arXiv subjects

D. Fischer

Publications and source records attributed to D. Fischer.

At least 19 recordsLinked to original sources

arXiv2026

Resonant photoionization dynamics during optical trapping of lithium atoms

Photoionization induced by trapping and auxiliary laser fields is an inherent feature of many laser-cooling and optical trapping experiments, yet its microscopic dynamics are rarely investigated directly. In this work, we employ a reaction microscope implementing an event-by-event photoionization time retrieval technique to extend momentum-resolved photoelectron spectroscopy to continuous-wave laser--atom interactions. We investigate low-intensity multicolor photoionization of laser-cooled lithium atoms confined in an all-optical trap. Complete three-dimensional electron momentum distributions and kinetic-energy spectra recorded for different laser wavelengths and polarization configurations identify resonant excitation of the $5p$ and $5f$ states and reveal an additional ionization channel following spontaneous decay from the $5f$ to the $4d$ state. A pronounced polarization dependence of the photoionization yield is explained by magnetic-sublevel selection rules and the coherent interference of different virtual excitation pathways. These results demonstrate that reaction microscopy combined with event-by-event time reconstruction provides a powerful approach for investigating microscopic electronic dynamics in laser-cooled atomic systems and offers new insight into photoionization processes occurring during optical trapping.

physics.atom-ph↗

arXiv2023

A pump-probe experiment in cw-mode on ionization of Rydberg atoms

Rydberg atoms are in the focus of intense research due to the peculiar properties which make them interesting candidates for quantum optics and quantum information applications. In this work we study the ionization of Rydberg atoms due to their interaction with a trapping laser field, and a reaction microscope is used to measure photoelectron angular and energy distributions. Reaction microscopes are excellent tools when brandished against atomic photoionization processes involving pulsed lasers; the timing tied to each pulse is crucial in solving the subsequent equations of motion for the atomic fragments in the spectrometer field. However, when used in pump-probe schemes, which rely on continuous wave probe lasers, vital information linked to the time of flight is lost. This study reports on a method in which the standard ReMi technique is extended in time through coincidence measurements. This is then applied to the photoionization of $^6$Li atoms initially prepared in optically pumped $2^{2}S_{1/2}$ and $2^{2}P_{3/2}$ states. Multi-photon excitation from a tunable femtosecond laser is exploited to produce Rydberg atoms inside an infrared optical dipole trap; the structure and dynamics of the subsequent cascade back towards ground is evaluated.

physics.atom-ph↗

arXiv2022

The HD 260655 system: Two rocky worlds transiting a bright M dwarf at 10 pc

We report the discovery of a multi-planetary system transiting the M0 V dwarf HD 260655 (GJ 239, TOI-4599). The system consists of at least two transiting planets, namely HD 260655 b, with a period of 2.77 d, a radius of R$_b$ = 1.240$\pm$0.023 R$_\oplus$, a mass of M$_b$ = 2.14$\pm$0.34 M$_\oplus$, and a bulk density of $\rho_b$ = 6.2$\pm$1.0 g cm$^{-3}$, and HD 260655 c, with a period of 5.71 d, a radius of R$_c$ = 1.533$^{+0.051}_{-0.046}$ R$_\oplus$, a mass of M$_c$ = 3.09$\pm$0.48 M$_\oplus$, and a bulk density of $\rho_c$ = 4.7$^{+0.9}_{-0.8}$ g cm$^{-3}$. The planets were detected in transit by the TESS mission and confirmed independently with archival and new precise radial velocities obtained with the HIRES and CARMENES instruments since 1998 and 2016, respectively. At a distance of 10 pc, HD 260655 becomes the fourth closest known multi-transiting planet system after HD 219134, LTT 1445 A, and AU Mic. Due to the apparent brightness of the host star (J = 6.7 mag), both planets are among the most suitable rocky worlds known today for atmospheric studies with the JWST, both in transmission and emission.

astro-ph.EP↗

arXiv2022

Two-path interference in the resonance-enhanced few-photon ionization of atoms

We investigate the resonance-enhanced few-photon ionization of atomic lithium by linearly polarized light whose frequency is tuned near the 2s-2p transition. Considering the direction of light polarization orthogonal to the quantization axis, the process can be viewed as an atomic "double-slit experiment" where the 2p states with magnetic quantum numbers m_l=+-1 act as the slits. In our experiment, we can virtually close one of the two slits by preparing lithium in one of the two circularly polarized 2p states before subjecting it to the ionizing radiation. This allows us to extract the interference term between the two pathways and obtain complex phase information on the final state. The experimental results show very good agreement with numerical solutions of the time-dependent Schroedinger equation. The validity of the two-slit model is also analyzed theoretically using a time-dependent perturbative approach.

physics.atom-ph↗

arXiv2021

Magnetic Dichroism in Few-Photon Ionization of Polarized Atoms

We consider few-photon ionization of atomic lithium by linearly polarized femtosecond laser pulses, and demonstrate that asymmetries of the electron angular distribution can occur for initially polarized (2p, m=+1) target atoms. The dependence of the photoelectron emission angle relative to the electric field direction is investigated at different laser intensities and wavelengths. The experimental spectra show excellent agreement with numerical solutions of the time-dependent Schroedinger equation. In the perturbative picture, the angular shift is traced back to interferences between partial waves with mean magnetic quantum number $\ne$0. This observation allows us to obtain quantum mechanical information on the electronic final state.

physics.atom-ph↗

arXiv2021

Multi point analysis of coronal mass ejection flux ropes using combined data from Solar Orbiter, BepiColombo and Wind

The recent launch of Solar Orbiter and BepiColombo opened a brief window in which these two spacecraft were positioned in a constellation that allows for the detailed sampling of any Earth-directed CMEs. Fortunately, two such events occurred with in situ detections of an ICME by Solar Orbiter on the 19th of April and the 28th of May 2020. These two events were subsequently also observed in situ by BepiColombo and Wind around a day later. We attempt to reconstruct the observed in situ magnetic field measurements for all three spacecraft simultaneously using an empirical magnetic flux rope model. This allows us to test the validity of our flux rope model on a larger and more global scale and allows for cross-validation of the analysis with different spacecraft combinations. Finally, we can also compare the results from the in situ modeling to remote observations obtained from the STEREO-A heliospheric imagers. We make use of the 3D coronal rope ejection model in order to simulate the ICME evolution. We adapt a previously developed ABC-SMC fitting algorithm for the application to multi point scenarios. We show that we are able to generally reconstruct the flux ropes signatures at three different spacecraft positions simultaneously using our model in combination with the flux rope fitting algorithm. For the well-behaved 19th of April ICME our approach works very well. The 28th of May ICME, on the other hand, shows the limitations of our approach. Unfortunately, the usage of multi-point observations for these events does not appear to solve inherent issues, such as the estimation of the magnetic field twist or flux rope aspect-ratios due to the specific constellation of the spacecraft positions. As our general approach can be used for any fast forward simulation-based model we give a blueprint for future studies using more advanced ICME models.

astro-ph.SR↗

arXiv2021

Circular Dichroism in Atomic Resonance-Enhanced Few-Photon Ionization

We investigate few-photon ionization of lithium atoms prepared in the polarized 2$p$($m_\ell=\!+1$) state when subjected to femtosecond light pulses with left- or right-handed circular polarization at wavelengths between 665 nm and 920 nm. We consider whether ionization proceeds more favorably for the electric field co- or counter-rotating with the initial electronic current density. Strong asymmetries are found and quantitatively analyzed in terms of "circular dichroism" ($CD$). While the intensity dependence of the measured $CD$ values is rather weak throughout the investigated regime, a very strong sensitivity on the center wavelength of the incoming radiation is observed. While the co-rotating situation overall prevails, the counter-rotating geometry is strongly favored around 800 nm due to the 2$p$-3$s$ resonant transition, which can only be driven by counter-rotating fields. The observed features provide insights into the helicity dependence of light-atom interactions, and on the possible control of electron emission in atomic few-photon ionization by polarization-selective resonance enhancement.

physics.atom-ph↗

arXiv2020

In situ multi-spacecraft and remote imaging observations of the first CME detected by Solar Orbiter and BepiColombo

On 2020 April 19 a coronal mass ejection (CME) was detected in situ by Solar Orbiter at a heliocentric distance of about 0.8 AU. The CME was later observed in situ on April 20th by the Wind and BepiColombo spacecraft whilst BepiColombo was located very close to Earth. This CME presents a good opportunity for a triple radial alignment study, as the spacecraft were separated by less than 5$^\circ$ in longitude. The source of the CME, which was launched on April 15th, was an almost entirely isolated streamer blowout. STEREO-A observed the event remotely from -75.1$^\circ$ longitude, which is an exceptionally well suited viewpoint for heliospheric imaging of an Earth directed CME. The configuration of the four spacecraft has provided an exceptionally clean link between remote imaging and in situ observations of the CME. We have used the in situ observations of the CME at Solar Orbiter, Wind, and BepiColombo, and the remote observations of the CME at STEREO-A in combination with flux rope models to determine the global shape of the CME and its evolution as it propagated through the inner heliosphere. A clear flattening of the CME cross-section has been observed by STEREO-A, and further confirmed by comparing profiles of the flux rope models to the in situ data, where the distorted flux rope cross-section qualitatively agrees most with in situ observations of the magnetic field at Solar Orbiter. Comparing in situ observations of the magnetic field between spacecraft, we find that the dependence of the maximum (mean) magnetic field strength decreases with heliocentric distance as $r^{-1.24 \pm 0.50}$ ($r^{-1.12 \pm 0.14}$), in disagreement with previous studies. Further assessment of the axial and poloidal magnetic field strength dependencies suggests that the expansion of the CME is likely neither self-similar nor cylindrically symmetric.

physics.space-ph↗

arXiv2020

Using circular dichroism to control energy transfer in multi-photon ionization

Chirality causes symmetry breaks in a large variety of natural phenomena ranging from particle physics to biochemistry. We investigate one of the simplest conceivable chiral systems, a laser-excited, oriented, effective one-electron Li target. Prepared in a polarized p state with |m|=1 in an optical trap, the atoms are exposed to co- and counter-rotating circularly polarized femtosecond laser pulses. For a field frequency near the excitation energy of the oriented initial state, a strong circular dichroism is observed and the photoelectron energies are significantly affected by the helicity-dependent Autler-Townes splitting. Besides its fundamental relevance, this system is suited to create spin-polarized electron pulses with a reversible switch on a femtosecond timescale at an energy resolution of a few meV.

physics.atom-ph↗

arXiv2019

Photo-ionization of polarized lithium atoms out of an all-optical atom trap: A complete experiment

An all-optical, near-resonant laser atom trap is used to prepare an electronically excited and polarized gas target at mK-temperature for complete photo-ionization studies. As a proof-of-principal experiment, lithium atoms in the 2$^2$P$_{3/2}$($m_l$=+1) state are ionized by a 266 nm laser source, and emitted electrons and Li$^+$ ions are momentum analyzed in a COLTRIMS spectrometer. The excellent resolution achieved in the present experiment allows not only to extract the relative phase and amplitude of all partial waves contributing to the final state, it also enables to characterize the experiment regarding target and spectrometer properties. Photo-electron angular distributions are measured for five different laser polarizations and described in a one-electron approximation with excellent agreement.

physics.atom-ph↗

arXiv2019

On the deviation from Maxwellian of the ion velocity distribution functions in the turbulent magnetosheath

The degree of deviation from the thermodynamic equilibrium in the ion velocity distribution functions (VDFs), measured by the Magnetospheric Multiscale (MMS) mission in the Earth's turbulent magnetosheath, is quantitatively investigated. Taking advantage of MMS ion data, having a resolution never reached before in space missions, and of the comparison with Vlasov-Maxwell simulations, this analysis aims at relating any deviation from Maxwellian equilibrium to typical plasma parameters. Correlations of the non-Maxwellian features with plasma quantities such as electric fields, ion temperature, current density and ion vorticity are very similar in both magnetosheath data and numerical experiments, and suggest that distortions in the ion VDFs occur close to (but not exactly at) peaks in current density and ion temperature. Similar results have also been found during a magnetopause crossing by MMS. This work could help clarifying the origin of distortion of the ion VDFs in space plasmas.

physics.space-ph↗

arXiv2018

The Fluxgate-Searchcoil Merged (FSM) Magnetic Field Data Product for MMS

The Fluxgate-Searchcoil Merged (FSM) data product for the Magnetospheric Multiscale (MMS) mission is created by combining the level-2 science quality data from the fluxgate and searchcoil magnetometers into a single level-3 data product. The merging method involves noise floor and calibration parameters determined both on the pre- and post-flight. Here, we describe the statistical inter-calibration process as well as the merging filter.

physics.space-ph↗

arXiv2017

An all-optical atom trap as a target for MOTRIMS-like collision experiments

Momentum-resolved scattering experiments with laser-cooled atomic targets have been performed since almost two decades with MOTRIMS (Magneto-Optical Trap Recoil Ion Momentum Spectroscopy) setups. Compared to experiments with gas-jet targets, MOTRIMS features significantly lower target temperatures allowing for an excellent recoil ion momentum resolution. However, the coincident and momentum-resolved detection of electrons was long rendered impossible due to incompatible magnetic field requirements. Here we report on a novel experimental approach which is based on an all-optical $^6$Li atom trap that -- in contrast to magneto-optical traps -- does not require magnetic field gradients in the trapping region. Atom temperatures of about 2 mK and number densities up to 10$^9$ cm$^{-3}$ make this trap ideally suited for momentum-resolved electron-ion coincidence experiments. The overall configuration of the new trap is very similar to conventional magneto-optical traps. It mainly requires small modifications of laser beam geometries and polarization which makes it easily implementable in other existing MOTRIMS experiments.

physics.atom-ph↗

arXiv2017

Electron scale structures and magnetic reconnection signatures in the turbulent magnetosheath

Collisionless space plasma turbulence can generate reconnecting thin current sheets as suggested by recent results of numerical magnetohydrodynamic simulations. The MMS mission provides the first serious opportunity to check if small ion-electron-scale reconnection, generated by turbulence, resembles the reconnection events frequently observed in the magnetotail or at the magnetopause. Here we investigate field and particle observations obtained by the MMS fleet in the turbulent terrestrial magnetosheath behind quasi-parallel bow shock geometry. We observe multiple small-scale current sheets during the event and present a detailed look of one of the detected structures. The emergence of thin current sheets can lead to electron scale structures where ions are demagnetized. Within the selected structure we see signatures of ion demagnetization, electron jets, electron heating and agyrotropy suggesting that MMS spacecraft observe reconnection at these scales.

physics.space-ph↗

arXiv2016

EXPRES: A Next Generation RV Spectrograph in the Search for Earth-like Worlds

The EXtreme PREcision Spectrograph (EXPRES) is an optical fiber fed echelle instrument being designed and built at the Yale Exoplanet Laboratory to be installed on the 4.3-meter Discovery Channel Telescope operated by Lowell Observatory. The primary science driver for EXPRES is to detect Earth-like worlds around Sun-like stars. With this in mind, we are designing the spectrograph to have an instrumental precision of 15 cm/s so that the on-sky measurement precision (that includes modeling for RV noise from the star) can reach to better than 30 cm/s. This goal places challenging requirements on every aspect of the instrument development, including optomechanical design, environmental control, image stabilization, wavelength calibration, and data analysis. In this paper we describe our error budget, and instrument optomechanical design.

astro-ph.IM↗

arXiv2013

Kepler-62: A five-planet system with planets of 1.4 and 1.6 Earth radii in the Habitable Zone

We present the detection of five planets -- Kepler-62b, c, d, e, and f -- of size 1.31, 0.54, 1.95, 1.61 and 1.41 Earth radii, orbiting a K2V star at periods of 5.7, 12.4, 18.2, 122.4 and 267.3 days, respectively. The outermost planets (Kepler-62e & -62f) are super-Earth-size (1.25 < planet radius/earth radius < 2.0) planets in the habitable zone (HZ) of their host star, receiving 1.2 +- 0.2 and 0.41 +- 0.05 times the solar flux at Earth's orbit. Theoretical models of Kepler-62e and -62f for a stellar age of ~7 Gyr suggest that both planets could be solid: either with a rocky composition or composed of mostly solid water in their bulk.

astro-ph.EP↗

arXiv2012

Seismic evidence for a rapidly rotating core in a lower-giant-branch star observed with Kepler

Rotation is expected to have an important influence on the structure and the evolution of stars. However, the mechanisms of angular momentum transport in stars remain theoretically uncertain and very complex to take into account in stellar models. To achieve a better understanding of these processes, we desperately need observational constraints on the internal rotation of stars, which until very recently were restricted to the Sun. In this paper, we report the detection of mixed modes - i.e. modes that behave both as g modes in the core and as p modes in the envelope - in the spectrum of the early red giant KIC7341231, which was observed during one year with the Kepler spacecraft. By performing an analysis of the oscillation spectrum of the star, we show that its non-radial modes are clearly split by stellar rotation and we are able to determine precisely the rotational splittings of 18 modes. We then find a stellar model that reproduces very well the observed atmospheric and seismic properties of the star. We use this model to perform inversions of the internal rotation profile of the star, which enables us to show that the core of the star is rotating at least five times faster than the envelope. This will shed new light on the processes of transport of angular momentum in stars. In particular, this result can be used to place constraints on the angular momentum coupling between the core and the envelope of early red giants, which could help us discriminate between the theories that have been proposed over the last decades.

astro-ph.SR↗

arXiv2012

Ion-lithium collision dynamics studied with an in-ring MOTReMi

We present a novel experimental tool allowing for kinematically complete studies of break-up processes of laser-cooled atoms. This apparatus, the 'MOTReMi', is a combination of a magneto-optical trap (MOT) and a Reaction Microscope (ReMi). Operated in an ion-storage ring, the new setup enables to study the dynamics in swift ion-atom collisions on an unprecedented level of precision and detail. In first experiments on collisions with 1.5 MeV/amu O$^{8+}$-Li the pure ionization of the valence electron as well as ionization-excitation of the lithium target has been investigated.

physics.atom-ph↗