Metastable Argon Atoms Research Articles

Multiheterodyne interference spectroscopy using a probing optical frequency comb and a reference single-frequency laser

We discuss multiheterodyne interference between the laser beams of optical frequency-comb (FC) and single-frequency (SF) lasers. The beat signal generated by the interference contains a dc component, discrete-frequency elements with the same frequency interval generated by interference between the two elements of the FC laser, and elements generated by the interference between the FC and the SF lasers. The beat-signal elements between them can be used for absorption spectroscopy around the SF-laser frequency, which is called frequency-comb interference spectroscopy (FCIS). In this paper, we compare the beat-signal spectrum predicted through theoretical descriptions of multiheterodyne interference with experimental results to improve understanding of FCIS and similar techniques. Our FCIS experiments use a probing Ti:sapphire mode-locked laser, which is the beam experiencing absorption, and a reference diode laser. We also discuss the potential of this method to measure the small shift in absorption profiles using the measurement results of argon metastable atoms.

A deep optical cavity trap for atoms and molecules with rapid frequency and intensity modulation

We describe a deep far-off resonance trap for metastable argon atoms utilizing a medium finesse cavity and a high input power (30 W) to produce trap depths of up to 11 mK. The depth can be rapidly modulated allowing efficient loading of the trap, characterization of trapped atom temperature, and reduction of intensity noise. We measure the change in radius of curvature of the mirrors due to heating by the high circulating intensity and show that trapping is not adversely effected by this for all input powers.

Kinetic simulations of argon dusty plasma afterglow including metastable atom kinetics

The afterglow of a dusty plasma of rf discharge in argon is simulated by the particle-in-cell-Monte Carlo collision (PIC-MCC) method. The experimental observation that heavy dust contamination of plasma leads to an anomalous increase in the electron density at the beginning of afterglow is explained by release of electrons from the dust surface. Under the assumption that the floating potential of particles is in equilibrium with plasma conditions, the fast cooling of electrons in afterglow plasma due to a rapid escape of hot electrons from the volume leads to a decrease in the magnitude of the floating potential and hence to a loss of charge by dust. The intensive desorption of electrons from nanoparticles is the origin of anomalous behavior of the electron density. At the next stage of afterglow, when the electrons become cool, the plasma decay is defined by ambipolar diffusion. The effect of metastable argon atoms is also considered. Additional ionization due to metastable atom collisions affects the electron temperature but does not change the behavior of the electron density qualitatively.

Experimental investigation of long-lived Rydberg states in ultracold argon

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 ${}^{2}{P}_{1/2}$ 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.

Emission characteristics of a barrier discharge in an Ar-H2O mixture

We have experimentally studied the UV radiation of a low-temperature barrier discharge plasma in an Ar-H2O mixture. The spectral interval 300–400 nm has been examined in detail. Addition of argon with a pressure of 24 kPa to a barrier discharge in water vapor at a pressure of ∼0.1 kPa leads to a ninefold increase in the UV radiation power of excited hydroxyl molecules. An increase in the duration of the UV radiation pulse of the mixture in the longitudinal discharge decay has been achieved for the first time, which may be direct evidence of energy transfer from metastable argon atoms to water molecules. An estimate of the upper boundary of the dissociative excitation rate constant of hydroxyl molecules OH*(A2Σ+) upon interaction of metastable argon atoms with water molecules has been obtained.

Diagnostics of VHF Argon Plasmas by Laser Thomson Scattering

The laser Thomson scattering (LTS) method has been applied to measure the electron density ne and electron temperature Te of very-high-frequency (VHF) argon plasmas. When the probing laser wavelength was 532 nm and the laser power density was ∼1015 W/m2, the Thomson scattering spectrum was obviously deformed by the effect of the photo-ionization of metastable argon atoms. The threshold laser power density at which the scattered light intensity from electrons in the plasma and that from electrons produced by photo-ionization are equivalent was found to be unexpectedly low (4× 1013 W/m2). To avoid the photo-ionization of metastable argon atoms, the laser power density was decreased to around 1×1013 W/m2 by using a cylindrical lens as the focusing lens. Then, the ne and Te values measured by LTS and the probe method were compared for a VHF plasma using argon gas at a pressure of 100 mTorr. This comparison confirmed that the LTS method gave reasonable ne and Te values.

Investigation on characteristics of argon corona discharge under atmospheric pressure

Spatially two-dimensional and axial-symmetric plasma model is developed to investigate the characteristics of a bar–plate corona discharge of argon gas under atmospheric pressure. An improved nonlocal collision-less electron heat flux is used in the model. Numerical simulation for the process of argon corona discharge under atmospheric pressure are carried out with a separation of 0.2 mm between the bar and plate and the discharge is excited by an external circuit. The results indicate that metastable argon atoms are mainly produced by ground state excitation reaction, the metastable step-wise ionization is the dominated reaction and the highest contribution to electron production during the discharge. In addition, the maximum of electron temperature arises in the cathode sheath due to Joule heating in the high electric field. Elastic collision is the dominant volumetric electron energy loss in atmosphere corona discharge, which is negligible in low pressure glow discharge. The discharge current–voltage characteristic is predicted and shows a good agreement with the results obtained by experiments.

On Both Spatial And Velocity Distribution Of Sputtered Particles In Magnetron Discharge

Abstract The kinetics of the sputtered atoms from the metallic target as well as the time-space distribution of the argon metastable atoms have been investigated for DC and high power pulse magnetron discharge by means of Tunable Diode – Laser Absorption Spectroscopy (TD-LAS) and Tunable Diode – Laser Induced Fluorescence (TD-LIF). The discharge was operated in argon (5-30 mTorr) with two different targets, tungsten and aluminum, for pulses of 1 to 20 μs, at frequencies of 0.2 to 1 kHz. Peak current intensity of ~100 A has been attained at cathode peak voltage of ~1 kV. The mean velocity distribution functions and particle fluxes of the sputtered metal atoms, in parallel and perpendicular direction to the target, have been obtained and compared for DC and pulse mode.

Time-resolved tunable diode laser absorption spectroscopy of excited argon and ground-state titanium atoms in pulsed magnetron discharges

Time-resolved investigations of excited argon atom density and temperature and ground-state titanium atom density during high-power impulse magnetron sputtering (HiPIMS, repetition frequency 100 Hz) and direct current pulsed magnetron (repetition frequency 2.5 kHz) discharges (PMDs) in argon employing a titanium target were performed. Atom density and temperature were measured with the help of tunable diode laser absorption spectroscopy. Excited argon atoms form during the discharge pulse and again by three-body electron ion recombination in the afterglow. Similarly, the temperature of excited (metastable) argon atoms rises during the plasma on phase and again during the afterglow. The observed temporal evolution of the temperature is faster than expected from thermal conductivity considerations, which is taken as an indication that metastable and ground-state argon atoms are not in thermal equilibrium. The time dependence of titanium atoms can be explained by recombination and diffusion. The results provide new insights into the physics of PMDs.

Frequency stabilization of an external-cavity diode laser to metastable argon atoms in a discharge

A laser stabilization scheme using magnetic dichroism in a RF plasma discharge is presented. This method has been used to provide a frequency stable external-cavity diode laser that is locked to the 4s[3/2](2) → 4p[5/2](3) argon laser cooling transition at 811.53 nm. Using saturated absorption spectroscopy, we lock the laser to a Doppler free peak which gave a locking range of 20 MHz when the slope of the error signal was maximized. The stability of the laser was characterized by determining the square root Allan variance of laser frequency fluctuations when the laser was locked. A stability of 129 kHz was measured at 1 s averaging time for data acquired over 6000 s.

On the high argon metastable atom density measured near the cathode surface of a hollow cathode discharge

We report laser absorption measurements of spatial density distributions of Ar metastable (Arm) and resonant state atoms in a cylindrical hollow-cathode discharge (HCD). The evaluation of the measured data by standard analysis indicates high Arm densities near the walls of the HCD. To obtain convincing evidence for these high densities, we critically examine the effect of diffraction phenomena on the measured distributions. This analysis confirms that—while diffraction has a significant effect—the Arm densities near the cathode wall are in the 1016 m−3 range. The measured values suggest that, in addition to the discharge volume, metastable atoms are likely to be created at the surface of the cathode. Further investigation is needed to clarify the processes responsible for the observed behavior.

Spectral intensity of the N2 emission in argon low-pressure arc discharges for lighting purposes

Nitrogen is discussed as an alternative to hazardous mercury in lamps for general lighting. Molecular nitrogen bands emit in both the near-UV (the second positive system C3Πu → B3Πg) and the visible spectral range (the first positive system B3Πg → A3Σ+u), which reduces conversion losses. To analyse the potential of nitrogen, low-pressure arc discharges in an argon background were characterized by means of optical emission spectroscopy. The spectral intensity of the molecular nitrogen emission rises with increasing nitrogen content in the discharge and shows a maximum around 4 mbar of absolute pressure. With regard to the application as a light source, radiation efficiencies were determined, which are around 5% at maximum. In order to identify the main population processes a collisional radiative model for the nitrogen–argon system was established which reveals the high relevance of heavy-particle collisions due to a pressure of a few mbar. The decisive excitation reactions for the state N2 C3Πu are the well-known processes of energy pooling between metastable nitrogen molecules and energy transfer from metastable argon atoms. For the state N2 B3Πg the main population channels are collision-induced crossings within the nitrogen states, where the collision partner can be either a nitrogen molecule or an argon atom, and the quenching collisions with argon.

Examination of argon metastable atom velocity distribution function close to a conducting wall

The spatial evolution of the 1s5 metastable argon atom velocity distribution function is recorded in the sheath and pre-sheath regions of a metallic wall using laser induced fluorescence (LIF) spectroscopy. Metastable argon atom temperature and fluid velocity are computed from measured data. Owing to the loss of metastable argon atom after a collision with the surface, the atom temperature seemingly decreases and the velocity increases when approaching the wall. These artifacts are carefully examined and explained in terms of changes in the metastable argon atom distribution function. In addition, the atom nonelastic reflection coefficient is computed from the ratio of outward to inward atom flux to the surface. This study indicates less than 1% of metastable atoms survive a collision with the metallic wall.

Populations of metastable and resonant argon atoms in radio frequency magnetron plasmas used for deposition of indium-zinc-oxide films

This work reports optical absorption spectroscopy measurements of the number density of Ar atoms in resonant (3P1, 1P1) and metastable (3P2, 3P0) states in rf magnetron sputtering plasmas used for the deposition of ZnO-based thin films. While the density of Ar 3P2 and 3P0 was fairly independent of pressure in the range of experimental conditions investigated, the density of Ar 3P1 and 1P1 first sharply increased with pressure and then reached a plateau at values close to those of the 3P2 and 3P0 levels at pressures above about 50 mTorr. At such pressures, ultraviolet radiation from resonant states becomes trapped such that these levels behave as metastable states. For a self-bias voltage of −115 V and pressures in the 5–100 mTorr range, similar number densities of Ar resonant and metastable atoms were obtained for Zn, ZnO, and In2O3 targets, suggesting that, over the range of experimental conditions investigated, collisions between these excited species and sputtered Zn, In, and O atoms played only a minor role on the discharge kinetics. The metastable-to-ground state number density ratios were also fitted to the predictions of a global model using the average electron temperature, Te, as the only adjustable parameter. For all targets examined, the values of Te deduced from this method were in excellent agreement with those obtained from Langmuir probe measurements.

The effect of matrix composition on radially resolved argon metastable atom populations in an emission ICP

We report radially resolved relative number densities for argon metastable atoms recorded by bleaching absorbers in a short segment of the absorption path of a continuous wave diode laser with a pulsed dye laser. The densities were measured in the presence and in the absence of high concentrations of metallic species. Mg, Ca, Sr, Ba, Y, Cu, La, Nd, Sm, and Gd were introduced into the plasma to model a variety of matrix species. At a height of 12 mm above the load coil, all of the introduced matrix species with the exception of copper caused significant increases in the steady-state argon metastable atom density. To examine the relationship between the argon metastable atoms and analyte or matrix ions, we recorded transient changes in argon metastable density in response to pulsed laser radiation tuned to a resonance ionic line of a selected metal in the plasma. Saturation of the ionic resonance lines caused transient decreases in the argon metastable atom density on a nanosecond time scale. The response of the argon metastable atoms to excitation of metal species is too fast to be due to collisions between excited metal ions and argon metastable atoms. We attribute the observed responses to heating of the electrons by suprathermal inelastic collisions between electrons and excited metal ions, followed by enhanced collisional excitation by electrons of argon atoms in the 4s levels.

Population inversion of molecular nitrogen in an Ar: N2 mixture by selective resonance-enhanced multiphoton ionization

Resonance-enhanced multiphoton ionization (REMPI) is shown to offer an attractive strategy for population inversion of molecular nitrogen in an Ar: N2 gas mixture. We present a detailed analysis of the key processes leading to a population inversion of molecular nitrogen in a REMPI-pumped Ar: N2 gas mixture, including a (3 + 1) REMPI of argon atoms, conversion of the REMPI-generated atomic argon ions into molecular ions, and generation of long-lived metastable excited-state argon atoms through dissociative recombination, populating the C3πu states of molecular nitrogen. Population inversion achieved for the second-positive-band laser transitions of molecular nitrogen enables stimulated emission of ultraviolet radiation at 337 nm. A high selectivity of the REMPI process helps to radically reduce the depletion of the working medium through the ionization of N2, providing a pump mechanism that is ideally suited for the creation of a new type of a highly efficient nitrogen laser.

Influence of Additive Gas on Electrical and Optical Characteristics of Non-equilibrium Atmospheric Pressure Argon Plasma Jet

Electrical and optical properties of an argon plasma jet were characterized. In particular, effects of an additive gas, namely nitrogen or oxygen, on these properties were studied in detail. The plasma jet was found to be of a glow-like discharge, which scarcely changed upon the injection of an additive gas, either directly or through a glass capillary. Optical emission spectroscopy characterization revealed that excited argon atoms were the predominant active species in this plasma jet. Metastable argon atoms were highly quenched, and N2(C3Πu) became the main energy carrier following nitrogen injection. When oxygen was added to the afterglow zone through a glass capillary, no significant quenching effect was observed and the number of oxygen atoms decreased with the increase in oxygen concentration. Finally, to demonstrate an application of this plasma jet, a high-density polyethylene surface was treated with argon, argon/nitrogen, and argon/oxygen plasmas.

Propagation of a surface microwave along the afterglow plasma column of a high-current pulsed discharge

It is demonstrated experimentally that the lifetime of the afterglow plasma of a high-current pulsed discharge in a dielectric tube filled with a mixture of argon with saturated mercury vapor is longer than 1 ms. Such a long lifetime, during which the electron density decreases from 1014 to 1012 cm−3, is explained by the chemi-ionization of mercury vapor by long-lived metastable argon atoms. During this time, the afterglow plasma can serve as a microwave waveguide for a weakly damped low-noise E0-type axisymmetric surface mode, which allows one to use it for transmission of signals in the centimeter wavelength range.

Trapping of a supersonic beam in a traveling magnetic wave

Here we report on a new approach to the magnetic deceleration of supersonic beams, based on the generation of a propagating wave of magnetic field. Atoms and molecules possessing a magnetic dipole moment, in so-called low field seeking quantum states, are trapped around a node of the propagating wave. The wave travels at a desired velocity in the direction of the supersonic beam, which can be chosen to match a velocity class populated in the beam. An additional quadrupole guide provides transverse confinement, independently of the decelerator itself. Our technique has been conceived to generate a smooth motion of the magnetic wave, which should optimize the efficiency of the trapping during a future Zeeman deceleration of the beam. We demonstrate the trapping of metastable argon atoms in a magnetic wave traveling at selected, constant velocities.

Photoassociative spectroscopy of ultracold metastable argon

We present results of photoassociative spectroscopy performed on ultracold metastable argon atoms in a magneto-optical trap. Ion spectra are obtained with laser detuning up to a few gigahertz below the $4s[3/2]{}_{2}\ensuremath{\rightarrow}4p[5/2]{}_{3}$ trapping transition at 811 nm and with intensities in a range of $~({10}^{2}\ensuremath{-}{10}^{5}){I}_{\mathrm{Sat}}$. We also compute dipole-dipole potentials for both singly and doubly excited diatomic molecules and use a Leroy-Bernstein analysis to determine the approximate vibrational spacings in the ($s+p$) and ($p+p$) manifolds. Based on this theoretical framework, we explain a broad background feature in our data and suggest that double-excitation mechanisms are likely responsible for sharp dips in the ion signal.