One-step model

The calculations were performed in the framework of a one-step model of photoe-mission derived from the one originally formulated by Pendry [1]. Nowadays the model includes relativistic effects [2-5], the possibility of having several atoms per unit cell [6], different types of layers and a realistic model for the surface potential [7]. It is further possible to consider ov erlayers on a surface. We will not review the theory here, which has been done already in several publications [2,4,6,8], but instead concentrate on the results. 

R26 are shown. From Fig. 4, we can see that the dynamics of the initial ET depends on the probing wavelengths. This phenomenon is usually called the wavelength-dependent kinetics. Obviously, the one-step model cannot account for this wavelength-dependent kinetics phenomenon because in this case only one rate constant is involved. In order to understand the origin of wavelength-dependent kinetics, the detailed mechanism of the initial ET in RCs should be constructed. 

Fig. 7. Deactivation of CPCR in microemulsion Marlipal 013-60/Water/Cyclohexane at y = 0.1 and Wq = 5 following the two-step-model (solid line). The one-step-model (dotted line) does not fit the experimental results appropriately...

In the mechanistic models used to predict toxic effects of time-variable exposure to organisms, a distinction can be made between 1-step models and 2-step models (Ashauer et al. 2006). One-step models only consider toxicokinetics, whereas 2-step models consider both toxicokinetics and toxicodynamics. One-step models try to describe the uptake and elimination of a given compound in an organism and relate the calculated internal concentration to the effect occurring. Usually, an average total body residue is calculated, assuming that the concentration at the actual site(s) of action will be linearly related to the total body concentration. In specific cases, it may be necessary to calculate the concentration at the site of action through the use of more refined multicompartment (PBPK) models. 

Westbrook and Dryer [203, 204] have studied the application of several global mechanisms for the oxidation of hydrocarbons in flames. Their one-step model was of the following form ... 

Parameters i, 2 and were dictated by the type of fuel, while parameters A, n, Ea, a and b were fitted to experimental flame velocity data. These parameters were determined for 17 different fuels and, in all cases, the predicted flame speeds agreed well with the experimental data over a wide range of conditions. However, these one-step models overpredict the burnt-gas temperature, mainly due to the lack of accurate calculation of CO2 and H2O concentrations at high temperatures. The applicability of one-step mechanisms for the description of flames was debated by Coffee [205]. 

The electronic structure of magnetic solids can be investigated in a very detailed way using spin- and angle-resolved photoemission. Adopting the so-called one-step model, the photocurrent at the detector is described by a 2 x 2 spin-density matrix (Braun 1996 Feder 1985),... 

The interpretation of data relevant to reactions (3.1)-(3.2) is currently in a state of flux. Experimental data (2) presented at this Congress on the initial step in the JR 6. sphaeroides bacterial photosynthetic reaction center at room temperature suggests two steps, (3.1)-(3.2), instead of the recently accepted one-step model (cf 17). Because of what may be a rapidly changing picture it is useful to summarize first the recent history and then the current results, the deductions from them, and questions which remain. 

The three-step model (Berglund and Spicer 1964a,b) divides the photoemission process into distinct parts propagation of the photon into the solid with photoexcitation of an electron, propagation of the excited electron to the surface, and escape of the electron into the vacuum. We use this model for its conceptual simplicity, knowing that quantum mechanics requires a one-step model. Such a one-step model has been developed and, under appropriate limiting conditions, it reduces to the more conceptual model. 

Two-photon photoemission involves the absorption of two photons, and the two optical transitions can usually be treated as independent consecutive processes. Energy and momentum conservation have been exploited already in Section 3.2.4.2. The matrix elements could be obtained according to the well-deveIop)ed theories for regular photoemission however, the one-step model of photoemission [47] has not yet been applied to two-photon-photoemission spectra. In the following paragraphs, we discuss a few aspects of time-resolved two-photon-photoemission with femtosecond pulses that do not occur in regular photoemission. 

Before leaving this section, mention should be made of attempts to devise quantum mechanical theories for a one-step model of photoemission, i.e., treating the excitation of the photoelectron, its transmission within the metal and through the interface, and its capture in the condensed phase as a single event. Grider has described this approach in some detail. The escape... 

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