time resolved s p ectroscopy l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Time-resolved S p ectroscopy PowerPoint Presentation
Download Presentation
Time-resolved S p ectroscopy

Loading in 2 Seconds...

play fullscreen
1 / 45

Time-resolved S p ectroscopy - PowerPoint PPT Presentation


  • 322 Views
  • Uploaded on

Time-resolved S p ectroscopy. A. Yartsev. 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.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Time-resolved S p ectroscopy' - monty


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
time resolved s p ectroscopy

Time-resolved Spectroscopy

A. Yartsev

16/04/2007

important factors for time resolved spectroscopy
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.

16/04/2007

direct temporal resolution
Direct Temporal Resolution
  • Absorption: Flash-photolysis – fast detector and fast oscilloscope.
  • Fluorescence: Time-Correlated Single Photon Counting (TCSPC).
  • STREAK camera.

16/04/2007

pump probe correlated temporal resolution
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

temporal resolution from data analysis
Temporal Resolution From Data Analysis.
  • Sub-instrumental response dynamics
  • Fluorescence phase-shift method
  • Excitation correlation fluorescence
  • Coherent: 3PEPS

16/04/2007

spectral resolution
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?

16/04/2007

short pump pulses
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

problems with short pump pulses
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

short probe pulses
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

problems with short probe pulses
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

what can we get from absorption
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!

16/04/2007

time resolved absorption
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.

16/04/2007

differential absorption weak probe
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.

16/04/2007

lock in technique
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.

16/04/2007

differential absorption
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 Iinis stable (Iin=I*in): A(t) = -Log(Iout/I*out)

If Iinis not stable (IinI*in) a reference Iref is needed: A(t) = -Log(I*out Iref/I*refIout)

16/04/2007

locked in refence scheme
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

polarized light
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)

16/04/2007

instrumental function and zero time
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

un correlated and correlated noise
Un-correlated and Correlated Noise.

Un-correlated (independent) noise when ΔA׀׀ and ΔA aremeasured after each other – noise is of two measurements is larger than for each of them.

ΔA׀׀ and ΔA aremeasured simultaneously for each laser pulse – may be much smaller (if noise is correlated).

Important: identical temporal- and spatial- overlap with pump!

16/04/2007

signal to noise detectors
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?

16/04/2007

slide22

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

0.00975

36.7

Scan

500

4

2000

284

0.327

13.2

0.0445

52.0

0.00977

13.6

Sweep

1

100

100

5652

0.314

3.6

0.0365

16.1

0.00962

3.9

16/04/2007

how strong should be pump and probe light
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?

16/04/2007

how weak should be probe
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.

16/04/2007

t ransient absorption
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.

16/04/2007

homodyne detection transient grating
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.

16/04/2007

time resolved fluorescence
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)

16/04/2007

time resolved fluorescence28
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.

16/04/2007

slide29

TCSPC

16/04/2007

tcspc
TCSPC
  • Advantages:
    • High sensitivity
    • Statistical noise
    • Electronics-limited
  • Disadvantage: low time resolution: 20-30 ps
  • Sensitivity: (much less than) single photon level.

16/04/2007

streak camera
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.

16/04/2007

up conversion
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.

16/04/2007

slide34

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

broad band up conversion with amplified short pulses
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

fluorescence kerr gating
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

slide37

S. Arzhantsev and M. Maroncelli

Applied Spectroscopy, V 59, N 2, 206-220,2005

Kerr gating

16/04/2007

strong pump strong probe
”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.

16/04/2007

time resolved raman and cars
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.

16/04/2007

new twist of time resolved spectroscopy
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

electro induced differential absorption
Electro-induced differential absorption
  • The signal reflects EF-induced changes in the photoinduced dynamics.

EDA(t) = -Log(IEFout Iref/IEFrefIout)

  • 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

scheme of eda experiment

pump pulse

probe pulse

to detector

SI from LASER

Scheme of EDA experiment

EF-generator

16/04/2007

optimal coherent control
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