Page 352 - Instant notes

Physical Chemistry 338

A characteristic feature of a molecule that phosphoresces is that it possesses an excited
triplet electronic state, T 1, of energy similar to the excited singlet state, S 1, and into
which the excited singlet state can convert. In a triplet state two electrons in different
orbitals have parallel spins ( ). Although normally forbidden, if a mechanism exists for
converting paired ( ) electron spins into unpaired ( ) electron spins the excited S1
state may undergo intersystem crossing into the T 1 state. (The usual mechanism for
intersystem crossing is spin-orbit coupling in which the magnetic field from the nucleus
of a heavy atom induces a neighboring electron to flip its spin orientation.) After
intersystem crossing the molecule continues to step down the vibrational ladder of the T 1
state by loss of energy in collisions with surrounding molecules. The molecule cannot
lose electronic energy by radiative transfer to the ground state because a triplet-singlet
transition is forbidden. However, the transition is not entirely forbidden because the same
mechanism that permits singlet to triplet intersystem crossing in the first place also
breaks the selection rule so that the molecules are able to emit weak phosphorescence
radiation on a longer timescale.

Photoelectron spectroscopy

The absorption of a photon of high enough energy may cause an electron to be ejected
entirely from a molecule. In photoelectron spectroscopy, molecules are irradiated with
high frequency, monochromatic light and the kinetic energy of the emitted photoelectrons
is analyzed. The resulting photoelectron spectrum provides information on the energy
levels of the orbitals from which the electrons were emitted. Conservation of energy
dictates that if the incoming photon has frequency, v, and the ionization energy for the
electron in an orbital is I, the kinetic energy of the emitted photoelectron is:

The kinetic energy of the electrons is determined from the strength of the electric or
magnetic field required to bend their path into a detector. The slower the ejected electron,
the lower in energy the molecular orbital from which it was ejected. Ultraviolet
photoelectron spectroscopy provides information on the energy levels of the molecular
orbitals of the valence electrons of molecules; X-ray photoelectron spectroscopy
provides information on the energy levels of core electrons. If the apparatus has
sufficient resolution of photoelectron kinetic energy, it may be possible to resolve fine
structure in the photoelectron spectrum associated with the vibrational levels of the
molecular cation formed by the ionization.