemical reactions in the atmosphere. Perhaps the single most important interaction involves the hydroxyl free radical, OH-. This extremely reactive radical is produced principally from the reactions of electronically excited atomic oxygen, 0( D), with water vapor. Photo-... 

Figure 5.4, one can easily understand why the interfacial electron transfer should take place in the 10-100 fsec range because this ET process should be faster than the photo-luminescence of the dye molecules and energy transfer between the molecules. Recently Zimmermann et al. [58] have employed the 20 fsec laser pulses to study the ET dynamics in the DTB-Pe/TiC>2 system and for comparison, they have also studied the excited-state dynamics of free perylene in toluene solution. Limited by the 20 fsec pulse-duration, from the uncertainty principle, they can only observe the vibrational coherences (i.e., vibrational wave packets) of low-frequency modes (see Figure 5.5). Six significant modes, 275, 360, 420, 460, 500 and 625 cm-1, have been resolved from the Fourier transform spectra of ultrashort pulse measurements. The Fourier transform spectrum has also been compared with the Raman spectrum. A good agreement can be seen (Figure 5.5). For detail of the analysis of the quantum beat, refer to Figures 5.5-5.7 of Zimmermann et al. s paper [58], These modes should play an important role not only in ET dynamics or excited-state dynamics, but also in absorption spectra. Therefore, the steady state absorption spectra of DTB-Pe, both in...

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