1,721,079 research outputs found
Mapping the fragmentation of acetylene with femtosecond resolution pump probe at LCLS using 2, 3, and 4 particle coincidences
No full textRydberg wave packets and half-cycle electromagnetic pulses.
No full textThis dissertation summarizes an examination of the dynamics of atomic Rydberg wave packets with coherent pulses of THz electromagnetic radiation consisting of less than a single cycle of the electric field. The bulk of the energy is contained in just a half-cycle. Previous work (1, 10) has shown how these half-cycle pulses can be used to ionize the highly excited states of an atom, and that a classical view of electronic motion in the atom explains the ionization mechanism. To further probe the boundary between classical trajectories and quantum mechanics, in this work we investigate dynamical combinations of Rydberg states, or Rydberg wave packets, and how they ionize under the influence of a half-cycle electromagnetic pulse. With time-domain techniques we are able to extract the dynamics of the wave packet from the ionization rate. We then observe wave packet motion in both the electronic radial and angular coordinates, and can view it directly with the half-cycle pulse anywhere on its trajectory. This is the unique feature of half-cycle pulse ionization. Semiclassical ideas of ionization in conjunction with quantum descriptions of the wave packet, are capable of reproducing the main trends in our data, and in the absence of a rigorous model we rely on these. Experiments of this nature provide examples of the ongoing effort to use the coherent properties of radiation to control electronic motion in an atom, as well as to probe the boundaries between quantum and classical mechanics.PhDAtomic physicsOpticsPhysicsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130336/2/9722068.pd
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Seeing sound: Dynamical effects in ultrafast X -ray diffraction.
No full textTime-resolved x-ray diffraction has become a very powerful tool in the study of structural dynamics of solids. The wave nature of x-rays allows the detection of milliAngstrom changes in crystalline structure. Coupling the sensitivity of x-ray scattering with ultrafast pump-probe techniques can provide unprecedented studies of ultrafast dynamics in solids. This thesis demonstrates how time-resolved x-ray diffraction can be used to view transient strains in crystalline solids. Several different x-ray scattering geometries are explored. In the Bragg scattering geometry, laser generated picosecond acoustic phonons in single crystal Ge and InSb are analyzed. Comparisons with dynamical diffraction theory have shown that the accepted theory of laser driven acoustic phonons is inconsistent with the observed data. It is also observed that the dispersion associated with pulse propagation is much larger than expected. In the Laue scattering geometry, studies using single crystal Ge, InSb, and GaAs have demonstrated a direct coupling between the ultrafast acoustic pulse and the diffracting x-ray pulse. Qualitative agreement is found between the observed data and a simple two crystal model. In the two crystal model the acoustic pulse is assumed to be a moving thin interface lying between two unstrained thick crystals. The resulting moving interface causes time-dependent Pendellosung oscillations in the diffracted beams. It is also shown that this coupling is dependent on the direction of the acoustic phonon when compared to the reciprocal lattice vector. The generation of an acoustic pulse causes an ultrafast coherent transfer of population between the two diffracting beams. The time scale for this diffraction effect ranges from 10--300ps. Several potential mechanisms describing the physics behind this phenomena are proposed, but exact nature of this diffraction effect is not known and requires further study.PhDCondensed matter physicsOpticsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/129355/2/3042061.pd
Learning from molecules: Coherent feedback control in liquid phase.
No full textCoherent control of light and matter is a major tool in current atomic physics and physical chemistry research. Technological advances in ultrafast laser pulse shaping allow control over the dynamics of molecular systems on sub-picosecond timescales. This thesis demonstrates feedback control over a variety of quantum systems and describes an adaptive genetic algorithm used in the experiments. In the gas phase, we examine control of multi-photon, dissociative ionization in diatomic sodium. The learning algorithm achieves control over the dissociative fraction primarily by adjusting the peak intensity of the laser field. We also investigate control of multi-mode stimulated Raman scattering in liquid phase methanol. Analysis of the results leads to identification of quasi-impulsive Raman coupling as a possible control mechanism. Further experiments, combined with numerical simulations of the interaction, demonstrate that an impulsive coupling of the two modes by the pump pulse can control the Raman gain. We extend the Raman control experiments to ethanol and isopropanol. Finally we apply feedback control techniques in an initial investigation of a liquid-phase chemical reaction. Specifically, we examine the ring-opening conversion of cyclohexadiene into hexatriene. The reaction dynamics occur on an excited state potential energy surface. Through control of the excitation pulse shape, we seek to guide the wavepacket evolution on the excited state to exert control over the reaction branching ratio.PhDMolecular physicsOpticsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124158/2/3122021.pd
Exploring multiple degrees of freedom in Rydberg wave packets.
No full textRecent advances in the study of Rydberg atoms have focused on the control, manipulation and detection of Rydberg wave packets using novel external fields such as half-cycle pulses. The radial degree of freedom has been controlled and used to process information encoded in Rydberg states. However, these previous experiments make use of only a single degree of freedom, restricting the potential of other degrees of freedom for quantum computing in Rydberg atoms. In this dissertation, we explore the control and detection of other degrees of freedom in an electron wave packet, such as the angular momentum quantum number ℓ, the magnetic quantum number m and the electron spin; so that the full range of quantum numbers can participate in information processing. We first propose an interferometric control of the population of angular momentum states using two time-delayed phase-locked ultrafast laser pulses. The population of arbitrary angular momentum states can be greatly enhanced by optimizing the time delay and the relative phases between two laser pulses. We then qualitatively measure the evolution of angular momentum components in Stark wave packets by a weak half-cycle pulse (HCP). This measurement utilizes a time-delayed HCP and is proved to be effective for detecting various aspects of wave packet dynamics, particularly, the evolution of non-stationary states. The technique relies on the fact that the HCP redistributes the eigenstate populations and induced the population variation which reflects the evolution of eigenstate phases. Finally, we find that the dynamics of m-states could be highly correlated with the internal degree of freedom of the electron, the spin. We study the effect of spin-orbit coupling on the wave packet dynamics and observe the angular precession of a Rydberg wave packet. The population redistribution from p to s states is highly sensitive to the polarization of the HCP and changes with the precession of the electron orbit. We obtain the selection rules and give an intuitive classical interpretation of angular momentum redistribution by an HCP. In short, new dynamics involving multiple degrees of freedom of an electron wave packet are discovered, observed and understood.PhDAtomic physicsOpticsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126323/2/3238113.pd
Strong field optical and quantum control.
No full textThis work presents the results of an effort to use unique forms of optical radiation to better probe and control matter. Results are presented of a study of intense field photo-ionization of krypton and xenon in a two-color field. The use of a two-color field provides a valuable probe, the relative optical phase, into the dynamics of the ionization process. It is found that phase dependent tunneling character is preserved even though the photoelectron spectra indicate that the experiments performed were well into the multi-photon regime of ionization. Evidence for core scattering of the departing electrons is seen in the changes to the phase dependent spectra as the polarization of the exciting light is varied from linear to slightly elliptical. To further control the optical field, a pulse shaper was constructed using liquid crystal modulators that allowed either spectral phase or spectral amplitude shaping of a short pulse. The results were characterized using cross-correlations. The shaped light was then subsequently amplified in a chirped pulse amplifier. This light was characterized using Frequency Resolved Optical Gating, a newly developed technique for the complete determination of the optical field in a short pulse. The shaped pulses were then used to tailor atomic radial wavepackets in cesium. The evolution of the wavepackets was monitored by measuring atomic auto-interferograms for the case of amplitude shaping, which was used to control the atomic states excited. Cross-interferograms were used for phase shaping, which was used to select the initial phase of the atomic states. The cross-interferograms required the simultaneous amplification of a shaped and an unshaped pulse in our amplifier.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/104541/1/9527737.pdfDescription of 9527737.pdf : Restricted to UM users only
High harmonic generation and applications.
No full textThis dissertation describes study of high-order harmonics of intense, ultrashort-pulse, 800nm laser radiation, generated in gas, and also describes the application of that harmonic radiation to novel, ultrafast, xuv-spectroscopy techniques in molecular dymanics. We describe the measurement of the polarization state of high-order harmonics of intense, ultrashort-pulse, 800nm laser radiation, generated in gas. The ellipticity of the polarization and the orientation of the polarization ellipse were found to be different than that of the laser, and to depend on the ellipticity of the polarization of the laser, and the type of gas. We discuss these results in the context of several theoretical models of harmonic generation. We also describe the application of ultrashort-pulse xuv harmonic radiation to the study of core-level shifts in molecules during photodissociation. We describe the design, assembly, and testing of a non-dispersive xuv monochromator for the study of shifts in the 4d > 5p absorbtion in iodine, during the dissociation of I2.PhDAtomic physicsMolecular physicsOpticsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132493/2/9963916.pd
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
