Metastable Rare Gas Atoms Research Articles

Asymptotically spin-forbidden optical transitions in quasimolecules

The results obtained recently on optical transitions that are induced in excited diatomic quasimolecules temporarily existing under thermal collisions of atoms of the Groups IIA and XII in the metastable atomic state 3P2 and rare gas atoms in the normal state 1S are summarized.

Pulsed-ramped-field-ionisation zero-kinetic-energy photoelectron spectroscopy of the metastable rare-gas atoms Ar, Kr and Xe.

The photoionisation of the metastable rare-gas atoms Rg = Ar, Kr and Xe is investigated at the Rg+ […](ns)2(np)5 2P3/2 ← Rg[…](ns)2(np)5((n + 1)s) 13P2 photoionisation threshold (n = 3, 4 and 5 for Ar, Kr and Xe) using the technique of pulsed-ramped-field-ionisation zero-kinetic-energy (PRFI-ZEKE) photoelectron spectroscopy. This technique, which monitors the field ionisation of high Rydberg states induced by a slowly growing electric-field ramp after a prepulse of opposite polarity, was recently introduced to record photoelectron spectra with high resolution and high sensitivity [Harper, Chen, Boyé-Péronne and Gans, Phys. Chem. Chem. Phys., 2022, 117, 9353]. A tunable UV laser system with a bandwidth of 30 MHz is used here to establish the factors determining the resolution and overall accuracy of this new method. In particular, the effects of stray electric fields, and of the magnitude of the prepulse and the field ramp on PRFI-ZEKE photoelectron spectra are studied by combining experiments with numerical simulations of the field-ionisation of high Rydberg states and of the electron flight times from the ionisation spot to the detector. A spectral resolution of 0.05 cm-1 is demonstrated in the photoelectron spectrum of metastable Ar.

Optically pumped argon metastable laser with repetitively pulsed discharge in a closed chamber

The optically pumped rare gas laser (OPRGL) is an innovative type of gas laser, which has the potential to be a candidate for high-energy lasers (HELs), in which metastable rare gas atoms generated by gas discharge are used as active particles. To realize these kinds of lasers as HELs in field application, high efficiency and low cost are necessary. For this purpose, we demonstrated a closed-chamber operation with repetitively pulsed gas discharge for an OPRGL of argon metastables. The pulsed discharge was powered by a home-made LC generator, which provided a high efficiency for the deposition of electric energy. The metastable density produced was higher than 1012 cm−3, quite enough for efficient laser operation. The dependencies of laser radiation on the parameters of discharge voltage, gas pressure, and argon concentration were studied, which showed an optimized argon concentration of 1%. Long-time operations were also performed, which demonstrated the practicability of the operation of OPRGL in a closed chamber, with the avoiding of the running away of rare gases in the open cycle of flowing operation.

Design and construction of a multistage Zeeman decelerator for crossed molecular beams scattering experiments.

Zeeman deceleration is a relatively new technique used to obtain full control over the velocity of paramagnetic atoms or molecules in a molecular beam. We present a detailed description of a multistage Zeeman decelerator that has recently become operational in our laboratory [Cremers et al., Phys. Rev. A 98, 033406 (2018)] and that is specifically optimized for crossed molecular beams scattering experiments. The decelerator consists of an alternating array of 100 solenoids and 100 permanent hexapoles to guide or decelerate beams of paramagnetic atoms or molecules. The Zeeman decelerator features a modular design that is mechanically easy to extend to arbitrary length and allows for solenoid and hexapole elements that are convenient to replace. The solenoids and associated electronics are efficiently water cooled and allow the Zeeman decelerator to operate at repetition rates exceeding 10 Hz. We characterize the performance of the decelerator using various beams of metastable rare gas atoms. Imaging of the atoms that exit the Zeeman decelerator reveals the transverse focusing properties of the hexapole array in the Zeeman decelerator.

Kinetic analysis of rare gas metastable production and optically pumped Xe lasers

Optically pumped all-rare-gas lasers use metastable rare gas atoms as the lasing species in mixtures with He or Ar buffer gas. The metastables are generated in a glow discharge, and we report model calculations for the optimal production of Ne*, Ar*, Kr* and Xe*. Discharge efficiency was estimated by solving the Boltzmann equation.Laser efficiency, gain and output power of the CW optically pumped Xe laser were assessed as functions of heavier rare gas content, pressure, optical pump intensity and the optical path length. It was found that, for efficient operation the heavier rare gas content has to be of the order of one percent or less, and the total pressure—in the range 0.3–1.5 atm. Output power and specific discharge power increase approximately linearly with pump intensity over the output range from 300–500 W cm−2. Ternary mixtures Xe:Ar:He were found to be the most promising. Total laser efficiency was found to be nearly the same for pumping the 2p8 or 2p9 state, reaching 61%–70% for a pump intensity of ~720 W cm−2 when the Xe fraction was in the range 0.001 ÷ 0.01 and Ar fraction—0.1 ÷ 0.5. However, when the 2p8 state was pumped, the maximum total efficiency occurred at larger pressures than for pumping of the 2p9 state. The discharge power density required to sustain a sufficient Xe* number density was in the range of tens of watts per cubic centimeter for 50% Ar in the mixture.

Stereodynamics in the Collisional Autoionization of Water, Ammonia, and Hydrogen Sulfide with Metastable Rare Gas Atoms: Competition Between Intermolecular Halogen and Hydrogen Bonds.

Recent experiments on the title subject, performed with a high-resolution crossed-beam apparatus, have provided the total ionization cross sections as a function of the collision energy between noble gas atoms, electronically excited in their metastable states (Ng*), and H2 O, H2 S, and NH3 reagents, as well as the emitted electron energy spectra. This paper presents a rationalization of all the experimental findings in a unifying picture to cast light on the basic chemical properties of Ng* under conditions of great relevance both from a fundamental and from an applied point of view. The importance of this investigation is that it isolates the selective role of the intermolecular halogen and hydrogen bonds, to assess their anisotropic effects on the stereodynamics of the promoted ionization reactions, and to model energy transfer and reactivity in systems of applied interest, such as planetary atmospheres, plasmas, lasers, and flames.

Optically pumped rare-gas lasers

The modern state of the research of a new promising optically pumped laser system with an active medium formed by metastable rare-gas atoms is briefly reviewed. The kinetics of these media is similar to that of laser media based on alkali metal vapour; however, the gas medium is inert. Metastable atoms can be produced in an electric discharge. As in alkali lasers, the specific laser power output under atmospheric pressure can be several hundreds of watts per . The lasing wavelengths lie in the near-IR range and fall in the transparency window of the terrestrial atmosphere. This new concept makes it possible to develop a closed-cycle cw laser with megawatt power levels and high beam quality.

Demonstration of a diode-pumped metastable Ar laser

Pulsed lasing from optically pumped rare gas metastable atoms (Ne, Ar, Kr, and Xe) has been demonstrated previously. The laser relies on a three-level scheme, which involves the (n+1)p[5/2](3) and (n+1)p[1/2](1) states from the np(5)(n+1)p electronic configuration and the metastable (n+1)s[3/2](2) level of the np(5)(n+1)s configuration (Racah notation). Population inversions were achieved using relaxation from ((n+1)p[5/2](3) to (n+1)p[1/2](1) induced by collisions with helium or argon at pressures near 1 atm. Pulsed lasing was easily achieved using the high instantaneous pump intensities provided by a pulsed optical parametric oscillator excitation laser. In the present study we examine the potential for the development of a continuous wave (CW) optically pumped Ar laser. We report lasing of the 4p[1/2](1)→4s[3/2](2) (912.547 nm) transition following CW diode laser excitation of the 4p[5/2](3)←4s[3/2](2) line (811.754 nm). A pulsed discharge was used to generate Ar 4s[3/2](2), and the time-resolved lasing kinetics provide insights concerning the radiative and collisional relaxation processes.

Kinetic study of a cw optically pumped laser with metastable rare gas atoms produced in an electric discharge

An analytical model of a cw diode pumped laser that uses metastable rare gas atoms as the lasing species is developed, providing a method for quick assessment of a rare gas laser performance. Ar mixtures with He are analysed as examples of active media. Ar metastables are supposed to be produced in a glow discharge. Laser efficiency, gain and output power of this novel system are assessed as functions of Ar content, pressure, pump intensity and the system size along the direction of pumping radiation. Efficiency of metastables' excitation in a glow discharge is estimated solving Boltzmann equation. It was found that for efficient operation at 300 K optimal Ar content is of the order of 1% and the pressure is near atmospheric. Output power and specific discharge power increase with pump intensity essentially linearly, starting from 300 W cm−2. With pump intensity of 840 W cm−2 optical and total efficiency reach ∼60%, specific output power—500 W cm−3. At the same time, the discharge power density required to sustain sufficient metastables' number density amounts to 18 W cm−3 only.

Ionization cross sections for electron scattering from metastable rare-gas atoms (Ne* and Ar*)

The optical-model approach has been used to investigate the electron-impact ionization of metastable rare-gas atoms. A complex equivalent-local polarization potential is obtained to describe the ionization continuum channels. We have calculated the cross sections for collisional ionization of the metastable atoms Ne* and Ar* by electrons in the energy range from threshold to 200 eV. The present results are in agreement with the available experimental measurements and other theoretical calculations.

Gain and lasing of optically pumped metastable rare gas atoms

Optically pumped alkali vapor lasers are currently being developed in several laboratories. The objective is to construct high-powered lasers that also exhibit excellent beam quality. Considerable progress has been made, but there are technical challenges associated with the reactivity of the metal atoms. Rare gas atoms (Rg) excited to the np(5)(n+1)s (3)P(2) configuration are metastable and have spectral properties that are closely similar to those of the alkali metals. In principle, optically pumped lasers could be constructed using excitation of the np(5)(n+1)p←np(5)(n+1)s transitions. We have demonstrated this potential by observing gain and lasing for optically pumped Ar(*), Kr(*) and Xe(*). Three-level lasing schemes were used, with He or Ar as the collisional energy transfer agent that established the population inversion. These laser systems have the advantage of using inert reagents that are gases at room temperature.

Improving metastable impact electron spectroscopy and ultraviolet photoelectron spectroscopy signals by means of a modified time-of-flight separation

The separation of ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES) is usually performed by a time-of-flight (ToF) separation using pre-set ToF for both types of signal. In this work, we present a new, improved ex situ signal separation method for the separation of MIES and UPS for every single measurement. Signal separation issues due to changes of system parameters can be overcome by changing the ToF separation and therefore allowing for the application of a wider range of measuring conditions. The method also enables to identify and achieve separation of the two signals without any time consuming calibration and the use of any special material for the calibration. Furthermore, changes made to the discharge source are described that enable to operate an existing MIES/UPS source over a broader range of conditions. This allows for tuning of the yield of UV photons and metastable rare gas atoms leading to an improved signal to noise ratio. First results of this improved setup are well in agreement with spectra reported in literature and show increased resolution and higher signal intensities for both MIE and UP spectra compared to the previous, non-optimized setup.

A novel vacuum ultra violet lamp for metastable rare gas experiments

We report on a new design of a vacuum ultra violet (VUV) lamp for direct optical excitation of high laying atomic states, e.g., for excitation of metastable rare gas atoms. The lamp can be directly mounted to ultra-high vacuum vessels (p ≤ 10(-10)mbar). It is driven by a 2.45 GHz microwave source. For optimum operation, it requires powers of ~20 W. The VUV light is transmitted through a magnesium fluoride window, which is known to have a decreasing transmittance for VUV photons with time. In our special setup, after a run-time of the VUV lamp of 550 h the detected signal continuously decreased to 25% of its initial value. This corresponds to a lifetime increase of two orders of magnitude compared to previous setups or commercial lamps.

Pulsed-jet discharge matrix isolation and computational study of CX 2Br + (X = H, F)

Pulsed-jet discharge matrix isolation spectroscopy and computational methods were used to characterize the CX 2Br + (X = H, F) ions. Our experiments combined matrix isolation techniques with a pulsed DC discharge nozzle, where a dilute CX 2Br 2:Ar sample was discharged and the products deposited onto a KBr window held at ∼8 K. This study represents the first spectroscopic observation of CH 2Br +, and the first report of the C–Br stretching frequency of CF 2Br +. Our results suggest that the primary mechanism for ion formation is ionization/fragmentation of the precursor by collision with metastable rare gas (Ar) atoms.

Mass Spectrometric Detection of Gaseous Hydrogen Peroxide in Ambient Air Using Dielectric Barrier Discharge as an Excitation Source

Abstract Real-time detection of gaseous hydrogen peroxide (H2O2) in ambient air is an important subject for public safety and security. We have developed a mass spectrometric detection system for gaseous H2O2 using dielectric barrier discharge as the excitation source for metastable rare gas atoms. The O2− ion produced by Penning ionization of ambient air forms a stable cluster ion [H2O2+O2]− which is detected by an orthogonal time-of-flight mass spectrometer. No interference by the moisture in the air was observed.

Sympathetic cooling by collisions with ultracold rare gas atoms, and recent progress in optical Stark deceleration

We propose a general scheme for sympathetic cooling of molecules to microK temperatures on a timescale of seconds. Experimental parameters have been estimated from theory, which indicate the viability of the scheme. This method, which is particularly suited to optical Stark deceleration, utilises ultracold, laser cooled metastable rare gas atoms quenched to their ground state as collision partners to co-trapped molecular species within a deep optical trap (150 mK). We also describe the measurement of the role of laser-induced molecular alignment on the dipole force in optical Stark deceleration and outline progress towards the realisation of chirped optical Stark deceleration for producing slow molecular beams with mK energy spreads.

Fragmentation of Singly Protonated Peptides via Interaction with Metastable Rare Gas Atoms

The effects of metastable energy level and peptide sequence on the fragmentation patterns of singly charged peptide ions dissociated in collisions with metastable rare gas atoms were studied. Fragmentation spectra of singly charged peptide ions were shown to be more structure-informative and very different from those obtained in low-energy collision-induced dissociation. Unusual odd-electron radical a- and x-ions were observed. Several fragment ions corresponding to a side-chain loss were also observed, which allowed differentiation between leucine and isoleucine. The fragmentation mechanism depends on electronic energy transferred during interaction with metastable gas atoms and proceeds either via Penning ionization with formation of radical odd-electron doubly charged molecular cation or via high-energy excitation of internal degrees of freedom of the peptide cation.

Atmospheric‐pressure Penning ionization of aliphatic hydrocarbons

A study has been made of the atmospheric-pressure Penning ionization (APPeI) of aliphatic hydrocarbons (pentane, hexane, heptane, and octane) with long-lived rare gas atoms (Rg*). The metastable rare gas atoms (He*, Ne*, Ar* and Kr*) were generated by the negative-mode corona discharge of atmospheric-pressure rare gases. In the Rg*APPeI mass spectra for aliphatic hyrocarbons, the relative abundances of fragment ions were found to increase in the order of He* --> Ne* --> Ar* --> Kr*. The order is in the opposite direction to the internal energies of the Rg*. The less fragmentation observed for He* may be because the nascent molecular ions [M(+.)]* formed by Penning ionization have lifetimes long enough for them to be collisionally deactivated in the atmospheric-pressure ion source. It was found that the relative abundances of fragment ions in Ar*APPeI increased when the sample pressure in the ion source was reduced. This is attributed to the collision of Ar* with molecular ions followed by fragmentation.

Rare Gas Metastable Atom Density in Diluted O2 RF Plasmas

Rare gas diluted O2 plasmas are gaining interests for application to high quality SiO2 film formation. The density of rare gas metastable atoms and O atom in rare gas diluted O2 radio frequency (RF) capacitively coupled plasma (CCP) was measured by optical absorption spectroscopy (OAS). Decreases of rare gas metastable densities due to addition of O2 indicate efficient O atom production by rare gas metastables via collisional quenching. Krypton metastable had highest density among four rare gas species for fixed RF power. The decrease of Ar metastable density due to O2 addition showed quantitative agreement with reported quenching rate coefficient. Detailed discussion on different gas pressures illustrates reduced O2 fraction is the key for selective production of O atoms through rare gas metastables.

Development of Sniffing Atmospheric Pressure Penning Ionization

The atmospheric pressure Penning ionization was applied to the analysis of atmospheric-pressure ambient gaseous samples. The metastable rare gas atoms (Rg*) were produced by a negative-mode corona discharge of reagent rare gas (Rg). The rare gas containing Rg* in the discharge cell flowed out into the ion source cell through a nozzle with an inner diameter of 1 mm as a jet stream. The jet stream acts as a gas-jet pump and the pressure of the ion source cell becomes somewhat lower than the atmospheric pressure. The sample gas at ambient atmospheric pressure was sucked into the reduced-pressure ion source cell through the stainless steel tube (10 cm in length, 1 mm in diameter). The sample gases introduced in the ion source cell are ionized by Penning ionization. This technique was found to be applicable to various gaseous samples such as aromatic compounds, oxygen- and nitrogen-containing compounds, etc. However, no ion signal could be detected for aliphatic hydrocarbons. This may be due to the much smaller ionization cross sections for these compounds than N2 and O2 molecules which are introduced into the ion source together with sample gases.