Groups | Seminars || Courses | Outreach

Seminars

Seminars will be held in room S-141 in the Physics and Astronomy Department building on Mondays at 4:00 PM, unless noted otherwise. When necessary, virtual seminar Zoom login instructions will be sent out via email.

Spring 2023

January 19, 2023, 4:00 PM Prof. Itzik Ben-Itzhak, J.R. Macdonald Laboratory, Physics Department, Kansas State University Coincidence momentum imaging of photo-induced multi-body fragmentation: a path toward understanding molecular dynamics with increasing complexity (Host: Tom Weinacht)

Experimental studies of the multi-body fragmentation of polyatomic molecules using coincidence momentum imaging techniques have become more attainable with advancements in detector and light source technologies. The goal of our research group is to apply these techniques to gain detailed insight into both concerted and sequential (also known as simultaneous and stepwise, respectively) molecular dynamics, and when possible, follow their time evolution. One common approach for relating the momentum images to the stationary or time-dependent molecular structure relies on the implementation of Coulomb explosion imaging (CEI), which inherently assumes a rapid concerted fragmentation. This assumption was originally fulfilled by stripping fast molecular ion beams in ultra-thin foils [1]. However, in photo-induced molecular processes the validity of this assumption is in question. Clearly sequential fragmentation is a process that can prevent the proper application of CEI, and therefore we need to separate concerted and sequential fragmentation–a goal that we accomplish by using native frames analysis [2]. The main advantage of the native-frames method is in providing an analysis and interpretation framework for multi-body fragmentation, which is typically challenging due to the high dimensionality inherent in such data. In one example, we study the sequential fragmentation of D2O molecules into D^++D^++O+2e^- following double ionization by a single photon [3]. Namely, we pinpoint the precise electronic states involved in each sequential breakup step and image the internal energy of the OD+ intermediate above the D^++O(^3P) dissociation limit. In addition, we show how the angular momentum of the intermediate molecule manifests as a threshold behavior in the kinetic energy release of the second breakup step. We will also discuss experimental progress toward imaging four-body sequential fragmentation and toward time-resolved studies. This work is in collaboration with the groups of Daniel Rolles, Artem Rudenko, Thorsten Weber, Bill McCurdy, Robert Lucchese, Tom Rescigno, Daniel Slaughter, Joshua Williams, Allen Landers, and Reinhard Dörner. References: [1] Z. Vager, R. Naaman, and E.P. Kanter, Science 244, 426 (1989) [2] J. Rajput, T. Severt, et al., Phys. Rev. Lett. 120, 103001 (2018) [3] T. Severt et al., Nature Communications 13, 5146 (2022) *Supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Award No. DE-FG02-86ER13491

February 20, 2023, 4:00 PM Dr. Marjan Mirahmadi, Fritz Haber Institute of the Max Planck Society Three-body recombination processes: From the cold to the thermal regime (Host: Jesús Pérez Ríos)

Three-body recombination or ternary association is a termolecular reaction in which three particles collide to form a bound state between two of them as a product state. This process plays an important role in many areas of physics and chemistry, such as stability of a quantum gases, formation of new charged products in hybrid atom-ion traps, formation of van der Waals molecules and different reactions in plasma physics, astrophysics and atmospheric physics. In the main part of my talk I will be discussing the ion-atom-atom three-body reaction A+A+B^+ with collision energies ranging from 100 µK to 10^5 K, by analyzing the two possible products: molecules and molecular ions. The dynamics is performed via a direct three-body formalism based on a classical trajectory method in hyperspherical coordinates [1]. As a result, the validity range for the previously derived threshold law for ion-neutral-neutral reactions is established. Furthermore, I introduce new intriguing scenarios in which the branching ratio of the product states deviates from the expected threshold law in the cold regime [2]. The second part of this talk is a brief discussion of ozone formation through direct (without invoking the existence of an intermediate complex) three-body recombination reactions O2+O+M→O3+M. I will provide the fully ab initio and pressure-independent rate for ozone formation derived for temperatures 50-900 K. Accounting for the process of vibrational quenching of the nascent population, a good agreement with available experimental data for temperatures 100-900 K is obtained [3]. [1] J. Pérez-Ríos, S. Ragole, J. Wang, and C. H. Greene, J. Chem. Phys. 140, 044307 (2014). [2] M. Mirahmadi, J. Pérez-Ríos, J. Chem. Phys. 158, 024103 (2023). [3] M. Mirahmadi, J. Pérez-Ríos, O. Egorov, V. Tyuterev, and V. Kokoouline, Phys. Rev. Lett. 128, 108501 (2022).

March 20, 2023, 4:00 PM Prof.Julia M. Mikhailova, ETOILES, Department of Mechanical and Aerospace Engineering, Princeton Ultrafast High-Field Science with Plasma Optics and Solid Materials (Host: Tom Weinacht)

I will present our latest developments in ultrafast ionization-based optical components as well as optical harmonic generation in solids. Ionization-based optics, such as plasma mirrors and diffraction gratings enable manipulation of ultra-intense light beyond the limitations imposed by the minimum beam size required to prevent optical damage. Harmonic generation in solids, both ionized and non-ionized, can serve as a diagnostic tool for plasmas and for new quantum materials, as well as a source of intense ultrafast short-wave radiation. Specifically, I will demonstrate our recent results on enhanced harmonic generation using a terawatt multi-color laser driver generated in a cascaded plasma mirror configuration and show that the enhancement can be controlled by adjusting the relative phase of the colors in the driving waveform. I will also show that ionization-based diffraction gratings can act as a controllable optical switch for high-power light, with a temporal contrast improvement of over five orders of magnitude and a switching time of less than 500 fs, which compares favorably to contrast enhancement achievable with other methods. Finally, I will present our experimental results on optical harmonic generation by solids and bulk topological materials.

May 5, 2023, 3:00 PM Prof. Oleg Pronin, Helmut Schmidt University, n2-Photonics GmbH, Hamburg Nonlinear optics in multipass cells  (Host: Tom Allison)

χ^(3) based nonlinear effects were extensively demonstrated in multipass cells over the last 6 years providing and establishing the field of χ^(3) multipass nonlinear optics [1,2,3] (also see schematic summary-picture below). This talk will address the recent developments in this field, including further peak and average power scalability towards sub-Joule energies and kW-level average power. Moreover, the first proof-of-concept demonstration of χ^(2) multipass nonlinear optics [4] where free space birefringent phase matching and quasi-phase matching can be realized with nearly all types of nonlinear materials, is also going to be presented in this talk. [1] J. Schulte, et al., “Nonlinear pulse compression in a multi-pass cell,” Opt. Lett. 41, 4511–4514 (2016). [2] K. Fritsch, et al., “All-solid-state multipass spectral broadening to sub-20 fs,” Opt. Lett. 43, 4643–4646 (2018). [3] M. Hanna, F. Guichard, N. Daher, Q. Bournet, X. Délen, and P. Georges Laser Photonics Rev. 15, 2100220 (2021). [4] N. Kovalenko, V. Hariton, K. Fritsch, O. Pronin https://arxiv.org/abs/2209.06207 [5] K. Fritsch, T. Hofer, J. Brons, et al. Dual-comb thin-disk oscillator. Nat. Commun. 13, 2584 (2022).

August 4, 2023, 10:00 AM Dr. Tom Tongue, Dr. Cecile Skoryna Kline, TOPTICA Photonics Working at TOPTICA (Host: Tom Allison)

Tom Tongue, VP of Sales and Marketing, and Cecile Skoryna Kline, a sales representative for the Eastern US, Canada, and Mexico, will give a 30-minute seminar about career opportunities at TOPTICA.

August 14, 2023, 10:00 AM Dr. Adam J. Fleisher, NIST Gaithersburg Precision molecular spectroscopy: Trace detection and line lists for astrophysics (Host: Tom Allison)

High-resolution molecular spectroscopy enables trace detection of greenhouse gases like CO2 and N2O, as well as an accurate determination of their isotopic composition. Further, new studies of the spectra of astrophysical molecules like HCN and HNC will be important to enable scientific discoveries using new infrared observatories like JWST. Here we will present recent advances in precision molecular spectroscopy performed at NIST, including the optical-feedback locking of quantum cascade lasers to high-finesse resonators and the application of electro-optic frequency combs and mode-locked lasers to direct, broadband spectroscopy.

August 14, 2023, 4:00 PM Prof. Ágnes Vibók, University of Debrecen, Hungary Controlling Cavity-Induced Non-Adiabatic Properties in Molecular Systems (Host: Tom Weinacht)

Molecular cavity quantum electrodynamics aims at studying and understanding the interaction of confined electromagnetic field modes with molecules. The coupling between photons and molecules gives rise to mixed light-matter states which are called polaritons carrying both photonic and molecular features. Experimental and theoretical works have demonstrated that polaritonic states can dramatically alter physical and chemical properties of molecular systems.. Among other effects, strong coupling can enhance charge and energy transfer, modify absorption spectra and give rise to strong nonadiabatic effects in molecules by providing ultrafast non-radiative decay channels. Results are presented how the strong non-adiabatic effect created by quantum light can modify and control the different topological, spectroscopic and dynamical properties of molecules.

August 15, 2023, 4:00 PM Prof. Dr. Thomas Baumert, University of Kassel Molecular Chirality in Light of Multi-Photon-Ionization on Different Time Scales (Host: Tom Weinacht)

Exploiting an electric dipole effect in ionization [1], photoelectron circular dichroism (PECD) is a highly sensitive enantioselective spectroscopy for studying chiral molecules in the gas phase using either single-photon ionization [2] or multiphoton ionization [3]. In the latter case resonance enhanced multiphoton ionization (REMPI) gives access to neutral electronic excited states. The PECD sensitivity opens the door to study control of the coupled electron and nuclear motion in enantiomers. A prerequisite is a detailed understanding of PECD in REMPI schemes. In this contribution I will report on our investigations on PECD with coherent light sources whose pulse durations span from femtoseconds to nanoseconds [4]. By this we address impulsive excitation on the femtosecond time scale to highly vibrational state selective excitation in mixtures with the help of high resolution nanosecond laser techniques [5]. The reflection of the number of absorbed photons in the PECD [6] will be discussed as well as subcycle effects in bichromatic fields [7]. [1] Ritchie, B. Theory of the angular distribution of photoelectrons ejected from optically active molecules and molecular negative ions. Phys. Rev. A 13, 1411–1415 (1976). [2] Böwering, N., Lischke, T., Schmidtke, B., Müller, N., Khalil, T. and Heinzmann, U. Asymmetry in Photoelectron Emission from Chiral Molecules Induced by Circularly Polarized Light. Phys. Rev. Lett. 86, 1187 (2001). [3] Lux, C., Wollenhaupt, M., Bolze, T., Liang, Q., Köhler, J., Sarpe, C. and Baumert, T. Circular dichroism in the photoelectron angular distributions of camphor and fenchone from multiphoton ionization with femtosecond laser pulses. Angew. Chem. Int. Ed. 51, 5001–5005 (2012). [4] Lee, H.-g., Ranecky, S. T., Vasudevan, S., Ladda, N., Rosen, T., Das, S., Ghosh, J., Braun, H., Reich, D. M., Senftleben, A. and Baumert, T. Pulse length dependence of photoelectron circular dichroism. Phys. Chem. Chem. Phys. 24, 27483–27494 (2022). [5] Ranecky, S. T., Park, G. B., Samartzis, P. C., Giannakidis, I. C., Schwarzer, D., Senftleben, A., Baumert, T. and Schäfer, T. Detecting chirality in mixtures using nanosecond photoelectron circular dichroism. Phys. Chem. Chem. Phys. 24, 2758–2761 (2022). [6] Lux, C., Senftleben, A., Sarpe, C., Wollenhaupt, M. and Baumert, T. Photoelectron circular dichroism observed in the above-threshold ionization signal from chiral molecules with femtosecond laser pulses. J. Phys. B: At. Mol. Opt. Phys. 49, 02LT01 (2016). [7] Demekhin, P. V., Artemyev, A. N., Kastner, A. and Baumert, T. Photoelectron Circular Dichroism with Two Overlapping Laser Pulses of Carrier Frequencies ω and 2ω Linearly Polarized in Two Mutually Orthogonal Directions. Phys. Rev. Lett. 121, 253201 (2018).

2023