Rotational stmcture

Such a series of lines is called a Rydberg series [26]. These lines also converge to the ionization energy of the atom or molecule, and fitting the lines to this fonuula can give a very accurate value for the ionization energy. In the case of molecules there may be resolvable vibrational and rotational stmcture on the lines as well. 

This general behaviour is characteristic of type A, B and C bands and is further illustrated in Figure 6.34. This shows part of the infrared spectrum of fluorobenzene, a prolate asymmetric rotor. The bands at about 1156 cm, 1067 cm and 893 cm are type A, B and C bands, respectively. They show less resolved rotational stmcture than those of ethylene. The reason for this is that the molecule is much larger, resulting in far greater congestion of rotational transitions. Nevertheless, it is clear that observation of such rotational contours, and the consequent identification of the direction of the vibrational transition moment, is very useful in fhe assignmenf of vibrational modes. 

Units are cm throughout. Measurements are of band heads, formed by the rotational stmcture, not band origins. Figures in parentheses are differences variations in differences (e.g. between the first two columns) are a result of uncertainties in experimental measurements. 

The three bands in Figure 9.46 show resolved rotational stmcture and a rotational temperature of about 1 K. Computer simulation has shown that they are all Ojj bands of dimers. The bottom spectmm is the Ojj band of the planar, doubly hydrogen bonded dimer illustrated. The electronic transition moment is polarized perpendicular to the ring in the — Ag, n — n transition of the monomer and the rotational stmcture of the bottom spectmm is consistent only with it being perpendicular to the molecular plane in the dimer also, as expected. 

Molecules vibrate at fundamental frequencies that are usually in the mid-infrared. Some overtone and combination transitions occur at shorter wavelengths. Because infrared photons have enough energy to excite rotational motions also, the ir spectmm of a gas consists of rovibrational bands in which each vibrational transition is accompanied by numerous simultaneous rotational transitions. In condensed phases the rotational stmcture is suppressed, but the vibrational frequencies remain highly specific, and information on the molecular environment can often be deduced from hnewidths, frequency shifts, and additional spectral stmcture owing to phonon (thermal acoustic mode) and lattice effects. 

Infrared spectroscopy has broad appHcations for sensitive molecular speciation. Infrared frequencies depend on the masses of the atoms iavolved ia the various vibrational motions, and on the force constants and geometry of the bonds connecting them band shapes are determined by the rotational stmcture and hence by the molecular symmetry and moments of iaertia. The rovibrational spectmm of a gas thus provides direct molecular stmctural information, resulting ia very high specificity. The vibrational spectmm of any molecule is unique, except for those of optical isomers. Every molecule, except homonuclear diatomics such as O2, N2, and the halogens, has at least one vibrational absorption ia the iafrared. Several texts treat iafrared iastmmentation and techniques (22,36—38) and thek appHcations (39—42). 

IRDLKS = infrared diode laser kinetic spectroscopy, investigation of the rotational stmcture of the V2 band MW = microwave spectroscopy ED = electron diffraction ES = electronic spectroscopy, investigation of the rovibronic stmcture of electronic transitions bond angles calculated from isotopic shifts of the uy bands of friatomic CAs are not presented here due to their large experimental error these values are reported in Section VI. 

A main field of activities is focused on structure and reactivity in two-dimensional adlayers at electrode surfaces. Significant new insights were obtained into the specific adsorption and phase formation of anions and organic monolayers as well as into the underpotential deposition of metal ions on foreign substrates. The in situ application of structure-sensitive methods with an atomic-scale spatial resolution, and a time resolution up to a few microseconds revealed rich, potential-dependent phase behavior. Randomly disordered phases, lattice gas adsorption, commensurate and incommensurate (compressible and/or rotated) stmctures were observed. Attempts have been developed, often on the basis of concepts of 2D surface physics, to rationalize the observed phase changes and transitions by competing lateral adsorbate-adsorbate and adsorbate-substrate interactions. 

Saksena et al. [02Sak] have reinvestigated the emission spectra of the electronic transition B IIi - X of the In Cl isotopomer. They observed and analyzed the 1-0, 2-1, 0-0, 0-1, 1-2, 0-2, and 1-3 bands, and obtained the upper-state A-doubling coefficients as well as several rotational equilibrium constants for the first time. Saksena and Deo [OlSak] measured and assigned the rotational stmcture of the 1-0, 2-1, 0-0, 0-1, 1-2, 0-2, and 1-3 bands of In Cl, and 1-0, 0-0, and 0-1 of In Cl of the A IIo - X electronic transition. They report accurate rotational equilibrium parameters. 

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