Photon deexcitation

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Fluorescence and phosphorescence are particular cases of luminescence (Table 1.1). The mode of excitation is absorption of a photon, which brings the absorbing species into an electronic excited state. The emission of photons accompanying deexcitation is then called photoluminescence (fluorescence, phosphorescence or delayed fluorescence), which is one of the possible physical effects resulting from interaction of light with matter, as shown in Figure 1.1. 

Resonance energy transfer between the aromatic amino acids proceeds by very weak coupling between the donor and acceptor.151,52) Very weak coupling implies that the interaction between the donor and acceptor wave functions is small enough so as not to perturb measurably the individual molecular spectra. This transfer process, which is distinct from the trivial process of absorption of an emitted photon, involves radiationless deexcitation of an excited-state donor molecule with concomitant excitation of a ground-... 

Fig. 1 Energy diagrams illustrating the process of one-photon absorption (IPA) and two-photon absorption (2PA) for (left) a molecule without an inversion center (right) a molecule with an inversion center. The length of each arrow is proportional to the photon energy. The dotted arrows represent possible deexcitation pathways...

One-step processes occur whenever the stochastic process consists of the absorption or emission of photons or particles, the excitation and deexcitation of atoms or nuclei, or of electrons in semiconductors, the birth and... 

Our present focus is on correlated electronic structure methods for describing molecular systems interacting with a structured environment where the electronic wavefunction for the molecule is given by a multiconfigurational self-consistent field wavefunction. Using the MCSCF structured environment response method it is possible to determine molecular properties such as (i) frequency-dependent polarizabilities, (ii) excitation and deexcitation energies, (iii) transition moments, (iv) two-photon matrix elements, (v) frequency-dependent first hyperpolarizability tensors, (vi) frequency-dependent polarizabilities of excited states, (vii) frequency-dependent second hyperpolarizabilities (y), (viii) three-photon absorptions, and (ix) two-photon absorption between excited states. 

The radiative lifetime rr is l/kT. It is the real emission lifetime of a photon that should be measured independently of the other processes that deactivate the molecule. However, since these processes occur in parallel to the radiative process, it appears impossible to eliminate them during radiative lifetime measurements. Therefore, we will measure a time characteristic of all deexcitation processes. This time is called the fluorescence lifetime and is lower than the radiative lifetime. A fluorophore can have one or several fluorescence lifetimes in this case, we can determine the fractional contribution of each lifetime and calculate the mean fluorescence lifetime r0 or (r) ... 

Xmax T = 3.67 x 106 nm K photon basis Deexcitation (first order) (4.11)... 

The absorption of light can lead to a photochemical reaction initiated by a molecule other than the one that absorbed the photon. This phenomenon suggests that electronic excitation can be transferred between molecules, resulting in the excitation of one and the deexcitation of the other. For instance, the excitation energy of might be transferred to a second molecule, represented in the ground state by S2... 

This second molecule thereby becomes excited, indicated by S2(W)W-), and the molecule that absorbed the photon becomes deexcited and is returned to its ground state. Such transfer of electronic excitation from molecule to molecule underlies the energy migration among the pigments involved in photosynthesis (see Chapter 5, Sections 5.3 and 5.4). We will assume that Equation 4.8 represents a first-order reaction, as it does for the excitation exchanges between chlorophyll molecules in vivo (in certain cases, Eq. 4.8 can represent a second-order reaction, i.e., dS /dt then equals fc S j). 

As we have just calculated, each chlorophyll molecule in an unshaded chloroplast can absorb a photon about once every 0.1 s. When there are 250 chlorophylls per reaction center, 12.5 of these molecules are excited every 5 ms (250 chlorophylls x 10 excitations per chlorophyll/1 s x 0.005 s). However, because the average processing time per reaction center is about 5 ms, only one of these 12.5 excitations can be used photochemically — the others are dissipated by nonphotochemical deexcitation reactions. Consequently, although the biochemical reactions leading to CO2 fixation operate at their maximum rates under such conditions of high PPF, over 90% of the electronic excitations caused by light absorption are not used for photosynthesis (Fig. 5-12). 

We observe that we obtain poles when either ( or co equals an excitation or deexcitation energy of one-photon transitions and when the sum of two frequencies, (On + is equal to an excitation or deexcitation energy of two-photon transitions. The residues provide information about one- and two-photon transition matrix elements. 

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