Energy sudden approximation

In the ESA-CSA method [26] the energy sudden approximation (ESA) is applied to the entrance reaction channel and the CSA to the exit channel. This technique gives considerable computational simplications compared to the CC and CSA methods, although retaining a good accuracy as the results of Section 4 show. 

Abstract. We present an overview of the influence of C.F. Curtiss on the theory of molecular collisions, as exemplified by the title paper. Both authors were graduate students of Curtiss and, as such, were strongly influenced by his ideas and approaches to theoretical chemistry. This resulted in a subsequent collaboration that provided the rigorous basis for understanding the success of the so-called centrifugal sudden and energy sudden approximations (the two combined being the infinite order sudden approximation). 

Goldflam R., Green S., Kouri D. J. Infinite order sudden approximation for rotational energy transfer in gaseous mixtures, J. Chem. Phys. 67, 4149-61, (1977). 

The evaluation of elements such as the M n,fin s is a very difficult task, which is performed with different levels of accuracy. It is sufficient here to mention again the so called sudden approximation (to some extent similar to the Koopmans theorem assumption we have discussed for binding energies). The basic idea of this approximation is that the photoemission of one-electron is so sudden with respect to relaxation times of the passive electron probability distribution as to be considered instantaneous. It is worth noting that this approximation stresses the one-electron character of the photoemission event (as in Koopmans theorem assumption). 

As opposed to the adiabatic limit, we assume in the sudden approximation that the internal motion is slow compared to the external (i.e., translational) motion. Most familiar is the rotational sudden approximation which is frequently exploited in energy transfer studies in full collisions (Pack 1974 Secrest 1975 Parker and Pack 1978 Kouri 1979 Gianturco 1979 ch.4). Its application to photodissociation is straightforward and will be outlined below for the model discussed in Section 3.2. 

Within the rotational sudden approximation we assume that the interaction time is much smaller than the rotational period of the fragment molecule so that the diatom BC does not appreciably rotate from its original position while the two fragments separate. In terms of energies, this requires the rotational energy, Erot, to be much smaller than the total available energy. If that is true, the operator for the rotational motion of BC, hrot, can be neglected in (3.16). The partial differential equation thus becomes an ordinary differential equation,... 

The main peaks in X-ray Photoelectron Spectroscopy (XPS) for molecules appear because of the photoionization of core electrons. In addition, satellite peaks on the high binding energy side of the main peak have often been observed. These peaks are generally referred to as shakeup satellite peaks. In the sudden approximation, the shakeup process which accompanies photoionization can be considered as a two-step process. First, a core electron is emitted as a photoelectron, creating an inner shell vacancy. In the next step, electron(s) in the same molecule transfer from valence orbital(s) to unoccupied orbital(s) with relaxation of orbital energies. It is important to study these satellites in order to understand the valence and excited states of molecules (1). 

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