HW 10-luminescence spectroscopy key

HW 10-luminescence spectroscopy key -...

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Unformatted text preview: HW:
Luminescence
Spectroscopy
 (1)  Explain
the
difference
between
a
fluorescence
emission
spectrum
and
a
fluorescence
excita@on
 spectrum.
Which
one
closely
resembles
an
absorp@on
spectrum?
 The
emission
spectrum
is
a
measure
of
the
light
emiFed
by
an
atom,
molecule,
par@cle
following
 absorp@on.
(An
excita@on
spectrum
is
an
absorp@on
spectrum.)
The
emission
spectrum
is
 stokes
shiJed
(or
red
shiJed)
from
the
excita@on
spectrum.

 (2)
Define
the
following:
 a)  Phosphorescence:
emission
resul@ng
from
relaxa@on
of
a
triplet
state
to
the
ground
state.
 b)  Triplet
state:
A
state
in
which
the
spin
mul@plicity
=
3
 c)  Vibra@onal
relaxa@on:
loss
of
energy
via
a
transi@on
from
a
higher
E
vibra@onal
state
to
a
lower
 E
vibra@onal
state.
 d)  Intersystem
crossing:
An
electronic
transi@on
from
a
singlet
excited
state
to
a
triplet
excited
 state.
 e)  Quantum
yield:
ra@o
of
the
number
of
photons
emiFed/number
absorbed
 (3)
Why
is
fluorescence
spectroscopy
more
sensi@ve
than
absorp@on
spectroscopy?
 In
fluorescence
the
detec@on
involves
measuring
a
large
signal
over
a
small
background.
Absorbance
 spectroscopy
measures
a
small
decrease
in
a
large
signal.

 (4)
Why
do
some
compounds
fluoresce
but
others
do
not?
 Molecules
will
not
fluorescence
or
phosphoresce
if
non‐radia@ve
routes
of
the
decay
(relaxa@on
of
 the
electron
from
the
excited
state
back
to
the
ground
state)
of
the
excited
state
are
faster
than
the
 radia@ve
routes
of
decay.
One
important
non‐radia@ve
relaxa@on
route
is
external
conversion,
in
 which
molecules
transfer
energy
to
the
solvent
via
collisions.
Rigid
aroma@c
molecules
typically
 undergo
radia@ve
decay
while
floppier
molecules
undergo
internal
conversion.
 ...
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