Broad vibrational bands

Besides a transition to a continuum level of an excited electronic state, dissociation can occur by another mechanism in electronic absorption spectroscopy. If the potential-energy curve of an excited electronic state A that has a minimum in UA(R) happens to be intersected by the U(R) curve of an unstable excited state B with no minimum in U, then a vibrational level of A whose energy lies near the point of intersection of UA and UB has a substantial probability to make a radiationless transition to state B, which then dissociates. This phenomenon is called predissociation. Predissociation shortens the lifetimes of those vibrational levels of A that are involved, and therefore by the uncertainty principle gives broad vibrational bands with rotational fine structure washed out. 

On the experimental side, the present state of the art is such that we can transfer virtually any molecular assembly into the gas phase, regardless of its mass, and observe its spectroscopic signals. However, spectral congestion can be an unbeatable limiting factor, particularly when H-bonded interactions lead to broad vibrational bands. The core pentasaccharide motif provides an illustration of this limitation and makes us wonder whether the limit of the approach is getting close. 

Excimers are often characterized by a broad emission band containing no vibrational structure, occurring at longer wavelengths than emission corresponding to the monomeric singlet state/41,87-89,71-73 ... 

Near-UV CD of denatured proteins also provides evidence for some order in the side chains, especially in urea- and cold-denatured proteins. Nolting et al (1997) found a broad positive band with possible vibrational... 

While the BC configuration for the B—H complex is now accepted, several aspects of the vibrational spectra of the acceptor-H complexes are not understood. The temperature dependence of the B—H complex has been examined by Raman spectroscopy (Stutzmann and Herrero, 1987) and IR absorption (Stavola et al., 1988a). The H-stretching vibration shifts from 1875 to 1903 cm 1 between room temperature and liquid He temperature. Frequency shifts of just a few cm 1 are more typical for local vibrational modes. The vibrational bands are also surprisingly broad. 

In the electronic transitions in visible and ultraviolet region with liquids or solutions, we do not get vibrational bands along with rotational fine structure, but we get a continuous broad electronic band and hence such curves do not give much valuable information. This is because the vibrational fine structure gets suppressed due to overlapping of vibrational spacings. 

A distinctive feature of the O2 and S2 luminescence spectra in minerals is a quasi-linear vibrational structure of the broad luminescence band (Tarash-chan 1978). The O2 and S2 molecular ions are isoelectronic. From the molecular orbital diagram describing their electron structure the emission transition Eg- n l2 is determined. When observing luminescence spectra at 77 K, a fine structure associated with the frequency of intra-molecular vibrations of O2 and S2 is detected. This frequency depends on the type of the molecular ion, on inter-nuclear distance and upon the particular position of the molecular ion in the structures. For S2 the maximum of the emission band lies within the range of 600-700 nm with a mean vibration frequency of 500-600 cm , while for O2 the respective maximum is 450-550 nm with a frequency in the 800-1,200 cm range. 

Time-resolved luminescence spectroscopy of sodalite evidences that the vibration structure has a very short decay time and disappears after a delay of 250 ns. Such structure is superimposed on the very broad IR band (Fig. 4.65). 

We showed in the preceding section that for solids with strong vibrational bands the position of features in absorption spectra can be shifted appreciably in going from the bulk to particulate states. Metallic particles can deviate even more markedly from the behavior of the bulk parent material they can have absorption features over broad frequency regions where none appear in the bulk. For a simple metal—one that is well described by the Drude formula... 

---