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FEMTOCHEMISTRY
Experimental observation and control
of molecular dynamics
időskála 2
CPU clock cycle time
atomic nucleus - neutrino interaction
nuclear motion in atomic nuclei
yocto-
zepto-
femto-
pico-
second
10 -3 10 -6 10 -9 10 -12 10 -15 10 -18 10 -21 10 -24
micro-
1
milli-
103
electronand
energytransfer
molecular
vibration
molecular
rotation
nano-
one minut
106
kilo-
length of a day
1012 109
giga-
1015
mega-
lifetime
of the
triplet
excited state
tera-
human lifespan
lifetime
of the
singulet
excited state
atto-
appearance of humans
vibrational
relaxation
solvation
peta-
the age of Earth
Time window of
elementary reactions
molecule - photon interaction
What is „femtochemistry” ?
időskála 3
molecular
vibration
molecular
rotation
lasermode amplified laser
mixing;
flow; flash
+ pulse compression
photolysis
locking
stopwatch
distance photolysis
delay time
oscilloscope
delay time
control optical path
yocto-
zepto-
femto-
pico-
nano-
10 -3 10 -6 10 -9 10 -12 10 -15 10 -18 10 -21 10 -24
mikro-
1
electronand
energytransfer
nuclear motion in atomic nuclei
lifetime
of the
triplet
excited state
milli-
103
kilo-
one minut
106
mega-
length of a day
1012 109
tera-
human lifespan
1015
giga-
lifetime
of the
singulet
excited state
atto-
appearance of humans
solvation
peta-
the age of Earth
vibrational
relaxation
molecule - photon interaction
1949 -1967
1967 -1972
1972 -1985
1985 18501900
-1900-1949
atomic nucleus - neutrino interaction
Time window of kinetic measurements
időfelbontás
increase in time resolution
idõ,
másodperc
seconds
time,
1011 times increase within 36 years!!
10
-15
10
-12
10
-9
10
-6
10
-3
amplified lasers
+ pulse compression
delay time
picosecond lasers
(ring lasers)
oscilloscope, delay
nanosecond lasers
(mode locking)
oscilloscope, delay
flash photolysis + relaxation
optical path length + oscilloscope
flow methods
distance control
1950
1960
1970
1980
year
év
Zewail
Ahmed Zewail, 1999 Nobel-prize in chemistry
Born 1946 in Egypt
B. Sc. at Alexandria University (Egypt), then
University of Pennsylvania (U.S.A.) Ph. D. 1974
1974–76 University of California Berkely
1976– California Institute of Technology
1990– professor, head of the Chemical Physics Division
Wolf-prize (1993), Nobel-prize (1999)
Professor & Doctor honoris causa (ELTE, 2009)
Nobel-prize for experimental studies at femtosecond
timescale in transition-state spectroscopy
történelem
Some history: dynamics of chemical reactions
1867
Pfaundler: collision theory + Maxwell-Boltzmann distribution
in interpreting chemical reactions. Reaction can occur only
if reactants have larger energy than needed to pass threshold.
1914
Marcelin: applying Lagrange-Hamilton (mechanical)
formalism and Gibbs-type statistical thermodynamics
N atoms in a 2N dimensional phase space
1935
Eyring and Polányi: transition state theory
(absolute rate theory, transition complex theory)
N atoms’ trajectory on a stationary potential energy surface
történelem 2
Experimental observation of the transition state
F + Na2
1986
[F····Na····Na ]‡
NaF + Na*
John Polanyi, sharing the chemistry Nobel-prize
NaD szárnyak
Experimental observation of the transition state
F + Na2
[F····Na····Na ]‡
NaF + Na*
NaD szárnyak 2
Experimental observation of the transition state
F + Na2
wing
[F····Na····Na ]‡
NaF + Na*
Na-D line intensity: 1
„wings” intensity: 0.000001
.....0.000002
D-line 
wing
REASON: FNa2‡ lifetime is approximately 10 – 13 s
detection time: 10 –7 s, random
formation of transition state molecules
lézerfotolízis
Introduction: basics of laser photolysis
Potential energy
A– B – C
A + BC
higher excited state
excited state
ground state
A – BC distance
pump-probe
Spectroscopy with femtosecond time resolution:
experimental arrangement
reference
detector
Nd:YAG
laser
probe
sample
Ar- ion
laser
excitation
H2O
amplifier
delay (1 fs = 0.3
CPM
laser
m path length)
Laser technics: http://femto.chem.elte.hu/kinetika/Laser/Laser.htm
pump-probe
1
Spectroscopy
with femtosecond time resolution:
experimental equipment
Femtochemistry
laboratory
Sherbrooke
University,
Canada
1988
1 m
Laser technics: http://femto.chem.elte.hu/kinetika/Laser/Laser.htm
pump-probe 2
Spectroscopy with femtosecond time resolution:
experimental equipment
prism
Ar-ion laser
prism
Ti-sapphire crystal
slit
birefrigent
filter
Laser technics: http://femto.chem.elte.hu/kinetika/Laser/Laser.htm
pump-probe 3
Spectroscopy with femtosecond time resolution:
experimental equipment
Faraday isolator
delay line
BBO
optical
fibre
Ti-sapphire laser
chopper
dicroic
mirror
monochromator
sample
parabolic
mirror
pump-probe 4
Spectroscopy with femtosecond time resolution:
experimental equipment
10 cm
Femtochemistry laboratory, MTA SZFKI, 2002 Hungary
Késleltetés 1
Spectroscopy with femtosecond time resolution:
delay line
probe
intensity
excitation
 delay time
time
Késleltetés 2
Spectroscopy with femtosecond time resolution:
delay line
probe
intensity
excitation
 delay time
time
Késleltetés 3
Spectroscopy with femtosecond time resolution:
delay line
probe
intensity
excitation
 delay time
time
Késleltetés 4
Spectroscopy with femtosecond time resolution:
delay line
probe
intensity
excitation
 delay time
time
pump-probe 5
Spectroscopy with femtosecond time resolution:
background of the experiment
potential energy
ultrashort pulse  coherence and selectivity
1 fs = 0.3 m path length
reaction coordinate
time
koherencia
incoherent
movement
coherent
movement
pump-probe
6
LIF signal
potential energy
Spectroscopy with femtosecond time resolution:
experimental results
reaction coordinate
delay time, fs
konvolúció
Spectroscopy with femtosecond time resolution:
experimental results
LIF signal
the laser pulse
broadens
– temporally
– spectrally
delay time, fs
time
OCR = optically coupled region
lassított felvétel
Spectroscopy with femtosecond time resolution:
construction of slow motion pictures
1 fs = 0.3 m path length
reference
detector
Nd:YAG
laser
probe
sample
Ar- ion
laser
excitation
amplifier
CPM
laser
delay line
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
lassított felvétel 2
Analogy: slow motion video of 100 metres sprint race
„femtosecond-like” technics of slow motion
1. the race starts following the starter pistol’s signal
2. following the start, runners arrive to the fixed position of the camera
3. the camera is registering one single picture
4. the next race will start only after 300 thousand years
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
I ··· CN
Reaction types, PES surfaces, ultrafast kinetics:
dissociation of the ICN molecule
[I····CN ]‡
I + CN
LIF signal
potential energy
ICN
OCR
reaction coordinate
delay time, fs
Direct experimental measurement of PES
classical mechanics approach
klasszikus
Bersohn, R. , Zewail, A. H.: Ber. Bunsenges. Phys. Chem. 92, 373 (1988)
potential
interatomic distance
reaction time
Direct experimental measurement of PES
kvantum
quantum mechanical approach
Williams, 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
Na ··· I
Reaction types, PES surfaces, ultrafast kinetics:
dissociation of the NaI molecule
[Na····I ]‡
Na + I
potential energy
ionic
covalent NaI
LIF signal
Na+I –
covalent
„avoided crossing”
(degeneráció)
free Na
ionic
delay time, fs
interatomic distance, nm
covalent
ionic
Na ··· I / 2
LIF signal
Reaction types, PES surfaces, ultrafast kinetics:
dissociation of the NaI molecule
delay time, fs
ciklobután
Reaction types, PES surfaces, ultrafast kinetics:
decomposition of cyclo butene
cyclo butene  2 ethenes
observed
Reaction types, PES surfaces, ultrafast kinetics:
bimolecular reactions
molekulasugár
Ahmed Zewail: Nobel lecture, December 8, 1999
Molecular beam
and laser beam
crossed
in vacuum
Reaction types, PES surfaces, ultrafast kinetics:
bimolecular reactions
bimolekulás1
IH · CO2 van der Waals complex
flying in the molecular beam
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
Reaction types, PES surfaces, ultrafast kinetics:
bimolecular reactions
bimolekulás2
formation of an H · · · CO2
transition state
products of the reaction:
OH radical and
CO molecule
get away from each other
the bimolecular reaction happens in a coherent way
bimolekulás
Reaction types, PES surfaces, ultrafast kinetics:
bimolecular reactions
1st step: initiation of the reaction:
2nd step : bimolecular reaction:
IH · CO2  I + H · CO2
H + OCO  [H···O···C–O ]‡  HO + CO
potential energy
energia
Potenciális
Result: fluorescence of the OH radical appears after about 5 ps only
[H···O···C – O ]‡
HO + CO
H + OCO
HOCO valley
reaction coordinate
reakciókoordináta
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 production
might lead to unprecedented perspectives in green chemistry.
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 state
of the molecule, two pulses of different frequency can excite each of them.
By varying the amplitude and phase between the two pulses, (the spectral and
temporal distribution of the pulse sequence), the outcome can be controlled.
Multiphoton CC: in case of two states having only slightly different energies, two
pulses 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 outcome
of the reaction can be controlled.
Fourier
Another possibility: Controlling the chirp of spectrally broadened pulses
Be f (t) and F () mutual Fourier-transformed in the time and frequency domains:
F ( ) 



f (t ) e  2 i t d t
f (t ) 

 2 i t
F
(

)
e
d


Let us define their “widths” the following way:
t   1
N
2

2
t
 f (t ) d t
2
 2

1

N

2

 F ( ) d
2


N is the second norm:
N
 f t 

2
dt 

If f is differentiable and
 F  
2
d

lim t f 2 t   0 , then
t 
1
(Heisenberg) uncertainty principle:  t    
2
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
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 pulses
the absorbing molecule feels the resultant rotating field strength.
centrifuga 2
optical centrifuge
Cl2 isotope separation
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
elektron
Electron solvation in polar solvents
T1
T2


e free

e ir 
e sol
20
15
10
5
0
-0.5
400
600
800
1.0
1000
Kés
1.5
sz,
s
1200
o
2.0
lelte
h
2.5 1400
té
llám
0.0
0.5
s, p
s
Hu
nm
Normalizált abszorbancia, M-1 cm-1
25
methanol
Normalizált abszorbancia, M-1cm-1
water
10
8
6
4
2
0
10
Kés 20 30
lelte
tés 40 50
, ps
0
600
800
, nm
1000
z
s
1200
hos
m
á
l
Hu l
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)
Tth
Tth
Tth
T1
T2


e1  
 ......  
 en  
 efree

 e   
 esolv
*
metanolban
e 1
Electron solvation in methanol
Tth
Tth
Tth
T1
T2





 ......  
 en  
 e free  
 e  
 e solv
*
Keszei et al. JCP 99, 2004 (1993)
ewb
cont. shift
C. Pépin, T. Goulet, D. Houde,
J.- P. Jay-Gerin, JPC 98, 7009 (1994)
Tcont.
_
eqf
Tstep
_ cont. shift
esb
Tcont.
_
Keszei et al. JPC 101, 5469 (1997):
either mechanisms can be fitted well
es
Normalizált abszorbancia, M-1 cm-1
10
8
6
4
2
0
10
Kés 20 30
lelte
tés 40 50
, ps
0
600
800
nm
1000
sz,
s
o
1200
h
lám
Hu l
normalizált abszorbancia
_
MeOH, termalizációs elágazó mehanizmus
7 kiválasztott hullámhosszon
10
700 nm
8
6
620 nm
800 nm
500 nm
4
900 nm
2
1100 nm
0
1350 nm
0
20
40
60
80 100 120 140 160 180 200
késleltetés, ps