Advances in Fingerprint Technology.pdf

purposes of understanding their photoluminescence, these molecules can be
thought of to good approximation as being two-electron systems. When the
molecule is in the ground (unexcited) state, the two optically active electrons
occupy the same molecular orbital, namely, the highest occupied molecular
orbital (HOMO). Their spins must be antiparallel to satisfy the exclusion
principle. The total spin (S) of the two electrons, namely the sum of the two
spins (1/2 each), is thus zero and the spin multiplicity (2S + 1) is 1. The state
is, accordingly, a singlet state. On optical excitation, one of the electrons is
promoted to a molecular orbital of higher energy via the absorption of the
excitation illumination, namely, to the lowest unoccupied molecular orbital
(LUMO). The molecule is now in the excited state that gives rise to photoluminescence.
If no electron spin flip has taken place during the excitation
process, the excited state is still a singlet state and the decay to the singlet
ground state (i.e., electron jump back to HOMO), accompanied by emission of
light, is termed fluorescence. If a spin flip has taken place, however, which is legal
in terms of the exclusion principle because the two electrons no longer occupy
the same orbital, the excited state becomes a triplet state (S = 1, 2S + 1 = 3) and
the decay to the singlet ground state is termed phosphorescence. The distinction
between fluorescence and phosphorescence on the basis of electron spins
is traced back to the nature of the electric dipole operator that describes the
emission of light. It does not operate on spin. Thus, an allowed transition,
namely one that can occur quickly, one that comes with a short lifetime,
involves ground and excited states of the same spin multiplicity (2S + 1).
When the spin multiplicities of the two states differ, the transition is forbidden
and thus does not occur quickly; hence, a long lifetime. This definition
of fluorescence and phosphorescence is not universally followed. For example,
the luminescences of lanthanides are often (sloppily) referred to as fluorescences
although the spin multiplicities of the involved states differ by
even more than discussed above, and the lifetimes of lanthanide luminescences
are very long compared to typical fluorescence lifetimes. Recombination
and trap luminescences, as found in semiconductor materials, are a
different matter still. In this chapter we reserve the term “fluorescence” to
the transition between molecular singlet states. Other transitions are referred
to by the catch-all terms “luminescence” and “emission,” or are called “phosphorescence”
when appropriate according to the above-described criterion.
Basics of Time-Gated Fingerprint Detection
Suppose a latent fingerprint has been treated such that it phosphoresces and
assume that it is located on a fluorescent article. Now illuminate the article
with an appropriate laser (filtered lamps are generally not useful for timeresolved
fingerprint detection) that is periodically turned on and off and
assume there is on hand an imaging device that can be turned on and off