Transcript Coherent Control by Femtosecond Laser Pulses Introduction

Coherent Control
by Femtosecond Laser Pulses
Shoichi Ito
Miyasaka Laboratory
Introduction
• In general, chemical reaction is a reconfiguration of
molecular structure and it could be couple with some
specific molecular vibrations.
• Laser pulse can induce coherent molecular vibrations
when the pulse width is sufficiently shorter than half the
period of the vibration.
Can femtosecond laser pulses
utilized to control chemical reaction?
Coherent Control
Photodissociation of NaI
Photoexcitation of the molecule by
femtosecond laser pulses
The excited state is generated.
[NaI ]*
[Na・・・I ]*
Dissociation to
Na＋I
Oscillation between
[NaI ]* and [Na・・・I ]*
A. H. Zewail JPC, B, 100,12701-12724 (1996).
Free-fragent detection
The product, Na, increases
with the oscillation period.
Activated-complex detection
The oscillation amplitude of the
excited state, [NaI ]*, is reduced as
time goes on.
Photodissociation to Na+I occurs
simultaneously with the NaI oscillation.
Stretching mode induces
photodissociation.
Can chemical reaction be controlled
by controlling the molecular
vibration?
A. H. Zewail JPC, B, 100,12701-12724 (1996).
Abstract
“Population and coherence control by
three-pulse four-wave mixing”
Emily J. Brown et al., J. Chem. Phys., 111, 3779 (1999).
This paper describes the coherent control of the iodine
molecular (I2) vibrations by three-pulse laser excitation
with degenerate four-wave mixing optical setup.
“Control of Chemical Reactions by Feedback-Optimized
Phase-Shaped Femtosecond Laser Pulses”
A. Assion et al., Science, 282, 919 (1998).
This paper reports application of pulse shaping
based on the evolutionary algorithm.
It was applied to control the photodissociation of
organometallic compounds in the gas phase.
Population and coherence control
by three-pulse four-wave mixing
Emily J. Brown et al., J. Chem. Phys., 111, 3779 (1999).
Transient grating
When two pump pulse arrive
simultaneously, interference
pattern is formed.
The third pulse gets diffracted
by the interferometric pattern.
(Bragg diffraction)
Stimulated photon echo
When two pump pulses are
separated in time, interference
cannot occur between the
electric fields and TG signal
will not be generated.
If the polarization is induced
coherently by the first pulse,
and electronic coherence is
preserved during the delay,
interference can take place
between the coherent
polarization and the second
electric field.
Photon echo signal will
be generated.
Laser Spec
•FWHM: 60 fs
•central wavelength: 620 nm
•pulse power: ~20 μJ
• Sample
I2 vapor at 140 ℃
Why coherent vibration can be observed in
femtosecond spectroscopy?
Molecules with different atomic distances can
absorb photons with different frequencies.
Either ground state or excited state vibration can be generated.
e
e’
e
ωe: vibrational frequency
of the excited states
g’
g
ωg: vibrational frequency of
the ground states
g
Sg∝(1+cos(ωeτab))(1+cos(ωgτ))
τab=(2n+1)π/ωe → Sg=0
Se∝(1+cos(ωgτab))(1+cos(ωeτ))
τab=(2n+1)π/ωg → Se=0
By proper selection of τab , a signal dominated by the
excited or the ground state vibration can be obtained.
Results
a) Withτab=460 fs ( 3/2τe),
vibration with a period of 307 fs
was observed.
b) Withτab=614 fs (2τe), vibration
with a period of 160 fs was
observed.
Fourier transform of the signal
Peak at 108 cm-1 is assigned to
the excited state vibration.
Peak at 208 cm-1 is assigned to
the ground state vibration.
Summary
• Selective enhancement of excited or ground state
vibration was achieved by controlling τab.
• Control of chemical reaction was not achieved.
Problems
As long as nonlinear optical method is utilized,
ensemble average over abundant number of molecules
(in the order of Avogadro number) is observed.
Is it really controlling the coherence of a single molecule?
Control of Chemical Reactions by Feedback-Optimized
Phase-Shaped Femtosecond Laser Pulses
A.Assion et al., Science, 282, 919 (1998).
•Optimized femtosecond pulse
generated from the experimental setup
shown in the right figure was utilized.
•Control of product yield was
attempted for two types of sample.
(1) Fe(CO)5
(2) CpFe(CO)2Cl
Laser Spec
•FWHM: 80 fs
•central wavelength: 800 nm
•pulse power: 1 mJ
Pulse shaper
Ultrashort pulse is
decomposed into a
spectrum by a grating.
D. Zeidler et al., PHYSICAL REVIEW A, 64, 023420 (2001).
Intensity at each
frequency is modified by
a liquid crystal filter.
The spectrum is passed
through another grating
and combined into a
pulse with new feature.
The liquid crystal filter is controlled by the evolutionary algorithm.
Results (1)
Photodissociation of Fe(CO)5
was controlled to optimize the
yield of Fe(CO)5+ and Fe+ .
Optimization of Fe(CO)5+ yield.
・Optimized in 30 generations.
・The pulse became short.
(Bandwidth-limited)
Optimization of Fe+ yield.
Fe(CO)5+ / Fe+ is maximized (solid blocks)
Fe(CO)5+ / Fe+ is minimized (open blocks)
・ Optimized in a few generations.
・The pulse became long.
(a few picoseconds)
Ratio of Fe(CO)5+ /Fe+ changed as much as 70 times between the two optimization.
Results (2)
Similary, CpFe（CO）2Cl was
controlled to optimize the yield of
CpFeCOCl+ or FeCl+.
The ratio of CpFeCOCl+ / FeCl+
was 4.9 at maximun and 1.2 at
minimum.
CpFeCOCl+ / FeCl+ is maximized (solid blocks)
CpFeCOCl+ / FeCl+ is minimized (open blocks)
With simple bandwidth-limited short pulse, the ratio was only 2.4.
On the other hand, arbitrary long picosecond pulse also did not achieve
maximum ratio.
It is not a simple effect of pulse duration.
The spectral phase of the laser pulse is important.
Pulses that achieved different product ratio.
A: A pulse for maximum ratio
of CpFeCOCl+ / FeCl+
B: A pulse for minimum ratio
of CpFeCOCl+ / FeCl+
C: A simple bandwidth-limited
pulse
Summary
• Spectral phase of a pulse was modified by a pulse
shaper which was controlled by the evolutionary
algorithm to optimize the product ratio of
photodissociation reactions.
• It was not a simple effect of a laser intensity or a pulse
duration.
Problems
Because the pulse shaper and the evolutionary algorithm
was utilized as a black box, the actual physical process
behind the observation is yet understood.
Abstract
一般に 化学反応は、分子の構造変化
を伴うので、特定の分子振動とカップル
している可能性がある。フェムト秒パル
スレーザーを用いれば、分子のコヒーレ
ントな振動を誘起することができる。分
子振動を制御することにより、化学反応
を制御する試みが盛んに行われている。
本発表では、それに関連した論文を2報
紹介する。
1報は、縮退四光波混合法によるもの
であり、３つのパルスにより振動のコ
ヒーレント制御を行う。これにより基底状
態、励起状態の振動を選択的に増強減
弱できることが示された。
もう1報は、解離反応のコヒーレント制
御である。レーザーパルスの形をパル
スシェイパーにより最適化し、ある特定
の生成物の収率を上げるというものであ
る。最適化には遺伝的アルゴリズムが
用いられ、数十世代にわたる最適化に
より収率は最高になることが示された。
収率はレーザー強度に依存するのでは
なく、その形に依存することが示された。
In general, chemical reactions couple with some
specific molecular vibrations through which molecular
structure is reconstructed into the product. Ultrafast
laser pulsed excitation can induce coherent molecular
vibrations. The control of the chemical reaction via the
coherent molecular vibrations are called “coherent
control of the chemical reaction” and are under vivid
investigations. On this coherent control, I will
introduce two papers at the presentation.
The first paper describes the coherent control of the
iodine molecule (I2) by three-pulse laser excitation
with degenerate four-wave mixing optical setup. It
was demonstrated thus the enhancement and the
supression
increased or decreased.
The second paper reports coherent control of
photodissociation. Laser pulses optimized by pulse
shaper can increase specific product yields. The
evolutionary algorithm was utilized for optimization. It
was shown that maximization of yields was achieved
within several generations of optimization and that the
yields were dependent not on laser intensity but on
Introduction
Chemical reactions closely
correlate molecular vibration
and progress.
It is known that femtosecond
laser pulses stimulate coherent
molecular vibration.
We discuss the influence of
coherent molecular vibration on
chemical reactions.