Chapter One - Detection of Metastable Atoms and Molecules using Rare Gas Matrices

Oxidation of silver using microwave-induced oxygen plasma and oxygen-ozone gas mixture was studied as a function of temperature and partial pressure. The oxide Ag2O was formed at temperatures well above its normal decomposition temperature in oxygen plasma at a pressure of 5 Pa. The higher oxide AgO1−x was formed in O2+O3 gas mixtures at lower temperatures. The oxygen chemical potentials for the oxidation of Ag to Ag2O, Ag2O to AgO1−x and AgO to Ag2O3 were evaluated from thermodynamic data and compared with the experimental results to obtain information on the chemical potential of oxygen in microwave plasma and gases containing ozone. The oxygen potential of the gas phase in microwave plasma operating at a pressure of 5 Pa was found to be in excess of 36 kJ/mol at 750 K. This is equivalent to a pressure of diatomic oxygen gas greater than 3 × 107 Pa. In the O2+O3 mixture at ambient pressure containing 5 mole percent O3, the oxygen potential is ∼112 kJ/mol at 465 K. The equivalent pressure of diatomic oxygen is ∼4 × 1017 Pa. Thus, metastable species such as O3 or charged species such as O− present in plasma can be used as a powerful reagent for the syntheses of metastable oxides. Similar techniques can be used for other metastable inorganic solids such as nitrides for functional applications.

Lifetimes of matrix-isolated NH/ND(a 1Δ) radicals have been measured as function of temperature and rare gas host. The metastable species were generated directly by in situ photolysis of hydrazoic or isocyanic acid, or by pulsed excitation of the b 1Σ+ state with a dye laser, which subsequently decays to the a state on a μs time scale. Rotation of NH/ND in the electronic ground state is perturbed or inhibited by the second photofragment in the photolysis systems. The lifetimes of NH(a 1Δ) in Ne, Ar, and Kr show little temperature dependence. This is consistent with a radiationless contribution to the overall relaxation in which the energy gap to the next lower vibrational level of the ground state is accepted by guest rotation and/or other local modes. The strong temperature dependent decay of ND(a 1Δ) in Ar and Kr is due to endothermic near resonant relaxation to the sixth vibrational level of the ground state, with an activation energy in the order of the energy gap, and a frequency factor of 20±5 s−1. The relaxation mechanism of NH/ND(a 1Δ) in solid Xe is of a different nature, the data being consistent with a temperature dependent external heavy atom effect. A lower limit of 1.9 s has been deduced for the radiative lifetime of the (a 1Δ) state in vacuo, in reasonable agreement with a recent ab initio calculation.

Hydroxyl (OH) radical is an important reactive agent in various plasma applications, such as plasma assisted combustion, plasma medicine, and material processing. Quantification of the absolute number densities of OH radical in atmospheric plasma jets can help better understand plasma chemistry, reaction kinetics as well as plasma design. A 2.45 GHz microwave plasma source along with UV pulsed cavity ringdown spectroscopy of OH (A - X) (1 - 0) band at 308 nm was employed to study the effect on OH radical generation due to addition of low water content to argon (feeding gas) and to measure the absolute number density of OH. With addition of 0, 3, 4, 6, 8, 11, and 14 ppm water vapor in argon gas, the OH number densities vary from 2.34 × 1015 to 2.85 × 1016, 3.73 × 1014 to 3.75 × 1016, 2.11 × 1014 to 3.04 × 1016, 1.73 × 1015 to 2.72 × 1016, 2.30 × 1015 to 3.12 × 1016, 1.31 × 1015 to 5.13 × 1016, and 1.20 × 1015 to 3.39 × 1016 molecules/cm3 in different locations along the plasma jet axis. The plasma jet length decreased from 11 mm to 4 mm with increase in water content. With the addition of water, the plasma gas temperature increased due to the global heating of plasma gas resulting from the collisional relaxation of H 2 O molecules. Optical emission spectra show a drastic increase in OH (A-X) emission intensity with increase in water fraction in the Ar/ H 2 O mixture. The formation mechanism of OH radicals is influenced by the water content in the gas mixture. Various reaction channels can be responsible for OH formation in different regions of the plasma jet, e.g., dissociative electron excitation of water and dissociative recombination of water ions in the plasma jet plume; dissociative recombination, electron impact excitation and dissociation of water due to radicals and metastable species in the vicinity of the jet tip; and dissociative recombination of water ions in the downstream/far downstream region.

A tunable cw laser has been used to selectively remove either of the two metastable species, 3P0,2, which are initially present in a neon metastable beam. The method is applicable to other rare gases and provides the opportunity for separate investigation of effects due to atoms in either the 3P0 or 3P2 state.

Total cross sections are theoretically evaluated for electron impact ionization of initially metastable states of nitrogen as well as oxygen atoms, known to play roles in auroral emissions observed in the Earth's polar regions as also in the cometary coma and airglows. If the long‐lived N(2P), N(2D), O(1S), and O(1D) metastable species get ionized in their interactions with energetic electrons, then they would not contribute to auroral emission. Therefore, we investigate electron ionization of the said metastable species by calculating relevant total cross sections. Our quantum mechanical calculations are based on projected approximate ionization contribution in the total inelastic cross sections. In all the four targets, the resulting cross sections are fairly large, indicating the importance of the electron ionization process.

Electron impact dissociative excitation of molecules is used to measure the lifetime of metastable species which cannot easily be produced in the laboratory by direct excitation. Two illustrative examples are given. The lifetime of the N2(a1 Pi g) state is measured to be 120+or-2 mu s and the lifetime of the O(5S) state to be 185+or-30 mu s, both in good agreement with earlier measurements. The method described may be extended to many other metastable species of current interest.

Optical Emission Spectroscopy (OES) was used to identify reactive species and their excitation states in low-temperature cascade arc plasmas of N2, CF4, C2F4, CH4, and CH3OH. In a cascade arc plasma, the plasma gas (argon or helium) was excited in the cascade arc generator and injected into a reactor in vacuum. A reactive gas was injected into the cascade arc torch (CAT) that was expanding in the reactor. What kind of species of a reactive gas, for example, nitrogen, are created in the reactor is dependent on the electronic energy levels of the plasma gas in the cascade arc plasma jet. OES revealed that no ion of nitrogen was found when argon was used as the plasma gas of which metastable species had energy less than the ionization energy of nitrogen. When helium was used, ions of nitrogen were found. While OES is a powerful tool to identify the products of the cascade arc generation (activation process), it is less useful to identify the reactive species that are responsible for surface modification of polymers and also for plasma polymerization. The plasma surface modification and plasma polymerization are deactivation processes that cannot be identified by photoemission, which is also a deactivation process. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1583–1592, 1998

Micro-scale plasmas are extremely interesting due to the spectrum of applications potentially feasible for these small scale plasmas. Due to the small physical size and short temporal scales, simulations can provide complementary and insightful tools to help understand the underlying physical processes. The present paper discusses a numerical model coupling the plasma, metastable species and gas dynamics in atmospheric microcavities in helium at atmospheric pressure. The self-consistent and time-dependant model is described with emphasis on terms involved in the close coupling among species (plasma, metastable and gas) and the applied field – both electric and magnetic fields. The microplasmas are studied from an initial cloud and transients are particularly important in the evolution. Gas heating, neutral depletion initiation and electrohydrodynamic effects are observed, highlighting the interaction between neutral gas and plasma species in governing the microplasmas. The crucial effects of the applied magnetic field and secondary emission are discussed – and surface and volume effects are compared in terms of spatial and temporal evolutions of the plasma and gas dynamics in atmospheric microplasmas.

Confronted with the stupefying flow of new information in the field of electronic spectroscopy, we have selected for review here three closely related topics likely to be less familiar to many of the readers of A nnual Reviews of Physical Chemistry than conventional spectroscopic methods. We do this in order to direct attention to a group of methods j ust now becoming especially fruitful for the study of electronic states of molecules. The methods are electron impact spectroscopy, photoelectron spectros­ copy, and Penning ionization spectroscopy. They have in common one facet that distinguishes them from conventional spectroscopy : all three of these methods use energy analysis of electrons, rather than energy analysis of photons, as the primary source of information. The three methods differ both in the manner by which excitation is induced, and in the kind of informa­ tion one obtains. I mpact spectroscopy obviously relies on transfer of kinetic energy from a free electron to the bound electrons of a target molecule or, as we shall often refer to it, a scatterer. This method is useful for studying normally empty levels, and particularly for studying transitions normally in conventional optical spectroscopy. Photoelectron spectros­ copy uses monoenergetic photons to remove bound electrons from targets and is particularly useful for studying normally occupied levels, including levels from the valence shells all the way to the most tightly bound K-shells. Penning spectroscopy is a new field, in which excitation is delivered to the target in the form of a quantum bound to an incoming atom, i.e., an excited, often metastable species. The Penning ionization process has the form A*+ M-7A+ M+ + e. This method is proving useful for the study of normally filled levels, particularly in the valence shell. Several reviews of topics related to electron spectroscopy should be noted. In connection with electron impact spectroscopy, the subject of forbidden transitions, the theory of excitation and ionization by electron impact and the measurement of these processes were reviewed in 1962 by Garstang (1) , Seaton (2) , and Fite (3) , respectively. Peterkop & Veldre have reviewed the theory of electron-atom collisions (4). Electron impact ionization cross sec­ tion data was surveyed by Kieffer & Dunn (5) , and Moiseiwitsch & Smith have reviewed electron impact excitation of atoms (6) . A review by Bardsley & Mandl of resonant electron-atom and electron-molecule collisions has

We use a global (volume averaged) model to study the dissociation processes and the presence of negative ions and metastable species in a low pressure high density O2/Ar discharge in the pressure range 1–100 mTorr. The electron density and the fractional dissociation of the oxygen molecule increases with increased argon content in the discharge. We relate this increase in fractional dissociation to an increase in the reaction rate for electron impact dissociation of the oxygen molecule which is due to the increased electron temperature with increased argon content in the discharge. The electron temperature increases due to higher ionization potential of argon than for molecular and atomic oxygen. We find the contribution of dissociation by quenching of the argon metastable Arm by molecular oxygen (Penning dissociation) to the creation of atomic oxygen to be negligible. The negative oxygen ion O− is found to be the dominant negative ion in the discharge. Dissociative attachment of the oxygen molecule in the ground state and in particular the metastable oxygen molecule O2(a 1Δg) are the dominating channels for creation of the negative oxygen ion O−.

Energy pooling mechanism for catalyst-free methane activation in nanosecond pulsed non-thermal plasmas

Nitrogen impurity seeding is a promising technique for increasing the radiative power dissipation rate in the edge plasma of a fusion device. It will be required in future fusion devices such as ITER to reduce the directed heat flux on the divertor strike-points to within erosion limits. However, chemical reactions between nitrogen and fuel isotopes may complicate tritium control measures by increasing in-vessel retention and impacting the gas-handling plant. To gain insight into the nitrogen–hydrogen plasma chemistry a volume-averaged (global) model is developed and compared with experimental measurements in the MAGnetised Plasma Interaction Experiment plasma device. A set of 702 reactions is compiled and used to model the population dynamics of 51 relevant neutral, ionic, electron, surface and metastable excited state species. Stable equilibrium values are compared to results from an experimental investigation in which a combination of mass spectrometry, Langmuir probe analysis and optical emission spectroscopy is used to determine neutral and positive-ionic trends under the same conditions. The dominant ammonia production mechanism is found to be the Langmuir–Hinshelwood reaction between adsorbed atomic hydrogen and above 25% hydrogen concentration. For lower hydrogen proportions the Eley–Rideal reaction between free atomic hydrogen and is found to dominate. The dominant loss mechanism (for all compositions) is found to be electron impact dissociation into neutral fragments.

The present Special issue collects original and review articles that were a matter of discussion at the workshop `All about metastables' organised as a subsection of the XVth ESCAMPIG conference held in Miskol-Lillafured in Hungary on 26-30 August 2000.The Europhysics Sectional Conference on Atomic and Molecular Physics of Ionised Gases (ESCAMPIG) is an important biennial meeting for scientists involved in fundamental and applied sciences based on ionised gases. The conference is organised under the sponsorship of the European Physical Society. The ESCAMPIG is a meeting rich with discussion opportunities. Besides lectures, hot topics and poster sessions, it is a custom of the ESCAMPIG to arrange during the meeting short workshops or round tables focused on topics of large scientific impact on this community. Such discussions are mainly based on the participation of scientists attending the conference.The International Scientific Committee of XVth ESCAMPIG envisaged a renewed interest in the topics concerning long-lived species, and particularly those concerning the atoms and molecules in electronically excited metastable states (metastable species). The importance of such species is largely known in different ionised gas environments like laboratory gas-discharges for lamps, lasers and panel display technologies, as well as in natural environments like the upper atmosphere. The energy transfers operated by such species are of primary importance in many kinds of afterglows containing nitrogen, oxygen and noble gases. Actually, most of the details (density temperature or internal state distributions, etc.) of long-lived species can be detected by many extremely sensitive techniques, most of them based on lasers. This knowledge therefore offers the possibility of approaching with more confidence the microscopic kinetic modelling of discharge and post-discharge systems containing long-lived species.Most of the matter discussed at the workshop `All about metastables' is addressed by the articles in this special issue. The issue focuses on some recent achievements in the topics concerning the diagnostics and modelling of elementary kinetics and energy transfers of electronically excited metastables and ground state atoms and vibrationally excited molecules in N2, O2, N2-O2 and N2-H2O systems. It also highlights the role of long-lived species in high-pressure discharge and in supersonic flow generated by N2 plasma jets as well as in oxygen-iodine lasers. Two reviews will help the reader to focus on the main aspects of the gas phase and the gas-surface kinetics modelling of the long-lived species in the glow and afterglow conditions.Finally, despite the appealing title, the workshop highlighted a restricted number of topics on metastables and more generally long-lived species.The issue of noble-gas metastables was not touched upon. For this reason we think the reader will appreciate also finding in this special issue two regular articles on topics not covered by the workshop. We hope this special issue contributes to an overview of the many links of the metastable issue in the physics and chemistry of partially ionised gases.Dr S De Benedictis, Guest EditorCentro di Studio per la Chimica dei Plasma CNR, c/o Dipartimento di Chimica Universita' di Bari, 4, Via Orabona, 70126 Bari, ItalyJune 2001

Noble gas anions of general formula FNgBN- (Ng = He-Xe) have been investigated by MP2, coupled-cluster, and multireference-CI calculations with correlation-consistent basis sets. These species reside in deep wells on the singlet potential energy surface and are thermodynamically stable with respect to the loss of F, F-, BN, and BN-. They are unstable with respect to Ng + FBN-, but at least for Ng = Ar, Kr, and Xe, the involved energy barriers are high enough to suggest their conceivable existence as metastable species. The stability of FNgBN- arises from the strong F--stabilization of the elusive NgBN. The character of the boron-noble gas bond passes from purely ionic for FHeBN- and FNeBN- to covalent for FXeBN-.

Post-discharge cleaning at atmospheric pressure of oxidized iron foils rinsed by acetone and methanol is studied by XPS. The influence of the temperature (T<450 K) and the UV photons on cleaning is negligible. When pure rare gases (He, Ne, Ar) are used, metastable species in post-discharge are transported downstream the plasma and relax their energy on the surface. C-C bonds are mainly removed by this process. When nitrogen is added to the rare gas (from 0 to 4.6 vol.% ), a selective influence on the etching process of the surface contaminants by nitrogen atoms is observed. The removal kinetics of chemical groups analysed by XPS is determined by using an exponential decay function corresponding to a first-order abstraction process. The decrease of C(1s) in N-C, O(1s) in N-C-O and N(1s) XPS peaks is correlated with the nitrogen atom concentration in the carrier gas. Reactions between nitrogen atoms and specific carbon containing groups occur (probably C-OH bonds of methanol). Stronger bonds like C=O (probably from acetone) are not removed by the post-discharge.