8 transient absorption

8/10/2019 8 Transient Absorption

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1

Ken Hanson

MWF 9:009:50 am

Office Hours MWF 10:00-11:00

CHM 5175: Part 2.8

Transient Absorption

Source

hn

Sample

DetectorSourcehn


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Excited State Decay

Non-radiative Decay

Absorption

Spectroscopy

Steady-state Emission

Time-resolved Emission

NMR

Mass-spec

x-ray


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Events in Time


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Source

hn

Sample

DetectorSourcehn

Transient Absorption

1) High intensity pulse of light.

2) Monitor absorption spectrum over time.

Transient Absorption Spectroscopy

Excitation

Internal Conversion

Fluorescence

Non-radiative decayIntersystem Crossing

Phosphorescence

S0

S1

S2

E T1

T2


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Steps

1) Excitation (sunlight)

2) Go inside

3) Monitor color change over time

TA of Photochromic Sunglasses(seconds to minutes)


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Spectroscopy Timeline


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The human eye and its brain interface, the

human visual system, can process 10 to 12

separate images per second (10 Hz),

perceiving them individually.

Time

Visual Spectroscopy

Perceived as green and then red.

10 ms or 0.01 s

Time

100 ms or 0.1 s

Perceived as yellow.


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We are missing out!

70 Hz

14 ms per cycle


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Time-resolved Spectroscopy

Eadweard Muybridge

The Horse in Motion (1872)


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Spectroscopy Timeline

150 years = 17 orders of magnitude

17 orders of magnitude (bacteria vs. size of the solar system)


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Time-Resolved Timeline


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Source

hn

Sample

DetectorSourcehn

Transient Absorption (Pump-Probe Experiment)

1) High intensity pulse of light.

2) Monitor absorption spectrum over time.



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hn

C+A-CA C*A


Electron Transfer Dynamics


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C+A-CA C*A


400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

A

bsorbance(a.u.)

Wavelength (nm)

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Absorbance(a.u.)

Wavelength (nm)

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Absorbance(a.u.)

Wavelength (nm)

Electron Transfer Dynamics


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Source

hn

Sample

DetectorSourcehn

Basics of TA Measurement

(1)

(2)

(3) (1)

Events:

1) Absorption Spectra

2) Excitation Flash

3) Absorption spectra(3)

Time

Ground State pump probe probe probe

Excited State


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A for xS0molecules

Difference Spectra

hn

4 excited states/100 molecules

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Absorbance(a.u.)

Wavelength (nm)

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Absorbance(a.u.)

Wavelength (nm)

A for (x - y)S0+ yS1molecules

S0

S1

E


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400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Abso

rbance(a.u.)

Wavelength (nm)

400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Abso

rbance(a.u.)

Wavelength (nm)

Difference Spectra

A(t) - A(0) = DA

- =400 450 500 550 600 650 700 750

-0.04

-0.03

-0.02

-0.01

0.00

0.01

DeltaA

Wavelength (nm)

DA at time tA(t) A(0)

A for

xS0

A for

(x - y)S0+ yS1

- yS0+ yS1

A(0) = absorption without laser pulseA(t) = absorption at time t after laser pulse


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Difference Spectra

S1generated

S0lost

We dont get to measure absorbance!


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Difference Spectra

A(t) - A(0) = DA

P0

Sample

(power in)

P

(power out)

Absorbance:

A = -log T = log P0/P

We measure transmittance!

A(t) = logP(t)

P0(t) A(0) = logP(0)

P0(0)

P0(t) = P0(0)

DA = logP(t)

P(0)

P(0) = power out before pump

P(t) = power out after pump

Probe sourceis the Same

Then:


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TA Measurement

Probehn

Sample

Detector

400 450 500 550 600 650 700 750-0.04

-0.03

-0.02

-0.01

0.00

0.01

DeltaOD

Wavelength (nm)

10 ns

750 ns

1490 ns

2230 ns

2970 ns

3710 ns

4450 ns

5190 ns

5930 ns

Full spectra detection

Pump

hn

Single detection

Probehn

Sample

DetectorPump

hn


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Single Wavelength Full Spectrum Data

Single Wavelength to Full Spectrum


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Events in Time

1 s1 ms1 ms1 ns1 ps1 fs

secondsmillimicronanopicofemto

Excitation

PhosphorescenceFluorescence

InternalConversion

Intersystem Crossing

PhotochemistryIsomerization

Nanosecond TAPicosecond TA

Femtosecond TA

Attosecond TA


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Nanosecond TA (10-9s)

high-intensity photography lamp

1 m quartz tube

Tungsten lamp

Photomultipliertube


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Q-switch laser

Nd:YAG, Ar Ion


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Picosecond TA (10-12s)

Flash Lamp

Mode-locked Laser

Or picosecond

white-light continua


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Femtosecond TA (10-15s)

First developed in the 1980s (A. H. Zawail)

1999 Nobel Prize in Chemistry for his studies of the

transition states of chemical reactions using

femtosecond spectroscopy"


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1) Femtosecond laser pulse

2) Beam splitter (into Pumpand Probe)

3) Probe Travels through Delay Stage

4) Pumphits sample (exciation)

5) Probehits sample

6) Transmitted Probehits detector

(1)Femtosecond TA (10-15s)

(2)Pump

Probe

Delay Stage(3)

(4)

Detector


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Intensity

Pump

Transient

Concentration

Intensity

pumpprobe

td1P(t)

Transmitted

Light at time 1

P(t1)

time

time time

Intensity

td2

Intensity

time time

DA

time

Graph of t vs DA

pump

DA= log P(0)/P(t)


probe Transmitted

Light at time 1

P(t2)

blank

P(0)


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DA= log P(0)/P(t)


P(t)

Decrease Transmitted

light P(t)

time

DA

time

Graph of t vs DA

Intensity

pumpprobe

td1

time

blank

P(0)

P(t) < P(0)

New species after laser pulse.

P(t) > P(0)

Intensity

pumpprobe

td1

time

blank

P(0)

P(t)

Increased

Transmitted

light P(t)

time

DA

time

Graph of t vs DA

Loss of species after laser pulse.


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Single Wavelength Full Spectrum Data

Single Wavelength to Full Spectrum

15


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d ( 18 )


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Attosecond Spectroscopy (10-18s)

However, the resolution offered by femtosecond spectroscopy is insufficient to

track the dynamics of electronic motion in atoms or molecules since they evolve

on an attosecond (1 as = 1018 s) to few-fs time scale and thus remain elusiveso far.

6-fs pulse

300 attosecond pulse

Nature Physics3, 381 - 387 (2007)


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f d


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Light Sources

Gain medium

Mirrors

R= 100% R< 100%

I0 I1

I2

I3 Laser medium

I

R. Trebino

Nano-femtosecond TA

Light Amplification by Stimulated Emission of Radiation

Mode-Locking Lasers

Pi f d TA


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http://www.youtube.com/watch?v=efxFduO2Yl8

Pico-femtosecond TA

A d TA
http://www.youtube.com/watch?v=efxFduO2Yl8http://www.youtube.com/watch?v=efxFduO2Yl8


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Attosecond TA

ad, An intense femtosecond near-infrared or visible (henceforth: optical) pulse (shown in yellow)

extracts an electron wavepacket from an atom or molecule. For ionization in such a strong field (a),

Newton's equations of motion give a relatively good description of the response of the electron.Initially, the electron is pulled away from the atom (a, b), but after the field reverses, the electron is

driven back (c) where it can 'recollide' during a small fraction of the laser oscillation cycle (d). The

parent ion sees an attosecond electron pulse. This electron can be used directly, or its kinetic energy,

amplitude and phase can be converted to an optical pulse on recollision.

Att d TA


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Attosecond TA

Electronic excitation and relaxation

processes in atoms, molecules andsolids, and possible ways of tracing

these dynamics in real time.

Att d S t


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Attosecond Spectroscopy

However, the resolution offered by femtosecond spectroscopy is insufficient to

track the dynamics of electronic motion in atoms or molecules since they evolve

on an attosecond (1 as = 1018 s) to few-fs time scale and thus remain elusiveso far.

6-fs pulse

300 attosecond pulse

Nature Physics3, 381 - 387 (2007)

T i t Ab ti E d


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Transient Absorption End

Any Questions?

8 transient absorption

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