Time-resolved absorption spectroscopy

Time-resolved absorption spectroscopy A. Penzkofer Graduate College Ring lecture SS 2008, 05.05.2008

Contents Introduction Methods Some applications

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 From B. Valeur Molecular Fluorescence

De-excitation pathways From B. Valeur Molecular Fluorescence After excitation follow development of absorption spectra

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-chirurgical applications

Photo-physical dynamics From B. Valeur Molecular Fluorescence

From P.W. Atkins, Physical Chemistry (see also lecture of Prof. Dick)

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

Photo-biological dynamics PQ = plastochinon Pc = plastocyanin NADP + = nicotinamide adenine dinucleotide phosphate, oxidized form ADP = adenosine diphosphate From P.W. Atkins, Physical Chemistry (see also lectures form Prof. Hauska)

Phototropism: Photo-morphogenesis Light from left no light normal light http://www.scienceclarified.com/oi-ph/phototropism.html (see also Kottke lecture)

Photo-medical dynamics Photo-dynamic therapy From P.W. Atkins, Physical Chemistry (Expert: Dr. U. Bogner)

From Haken und Wolf, Molkülphysik und Quantenchemie, Springer, 2005

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 Phototropin (LOV domain photocyle), Cryptochrome photocycle

2. Absorption Spectroscopic Methods 2.1 Linear (Conventional) absorption spectroscopy 2.2 Multiphoton absorption spectroscopy 2.3 Nonlinear absorption spectroscopy (timeresolved absorption spectroscopy) 2.4 Excitation spectroscopy 2.5 Site selection spectroscopy

2.1 Linear absorption spectroscopy Spectral range: 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 molecules, inner atomic shell transitions

Characterization: Low excitation intensity Molecules remain in their thermal groundstate Ground-state absorption is measured Excited-state level scheme is detected Transition dipole moment strengths are found out

Spectrophotometer setup

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 Experimental setup Level scheme (Expertise in this field: Dick, Penzkofer)

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 with spectrometer and gated CCD camera

2.3.1 saturable/ reverse saturable absorption See ring lecture A. Tyagi, WS 2007/8

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). Experimental setup:

NC NH 2 NC N C H OH Diamino Maleonitrile DAMND P1 P2 Pump-probe transmission result Photoisomerisation scheme

2.3.3 Double frequency pump- time-variable probe absorption methods OPG = optical parametric generator OPA = optical parametric amplifier KDP = potassium dihydrogen phosphate BBO = beta barium borate Experimental setup:

2.3.4 Pump continuous single-wavelength probe a) With streak camera detection

b) With multichannel scaler repetitive

2.3.5 Pump multicolor time-variable probe absorption spectroscopy Experimental setup:

2.3.6 Pump continuous multicolor probe a) With streak camera Used in lab of Prof. Dick

b) With gated CCD

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 (See lecture note Fluorescence lifetime II from SS 2003)

Setup:

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 Expert: U. Bogner

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

Flashlamp photolysis

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 μs flashlamps. Longer probe light pulses (or cw white light) may be used for probing in connection with time-delayed electrical gating (Was used by Dick, Kottke for phototropin photocycle studies)

Setup:

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. Letters 31 (1973) 863 )

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 amplifiers Lit: Riedle et al., Appl. Phys. B71 (2000) 457)

Example:

Photocycle:

References G. R. Fleming, Chemical Applications of Ultrafast Spectroscopy, Oxford University Press, New York, 1986. B. Valeur, Molecular Fluorescence, Wiley-VCH, Weinheim, 2002 P. W. Atkins, J. de Paula, Physikalishe Chemie, Wiley- VCH, Weinheim, 2006 J. R. Lakowicz, Topics in Fluorescence Spectroscopy. Vol.2: Principles, Plenum Press, New York, 1991.

F.C. De Schryver,S. De Feyter, G. Schweitzer (Eds.), Femtochemistry, Wiley-VCH, Weinheim, 2001 H. Rettig, B. Strehmel, S. Schrader, H. Seifert (Eds.), Applied Fluorescence in Chemistry, Biology and Medicine, Springer, Berlin, 1999. C. Rullière (Ed.), Femtosecond Laser Pulses. Principles and Experiments. 2nd Ed. Springer, Berlin, 2005. S. L. Shapiro (Ed.), Ultrashort light pulses. Picosecond techniques and applications, Springer, Berlin, 1977. W. Kaiser (Ed.) Ultrashort Laser Pulses and Applications Springer, Berlin, 1988.