Control of quantum phenomena: Past, present, and future

Quantum control is concerned with active manipulation of physical and chemical processes on the atomic and molecular scale. This work presents a perspective of progress in the field of control over quantum phenomena, tracing the evolution of theoretical concepts and experimental methods from early developments to the most recent advances. The current experimental successes would be impossible without the development of intense femtosecond laser sources and pulse shapers. The two most critical theoretical insights were (1) realizing that ultrafast atomic and molecular dynamics can be controlled via manipulation of quantum interferences and (2) understanding that optimally shaped ultrafast laser pulses are the most effective means for producing the desired quantum interference patterns in the controlled system. Finally, these theoretical and experimental advances were brought together by the crucial concept of adaptive feedback control, which is a laboratory procedure employing measurement-driven, closed-loop optimization to identify the best shapes of femtosecond laser control pulses for steering quantum dynamics towards the desired objective. Optimization in adaptive feedback control experiments is guided by a learning algorithm, with stochastic methods proving to be especially effective. Adaptive feedback control of quantum phenomena has found numerous applications in many areas of the physical and chemical sciences, and this paper reviews the extensive experiments. Other subjects discussed include quantum optimal control theory, quantum control landscapes, the role of theoretical control designs in experimental realizations, and real-time quantum feedback control. The paper concludes with a prospective of open research directions that are likely to attract significant attention in the future.Comment: Review article, final version (significantly updated), 76 pages, accepted for publication in New J. Phys. (Focus issue: Quantum control

Laser-initiated Coulomb explosion imaging of small molecules

Momentum vectors of fragment ions produced by the Coulomb explosion of CO2z+ (z = 3 - 6) and CS2z+ (z = 3 - 13) in an intense laser field (~50 fs, 1 x 1015 W/cm2) are determined by the triple coincidence imaging technique. The molecular structure from symmetric and asymmetric explosion channels is reconstructed from the measured momentum vectors using a novel simplex algorithm that can be extended to study larger molecules. Physical parameters such as bend angle and bond lengths are extracted from the data and are qualitatively described using an enhanced ionization model that predicts the laser intensity required for ionization as a function of bond length using classical, over the barrier arguments. As a way of going beyond the classical model, molecular ionization is examined using a quantum-mechanical, wave function modified ADK method. The ADK model is used to calculate the ionization rates of H2, N2, and CO2 as a function of initial vibrational level of the molecules. A strong increase in the ionization rate, with vibrational level, is found for H2, while N2 and CO2 show a lesser increase. The prospects for using ionization rates as a diagnostic for vibrational level population are assessed

Excited state non-adiabatic dynamics of pyrrole:A time-resolved photoelectron spectroscopy and quantum dynamics study

The dynamics of pyrrole excited at wavelengths in the range 242-217 nm are studied using a combination of time-resolved photoelectron spectroscopy and wavepacket propagations performed using the multi-configurational time-dependent Hartree method. Excitation close to the origin of pyrrole's electronic spectrum, at 242 and 236 nm, is found to result in an ultrafast decay of the system from the ionization window on a single timescale of less than 20 fs. This behaviour is explained fully by assuming the system to be excited to the A2(\u3c0\u3c3 17) state, in accord with previous experimental and theoretical studies. Excitation at shorter wavelengths has previously been assumed to result predominantly in population of the bright A1(\u3c0\u3c0 17) and B2(\u3c0\u3c0 17) states. We here present time-resolved photoelectron spectra at a pump wavelength of 217 nm alongside detailed quantum dynamics calculations that, together with a recent reinterpretation of pyrrole's electronic spectrum [S. P. Neville and G. A. Worth, J. Chem. Phys. 140, 034317 (2014)], suggest that population of the B1(\u3c0\u3c3 17) state (hitherto assumed to be optically dark) may occur directly when pyrrole is excited at energies in the near UV part of its electronic spectrum. The B1(\u3c0\u3c3 17) state is found to decay on a timescale of less than 20 fs by both N-H dissociation and internal conversion to the A2(\u3c0\u3c3 17) state.Peer reviewed: YesNRC publication: Ye

Molecular movies and geometry reconstruction using Coulomb explosion imaging

Coulomb explosion imaging is a technique of imaging the structure of small molecules in the gas phase and their ultrafast dynamics by inducing the rapid ionization and dissociation of the molecule into its constituent atomic fragments. The momentum vectors of the atomic fragments facilitate the retrieval of the molecule's structure, however, few attempts at geometry reconstruction appear in the published literature, whose vague methodology casts serious doubts on the geometry reconstructions that have been performed, and motivating the need for an investigation into the feasibility of geometry reconstruction. We develop a method for the fast and precise reconstruction of triatomic molecular geometries by casting the task as a nonlinear constrained optimization problem. We use this method to investigate the uncertainty in geometry reconstructions as a function of measurement uncertainty as well as the existence and nature of multiple solutions to the geometry reconstruction problem. We map out the conditions under which molecular geometries may be accurately reconstructed and propose a framework for reconstructing geometries, and therefore producing molecular movies using Coulomb explosion imaging

Fragmentation Dynamics of Triatomic Molecules in Femtosecond Laser Pulses Probed by Coulomb Explosion Imaging

In this thesis we have utilized few-cycle pulses in the range 10-15s, to initiate CE to allow us to image the structure, dynamics, and kinetics of ionization and dissociation of triatomic molecules. We have made a series of measurements of this process for CO2 and N2O, by varying the laser pulse duration from 7 to 500 fs with intensity ranging from 2.5×1014 to 4×1015 (W/cm2), in order to identify the charge states and time scales involved. This is a new approach in CEI introducing a multi-dimensional aspect to the science of non-perturbative laser-molecule interaction. We refer to this approach as FEmtosecond Multi-PUlse Length Spectroscopy (FEMPULS). The use of a time and position sensitive detector allow us to observe all fragment ions in coincidence. By representing the final fragmentation with Dalitz and Newton plots, we have identified the underlying break up dynamics. Momentum conservation has been used to extract the correlated fragment ions which come from a single parent ion. This is achieved by considering that the total momentum of all correlated fragments must add up to zero. One of the main outcomes of our study is observation of charge resonance enhanced ionization (CREI) for triatomic molecules. In the case of CO2, we found that for the 4+ and higher charge states, 100 fs is the time scale required to reach the critical geometry RCO= 2.1Å and ӨOCO =163º (equilibrium CO2 geometry is RCO= 1:16Å and ӨOCO =172º. The CO23+ molecule, however, appears always to begin dissociation from closer than 1.7 Å indicating that dynamics on charge states lower than 3+ is not sufficient to initiate CREI. Finally, we make quantum ab initio calculations of ionization rates for CO2 and identify the electronic states responsible for CREI. Total kinetic energy (KER) has been measured for channels (1, 1, 1) to (2, 2, 2) and it was found that the (1, 1, 1) channel is not Coulombic, while (2, 2, 2) channel is very close to Coulombic (KER close to 90% of the coulombic potential). As another outcome of our study, for the case of N2O, we observed for the first time that there are two stepwise dissociation pathways for N2O3+: (1) N2O3+ → N++ NO2+ → N+ + N++ O+ and (2) N2O3+ → N22++O+ → N+ + N++ O+ as well as one for N2O4+ → N2++ NO2+ → N2+ + N++ O+. The N22+ stepwise channel is suppressed for longer pulse length, a phenomenon which we attribute to the influence which the structure of the 3+ potential has on the dissociating wave packet propagation. Finally, by observing the KER for each channel as a function of pulse duration, we show the increasing importance of CREI for channels higher than 3+.1 yea

Improvements to detection efficiency and measurement accuracy in Coulomb Explosion Imaging experiments

An algorithm for extracting event information from a Coulomb Explosion Imaging (CEI) position sensitive detector (PSD) is developed and compared with previously employed schemes. The PSD is calibrated using a newly designed grid overlay and validates the quality of the described algorithm. Precision calculations are performed to determine how best the CEI apparatus at The University of Waterloo can be improved. An algorithm for optimizing coincidence measurements of polyatomic molecules in CEI experiments is developed. Predictions of improved efficiency based on this algorithm are performed and compared with experiments using a triatomic molecule. Analysis of an OCS targeted CEI experiment using highly charged Argon ions to initiate ionization is performed. The resulting measurements are presented using a variety of visualization tools to reveal asynchronous and sequential fragmentation channels of OCS3+

Eighteenth annual West Coast theoretical chemistry conference

Abstracts are presented from the eighteenth annual west coast theoretical chemistry conference. Topics include molecular simulations; quasiclassical simulations of reactions; photodissociation reactions; molecular dynamics;interface studies; electronic structure; and semiclassical methods of reactive systems