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Coherent control over strong-field dissociation of heteronuclear diatomic molecules

Brandon Rigsbee

Published Apr 24, 2015• 0 citations
OpenAlex

Abstract

In the last 20 years, advancements in laser technology have allowed for the production of intense laser pulses with durations in the femtosecond (10⁻¹⁵ second) regime, giving scientists the ability to probe nuclear dynamics on their natural time scale. Study of the dissociated fragments created by these intense fields can be used to learn about the molecular structure and dynamics. The work presented in this thesis focuses on controlling this light–molecule interaction in such a way that we can preferentially dissociate the molecule to a desired final product. The hydrogen molecular ion, HD⁺, as well as LiF serve as simple systems that can be studied theoretically for a broad range of laser parameters. Our goal in using these relatively simple systems is to capture the essential physics of the light–molecule interaction and develop general methods to describe these interactions in more complex systems.

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Whenever scientific tools are significantly advanced; new discoveries soon follow. My thesis work relies on advances in ultrafast laser technology that allow precise control over the shape of sub 20-fs laser pulses. This laser system enabled many new experiments that yielded significant advances in attosecond science, atomic physics; extreme nonlinear optics, quantum chemistry, coherent control, molecular physics, ultrafast optical phase modulation, EUV coherent imagines, and EUV radiation sources. By using an automated learning control algorithm coupled to our deformable mirror pulse shaper; we have demonstrated the control of a process on an attosecond time-scale for the first time. We demonstrate the manipulation of the phase of an electron wave packet, allowing us to tailor the EUV spectrum generated by high-order harmonic generation. These optimal fields were discovered by a learning algorithm that both found an unknown optimal solution; but also discovered an unknown phase-matching mechanism that occurs in the interaction of a single atom and light pulse. We control molecular vibrational and rotational coherences with the same learning algorithm; demonstrating selective control in room temperature molecules at atmospheric pressures. Moreover, we demonstrate that modifications to the algorithm can help automatically uncover the physics of the control mechanism found by the learning algorithm. We also demonstrate a new pulse-compression technique using rotational phase modulation, where optical pulses were compressed by an order of magnitude after phase modulation by a molecular rotational coherence and subsequent propagation through a transparent window. This technique will enable efficient pulse compression in the deep-UV. Finally, we measure the coherence of EUV light generated by high-harmonic upconversion of a femtosecond laser. In phase-matched hollow-fiber geometry, the EUV light exhibits the highest inherent spatial coherence of any source in this region of the spectrum. We use this source to demonstrate the coherent image with first table-top EUV radiation. Since this source exhibits full spatial coherence at very short wavelength, this light source represents has smallest inherent effective source-size of any light source yet created. We also demonstrate that the spectrum of an optical field can be determined by measuring the interference of a double-slit pattern.

•Jan 1, 2002•5 citations

Identification in the Frequency Domain of Molecular Dissociation Fragments Detected by a Wavepacket

Y. Xiao, Brian J. Ricconi, Ju Gao +1 more

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Strong-field control and attosecond probing of multielectron dynamics

Zengxiu Zhao

The past thirty years witnessed rapid developing of strong field physics that leads to the first generation of attosecond (10−18 s) pulses. The duration of available pulses has recently been reduced to less than 100 as, approaching the Kepler period of a classical electron revolving around the nucleus. For the first time, electron motions inside atoms, molecules and solids can be traced, probed and even steered in real time. It pushed the study on ultrafast dynamic processes into a new frontier, giving birth to attosecond physics. The development of attosecond pulses provides unprecedented coherent ultrashort XUV (extreme ultra-violet) of soft X-ray sources that enable imaging the atomic structure and resolving the related quantum dynamics for matter science. In particular, it allows the probing and monitoring of electron dynamics in the natural time scale of attoseconds with atomic spatial resolution. The generation and applications of attosecond pulses are directly related to the control and manipulation of the sub-cycle electron dynamics. Recent studies on the correlated multielectron dynamics have provided insight into the optical and dynamical properties of matter in the perspectives of time, phase and entanglement. It deepens our understanding on some of the fundamental questions, e.g., how the single or multiple photons are absorbed and how short are the quantum processes such as photoionization, tunneling ionization and charge migration. Although currently attosecond pulses are limited by the flux and pulse duration, the combination of attosecond pulses with intense infrared laser pulses paves new ways of controlling and probing electron dynamics in the domains of energy, time and space that lays the foundation for petahertz optoelectronics and attosecond pulse based transient spectroscopy. In this review, we will first briefly summarize the advancement of strong field and ultrafast physics including the relevant experimental findings, the theoretical explorations, the related technologies and the possible applications. Then we focus on the sub-cycle electron or multielectron dynamics and their manifestation on the ionization and coherent radiation properties. We start with the time behavior of Fano resonance and how it can be probed in time domain. In section 2, the correlated electron dynamics is discussed in term of anti-screening effect and Pauli effect that influence strong field ionization and high harmonic generation processes. In section 3, the coherent emission with frequency from terahertz to soft XUV and their joint measurement are discussed by emphasizing the generation mechanism of terahertz wave in two-color laser pulses. The developed high-harmonic and terahertz wave spectroscopy (HATS) is shown capable of imaging the molecular structure and dynamics. In section 4, the time-resolving of electron and hole dynamics and their coherence is proposed based on attosecond transient absorption technique. The coherence of ionization and the correlated electron dynamics is hinted crucial for electron-hole interaction. The topics are very limited and mainly selected from the perspectives of our group.

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Recent advances in the ultrashort laser technology have spawned a series of new directions of research in atomic and molecular physics. This work compounds two studies on the control and probing of atomic matter by ultrafast laser pulses. The first study is concerned with the control of quantum wavepackets focusing by chirped, ultrashort laser pulses. The ability to control wavepacket focusing is important for the achievement of specific excitation pathways, for controlling chemical reactivity or even molecular synthesis. In this work, we present a general method for wavepacket control in the weak field limit. Numerical simulations involving the ICN molecule confirm our theoretical framework. The second study refers to the ionization of sodium Rydberg states via half-cycle pulses (HCPs). For this process, our full quantum-mechanical calculations demonstrate an asymmetry in both ionization rates and ionization spectrum between the downhill (Stark red-shifted) and uphill (Stark blue- shifted) states. The asymmetry is explained by appealing to a semi-classical model of the interfering ionization pathways.

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Laser Induced Dissociation Control of $H_2^+$ and $H_3^{2+}$

Z.-M. Jia, Zhinan Zeng

Laser induced dissociation control of the symmetric diatomic molecular ion $H_2^+$ and the triatomic molecular ion $H_3^{2+}$ is discussed. The simulation results demonstrate that a long-wavelength terahertz or mid-infrared laser pulse can be used to control the electrons of the dissociative states after the excitation of an ultrashort ultraviolet (UV) laser pulse. For $H_2^+,$ there exists an effective time, which increases with increasing pulse duration of the UV laser pulse, for controlling the molecular dissociation. For the electrons of the $1sσ_g$ state, they move along the polarization direction of the dissociation control electric field. In contrast, for the electrons of the $2pσ_u$ state, they move in the opposite direction to that of the electric force. And for the triatomic molecule $H_3^{2+}$, the electron dissociation control can also be realized by changing the central wavelength of the exciting UV pulse.

The Journal of Atomic and Molecular Sciences•Aug 1, 2017•0 citations
In the last 20 years, advancements in laser technology have allowed for the production of intense laser pulses with durations in the femtosecond (10⁻¹⁵ second) regime, giving scientists the ability to probe nuclear dynamics on their natural time scale. Study of the dissociated fragments created by these intense fields can be used to learn about the molecular structure and dynamics. The work presented in this thesis focuses on controlling this light–molecule interaction in such a way that we can preferentially dissociate the molecule to a desired final product. The hydrogen molecular ion, HD⁺, as well as LiF serve as simple systems that can be studied theoretically for a broad range of laser parameters. Our goal in using these relatively simple systems is to capture the essential physics of the light–molecule interaction and develop general methods to describe these interactions in more complex systems.
Whenever scientific tools are significantly advanced; new discoveries soon follow. My thesis work relies on advances in ultrafast laser technology that allow precise control over the shape of sub 20-fs laser pulses. This laser system enabled many new experiments that yielded significant advances in attosecond science, atomic physics; extreme nonlinear optics, quantum chemistry, coherent control, molecular physics, ultrafast optical phase modulation, EUV coherent imagines, and EUV radiation sources. By using an automated learning control algorithm coupled to our deformable mirror pulse shaper; we have demonstrated the control of a process on an attosecond time-scale for the first time. We demonstrate the manipulation of the phase of an electron wave packet, allowing us to tailor the EUV spectrum generated by high-order harmonic generation. These optimal fields were discovered by a learning algorithm that both found an unknown optimal solution; but also discovered an unknown phase-matching mechanism that occurs in the interaction of a single atom and light pulse. We control molecular vibrational and rotational coherences with the same learning algorithm; demonstrating selective control in room temperature molecules at atmospheric pressures. Moreover, we demonstrate that modifications to the algorithm can help automatically uncover the physics of the control mechanism found by the learning algorithm. We also demonstrate a new pulse-compression technique using rotational phase modulation, where optical pulses were compressed by an order of magnitude after phase modulation by a molecular rotational coherence and subsequent propagation through a transparent window. This technique will enable efficient pulse compression in the deep-UV. Finally, we measure the coherence of EUV light generated by high-harmonic upconversion of a femtosecond laser. In phase-matched hollow-fiber geometry, the EUV light exhibits the highest inherent spatial coherence of any source in this region of the spectrum. We use this source to demonstrate the coherent image with first table-top EUV radiation. Since this source exhibits full spatial coherence at very short wavelength, this light source represents has smallest inherent effective source-size of any light source yet created. We also demonstrate that the spectrum of an optical field can be determined by measuring the interference of a double-slit pattern.
The past thirty years witnessed rapid developing of strong field physics that leads to the first generation of attosecond (10−18 s) pulses. The duration of available pulses has recently been reduced to less than 100 as, approaching the Kepler period of a classical electron revolving around the nucleus. For the first time, electron motions inside atoms, molecules and solids can be traced, probed and even steered in real time. It pushed the study on ultrafast dynamic processes into a new frontier, giving birth to attosecond physics. The development of attosecond pulses provides unprecedented coherent ultrashort XUV (extreme ultra-violet) of soft X-ray sources that enable imaging the atomic structure and resolving the related quantum dynamics for matter science. In particular, it allows the probing and monitoring of electron dynamics in the natural time scale of attoseconds with atomic spatial resolution. The generation and applications of attosecond pulses are directly related to the control and manipulation of the sub-cycle electron dynamics. Recent studies on the correlated multielectron dynamics have provided insight into the optical and dynamical properties of matter in the perspectives of time, phase and entanglement. It deepens our understanding on some of the fundamental questions, e.g., how the single or multiple photons are absorbed and how short are the quantum processes such as photoionization, tunneling ionization and charge migration. Although currently attosecond pulses are limited by the flux and pulse duration, the combination of attosecond pulses with intense infrared laser pulses paves new ways of controlling and probing electron dynamics in the domains of energy, time and space that lays the foundation for petahertz optoelectronics and attosecond pulse based transient spectroscopy. In this review, we will first briefly summarize the advancement of strong field and ultrafast physics including the relevant experimental findings, the theoretical explorations, the related technologies and the possible applications. Then we focus on the sub-cycle electron or multielectron dynamics and their manifestation on the ionization and coherent radiation properties. We start with the time behavior of Fano resonance and how it can be probed in time domain. In section 2, the correlated electron dynamics is discussed in term of anti-screening effect and Pauli effect that influence strong field ionization and high harmonic generation processes. In section 3, the coherent emission with frequency from terahertz to soft XUV and their joint measurement are discussed by emphasizing the generation mechanism of terahertz wave in two-color laser pulses. The developed high-harmonic and terahertz wave spectroscopy (HATS) is shown capable of imaging the molecular structure and dynamics. In section 4, the time-resolving of electron and hole dynamics and their coherence is proposed based on attosecond transient absorption technique. The coherence of ionization and the correlated electron dynamics is hinted crucial for electron-hole interaction. The topics are very limited and mainly selected from the perspectives of our group.
Quantum coherent control of ultrafast bond making and the subsequent molecular dynamics is crucial for the realization of a new photochemistry, where a shaped laser field is actively driving the chemical system in a coherent way from the thermal initial state of the reactants to the final state of the desired products. We demonstrate here coherent control over the relative yields of Mg2 molecules that are generated via photoassociation and subsequently photodriven into different groups of final states. The strong-field process involves non-resonant multiphoton femtosecond photoassociation of a pair of thermally hot magnesium atoms into a bound Mg2 molecule and subsequent molecular dynamics on electronically excited states. The branching-ratio control is achieved with linearly chirped laser pulses, utilizing the different chirp dependence that various groups of final molecular states display for their post-pulse population. Our joint experimental and theoretical study establishes the feasibility of high degree coherent control over quantum molecular dynamics that is initiated by femtosecond photoassociation of thermal atoms.
Recent advances in the ultrashort laser technology have spawned a series of new directions of research in atomic and molecular physics. This work compounds two studies on the control and probing of atomic matter by ultrafast laser pulses. The first study is concerned with the control of quantum wavepackets focusing by chirped, ultrashort laser pulses. The ability to control wavepacket focusing is important for the achievement of specific excitation pathways, for controlling chemical reactivity or even molecular synthesis. In this work, we present a general method for wavepacket control in the weak field limit. Numerical simulations involving the ICN molecule confirm our theoretical framework. The second study refers to the ionization of sodium Rydberg states via half-cycle pulses (HCPs). For this process, our full quantum-mechanical calculations demonstrate an asymmetry in both ionization rates and ionization spectrum between the downhill (Stark red-shifted) and uphill (Stark blue- shifted) states. The asymmetry is explained by appealing to a semi-classical model of the interfering ionization pathways.
Laser Induced Dissociation Control of H2+H_2^+H2+​ and H32+H_3^{2+}H32+​
Laser induced dissociation control of the symmetric diatomic molecular ion H2+H_2^+H2+​ and the triatomic molecular ion H32+H_3^{2+}H32+​ is discussed. The simulation results demonstrate that a long-wavelength terahertz or mid-infrared laser pulse can be used to control the electrons of the dissociative states after the excitation of an ultrashort ultraviolet (UV) laser pulse. For H2+,H_2^+,H2+​, there exists an effective time, which increases with increasing pulse duration of the UV laser pulse, for controlling the molecular dissociation. For the electrons of the 1sσg1sσ_g1sσg​ state, they move along the polarization direction of the dissociation control electric field. In contrast, for the electrons of the 2pσu2pσ_u2pσu​ state, they move in the opposite direction to that of the electric force. And for the triatomic molecule H32+H_3^{2+}H32+​, the electron dissociation control can also be realized by changing the central wavelength of the exciting UV pulse.