arXiv subjects

Frank Jensen

Publications and source records attributed to Frank Jensen.

11 recordsLinked to original sources

arXiv2025

Laser-induced Coulomb explosion of the LiI molecule and of its dimer

A gas-phase sample consisting of lithium iodide, $\mathrm{LiI}$, molecules and their dimer $\mathrm{(LiI)}_2$, are Coulomb exploded by an intense 25 femtosecond laser pulse. In the case of $\mathrm{LiI}$, we focus on the double ionization that creates a pair of $\mathrm{Li}^+$ and $\mathrm{I}^+$ recoil ions. From the kinetic energy distribution of the $\mathrm{Li}^+$ ions, extracted using coincidence filtering, we determine the distribution of internuclear distances $P(R)$ via the ground state potential curve of $\mathrm{LiI}^{2+}$ obtained from an ab initio calculation that accounts for non-Coulombic effects. We find that the center of $P(R)$ is close to the expected internuclear separation based on the three vibrational states of $\mathrm{LiI}$ populated, whereas the width of $P(R)$ exceeds the theoretical value by $\sim$ 52 %. We discuss if fragmentation via excited $\mathrm{LiI}^{2+}$ potential curves affects the determination of $P(R)$. In the case of the dimer, $\mathrm{(LiI)}_2$, we observe kinetic energies and relative emission directions of $\mathrm{Li}^+$, $\mathrm{I}^+$, and $\mathrm{I}^{2+}$ recoil ions consistent with Coulomb explosion of the parallelogram-shaped dimer after removing up to six electrons by the laser pulse.

physics.atom-ph↗

arXiv2025

Nonadiabatic laser-induced alignment dynamics of alkali dimers on the surface of a helium droplet

Alkali dimers, $\mathrm{Ak}_2$, located on the surface of a helium nanodroplet, are set into rotation through the polarizability interaction with a nonresonant 1-ps-long laser pulse. The time-dependent degree of alignment is recorded using femtosecond-probe-pulse-induced Coulomb explosion into a pair of $\mathrm{Ak}^+$ fragment ions. The results, obtained for $\mathrm{Na}_2$, $\mathrm{K}_2$, and $\mathrm{Rb}_2$ in both the ground state $1 ^1\Sigma_g^+$ and the lowest-lying triplet state $1 ^3\Sigma_u^+$, exhibit distinct, periodic revivals with a gradually decreasing amplitude. The dynamics differ from that expected for dimers had they behaved as free rotors. Numerically, we solve the time-dependent rotational Schr\"odinger equation, including an effective mean-field potential to describe the interaction between the dimer and the droplet. The experimental and simulated alignment dynamics agree well and their comparison enables us to determine the effective rotational constants of the alkali dimers with the exception of $\mathrm{Rb}_2$($1 ^3\Sigma_u^+$) that only exhibits a prompt alignment peak but no subsequent revivals. For $\mathrm{Na}_2$($1 ^3\Sigma_u^+$), $\mathrm{K}_2$($1 ^1\Sigma_g^+$), $\mathrm{K}_2$($1 ^3\Sigma_u^+$) and $\mathrm{Rb}_2$($1 ^1\Sigma_g^+$), the alignment dynamics are well-described by a 2D rotor model. We ascribe this to a significant confinement of the internuclear axis of these dimers, induced by the orientation-dependent droplet-dimer interaction, to the tangential plane of their residence point on the droplet.

physics.atm-clus↗

arXiv2024

Calculating the energy profile of an enzymatic reaction on a quantum computer

Quantum computing (QC) provides a promising avenue toward enabling quantum chemistry calculations, which are classically impossible due to a computational complexity that increases exponentially with system size. As fully fault-tolerant algorithms and hardware, for which an exponential speedup is predicted, are currently out of reach, recent research efforts are dedicated to developing and scaling algorithms for Noisy Intermediate-Scale Quantum (NISQ) devices to showcase the practical utility of such machines. To demonstrate the utility of NISQ devices in the field of chemistry, we apply our recently developed FAST-VQE algorithm and a novel quantum gate reduction strategy based on propositional satisfiability together with standard optimization tools for the simulation of the rate-determining proton transfer step for CO2 hydration catalysed by carbonic anhydrase resulting in the first application of a quantum computing device for the simulation of an enzymatic reaction. To this end, we have combined classical force field simulations with quantum mechanical methods on classical and quantum computers in a hybrid calculation approach. The presented technique significantly enhances the accuracy and capabilities of QC-based molecular modeling and finally pushes it into compelling and realistic applications. The framework is general and can be applied beyond the case of computational enzymology.

quant-ph↗

arXiv2023

Nonadiabatic Laser-Induced Alignment Dynamics of Molecules on a Surface

We demonstrate that a sodium dimer, Na$_{2}$($1^3Σ_{u}^+$), residing on the surface of a helium nanodroplet, can be set into rotation by a nonresonant 1.0 ps infrared laser pulse. The time-dependent degree of alignment measured, exhibits a periodic, gradually decreasing structure that deviates qualitatively from that expected for gas phase dimers. Comparison to alignment dynamics calculated from the time-dependent rotational Schrödinger equation shows that the deviation is due to the alignment dependent interaction between the dimer and the droplet surface. This interaction confines the dimer to the tangential plane of the droplet surface at the point where it resides and is the reason that the observed alignment dynamics is also well-described by a 2D quantum rotor model.

physics.atm-clus↗

arXiv2022

Laser-induced Coulomb Explosion Imaging of Alkali Dimers on Helium Nanodroplets

Alkali dimers, $\mathrm{Ak}_2$, residing on the surface of He nanodroplets are doubly ionized due to multiphoton absorption from an intense, 50-fs laser pulse leading to fragmentation into a pair of alkali cations. Based on the measured kinetic energy distributions, $P(E_{\text{kin}})$, of the $\mathrm{Ak}^+$ fragment ions, we retrieve the distribution of internuclear distances, $P(R)$, via the $\mathrm{Ak}_2^{2+}$ potential curve. Results are obtained for $\mathrm{{Na}_2}$, $\mathrm{K}_2$, $\mathrm{Rb}_2$, and $\mathrm{Cs}_2$ in both the 1 $^1Σ_{g}^+$ ground state and in the lowest-lying triplet state 1 $^3Σ_{u}^+$, and for $\mathrm{Li}_2$ in the 1 $^3Σ_{u}^+$ state. For $\mathrm{Li}_2$, $\mathrm{K}_2$, and $\mathrm{Rb}_2$, the center of the measured $P(R)$'s is close to the center of the wave function, $Ψ(R)$, of the vibrational ground state in the 1 $^1Σ_{g}^+$ and 1 $^3Σ_{u}^+$ states, whereas for $\mathrm{{Na}_2}$ and $\mathrm{{Cs}_2}$ small shifts are observed. For all the $\mathrm{{Ak}_2}$, the width of the measured $P(R)$ is broader than $|Ψ(R)|^2$ by a factor of 2-4. We discuss that resonance effects in the multiphoton ionization and interaction of the $\mathrm{Ak}^+$ ion with the He droplet give rise to the observed deviations of $P(R)$ from $|Ψ(R)|^2$. Despite these deviations, we deem that timed Coulomb explosion will allow imaging of vibrational wave packets in alkali dimers on He droplets surfaces.

physics.atom-ph↗

arXiv2017

How Large is the Elephant in the Density Functional Theory Room?

A recent paper compares density functional theory results for atomization energies and dipole moments using a multi-wavelet based method with traditional Gaussian basis set results, and concludes that Gaussian basis sets are problematic for achieving high accuracy. We show that by a proper choice of Gaussian basis sets they are capable of achieving essentially the same accuracy as the multi-wavelet approach, and identify a couple of possible problems in the multi-wavelet calculations.

physics.comp-ph↗

arXiv2013

Approximations in Bayesian Belief Universe for Knowledge Based Systems

When expert systems based on causal probabilistic networks (CPNs) reach a certain size and complexity, the "combinatorial explosion monster" tends to be present. We propose an approximation scheme that identifies rarely occurring cases and excludes these from being processed as ordinary cases in a CPN-based expert system. Depending on the topology and the probability distributions of the CPN, the numbers (representing probabilities of state combinations) in the underlying numerical representation can become very small. Annihilating these numbers and utilizing the resulting sparseness through data structuring techniques often results in several orders of magnitude of improvement in the consumption of computer resources. Bounds on the errors introduced into a CPN-based expert system through approximations are established. Finally, reports on empirical studies of applying the approximation scheme to a real-world CPN are given.

cs.AI↗

arXiv2013

Analysis in HUGIN of Data Conflict

After a brief introduction to causal probabilistic networks and the HUGIN approach, the problem of conflicting data is discussed. A measure of conflict is defined, and it is used in the medical diagnostic system MUNIN. Finally, it is discussed how to distinguish between conflicting data and a rare case.

cs.AI↗

arXiv2013

Optimal Junction Trees

The paper deals with optimality issues in connection with updating beliefs in networks. We address two processes: triangulation and construction of junction trees. In the first part, we give a simple algorithm for constructing an optimal junction tree from a triangulated network. In the second part, we argue that any exact method based on local calculations must either be less efficient than the junction tree method, or it has an optimality problem equivalent to that of triangulation.

cs.AI↗

arXiv2013

From Influence Diagrams to Junction Trees

We present an approach to the solution of decision problems formulated as influence diagrams. This approach involves a special triangulation of the underlying graph, the construction of a junction tree with special properties, and a message passing algorithm operating on the junction tree for computation of expected utilities and optimal decision policies.

cs.AI↗

arXiv2006

Efficient coherent internal state transfer in trapped ions using Stimulated Raman Adiabatic Passage

We demonstrate experimentally how the process of Stimulated Raman Adiabatic Passage (STIRAP) can be utilized for efficient coherent internal state transfer in single trapped and laser-cooled $^{40}$Ca$^+$ ions. The transfer from the D$_{3/2}$ to the D$_{5/2}$ state, is detected by a fluorescence measurement revealing the population not transfered to the D$_{5/2}$ state. A coherent population transfer efficiency at the level of 95 % in a setup allowing for the internal state detection of individual ions in a string has been obtained.

quant-ph↗