Quantum Initiative

Prof. Anh-Thu Le, Department of Physics, University of Connecticut

Following electron-nuclear dynamics with ultrafast intense lasers

Recent progress in laser technology has led to new coherent light sources that can be used to investigate ultrafast processes in matter. To take advantage of these new light sources, different experimental techniques have been developed to reveal the inner-workings of coupled electron-nuclear dynamics in molecules. Concurrently, theoretical and computational tools have also been developed to understand and decode hidden information from experimental measurements. In this talk, I will present our group’s recent progress in understanding intense laser-atom/molecule interactions by using some of the most promising techniques such as laser-induced electron diffraction, high-harmonic generation spectroscopy, and attosecond transient absorption spectroscopy. I will also address the challenges and opportunities in this field for practical realization of molecular “movies” with atomic resolution in space and time that can provide new insights into fundamental chemical reactions.

Dr. Jakob den Brok, Harvard Smithsonian Center for Astrophysics

Title and abstract TBA

Prof. Philip Mannheim, University of Connecticut

The Accelerating Universe

I will describe some of the background that led to the award of the Nobel prize to Dr. Adam Riess, who will be our 2024 Katzenstein speaker on November 15.

Prof. Luca Argenti, University of Central Florida

ASTRA: A Transition-Density-Matrix Approach to Time-Resolved Molecular Ionization

Attosecond science, which investigates the time-resolved correlated motion of electrons in atoms, molecules, and solids, is rapidly advancing toward larger molecular systems and more complex processes, such as multiple ionization and molecular fragmentation. Theoretical methods capable of addressing both multiple excitations and photofragment entanglement are essential to capture these phenomena. Among the most promising theoretical approaches are ab initio wave-function-based close-coupling (CC) methods, increasingly adopted by the AMO community.

Despite significant progress from codes like XCHEM [1,2], tRecX [3], RMT [4], and UKRmol+ [5], scaling remains a major challenge – whether in handling ionic correlation, accounting for many atoms, or for distant fragments. To address these limitations, we developed ASTRA [6] (AttoSecond TRAnsitions), an ab initio CC molecular ionization code based on high-order transition density matrices between correlated ionic states of arbitrary multiplicity [7], and hybrid Gaussian-B-spline integrals [5,9]. ASTRA integrates multiple state-of-the-art codes, such as DALTON [8], a general-purpose quantum chemistry code, LUCIA [7], a large-scale CI code, and GBTOlib [5], a hybrid integral library suited for slow photoelectrons and comparatively small molecules.

ASTRA has successfully reproduced total and partial photoionization cross sections, photoemission asymmetry parameters, and molecular-frame photoelectron angular distributions for molecules such as N ₂ , CO, H ₂ CO, and Pyrazine, showing excellent agreement with existing benchmarks. Currently, ASTRA is being applied to study attosecond transient absorption spectra of CO and O ₂ , as well as sequential XUV-pump IR-probe ionization of C ₂ H ₄ . Its formalism naturally extends to molecular double ionization and can efficiently model electron exchange between multiple disjoint molecular fragments − relevant for describing ionization in weakly bound clusters like (H ₂ O) _n .

Looking ahead, continued integration with tools tailored to high-energy photoemission, non-adiabatic nuclear dynamics, and strong fields ionization will be critical for addressing emerging challenges in ultrafast many-body dynamics. Free-electron lasers enable time-resolved studies of core ionization, while table-top attosecond pump-probe experiments are targeting increasingly larger molecules, monitoring both electron dynamics and nuclear rearrangements throughout chemical reactions with intense probe pulses [10]. To reproduce these complex experiments, we are collaborating with NIST to replace GBTOlib with a more efficient hybrid library capable of handling larger molecules and higher orbital angular momenta. We are also pairing ASTRA with surface-hopping methods [11], where multiphoton ionization is typically not available. Additionally, to track the asymptotic evolution of weakly coupled photofragments under strong light fields − without incurring prohibitive computational costs − we are considering integrating separate optimized propagators for each fragment, which will open the door for us to simulate strong-field multichannel molecular-ionization processes.

[1] M. Klinker et al., J. Phys. Chem. Lett. 9, 756 (2018).

[2] V. J. Borràs et al., Science Advances 9, eade3855 (2023).

[3] A. Scrinzi, Comput. Phys. Commun. 270, 108146 (2022).

[4] A. C. Brown et al., Comput. Phys. Commun. 250, 107062 (2020).

[5] Z. Masin et al., Comp. Phys. Commun. 249, 107092 (2020).

[6] J. M. Randazzo et al., Phys. Rev. Res. 5, 043115 (2023).

[7] J. Olsen et al., J. Chem. Phys. 89, 2185 (1988); ibid. 104, 8007 (1996).

[8] K. Aidas et al., Comp. Mol. Sci. 4, 269 (2014).

[9] H. Gharibnejad et al., Comp. Phys. Commun. 263, 107889 (2021).

[10] F. Vismarra et al., Nature Chemistry (2024).

[11] L. Fransén et al., J. Phys. Chem. A 128, 1457 (2024).

Madisyn Brooks, UConn

Title and abstract TBA