c

1
címlap
FEMTOCHEMISTRY
Experimental observation and controlof molecular
dynamics
2
idoskála 2
What is femtochemistry ?
appearance of humans
the age of Earth
second
3
idoskála 3
Time window of kinetic measurements
4
idofelbontás
increase in time resolution
1011 times increase within 36 years!!
amplified lasers pulse compression
delay time
time, seconds
picosecond lasers (ring lasers)oscilloscope
, delay
nanosecond lasers (mode locking)oscilloscope,
delay
flash photolysis relaxation optical path
length oscilloscope
flow methodsdistance control
year
5
Zewail
Ahmed Zewail, 1999 Nobel-prize in chemistry
Born 1946 in EgyptB. Sc. at Alexandria
University (Egypt), then
University of Pennsylvania (U.S.A.) Ph. D. 1974
197476 University of California Berkely1976
California Institute of Technology1990
professor, head of the Chemical Physics
DivisionWolf-prize (1993), Nobel-prize
(1999)Professor Doctor honoris causa (ELTE,
2009)
Nobel-prize for experimental studies at
femtosecond timescale in
transition-state spectroscopy
6
történelem
Some history dynamics of chemical reactions
Pfaundler collision theory Maxwell-Boltzmann
distributionin interpreting chemical reactions.
Reaction can occur onlyif reactants have larger
energy than needed to pass threshold.
1867
Marcelin applying Lagrange-Hamilton
(mechanical)formalism and Gibbs-type statistical
thermodynamics N atoms in a 2N dimensional phase
space
1914
Eyring and Polányi transition state
theory(absolute rate theory, transition complex
theory)N atoms trajectory on a stationary
potential energy surface
1935
7
történelem 2
Experimental observation of the transition state
John Polanyi, sharing the chemistry Nobel-prize
1986
8
NaD szárnyak
Experimental observation of the transition state
9
NaD szárnyak 2
Experimental observation of the transition state
Na-D line intensity 1 wings intensity
0.000001
.....0.000002
D-line ?
REASON FNa2 lifetime is approximately 10 13
s detection time 10 7 s, random
formation of transition state molecules
10
lézerfotolízis
Introduction basics of laser photolysis
Potential energy
higher excited state
excited state
ground state
A BC distance
11
pump-probe
Spectroscopy with femtosecond time
resolutionexperimental arrangement
(1 fs 0.3 ?m path length)
Laser technics http//femto.chem.elte.hu/kinetika
/Laser/Laser.htm
12
pump-probe 1

Spectroscopy with femtosecond time
resolutionexperimental equipment
Femtochemistry laboratory Sherbrooke
University, Canada1988
1 m
Laser technics http//femto.chem.elte.hu/kinetika
/Laser/Laser.htm
13
pump-probe 2
Spectroscopy with femtosecond time
resolutionexperimental equipment
Laser technics http//femto.chem.elte.hu/kinetika
/Laser/Laser.htm
14
pump-probe 3
Spectroscopy with femtosecond time
resolutionexperimental equipment
15
pump-probe 4
Spectroscopy with femtosecond time
resolutionexperimental equipment
10 cm
Femtochemistry laboratory, MTA SZFKI, 2002 Hungary
16
Késleltetés 1
Spectroscopy with femtosecond time
resolutiondelay line
17
Késleltetés 2
Spectroscopy with femtosecond time
resolutiondelay line
18
Késleltetés 3
Spectroscopy with femtosecond time
resolutiondelay line
intensity
time
19
Késleltetés 4
Spectroscopy with femtosecond time
resolutiondelay line
intensity
time
20
pump-probe 5
Spectroscopy with femtosecond time
resolutionbackground of the experiment
ultrashort pulse ? coherence and selectivity
1 fs 0.3 ?m path length
potential energy
reaction coordinate
time
21
koherencia
incoherent movement
coherentmovement
22
pump-probe 6
Spectroscopy with femtosecond time
resolutionexperimental results
LIF signal
potential energy
reaction coordinate
delay time, fs
23
konvolúció
Spectroscopy with femtosecond time
resolutionexperimental results
the laser pulse broadens
temporally spectrally
LIF signal
delay time, fs
time
OCR optically coupled region
24
lassított felvétel
Spectroscopy with femtosecond time
resolutionconstruction of slow motion pictures
1 fs 0.3 ?m path length
time
1. an excitation pulse is released towards the
sample
2. the excitation pulse is followed after some
delay by a probe pulse
3. the detector measures the (integrated)
laser-induced fluorescence
4. the next excitation pulse is released only
after 0.1-0.01 seconds
25
lassított felvétel 2
Analogy slow motion video of 100 metres sprint
race femtosecond-like technics of
slow motion
26
I CN
Reaction types, PES surfaces, ultrafast
kineticsdissociation of the ICN molecule
LIF signal
OCR
potential energy
reaction coordinate
delay time, fs
27
klasszikus
Direct experimental measurement of PESclassical
mechanics approachBersohn, R. , Zewail, A. H.
Ber. Bunsenges. Phys. Chem. 92, 373 (1988)
potential
interatomic distance
reaction time
28
kvantum
Direct experimental measurement of PESquantum
mechanical approachWilliams, S. O. , Imre, D.
G. J. Phys. Chem. 92, 6648 (1988)
0
time (fs)
20
wave function
40
60
80
100
140
180
potential of the excited state
0
8
10
4
C I interatomic distance
29
Na I
Reaction types, PES surfaces, ultrafast
kineticsdissociation of the NaI molecule
ionic
LIF signal
covalent NaI
potential energy
covalent
free Na
ionic
delay time, fs
interatomic distance, nm
30
Na I / 2
Reaction types, PES surfaces, ultrafast
kineticsdissociation of the NaI molecule
LIF signal
delay time, fs
31
ciklobután
Reaction types, PES surfaces, ultrafast
kineticsdecomposition of cyclo butene
cyclo butene ? 2 ethenes
?
observed
32
molekulasugár
Reaction types, PES surfaces, ultrafast
kineticsbimolecular reactions
Ahmed Zewail Nobel lecture, December 8, 1999
Molecular beamand laser beam crossed
in vacuum
33
bimolekulás1
Reaction types, PES surfaces, ultrafast
kineticsbimolecular reactions
due to the exciting pulse, the IH molecule
dissociates ? the H-atom is projected onto the
CO2 molecule
the exciting (clocking) pulse initiates a
bimolecular reaction
34
bimolekulás2
Reaction types, PES surfaces, ultrafast
kineticsbimolecular reactions
formation of an H CO2
transition state
products of the reaction OH radical and
CO moleculeget away from each other
the bimolecular reaction happens in a coherent way
35
bimolekulás
Reaction types, PES surfaces, ultrafast
kineticsbimolecular reactions
IH CO2 ? I H CO2
1st step initiation of the reaction
H OCO ? HOC O ? HO CO
2nd step bimolecular reaction
Result fluorescence of the OH radical appears
after about 5 ps only
HOC O
potential energy
Potenciális energia
HO CO
H OCO
HOCO valley
reaction coordinate
reakciókoordináta
36
kontroll
Coherent control of chemical reactions
shaping the wave function of the transition state
aka quantum control
Most (industrially important) reactions have
different pathways (products)
quantum control with specific shaping of the
transition state, it is possible
to enable only the desired reaction
path, i. e. to get only the desired product
Technics applying specifically shaped and
timed pulses (temporal shape,
polarisation, spectral distribution, delay)
the shape of the transition state
evolves differently, i. e. the
reaction path changes, resulting in a different
product
If applied properly, by selecting the desired
reaction pathways, clean, environmentally
friendly, wasteless chemical productionmight
lead to unprecedented perspectives in green
chemistry.
37
kontroll 2
Technical possibilities of coherent control
Problem when selectively exciting one specific
bond, excitation energy is quickly
distributed onto the other bonds as well (IVR
Internal Vibrational Relaxation 1 ps)
Solution interferences between the different
molecular modes should be influenced in a way
that a constructive interference occures in
the molecular mode leading to the desired
reaction path
We have to know interactions between the pulses
and the molecules, as well as between the
different modes of the molecules
Technique internal coherence of the molecules is
achieved by properly applying the coherence
of the external field in the form of the puls(es)
Some possibilities
Frequency Resolved Coherent Control (CC) in case
of two dissociative stateof the molecule, two
pulses of different frequency can excite each of
them. By varying the amplitude and phase between
the two pulses, (the spectral andtemporal
distribution of the pulse sequence), the outcome
can be controlled.
Multiphoton CC in case of two states having only
slightly different energies, twopulses can
excite each of them, but with a different number
of absorbed photons.Changing the ratio of the
higher harmonic components of the pulses, the
outcomeof the reaction can be controlled.
38
Fourier
Another possibility Controlling the chirp of
spectrally broadened pulses
Be f (t) and F (?) mutual Fourier-transformed in
the time and frequency domains
Let us define their widths the following way
N is the second norm
If f is differentiable and
, then
(Heisenberg) uncertainty principle
39
vibrációs fókusz
Another possibility Controlling the chirp of
spectrally broadened pulses vibrational
focusing of the exciting pulse on the anharmonic
PES
example selective excitation of the vibrational
mode of the I2 molecule Krause, J. L. et al.
in Femtosecond Chemistry, editors Manz, J.,
Wöste, L., p. 743-777, VCH, Weinheim (1995)
optimális lokalizáció
40
centrifuga
An interesting control type the optical
centrifuge
Villeneuve, D. M. , et al. Phys. Rev. Letters
85, 542 (2000)
Control of the chirp of two circularly polarized,
spectrally broadened pulsesthe absorbing
molecule feels the resultant rotating field
strength.
41
centrifuga 2
optical centrifuge
Cl2 isotope separation
42
ED, EC, EM
Further achievements
Annu. Rev. Phys. Chem. 2006. 57
UED ultrafast electron diffraction a
photocathode is illuminated by the detecting
laser pulse, electrons leaving the
cathode are used to determine structure
UEC ultrafast electron crystallography
same as UED, but the electron beam is scattered
not by moleculecules but crystals (e.
g. phase transition)
UEM ultrafast electron microscopy
similar to UED, but instead of diffraction,
ultrafast transmission electron microscopy
UXD ultrafast X-ray diffraction
similar to UED, but ultrafast laser pulses
produce X-ray pulses to determine
molecular structure
43
elektron
Electron solvation in polar solvents
water
methanol
44
elektron vízben
Electron solvation in water
E. Keszei, S. Nagy, T. H. Murphrey, P. J. Rossky,
J. Chem. Phys. 99, 2004 (1993)
diabatic quantum dynamical simulations in water
indirect solvation
direct solvation
E. Keszei, T. H. Murphrey, and P. J. Rossky, J.
Phys. Chem., 99, 22 (1995)
45
metanolban
Electron solvation in methanol
C. Pépin, T. Goulet, D. Houde,J.- P. Jay-Gerin,
JPC 98, 7009 (1994)
Keszei et al. JPC 101, 5469 (1997)either
mechanisms can be fitted well