Plasma chemistry in laser ablation processes

Based on the results of quantitative spectroscopic diagnostics (LIF in combination with time resolved emission spectroscopy) chemical dynamics in laser-produced plasmas of metallic (Ti, Al,), and graphite samples have been examined. The Nd-YAG (1064 nm, 10 ns, 100 mJ) and excimer XeCl (308 nm, 10 ns, 10 mJ) lasers were employed for ablation. The main attention was focused on the elucidation of a role of oxide and dimer formation in controlling spatio-temporal distributions of different species in the ablation plume. The results of the spatial and temporal analysis of a laser-produced plasma in air indicates the existence of diatomic oxides in the ablation plume both in the ground and excited states, which are formed from reactions between ablated metal atoms and oxygen. The efficiency of the oxidation reaction depends on the intensity and spot diameter of the ablation laser beam. The maximal concentration of TiO molecules are estimated to be of 1×10¹⁴ cm⁻³ at the time of 10 μs after the start of the ablation pulse. A comparison of spatial–temporal distributions of Ti atoms and excited TiO molecules allow us to find a correlation in their change, which proves that electronically excited Ti oxides are most probably formed from oxidation of atoms in the ground and low lying metastable states. The spectroscopic characterization of pulsed laser ablation carbon plasma has also been performed. The time–space distributions as well as the high vibrational temperature of C₂ molecules indicate that the dominant mechanism for production of C₂ is the atomic carbon recombination.     

Effects of oxygen addition to argon glow discharges: A hybrid Monte Carlo-fluid modeling investigation

A hybrid model is developed for describing the effects of oxygen addition to argon glow discharges. The species taken into account in the model include Ar atoms in the ground state and the metastable level, O₂ gas molecules in the ground state and two metastable levels, O atoms in the ground state and one metastable level, O₃ molecules, Ar⁺, O⁺, O₂⁺ and O^? ions, as well as the electrons. The hybrid model consists of a Monte Carlo model for electrons and fluid models for the other plasma species. In total, 87 different reactions between the various plasma species are taken into account. Calculation results include the species densities and the importance of their production and loss processes, as well as the dissociation degree of oxygen. The effect of different O₂ additions on these calculation results, as well as on the sputtering rates, is discussed.     

Physical phenomena and analytical applications of helium microwave discharges

The intensity of atomic emission from a microwave-induced helium gas discharge as a function of pressure in the range 13–130 mbar is described. Two of the spectra studied were given by excited helium atoms, and one by (a species of) an excited triplet helium molecule (He₂^*). The pressure dependence of the concentration of helium atoms in the triplet metastable state was studied by absorption spectroscopy. After introduction of known quantities of mercury, chlorine, and iodine into the helium plasma, the emission was measured for some intense lines in the visible and near-u.v. Comparison of the data suggests that the atoms of these elements can be excited by helium atoms and molecules to levels at which they emit light in the visible and near-u.v. The use of a helium discharge tube for the detection of single elements in gas chromatographic fractions is described. Selectivity is greatly improved by wavelength modulation. The method allows a highly sensitive and selective determination of nanogram amounts of organic compounds which contain the elements sought, including stable isotopes such as deuterium, carbon-13 and nitrogen-15.     

Hybrid Monte Carlo — Fluid model for studying the effects of nitrogen addition to argon glow discharges

A computer model is developed for describing argon/nitrogen glow discharges. The species taken into account in the model include electrons, Ar atoms in the ground state and in the 4s metastable levels, N₂ molecules in the ground state and in six different electronically excited levels, N atoms, Ar⁺ ions, N⁺, N₂⁺, N₃⁺ and N₄⁺ ions. The fast electrons are simulated with a Monte Carlo model, whereas all other species are treated in a fluid model. 74 different chemical reactions are considered in the model. The calculation results include the densities of all the different plasma species, as well as information on their production and loss processes. The effect of different N₂ additions, in the range between 0.1 and 10%, is investigated.     

Ionization processes accompaning interaction of excited atoms with CH3CN,CH3COOH,and HCOOH molecules

The fragmentation processes of the molecular ions formed as a result of single collisions of metastable and highly excited Rydberg atoms of noble gases with molecules of acetonitrile, formic acid, and acetic acid have been investigated by a mass-spectrometric method. The correlation between the observed Penning-dissociativeionization mass spectra and the degree of overlap of the moelcular orbitals with vacant orbitals of the metastable atoms determined from the available energy spectra of the electrons formed during Penning ionization has been examined. Complex ions appearing during associative ionization have been discovered. The mechanisms for the formation of the observed ions have been discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 1, pp. 39–46, January–February, 1990.     

Deposition of silicon nitride thin films by reaction of SiH4 in a nitrogen post-discharge

Silicon nitride thin films are deposited on silicon wafers at room temperature when silane gas is injected in a nitrogen flowing post-discharge. Reactive processes involving siane molecules and long-lifetime nitrogen species are studied, pointing out the nonreactivity of the N₂(A³ _u ⁺ ) metastable state, the low contribution of the vibrationally excited nitrogen ground-state molecules, and the high reactivity of N(⁴S) atoms. Spectroscopic observations performed in the reaction region are correlated with thin-film characteristics.     

Studies on lifetime measurements and collisional processes in an inductively coupled argon plasma using laser induced fluorescence

Lifetimes of excited atoms and ions in an inductively coupled argon plasma are obtained by the laser excited fluorescence technique, and quantum efficiencies are calculated for each species. Thermally assisted fluorescence is measured in order to explain the decrease in quantum efficiency with the existence of other excited levels of the same element near the laser excited level. Finally, a discussion is given on the possible collisional population processes.     

Electronic perturbation investigations into excitation and ionization in the millisecond pulsed glow discharge plasma

This study employed a power perturbation method to examine the energy transfer processes at different locations within the afterpeak regime of a millisecond pulsed glow discharge plasma. Brief power perturbation pulses were applied during the afterpeak regime altering the environment of the collapsing plasma. Responses of several transitions to the power perturbations were measured via atomic emission and absorption spectroscopic methods at various distances from the surface of the cathode. The experimental data provide further insight into the energy transfer processes that occur at different spatial locations and in different temporal regimes of these pulsed glow discharge plasmas. Although the enhancement of the large population of metastable argon atoms is again confirmed, the mechanism responsible for this enhancement remains unclear. The most likely possibility involves some form of ion–electron recombination followed by radiative relaxation of the resulting species. The metastable argon atoms subsequently Penning ionize sputtered copper atoms which then appear to undergo a similar ion–electron recombination process yielding variable degrees of observable afterpeak emission for copper atom transitions. The kinetic information of these processes was approximated from the corresponding relaxation time. The electron thermalization time allowing for recombination with ions was found to be ∼25 μs after the discharge power termination.     

Two-Temperature Chemical-Nonequilibrium Modelling of a High-Velocity Argon Plasma Flow in a Low-Power Arcjet Thruster

A numerical simulation has been performed of a high-velocity argon plasma arc flow in a low power arcjet including a finite-rate chemical kinetic model. Electrons, ions, molecular ions ( $ {\text{Ar}}_{2}^{ + } $ ), neutral atoms including the ground and excited argon atoms (Ar^*) are treated as separate species in the plasma mixture. The chemical reactions considered are excitation, de-excitation, ionization and recombination processes, in which reactions involving excited argon atoms (Ar^*) and molecular ions ( $ {\text{Ar}}_{2}^{ + } $ ) are taken into account. The relative importance of different production and loss processes in determining the densities of excited argon atoms and ions is calculated inside the constrictor and expansion portion of the nozzle. The roles of the excited argon atoms and molecular ions are investigated. It is found that excited argon atoms play an important role in the ionization of argon atoms in the core of plasma arc, while the molecular ions have a significant effect on the recombination process at the arc fringes inside the constrictor and in the arc attachment zone of the anode.     

Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. II. H-physisorbed case

Following previous investigation of collision induced (CI) processes involving hydrogen atoms chemisorbed on graphite [R. Martinazzo and G. F. Tantardini, J. Chem. Phys. 124, 124702 (2006)], the case in which the target hydrogen atom is initially physisorbed on the surface is considered here. Several adsorbate-substrate initial states of the target H atom in the physisorption well are considered, and CI processes are studied for projectile energies up to 1 eV. Results show that (i) Eley-Rideal cross sections at low collision energies may be larger than those found in the H-chemisorbed case but they rapidly decrease as the collision energy increases; (ii) product hydrogen molecules are vibrationally very excited; (iii) collision induced desorption cross sections rapidly increase, reaching saturation values greater than 10 A2; (iv) trapping of the incident atoms is found to be as efficient as the Eley-Rideal reaction at low energies and remains sizable (3-4 A2) at high energies. The latter adsorbate-induced trapping results mainly in formation of metastable hot hydrogen atoms, i.e., atoms with an excess energy channeled in the motion parallel to the surface. These atoms might contribute in explaining hydrogen formation on graphite.     

Spectral, spatial and temporal characteristics of a millisecond pulsed glow discharge: metastable argon atom production

Time resolved atomic emission, atomic absorbance, and laser-induced atomic fluorescence measurements of a millisecond pulsed glow discharge, made perpendicular to the insertion probe, provide temporal profiles of 1s₅ (³P₂) and 1s₃ (³P₀) metastable argon atom populations. Acquisition of these profiles at different spatial positions in the plasma provides data from which two-dimensional spatial plots of relative populations are constructed. Each map, the result of 368 individual pulse profiles, provides insight into the production of metastable argon atoms as a function of time and position within the plasma. During power application, intensities plateau after 3 ms as the plasma reaches a steady state condition. Metastable argon atoms are most abundant 1–2 mm above the cathode surface during this time. Excitation mechanisms such as electron excitation and fast atom/ion impact appear to dominate in this temporal regime. In contrast, argon ion–electron recombination dominates metastable formation after pulse termination. The relative population maximum for metastable argon atoms in the afterpeak shifts to 5–9 mm above the cathode surface. This shift should impact signals for analyte species generated by Penning processes in the plasma. Absorption and fluorescence measurements of the ³P₂ (11.55 eV) and the ³P₀ (11.72 eV) metastable argon atom states indicate possible differences in the populations of these two states between the plateau and afterpeak time regimes.     

稀有气体亚稳态原子与CCl4和CBr4的能量转移反应

CBr_4和CCl_4分子的解离反应前人已做了许多工作,他们分别采用射频放电、电子轰击、He~ 的传能反应等方法研究了CCl_4和CBr_4的解离反应,得到了CCI(A)、CCl~ 、CBr~ 等碎片的发射光谱。有关亚稳态原子与它们的传能反应,只有某些较简单的报道,对传能反应机理也未作深入探讨。本文研究了各种亚稳态原子He(2~3S)、Ne(~3P_(0.2))、Ar(~3P_(0.2))与CCl_4和CBr_4分子的传能反应,并对反应机理进行了初步的讨论和分析。     

Magnetic Analysis of Atomic and Molecular Beams. Recent Applications

A progress report is given on recent advances in the magnetic analysis techniques for atomic and molecular beams. The cases of halogen atoms and of oxygen molecules are considered in detail. Applications are outlined to discharge plasma diagnostics, to the measurements of molecular alignment in seeded beams and to scattering studies of the interaction of open shell atoms.     

Plasma diagnostics of barrier-torch discharge in argon flow in a capillary by cross-correlation spectroscopy

The radiation kinetics of the plasma of barrier-torch disrcharge in argon flow in a capillary has been studied by cross-correlation spectroscopy. It was established that the discharge emission spectrum consists of peaks of electronically excited states of argon, bands of hydroxyl radicals, and a second positive system of nitrogen. An analysis of the spatio-temporal distributions of emission intensity for the selected spectral indicators showed that the causes of the torch are ionization waves that extend through the capillary from the electrode system with a speed of 10⁵ m/s and project up to 3–4 mm. It was established that the formation of electronically excited molecules of nitrogen N₂(C ³Π _u ) in the torch of discharge occurs mainly on the reaction between metastable electronically excited atoms of argon and molecules of nitrogen in the electronic ground state.     

Physical chemistry of plasma-solution systems

The basic physical, physicochemical, and chemical processes occurring in the plasma-solution systems are considered. Data on correlation between the emission intensity of electrolyte-cathode glow discharge and the rate of nonequilibrium discharge-induced vaporization of the solution are presented. A mechanism for the appearance of the atomic emission threshold of metal atoms in the plasma zone is proposed. The role of chemically active species generated by ion bombardment in chemical processes occurring in solutions is shown.     

Novel techniques for plasma diagnostics: Electron paramagnetic resonance and laser-induced fluorescence spectroscopy

Diagnostic techniques for low-pressure, cold plasmas have mostly been limited to emission and mass spectrometry. Herein, the techniques of gas-phase electron paramagnetic resonance and laser-induced fluorescence spectroscopy are briefly reviewed. Particular attention is paid to their attributes which make them good candidates for plasma diagnostic tools. It is found that gas-phase electron paramagnetic resonance can be used to determine and monitor the absolute concentration of a number of important plasma species, e.g., free radicals and atoms. Laser-induced fluorescence can also monitor, with even more sensitivity, but perhaps not so well absolutely, the concentrations of many plasma species, e.g., free radicals, metastable excited states, and molecular ions.     

Metastable fragment production by electron-impact dissociation of SO2

Time-of-flight detection of neutral metastable fragments has been applied to the study of electron-impact dissociation of SO₂ molecules. The time-of-flight distributions indicate that a variety of processes are involved in the production of metastable fragments with a wide range of energies. Rydberg S and O atoms and SO molecules have been identified. Data for these processes have been measured up to 300 eV. The results presented are compared with other studies of fragmentation of SO₂.     

Excitation of singly ionized argon species in helium-matrix glow discharge plasma—role of the energy transfer from helium metastables

When a small amount of argon is added to the helium plasma in a Grimm-type glow discharge radiation source, the interaction between helium and argon species is investigated from analyzing the intensities of emission lines of of argon ion (ArII). The excitation energy as well as the term multiplicity concerning the optical transitions to which the ArII emission lines are identified are significant factors for determining their emission intensities in the helium-matrix plasma. In the case where the excitation energy of ArII lines is higher than the internal energy of the helium metastable states, the emission intensity in the helium-matrix plasma is observed to be much weaker than that obtained only with argon gas. On the other hand, the intensity is enhanced when the excitation energy is slightly lower. In the excited levels of argon ion having quartet multiplicity, closer interactions with the triplet rather than the singlet metastable level of helium atom are recognized, with the singlet helium metastable in the argon excited levels having doublet multiplicity.     

氧气常压介质阻挡放电的发射光谱及能量传递机理

为研究氧气常压介质阻挡放电中的物理化学行为, 以纯氧作为放电体系, 用发射光谱(optical emission spectroscopy)诊断技术分析了等离子体中可能存在的化学活性物种. 利用在500-950 nm范围的氧原子发射光谱计算出等离子体中的电子温度为(1.02±0.03) eV; 观测了760 nm处的具有清晰转动结构的氧气A带(atmospheric band)O2(b1∑+g-X3∑-g), 并用其转动结构计算了转动温度(气体温度)为(650±20) K; 在500-700 nm范围观测了氧气的第一负带系(first negative system) O+2(b4∑-g-a4∏u), 在190-240 nm范围观测了微弱但特征清晰的氧气的Hopfield带系O+2(c4∑+u-b4∑-g). 研究发现, 在氧气常压介质阻挡放电等离子体中存在多种激发态氧原子、激发态氧气分子、基态和激发态氧气分子离子等反应活性物种, 这些活性物种的形成涉及氧气分子的激发、解离和电离等多种过程, 每个过程都包含多个能量传递步骤, 氧分子解离产生的氧原子是导致一系列高激发态氧原子生成和氧气电离激发的主要因素.     

Vibrationally excited molecules and mechanisms of chemical and physical processes in non-equilibrium plasmas

The results of theoretical and experimental investigations of some physical and chemical processes caused by collisions between vibrationally excited molecules (such as molecular dissociation, molecular electronic excitation, ionization and ion conversion, gas heating) under non-equilibrium plasma conditions of low-pressure electrical discharges are presented. It is shown that the role of vibrationally excited molecules in these processes is very large for nitrogen molecules, in which the probability of V—T exchange is much smaller than in other molecules for low gas temperatures.