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Chinese Physics Letters

Ultracold Neutral Plasma Produced by One-Color Two-Photon Ionization in Supersonic Nitric Oxide Molecular Beam

Hui Li, Zhe Shi, Jinbo Liu, Jingwei Guo, Can‐Hua Zhou, Hongxing Cai, Xianglong Cai

Published Dec 1, 2013• 1 citation
Publisher

Abstract

Nitric oxide (NO) is cooled to 1 K in a seeded Ar supersonic molecular beam. NO molecules are ionized by one-color two-photon resonant enhanced multiphoton ionization (REMPI) process to form an ultracold plasma. The density of the ionized molecules in the illuminated volume approaches to 1.5 × 10 13 cm −3 . Prompt electrons, plasma electrons and intact NO + ions are produced during this process. Experimental results confirm that the lifetime of this ultracold plasma is longer than 18.3 μs. This is the first report of NO ultracold plasma with significant lifetime produced by one-color two-photon REMPI process.

DOI
10.1088/0256-307x/30/12/125201

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AIP conference proceedings•Jan 1, 1999•1 citation
Nitric oxide (NO) is cooled to 1 K in a seeded Ar supersonic molecular beam. NO molecules are ionized by one-color two-photon resonant enhanced multiphoton ionization (REMPI) process to form an ultracold plasma. The density of the ionized molecules in the illuminated volume approaches to 1.5 × 10 13 cm −3 . Prompt electrons, plasma electrons and intact NO + ions are produced during this process. Experimental results confirm that the lifetime of this ultracold plasma is longer than 18.3 μs. This is the first report of NO ultracold plasma with significant lifetime produced by one-color two-photon REMPI process.
We report the spontaneous formation of a plasma from a gas of cold Rydberg molecules. Double-resonant laser excitation promotes nitric oxide, cooled to 1 K in a seeded supersonic molecular beam, to single Rydberg states extending as deep as $80\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ below the lowest ionization threshold. The density of excited molecules in the illuminated volume approaches $1\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$. This population evolves to produce free electrons and a durable cold plasma of electrons and intact NO+{\mathrm{NO}}^{+}NO+ ions.
The double-resonant laser excitation of nitric oxide, cooled to 1 K in a seeded supersonic molecular beam, yields a gas of $\ensuremath{\approx}{10}^{12}$ molecules per cubic centimeter in a single selected Rydberg state. This population evolves to produce prompt free electrons, and a durable cold plasma of electrons and intact NO+{\text{NO}}^{+}NO+ ions. This plasma travels with the molecular beam through a field-free region to encounter a grid. The atomic weight of the expansion gas controls the beam velocity and hence the flight time from the interaction region to the grid. Monitoring electron production as the plasma traverses this grid measures its longitudinal width as a function of flight time. Comparing these widths to the width of the laser beam that defines the initial size of the illuminated volume allows us to gauge the rate of expansion of the plasma. We find that the plasma created from the evolution of a Rydberg gas of NO expands at a small but measurable rate, and that this rate of expansion accords with the Vlasov equations for an initial electron temperature of ${T}_{e}\ensuremath{\approx}7\text{ }\text{K}$.
Double-resonant photoexcitation of nitric oxide in a molecular beam creates a dense ensemble of 50f(2) Rydberg states, which evolves to form a plasma of free electrons trapped in the potential well of an NO + space charge. The plasma travels at the velocity of the molecular beam and, on passing through a grounded grid, yields an electron time-of-flight signal that gauges the plasma size and quantity of trapped electrons. This plasma expands at a rate that fits with an electron temperature as low as 5 K, colder than typically observed for atomic ultracold plasmas. The recombination of molecular NO + cations with electrons forms neutral molecules excited by more than twice the energy of the NO chemical bond, and the question arises whether neutral fragmentation plays a role in shaping the redistribution of energy and particle density that directs the short-time evolution from Rydberg gas to plasma. To explore this question, we adapt a coupled rate-equations model established for atomic ultracold plasmas to describe the energy-grained avalanche of electron–Rydberg and electron–ion collisions in our system. Adding channels of Rydberg predissociation and two-body, electron–cation dissociative recombination to the atomic formalism, we investigate the kinetics by which this relaxation distributes particle density and energy over Rydberg states, free electrons and neutral fragments. The results of this investigation point to conditions under which such processes can effect the steady-state temperature of plasma electrons.
Ultracold plasma provides a possible route to approach the strongly-coupled regime under laboratory conditions.Normally,the lifetime of ultracold plasma is very limited due to plasma heating and expansion mechanisms.We present a new method to generate long lifetime ultracold plasmas consisting mainly of cations and anions.This plasma demonstrates a capability of traversing a DC barrier of up to 5(or-3) V.The lifetime of this plasma is expected to be more than 250 μs.Finally,molecular dynamics(MD) simulation is used to explain how anions slow the expansion rate and prolong the lifetime of this plasma.
Ultracold neutral plasmas (UNPs) are highly correlated, charged-particle systems studied in standard atomic-molecular optical physics laboratories. Advances in ultracold physics techniques have led to the development of magneto-optical traps that permit the localization of micro-Kelvin temperature atomic samples (10⁸ - 10¹⁰ cm -³ that can be photoionized with a defined excess laser energy. Magneto-optical trapping of molecules proves difficult, and to date, lags in reaching conditions attainable for atoms. Molecules possess additional degrees of freedom where energy can flow during laser trapping and cooling. Alternatively, elastic collisions with inert gases offer another route to low temperatures. Super-sonic expansions of target molecules seeded in a noble gas can reduced the translational temperature to values as low as 0.1 K. Here I report on the formation of a molecular ultracold plasma in a molecular beam of nitric oxide seeded in an inert gas. Our method does not directly form a plasma, but instead, I create a Rydberg gas that spontaneously evolves to a plasma. I have verified the transition to a collective plasma state by applying pulsed-fields to the field-free flight region, and observe a signal that persists well beyond classical field amplitudes required to ionize individual Rydberg states. Electric field screening is a collective effect occurring over a characteristic distance, quantified by the Debye length, λ_D. Molecular ultracold plasmas described here exhibit Debye lengths smaller than the overall size, λ_D<<σ. Plasmas of this nature offer an exotic environment for which to study fundamental phenomena. While plasmas are generally thought of as hot, UNPs exist at low enough temperatures that electrostatic interaction energy between particles is greater than their kinetic energy. At this point, charged particles begin to find themselves in local potential energy minima. Over the entire sample this translates to long-range order, or correlation. It is of great interest for researchers studying UNPs to maximize particle correlation, which requires UNPs to remain very cold, and dense through their lifetime.
We present the first experiment to observe an ultracold neutral plasma. The plasma, which was created by photoionization of laser cooled atoms, has charge densities as high as 2×109 cm−3, and the temperatures of electrons and ions are as low as 100 mK and 10 μK, respectively. The plasma has a lifetime of about 100 μs, much longer than predicted by recombination rates. When the laser that excites the atoms is tuned below the ionization limit we create a sample of very highly excited cold Rydberg atoms. At our highest densities and during a time of a few microseconds, in which the Rydberg atoms are essentially stationary, the ensemble evolves towards an unbound plasma-like state.