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Laser Induced Fluorescence
Structural information about the ground and excited states of molecules.
Excitation experiments  Excited state information
Emission experiments  Ground state information
Internal
conversion
S2
Vibrational
relaxation
Intersystem
crossing
S1
T1
Absorption
Fluorescence
S0
A Jablonski diagram
Phosphorescence
Laser Induced Fluorescence
Excitation
3
2
1
h
v`= 0
3
Essentially, the observation of fluorescence, is
used to infer the presence of a vibronic level.
2
1
v``= 0
Nuclear conformation
Excitation v``  v`
/
Potential Energy
• Tune incident radiation.
• When incident radiation matches a vibronic
transition, radiation is absorbed.
• The excited state fluoresces.
• The total fluorescence is collected by a
photomultiplier tube (PMT).
Potential Energy
Excitation process
Laser Induced Fluorescence
Excitation
Potential Energy
• The laser is fixed at one of the
3
excitation/absorption frequencies.
2
• The emitted fluorescence is dispersed into
1
its component wavelengths by a
v`= 0
monochromator.

h
• The spacing between the observed bands
gives the spacing between the vibrational
levels in the ground state.
3
Potential Energy
Emission process
Emission
3
2
1
h
v`= 0
3
2
2
1
Need to do an excitation experiment first to
determine the absorption frequencies. v``= 0
v``= 0
Nuclear conformation
Nuclear conformation
Excitation v``  v`
Emission v`  v``
/
1
/
/
Supersonic Jets
Method of producing internally (vibrationally, rotationally) cold molecules.
High-pressure
gas reservoir
Nozzle
• Molecules emerge with a narrow spread
of velocities.
• Collisions partition vibrational and
rotational energy into translational.
• Effect is an increase in the translational
velocity, u, of the gas.
• Drop in density, reduces the local speed
of sound, a.
The Mach number, M, is
Laser Beam
u
M
a
When M>1, beam is termed
supersonic.
Supersonic Jets - Laser Desorption.
How to get the molecules of interest into the supersonic jet?
If molecules are thermally stable or relatively volatile,
• Thermally vaporise sample and mix with carrier gas in reservoir.
If sample is thermally labile or involatile,
• Use laser desorption to vaporise sample.
• Desorption affected by a pulsed CO2
laser, from a solid sample.
• Sample molecules and carrier gas mix
in a device called a “faceplate”.
• The narrow channels promote
collisional cooling.
Faceplate
The Franck-Condon Principle
Overlap of the wavefunctions in the initial and final states determine
whether the transition will occur.
Rif   v*, f  v , i d
S1
v`= 3
v`= 2
v`= 1
Energy
v`= 0
S0
v``= 3
v``= 2
v``= 1
v``= 0
Internuclear Distance
Vibrational overlap integral
If S0 and S1 similar in shape
• Biggest overlap between v’’=0 and v’=0.
• Single band seen in the excitation spectrum.
The Franck-Condon Principle
S1
v`= 3
v`= 2
v`= 1
Energy
v`= 0
S0
v``= 3
v``= 2
v``= 1
v``= 0
Internuclear Distance
When S0 and S1 different
• Many levels in S1 have overlap with the
v’’=0 wavefunction.
• Several vibronic bands observed in the
excitation spectrum.
• Most intense band is that with greatest
overlap.
• Distribution is called the Franck-Condon
envelope.
4-hydroxyl biphenyl
LIF excitation spectrum
In S0 =42 (electron diffraction)
I
• Long vibronic progression  big change between S0 and S1 electronic states.
• Constant spacing implies S1 state is a harmonic potential.
• Spacing has low frequency (56cm-1) low frequency mode excited in S1.
• Progression probably due to torsional motion.
Biphenyl
4000
• Can also use ab initio methods to
model the torsional potentials.
• Origin of the change in potential
shapes is related to the shape of the
HOMO and the LUMO.
SS1
3000
3000
1
2000
2000
Energy / wavenumbers
• Analysis of the Franck-Condon
envelope shows that the torsional
angle changes by 42 between S0
and S1.
• In S1 molecule is flat, =0 .
4000
1000
1000
00
4000
4000
3000
3000
2000
2000
S0
S0
1000
1000
00
-100
100
-50
50
0
0
50
50
Torsional Angle / degrees
HOMO
100
100
LUMO
Tyramine
LIF excitation spectrum
• Six bands observed.
• Not vibronic structure, but due to six different molecular conformers.
• Confirmed by power saturation experiments and “hole-burning” experiments.
• Very narrow line-width lasers can resolve the bands to rotational resolution.
• Can get rotational constants for each molecular conformer - this is hard!
Tyramine
LIF emission spectra
• Disperse the emission from each
band in the excitation spectrum.
• Each conformer has a slightly
different pattern of vibrational
bands in the ground state.
• Different structures have
different vibrational frequencies.
Now have rotational and vibrational
information about each conformer.
A
B
C
D
E
F
Wavenumber / cm-1
Tyramine
1
2
6
5
• Again, make recourse to ab initio
methods.
• Compare calculated vibrational and
rotational frequencies with
information gained from experiment.
• Allows assignments of bands to
different conformer structures.
Interchange between conformer structures
obtained by rotation of the tail segments.
3
4
Summary
Fluorescence is :
• A sensitive probe of molecular structure in different electronic
states.
• A useful tool to study conformational behaviour in flexible
molecules.
• Applicable to both thermally stable and labile molecules.
Next Week- What happens if the molecules are not fluorescent?
Alternative absorption methods and the usefulness of ionisation.