Molecular electronic transitions band broadening

Apart from molecular vibrations, also rotational states bear a significant influence on the appearance of vibrational spectra. Similar to electronic transitions that are influenced by the vibrational states of the molecules (e.g. fluorescence, Figure 3-f), vibrational transitions involve the rotational state of a molecule. In the gas phase the rotational states may superimpose a rotational fine structure on the (mid-)IR bands, like the multitude of narrow water vapour absorption bands. In condensed phases, intermolecular interactions blur the rotational states, resulting in band broadening and band shifting effects rather than isolated bands. 

So far, this discussion of selection rules has considered only the electronic component of the transition. For molecular species, vibrational and rotational structure is possible in the spectrum, although for complex molecules, especially in condensed phases where collisional line broadening is important, the rotational lines, and sometimes the vibrational bands, may be too close to be resolved. Where the structure exists, however, certain transitions may be allowed or forbidden by vibrational or rotational selection rules. Such rules once again use the Born-Oppenheimer approximation, and assume that the wavefunctions for the individual modes may be separated. Quite apart from the symmetry-related selection rules, there is one further very important factor that determines the intensity of individual vibrational bands in electronic transitions, and that is the geometries of the two electronic states concerned. Relative intensities of different vibrational components of an electronic transition are of importance in connection with both absorption and emission processes. The populations of the vibrational levels obviously affect the relative intensities. In addition, electronic transitions between given vibrational levels in upper and lower states have a specific probability, determined in part... 

Initially, the PL mechanism is mainly studied by the molecular orbit theory, and this theory only treats some high-symmetry crystal. For intrinsic PL materials, first-principles calculations are used extensively to discuss the PL origin. From the calculation result, the fundamental crystal information and electronic properties can be obtained. The electronic-transition modes and their allowed or forbidden transition nature can be revealed. Thus, the theoretical results can predict the excitation and emission band positions approximately, which helps to perform the band assignment in the experimental spectra. After knowing the luminescent mechanism, we can modify the luminescence intensify and shift peak position as well as broaden the emission ranges by utilizing various experimental strategies. 

The atomic lines in the spectrum appear as vertical lines or peaks due to the nature of the electronic transition involved. That is, in molecules an electronic transition is usually accompanied by simultaneous changes in the molecule s vibrational and rotational energy levels sometimes all the three energy types may change simultaneously in an electronic transition in a molecule. The many different transition possibilities allowed in this way and the solvent effect derived from the aggregation state of the sample (the excited sample is in liquid form) determines that in UV-Vis molecular absorption (or emission) the corresponding peaks in the spectrum are widely broadened. Typically, the half-bandwidth of an absorption band in such molecular UV-Vis spectra is around 40 nm (or 400 A), whereas in atomic lines the half-bandwidth observed as a result of pure electronic transitions is of a few hundredths of an angstrom (typically 0.03-0.05 A). 

The far-UV to near-IR EF.T, spectrum of the unexposed C60 film is exhibited in Fig. 2.2a, in close agreement with spectra reported previously for thick C60 films on Si(100) (Gensterblum 1991). The characteristic camel back features, which exclude C60 as a carrier of the interstellar extinction, are readily observed at 195 and 260 nm. The three peaks at 260, 335 and 420 nm are seen in absorption spectroscopy (AS), and correspond to dipole-allowed,1A - Tlu, single-electron n-n transitions in the C60 molecule (Leach 1992 Hare et al. 1991). The intense peak at 195 nm is the so-called ir-plasmon that results from a collective excitation of the molecular TT-elcctron subsystem. The collective nature of this excitation is affirmed by our experimental observation that its intensity is variable upon changing the primary electron energy of the EEL spectrometer (Lucas 1992). Additionally, this band is broadened and blue-shifted when compared to the narrower transition seen in AS... 

The binding energy of 2.7 eV for the 0 2p electrons is unusually small compared to NaaO (4.2 eV) (70) or transition metal oxides (4-lOeV) (71). Indeed, it is the smallest known value which has to be interpreted as due to the comparably weak electrostatic field of positive charges acting on the 0 " ion in the suboxides. The very narrow structure of the 0 2p band is unusual, too. A band width of 5 eV is reported e.g. for NbO. (73) The narrow 0 2p band is characteristic of crystalline as well as amorphous suboxides. Obviously, it is the well-defined atomic environment and the isolation of the nearly gas-like 0 ions in the quasi-molecular clusters, which is responsible for the narrow 0 2p band in the suboxides, as this band is significantly broadened with the oxide CS2O itself. 

Molecular two-photon spectroscopy can also be applied in the infrared region to induce transitions between rotational-vibrational levels within the electronic ground state. One example is the Doppler-free spectroscopy of rotational lines in the V2 vibrational bands of NH3 [258]. This allows the study of the collisional properties of the V2 vibrational manifold from pressure broadening and shifts (Vol. 1, Sect. 3.3) and Stark shifts. 

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