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Chinese Physics Letters

Angular-Momentum-Resolved Rydberg State Population of Argon Ions in Strong Laser Fields

Zhigang Zheng, Di Wang, Ming-Yue Wang, Li Wang, Xu Sun, Jing Zhao, Zhihui Lv, Xiaowei Wang, Zengxiu Zhao

Published Mar 25, 2025• 3 citations
Publisher

Abstract

Abstract This letter demonstrates an experimental approach to measuring the angular-momentum-resolved population of excited states in laser-produced argon ions. By measuring the spectra of free induction decay emissions corresponding to the resonant transitions between Rydberg states, the relative population of the Rydberg states is obtained with known Einstein A-coefficients. This study deepens the mechanistic understanding of coherent dynamics in laser-driven ionic excited states, and establishes experimental benchmarks essential for validating and refining advanced quantum kinetic models in strong-field physics.

DOI
10.1088/0256-307x/42/5/053702

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Physical Review A•Mar 25, 2013•1 citation
Abstract This letter demonstrates an experimental approach to measuring the angular-momentum-resolved population of excited states in laser-produced argon ions. By measuring the spectra of free induction decay emissions corresponding to the resonant transitions between Rydberg states, the relative population of the Rydberg states is obtained with known Einstein A-coefficients. This study deepens the mechanistic understanding of coherent dynamics in laser-driven ionic excited states, and establishes experimental benchmarks essential for validating and refining advanced quantum kinetic models in strong-field physics.
Abstract We measure spontaneous emission from Ar II ions produced by femtosecond strong-field excitation of argon and reconstruct state-resolved excited-state populations from selected emission lines spanning upper-level energies of ~19–23 eV above the Ar II ground state. The ionic-line intensities follow a power-law dependence on pulse energy and a non-monotonic dependence on gas pressure, and the extracted populations deviate significantly from a Boltzmann distribution. The deviation grows with increasing pulse energy, reflecting a stronger nonequilibrium character of the initial population prepared by the strong field, and weakens with increasing gas pressure, consistent with collisional redistribution driving the system toward thermal equilibrium. These results establish ionic fluorescence as a practical state-resolved probe of nonequilibrium population distributions in strong-field-ionized argon.
The AC Stark shift of electronic levels is ubiquitous in the interaction of intense light fields with atoms and molecules. As the light intensity changes on the rising and falling edges of a femtosecond laser pulse, it shifts the Rydberg states in and out of multiphoton resonances with the ground state. The two resonant pathways for transient excitation arising at the leading and the trailing edges of the pulse generate Young's type interference, generally referred to as the St\"uckelberg oscillations. Here we report the observation of the St\"uckelberg oscillations in the intensity of the coherent free-induction decay following resonant multiphoton excitation. Moreover, combining the experimental results with accurate numerical simulations and a simple model, we use the St\"uckelberg oscillations to recover the population dynamics of strongly driven Rydberg states inside the laser pulse by all-optical measurements after the end of the pulse. We demonstrate the potential of this spectroscopy to characterize lifetimes of Rydberg states dressed by laser fields with strengths far exceeding the Coulomb field between the Rydberg electron and the core.
There is considerable interest in the dynamics of ultracold plasmas and ultracold Rydberg gases. Ultracold plasmas are typically formed by photo-excitation of ultracold atoms to an energy region near (both above and below) an ionization threshold. Excitation to bound, highly-excited Rydberg states can lead to formation of a plasma via several processes, including collisions between Rydberg atoms. Three-body recombination in an ultracold plasma can also result in the production of ultracold Rydberg atoms. Understanding the dynamics of ultracold Rydberg gases is therefore important for understanding the dynamics of ultracold plasmas. In this dissertation, we have investigated the formation and survival of a particular class of Rydberg atoms. These atoms are known as ZEKE state Rydberg atoms, where the term ZEKE is derived from "Zero Kinetic Energy." ZEKE Rydberg states are high angular momentum and high angular momentum projection excited states which can be formed by laser excitation in the presence of electric fields. Without the electric fields, these states would be optically dark. We have investigated ZEKE Rydberg states in ultracold argon in an energy region just below the second ionization threshold of the atoms in our magneto-optical trap. Here, low angular momentum states decay very quickly (< 1 ns) by auto-ionization through a core spin flip due to the Rydberg electron-core interaction. This interaction has been significantly reduced due to the dilution of low l and m states as a result of l and m mixing during excitation. Hence, ZEKE Rydberg states live orders of magnitude longer than low angular momentum states. We are reporting on our experimental investigation of these states in the ultracold regime and prospects for future studies in which external control of these states may be used to control ultracold plasma dynamics.
Cold, dense Rydberg gases produced in a cold-atom trap are investigated using spectroscopic methods and time-resolved electron counting. Optical excitation on the discrete Rydberg resonances reveals long-lasting electron emission from the Rydberg gas ( >20 ms). Our observations are explained by lm-mixing collisions between Rydberg atoms and slow electrons that lead to the population of long-lived high-angular-momentum Rydberg states. These atoms thermally ionize slowly and with large probabilities.
Using field ionization combined with the direct detection of excited neutral atoms we measured the distribution of principal quantum number n of excited He Rydberg states after strong-field excitation at laser intensities well in the tunneling regime. Our results confirm theoretical predictions from semiclassical and quantum mechanical calculations and simultaneously underpin the validity of the semiclassical frustrated tunneling ionization model. Moreover, since our experimental detection scheme is spin sensitive in the case of He atoms, we show that strong-field excitation leads to strong population of triplet states. The origin of it lies in the fact that high angular momentum states are accessible in strong-field excitation. Thus, singlet-triplet transitions become possible due to the increased importance of spin-orbit interaction rather than due to direct laser induced spin-flip processes.
We report on our investigation of the formation and survival of long-lived Rydberg states in argon produced by pulsed laser excitation of ultracold metastable state argon atoms in a magneto-optical trap. The states studied have a 2P1/2{}^{2}{P}_{1/2}2P1/2​ core. Low angular momentum Rydberg states with this core normally autoionize rapidly. If, however, atoms are excited in the presence of electric fields, higher angular momentum states, traditionally termed ``ZEKE states'' (ZEKE is derived from ``zero kinetic energy'') can be formed. The lifetime of these states can be orders of magnitude greater than low angular momentum states. In this paper, we report on the time dependence of ZEKE Rydberg state population in an ultracold environment.