Virtual intermediate

The fact that the single photon transitions require so little power prompts us to consider two photon transitions. Consider the Na 16d — 16g transition via the virtual intermediate 16f state which is detuned from the real intermediate 16f state as shown by the inset of Fig. 16.3. If the detuning between the real and virtual intermediate states is A and the matrix elements between the real states arefxx and fi2, the expression analogous to Eq. (16.5) for a two photon transition is... 

Primary photochemical events in reaction centers from the Rb. sphaeroides wild type and site directed mutant RCs, where the tyrosine at the M210 position was replaced by phenylalanine and leucine, were investigated by femtosecond time-resolved absorbance and ENDOR/TRIPLE spectroscopy techniques (Wachtveitl, et al., 1998). The results allowed the authors to suggest that primary electron transfer follows a stepwise mechanism and P+Bcl state is the first electron transfer intermediate in these mutants. Independent evidence in favor of the anion radical Bchl as the first material (but not virtual ) intermediate, was obtained (Yakovlev et al., 2000). It was demonstrated that in the porphyrin-modified RCs of Rb. spheroidas R-26, the femtosecond oscillations in the excited primary donor emission occur (Vos et al, 1994). 

This holds for the realm of linear optics. There is, however, new theoretical [4,5] and some experimental [4,6] work that seems to indicate that for 2-photon excitation, the existence of a CD effect can be due to pure electronic-dipole allowed transitions. Damping of fhe virtual intermediate state seems to be a requirement [5]. This phenomenon waits for broader investigation. 

Figure 5. Schematic representation of bridge-mediated superexchange of the "electron" (top, left to right) and hole (bottom, right to left) type, illustrated for the case of intermolecular electron transfer between two metal/ligand (M/L) complexes, where D = M/ B = L/ L, and A = M,. The virtual intermediate states for the hole and eleetron processes involve, respectively, charge localization based on the filled ( valence") and empty ("conduction ) bands of the bridge.

Figure 2. (a) The ET pathways in the bacterial photosynthetic reaction are shown proceeding from the special pair (B) via the bridging bacteriochlorophyll (BC) to the bacteriopheophytin (BP) (the right-hand branch is active). Arrows represent the nearest-neighbor interactions between chromo-phores. The B-BC and BC- BP interactions may produce either a real or a virtual intermediate localized on BC during the reaction [88]. (b) A proposed vibronically-coupled holehopping mechanism for ET in DNA is shown [69]. This mechanism emphasizes the delocalization of the hole. 

Basically, this set of models claims that SERS is a result of a resonance Raman process, where the molecule avails itself of the unoccupied states of the metal, or vice versa. Thus one can envisage a virtual intermediate transition to occur not between pure molecular states, but between a molecular and a metal state. This model has the advantage of a clear chemical picture, but it does not easily lend itself to rigorous calculations leading to quantitative predictions. 

Given that the molecule is initially in its ground state, there are initially q photons of the pump mode (k, X) and q photons of the harmonic mode (k, X ). There are three possible sequences of photon annihilation and creation (a, b, and c) that can provide a route from the initial to the final state, each involving different virtual intermediate states. To avoid confusion, the intermediate state labels r[Pg.619]

Figure 6.2 Allowed two-photon pathways via the NO A2E+ virtual state for the two-photon Oie2e and Oif2f rotational branches. Solid lines indicate a strongly allowed pathway and dashed lines indicate a weakly allowed pathway. The fi, 1 2 character of the virtual intermediate state is specified in parentheses in the one-photon branch labels. The A—X P( l)2e nominally satellite transition is strong because the A—X transition is case(b)-case(a) and the C—A -Pi/(2) satellite transition is weak because the C—A transition is case(b)-case(b).

A Monte Carlo simulation was used to model the results of the experiment. The parameters required for this simulation were the anisotropy parameter of the fragmentation (/ ), and the rotational quantum number and branch used in the first step in the REMPI scheme. The two-photon probabilities were calculated by taking the product of two one-photon probabilities via a virtual intermediate allowed transition. Five branches are possible 0, P, Q, R, and S. The dissociation process was assumed to be impulsive, therefore v 1 j. Finally, it was assumed that no p-v-j correlation exists. This finding is not strictly true, as pointed out by Hall et al. [38], but it was found that this correlation can be ignored in the calculation. 

The problem of bacterial photosynthesis has attracted a lot of recent interest since the structures of the photosynthetic reaction center (RC) in the purple bacteria Rhodopseudomonas viridis and Rhodobacterias sphaeroides have been determined [56]. Much research effort is now focused on understanding the relationship between the function of the RC and its structure. One fundamental theoretical question concerns the actual mechanism of the primary ET process in the RC, and two possible mechanisms have emerged out of the recent work [28, 57-59]. The first is an incoherent two-step mechanism where the charge separation involves a sequential transfer from the excited special pair (P ) via an intermediate bacteriochlorophyll monomer (B) to the bacteriopheophytin (H). The other is a coherent one-step superexchange mechanism, with P B acting only as a virtual intermediate. The interplay of these two mechanisms can be studied in the framework of a general dissipative three-state model (AT = 3). 

The high intensity and coherence of laser radiation can lead to more elaborate photon scattering processes than those involved in the conventional Raman effect. The simplest example is second harmonic generation (hyper-Rayleigh scattering) and the associated hyper-Raman effect in which two laser photons of frequency interact simultaneously with the molecule to produce a scattered photon at frequency (hyper-Rayleigh), or at XiOi.-tO (Stokes hyper-Raman) or at (antiStokes hyper-Raman). As illustrated in figure 1.3, these processes involve two virtual intermediate excited states. 

The third step after desorption and cooling is the multiphoton ionization. As demonstrated later, in most cases the ionization takes place by a resonance enhancement of an intermediate excited state but in several cases the ionization can occur via a virtual intermediate state. 

The two-photon absorption (TPA) phenomenon was proposed by Gbeppert-Mayer in 1931 [118] and experimentally first observed by Kaiser and Garrett [119]. TPA is one of the important third-order NLO features. When a molecule is exposed to an intense optical field such as from a pulse laser, it can absorb two photons simultaneously by involving a virtual intermediate state. Figure 49.9 schematically represent this process. If the two photons are of the same frequency, the process is called degenerate TPA if they are of different frequency, the process is a nondegenerate TPA [120]. 

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