Optical observation of the C, 3ssigma(g)F3, and 3ppi(u)G3 3Pi(u) states of N2

Diabatic Valence-Hole Concept

A global diabatization scheme, based on the "valence-hole" concept, has been previously applied to model webs of avoided crossings that exist in four electronic-state symmetry manifolds of C2 (1Πg, 3Πg, 1Σu+, and 3Σu+). Here, this model is extended to the electronically excited states of four more molecules: CN (2Σ+), N2 (3Πu), SiC (3Π), and Si2 (3Πg). Many strangenesses in the spectroscopic observations (e.g., energy level structure, predissociation linewidths, and radiative lifetimes) for all four electronic state systems discussed here are accounted for by this unified model. The key concept of the model is valence-hole electron configurations: 3σ24σ11π45σ2 in CN (2Σ+), 2σg22σu11πu43σg21πg1 in N2 (3Πu), 5σ26σ17σ22π3 in SiC (3Π), and 4σg24σu15σg22πu3 in Si2 (3Πg), all of which have a triply occupied "valence-core" (i.e., 2σg22σu1 or the equivalent). These valence-hole configurations have a nominal bond order of three or higher and correlate with high-energy separated-atom limits with an np ← ns (n = 2, 3) promotion in one of the atomic constituents. On its way to dissociation, the strongly bound diabatic valence-hole state crosses multiple weakly bound or repulsive states, which belong to electron configurations with a completely filled valence-core. These curve crossings between diabatic potentials result in a network of many avoided crossings among multiple electronic states, analogous to the well-studied electronic structure landscape of ionic-covalent crossings in strongly ionic molecules. Considering the unique role of valence-hole states in shaping the global electronic structure, the valence-hole concept should be added to our intuitive framework of chemical bonding.

The spin-forbidden vacuum-ultraviolet absorption spectrum of 14N15N

Photoabsorption spectra of ¹⁴N¹⁵N were recorded at high resolution with a vacuum-ultraviolet Fourier-transform spectrometer fed by synchrotron radiation in the range of 81-100 nm. The combination of high column density (3 × 10¹⁷ cm^-2) and low temperature (98 K) allowed for the recording of weak spin-forbidden absorption bands' exciting levels of triplet character. The triplet states borrow intensity from ¹Π_u states of Rydberg and valence character while causing their predissociation. New predissociation linewidths and molecular constants are obtained for the states C³Π_u(v = 7, 8, 14, 15, 16, 21), G³Π_u(v = 0, 1, 4), and F³Π_u(v = 0). The positions and widths of these levels are shown to be well-predicted by a coupled-Schrödinger equation model with empirical parameters based on experimental data on ¹⁴N₂ and ¹⁵N₂ triplet levels.

High-resolution Fourier-transform extreme ultraviolet photoabsorption spectroscopy of 14N15N

The first comprehensive high-resolution photoabsorption spectrum of (14)N(15)N has been recorded using the Fourier-transform spectrometer attached to the Desirs beamline at the Soleil synchrotron. Observations are made in the extreme ultraviolet and span 100 000-109 000 cm(-1) (100-91.7 nm). The observed absorption lines have been assigned to 25 bands and reduced to a set of transition energies, f values, and linewidths. This analysis has verified the predictions of a theoretical model of N(2) that simulates its photoabsorption and photodissociation cross section by solution of an isotopomer independent formulation of the coupled-channel Schrödinger equation. The mass dependence of predissociation linewidths and oscillator strengths is clearly evident and many local perturbations of transition energies, strengths, and widths within individual rotational series have been observed.

A coupled-channel model of the 3Pi(u) states of N2: structure and interactions of the 3ssigma(g)F3 3Pi(u) and 3ppi(u)G3 3Pi(u) Rydberg states

New and existing spectroscopic data on N(2), obtained using a wide variety of experimental techniques, are interpreted using a coupled-channel Schrodinger-equation (CSE) model of the structure and predissociation dynamics for the interacting Rydberg and valence states of (3)Pi(u) symmetry. As a result, v>0 levels of the 3ppi(u)G(3) (3)Pi(u) Rydberg state are assigned correctly for the first time, leading to the identification of very strong perturbations in the G(3)-state vibrational structure. A four-channel CSE model, which includes the 3ssigma(g)F(3) (3)Pi(u) and 3ppi(u)G(3) (3)Pi(u) Rydberg states and the C(') (3)Pi(u) and C (3)Pi(u) valence states, indicates strong Rydberg-Rydberg coupling between the F(3) and G(3) states, strong Rydberg-valence coupling between the G(3) and C(') states, and weaker coupling between the F(3) and C(') states.