of Stimulation Electrodes: Part
I: Page 4
Electron Transfer in a Hypothetical System
Containing a Single Molecule.
Quantum theory tells us that electrons in atomic orbitals
occupy discrete levels of energy that may be determined from
the wave function ().
The probability of electron being in an elementary volume
is a function of ||^2. Theoretical
and experimental value for the lowest energy level of the
1s orbital of the Hydrogen atom has been found to be –13.6
eV (electron-volt, 1 eV = 1.6 x 10^-19 J). For spherically
symmetric orbitals, the discrete energy levels are given
E(n) = -13.6/n^2 ( for n = 1,2,3 ……).
Orbital electrons are characterized by the quantum numbers,
n, l and m, which are eigenvalues of the general solution
of the wave equation. Following the Pauli Exclusion Principle,
no discrete level may be occupied by electrons with the
same quantum number. Each atomic orbital can have a maximum
two electrons with opposite spin quantum number. In the
ground state (minimum energy) the orbitals are filled
from the lowest
In the hypothetical case of a single molecule shown on the
Left Panel, the electron states are filled up to a certain
energy level (Highest occupied molecular orbital, HOMO) and
the next possible energy level is unoccupied (Lowest unoccupied
molecular orbital, LUMO).
If this molecule were sufficiently close to a metal electrode
and the Fermi Level of the electrons in the electrode was
elevated to the same level as the unoccupied energy state
(LUMO) of the molecule, an electron could move from the electrode
to the unoccupied state of the molecule (Right Panel). In
this manner, current applied to an electrode can alter the
ionic state of the molecules in the vicinity of the electrode.
It is also worth noting that electron transfer does not occur
for lower energy levels because the allowable states in the
metal and that of the molecule are full at the matched energy
Similarly, if the Fermi Level of the electrons in the electrode
were lowered to the level of the highest occupied states,
by the application of anodic current (positive potential),
an electron can transfer from the molecule to the electrode
when the unoccupied energy states are matched in the metal
with the occupied state of the molecule. This electron
transfer would alter the ionization state of a molecule.