Time-resolved Spectroscopy

A. Yartsev 16/04/2007 1

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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Direct Temporal Resolution

 Absorption: Flash-photolysis – fast detector and fast oscilloscope.

 Fluorescence: Time-Correlated Single Photon Counting (TCSPC).  STREAK camera.

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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 16/04/2007 4

Temporal Resolution From Data Analysis.

 Sub-instrumental response dynamics  Fluorescence phase-shift method  Excitation correlation fluorescence  Coherent: 3PEPS 16/04/2007 5

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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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. 16/04/2007 7

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. 16/04/2007 8

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. 16/04/2007 9

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. 16/04/2007 10

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  [dm 3 mol -1 cm -1 ] Or extinction  from concentration

c

.   Molecular cross-section  [cm -2 ] from

C

[cm -3 ].

transition dipole moment  f rom spectral shape of  .

 And with short pulses we can time-resolve this all!

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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 λ probe : point-by-point.

Single shot – whole spectrum.

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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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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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Differential absorption.

Differential absorption  A(t) (transmission  T(t)):  A(t) = A(t) – A*(t) = -Log(I* out /I* in ) + Log(I out /I in ) How to convert  A(t) into  T(t)?

How to measure  A(t) with I out only?

If I in is stable (I in = I* in ):  A(t) = -Log(I out /I* out ) If I in is not stable (I in  I* in ) a reference I ref  A(t) = -Log(I* out I ref /I* ref I out ) is needed: 16/04/2007 15

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. 16/04/2007 16

Polarized Light

Pump ׀׀ Probe:

ΔA ׀׀

and Pump  Probe:

ΔA

 Magic Angle signal (MA) is sensitive to population dynamics only

MA = ( ΔA ׀׀ + 2 ΔA



)/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 ׀׀ + 2 ΔA



)

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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 16/04/2007 18

Un-correlated and Correlated Noise.

Un-correlated (independent) noise when ΔA ׀׀ ΔA  are measured after each other and – 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!

16/04/2007 19

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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Dependence of the noise level on Probe-pulse energy.

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0.0001

0.0000

-0.0001

-0.0002

-0.0003

-0.0004

400 Sweep 500ps Sweep 22ps Sweep 1ps Scan 284 Scan 116 -0.0005

300 350

Time (ps)

Type of experim ent

Scan Scan Swee p

Measur ements per point

300

Number of repeats

12

Total measur ements

3600

Number of time point used

116

k1 (1/ps)

0.309

Uncer tainty 1 ± % k2 (1/ps)

13.3

0.0348

Uncer tainty 2 ± % k3 (1/ps)

89.8

500 1 4 100 2000 100 284 5652 0.327

0.314

13.2

3.6

0.0445

0.0365

52.0

16.1

0.0097

5 0.0097

7 0.0096

2 16/04/2007

Uncer tainty 3 ± %

36.7

13.6

3.9

22

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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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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Transient absorption:

Advantages:     Probe pulse is relatively easy to tune. Even “dark” excited states can be seen by S 1 → S n 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. I det  |E s (t)| 2   A(t)  [R(t)*K(t)] R(t) – rotational correlation function, 2 K(t) – populational decay function  I s In heterodyne scheme (i.e. Differential absorption) additional light field (Local oscillator) is added. I det  |E LO +E s | 2 = I s + I LO + nc/4  Re[E* LO (t)  E s (t)] is realtively weak, I LO  can be removed by chopping detected signal is linearized against Pump. 16/04/2007 26

Time-resolved fluorescence.

Clear method: emissive excited state dynamics.  Isotropic decay: MA(t) = (

I

par +2 

I

per )/3  Anisotropy decay: r(t) = (

I

par -

I

per )/(

I

par +2 

I

per ) 16/04/2007 27

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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TCSPC

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TCSPC

 Advantages:  High sensitivity  Statistical noise  Electronics-limited  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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STREAK camera

 Advantages:  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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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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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 16/04/2007 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  Broad phase-matching by type II crystal  Tilted gate pulses for sub-100 fs resolution  Optimized scheme: ~ 1 count/channel per pulse 16/04/2007 35

Fluorescence Kerr gating

 Advantages:  Complete spectra – no phase matching  Good time resolution: 200-400 fs  Reasonable sensitivity  Disadvantage:  large background  Better resolution gives less signal 16/04/2007 36

Kerr gating

S. Arzhantsev and M. Maroncelli

Applied Spectroscopy, V 59, N 2, 206-220, 2005 16/04/2007 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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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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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 16/04/2007 40

Electro-induced differential absorption

 The signal reflects EF-induced changes in the photoinduced dynamics.

EDA

(t) = -Log(I EF out I ref /I EF ref I out )  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 16/04/2007 41

Scheme of EDA experiment

pump pulse probe pulse to detector SI from LASER EF-generator

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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. 16/04/2007 43

Pump - Shaped Dump – Probe Scheme

trans

+ N C 2 H 5 I N C 2 H 5

cis

515 nm pump 630 nm

sink region

shaped dump probe 550 nm delay time 16/04/2007 44

CI seem 1 2 delay time 16/04/2007 45