1. Time-resolved Spectroscopy
A. Yartsev
16/04/2007

2. Important Factors for Time-resolved Spectroscopy.
Temporal resolution – pulse duration.
Spectral resolution – bandwidth and tunability.
Efficient start of the dynamics of interest – high intensity of ”Pump” pulse.
Fast process to probe the dynamics – tunability and intensity of ”Probe” pulse.
Sensitive detection.
Data analysis and modelling.
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3. Direct Temporal Resolution
Absorption: Flash-photolysis – fast detector and fast oscilloscope.
Fluorescence: Time-Correlated Single Photon Counting (TCSPC).
STREAK camera.
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4. Pump-Probe Correlated Temporal Resolution.
”Strong” Pump – ”weak” Probe
Transient absorption
Transient gaiting
”Strong” Pump – ”strong” Probe
Multiphoton ionization
Integrated fluorescence
RAMAN etc.
”Strong” Pump – ”strong” Gate
Fluorescence up-conversion
Optical Kerr Effect
Coherent methods
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5. Temporal Resolution From Data Analysis.
Sub-instrumental response dynamics
Fluorescence phase-shift method
Excitation correlation fluorescence
Coherent: 3PEPS
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6. Spectral Resolution.
Uncertainty principle limitation: short time needs broad spectrum!
Tunability of the pump and probe light is available through various lasers, frequency conversion and fs-continuum.
Spectral sensitivity of detector.
Is the uncertainty principle applied for detection as well?
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7. Short Pump Pulses.
Fast excitation – temporally ”clean” start of process.
High intensity of the light – non-linear effects can be used for excitation and probing.
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8. Problems with Short Pump Pulses.
Broad spectrum – lack of spectral selectivity.
Non-linearity may induce complications in the dynamics of interest.
Artefacts: - may complicate early time scale dynamics.
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9. Short Probe Pulses.
Make fine grid to accuratelly resolve the dynamics.
Short probe pulse + fine time grid –accurately resolved dynamics.
Broad probe spectrum is good to resolve new transitions.
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10. Problems with Short Probe Pulses.
Broad spectrum – lack of spectral selectivity.
Non-linearity may the probe-induced dynamics.
Artefacts: - spectrally non-even detection efficiency may lead to XFM.
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11. What can we get from absorption?
Absorption spectrum as a fingerprint of a molecule.
From absorption, path length and Beer-Lambert law:
Concentration c[mol*dm-3] from extinction [dm3mol-1cm-1]
Or extinction  from concentration c.
Molecular cross-section [cm-2] from C[cm-3].
transition dipole moment  from spectral shape of .
And with short pulses we can time-resolve this all!
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12. Time-resolved Absorption
Single colour
Shot – to – shot.
Lock-in technique: chopped pump or both pump and probe are chopped at different frequencies and the signal is measured at differential frequency.
Pseudo two-colour.
Multiple colour
Single shot – single λprobe: point-by-point.
Single shot – whole spectrum.
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13. Differential absorption: ”weak” Probe.
Lock-in technique: filter the Probe light noise out, keep the Pump contribution only.
Differential absorption A(t) as a difference in transmission with- and without Pump.
Reference beam: bypassing or passing through the sample?
Locked-in reference beam scheme.
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14. Lock-in Technique Investigate the Probe beam fluctuations.
Modulating the Pump beam at a frequency in a ”silent” part of noise frequency spectrum.
Biuld a narrow frequency filter to transmit only the frequency of Pump modulation.
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15. Differential absorption.
Differential absorption A(t) (transmission T(t)):
A(t) = A(t) – A*(t) = -Log(I*out/I*in) + Log(Iout/Iin)
How to convert A(t) into T(t)?
How to measure A(t) with Iout only?
If Iin is stable (Iin= I*in): A(t) = -Log(Iout/I*out)
If Iin is not stable (Iin I*in) a reference Iref is needed: A(t) = -Log(I*out Iref /I*ref Iout)
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16. Locked-in Refence Scheme.
When collecting large
number of shots for
averaging out the noise
each pair of pulses with-
and without- Pump is
treated separately.
The the long-time noise
is then filtered out.
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17. Polarized Light Pump ׀׀ Probe: ΔA׀׀ and Pump  Probe: ΔA
Magic Angle signal (MA) is sensitive to population dynamics only
MA = (ΔA׀׀+ 2ΔA)/3
Why MA signal can be measured at
~54.7 between pump and probe?
And anisotropy signal r(t) is sensitive to dipole orientation only
r(t) = (ΔA׀׀- ΔA)/(ΔA׀׀+ 2ΔA)
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18. Instrumental function and zero-time.
Often a very good time-resolution has to be characterized ”at the spot”.
Instrumental response is generally varied over the wide probe spectrum.
Zero time position is crytical and often difficult to define.
Several options to characterize both:
SHG of Probe and Pump
Two-photon (one from Pump, one from Probe) absorption
Set of reference samples
OKE in samples with little nuclear response
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19. Un-correlated and Correlated Noise.
Un-correlated (independent) noise when ΔA׀׀ and ΔA are measured after each other – noise is of two measurements is larger than for each of them.
ΔA׀׀ and ΔA are measured simultaneously for each laser pulse – may be much smaller (if noise is correlated).
Important: identical temporal- and spatial- overlap with pump!
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20. Signal-to-Noise: detectors
Two types of noise: Probe and Pump.
Light level: how accurate one can count photons?
Integrated- or spectrally- resolved detection?
Dark noise and digitizing – limitations of electronics.
Peak- or Integrating- detector?
Pump noise: normalization and spatial fluctuations.
Pump noise: one time-point – many shots or many time-points – few shots?
Averaging... For how long?
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21. Dependence of the noise level on Probe-pulse energy.
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22. Type of experiment
Measurements per point
Number of repeats
Total measurements
Number of time point used
k1 (1/ps)
Uncertainty 1 ±%
k2 (1/ps)
Uncertainty 2 ±%
k3 (1/ps)
Uncertainty 3 ±%
Scan
300 12
3600
116
0.309
13.3
0.0348
89.8
36.7
500 4
2000
284
0.327
13.2
0.0445
52.0
13.6
Sweep 1
100
5652
0.314
3.6
0.0365
16.1
3.9
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23. How Strong Should Be Pump and Probe Light?
Pump intensity: ”linear” and ”non-linear” signal.
Non-linear absorption: multi-photon absorption and absorption saturation
Sequential (two-steps) absorption
Concentration-dependent dynamics.
Relative Pump/Probe intensity:
Strong Pump – Weak Probe?
Probe intensity: how weak should be Probe?
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24. How weak should be Probe?
Additional ΔA amplitude induced by Probe itself has to be smaller than the noise level needed to resolve Pump-induced changes.
Easily achievable out of absorption region.
In the absorption region: possible Probe self-induced effect in differential absorption.
What should be the relative density of photons
to induce 10-4 differential signal by Pump or by Probe itself?
Decrease Probe intensity by reducing number of photons or by increasing beam diameter.
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25. Transient absorption:
Advantages:
Probe pulse is relatively easy to tune.
Even “dark” excited states can be seen
by S1 → Sn absorption.
Gives total picture of the involved components.
Very good temporal resolution and signal-to-noise.
Disadvantage:
Sometimes too much information – difficult to interpret.
Good to combine with time-resolved fluorescence.
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26. Homodyne detection: transient grating.
In homodyne transient absorption (i.e. transient grating or OKE) only the signal field is recorded.
Idet  |Es(t)|2  A(t)  [R(t)*K(t)]2
R(t) – rotational correlation function,
K(t) – populational decay function
In heterodyne scheme (i.e. Differential absorption) additional light field (Local oscillator) is added.
Idet  |ELO+Es|2 = Is + ILO + nc/4 Re[E*LO(t)Es(t)]
Is is realtively weak, ILO can be removed by chopping  detected signal is linearized against Pump.
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27. Time-resolved fluorescence.
Clear method: emissive excited state dynamics.
Isotropic decay: MA(t) = (Ipar+2Iper)/3
Anisotropy decay: r(t) = (Ipar-Iper)/(Ipar+2Iper)
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28. Time-resolved fluorescence.
Direct, electronic resolution.
Fast photodiode (PMT) + fast oscilloscope.
Time-correlated single photon counting
STREAK camera
Inderect methods.
Fluorescence gaiting (up-conversion, etc.).
Excitation correlation method.
Phase-shift method.
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29. TCSPC
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30. TCSPC Advantages: Disadvantage: low time resolution: 20-30 ps
High sensitivity
Statistical noise
Electronics-limited
Disadvantage: low time resolution: ps
Sensitivity: (much less than) single photon level.
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31. Schematic of STREAK camera
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32. STREAK camera Advantages: Disadvantage:
Direct two-dimensional resolution.
Sensitivity down to single photon.
Very productive.
Disadvantage:
Depends on high stability of laser.
Limited time resolution: 2-10 ps.
Needs careful and frequent calibration.
Expensive.
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33. Up-conversion
Advantage: (very) high time resolution, limited mainly by laser pulse duration.
Disadvantages:
Demanding in alignment.
Limited sensitivity, decreasing with increasing time resolution (crystal thickness).
Required signal calibration.
J. Shah, IEEE J. Quant. Electr., 1988, 24, 276–288.
M. A. Kahlow, W. Jarzeba, T. P. DuBruil and P. F. Barbara, Rev. Sci. Instr., 1988, 59, 1098–1109.
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34. Fluorescence up-conversion set-up.
L. Zhao, J. L. Perez Lustres, V. Farztdinov and N. P. Ernsting
Phys . Chem. Chem. Phys . , v. 7 , 1716 – 1725, 2005
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35. Broad-band up-conversion with amplified short pulses.
L. Zhao, J. L. Perez Lustres, V. Farztdinov and N. P. Ernsting
Phys . Chem. Chem. Phys . , v. 7 , 1716 – 1725, 2005
Broad phase-matching by type II crystal
Tilted gate pulses for sub-100 fs resolution
Optimized scheme: ~ 1 count/channel per pulse
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36. Fluorescence Kerr gating
Advantages:
Complete spectra – no phase matching
Good time resolution: fs
Reasonable sensitivity
Disadvantage:
large background
Better resolution gives less signal
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37. Kerr gating S. Arzhantsev and M. Maroncelli
Applied Spectroscopy, V 59, N 2, , 2005
Kerr gating
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38. ”Strong” Pump – ”strong” Probe
Pump-induced intermediate is selectivelly in time and wavelength transfered into an easily detectable state.
Multiphoton ionization: very sensitive and accurate TOF detection
Pump-Probe induced fluorescence: measured by a sensitive integrating detector (PMT).
Probe-induced RAMAN or CARS scattering.
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39. Time-resolved RAMAN and CARS
Record changes in vibrations to follow dynamics of the process.
Spontaneous scattering in RAMAN is amplified in CARS: stronger and spatially selected signals .
As CARS is strong and is often a molecule-specific time-resolved CARS of excited state can be used as a sensitive probe tool.
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40. New twist of time-resolved spectroscopy.
By a first pulse prepare a particular state;
By the second pulse induce some dynamics in this state;
By a Probe pulse (strong or weak) resolve the dynamics in this new state.
Pump-Dump-Probe;
Pump-Re-Pump-Probe
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41. Electro-induced differential absorption
The signal reflects EF-induced changes in the photoinduced dynamics.
EDA(t) = -Log(IEFout Iref /IEFref Iout)
In this way, one can study a dynamic effect of EF not switching ON or OFF EF but rather by timely ”injection” of system of interest into EF
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42. Scheme of EDA experiment
SI from LASER
Scheme of EDA experiment
pump pulse
EF-generator
probe pulse
to detector
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43. Optimal (Coherent) Control
This is another technique where the effect of Pump is specially treated.
The shape of the Pump pulse is optimized so that it has MAX influense on a particular process of interest.
In such a way, a minor part of regular sample response (for TL pulse) could be expressed and become dominant.
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44. Pump - Shaped Dump – Probe Scheme
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