Ionization potential, vertical adiabatic

FIGURE 4.1 Illustration of adiabatic and vertical ionization potentials. Adiabatic I.P. refers to the energy difference between the lowest quantum states of the molecule and its positive ion. Often, Franck-Condon (vertical) transitions lead to a higher value, the vertical ionization potential. 

We turn to the chemical behavior of cycloalkane holes. Several classes of reactions were observed for these holes (1) fast irreversible electron-transfer reactions with solutes that have low adiabatic IPs (ionization potentials) and vertical IPs (such as polycyclic aromatic molecules) (2) slow reversible electron-transfer reactions with solutes that have low adiabatic and high vertical IPs (3) fast proton-transfer reactions (4) slow proton-transfer reactions that occur through the formation of metastable complexes and (5) very slow reactions with high-IP, low-PA (proton affinity) solutes. 

The electron alfinity and ionization potential can be either for vertical excitations or adiabatic excitations. For adiabatic potentials, the geometry of both ions is optimized. For vertical transitions, both energies are computed for the same geometry, optimized for the starting state. 

The ionization being accompanied by a vibrational excitation, the fine structure of bands can be exploited for determination of vibrational levels of an ionized system in the ground and excited states. Of course, the first (0-0) and the strongest vibrational bands are the most important because they determine adiabatic and vertical ionization potentials of radicals. 

Another consequence of the stronger interactions upon ionization is that the equilibrium geometry of the ionized complex may differ signihcantly from that of the neutral states. Broadened ionization onsets are frequently attributed to the spectral superposition of ionization into several vibrational levels for which Franck-Condon factors are more favorable. As a result, the adiabatic ionization potential may be considerably lower than the vertical potential, and the observed ionization onsets may occur above the adiabatic potential. Another factor to be considered is the conformation-dependent efifect, due to the different conformations of the solvent molecules. The most populated form of a complex may involve a less stable form of the solvent. After photoionisation, the lowest-energy dissociation channel in the complex ion leads to the most stable form of isolated solvent, which has to be taken into account for the estimate of the binding energy. 

Fig. 9a, b. A portion of a photoelectron spectrum (idealized) showing (a) the identification of adiabatic and vertical ionization potentials with resolved (1) and unresolved (2) vibrational structures, (b) the identification of a higher adiabatic ionization potential with a break . 

Fio. 12. Fhotoelectron spectrum of methanol vapour using the helium resonance line (21-21 e.v.). Ionization energy increases from left to right. The adiabatic ionization potentials measured (Al-Jobomy and Turner, 1964) are indicated by vertical arrows, and can be compared with (probably) vertical I.P. values derived from electron impact appearance potentials by Collin (1961) (dotted arrows). 

The test set used for most comparisons in the present paper is Database/3 18), which was introduced elsewhere. It consists of 109 atomization energies (AEs), 44 forward and reverse reaction barrier heights (BHs) of 22 reactions, 13 electron affinities (EAs), and 13 ionization potentials (IPs). There are a total of 513 bonds among the 109 molecules used for AEs, where double or triple bonds are only counted as a single bond. Note that all ionization potentials and electron affinities are adiabatic (not vertical), i.e., the geometry is optimized for the ions... 

Photoelectron spectra of 2,5-di-(fluoroalkyl)-l,3,4-oxadiazoles were studied and adiabatic and vertical ionization potentials were given <89JGU24il>. 

Figure 3 The photoionization quantum yields (rji) of CH4 as a function of the incident photon energy measured via the double ionization chamber and synchrotron radiation as mentioned in Section 2.1. The bandpass was 0.1 nm, which corresponds to the energy width of 32 meV at the incident photon energy of 20 eV. The vertical ionization potentials of the ionic states involved are indicated by the vertical bars [11] along with the first adiabatic ionization potential by the arrow [17]. (From Ref. [7]. Reprinted with permission from Elsevier Science.)...

Two different values of the ionization potential / have to be considered2 3,7 (1) the adiabatic Ip, which corresponds to a transition from the zero-vibration level of the ground state of the molecule to that of the ground state of the molecular ion (2) the vertical which corresponds... 

The most probable transitions, according to the Franck-Condon principle are the vertical ones. They correspond to the maxima of electron groups in the kinetic spectrum. The upper limits in the kinetic energies for each group correspond to the adiabatic ionization potentials. Thus from the difference of these energy values one can get the difference Ip between the vertical and adiabatic potentials (Table IV). 

An SCF MO calculation gives the following energies for the occupied H20 MOs (Section 1.20) la, 2a]t 1 b2, 3av 1 bx —20.56, —1.28, — 0.62, —0.47, -0.40 hartrees, respectively. For the He 1 PES of H20, answer the following questions, (a) Give the band locations, (b) For each band, state whether or not the adiabatic and vertical ionization potentials will coincide, (c) For each band, state whether the vibrational frequencies of the upper state should be less than, about the same as, or greater than those in the lower state. 

FIGURE 11. He(I) photoelectron spectra of some nitriles. Adiabatic N lone-pair o ionization potentials predicted by equation 27 are indicated by the vertical line above each spectrum... 

Figure 10. Ionization potentials of Si clusters. Solid and dashed lines are the boundaries obtained in experiment. Triangles are the vertical and adiabatic IPs computed using PWB filled for the global minima and empty for some other low-energy neutrals (Ref. [45]).

Reference data on total energies of forms 19-23 optimized by means of different theoretical methods in the gas phase are given in Table 2. Various energetic characteristics of tetrazoles can be successfully estimated. The vertical adiabatic ionization potentials of both neutral tautomers 20 and 21 were calculated for a- and Tt-radical cations <2000CPL(330)212>. The standard molar thermodynamic functions (enthalpies, heat capacities, and entropies) of... 

The effect of a range of substituents on the electron-donor capability of TTFs (tetrathiafulvalenes) was the subject of a report by Bowadt and Jensen <89SM179>. Semiempirical MNDO calculations have been conducted on 5-[l,3]dithiolo[4,5-d]-l,2,3-trithiol-5-ylidine (12) and the following conclusions were adopted the molecule is planar and has Dlh symmetry its vertical ionization potential is 8.61 eV and its adiabatic ionization potential is 8.02 eV. The calculated ionization potentials are in good agreement with the experimental values measured in solution. 

One radiation source, the He(Ia) photon energy line, at 21.22 eV, provides vertical and adiabatic ionization potentials. In l,2-dithiole-3-thione (3b), a first ionization band at 8.42 eV has high intensity and is assigned to at least two electronic transitions (75MI43100). Data are also available for other dithiole-3-thiones and two fused dithioles (26) and (27a) (79MI43100). 

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