MadSci Network: Physics

Re: What is the reference point when measuring ionization potentials of atoms

Date: Tue Mar 30 10:30:00 2004
Posted By: Kenneth Beck, Staff, Chemistry and Physics of Complex Systems (C&PCS), Pacific Northwest National Laboratory
Area of science: Physics
ID: 1079633328.Ph


Rigorously, the first ionization energy of an atom in vacuum is the 
amount of energy necessary to remove the outer valence electron from its 
orbital to a distance of infinity. I say "first" because an atom can have 
as many ionization energies as it has electrons. The second ionization 
energy, for example, is the amount of energy necessary to remove the 
second outer valence electron to infinity after the first electron has 
previously been removed.  See, you just work your way through the atom, 
one electron at a time, all the way through the valence and core 
electrons.  You can imagine that the more electrons you remove and the 
closer the electrons are to the nucleus, the more energy it will take to 
remove them to infinity. Also note, ionization energy is sometimes 
confusingly called "ionization potential" since only the charge of one 
electron is considered. But stick to ionization energy since eV is an 
energy term, while V is a potential term (e = charge of electron; V = 
potential.  So, e times V = energy of an electron, or eV).

In reality, a typical researcher working with a finite apparatus probably 
takes "infinity" to mean a few meters separation max.  At that distance 
the interaction of the removed electron and the atom are nearly non-
existent.  The electron is essentially a "free electron".  One of the most 
accurate means to measure ionization potentials (and electron 
affinities) is via ZEKE spectroscopy (do a Google web search for that 
term if you're interested).  Using ZEKE, ionization potentials of 1meV 
resolution are possible for even large molecules.

Remember however, for single atoms in a vacuum, there are physical 
constants associated with the process of ionization. These terms are also 
used to describe ionization of molecules and solids, but the values are 
not constant because their ionization can be affected by the local 
chemistry, geometry, and temperature.

Here are a couple of URLs that may be useful...

=> http://www.chem.uidaho.ed
=> http://spectr-

---* Dr. Ken Beck

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