Excited Rare Gas Atoms Research Articles

2D collisional-radiative model for non-uniform argon plasmas: with or without ‘escape factor’

Collisional-radiative models for excited rare-gas atoms in low-temperature plasmas are a widely investigated topic. When these plasmas are optically thick, an ‘escape factor’ is introduced into the models to account for the reabsorption of photons (so-called radiation trapping process). This factor is usually obtained assuming a uniform density profile of the excited species; however, such an assumption is often not satisfied in a bounded plasma. This article reports for the first time a self-consistent collisional-radiative model without using an ad hoc ‘escape factor’ for excited Ar atoms in the 2p states (in Paschen’s notation). Rather, the rate balance equations—i.e. the radiation transfer equations—of the 2p states are numerically solved to yield the actual density profiles. The predictions of this self-consistent model and a model based on the escape factor concept are compared with spatially-resolved emission measurements in a low-pressure inductive Ar plasma. The self-consistent model agrees well with the experiment but the ‘escape factor’ model shows considerable deviations. By the comparative analysis the limitations and shortcomings of the escape factor concept as adopted in a significant number of works are revealed.

Time-resolved microplasma electron dynamics in a pulsed microwave discharge

Microwave-driven microplasmas are typically operated in a steady-state mode in which the electron temperature is constant in time. Transient measurements of excitation temperature and helium emission lines, however, suggest that short microwave pulses can be used to increase the electron energy by 20–30%. Time-resolved optical emission spectrometry reveals an initial burst of light emission from the igniting microplasma. This emission overshoot is also correlated with a measured increase in excitation temperature. Excimer emission lags atomic emission, however, and does not overshoot. A simple model shows that an increase in electron temperature is responsible for the overshoot of atomic optical emission at the beginning of each microwave pulse. The formation of dimers and subsequent excimer emission requires slower three-body collisions with the excited rare gas atoms, which is why excimer emission does not overshoot the steady-state values. Similar results are observed in argon gas. The overshoot in electron temperature may be used to manipulate the collisional production of species in microplasmas using short, low-duty cycle microwave pulses.

Energy Transfer Kinetics of the np5(n + 1)p Excited States of Ne and Kr

Energy transfer rate constants for Ne(2p(5)3p) and Kr(4p(5)5p) atoms colliding with ground state rare gas atoms (Rg) have been measured. In part, this study is motivated by the possibility of using excited rare gas atoms as the active species in optically pumped laser systems. Rg(np(5)(n + 1)s) metastable states may be produced using low-power electrical discharges. The potential then exits for optical pumping and laser action on the np(5)(n + 1)p ↔ np(5)(n + 1)s transitions. Knowledge of the rate constants for collisional energy transfer and deactivation of the np(5)(n + 1)p states is required to evaluate the laser potential for various Rg + buffer gas combinations. In the present study we have characterized energy transfer processes for Ne (2p(5)3p) + He for the six lowest energy states of the multiplet. Rate constants for state-to-state transfer have been determined. Deactivation of the lowest energy level of Kr (4p(5)5p) by He, Ne, and Kr has also been characterized. Initial results suggest that Kr (4p(5)5p) + Ne mixtures may be the best suited for optically pumped laser applications.

Discharge instability induced by external laser preionization of a XeF discharge laser

External-laser-induced preionization of excimer lasers was investigated. A discharge XeF laser was preionized by two different UV lasers [a KrF laser (λ=249 nm) and an ArF laser (λ=193 nm)], and the improvements in performance of the XeF laser were compared. The XeF laser beam profiles were measured by an intensified CCD (ICCD) camera with temporal resolution of 10 ns. Striated XeF laser profiles were obtained with 249 nm laser preionization, whereas there was no striation in the profiles for 193 nm laser preionization. These striations originated from discharge in the XeF laser induced by laser preionization. The influence of excited rare-gas atoms on the discharge instability was examined.

Collisional deexcitation of the excited rare gas atoms in resonant states: The Watanabe–Katsuura theory revisited

The cross sections for the collisional deexcitation of neon atoms in the lowest excited P11 state by Ar, Kr, Xe, N2, O2, CO, NO, and CH4, and in the lowest excited P13 state by O2 and CH4 have been measured at a mean collisional energy corresponding to room temperature. Data are also included for collisions of argon atoms in the lowest excited P11 and P13 states by C2H4, cyclo-C3H6, and C3H8, and collisions of krypton atoms in the lowest excited P11 and P13 states by C2H4 and cyclo-C3H6. The measured cross sections, together with those obtained in our previous studies, are compared with the cross sections calculated using the Watanabe–Katsuura theory. An extension of the Watanabe–Katsuura theory to the deexcitation of excited rare gas atoms in collisions with molecular quenchers, not atoms, is examined.

OPHTHALMIC LASERS FOR THE NEXT MILLENNIUM

Ophthalmologists, with respect to both therapeutic and diagnostic applications, have traditionally been at the forefront of laser technology. The proliferation of new technologies and the ingenious application of these technologies over the last several decades ensures that ophthalmologists will continue to be at the vanguard of laser technology well into the next millennium. This article reviews both past and present applications of laser technology and provides an overview of future developments in laser technology within the field of ophthalmology. The history of lasers dates to 1960, when the first functioning laser was demonstrated by Maiman. 76 It was not until 1963 and 1964 when Campbell 19 and Zweng, 141 respectively, reported the first clinical ophthalmic use of a laser in humans, however. They employed a pulsed ruby laser for retinal photocoagulation. They found that laser photocoagulation was efficient, effective, and did not require anesthesia or akinesia. They found that the use of a pulsed laser led to the formation of retinal hemorrhages and had difficulty producing full-thickness retinal burns; this led to the development of continuous wave, gas ion lasers. In 1964, the argon laser, which provides an emission spectrum that is well absorbed by hemoglobin and is used in a continuous mode, was developed. L'Esperance conducted the first human photocoagulation trial for ophthalmic disease using the argon laser in 1968 69 ; he also introduced the frequency-doubled neodymium:yttrium-aluminum-garnet (Nd:YAG) and krypton lasers in 1971 and 1972, respectively. The use of Q-switched and mode-locked Nd:YAG lasers in 1980 and 1981, respectively, provided the ability to produce short bursts of energy that allowed transparent membranes (vitreous, posterior capsules) to be cut. The tunable dye laser was introduced in 1981 and provided the theoretical advantage of a variable output wavelength to match the absorption spectra of specific ocular tissue. The semiconductor infrared diode laser was developed in 1962. Since then, the diode laser has been employed in multiple delivery modes: transpupillary slit lamp, transpupillary laser indirect, transcleral, and endophotocoagulation. It has proven useful in the treatment of choroidal neovascularization, proliferative diabetic retinopathy, retinopathy of prematurity, macular edema, and choroidal melanoma. 84,103 The midinfrared CO 2 laser was developed in 1964 and has been used to perform corneal ablations. 102 This application has been limited by tissue shrinkage and thermal damage to surrounding tissue. 57 In the 1980s, the CO 2 laser was used in skin resurfacing and is now a common technique of rhytid and scar removal. More recently, the CO 2 laser has been used to perform soft-tissue incisional surgery, such as for incisional blepharoplasty. A variety of excimer lasers have been developed. These lasers are capable of emitting pulses of 10 ns and the active medium consists of an excited rare-gas atom with a halogen molecule, resulting in an excited dimer, or excimer. 18 The initial use of the excimer laser, in the early 1980s, was to precisely etch submicrometer patterns into polymer materials for use in the semiconductor industry. 31,61 Srinivasan 120 has termed this controlled removal of material, in which molecules on the irradiated surface are broken into small volatile fragments, ablative photodecomposition. The first ophthalmic use of the excimer laser was reported in 1983 by Trokel, who used the excimer laser to achieve precise etching of the cornea. 130 Since then, a wealth of activity has centered on corneal ablation by the excimer laser, leading to the current interest in laser refractive surgery. A host of new lasers, such as the erbium:YAG (Er:YAG), Q-switched 532 nm Nd:YAG and holmium:YAG lasers promise either improvements in current techniques or novel applications. These new laser techniques may widen the indications for refractive surgery, improve outcomes in glaucoma surgery, obviate the need for vitroretinal surgery in some cases, improve outcomes in oculoplastic procedures, and even help to remove cataracts. Table 1 describes these new ophthalmic lasers and their associated features. Table 2 lists novel applications of existing laser technology.

Excitation of vacuum ultraviolet transitions in a discharge of rare-gas mixtures with alkali-metal vapours

Collisional energy transfer of excited rare-gas atoms with alkali-metal atoms was studied in a gas discharge of their mixtures by monitoring the vacuum ultraviolet (VUV) line intensities and by a comparison with the approximate solutions of the rate equations. The method used enabled us to obtain depletion cross sections of rare-gas atom resonance states by alkali-metal atoms in a discharge of their mixtures for the following atom pairs: He* by Rb sigma d=8.8*10-15 cm2, Ne* by Rb sigma d=2.0*10-16 cm2 and Ar* by Cs sigma d=1.2*10-15 cm2. The fundamental processes of atomic collisions producing alkali-metal quartet quasimetastable highly excited states in a discharge plasma were studied. The cross section of Rb(4p55s5p)4S32/ quasimetastable excitation in non-elastic collisions with neon metastable atoms was measured to be 2.0(+or-0.8)*10-15 cm2. The quasiresonant charge transfer from a caesium ion metastable to a caesium atom produces Cs(5p55d6s)4P52/ quasimetastable states with a cross section of 9(+or-3)*10-15 cm2. These reactions lead to quasimetastable level population in the discharge, which is important for applications in VUV laser schemes.

Interaction processes with creation of fast electrons in the low temperature plasma

The methods and the results of investigations of the reactions in which electrons appear with energies surpassing the average electron distribution's energy in a plasma are considered. Particular attention is paid to the plasma electron spectroscopy method, which combines the advantages of the elementary processes examination in a plasma with the possibilities of conventional electron spectroscopy. Data of the study of chemiionization reactions involving two excited rare gas atoms, Penning ionizations of atoms and molecules by the helium metastable atoms and quenching of excited inert gases and mercury atoms by the electrons are given. The influence of the processes of creating fast electrons on the plasma properties is discussed.

Inelastic processes in the scattering of metastable rare gas atoms at metal surfaces

Electron spectra of various metastable rare gas atoms systematically measured on a Pt(111) surface with Rb coverages ranging from submonolayers (3%) to multilayers are presented. The different decay channels of the excited particles are discussed in terms of resonant electron exchange processes between the substrate and the projectile in relation to the work function. It is shown that below a certain value of the work function a highly excited negative rare gas atom is formed which can undergo different de-excitation processes. A careful discussion of the branching ratios into the decay channels offers a natural explanation of the variations in the electron spectra induced by alkali metal adsorption. Additionally, an attempt is made to extract information about the alkali metal chemisorption state from the observed electron spectra.

The A 2Σ+ state of rare gas–NO van der Waals molecules probed by 1+1 multiphoton ionization spectroscopy

The previously unobserved bound–bound spectra of ArNO, KrNO, and XeNO have been observed slightly blue shifted from the (1,0) and (0,0) bands of the A 2Σ+←X 2Π1/2 transition of uncomplexed nitric oxide. Although the structured but incomplete spectra cannot be assigned with certainty, limits to the ground and excited state bond dissociation energies, D″0 and D0, respectively, can be estimated. For ArNO these limits are D″0 ≥89 and 54 cm−1≤D0 ≤101 cm−1. The observed fragmentation of the KrNO and XeNO molecules, coupled with earlier results for MPI via the C 2Π state, suggests that superexcited, autoionizing states of the van der Waals molecules dissociate to yield excited rare gas atoms whenever energetically possible.

Gas-phase collision dynamics by means of pulse-radiolysis methods

After a brief survey of recent advances in gas-phase collision dynamics studies using pulse radiolysis methods, the following two topics in our research programs are presented with emphasis on the superior advantages of the pulse radiolysis methods over the various methods of gas-phase collision dynamics, such as beam methods, swarm methods and flow methods. One of the topics is electron attachment to van der Waals molecules. The attachment rates of thermal electrons to O 2 and other molecules in dense gases have been measured in wide ranges of both gas temperatures and pressures, from which experimental evidence has been obtained for electron attachment to van der Waals molecules. The results have been compared with theories and discussed in terms of the effect of van der Waals interaction on the electron attachment resonance. The obtained conclusions have been related with investigations of electron attachment, solvation and localization in the condensed phase. The other is Penning ionization and its related processes. The rate constants for the de-excitation of He(2 1P), He(2 3S), Ne( 3P 0), Ne( 3P 1), Ne( 3P 2), Ar( 1P 1), Ar( 3P 1), by atoms and molecules have been measured in the temperature range from 100 to 300 K, thus obtaining the collisional energy dependence of the de-excitation cross sections. The results are compared in detail with theories classified according to the excited rare gas atoms in the metastable and resonance states.

Dynamic effects of surface plasmons on the lifetimes of neutral excited rare-gas atoms physisorbed on metal surfaces

Numerical calculations are presented for the optical spectra of neutral rare-gas atoms physisorbed on metal surfaces. The excitation spectra of adsorbates on metal surfaces have been calculated at low coverage and when the nonradiative processes due to the damping of the surface plasmons dominate. Numerical results indicate that the peaks of the symmetric and antisymmetric modes arising from two nearby excited states of the physisorbed atom on the metal surface cancel each other out provided that the frequency profiles of the peaks in question overlap. The resulting excited geometrical configurations arising from the cancellation process between the spectra of the symmetric and antisymmetric modes have been computed as a function of the distance R from the atom to the metal surface, and they are presented graphically. The vanishing computed spectra of Xe and Al and Au and the persistence of the spectra of Xe on Mg, Cu and Ar on Au and Mg are compatible with experimental observations. Predictions have been...

Dynamic effects of surface plasmons on the lifetimes of neutral excited rare-gas atoms physisorbed on metal surfaces Radiative and nonradiative processes

We have considered the excitation spectra of neutral rare-gas atoms physisorbed on metal surfaces. The adsorbed atom and its image interact through the dipole-dipole interaction and radiate to each other as well. The charge of the image atom is screened by the dielectric function of the surface plasmons. Due to the common radiation field between the atom and its image, the excitation spectra consist of the symmetric and antisymmetric modes, respectively. Each of them splits into two excitations: the atomic-like and the surface plasmon-like excitations, which arise because of the presence of the surface plasmons. The surface plasmon-like excitations appear near the surface plasmon frequencies and consist of broad spectral lines, which have large radiative widths and small relative intensities in comparison with those of the atomic like excitations, that emerge near the atomic frequencies. The spectral functions describing the symmetric and antisymmetric modes have been calculated in the presence of the plasmon damping and consist of asymmetric Lorentzian lines, where the extent of the asymmetry depends on the strength of the surface plasmons. Competition between the cooperative radiative and non-radiative processes takes place. In the absence of plasmon damping or when the effective radiative damping is greater than the damping of the surface plasmons, the largest enhancement of the relative intensities per atom occurs for the spectra of the symmetric modes of the excited Xe, Kr and Ar when they are physisorbed on Mg with K and Li holding the second and third place, respectively. The relative intensities per atom for the spectra of Xe, Kr and Ar on the surfaces of A1, Cu, Ag and Au are much less than the corresponding ones for the single free atoms in question, respectively. The enhancement or the decrease of the maximum relative intensity per atom is due to the dynamic effect arising from the presence of the surface plasmons. In the opposite limit, when the nonradiative process due to the damping of the surface plasmons dominates, the relative intensities per atom for the peaks of the symmetric and anti-symmetric modes take positive and negative values describing the physical processes of absorption and stimulated emission, respectively. Hence, a cancellation is expected to occur between the peaks of the symmetric and anti-symmetric modes, which arise from two nearby excited states of the physisorbed atom on the metal surface, provided that an overlap exists between the frequency profiles of the two peaks in question. The red (blue) shifted peak of the symmetric mode of the higher (lower) excited state and the blue (red) shifted peak of the antisymmetric mode of the lower (higher) excited state of the atom cancel each other out provided that the frequency profiles of their lineshapes nearly coincide. This may be a possible explanation of the persistence-extinction phenomenon that has been observed for a number of rare-gas metal substrate systems in the low coverage limit, where it has been proposed that a charge-transfer instability exists. The computed spectra of the symmetric and antisymmetric modes for excited Xe, Kr and Ar physisorbed on the metal surfaces of K, Li, Mg, Al, Cu, Ag, Au and Ti are presented graphically and discussed. Results of numerical calculations indicate that at low coverage the peaks of excited Xe on Al, Ti and Au and excited Kr on Au, vanish; this is compatible with the experimental observations. At low coverage, the height of the peaks of the spectral lines is shown to be proportional to the number of atoms at the surface of the metal N and, hence, the total spectral function describing the exctiation spectra varies as N2; this is compatible with the observed data.

Angular-momentum-transfer formalism for photoionization of excited rare-gas atoms.

Expressions for photoionization channel amplitudes are derived applying the Dill and Fano angular-momentum--transfer formalism to the j-l coupling scheme. This expression will be useful in understanding excited-state photoeffects of rare-gas atoms. The situation in which the present formula reduces to that of the Cooper-Zare model is also discussed.

Preparation of polarized excited rare gas atom statesR(np 5(n+1)s) andR(np 5(n+1)p) by laser optical pumping

On the basis of rate equations we have theoretically analyzed all optical pumping schemes, which arise from the rare gas transitionsR(np 5(n+1)s,J i =0, 2) toR(np 5(n+1)p,J f =1, 2, 3) for linearly, circularly and unpolarized light propagating in one direction. Analytical formulae are given for the time-averaged or steady state populations of the magnetic substates and their state multipoles, as a function of the partial decay rateA fi , the natural lifetime τ of the upper level, and the laser induced pumping rateb. Numerical values of these quantities are presented for the case of actual interestR=Ne.

Charge transfer dynamics of adsorbed excited rare gas atoms: A simple model calculation

Optical spectra of rare gas atoms adsorbed on metal surfaces exhibit a bimodal behavior, which might be correlated with the differenceI*-ϕ between the ionization energy of the excited rare gas atomI* and the metal work function ϕ. In this paper we propose a simple model which allows to treat this charge transfer and some accompanying many body effects in detail. The model consists of three coupled oscillators: The atomic dipole, the particle-hole excitations of the combined system (atomicp-hole plus metallic particle states above the Fermi level) and the plasmons of the metal. An improved charge transfer criterion is obtained which, besides the parameterI*-ϕ, involves additional parameters such as the adsorbate-metal coupling strength and the plasmon frequency.

The rare gas sensitized radiolysis of hydrogen chloride

The γ-radiolysis of hydrogen chloride in the presence of argon, krypton and xenon has been investigated. The experimental results have shown that the decomposition of hydrogen chloride in collision with the excited rare gas atoms is inefficient, most probably because excitation transfer occurs via (RHCl∗) complexes. Rate constant ratios of the collisional deactivation of these complexes have been evaluated. Reactions of the electron involved in the Xe-HCl system are discussed. It has been found that only part of the thermal electrons captured by HCl forms H 2 as a final product.

Matrix-isolation study of the decomposition of CF3NNCF3 by photons and by excited rare-gas atom bombardment at energies between 4.9 and 16.8 eV

Abstract When dilute solid solutions of CF 3 NNCF 3 in argon at 14 K are irradiated by the full light of a medium-pressure mercury arc, no net photolysis occurs, suggesting that the primary photodecomposition products, CF 3 N 2 + CF 3 , undergo cage recombination. On 122 nm photolysis, cage recombination leads instead to the appearance of prominent infrared absorption of C 2 F 6 , suggesting an initial photodecomposition process to form 2 CF 3 + N 2 . Prominent infrared absorptions of CF 4 and CF 2 were shown to result from the photodecomposition of C 2 F 6 . Argon-resonance radiation does not penetrate solid Ar:CF 3 NNCF 3 deposits, but when photolysis is conducted concurrently with deposition a high yield of CF 3 is isolated in the argon matrix. Circumvention of the cage effect in this system is attributed to the large amount of excess energy with which the photofragments are formed and to a very short lifetime for the excited electronic state of CF 3 NNCF 3 before dissociation. Even higher yields of CF 3 are formed when the Ar:CF 3 NNCF 3 sample is codeposited with a beam of argon atoms excited in a microwave discharge. The effects of concentration and experimental configuration on the product yield are discussed. When the sample is codeposited with a beam of excited neon atoms (16.6–16.8 eV); the most prominent product absorption is that of CF 3 + , with relatively weak absorptions of CF 3 , suggesting that at this energy the primary photodecomposition process leads to the formation of CF 3 + CF 3 + + N 2 + e.

Energy Transfer and the Choice of Gas Fillings for the Photoionization Proportional Scintillation Chamber

Energy transfer between excited rare gas atoms and/ or molecules to rare gas atoms is exploited in order to get suitable cheap fillings for the photoionization proportional scintillation chamber (PIPS). In particular, variation of the emission with the pressure of both donor and acceptor is studied. Xenon doped argon and krypton doped argon are considered and its behaviour compared to pure argon.

Dynamics of localized excitations from energy and time resolved spectroscopy

The electronic structure and the excited state relaxation phenomena in condensed rare gases have been studied by absorption spectroscopy, photoelectron spectroscopy, and time resolved luminescence spectroscopy, all using selective excitation by monochromatized synchrotron radiation. Radiationless and radiative electronic transitions in excited states and the rearrangement of surrounding matrix atoms will be discussed for excited rare gas atoms in rare gas matrices. Furthermore, experimental results concerning the scattering of excitons within the free exciton branches, localization of excitons, relaxation within the self-trapped exciton states, and energy transfer to guest atoms and boundaries will be presented.