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Physica Scripta

Slowdown of the recombination of ultracold xenon plasma created by two -step photoionization of Xe-atoms

Н. И. Косарев

Published Jun 2, 2025• 0 citations
Publisher

Abstract

Abstract This paper discusses the results of a numerical study of the two-step photoionization kinetics of xenon atoms and plasma recombination. At initial electron temperatures T e ≥ 0.5 K during the pump laser ( λ 1 = 882 nm ) and the ionizing laser ( λ 2 = 514 nm ) action, the electrons are heated in superelastic collisions and in three-body recombination. As a result, the electron temperature increases by almost two order of magnitude. This causes the plasma recombination to slowdown, which fits well into the T e − 9 / 2 dependence of the three-body recombination coefficient. At times t ≥ 1 μ s , the electron temperature decreases due to the adiabatic expansion of the plasma droplet. The method of resonance fluorescence of the atomic line ( λ 1 = 882 nm ) allows extracting the temperature dependence of electrons on time.

DOI
10.1088/1402-4896/addf1b

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AIP conference proceedings•Jan 1, 2002•1 citation
Abstract This paper discusses the results of a numerical study of the two-step photoionization kinetics of xenon atoms and plasma recombination. At initial electron temperatures T e ≥ 0.5 K during the pump laser ( λ 1 = 882 nm ) and the ionizing laser ( λ 2 = 514 nm ) action, the electrons are heated in superelastic collisions and in three-body recombination. As a result, the electron temperature increases by almost two order of magnitude. This causes the plasma recombination to slowdown, which fits well into the T e − 9 / 2 dependence of the three-body recombination coefficient. At times t ≥ 1 μ s , the electron temperature decreases due to the adiabatic expansion of the plasma droplet. The method of resonance fluorescence of the atomic line ( λ 1 = 882 nm ) allows extracting the temperature dependence of electrons on time.
The kinetics of ionization and recombination of an ultracold barium plasma created in a two-step process, taking into account the transfer of resonant radiation in 3D cylindrical geometry, is studied by numerical simulation. At the first step, a pump laser excites the upper level of the resonant transition 6s2 S10↔6s6p P11 (λ1=553.5 nm). At the second step, the laser with quantum energy exceeding the ionization potential from the level 6s6p P11 (λ2=417.79 nm) ionizes the atoms. A scheme is proposed for increasing the efficiency of electron yield: at the second ionization step, the laser radiation with frequency corresponding to the continuum from the metastable D32 is used. The electron temperature from the initial value 0.1 K during the action of the pump and ionizing lasers increases by more than 200 times due to superelastic processes. As a result, the time of three-body recombination of plasma increases significantly. The results of numerical simulation indirectly confirm the fact of Killian et al. [Phys. Rev. Lett. 83(23), 4776 (1999)] that the deceleration of recombination of ultracold xenon plasma can be explained by the heating of electrons in superelastic quenching collisions.
Signals of ultracold plasma are observed by two-photon ionization of laser-cooled caesium atoms in a magneto-optical trap. Recombination of ions and electrons into Rydberg atoms during the expansion of ultracold plasma is investigated by using state-selective field ionization spectroscopy. The dependences of recombination on initial electron temperature (1–70 K) and initial ion density (∼10 10 cm −3 ) are investigated. The measured dependence on initial ion density is N 1.547+0.004 at a delay time of 5 μs. The recombination rate rapidly declines as initial electron temperature increases when delay time is increased. The distributions of Rydberg atoms on different values of principal quantum number n, i.e. n = 30–60, at an initial electron temperature of 3.3 K are also investigated. The main experimental results are approximately explained by the three-body recombination theory.
Three-body recombination, an important collisional process in plasmas, increases dramatically at low electron temperatures, with an accepted scaling of Te(-9/2). We measure three-body recombination in an ultracold neutral xenon plasma by detecting recombination-created Rydberg atoms using a microwave-ionization technique. With the accepted theory (expected to be applicable for weakly coupled plasmas) and our measured rates, we extract the plasma temperatures, which are in reasonable agreement with previous measurements early in the plasma lifetime. The resulting electron temperatures indicate that the plasma continues to cool to temperatures below 1 K.
By photoionizing a sample of laser-cooled xenon atoms, we create ultracold neutral plasmas with initial temperatures of 1–1000 K and densities as high as 1010 cm−3. The plasma is formed by the trapping of electrons by the residual positive charge that is left after some electrons initially leave the sample. We excite plasma oscillations with applied radio frequency fields and use this to monitor the expansion of the unconfined plasma. We have observed significant recombination of the plasma into Rydberg atoms (up to 20%). At these low temperatures, the only traditional form of recombination that could be significant is three-body recombination (TBR), but we see a number of trends in direct contradiction to what one would expect from classical TBR theory.
Summary form only given. We have observed recombination into a large range of Rydberg levels in an ultracold plasma. The plasma is created by two-photon ionization of laser-trapped and -cooled metastable xenon atoms. By changing the intensity and detuning of the ionizing laser pulse, the initial temperature and density of the plasma can be varied. This allows us to access plasma coupling parameters (???) between 0.1 and 5. In this range of values, we previously reported unexplained heating of the expanding cloud, between 10 and 100 ???s after creation of the plasma. The recently observed recombination appears to provide much of the extra energy responsible for this behavior. The recombined atoms, up to 20% of the created plasma, populate energy levels down to n=40. They survive for /spl sim/1 ms indicating that high angular momentum states are present. However, standard scaling laws for three-body recombination do not agree with the observed dynamics. Furthermore, Rydberg atom formation is incomplete at the time when the plasma expansion velocity has reached its maximum value. Various effects are under consideration as possible remedies to these inconsistencies.
By photoionizing a sample of laser‐cooled xenon atoms, we create ultracold neutral plasmas with initial temperatures of 1–1000 K and densities as high as 1010 cm−3. The plasma is formed by the trapping of electrons by the residual positive charge that is left after some electrons initially leave the sample. We excite plasma oscillations with applied radio frequency fields and use this to monitor the expansion of the unconfined plasma. We have observed significant recombination of the plasma into Rydberg atoms (up to 20 %). At these low temperatures, the only traditional form of recombination that could be significant is three‐body recombination (TBR). Recent theoretical studies of ultracold plasmas, and prospects for future experiments, are discussed.