Laser-driven Rotational Dynamics of Gas-phase Molecules PDF Download
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Author: Xiaoming Ren
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
In this thesis, our work on developing new techniques to measure and enhance field-free molecular alignment and orientation is described. Non-resonant femtosecond laser pulses are used to align and orient rotationally-cold gas-phase molecules. The time-dependent Schrodinger equation is solved to simulate the experimental results. A single-shot kHz velocity map imaging (VMI) spectrometer is developed for characterizing 1D and 3D alignment. Stimulated by a novel metric for 3D alignment proposed by Makhija et al. [Phys. Rev. A 85,033425 (2012)], a multi-pulse scheme to improve 3D alignment is demonstrated experimentally on difluoro-iodobenzene molecules and the best field-free 3D alignment is achieved. A degenerate four wave mixing probe is developed to overcome limitations in VMI measurement; experiments on different types of molecules show good agreement with computational results. Highly aligned linear molecules are used for high harmonic generation experiments. Due to the high degree of alignment, fractional revivals, variation of revival structure with harmonic order and the shape resonance and Cooper minimum in the photoionization cross section of molecular nitrogen are all observed directly in experiment for the first time. Enhanced orientation from rotationally cold heteronuclear molecules is also demonstrated. We follow the theory developed by Zhang et al. [Phys. Rev. A 83, 043410 (2011)] and demonstrate experimentally for the first time that for rotationally cold carbon monoxide an aligning laser pulse followed by a two-color laser pulse can increase field-free orientation level by almost a factor of three compared to using just the two-color pulse.
Author: Xiaoming Ren
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
In this thesis, our work on developing new techniques to measure and enhance field-free molecular alignment and orientation is described. Non-resonant femtosecond laser pulses are used to align and orient rotationally-cold gas-phase molecules. The time-dependent Schrodinger equation is solved to simulate the experimental results. A single-shot kHz velocity map imaging (VMI) spectrometer is developed for characterizing 1D and 3D alignment. Stimulated by a novel metric for 3D alignment proposed by Makhija et al. [Phys. Rev. A 85,033425 (2012)], a multi-pulse scheme to improve 3D alignment is demonstrated experimentally on difluoro-iodobenzene molecules and the best field-free 3D alignment is achieved. A degenerate four wave mixing probe is developed to overcome limitations in VMI measurement; experiments on different types of molecules show good agreement with computational results. Highly aligned linear molecules are used for high harmonic generation experiments. Due to the high degree of alignment, fractional revivals, variation of revival structure with harmonic order and the shape resonance and Cooper minimum in the photoionization cross section of molecular nitrogen are all observed directly in experiment for the first time. Enhanced orientation from rotationally cold heteronuclear molecules is also demonstrated. We follow the theory developed by Zhang et al. [Phys. Rev. A 83, 043410 (2011)] and demonstrate experimentally for the first time that for rotationally cold carbon monoxide an aligning laser pulse followed by a two-color laser pulse can increase field-free orientation level by almost a factor of three compared to using just the two-color pulse.
Author: Varun Makhija
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
In general, molecules in the gas phase are free to rotate, and measurements made on such samples are averaged over a randomly oriented distribution of molecules. Any orientation dependent information is lost in such measurements. The goal of the work presented here is to a) mitigate or completely do away with orientational averaging, and b) make fully resolved orientation dependent measurements. In pursuance of similar goals, over the past 50 years chemists and physicists have developed techniques to align molecules, or to measure their orientation and tag other quantities of interest with the orientation. We focus on laser induced alignment of asymmetric top molecules. The first major contribution of our work is the development of an effective method to align all molecular axes under field-free conditions. The method employs a sequence of nonresonant, impulsive laser pulses with varied ellipticities. The efficacy of the method is first demonstrated by solution of the time dependent Schrödinger equation for iodobenzene, and then experimentally implemented to three dimensionally align 3,5 difluoroiodobenzene. Measurement from molecules aligned in this manner greatly reduces orientational averaging. The technique was developed via a thorough understanding and extensive computations of the dynamics of rotationally excited asymmetric top molecules. The second, and perhaps more important, contribution of our work is the development of a new measurement technique to extract the complete orientation dependence of a variety of molecular processes initiated by ultrashort laser pulses. The technique involves pump-probe measurements of the process of interest from a rotational wavepacket generated by impulsive excitation of asymmetric top molecules. We apply it to make the first measurement of the single ionization probability of an asymmetric top molecule in a strong field as a function of all relevant alignment angles. The measurement and associated calculations help identify the orbital from which the electron is ionized. We expect that this technique will be widely applicable to ultrafast-laser driven processes in molecules and provide unique insight into molecular physics and chemistry.
Author: Igor Pastirk
Publisher:
ISBN:
Category : Lasers in chemistry
Languages : en
Pages : 372
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Author: Huynh Van Sa Lam
Publisher:
ISBN:
Category :
Languages : en
Pages :
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One of the main goals of ultrafast atomic, molecular, and optical physics is to monitor and control chemical reactions in real time. Ultrashort laser pulses (time scales in picoseconds or shorter) provide sufficiently high spatio-temporal resolution to study the reaction dynamics. Together with the development of shorter pulses, studies of these reactions in three-dimensional (3D) space are also crucial since the 3D structures determine the physical and chemical properties of molecules. For example, stereoisomers, such as chiral molecules, have the same molecular formula but can behave very differently in reactions with other stereoisomers or optical pulses because of the different orientations of their atoms in space. However, in a gas-phase experiment, the orientation-dependent information is usually lost after averaging over a randomly distributed molecular sample. Many different methods have been investigated to solve this important problem. In 2014, Makhija et al. demonstrated that the angle-dependent strong-field ionization of ethylene (C2H4), an asymmetric top molecule, can be retrieved from a time-resolved measurement of the yield of the cation. In this pump-probe experiment, the pump aligns and the probe ionizes the molecules, and the ion yield is measured as a function of pump-probe delay. The angle dependence is retrieved from fitting to this delay-dependent ion yield. This time-domain approach, called Orientation Resolution through Rotational Coherence Spectroscopy (ORRCS), has many advantages that can be exploited in other applications. The main theme of this work is the further development of ORRCS for extracting orientation-resolved information of different processes from rotational wave packet dynamics. The first goal of this dissertation is to systematically investigate and develop the ORRCS retrieval algorithm, since the retrieval of the angle dependence is sensitive to many parameters. We perform a series of experiments and statistical analyses to evaluate different types of errors, determine the appropriate size of the model, and check the consistency of the retrieval. Specifically, we look at the angle-dependent strong-field ionization of carbon dioxide (CO2, a linear molecule) and sulfur dioxide (SO2, an asymmetric top molecule). Strong-field ionization of CO2 has been discussed extensively in the literature because there were significant discrepancies between different experiments and theories, while SO2 has been used extensively in other experiments. The second goal of this dissertation is to expand the time-domain approach to momentum measurements. With this new development, we present two applications of ORRCS to the dissociation and photoionization of molecules. In the dissociation of molecules, the axial recoil approximation is often used without validation. We show that this approximation can be tested by measuring the momentum distributions of the fragment ions as a function of pump-probe delay. In particular, we examine the dissociation of CO2 and N2 with 800 nm and 262 nm laser pulses, respectively. In each case, we determine how the likelihood of dissociation depends on the initial orientation of the molecule and the effect of the laser field on the momentum distribution of the fragment ions. With a similar framework but different interpretation, we show that substantial information about molecular-frame photoelectron angular distributions can be obtained using rotational wave packets. We retrieve the alignment dependence of photoelectron angular distributions from N2, CO2, and C2H4 in the few-photon ionization regime. We also compare few-photon ionization with single-photon ionization and strong-field ionization to enrich our knowledge in this regime, which is not very well understood. We believe that the time-domain approach discussed in this work is useful in many areas of ultrafast physics and chemistry. With the rapid development of high-repetition-rate light sources in recent years, we expect that many measurements, including those using x-ray free-electron lasers and ultrafast electron beams, will have the ability to apply our method and gain valuable insights into molecular structures and dynamics in the near future.
Author: Hai-Woong Lee
Publisher:
ISBN:
Category :
Languages : en
Pages : 16
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Book Description
A semiclassical study presented here indicates that a sufficiently short and intense pulse can be much more effective in inducing a collisional radiative transition than cw radiation or a long pulse, although the intensity must not be too high because the Rabi oscillation can bring down the probability. For the situation of a molecule physisorbed on a crystalline surface and irradiated by a laser, a master equation approach, used to describe the time evolution of the population of the vibrational adbond levels, shows that for high intensities laser-induced vibrational excitation is the same for pulsed and cw lasers. Keywords: Molecular dynamics, Laser induced, Short pulses, Molecular collisions, Electronic transitions, Desorption.
Author: André D. Bandrauk
Publisher: CRC Press
ISBN: 9780824791759
Category : Science
Languages : en
Pages : 480
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Book Description
This text presents the major advances in both intense laser fields phenomena and laser control of photochemical reactions - highlighting experimental and theoretical research on the interaction of simple molecules with intense laser fields. The book introduces new concepts such as above-threshold ionization (ATI), above-threshold dissociation (ATD), laser-induced avoided crossings, and coherent control.
Author:
Publisher:
ISBN:
Category : Aeronautics
Languages : en
Pages : 836
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Author: Jui-teng Lin
Publisher:
ISBN:
Category :
Languages : en
Pages : 155
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Book Description
Various theoretical approaches to laser-induced molecular dynamics in the context of multiphoton processes are reviewed. The presentation is divided into two general categories: gas-phase processes and surface processes. Within the first category, unimolecular dynamics and molecular collisions are addressed. Within the second category, energy flow in adspecies-surface systems is examined, and laser applications to surface chemistry are discussed.
Author: Y.I. Khanin
Publisher: Newnes
ISBN: 0444598871
Category : Technology & Engineering
Languages : en
Pages : 420
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Book Description
This monograph summarizes major achievements in laser dynamics over the past three decades. The book begins with two introductory Chapters. Chapter 1 offers general considerations on quantum oscillators, formulates the requirements for the laser key elements and shows how these requirements are met in different laser systems. The second Chapter proposes the mathematical models used in semiclassical laser theory, discusses the approximations and simplifications in particular cases, and specifies the range of applicability of these models. In Chapters 3-5 attention is given primarily to the steady states and their stability, the laser behavior in the instability domain, the characteristics of regular and chaotic pulsations and the nature of their mechanisms. Chapter 6 deals with the processes in a laser, accompanying the time variance of laser parameters. Considerable attention is given to a laser response to weak, low-frequency modulation of the parameters. The problems addressed therein are resonant modulation enhancement, transition to the nonlinear regime, chaotic response to periodic impact, spike-like generation due to variation of the cavity geometry and a laser rod temperature drift. Laser behavior is subject to qualitative changes if its optical elements exhibit nonlinear properties. The action of a saturable absorber, which leads to a loss of laser stability and provides passive Q-modulation, is investigated. To a much lesser degree the researchers' attention has been attracted by other nonlinear effects such as self-focusing, e.g., which may have a strong influence on laser dynamics. All of these issues are covered in Chapter 7. The book is intended for researchers, engineers, graduate and post-graduate students majoring in quantum electronics.
Author: K. P. Lawley
Publisher: John Wiley & Sons
ISBN: 0470141867
Category : Science
Languages : en
Pages : 398
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Book Description
This text is the first of a two-volume work on molecule surface interactions addressing topics in chemical physics, surface science, physical chemistry, materials science, and electronics and semiconductor manufacture. As with the other titles in the Advances in Chemical Physics series, the chapters are written by an international group of contributors and cover a wide range of important issues in the field.
