Some Experimental Observations Photoelectron Spectroscopy

In the proper development of a scientific theory it is to be expected that theory and experimental observations should be consistent with one another, preferably the theory being refined by further observations. It is essential to obtain experimental observations by which ideas such as molecular orbital theory may be tested and possibly refined. This section describes a method of studying molecules using electromagnetic radiation. 

The relationship between frequency and wavelength is given by the equation  

Wavenumbers, represented by the symbol v (nu-bar), have the units of reciprocal length and are usually quoted in cm-1, although the strictly S.I. unit is reciprocal metres, nr1. For example, 3650 cm1 = 365000 m-1. A wavenumber represents the number of full waves that occur in the unit of length. For example, the wavenumber for the stretching vibration of a C-H bond in a hydrocarbon is around 3650 cnr1 and means that there are 3650 wavelengths in one centimetre. 

When dealing practically with equation 3.44 it is usual to work in molar quantities so that the energy has units of J mol1, and to achieve this it is necessary to use the equation  

Q What is the quantum energy of visible light of wavelength 500 nm  

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The electron affinities Ea of the main group atoms are the most precisely measured values. Recall that the Ea is the difference in energy between the most stable state of the neutral and a specific state of a negative ion. It was once believed that only one bound anion state of atoms and molecules could exist. However, multiple bound states for atomic and molecular anions have been observed. This makes it necessary to assign the experimental values to the proper state. The random uncertainties of some atomic Ea determined from photodetachment thresholds occur in parts per million. These are confirmed by photoelectron spectroscopy, surface ionization, ion pair formation, and the Born Haber cycle. Atomic electron affinities illustrate the procedure for evaluating experimental Ea. 

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