For ultraviolet and visible wavelengths, one should expect from this discussion that the absorption spectrum of a molecule (i.e.,
a plot of its degree of absorption against the wavelength of the incident radiation) should show a few very sharp lines. Each line
should occur at a wavelength where the energy of an incident photon exactly matches the energy required to excite an
electronic transition.
In practice it is found that the ultraviolet and visible spectrum of most molecules consists of a few humps rather than sharp lines.
These humps show than the molecule is absorbing radiation over a band of wavelengths. One reason for this band, rather than
line absorption is that an electronic level transition is usually accompanied by a simultaneous change between the more
numerous vibrational levels. Thus, a photon with a little too much or too little energy to be accepted by the molecule for a 'pure'
electronic transition can be utilized for a transition between one of the vibrational levels associated with the lower electronic
state to one of the vibrational levels of a higher electronic state.
If the difference in electronic energy is 'E' and the difference in vibrational energy is 'e', then photons with energies of E, E+e,
E+2e, E-e, E-2e, etc. will be absorbed.
Furthermore, each of the many vibrational levels associated with the electronic states also has a large number of rotational
levels associated with it. Thus a transition can consist of a large electronic component, a smaller vibrational element and an
even smaller rotational change. The rotational contribution to the transition has the effect of filling in the gaps in the vibrational
fine structure.
In addition, when molecules are closely packed together as they normally are in solution, they exert influences on each other
which slightly disturb the already numerous, and almost infinite energy levels and blur the sharp spectral lines into bands. These
effects can be seen in the spectra of benzene as a vapor and in solution. In the vapor, the transitions between the vibration
levels are visible as bands superimposed on the main electronic transition bands.
In solution they merge together and at high temperature or pressure even the electronic bands can blur to produce single wide
band such as that enclosed by the dotted line in Figure 6.