Electron Spectroscopic Methods in Teaching Michael Allan lnstitut de Chimie Physique de I'Universite, CH-1700 Fribourg, Switzerland The quantum theory of matter is one of the fundamental topics to which an undergraduate student of chemistry or physics is exposed. The experimental evidence on which the treatment of this suhiect is based is usually the internal structure of atoms andmolecules, in particular the quantization of excitation energies. Some of the clearest experimental evidence for quantization came from the classic experiment performed in 1918 by James Franck and Gustav Hertz (Fig. 1). Although historically it came after much other evidence for quantization, it is a particularly straightforward example of a quantum process, both conceptually and practically. For this reason it is often quoted in introductory texts on the subject, for example in the excellent textbooks of Berry, Rice, and Ross ( I ) , and Hellwege (2).Unfortunately, the original Franck-Hertz experiment can only he used to introduce-the concept of discrete energy losses. i t s very low resolution and the complications brought by multiple collisions and multiple e n e h losses in the relatively dense gas do not permit the visualization of any details of the excitation energies. Further discussion of the energy levels of atoms and molecules are therefore illustrated in most textexperiments onlv. books hv. optical . Electron energy-loss spectroscopy, the modern version of the Franck-Hertz experiment, provides an excellent connection to experiment in elementary discussions of excitations in atoms and molecules. It is a great improvement on the original Franck-Hertz experiment because the single collision conditions of the modern experiment make it conceptually simpler and only one peak in the spectrum now &responds to a given excited state. The higher resolution also permits the discussion of vibrational motions in molecules. The basic principles of this method and its application to chemistry is described for example in the review article of Kupperman e t al. (3). Photon emission and absorption spectroscopy suffers from some problems from the didactic point of view, which are overcome by the electron energy-loss spectroscopy; 1. The optical methods usually cover relatively narrow energy
ranges, and the global view of the phenomena is often lost. 2. Many of the electronically excited states of simple atoms and molecules fall into the vacuum-UV region, where spectra are
often not readily available. 3. The emission spectra of simple molecules and atoms usually
consist oftransitions between excited states and illustrate onlv in
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Figure 1. The original Franck-Herb experiment in Hg vapor. In this experiment electrons emined from a hot filament were accelerated to a variable energy by means of a variable vaitaoe The aooaratus " between the filament and a arid. " .. was filled with a low-pressure mercury vapor and the electron could lase energy in collisions with the Hg atoms. me energies of the electrons alter passage through the gas were analyzed by applying a variable retarding voltage to the collector plate. The experiment showed that the electrons lose only energies amountingto a multiple (because several consecutive collisions may occur)of 5 eV, indicating a discrete excitation energy of 5 eV in mercury.
separation in the valence transitions as compared to the Rydherg transitions),are not affected by complex overlap and mixing with them and are conseouentlv . . easilv identified and discussed in termsof orbitaloccupation. Asan example the r' r transitions in formaldehyde may be quoted. The 3(rn*)(3A1) hand is a well developed band with maximum at 5.8 eV, whereas the '(ra')('A~)transition has been predicted by theary to be around 11 eV, in a region heavily populated by Rydberg transitions with which it apparently strongly interacts so that it cannot he identified in the spectrum at all (4).A similar situation is encountered in ethylene, the prototype r-electron system (5).A further examole is the N"molecule discussed in this .oaner. . . where the sinelet" triplet transitions may Iw sucv~\slullyrationali,ed i u terms oi qualitative mulerular orbitals, u,herrdr thcuptirdlly al~crnt.
