Time-resolved absorption spectroscopy

Time-resolved absorption

spectroscopy

A. Penzkofer

Contents

•

Introduction

•

Methods

1. Introduction

• The behavior of a system (atom, molecule, macromolecule, aggregate,polymer, gas, liquid, solid,…) after excitation (optical, electrical, collissional …) shall be studied by optical

absorption spectroscopy. • Questions:

- which state was excited?

- does it form reaction products? - does it form intermediates?

- is it stable?

- what is its relaxation dynamics? - …

Light matter interaction

De-excitation pathways

From B. Valeur Molecular Fluorescence

Classification

• Photo-physical dynamics (photo-physics)

excitation and relaxation (isomerisation, triplets, ionization) • Photo-chemical dynamics (photo-chemistry)

photo-fragmentation (photolysis), photoproduct formation

photo-catalysis dynamics

catalyzer in photo-excited state enables chemical reaction (recovers in dark to orginal state)

• Photo-biological dynamics

- photosynthesis

- photo-morphogenesis photoreceptors

photoreceptor excitation triggers reguatory/effector chain Photo-induced intermediate formation and dark recover • Photo-medical dynamics

photo-dynamical therapy

Photo-physical dynamics

From P.W.

Atkins, Physical Chemistry

(see also lecture of Prof. Dick)

Photochemical dynamics

From P.W. Atkins, Physical Chemistry (see also lectures of Prof. König) retinal

Photo-biological dynamics

From P.W. Atkins, Physical Chemistry (see also lectures form Prof. Hauska)

PQ = plastochinon Pc = plastocyanin

NADP+ _{= nicotinamide adenine dinucleotide}

phosphate, oxidized form ADP = adenosine diphosphate

Photo-morphogenesis

http://www.scienceclarified.com/Oi-Ph/Phototropism.html (see also Kottke lecture)

Phototropism:

Photo-medical dynamics

• Photo-dynamic therapy

From P.W. Atkins, Physical Chemistry (Expert: Dr. U. Bogner)

Time regime

• fs

non-radiative relaxation of higher electronic states • ps

vibrational relaxation • ns

radiative relaxation of first excited singlet state (fluorescence) • μs

spin-forbidden relaxation of first excited triplet state (phosphorescence)

• ms

fast photocycles (photolyase, phytochrome, rhodopsin) • s

BLUF domain photocycle • min

2. Absorption Spectroscopic Methods

2.1 Linear (Conventional) absorption spectroscopy

2.2 Multiphoton absorption spectroscopy

2.3 Nonlinear absorption spectroscopy (time-resolved absorption spectroscopy)

2.4 Excitation spectroscopy

2.1 Linear absorption spectroscopy

Far infrared FIR (> 10 μm)

rotations, librations, H-bonding Mid infrared MIR ( 2-10 μm)

vibrations

Near infrared NIR ( 780 nm – 2000 nm)

overtone vibrations

Visible VIS (390 nm – 780 nm)

electronic transitions of chromophores

Ultraviolet UV (3 nm – 390 nm)

electronic transitions in atoms and

Characterization:

• Low excitation intensity

• Molecules remain in their thermal

ground-state

• Ground-state absorption is measured

• Excited-state level scheme is detected

• Transition dipole moment strengths are

Absorption spectrum

Example

From

P.Zirak et al., Chem. Phys. 335 (2007) 15.

2.2 Multiphoton absorption spectroscopy

• Two-photon absorption

Material is transparent at wavelength of excitation light Selection rules different from single photon absorption

Level scheme

Experimental setup

2.3 Nonlinear absorption spectroscopy

(time-resolved absorption spectroscopy)

• Excited state is populated sufficiently

• Ground-state population is decreased

measurably

• Fate of excited states is probed by

absorption measurements

distinctions

1) Single frequency, single pulse instantaneous absorption methods

saturable absorption

reverse saturable absorption

2) Single frequency pump- time-variable probe absorption methods

3) Double frequency pump- time-variable probe absorption methods

4) Pump – continuous single-wavelength probe methods with streak camera

with multichannel scaler

5) Pump – multicolor time-variable probe absorption spectroscopy

6) Pump –continuous multicolor probe methods with spectormeter and streak camera

2.3.1 saturable/ reverse saturable absorption

2.3.2 Single frequency pump- time-variable probe absorption methods

Example: Photo-isomerisation study on a diamino-maleonitrile derivative (T. Susdorf et al., Chem. Phys. 333 (2007) 49).

OH C H N NC NH₂ NC P1 P2

Diamino Maleonitrile DAMND

Photoisomerisation scheme

2.3.3 Double frequency pump- time-variable probe absorption methods

Experimental setup:

OPG = optical parametric generator OPA = optical parametric amplifier

KDP = potassium dihydrogen phosphate BBO = beta barium borate

2.3.4 Pump – continuous single-wavelength probe

b) With multichannel scaler

2.3.5 Pump – multicolor time-variable probe absorption spectroscopy

2.3.6 Pump – continuous multicolor probe

a) With streak camera

2.4 Excitation spectroscopy

• Excitation is varied and luminescence at a

fixed wavelength is probed

• Used in the case of very weak

absorbance, but reasonable fluorescence

2.5 Site selection spectroscopy

• Belongs to high resolution spectroscopy

• Laser linewidth is small compared to

inhomogeneous absorption linewidth

• Applied in

photo-physical spectral hole burning

persistent spectral hole burning

transient spectral hole burning

3. Some applications

3.1 Flashlamp photolysis

3.2 Laser flash photolysis

3.2.1 Nanosecond photolysis

3.2.2 Picosecond photolysis

3.2.3 Femtosecond photolysis

3.1 flash photolysis

• Intense burst of light (flash lamp) excites sample and creates radicals

A time-delayed probe flash-lamp is used to record the spectra of these radicals and their change in time

Method developed starting 1950 by Manfred Eigen, R.G. W. Norrish, and G. Porter

They got Nobel Prize 1967

Lit: Norrish and Porter, Nature 164 (1949) 658 M. Eigen, Discuss. Faraday Soc. 17 (1954) 194

3.2 Laser Photolysis

3.2.1 Nanosecond Laser Photolysis

• Q-switched lasers (ns-lasers) are used for

lysis (photo-excitation)

Probing is carried out with ns to

μ

s

flash-lamps.

Longer probe light pulses (or cw white light)

may be used for probing in connection with

time-delayed electrical gating

3.2.2 Picosecond Laser Photolysis

• Mode-locked lasers (ps lasers) are used for lysis (photo-excitation)

time-delayed picosecond white-light continua (super-continua) are used for probing the

spectral changes

(intense ps laser pulses generate ps light

continua in transparent dielectrics by parametric four-photon interaction Lit: A. Penzkofer et al, Phys. Rev.

Example: Triplet-triplet Absorption spectroscopy, H. Gratz and A. Penzkofer, J. Photochem. Photobiol. A: Chem. 127 (1999) 21

3.2.3 Femtosecond laser photolysis

• Mode-locked lasers (fs lasers) are used for lysis (photo-excitation)

time-delayed femtosecond white-light continua (super-continua) are used for probing the

spectral changes

(intense fs laser pulses generate fs light

continua in transparent dielectrics by parametric four-photon interaction Lit A. Penzkofer and M. Wittmann, Opt. Commun. 126(1996) 308)

Frequency tunable fs probe light may be generated in (three-photon) parametric generator- amplifier systems

Shortest fs probe continua are generated in NOPAs (non-colinear optical parametric

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