Coherence Spectroscopy Continued: Double Resonance, 2D-Techniques, Relaxation

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Coherence Spectroscopy ContinuedDouble
Resonance,2D-Techniques,Relaxation
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Energy Level Diagram of the Ar-CO Complex
Y. Xu and A. R. W. McKellar, Mol. Phys. 88, 859
(1996).
M. Hepp, R. Gendriesch, I. Pak, F. Lewen, and G.
Winnewisser, JMS 183, 295 (1997).
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Experimental Configuration
Excitation pulse generation
Pulsed nozzle
to detection system
Backward wave oscillator
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Double Resonance Experiments, Ar-CO
w
u
FFT
Single resonance experiments, 2000 cycles
v
w
u
FFT
Double resonance experiments, 10 cycles
v
w
u
FFT
Double resonance experiments, 300 cycles
v
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Three Level System
Ec
Pump transition ?bc0 (Ec-Eb)/ S
?bc0
Eb
Signal transition ?ab0 (Eb-Ea)/ S
?ab0
??ab ?ab0 - ?ab
Ea
??bc ?bc0 - ?bc
?ab, ?bc frequencies of the external
radiation fields
Transition dipole moments µab, µbc ? 0 µac
0. The detector can only see polarization from
the ab transition.
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Three-Level Bloch Equations
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Application of External Radiation
vac
uac
wbc
vbc
ubc
wab
vab
uab
xab 0
xab? 0
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Pulse Sequence for 2D-Spectroscopy
p/2-pulse
Time
Signal
p-pulse
p-pulse
Pump
Evolution
Mixing
Preparation
Detection
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Application of 2D Pulse Sequence
Signal
Pump
Detected Signal
vac
uac
wbc
vbc
ubc
wab
vab
uab
xab xbc 0
xab xbc 0
xab xbc 0
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Development ofCoherence and Population Terms
Detected Signal
vac
uac
wbc
vbc
ubc
wab
vab
uab
vac
uac
wbc
vbc
ubc
wab
vab
uab
vac
uac
wbc
vbc
ubc
wab
vab
uab
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2D-Experiment
FT
Signal Pulse
Pump Pulse
24382.19 24379.75 MHz
(l-type transitions, OCS)
100 ns
FT
t1
t2
1/??ac
13.47 MHz
OCS, ?211 Signal J 2-1, l 1f-1f _at_ 24381.00
MHz Pump J 1-1, l 1f-1e _at_ 12.72 MHz
t1
11.03 MHz
24382.19
24379.75 MHz
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Javan Doublet (Pump Radiation during Emission)
Detected Signal
vac
uac
wbc
vbc
ubc
wab
vab
uab
xbc? 0, xab 0
xab xbc 0
xab? 0
xbc 0
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Relaxation

• Molecular collisions lead to relaxation to
  thermal equilibrium
• Relaxation of population terms exp(-t/T1)
• Relaxation of coherence terms exp(-t/T2)
• T1-relaxation only J-changing collisions
  contribute to T1
• T2-relaxation also elastic collisions can
  contribute to T2

Usually T2ltT1
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Bloch Equations with Relaxation
?? ?0 ? (off-resonance) x µ e0 / S (Rabi
frequency)
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Relaxation in Modulation Experiment
on-resonance
off-resonance
on-resonance
off-resonance
w
v
u
1/T1 5 MHz, x 20 MHz
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Relaxation to Steady State
on-resonance
off-resonance
on-resonance
off-resonance
w
v
u
1/T1 20 MHz, x 20 MHz
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Fast Relaxation
on-resonance
off-resonance
on-resonance
off-resonance
w
v
u
1/T1 200 MHz, x 20 MHz
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The End