arXiv subjects

Yen-Cheng Lin

Publications and source records attributed to Yen-Cheng Lin.

5 recordsLinked to original sources

arXiv2025

Probing autoionization decay lifetimes of the $\mathbf{4d^{-1}6\boldsymbol{\ell}}$ core-excited states in xenon using attosecond noncollinear four-wave-mixing spectroscopy

The decay of core-excited states is a sensitive probe of autoionization dynamics and correlation effects in many-electron systems, occurring on the fastest timescales. Xenon, with its dense manifold of autoionizing resonances that can be coupled with near-infrared light, provides a platform to investigate these processes. In this work, the autoionization decay lifetimes of $4d^{-1}6\ell$ $(\ell = s, p, d, ...)$ core-excited states in xenon atoms are probed with extreme ultraviolet (XUV) attosecond noncollinear four-wave-mixing (FWM) spectroscopy. The $4d^{-1}_{\{5/2,\, 3/2\}}6p$ XUV-bright states (optically dipole allowed) exhibit decay lifetimes of $\sim$6 fs, which is consistent with spectator-type decay. In contrast, the $4d^{-1}_{\{5/2,\, 3/2\}}6s$ and $4d^{-1}_{\{5/2,\, 3/2\}}6d$ XUV-dark states (optically dipole forbidden) show longer decay lifetimes of $\sim$20 fs. Photoionization calculations confirm that all core-hole states with $4d$ character should decay via spectator channels in $\leq$ 6 fs, suggesting that the apparent longer dark state decay times arise from an alternative mechanism. A few-level simulation of the FWM process shows that the inclusion of a nearby, longer-lived dark state can mimic the experimental FWM signal, suggesting population cycling with a second electronic state with non-$4d$ character. Ab-initio calculations support the presence of such multi-electron excited states in the 60$-$70 eV range. These results demonstrate that FWM signals can encode coupled-state dynamics when probing complex systems, highlighting the importance of combining theoretical and experimental approaches to disentangle accurate core-level decay pathways and lifetimes.

physics.atom-ph↗

arXiv2024

Addressing bedload flux variability due to grain shape effects and experimental channel geometry

The study-to-study variability of bedload flux measurements in turbulent sediment transport borders an order of magnitude, even for idealized laboratory conditions. This uncertainty stems from physically poorly supported, empirical methods to account for channel geometry effects in the determination of the transport-driving bed shear stress, and from study-to-study grain-shape variations. Here, we derive a universal method of bed shear stress determination. It consists of a granular-physics-based definition of the bed surface and a channel sidewall correction based on linking Reynolds stress to bulk flow properties via Kolmogorov's theory of turbulence. Application of this method to bedload transport of spherical grains---to rule out grain-shape effects---collapses data from existing laboratory measurements and grain-resolved computation fluid dynamics (CFD) and discrete element method (DEM) simulations for various channel geometries onto a single curve. In contrast, classical sidewall corrections, as well as an alternative bed surface definition, are unable to universally capture these data, especially those from shallow or very narrow channel flows. We then apply our method to an extended grain-shape-controlled data compilation, complemented by literature data for non-spherical grains and from grain-unresolved CFD-DEM simulations. This compilation covers a very diverse range of transport conditions, ranging from very narrow to infinitely wide channels, from shallow to deep channel flows, from mild to steep bed slopes, and from weak to intense transport. We generalize an existing physical bedload flux model to account for grain-shape effects and show that it explains almost all the compiled data within a factor of only $1.3$.

physics.geo-ph↗

arXiv2022

Measuring autoionization decay lifetimes of optically forbidden inner valence excited states in neon atoms with attosecond noncollinear four wave mixing spectroscopy

Attosecond noncollinear four wave mixing spectroscopy with one attosecond extreme ultraviolet (XUV) pulse and two few-cycle near-infrared (NIR) pulses was used to measure the autoionization decay lifetimes of inner valence electronic excitations in neon atoms. After a 43-48 eV XUV photon excites a 2s electron into the 2s2p6[np] Rydberg series, broadband NIR pulses couple the 2s2p6[3p] XUV-bright state to neighboring 2s2p6[3s] and 2s2p6[3d] XUV-dark states. Controllable delays of one or both NIR pulses with respect to the attosecond XUV pulse reveal the temporal evolution of either the dark or bright states, respectively. Experimental lifetimes for the 3s, 3p, and 3d states are measured to be 7 +/- 2 fs, 48 +/- 8 fs, and 427 +/- 40 fs, respectively, with 95% confidence. Accompanying calculations with two independent ab initio theoretical methods, NewStock and ASTRA, verify the findings. The results support the expected trend that the autoionization lifetime should be longer for states that have a smaller penetration in the radial region of the 2s core hole, which in this case is for the higher angular momentum Rydberg orbitals. The underlying theory thus links the lifetime results to electron correlation and provides an assessment of the direct and exchange terms in the autoionization process.

physics.atom-ph↗

arXiv2022

Non-resonant Coherent Amplitude Transfer in Attosecond Four-Wave Mixing Spectroscopy

Attosecond four-wave mixing spectroscopy using an XUV pulse and two noncollinear near-infrared pulses is employed to measure Rydberg wavepacket dynamics resulting from extreme ultraviolet excitation of a 3s electron in atomic argon into a series of autoionizing 3s-1np Rydberg states around 29 eV. The emitted signals from individual Rydberg states exhibit oscillatory structure and persist well beyond the expected lifetimes of the emitting Rydberg states. These results reflect substantial contributions of longer-lived Rydberg states to the four wave mixing emission signals of each individually detected state. A wavepacket decomposition analysis reveals that coherent amplitude transfer occurs predominantly from photoexcited 3s-1(n+1)p states to the observed 3s-1np Rydberg states. The experimental observations are reproduced by time-dependent Schr\"odinger equation simulations using electronic structure and transition moment calculations. The theory highlights that coherent amplitude transfer is driven non-resonantly to the 3s-1np states by the near-infrared light through 3s-1(n+1)s and 3s-1(n-1)d dark states during the four-wave mixing process.

physics.atom-ph↗

arXiv2021

Solid-state core-exciton dynamics in NaCl observed by tabletop attosecond four-wave mixing spectroscopy

Nonlinear wave-mixing in solids with ultrafast x-rays can provide new insight into complex electronic dynamics of materials. Here, tabletop-based attosecond noncollinear four-wave mixing (FWM) spectroscopy using one extreme ultraviolet (XUV) pulse from high harmonic generation and two separately timed few-cycle near-infrared (NIR) pulses characterizes the dynamics of the Na+ L2,3 edge core-excitons in NaCl around 33.5 eV. An inhomogeneous distribution of core-excitons underlying the well-known doublet absorption of the Na+ \Gamma-point core-exciton spectrum is deconvoluted by the resonance-enhanced nonlinear wave-mixing spectroscopy. In addition, other dark excitonic states that are coupled to the XUV-allowed levels by the NIR pulses are characterized spectrally and temporally. Approximate sub-10 femtosecond coherence lifetimes of the core-exciton states are observed. The core-excitonic properties are discussed in the context of strong electron-hole exchange interactions, electron-electron correlation, and electron-phonon broadening. This investigation successfully indicates that tabletop attosecond FWM spectroscopies represent a viable technique for time-resolved solid-state measurements.

cond-mat.mtrl-sci↗