Molecular Rydberg states description

Ab initio calculations were carried out for all the low-lying non-Rydberg states of the systems N2, 02, NO, Of, and NO+. In N2, for example, there are 102 molecular states that result from nitrogen atoms in the lowest 4S, 2D, and 2P states. These states were all uniformly described using VCI wave functions constructed as described in Section II. Minimum basis, double-f basis and double-f-plus-polarization basis sets were employed for these studies. For the minimum basis-set calculations, which were always carried out first, the VCI wave functions represent full Cl projections with the constraint that the K shells were kept frozen for all states. However, no constraint on the 2og and 2ou orbitals was made since a Cl among these orbitals is necessary to ensure proper description of the hole states in these molecules, such as C3n of N2. The calculations all have the property of asymptotically connecting with the correct atomic states. This computational method has previously been applied, with reliable results, to both closed- and open-shell systems.6 9 11... 

In writing this equation, we have made use of Van Vleck s pure precession hypothesis [12], in which the molecular orbital /.) is approximated by an atomic orbital with well-defined values for the quantum numbers n, l and /.. Such an orbital implies a spherically symmetric potential and its use is most appropriate when the electronic distribution is nearly spherical. Examples of this situation occur quite often in the description of Rydberg states. It is also appropriate for hydrides like OH where the molecule is essentially an oxygen atom with a small pimple, the hydrogen atom, on its side. Accepting the pure precession hypothesis allows the matrix elements of the orbital operators to be evaluated since... 

A. H. Zewail Are we supposed to think, according to the discussion by Prof. Schlag, that for high-n Rydberg orbitals we have an electronic manifold within each vibrational state, the inverse of conventional molecular description ... 

The potential curves derived from such calculations can often be empirically improved by comparison with so-called experimental curves derived from observed spectroscopic data, using Rydberg-Klein-Rees (RKR) or other inversion procedures. It is often found, particularly for the atmospheric systems, that the remaining correlation errors in a configuration interaction (Cl) calculation are similar for many excited electronic states of the same symmetry or principal molecular-orbital description. Thus it is often possible to calibrate an entire family of calculated excited-state potential curves to near-spectroscopic accuracy. Such a procedure has been applied to the systems described here. 

The SAC-CI method can be applicable to various kinds of molecular spectroscopy excitation, ionization and electron-attached states in high accuracy. First, we overview the excitation [51 -53] and ionization spectra [51,54] of five-membered ring compounds, furan and thiophene. For these spectra, pioneering theoretical spectroscopy was performed by the SAC-Cl method [51] and has been followed by many theoretical works [52,53,55-58]. Comparisons of our old and new calculations show the importance of using the sufficiently flexible basis sets with valence and Rydberg functions as well as the reliable theory for the accurate descriptions of these spectra. We also explain the spectroscopy of p-benzoquinone (p-BQ), its anion radical [59], and aniline [60]. 

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