bei48482_FM

292 Chapter Eight

Representative coordinate

Excited state

Energy

Vibrational transition

Ground state

Figure 8.24The origin of fluorescence. The emitted radiation is lower in frequency than the absorbed

radiation.

vibrational spectra because they lack permanent dipole moments, nevertheless have

electronic spectra whose rotational and vibrational fine structures enable moments of

inertia and bond force constants to be found.

Electronic excitation in a polyatomic molecule often leads to a change in the mol-

ecule’s shape, which can be determined from the rotational fine structure in its band

spectrum. The origin of such changes lies in the different characters of the wave

functions of electrons in different states, which lead to correspondingly different bond

geometries. For example, the molecule beryllium hydride, BeH 2 , is linear (H—Be—H)

in one state and bent (H—Be) in another.



H

Fluorescence

A molecule in an excited electronic state can lose energy and return to its ground state

in various ways. The molecule may, of course, simply emit a photon of the same frequency

as that of the photon it absorbed, thereby returning to the ground state in a single step.

Another possibility is fluorescence.Here the molecule gives up some of its vibrational

energy in collisions with other molecules, so that the downward radiative transition orig-

inates from a lower vibrational level in the upper electronic state (Fig. 8.24). Fluorescent

radiation is therefore of lower frequency than that of the absorbed radiation.

Fluorescence excited by ultraviolet light has many applications, for instance to help

identify minerals and biochemical compounds. Fabric “brighteners” that are sometimes

bei4842_ch08.qxd 1/23/02 8:37 AM Page 292