Atomic spectroscopy compared with molecular

The quantum theory of spectral collapse presented in Chapter 4 aims at even lower gas densities where the Stark or Zeeman multiplets of atomic spectra as well as the rotational structure of all the branches of absorption or Raman spectra are well resolved. The evolution of basic ideas of line broadening and interference (spectral exchange) is reviewed. Adiabatic and non-adiabatic spectral broadening are described in the frame of binary non-Markovian theory and compared with the impact approximation. The conditions for spectral collapse and subsequent narrowing of the spectra are analysed for the simplest examples, which model typical situations in atomic and molecular spectroscopy. Special attention is paid to collapse of the isotropic Raman spectrum. Quantum theory, based on first principles, attempts to predict the. /-dependence of the widths of the rotational component as well as the envelope of the unresolved and then collapsed spectrum (Fig. 0.4). 

One of the most promising tools in the study of the nature and structure of adsorbed molecules is photoelectron spectroscopy (40), and results from such experiments can be compared with EHT calculations. As an example, the experimental and calculated spectra of ethylene on Ni(l 11) are compared in Fig. 40. In the calculations, the model surface consisted of a Ni atom surrounded by six nearest neighbours in the surface plane and three in the plane below. The molecular plane of ethylene was taken to be parallel to the surface. 

The use of solid state NMR for the investigation of polymorphism is easily understood based on the following model. If a compound exists in two, true polymorphic forms, labeled as A and B, each crystalline form is conformationally different. This means for instance, that a carbon nucleus in form A may be situated in a slightly different molecular geometry compared with the same carbon nucleus in form B. Although the connectivity of the carbon nucleus is the same in each form, the local environment may be different. Since the local environment may be different, this leads to a different chemical shift interaction for each carbon, and ultimately, a different isotropic chemical shift for the same carbon atom in the two different polymorphic forms. If one is able to obtain pure material for the two forms, analysis and spectral assignment of the solid state NMR spectra of the two forms can lead to the origin of the conformational differences in the two polymorphs. Solid state NMR is thus an important tool in conjunction with thermal analysis, optical microscopy, infrared (IR) spectroscopy, and powder... 

Studies by Teplyakov et al. provided the experimental evidence for the formation of the Diels-Alder reaction product at the Si(100)-2 x 1 surface [239,240]. A combination of surface-sensitive techniques was applied to make the assignment, including surface infrared (vibrational) spectroscopy, thermal desorption studies, and synchrotron-based X-ray absorption spectroscopy. Vibrational spectroscopy in particular provides a molecular fingerprint and is useful in identifying bonding and structure in the adsorbed molecules. An analysis of the vibrational spectra of adsorbed butadiene on Si(100)-2 x 1 in which several isotopic forms of butadiene (i.e., some of the H atoms were substituted with D atoms) were compared showed that the majority of butadiene molecules formed the Diels-Alder reaction product at the surface. Very good agreement was also found between the experimental vibrational spectra obtained by Teplyakov et al. [239,240] and frequencies calculated for the Diels-Alder surface adduct by Konecny and Doren [237,238]. 

However, what unite all applications of NIRS for PAC are the unique features of the NIR spectrum. The NIR is in effect the chemical spectroscopy of the hydrogen atom in its various molecular manifestations. The frequency range of the NIR from about 4000 cm-1 up to 12 500 cm-1 (800-2500 nm) covers mainly overtones and combinations of the lower-energy fundamental molecular vibrations that include at least one X—H bond vibration. These are characteristically significantly weaker in absorption cross-section, compared with the fundamental vibrational bands from which they originate. They are faint echoes of these mid-IR absorptions. Thus, for example, NIR absorption bands formed as combinations of mid-IR fundamental frequencies (for example v + u2), typically have intensities ten times weaker than the weaker of the two original mid-IR bands. For NIR overtone absorptions (for example 2v, 2v2) the decrease in intensity can be 20-100 times that of the original band. 

It has been shown that, under appropriate conditions, the momentum distribution uj py for an individtial electronic state is directly meastired by electron-momentum spectroscopy (ref. 29). Figure 3.8 compares the experimental momentum distribution for the hydrogen atom ground state with the function calculated by the Fourier transform of the hydrogen Is orbital. In general, the electron-momentum spectroscopy results serve to evaluate wavefunctions at various levels of theory for a variety of atomic (and molecular) systems. 

Commonly, the vibrational spectroscopy covers a wavenumber range from 200 to 4000 cm-1. We should know that crystalline solids also generate lattice vibrations in addition to molecular vibrations. The lattice vibrations refer to the vibrations of all the atoms in crystal lattice in a synchronized way. Such vibrations exhibit lower frequencies compared with those of common molecular vibrations and have a wavenumber range of about 20-300 cm-1. Coupling between lattice and molecular vibrations can occur if the molecular vibrations lie in such a low wavenumber range. Molecular vibrations can be distinguished from the lattice vibrations because they are not as sensitive to temperature change as lattice vibrations. 

Our previous work has shown that spin-coated films of zinc phthalocyanines exhibited detectable shifts in the optical absorption spectra during alcohol vapor exposure [5], We here report an investigation of the molecular interactions between zinc phthalocyanines and alcohols by the x-ray absorption spectroscopy, the phase contrast optical microscopy and the transmission electron microscopy. The experimental results are also compared with our Density Functional Theory (DFT) calculations of the interactions between alcohol molecules and the zinc atom of the phthalocyanines [5],... 

Either two or more molecular levels of a molecule are excited coherently by a spectrally broad, short laser pulse (level-crossing and quantum-beat spectroscopy) or a whole ensemble of many atoms or molecules is coherently excited simultaneously into identical levels (photon-echo spectroscopy). This coherent excitation alters the spatial distribution or the time dependence of the total, emitted, or absorbed radiation amplitude, when compared with incoherent excitation. Whereas methods of incoherent spectroscopy measure only the total intensity, which is proportional to the population density and therefore to the square ir of the wave function iff, the coherent techniques, on the other hand, yield additional information on the amplitudes and phases of ir. 

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