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Author: Varun Makhija
Publisher:
ISBN:
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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: 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: Paul Hockett
Publisher: Morgan & Claypool Publishers
ISBN: 1681746875
Category : Science
Languages : en
Pages : 204
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Book Description
Since the turn of the century, the increasing availability of photoelectron imaging experiments, along with the increasing sophistication of experimental techniques, and the availability of computational resources for analysis and numerics, has allowed for significant developments in such photoelectron metrology. Quantum Metrology with Photoelectrons, Volume 1: Foundations discusses the fundamental concepts along with recent and emerging applications. The core physics is that of photoionization, and Volume 1 addresses this topic. The foundational material is presented in part as a tutorial with extensive numerical examples and also in part as a collected reference to the relevant theoretical treatments from the literature for a range of cases. Topics are discussed with an eye to developing general quantum metrology schemes, in which full quantum state reconstruction of the photoelectron wavefunction is the goal. In many cases, code and/or additional resources are available online. Consequently, it is hoped that readers at all levels will find something of interest and that the material provides something rather different from existing textbooks.
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: S. M. Purcell
Publisher:
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Category :
Languages : en
Pages :
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The optical dipole force from a singe focussed laser beam was used to study the role of laser-induced molecular alignment on the centre-of-mass motion of carbon disulphide molecules in a molecular beam. The translational, rotational and vibrational temperatures of the carbon disulphide molecules were measured to be 3.4\pm0.2 K, 35\pm10 K and 250\pm14 K respectively. The velocity of the beam was measured to be 542\pm22 m s {-1}. Time-of-flight mass spectroscopy was used to measure the acceleration and deceleration of the molecules. Maximum velocity changes of 7.5 m s {-1} and 10 m s {-1} were recorded for linearly and circularly polarised light respectively. These results showed that the dipole force, \digamma \alpha \bigtriangledown [\alpha_e_f_f(I)I(r)], where \alpha_e_f_f is the effective polarisability and determined through laser-induced alignment, can be modified by changing the laser polarisation. For linearly and circularly polarised light, a 12% difference in effective polarisability was measured to produce a 20% difference in dipole force. The dipole force from a single focussed laser beam produces a molecular optical lens and the downstream density of the molecular focus was probed by measuring the ion signal for both laser polarisations. The focal lengths for linearly and circularly polarised light were found to be separated by \approx 100 \mu m. By altering the laser polarisation from linearly through elliptically to circularly polarised light, the focal length of the molecular optical lens could be smoothly altered over the \approx 100 \mu m focal range. The role of the effective polarisability of each rotational state was also studied numerically. Separate rotational states were found to significantly alter the focal properties of a molecular optical lens. In carbon disulphide, higher rotational states (J > 10), exhibit less molecular alignment and when occupied, the focal length of the molecular optical lens for these states was increased by 60 % compared to the ground state.
Author: Xiaoming Ren
Publisher:
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Languages : en
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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: James Patrick Cryan
Publisher:
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Languages : en
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Atomic and molecular photo-reactions are best described in a reference frame which is fixed with respect to the atom or molecule. In general, this molecular frame does not coincide with the laboratory frame where measurements of these reactions are made. The random orientation of the molecules in the laboratory frame leads to diminished resolution in studying chemical reactions. Although a wealth of theoretical and experimental effort has been devoted to measuring the orientation of the molecule during and following the photo-reaction, an alternative approach would be to know the orientation of the molecular frame \textit{a priori}. Here I describe a method to bring the molecular frame into a predefined orientation with respect to the laboratory. I also present preliminary measurements of molecular processes made in this molecular frame. My discussion of molecular frame measurement begins with a description of strong-field laser-induced molecular alignment. I apply laser induced molecular alignment to make angle-resolved molecular frame measurements of x-ray excited molecules. I present a technique which uses a train of laser pulses, which take advantage of the periodic nature of rotational wavepackets, to create a highly aligned ensemble of molecules. Besides being useful for making molecular frame measurements, these highly aligned ensembles are a novel medium for studying wavepacket decoherence. Using my multiple pulse technique I am able to measure the population relaxation of highly non-thermal rotational wavepackets, and show that the population lifetime increases with $J$-state. The alignment technique described in this thesis produces a window~($\sim200$~fs) of field-free alignment in a molecular ensemble. The Linac Coherent Light Source~(LCLS), a high brightness, x-ray free electron laser (xFEL), can be focused to produce extremely high x-ray power densities. These high x-ray power densities saturate $K$-shell absorption in low-$Z$ atoms inside this alignment window. Given this impressive source I am able to measure the angle-resolved Auger electron emission pattern from diatomic nitrogen. The capability to resolve Auger emission angular distributions in the molecular frame provides a new tool for spectral assignments in the congested normal $KLL$-Auger electron spectra and takes advantage of the symmetries of the final dication states. The high brightness x-rays from the LCLS, in addition to providing sufficient flux to perform experiments on pre-aligned molecular ensembles, are capable of producing double core vacancies through rapid sequential ionization. This enables double core vacancy Auger electron spectroscopy, an entirely new way to study femtosecond chemical dynamics with Auger electrons that probe the local valence structure of molecules near a specific atomic core. I observed a rich single-site double core vacancy Auger electron spectrum in good agreement with ab initio calculations, and I measured the corresponding Auger electron angle dependence in the molecular frame.
Author: Philip Ball
Publisher: Princeton University Press
ISBN: 9780691029009
Category : Science
Languages : en
Pages : 404
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Book Description
Molecular chemistry.
Author: K. P. Lawley
Publisher: John Wiley & Sons
ISBN: 0470143177
Category : Science
Languages : en
Pages : 676
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Book Description
The Advances in Chemical Physics series provides the chemical physics and physical chemistry fields with a forum for critical, authoritative evaluations of advances in every area of the discipline. Filled with cutting-edge research reported in a cohesive manner not found elsewhere in the literature, each volume of the Advances in Chemical Physics series serves as the perfect supplement to any advanced graduate class devoted to the study of chemical physics.
Author: Gordon W. F. Drake
Publisher: Springer Nature
ISBN: 3030738930
Category : Science
Languages : en
Pages : 1436
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Book Description
Comprises a comprehensive reference source that unifies the entire fields of atomic molecular and optical (AMO) physics, assembling the principal ideas, techniques and results of the field. 92 chapters written by about 120 authors present the principal ideas, techniques and results of the field, together with a guide to the primary research literature (carefully edited to ensure a uniform coverage and style, with extensive cross-references). Along with a summary of key ideas, techniques, and results, many chapters offer diagrams of apparatus, graphs, and tables of data. From atomic spectroscopy to applications in comets, one finds contributions from over 100 authors, all leaders in their respective disciplines. Substantially updated and expanded since the original 1996 edition, it now contains several entirely new chapters covering current areas of great research interest that barely existed in 1996, such as Bose-Einstein condensation, quantum information, and cosmological variations of the fundamental constants. A fully-searchable CD- ROM version of the contents accompanies the handbook.
Author: Ahmed H Zewail
Publisher: World Scientific
ISBN: 981450212X
Category : Science
Languages : en
Pages : 373
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Book Description
These two volumes on Femtochemistry present a timely contribution to a field central to the understanding of the dynamics of the chemical bond. This century has witnessed great strides in time and space resolutions, down to the atomic scale, providing chemists, biologists and physicists with unprecedented opportunities for seeing microscopic structures and dynamics. Femtochemistry is concerned with the time resolution of the most elementary motions of atoms during chemical change - bond breaking and bond making - on the femtosecond (10-15 second) time scale. This atomic scale of time resolution has now reached the ultimate for the chemical bond and as Lord George Porter puts it, chemists are near the end of the race against time. These two volumes cover the general concepts, techniques and applications of femtochemistry.Professor Ahmed Zewail, who has made the pioneering contributions in this field, has from over 250 publications selected the articles for this anthology. These volumes begin with a commentary and a historical chronology of the milestones. He then presents a broad perspective of the current state of knowledge in femtochemistry by researchers around the world and discusses possible new directions. In the words of a colleague, ';it is a must on the reading-list for all of my students ...; all readers will find this to be an informative and valuable overview.';The introductory articles in Volume I provide reviews for both the non-experts as well as for experts in the field. This is followed by papers on the basic concepts. For applications, elementary reactions are studied first and then complex reactions. Volume I is complete with studies of solvation dynamics, non-reactive systems, ultrafast electron diffraction and the control of chemical reactions.Volume II continues with reaction rates, the concept of elementary intramolecular vibrational-energy redistribution (IVR) and the phenomena of rotational coherence which has become a powerful tool for the determination of molecular structure via time resolution. The second volume ends with an extensive list of references, according to topics, based on work by Professor Zewail and his group at Caltech.These collected works by Professor Zewail will certainly be indispensable to both experts and beginners in the field. The author is known for his clarity and for his creative and systematic contributions. These volumes will be of interest and should prove useful to chemists, biologists and physicists. As noted by Professor J Manz (Berlin) and Professor A W Castleman, Jr. (Penn State): femtochemistry is yielding exciting new discoveries from analysis to control of chemical reactions, with applications in many domains of chemistry and related fields, e.g., physical, organic and inorganic chemistry, surface science, molecular biology, ...; etc.
