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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.