The Shell Model and PES

The Shell Model
And Photoelectron Spectroscopy
and the Structure of Atoms
Ch 7 part 1.5
A little history
 Einstein earned the Noble Prize for his
work on the Photoelectric effect.
 The photoelectric effect is when electrons
are emitted from a solid liquid or gas when
they absorb energy in the form of light.
 This has been determined experimentally
by shining light on metal.
Photoelectric Effect
By shining a light on metal,
one may create a current
Photoelectric effect
 Increasing the intensity of the light
increased the number of photoelectrons,
but not their maximum kinetic energy
 Red light will not cause the ejection of
electrons, no matter what the intensity!
 A weak violet light will eject only a few
electrons, but their maximum kinetic
energies are greater than those for intense
light of longer wavelengths!
Ephoton = hv
 PES provides a useful means to extract
information on atomic structure using
spectroscopic data.
 By looking at the data chart one may
determine the structure of the atom.
 How? The chart gives evidence for the
shell model.
Spectroscopy (PES)
 Do all electrons in a
given shell have the
same energy?
 To answer this a
technique using
radiation on a neutral
atom in the gas
phase called
Spectroscopy is
 The technique shines
radiation (UV or Xray) on an atom to
excite it to the point
that it will eject an
electron from its shell
and form a cation.
Shell Model
 In the shell model,
we see the ionization
energy is decreased
the farther the
electron is from the
 By the way of
reasoning, it takes
more energy to
remove an electron
from n=2 than n=3.
Spectroscopy (PES)
 The energy of the
photon used is
higher than the
ionization energy.
 The excess energy is
converted to the
kinetic energy on the
ejected electron.
 The energy required
to remove the
electron is equal to
the difference
between the energy
absorbed by the
atom (hv) and the
kinetic energy of the
ejected electron
 IE = hv - KE
Spectroscopy (PES)
 This technique
allows the researcher
to remove one
electron from any
 Therefore, One may
remove electrons
from the inner core
without moving the
outer electrons.
The apparatus
Reading a PES spectra
 On the PES spectra, energy increases as one
moves to the left. The height refers to the
number of electrons.