Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 68, Number 7
Monday–Friday, June 5–9, 2023; Spokane, Washington
Session S06: Strong-Field Ionization and High Harmonic Generation |
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Chair: Francois Mauger, Louisiana State University Room: 206 A |
Thursday, June 8, 2023 10:30AM - 10:42AM |
S06.00001: H3+ Formation From Methyl Halogens and Pseudohalogens Jacob Stamm, Sunny Kwon, Clayton Wicka, Swati Priyadarsini, Arnab Chakraborty, Jun Shen, Piotr Piecuch, Marcos Dantus The H3+ ion is of critical importance to interstellar chemistry due to its reactivity and ability to protonate other species. This ion is highly abundant in interstellar space and plays a key role in the formation of small molecules which are important for the formation of stars. Due to the relevance of H3+, understanding the nature of its molecular dynamics is critical to gaining insight into astrochemistry. Here, we study H3+ formation from compounds of the form CH3X, where X = (Cl, I, OH, NCS, SCN) following strong-field double ionization. The nature of the X group affects the CH3 moiety in different ways, altering the formation timescale and yield of H3+. We find that CH3I and CH3SCN generate no appreciable H3+, while the remaining compounds generate H3+ in increasing yield: CH3NCS, CH3OH, CH3Cl. We performed time-resolved strong field measurements and find the timescale of H3+ formation to be 150 fs from CH3OH, 175 fs from CH3NCS, and 415 fs from CH3Cl. High-level computation work is also done to analyze the possible formation mechanisms of H3+ from these systems, and to help elucidate the differences in yield and timescale of H3+ from these different methyl halogens and pseudohalogens. |
Thursday, June 8, 2023 10:42AM - 10:54AM |
S06.00002: High-Harmonic Spectroscopy as a Tool to Extract Hole Location in Particle-Like Charge Migration Kyle A Hamer, Mette B Gaarde, Francois Mauger We will show that particle-like charge migration [1] can be effectively tracked using high-harmonic spectroscopy when used in a pump-probe, frequency-matched configuration, such that the location of the hole at any given time can be accurately extracted from the delay-dependent harmonic spectra [2]. Due to the attochirp of the harmonic radiation, different harmonic frequencies see distinct molecular landscapes upon recombination; thus, we observe a delay- and harmonic-frequency-dependent modulation of the HHG yield due to the CM. Using the semiclassical model of HHG as a guide, this modulation allows us to determine the absolute “time zero” at which the CM dynamics was initiated. |
Thursday, June 8, 2023 10:54AM - 11:06AM |
S06.00003: Generation and control of elliptically polarized harmonics in atoms interacting with two-color linear cross polarized pulses. Spencer R Walker, Bejan Ghomashi, Andreas Becker We have studied the generation of ellipticity in high harmonic generation in atoms with spherically symmetric ground states by two intense linearly polarized crossed beams, as observed recently in experiment [1]. Numerical simulations of the time-dependent Schrodinger equation as a function of their intensity ratio, relative phase, and pulse duration show that for sufficiently short pulse durations and experimentally feasible intensity ratios large amounts of ellipticity are possible. We describe the underlying physical mechanism and provide a model for how an experimentalist may tailor the ellipticity of their harmonics. |
Thursday, June 8, 2023 11:06AM - 11:18AM |
S06.00004: Strong Field Double Ionization of Formaldehyde Vaibhav Singh Strong field Ionization (SFI) plays a key role in many time resolved experiments, such as high harmonic generation and Coulomb explosion imaging. In our work, momentum resolved covariance measurements were performed on the fragments produced from the double ionization of formaldehyde (H2CO) using SFI.1 The momentum resolved covariance measurements allow for complete characterization of the dissociation products, and recovery of the molecular recoil frame ionization yields. But, understanding the underlying cause for the formation of fragments and getting state-resolved ionization yields through experiments is limited. Computational studies can help overcome these limitations. We have performed excited states molecular dynamics on several singlet and triplet states using trajectory surface hopping to calculate the kinetic energies (KE) of fragments formed from H2CO2+. Our KE distributions were then fitted with experimental KE releases to get state-resolved double ionization yields. Electronic structure calculations were also performed in order to understand the underlying mechanics through which the dissociations occur. One of the interesting findings of this work is the presence of metastable H2CO2+ found on the first excited singlet state (S1).2 This work is an example of how computational and experimental results can be combined to verify each other and fill up knowledge gaps. |
Thursday, June 8, 2023 11:18AM - 11:30AM |
S06.00005: Imaging Isotopic Effects in the Enhanced Ionization of Water Andrew J Howard, Mathew Britton, Zachary L Streeter, Chuan Cheng, Ruaridh Forbes, Joshua L Reynolds, Felix Allum, Ian Gabalski, Robert R Lucchese, William McCurdy, Thomas Weinacht, Philip H Bucksbaum A strong enhancement can be achieved in the sequential multiple ionization of water by stretching and unbending the molecule. Selectively ionizing two electrons can induce these geometrical distortions and facilitate the ionization of a third electron. Studying this effect in D2O, one of the heavier isotopologues of water, we uncovered a critical geometry at which this enhancement is maximal. Here the molecule was entirely unbent and the OD bond lengths were approximately doubled in length (rOD ~ 2.2 Å). However, the sequential multiple ionization process in water should differ for its other isotopologues, e.g. H2O and HOD. The width of the nuclear wave packets involved, the speed at which they move, and the paths they take will all differ. By performing a comparative analysis of all three isotopologues and utilizing both experimental data and ab initio theory, we seek to identify how the enhanced ionization effect differs, and identify the key contributing factors. For each isotopologue, we initiate sequential multiple ionization by using 6-fs 800-nm pulse pairs with variable interpulse delay. The first pulse forms the dication; the second pulse forms the trication, and nuclear rearrangement occurs in the time between the two. After formation of the trication, the molecule undergoes a Coulomb explosion and the 3D momentum of each fragment is measured in coincidence and used to reconstruct the molecule’s internuclear geometry. |
Thursday, June 8, 2023 11:30AM - 11:42AM |
S06.00006: Resolving Optimal Electron-Hole Dynamics in IR-assisted XUV Photoionization for Quantum Control of High-Order Harmonic Generation R. Esteban Goetz, Anh-Thu Le In photoionization, the correlated dynamics of low-lying orbital electrons is affected by the interaction between the parent ion and the ejected photoelectron. For slow photoelectrons, interchannel couplings affect the degree of coherence between hole states [1]. Recent works have suggested that such couplings may be the missing link to understanding the dynamics of high order harmonic generation before the photoionized electron recombines with the parent atom [2]. It has also been shown that details of the electron-hole dynamics are imprinted in the HHG yield and can be probed using a combination of XUV and IR fields [3]. In this work, we theoretically investigate IR-assisted XUV-photoionization schemes for quantum control of harmonic generation under the influence of the hole dynamics in the photoion. The XUV field configuration promotes the photoelectron wave packet to the same continuum state following different photoionization pathways prior to recombination. We analyze and time-resolve the details of electron-density dynamics yielding to the optimized HHG spectrum using an interferometric scheme that involves two atomic targets generating HHG radiation that interfere in the same detection plane as a function of the time delay between the fields acting on each target. We show a direct mapping between the optimized electron-density dynamics and interfering HHG radiation phase spectrum. |
Thursday, June 8, 2023 11:42AM - 11:54AM |
S06.00007: Quantum Trajectory Selector: Attosecond Study of Strong Field Ionization Qiaoyi Liu, Andrew J Piper, Yaguo Tang, Abraham Camacho Garibay, Dietrich Kiesewetter, Pierre Agostini, Kenneth J Schafer, Louis F DiMauro The dynamics of an electron photoionized by an intense laser field is commonly understood using the semi-classical model: the electron first tunnels out of the atomic potential, then is accelerated in the strong field, and finally recollides with its parent ion. One constraint of studying strong field processes is that tunneling can occur at times within the strong field cycle that are weighted by the instantaneous ionization probability proportional to the field strength, thus only a limited phase space of all possible trajectories contributes to the measurement. We introduce the quantum trajectory selector (QTS) method capable of decoupling ionization from acceleration and recollision using an extreme ultraviolet (XUV) attosecond pulse train to ionize the target dressed by a strong near-infrared (NIR) pulse that accelerates the electron and drives recollision. A variable delay between the XUV and NIR pulses enables control of the electron's release time in the strong field, accessing a well-defined set of trajectories. Here, we apply the QTS method to study non-sequential double ionization in argon. Our work shows the promise of the QTS method for studying strong field processes and providing control of strong field phenomena with unprecedented breadth and sensitivity. |
Thursday, June 8, 2023 11:54AM - 12:06PM |
S06.00008: Fourier spectroscopy of rotational coherences in <!--[if gte msEquation 12]> style='font-size:14.0pt;mso-ansi-font-size:14.0pt;mso-bidi-font-size:14.0pt; font-family:"Cambria Math",serif;mso-ascii-font-family:"Cambria Math"; mso-hansi-font-family:"Cambria Math";color:windowtext'> lang=EN-IN style='font-size:14.0pt;line-height:107%;font-family:"Cambria Math",serif; color:windowtext'>D lang=EN-IN style='font-size:14.0pt;line-height:107%;font-family:"Cambria Math",seri Tomthin Nganba Wangjam, Huynh Van Sa V Lam, Vinod Kumarappan We study the bound wave packet launched by an ionizing 30-fs, 800-nm pulse in the ground state of by using a lower intensity pulse to dissociate the ion. The momentum distribution of ions is measured using a velocity map imaging spectrometer. A long delay scan provides enough resolution in the rotational quantum beat spectrum that we can unambiguously separate contributions from vibrational levels between to. By filtering the momentum spectra in the Fourier domain, we reconstruct momentum distributions from either a single vibrational state, or even from a single rotational coherence. These spectra provide us the opportunity to investigate the passage of coherently-prepared ions through light-induced conical intersections between light-dressed and states. |
Thursday, June 8, 2023 12:06PM - 12:18PM |
S06.00009: H3+ Formation from Cyclopropane Sunny Kwon, Jacob Stamm, Clayton Wicka, Marcos Dantus H3+ is a triatomic cation found in interstellar medium that can act as a proton donor to form water and small organic molecules. These products are crucial to begin the formation of stars and highlight the importance of H3+ in interstellar chemistry. Examining H3+ formation dynamics grants a better understanding of the chemical dynamics of this ion in space through various mechanisms. Previous literature has looked at the formation of H3+ from cyclopropane experimentally and suggested that H3+ formation resulted from a concerted grouping of three hydrogen atoms via a ‘ring-closing’ mechanism but did not measure the timescale of formation. In this study, the time of formation of H3+ was measured to take place in 174 fs through strong-field double ionization followed by femtosecond time resolved disruptive probing. The focus of this study is to settle if the formation mechanism of H3+ from cyclopropane is a roaming, concerted, or migration mechanism. Based on timescales of H3+ from cyclopropane and time resolved data as well as mass spectra data from propane and propene, we hypothesize the reaction mechanism of H3+ to be different from the concerted ring-closing. |
Thursday, June 8, 2023 12:18PM - 12:30PM |
S06.00010: The induced polarization effect in ionization simulated by the weak-field asymptotic theory Imam S Wahyutama, Denawakage D Jayasinghe, Kenneth Lopata, Kenneth J Schafer, Henrik R Larsson Accurate and efficient ionization simulations are central in attodynamics. Following ionization, the outgoing electron moves under the influence of the laser-distorted charge density of the ion – in the first order of the field strength, this distortion accounts for the induced-core polarization effect (IPE). Here, we study the IPE in the angle-dependent ionization calculated by the many-electron weak-field asymptotic theory in the first order correction (ME-WFAT(1)). We highlight the potential application of ME-WFAT(1) to highly correlated molecules, which owes, partly, to the modularity in the wavefunction input. Here, modularity means that in ME-WFAT, the wavefunction input follows a plug-and-play principle [1]. We also highlight the prospect of the density matrix renormalization group (DMRG) to obtain the wavefunctions of highly correlated molecules for ME-WFAT. The ME-WFAT/DMRG combination will be a significant development of an accurate ionization theory in strongly correlated molecules with affordable computational cost. |
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