In a photoionization experiment, several different ionization energies can be measured, depending on the degree of vibrational excitation of the cations. In general, the following two types of ionization energies are considered:
1.
Adiabatic ionization energy, EIa
The
energy corresponding to the transition
M(X, v” = 0) + hn
à
M+(x, v’ = 0) + e-
that is, the minimum energy required to eject an electron from a molecule
in its ground vibrational state and transform it into a cation in the lowest
vibrational level of an electronic state x of the cation.
2.
Vertical ionization energy, EIv
The
vertical ionization energy corresponds to the transition
M(X, v” = 0) + hn
à
M+(x, v’ = n) + e-
where, the value n of the vibrational quantum number v’ corresponds to
the vibrational level whose wavefunction gives the largest overlap with the v”
= 0 wavefunction. This is the most probable transition and usually corresponds
to the vertical transition where the internuclear separations of the ionic state
are similar to those of the ground state.
It is clear that transitions to each
of the ionic states x,a,b, c, … will have individual adiabatic and
vertical ionization energies. In the ZEKE or MATI experiments, one obtains
the adiabatic ionization energy of a molecule.
背景音樂: [給愛麗絲, 貝多芬]
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