The vibrational structure of the highly symmetrical octahydridosilasesquioxane has been investigated in detail, and a harmonic force field in terms of internal force constants has been determined, based on extensive IR and FT-Raman data and on a normal coordinate analysis of H~SisOl2 and DsSigOl2. Group frequencies have been assigned according to a potential energy analysis, and relations to group frequencies of comparable silicon compounds have been discussed. A step by step procedure starting from Oh-H~Sig012 to the frameworks Oh-(-O)8SigO12, D4h-(-0)8Si~012, D4h-(=Si0)8Si8012, and finally to Dw(*0)8T8012, T = Si or A1 and Si/Al = 1, has turned out to be an excellent, enlightening approach for qualitatively and quantitatively describing the vibrational structure of the zeolite A framework. NIR FT-Raman spectra of Na+-exchanged zeolite Y and of Na+-and Ag+-exchanged zeolite A have been measured and compared with each other. An improved force field is reported, and a correlation of the experimental and the calculated IR and Raman spectra of zeolite A has allowed one to assign group frequencies to all fundamental modes. All fundamentals between 11 10 and 950 cm-l belong to antisymmetric T-0-T stretching vibrations while the symmetric T-O-T stretching modes are at 860-830 cm-l, 740-680 cm-l, and 610-570 cm-l. The fundamentals between 490 and 100 cm-l can be described as 0-T-0 bendingvibrations-with the exception of a double four ring and two sodalite cage breathing modes and the 8-ring pore opening-whereas the T-O-T bending vibrations and the torsional modes are below 100 cm-1.

In this work we present a deperturbation study of the $d\,^3\Pi _g, v=6$d3Πg,v=6 state of C$_{2}$2 by double-resonant four-wave mixing spectroscopy. Accurate line positions of perturbed transitions are unambiguously assigned by intermediate level labeling. In addition, extra lines are accessible by taking advantage of the sensitivity and high dynamic range of the technique. These weak spectral features originate from nearby-lying dark states that gain transition strength through the perturbation process. The deperturbation analysis of the complex spectral region in the (6,5) and (6,4) bands of the Swan system ($d^3\Pi _g-a\,^3\Pi _u$d3Πg−a3Πu) unveils the presence of the energetically lowest high-spin state of C$_{2}$2 in the vicinity of the $d\,^3\Pi _g, v=6$d3Πg,v=6 state. The term energy curves of the three spin components of the d state cross the five terms of the $1\;^5\Pi _g$15Πg state at rotational quantum numbers N ⩽ 11. The spectral complexity for transitions to the v = 6 level of $d\,^3\Pi _g$d3Πg state is further enhanced by an additional perturbation at N = 19 and 21 owing to the $b\, ^3\Sigma _g^-, v=19$b3Σg−,v=19 state. The spectroscopic characterization of both dark states is accessible by the measurement of 122 “window” levels. A global fit of the positions to a conventional Hamiltonian for a linear diatomic molecule yields accurate molecular constants for the quintet and triplet perturber states for the first time. In addition, parameters for the spin-orbit and L-uncoupling interaction between the electronic levels are determined. The detailed deperturbation study unravels major issues of the so-called high-pressure bands of C$_{2}$2. The anomalous nonthermal emission initially observed by Fowler in 1910 [Mon. Not. R. Astron. Soc. 70, 484 (1910)] and later observed in numerous experimental environments are rationalized by taking into account “gateway” states, i.e., rotational levels of the $d\,^3\Pi _g, v=6$d3Πg,v=6 state that exhibit significant $^5\Pi _g$5Πg character through which all population flows from one electronic state to the other.

The first high-spin transition in C2 (1 (5)Πu - 1 (5)Πg) is observed by perturbation-facilitated optical-optical double resonance spectroscopy. The experiment is performed by applying unfolded two-color resonant four-wave mixing. C2 radicals in the initial a (3)Πu, v = 5 state are produced by using a discharge source in a molecular beam environment. The final quintet state is excited via intermediate "gateway" states exhibiting both substantial triplet and quintet character due to a perturbation between the 1 (5)Πg, v = 0 and the d (3)Πg, v = 6 states. Fifty seven rotational transitions in the P, Q, and R branches of all spin sub-states are measured and yield accurate molecular constants of the newly found upper level 1 (5)Πu. In addition, satellite transitions (ΔJ ≠ ΔN) are observed and allow an accurate determination of the spin-orbit constant. The results are compared with high-level ab initio computations at the multi-reference configuration interaction level of theory. The high-lying quintet state is found to be predissociative and displays a shallow potential that accommodates three vibrational levels only.

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