The Negative-Ion States of

The first step in assigning the experimental Ea to electronic states is to determine the number of theoretical states. The H2 anion has a polarization ground state and 

Electron scattering data give a VEa of —3.3 eV for the bonding state [31]. The ESR spectrum of the valence-state anion has been observed in irradiated solid H2 [32], The combination of these data yields re = 153 pm and ve 1,700 cm 1 for the bonding valence-state anion. The H(—) distribution and an assumed dissociation energy 

TABLE 7.3 Morse Parameters, Dimensionless Constants, and Experimental Data for Neutral and Ionic H2 and He2(+) 

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Curves for the negative-ion states of H2 and L are chosen to illustrate the procedures for the homonuclear diatomic molecules. Curves for benzene and naphthalene are examples of excited states for larger molecular negative ions. These illustrate the relationship between gas phase acidities and thermal electron attachment reactions. Such correlation procedures can be applied to systematic predictions for many different problems. 

This paper summarizes the first investigations of three- and four-membered ring compounds by the technique of electron transmission spectroscopy (ETS). He will briefly discuss two general areas associated with the negative ion states of small ring hydrocarbons ... 

A sharp decrease in the transmitted current is seen at energies which correspond to the energies of the negative ion states of these molecules. In the context of Koopmans theorem (j(), these resonances are associated with the negative of the SCF molecular orbital energies of the neutral molecule. In order to accentuate the variations in the transmitted current, a small (20-50 meV) AC volt-age (J.) is applied to the collision chamber. Thus, the derivative of the transi tted current with respect to energy is obtained. 

In order to better understand bonding in main-group organo compounds and to obtain data on main-group species which can act as ligands in transition metal complexes, study of the negative ion states of various saturated and unsaturated Group IV, V, VI, and VII hydrocarbons was undertaken. One example of this work is the series of para-disubstituted benzenes ... 

We performed Hartree-Fock self-consistent-field (HF-SCF) calculation and obtained PES s corresponding to T, Csv and Did deformation modes of the cluster as shown in Fig. l(a)-(c).Douhle-zeta basis functions, which express each valence orbital of the atom with two functions, are employed with two d functions for silicon and p functions for the negative ion state of oxygen. We assume no electron orbitals around the point charges. 

Figure Bl.6.11 Electron transmission spectrum of 1,3-cyclohexadiene presented as the derivative of transmitted electron current as a fiinction of the incident electron energy [17]. The prominent resonances correspond to electron capture into the two unoccupied, antibonding a -orbitals. The negative ion state is sufficiently long lived that discrete vibronic components can be resolved.

It is perhaps important to note here that PE exhibits a negative electron affinity (Bloor, 1976), a feature shared with only a very restricted set of materials. This means that excess electrons prefer energetically to reside outside PE rather than be in any way bound to the PE molecular structure. Nevertheless, recent calculations show that there are electronic surface states lying below the vacuum level in the forbidden band gap of PE (Righi et al., 2001). These surface states will certainly be expected to act as traps for transferred or injected electrons, and they will therefore be involved in contact charging. Their resonance interaction with negative ion states of typical PE dopants (02 and H20) may be very important too. 

Dewar and Rzepa have calculated with the MINDO semiempirical SCF—MO method that TCNQ is only about 0.43 eV less stable than TCNQ , and is more stable than the tetracyanoquinodimethan (TCNQ) molecule. Compton and Cooper ) make no mention of a doubly-charged negative ion in their report of the negative ion properties of TCNQ. They do, however, comment upon the importance of excited states in complex negative ion systems. An omegatron or ICR search for the TCNQ ion would be appropriate, particularly as a test of the utility of this semiempirical approach. 

Figure 2.1 Morse potential energy curves for the neutral and negative-ion states of F2. The vertical electron affinity VEa, adiabatic electron affinity AEa, activation energy for thermal electron attachment E, Err — AEa — VEa, EDEA — Ea(F) — D(FF), and dissociation energy of the anion Ez are shown.

Figure 2.3 Morse potential energy curves for the neutral and negative-ion states of CC14. The new quantity illustrated in this figure is photodetachment energy. It is larger than AEa and is the peak in the photodetachment spectmm. Thermal electron attachment is exothermic, that is, EDEA = a positive quantity. Two other states dissociating to Cl + CC13(—) and the polarization curve are not shown.

Theoretical potential energy curves were calculated for both NO and O2 that included numerous excited states, only one of which had been observed experimentally. Freeman and independent investigators measured the Ea of what is now thought to be an excited state in the ECD [52, 53]. Freeman obtained unexplained results for electron attachment to both 02 and NO that were included in the thesis, but were not understood until the rediscovery of a paper predicting the multiple negative-ion states of 02(—) by H. H. Michels [54]. The data for 02 have been verified and the analysis published. A similar analysis for NO will be carried out in Chapter 9. 

The dimensionless constants for the ground state for I2(—) correspond to an increase in the attractive term but a larger increase in the repulsive term to give a smaller bond dissociation energy. In the case of bonds for aromatic hydrocarbons the attractive terms are decreased and the repulsive terms increased, but by a smaller amount than for I2(—) or H2(—). Notice that the relative bond order is given by BO = D,.(A 2( j)/De Xi) = [k /kR], Thus, the bond orders for the aromatic hydrocarbons are larger than for the diatomic molecules. If there are no experimental data for the construction of negative-ion states of aromatic hydrocarbons, then these values can be used as first approximations or the theoretical values could be used. 

Figure 9.11 Morse potential energy curves for 24 negative-ion states of 02. The parameters used to calculate the curves and assignments are given in Table 9.4 [5].

Figure 9.16 Morse potential energy curves for the neutral and negative-ion states of NO.

In this review, we have shown that electron transmission spectroscopy provides a useful and relatively simple way to survey the temporary negative ion states of complex molecules. The utility of the technique has been further enhanced by a number of recent developments which we briefly mention here. 

The somewhat low value for the first negative Ion state of 3 (1.73 eV) can be understood on the basis of coaq>en-sating effects due to the short C=C bond and the "reversed" polarity of 3 relative to 4-6. The lowest resonances observed In 3i 4, 7 and 8 have been assigned on the basis of ab initio 6-31C molecular orbital calculations and by consideration of the components of the angular momentum associated with each resonance. 

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