Shoichi Ito Miyasaka Laboratory

1. Coherent Control by Femtosecond Laser Pulses Shoichi Ito Miyasaka Laboratory

2. 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

3. Photodissociation ofNaI Photoexcitation of the molecule byfemtosecond laser pulses The excited state is generated. [NaI ]* [Na・・・I ]* Dissociation toNa＋I Oscillation between[NaI ]*and[Na・・・I ]* A. H. Zewail JPC, B, 100,12701-12724(1996).

4. 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 toNa+Ioccurs simultaneously with theNaI oscillation. Stretching mode induces photodissociation. A. H. Zewail JPC, B, 100,12701-12724(1996). Can chemical reaction be controlled by controlling the molecular vibration?

5. 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.

6. Emily J. Brown et al., J. Chem. Phys., 111, 3779 (1999). Population and coherence control by three-pulse four-wave mixing Transient grating When two pump pulse arrive simultaneously, interference pattern is formed. The third pulse gets diffracted by the interferometric pattern. (Bragg diffraction)

7. 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. • Laser Spec • FWHM: 60 fs • central wavelength: 620 nm • pulse power: ~20 μJ • Sample • I2 vapor at 140 ℃ Photon echo signal will be generated.

8. Why coherent vibration can be observed in femtosecond spectroscopy? Molecules with different atomic distances can absorb photons with different frequencies.

9. Either ground state or excited state vibration can be generated. e e’ e ωg: vibrational frequency of the ground states ωe: vibrational frequency of the excited states g’ g 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.

10. Results • 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.

11. 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?

12. 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

13. 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.

14. 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. ・ Optimized in a few generations. ・The pulse became long. (a few picoseconds) Fe(CO)5+ / Fe+ is maximized (solid blocks) Fe(CO)5+ / Fe+ is minimized (open blocks) Ratio of Fe(CO)5+ /Fe+ changed as much as 70 times between the two optimization.

15. 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.

16. 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

17. 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.

18. Abstract 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 the pulse envelope. 　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。

19. 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.