Time-resolved
Spectroscopy
A. Yartsev
16/04/2007
1
Important Factors for Timeresolved Spectroscopy.
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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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2
Direct Temporal Resolution
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Absorption: Flash-photolysis – fast detector
and fast oscilloscope.
Fluorescence: Time-Correlated Single
Photon Counting (TCSPC).
STREAK camera.
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3
Pump-Probe Correlated
Temporal Resolution.
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”Strong” Pump – ”weak” Probe
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”Strong” Pump – ”strong” Probe
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Multiphoton ionization
Integrated fluorescence
RAMAN etc.
”Strong” Pump – ”strong” Gate
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Transient absorption
Transient gaiting
Fluorescence up-conversion
Optical Kerr Effect
Coherent methods
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4
Temporal Resolution From
Data Analysis.
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Sub-instrumental response dynamics
Fluorescence phase-shift method
Excitation correlation fluorescence
Coherent: 3PEPS
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5
Spectral Resolution.
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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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6
Short Pump Pulses.
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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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7
Problems with Short Pump Pulses.
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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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Short Probe Pulses.
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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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Problems with Short Probe Pulses.

Broad spectrum – lack of spectral selectivity.
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Non-linearity may the probe-induced dynamics.
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Artefacts: - spectrally non-even detection
efficiency may lead to XFM.
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10
What can we get from absorption?
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Absorption spectrum as a fingerprint of a molecule.
From absorption, path length and Beer-Lambert law:
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Concentration c[mol*dm-3] from extinction [dm3mol-1cm-1]
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Or extinction  from concentration c.
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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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11
Time-resolved Absorption
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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.

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Multiple colour
Single shot – single λprobe: point-by-point.
 Single shot – whole spectrum.

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Differential absorption: ”weak” Probe.
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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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Lock-in Technique
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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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14
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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Locked-in Refence Scheme.
When collecting large
number of shots for
averaging out the noise
each pair of pulses withand without- Pump is
treated separately.
The the long-time noise
is then filtered out.
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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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17
Instrumental function and zero-time.
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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:
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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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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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Signal-to-Noise: detectors
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Two types of noise: Probe and Pump.
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Light level: how accurate one can count photons?
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Integrated- or spectrally- resolved detection?
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Dark noise and digitizing – limitations of electronics.
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Peak- or Integrating- detector?
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Pump noise: normalization and spatial fluctuations.
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Pump noise: one time-point – many shots or many
time-points – few shots?
Averaging... For how long?
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Dependence of the noise level
on Probe-pulse energy.
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0.0001
0.0000
²Abs
-0.0001
-0.0002
Sweep 500ps
Sweep 22ps
Sweep 1ps
Scan 284
Scan 116
-0.0003
-0.0004
-0.0005
300
350
400
Time (ps)
Type of
experim
ent
Measur
ements
per
point
Number
of
repeats
Total
measur
ements
Number
of time
point
used
k1
(1/ps)
Uncer
tainty
1 ±%
k2
(1/ps)
Uncer
tainty
2 ±%
Scan
300
12
3600
116
0.309
13.3
0.0348
89.8
Scan
500
4
2000
284
0.327
13.2
0.0445
52.0
Swee
p
1
100
100
5652
0.314
3.6
0.0365
16.1
16/04/2007
k3
(1/ps)
0.0097
5
0.0097
7
0.0096
2
Uncer
tainty
3 ±%
36.7
13.6
3.9
22
How Strong Should Be Pump and
Probe Light?
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Pump intensity: ”linear” and ”non-linear” signal.
Non-linear absorption: multi-photon absorption
and absorption saturation
 Sequential (two-steps) absorption
 Concentration-dependent dynamics.
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Relative Pump/Probe intensity:
Strong Pump – Weak Probe?
Probe intensity: how weak should be Probe?
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23
How weak should be Probe?
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Additional ΔA amplitude induced by Probe itself
has to be smaller than the noise level needed to
resolve Pump-induced changes.
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Easily achievable out of absorption region.
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In the absorption region: possible Probe selfinduced effect in differential absorption.
What should be the relative density of photons
to induce 10-4 differential signal by Pump or by Probe itself?
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Decrease Probe intensity by reducing number of
photons or by increasing beam diameter.
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24
Transient absorption:
Advantages:
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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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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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Time-resolved fluorescence.
Clear method: emissive excited state dynamics.
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Isotropic decay:
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Anisotropy decay:
MA(t) = (Ipar+2Iper)/3
r(t) = (Ipar-Iper)/(Ipar+2Iper)
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Time-resolved fluorescence.
Direct, electronic resolution.
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Fast photodiode (PMT) + fast oscilloscope.
Time-correlated single photon counting
STREAK camera
Inderect methods.
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Fluorescence gaiting (up-conversion, etc.).
Excitation correlation method.
Phase-shift method.
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TCSPC
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TCSPC
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Advantages:
High sensitivity
 Statistical noise
 Electronics-limited
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Disadvantage: low time resolution: 20-30 ps
Sensitivity: (much less than) single photon level.
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Schematic of STREAK camera
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31
STREAK camera
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Advantages:
Direct two-dimensional resolution.
 Sensitivity down to single photon.
 Very productive.
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Disadvantage:
Depends on high stability of laser.
 Limited time resolution: 2-10 ps.
 Needs careful and frequent calibration.
 Expensive.

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Up-conversion
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Advantage: (very) high time resolution, limited
mainly by laser pulse duration.
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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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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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34
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
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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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35
Fluorescence Kerr gating
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Advantages:
Complete spectra – no phase matching
 Good time resolution: 200-400 fs
 Reasonable sensitivity
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Disadvantage:
large background
 Better resolution gives less signal

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Kerr gating
16/04/2007
S. Arzhantsev and M. Maroncelli
Applied Spectroscopy, V 59, N 2, 206-220, 2005
37
”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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38
Time-resolved RAMAN and
CARS
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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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New twist of time-resolved
spectroscopy.
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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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40
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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Scheme of EDA
experiment
SI from
LASER
pump pulse
EF-generator
probe
pulse
to detector
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Optimal (Coherent) Control
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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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Pump - Shaped Dump – Probe Scheme
I-
trans
cis
+
N
N
potential energy
C 2H 5
C2H5
550 nm
515 nm
630 nm
sink region
pump
shaped dump
probe
delay time
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CI seem
1
intensity
2
delay time
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