Diabatic process

Chemical processes, such as bond stretching or reactions, can be divided into adiabatic and diabatic processes. Adiabatic processes are those in which the system does not change state throughout the process. Diabatic, or nonadiabatic, processes are those in which a change in the electronic state is part of the process. Diabatic processes usually follow the lowest energy path, changing state as necessary. 

Considering transient phenomena of St" (D2) cannot be assumed on the basis of those for St(Si). Judging from Xc 120 psec and T 240 psec, a diabatic process is proposed for the isomerization of c-Sf to It has been proposed the analogous process in which c-... 

In contrast to electron scattering, which is an extremely diabatic process, the quenching is certainly closer to the adiabatic picture. Therefore, scattering amplitudes are often described in a body-fixed system, rotating during the collision ZUxiy, Y, as opposed to the... 

See - nonadiabatic (diabatic) process, -> Marcus theory, - Randles, and - Gurney, - adiabatic process (thermodynamics). 

An important factor is the electron coupling between the electrode metal and the redox species or between the two members of the redox couple. If this coupling is strong the reaction is called adiabatic, i.e., no thermal activation is involved. For instance, electrons are already delocalized between the metal and the redox molecule before the electron transfer therefore, in this case no discrete electron transfer occurs [see also -> adiabatic process (quantum mechanics), - nonadiabatic (diabatic) process]. 

Non-adiabatic (diabatic) process (quantum mechanics) — In quantum mechanics a process is called non-adiabatic (diabatic) if one or more electrons fail to equilibrate with nuclei as they move. In a widely-used extension of this terminology, non-adiabatic electron transfer is said to occur when an electron tunnels out of one electronically non-equilibrated state into another. Due... 

On triplet sensitization the resulting cis ( c ) and trans triplet excited state (H ) convert to the perpendicular excited triplet state ( p ) p is equilibrated with by an equilibrium constant of = [ p ]/[H ] and deactivated to the ground state p, giving c and t [6,13-15,24,33-41]. Thus, the isomerization of stilbene takes place as a diabatic process. 

Table 6 summarizes the features of one-way and two-way isomerization of arylethenes depending on the aryl substituents. Thus, the typical features of one-way and two-way isomerizations shown in Table 1 are modified, but oneway isomerization does not necessarily accompany the quantum chain process and two-way isomerization does not necessarily involve the diabatic process. For some compounds two-way isomerization can take place as an adiabatic process by mutual conversion of c and H followed by deactivation to the ground state. The short triplet lifetime due to the heavy atom effect may cause a very inefficient one-way isomerization. Furthermore, the quantum chain processes can be observed in many compounds when proper experimental conditions can be established. 

The ZEKE detection scheme is equivalent to ionization of high-n Rydberg states by Pulsed Field Ionization (PFI). If one assumes that the pulsed field ionization of the Rydberg electron follows a diabatic process (Chupka, 1993), then the ionization threshold is lowered by... 

The potential energy surface of the isomerization is discussed in terms of adiabatic and diabatic processes [1-4,12,18,71]. Two-way isomerization in the triplet manifold without a quencher takes place as a diabatic process by deactivation at p. However, as mentioned above, photochemical cis- trans one-way isomerization in the triplet state proceeds by an adiabatic process where the excited state of a starting material, c, undergoes adiabatic conversion to the excited state of the product, t, followed by either unimolecular deactivation to the, ground state of the product, t, or energy transfer to c to give t and c. The isomerization of 5b also proceeds partly by way of an adiabatic process. Deactivation from t occurs as an adiabatic process, but that from p proceeds as a diabatic process [25]. Therefore, two-way photoisomerization usually takes place as a diabatic process, whereas one-way photoisomerization and isomerization... 

This one-way trans - cis isomerization seems to take place through a diabatic process via deactivation at the perpendicular excited singlet state ( p ) [process (1) in Figure 9] or through adiabatic t c conversion... 

There are variations on the diabatic process. One important feature is that some reactions will have a small barrier on S that separates the initial excited state geometry from the funnel geometry. This can adversely affect photochemical efficiency and produce temperature dependent quantum yields. Still, the basic idea of finding geometries in which the excited state and ground state are close in energy is central to photochemistry. 

The roots of QSM theory lie in Mori s statistical mechanical theory of transport processes and Kubo s theory of thermal disturbances. The version of QSM theory given here with its refinements and modem embellishments was used by the author to develop an irreversible thermodynamic theory for photophysical phenomena and a quantum stochastic Fokker-Planck theory for adiabatic and nona-diabatic processes in condensed phases. 

Our recent activity in this area has also triggered our thoughts on factors controlling efficiency of diabatic processes such as HT. But it is clear that the HT concept is stiU at a nascent stage awaiting new information from reliable experimental data and exact theoretical models. Nevertheless, we are hopeful that a more thorough understanding of the intricate details and scope and limitation of the process can be achieved in the near future. 

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