Energy levels electronic transitions

The interaction of a molecular species with electromagnetic fields can cause transitions to occur among the available molecular energy levels (electronic, vibrational, rotational, and nuclear spin). Collisions among molecular species likewise can cause transitions to occur. Time-dependent perturbation theory and the methods of molecular dynamics can be employed to treat such transitions. 

Ultraviolet absorption spectra appear when outer electrons of atoms or molecules absorb radiant energy and undergo transition to higher energy levels. These transitions are quantised and depends on the compound under examination. 

In addition to redox reactions due to the direct transfer of electrons and holes via the conduction and valence bands, the transfer of redox electrons and holes via the surface states may also proceed at semiconductor electrodes on which surface states exist as shown in Fig. 8-31. Such transfer of redox electrons or holes involves the transition of electrons or holes between the conduction or valence band and the surface states, which can be either an exothermic or endothermic process occurring between two different energy levels. This transition of electrons or holes is followed by the transfer of electrons or holes across the interface of electrodes, which is an adiabatic process taking place at the same electron level between the surface states and the redox particles. 

Laser-induced fluorescence (LIF). Laser-induced fluorescence measurements have been applied to the atmosphere since the suggestion of Baardsen and Ter-hune in 1972 that this method should be feasible. Figure 11.43 shows the energy levels and transitions involved in LIF measurements. OH is excited from its ground X2n state into the first electronically excited A22 state. The v" = 0 to r = 0 transition is around 308 nm and the v" = 0 to v = 1 at 282 nm. Two schemes have been used excitation using 282 nm into v = 1 of the upper electronic state, or excitation using 308 nm into v = 0 of the upper state. Collisional quenching deactivates some of the v = 1 into u = 0 in competition with fluorescence, mainly in the (1,1) band of the electronic transition (that is, from v = 1 of the upper state into v" =1 of the lower state). Collisional deactivation of v = 0 then occurs in competition with fluorescence in the (0,0) band at 308 nm... 

The energy of electronic transition is equal to the energy difference between the starting energy level and the final level. Therefore, the transition energy E (J mol-1) is ... 

Ha emission a red emission line at 6562.8 A emitted by a hydrogen atom when its excited electron jumps from the n = 3 energy level to the n = 2 energy level. This transition line is a powerful and easily accessible probe of hot hydrogen gas. 

Electrons not only orbit around the nucleus of an atom, but they also move to higher and lower orbits also called higher and lower energy levels. These transitions require energy to move to a higher orbit and release energy to move to a lower orbit. The position of an electron can be identified by its unique set of four quantum numbers. 

Molecular spectra are not solely derived from single electronic transitions between the ground and excited states. Quantised transitions do occur between vibrational states within each electronic state and between rotational sublevels. As we have seen, the wavelength of each absorption is dependent on the difference between the energy levels. Some transitions require less energy and consequently appear at longer wavelengths. 

We briefly summarize the parameterization schemes for f-electron energy levels, intraconfiguration transition probabilities, and the electron-phonon interaction, and review the current experimental situation for each area. We shall also speculate on potentially fertile areas of future investigation. 

The development of correlation schemes at the highest levels of theory (the CCSD(T) technique) allowed for very accurate DCB predictions of atomic properties for the heaviest elements up to Z=122 (see Chapter 2 in this book). Reliable electronic configurations were obtained assuring the position of the superheavy elements in the Periodic Table. Accurate ionization potentials, electron affinities and energies of electronic transitions (with the accuracy of below 0.01 eV) are presently available and can be used to assess the similarity between the heaviest elements and their lighter homologs in the Periodic Table. 

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