Manipulating Quantum Systems An Assessment of Atomic, Molecular, and Optical Physics in the United States (2020) / Chapter Skim
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5 Harnessing Quantum Dynamics in the Time and Frequency Domains
5 Harnessing Quantum Dynamics in the Time and Frequency Domains
Pages 146-180
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From page 146... ...
The observation and control of such coupled dynamics thus require advanced investigative tools both experimental and theoretical. For direct time domain access, the unprecedented development of ultrafast light sources as described in Chapter 2 has revolutionized the capabilities of AMO science to make molecular movies.
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From page 147... ...
Inner-shell ionization that initiates electron dynamics, which in turn leads to chemical and structural dynamics, happens at photon energies of hundreds to thousands of electronvolts. The ionization from outer shells, lead ing to the fastest electron dynamics measured to date, can be done with soft X-ray light with photon energies below 100 eV.
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From page 148... ...
Ultrafast pulses of electrons have the same advantage, and recent developments have also allowed the making of molecular movies in which such electron pulses are taking the pictures. This chapter begins with a discussion on the making of molecular movies, starting with the attosecond electron dynamics and continuing with the femtosecond molecular dynamics.
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From page 149... ...
• Complex reaction dynamics and collision physics: The improvement of theoretical and experimental techniques that can predict reaction rates and unravel complex reaction dynamics of increasingly complex systems of atoms, molecules, ions, and photons is needed for deepening our under standing of chemical transformation processes and plasma environments. • Extreme physics with extreme light sources: Light sources with extreme in tensities, both in the infrared/optical regime and the X-ray regime, will allow for unprecedented studies of matter in extreme conditions with potential for numerous applications.
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From page 150... ...
Fundamental Questions on the Attosecond Time Scale The photoelectric effect, which was first explained by Einstein more than 100 years ago, describes the emission of an electron following the absorption of a photon whose energy exceeds the system's binding energy. Starting in 2010, a series of experimental and theoretical works have explored the dynamics of the photo electric effect.
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From page 151... ...
FIGURE 5.1.1 Timing the photoelectric effect in metals. The absolute photoemission time from a tungsten surface is measured in a two-step process by relating it to photoemission times from other species: (a)
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From page 152... ...
Charge migration thus provides an exciting challenge for both experiment and theory in ultrafast AMO science. Intense Laser-Matter Interactions: A Gateway to Attosecond Science In the first decade of the new millennium, the field of attosecond science was synonymous with the generation of attosecond light pulses in atomic gases via high harmonic generation.
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From page 153... ...
However, the attosecond light pulses are only one possible outcome of this strong-field interaction, and as illustrated above, attosecond science now drives a range of applications ranging from fundamental questions in quantum mechanics to filming molecular movies. The physics of intense laser-matter interaction is well established by a plethora of experiments and theoretical analyses, which culminated in a widely accepted intuitive picture, the semiclassical recollision model.
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From page 154... ...
Strong-Field Attosecond Physics in Solids and Nanostructures The insights of the recollision model are applicable not only to describing gas-phase processes, but also to the physics associated with a crystalline material interacting with an intense long-wavelength laser field. For example, nonlinear absorption across the bandgap in semiconductors and insulators generates emis sion through electron-hole recombination.
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From page 155... ...
Nanostructures interacting with intense, few-cycle pulses also exhibit recollision model behavior, such as producing high-energy electrons and high-harmonic photons on very fast time scales as described above, with the physics of the dynamics again richer than in atoms. Exposing a metal nanotip to an intense field produces an electron energy distribution quite analogous to the well-known phenomenon of above-threshold ionization in atoms.
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From page 156... ...
time scale, and the resulting chirp of the emitted harmonic radiation has been dubbed the attochirp. HHS has been used in a wide range of applications, measuring atomic and mo lecular structure and dynamics, both at the electronic (charge migration)
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From page 157... ...
TDDFT has been shown to be accurate enough for single ionization and charge migration studies, but also raises open questions about how to calculate certain observables from the density, as well as its reliability to describe processes with correlations beyond the mean-field level. The next-level problem in calculating coherent electron dynamics will be to include the coupling to the inevitable nuclear dynamics and the ensuing loss of electronic coherence.
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From page 158... ...
Given that strong laser fields can be produced with optical periods close to the petahertz level, this suggests that lightwave-driven currents in transistors could result in speed-up of many orders of magnitude in the near future. Another exciting prospects for attosecond science is the impending availability of intense attosecond pulses in the soft and hard X-ray regime from XFELs, which would enable the initiation of electron dynamics (e.g., charge migration)
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From page 159... ...
Light harvesting in chromophores, the photoprotection mechanism in our DNA, and more generally optically driven molecular devices are all examples of processes in which an initial electronic excitation is coupled to nuclear motion that leads to chemical or structural changes. In the previous section, the committee discussed that the initial electron motion takes places on sub-to-few femtosecond time scales and often involves electron-electron correlation.
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From page 160... ...
are so short and so bright that the diffraction pattern samples the molecular structure at a particular time after excitation. Calcula tions show that this electron-assisted structural change happens through a conical intersection, and that it involves a reshuffling of electron bonds: In the ring-shaped molecule there are double-bonds between two of the carbon sites, whereas in the chain-shaped molecule there are three double-bonds.
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From page 161... ...
Soft and hard X-ray pulses generally probe different aspects of the electronic and nuclear dynamics, and different types of experiments can therefore elucidate the connection between the initial electronic dynamics and the subsequent molecular dynamics in a molecule that has been excited by an optical or X-ray pump pulse. As also discussed in the section "Attosecond Science: The Time Scale of the Electron," soft X-ray and XUV pulses address the structure and dynamics of valence electrons with their moderate binding energies, whereas hard X-ray pulses with their very short wavelengths can be used to resolve the location of nuclei directly through scattering images, as was illustrated in Box 5.2.
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From page 162... ...
The right side illustrates how this fragmentation can be followed via time-resolved X-ray absorption spectroscopy. The characteristic X-ray absorption energy is sensitive to the length of the C-F bond, so that a new absorption peak appears and changes energy as the delay between the initial ionization laser pulse and the probe X-ray pulse is changed.
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From page 163... ...
state via a conical intersection in less than 100 fs. Hard X-ray radiation is ideal for taking direct pictures of molecular structure through scattering images, because their short wavelengths, comparable to common bond lengths, permit very high spatial resolution.
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From page 164... ...
has long been a dream of several communities, both as an element of making molecular movies, but more generally for imaging in structural biology and materials science. Inroads are now being made toward this goal, thanks to the advent of the very intense and short pulses of X-ray light available at XFEL facilities.
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From page 165... ...
Early experiments used precise shaping of the driving laser pulse to control ionization, dissociation, or emission properties in small quantum systems, as well as reaction pathways in chemical reactions, for example, via light-induced conical intersections. For current and future studies, the continued progress in machine learning and big-data analysis enables the control of a much larger range of laser and environmental parameters, potentially making outcomes more robust and repeatable.
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From page 166... ...
Improvements in the treatment of quantum effects in the nuclear dynamics and the solution of the electronic structure problem for large molecules are both critical to further progress. In order to even qualitatively model conical intersections, where multiple electronic states are degenerate, multireference electronic structure methods are needed.
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From page 167... ...
Similarly, DMRG methods have been gaining attention in the quantum chemistry community recently, also for their potential to both increase precision and computational efficiency. A recent example of the latter includes studies of long chain molecules for which the computational time scales linearly with chain length.
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From page 168... ...
Nonlinear spectroscopy -- for example, Coherent Anti-Stokes Raman Scattering -- using a carefully timed series of optical pulses, has long been a favorite tool for studying electron and molecular dynamics, and its extension to the X-ray regime is an active area of research. Its extension to the X-ray regime would allow making "complete" molecular movies spanning both spatially resolved, attosecond, core, and valence electron dynamics, as well as couplings to femtosecond nuclear dynamics.
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From page 169... ...
Recent exciting examples include the demonstration of light-induced superconductivity, and control of topological phases via the polarization of the excitation light source. A theoretical prediction for the ability to switch a material between a topological insulator and a conducting semi-metal using femtosecond laser sources could potentially be experimentally validated using diffraction of hard X-ray pulses from soon-to-be-available XFEL sources.
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From page 170... ...
Scientists can thus measure atoms and molecules by combining high-sensitivity, precise frequency control, broad spectral coverage, and high resolution in a single experimental platform. Recent applications of cavity-enhanced direct frequency comb spectroscopy include sensitive and multiplexed trace-molecule detection for various species, as well as precise quantum control of atomic transitions via coherent pulse ac cumulations.
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From page 171... ...
on a multidimensional potential energy surface, this reaction has been challenging to understand. Mid-infrared frequency comb spectroscopy allowed the first direct observation of trans-DOCO and cis-DOCO intermediates from OD+CO at thermal reaction conditions.
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From page 172... ...
Reactive Processes Involving Physics: Topological Physics Examples of excellent progress in the past decade include far better under standing of reactive processes that involve topological physics, which include physics related to conical intersections. In ultracold AMO physics context, as in condensed-matter physics, the word "topological" is often used to refer to many particle phases of matter, but even systems with a few electrons and atoms involved exhibit topological features in their quantum mechanical behavior, especially in the context of reactive collisions.
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From page 173... ...
Broad Impact of Collision Dynamics Studies While collision physics has tremendous intellectual challenges remaining -- for example, to achieve a deeper understanding of reactions involving polyatomic molecules -- there are additional strong practical motivations for advancing theory and experiment in this arena. The following examples showcase some of those subjects in need of better understanding of collision physics, especially reaction rates in a variety of environments: • Impact on astrophysics: Extensive experimental and theoretical headway has emerged from studies of electron collisions with a variety of hydrogen rich molecules that are very important in astrophysical environments -- molecular ions like HeH+, H2+, H3+, CH3+, NH4+, to name just a few.
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From page 174... ...
Likewise, ion-atom collisions leading to charge-exchange processes emitting X rays that are detected by space-based telescopes can yield infor mation on both the projectile and the target. Highly energetic events such as in the solar wind or supernova explosions lead to highly charged ions propagating through space.
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From page 175... ...
Also in the frequency domain, with the revolutionary impact on precision metrology and ultrafast science brought by the recent development of optical frequency combs in the visible and mid-infrared spectral regions, we can naturally expect to extend this coherent spectral coverage to the extreme ultraviolet (XUV) ends of the spectrum.
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From page 176... ...
Extreme Physics with XFEL Laser Light XFEL sources provide coherent laser pulses of X-ray light with record-setting intensities and pulse durations. A number of XFEL experiments in this first decade have been performed on AMO systems, addressing fundamental questions and characterizing the capabilities of the light source.
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From page 177... ...
The X-ray pulse is so short that the molecule does not have time to dissociate, and so intense that the ionization from and replenishing of the iodine atom can happen multiple times before the pulse ends, leading to a highly charged molecular ion with electrons missing from both the iodine and the methyl end of the molecule. SOURCE: Left: See DESY, "X-ray Pulses Create ‘Molecular Black Hole,'" News, June 1, 2017, http://www.desy.de/news/news_search/index_eng.html, courtesy of DESY/Science Communication Lab.
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From page 178... ...
Future of Applications of Extreme Light Sources The field of short-wavelength nonlinear optics is only just opening up. With next-generation HHG sources of intense XUV light, and current-and-next genera tion XFEL sources of intense soft and hard X-ray light, we will be learning much more about the nonlinear response of atoms, molecules, and solids to intense short-wavelength radiation.
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From page 179... ...
FINDINGS AND RECOMMENDATIONS In this chapter, the committee discussed the study of dynamics spanning the attosecond to the microsecond time scale, and how to access it in both the time domain and the frequency domain. The committee has discussed current and future progress in the making of time-domain molecular movies, spanning attosecond, coherent electron dynamics, through femtosecond molecular dynamics and beyond.
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From page 180... ...
Finding: There has been widespread use of data typically gathered in collision physics, which are needed for applications and analysis in astrophysics, plasma physics, and nuclear medicine among others, as well as a decline in university supported collision physics groups. Recommendation: National laboratories and NASA should secure the con tinuation of collision physics and spectroscopy expertise in their research portfolios.
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