Molecular orbitals electron spectra

The experimental spectrum is a plot of intensity (number of ejected electrons) as a function of ionization potential. This gives a representation of the occupied molecular orbitals. The spectrum for H2O, the orbitals of which we have just considered in Section 9.3, is shown in Figure 9.3. The energies of the MOs cannot, of course, be derived purely from symmetry arguments. They are obtained experimentally by photoelectron spectroscopy and they can also be obtained from a quantum mechanical calculation, which, along with the atomic orbital weighting coefficients, allows pictorial representations of the resulting molecular orbitals to be made (Section 3.8.3). 

Figure Bl.6.12 Ionization-energy spectrum of carbonyl sulphide obtained by dipole (e, 2e) spectroscopy [18], The incident-electron energy was 3.5 keV, the scattered incident electron was detected in the forward direction and the ejected (ionized) electron detected in coincidence at 54.7° (angular anisotropies cancel at this magic angle ). The energy of the two outgoing electrons was scaimed keeping the net energy loss fixed at 40 eV so that the spectrum is essentially identical to the 40 eV photoabsorption spectrum. Peaks are identified with ionization of valence electrons from the indicated molecular orbitals.

The majority of photochemistry of course deals with nondegenerate states, and here vibronic coupling effects aie also found. A classic example of non-Jahn-Teller vibronic coupling is found in the photoelection spectrum of butatiiene, formed by ejection of electrons from the electronic eigenfunctions [approximately the molecular orbitals). Bands due to the ground and first... 

The photoelectron spectrum of nitrogen (N2) has several peaks, a pattern indicating that electrons can be found in several energy levels in the molecule. Each main group of lines corresponds to the energy of a molecular orbital. The additional "fine structure" on some of the groups of lines is due to the excitation of molecular vibration when an electron is expelled. 

The naphthalene anion radical spectrum (Figure 2.2) provided several surprises when Samuel Weissman and his associates1 first obtained it in the early 1950s at Washington University in St. Louis. It was a surprise that such an odd-electron species would be stable, but in the absence of air or other oxidants, [CioHg]- is stable virtually indefinitely. A second surprise was the appearance of hyperfine coupling to the two sets of four equivalent protons. The odd electron was presumed (correctly) to occupy a it molecular orbital... 

FIGURE 5.15 Molecular orbitals for ethylene. Promotion of an electron from the ground state to the excited state is known as a n - n transition and is usually accompanied by an absorption of radiation in the ultraviolet region of the spectrum. 

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