Illustrative applications to NO excitations

Before looking at details of CIS-level excitation features, we might askhow these compare with experimental reality. As shown in Table 11.1, such comparisons are far from reassuring. The agreement with experimentally inferred excitation (Tg), vibration (Vg), and geometry (Rg) values is so poor that even the presumed state associations 

Let us first consider the two ground-state levels (solid and dashed curves of Fig. 11.1), which exhibit a characteristic idiosyncrasy of UHF and CIS-level description. The expected ground-state configuration of NO, namely, 

UHF configuration, and hence never appear in CIS-level description.] Such symmetry breaking artifacts are intrinsic to UHF/CIS description, and should remind us that CIS-level wavefunctions are at best useful only for cmde qualitative purposes. 

The characteristic symmetry breaking of UHF/CIS theory is also displayed by significant deviations from expected doublet spin symmetry, even in the near-neighborhood of equilibrium. The number of unpaired electrons ( ) of spin 1/2 can be related to the expectation value of total squared spin angular momentum ((5 )) by the following equation  

As shown in Table 11.2, the NLS description of the ground X-state corresponds to the expected form (Eqs. 11.8a and 11.8b), with double-bonded a 

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The properties of the band gap in semiconductors often control the applicability of these materials in practical applications. To give just one example, Si is of great importance as a material for solar cells. The basic phenomenon that allows Si to be used in this way is that a photon can excite an electron in Si from the valence band into the conduction band. The unoccupied state created in the valence band is known as a hole, so this process has created an electron-hole pair. If the electron and hole can be physically separated, then they can create net electrical current. If, on the other hand, the electron and hole recombine before they are separated, no current will flow. One effect that can increase this recombination rate is the presence of metal impurities within a Si solar cell. This effect is illustrated in Fig. 8.4, which compares the DOS of bulk Si with the DOS of a large supercell of Si containing a single Au atom impurity. In the latter supercell, one Si atom in the pure material was replaced with a Au atom,... 

Figure 11.18 illustrates the principles of application of REMPI to NO (discussed in more detail later). The electronically excited states of NO are shown in Fig. 11.18a and some potential ionization schemes in Fig. 11.18b (Pfab, 1995). Pulsed tunable lasers with wavelengths from 190 to 1000 nm and spectral resolutions of 0.1 cm 1 are readily available. To ionize NO, the absorption of two, three, or four photons is needed. The first photon excites the NO into an intermediate state from which it is ionized using a second or, in some cases, two more photons. The transitions are described as an (n + m) transition, where n is the...

The excited states of NO can also be approximated by complete active space (CAS) calculations (Sidebar 11.2) employing NBOs as starting orbitals ( CAS/NBO method ). As an illustrative application we consider full-valence CAS(11,8) active space (with /V= 11 electrons, 8 orbitals) to optimize and analyze the nroot=6 (fifth excited root) of NO, as shown in the Gaussian input file below ... 

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