Molecular spectral fine structures

The spectrum of Figure lb is a fingerprint of the presence of a CO molecule, since it is different in detail from that of any other molecule. UPS can therefore be used to identify molecules, either in the gas phase or present at surfaces, provided a data bank of molecular spectra is available, and provided that the spectral features are sufficiently well resolved to distinguish between molecules. By now the gas phase spectra of most molecules have been recorded and can be found in the literature. Since one is using a pattern of peaks spread over only a few eV for identification purposes, mixtures of molecules present will produce overlapping patterns. How well mixtures can be analyzed depends, obviously, on how well overlapping peaks can be resolved. For molecules with well-resolved fine structure (vibrational) in the spectra (see Figure lb), this can be done much more successfiilly than for the broad. 

To identify a colorant, its excitation and emission spectra must be measured. This can be done under standard conditions if the colorant has been extracted from a foodstuff. Usually the spectral patterns taken from real conditions will not deviate too much from standard conditions. One must be aware that the main spectral patterns are determined by the chromophore of the colorant and that further molecular identification needs to recognize special fine structures of the spectra or employ additional analytical tools. 

Fig. 6.3. The significant free —> free components of the spectral functions of molecular hydrogen pairs at 77 K. For a given set of expansion parameters A1A2AL, a different line type is chosen. When two curves of the same type are shown, the upper one represents the free — free, the lower the bound —< free contributions their sum is the total FG al T). The extreme low-frequency portion of the bound — free contributions with the dimer fine structures is here suppressed [282],...

It has been necessary to understand the relationship between molecular fine structure of cyanine dyes and important properties such as colour, dye aggregation, adsorption on silver halide and electrochemical potentials in order to design and prepare sensitizers with optimum performance. For general discussion of these topics and the mechanism of spectral sensitization, the reader is referred to recent surveys on the subject (B-77MI11401, 77HC(30)441). 

The study of the rotation-vibration spectra of polyatomic molecules in the gas phase can provide extensive information about the molecular structure, the force field and vibration-rotation interaction parameters. Such IR-spectra are sources of rotational information, in particular for molecules with no permanent dipole moment, since for these cases a pure rotational spectrum does not exist. Vibrational frequencies from gas phase spectra are desirable, because the molecular force field is not affected by intermolecular interactions. Besides, valuable support for the assignment of vibrational transitions can be obtained from the rotational fine structure of the vibrational bands. Even spectra recorded with medium resolution can contain a wealth of information hot bands , for instance, provide insight into the anharmonicity of vibrational potentials. Spectral contributions of isotopic molecules, certainly dependent on their abundance, may also be resolved. 

The experimentally achievable localized excitations are typically described by one of the zero-order basis states (see Section 3.2), which are eigenstates of a part of the total molecular Hamiltonian. Localization can be in a part of the molecule or, more abstractly, in state space . The localized excitations are often described by extremely bad quantum numbers. The evolution of initially localized excitations is often more complex and fascinating than an exponential decay into a nondescript bath or continuum in which all memory of the nature of the initial excitation is monotonically lost. The terms in the effective Hamiltonian that give birth to esoteric details of a spectrum, such as fine structure, lambda doubling, quantum interference effects (both lineshapes and transition intensity patterns), and spectroscopic perturbations, are the factors that control the evolution of an initially localized excitation. These factors convey causality and mechanism rather than mere spectral complexity. 

UV-VIS spectra are complex as they contain the fine structure from rovibrational transitions. Upon adsorption this structure can only rarely be observed. Due to the blurring of the rovibrational levels mainly broad unresolved bands remain with the complete loss of structural information. The assignment of bands to certain electronic transitions is carried out in correspondence with the direction of the frequency shift or with the aid of semiempirical concepts. At present there is no strict theory relating the spectral changes to the nature of molecular interaction. New surface complexes and intermediate surface compounds have to be determined by comparison with the spectra of the same compounds in solution. 

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