Vacuum Ultraviolet Absorption Research Articles

Vacuum ultraviolet absorbance of alkanes: an experimental and theoretical investigation

The electronic absorption spectra of alkanes are known to be broad and lack discrete band structures. Previous studies have suggested their HOMO-LUMO energy gaps could be used to understand the absorbance edges of these spectra. With the advent of a new benchtop vacuum ultraviolet (VUV) spectroscopic absorption detector, it is now possible to collect VUV absorption spectra (from 120 to 240 nm) for an extended range of structures under an inert gas phase environment more conveniently. The previously reported hypothesis was revisited and its limits were explored using a combination of new extended experimental VUV spectral data and theoretical quantum mechanics calculations. It was pointed out from this study that the first strong excitations of alkanes are not always the HOMO-LUMO transition. As a result, the HOMO-LUMO energy gap of alkanes should not be used directly as a universal and reliable parameter to understand their experimental absorbance edges. VUV spectral data for a larger variety of alkanes structures were reported and the relation between these structures and spectra was discussed.

Vacuum-ultraviolet absorption cross-sections of functionalized cyclic hydrocarbons: Six-membered rings

Absorption cross-sections were measured in the vacuum ultraviolet (9.92–5.17 eV) using differential absorption spectroscopy for a series of six-membered hydrocarbon rings: cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, benzene, tetrahydropyran, 3,4-dihydro-2H-pyran, 1,4-dioxane, 1,4-dioxene, cyclohexanol, cyclohexen-4-ol, cyclohexen-3-ol, cyclohexanone, cyclohexen-4-one, cyclohexen-3-one, 3,4-epoxycyclohexene, and 4,5-epoxycyclohexene. In addition, cross-sections for 1-butene and acetic acid were measured for comparison with the literature. Uncertainties were quantified in all cases by accounting for errors in gas-phase concentration, experimental repeatability, and signal-to-noise ratio as a function of photon energy. Convolving the sources of error using the root-sum-square method led to an upper limit of 5% uncertainty above the detection limit, which is largely attributable to chemical purity.The primary objective of the present work is to provide absolute cross-sections along with quantified uncertainty limits. The majority of the absorption spectra, which reflect electronic transitions such as σ → σ* and n → σ*, are reported for the first time and provide insight into fundamental chemical physics, such as vibrational band structure and Rydberg transitions. In addition, the absorption cross-sections enable quantitative isomer-resolved speciation measurements in combustion chemistry, photolysis calculations for laser spectroscopy and atmospheric chemistry, and spectral assignments in astrochemistry.

Correlation between the Carbon Nanotube Growth Rate and Byproducts in Antenna-Type Remote Plasma Chemical Vapor Deposition Observed by Vacuum Ultraviolet Absorption Spectroscopy.

For sp2 or sp3 carbon material growth, it is important to investigate the precursors or intermediates just before growth. In this study, the density of ethylene (C2 H4 ) outside the plasma discharge space and just before reaching the carbon nanotube (CNT) growth region is investigated by vacuum ultraviolet absorption spectroscopy for plasma discharge in an antenna-type remote plasma chemical vapor deposition with a CH4 /H2 system, with which the growth of very long (≈0.5 cm) CNT forests is achieved. Single-wall CNT forests have the potential for application as electrodes in battery cells, vertical wiring for high current applications, and thermal interface materials. It is observed that the plasma discharge decomposes the CH4 source gas and forms C2 Hx species, which reversibly reform to C2 H4 in the plasma-off state. In addition, the density of the formed C2 H4 has a strong correlation to the CNT growth rate. Therefore, the C2 H4 density is a good indicator of the density of C2 Hx species for CNT growth in the CH4 /H2 plasma system.

Experimental and theoretical studies on the absorption spectra of n-dodecane in the IR and VUV regions

We report here a comprehensive spectroscopic study of the absorption spectrum of the n-dodecane molecule using synchrotron radiation based photoabsorption and FTIR spectroscopy. Quantum chemical calculations using the DFT and TDDFT methodologies are used to predict relevant ground and excited state properties and correlate them with the experimental results. In the IR spectrum, assignment of the prominent CH stretching bands in the 2800–3000 cm−1 region are consolidated while a few new vibrational assignments are made in the 700–1500 cm−1 region. The electronic absorption spectrum of n-dodecane lies entirely in the vacuum ultraviolet (VUV) region and consists of a broad continuous absorption band starting from ∼7.5 eV, with no discernible structure. TDDFT calculations of vertical excited states predict that most of the excited states are Rydberg in nature. Potential energy curves of the first few excited states with respect to various bond lengths and bond angles are studied in order to gain additional insights into the nature of the excited states. The broad nature of the absorption is attributed partly to the effect of several conformers as well as hot bands from low frequency modes in the ground state and partly due to overlap of several Rydberg series converging to the first few ionization potentials (IPs). Energy separation between the first and second IP is theoretically predicted to be ∼0.65 eV, while successive separation between the second, third, fourth and fifth IPs is ∼0.03 eV. To the best of our knowledge this is the first report of the VUV absorption spectrum of n-dodecane in the region from 6 to 10 eV, as also the first report of theoretical calculations of its electronically excited states.

Application and evaluation of nitriding treatment using active screen plasma

Plasma nitriding was performed on samples of JIS SKD61 die steel, and the microstructure near the surface was analyzed. The plasma nitriding was performed using active screen plasma (ASP). In the ASP nitriding process, a dependency on the bias current applied to the sample was found.Emission spectra showed the presence of atomic nitrogen ions (N+), molecular nitrogen ions (N2+), hydrogen radicals (H), nitrogen radicals (N), and radicals consisting of nitrogen and hydrogen (NiHi). The densities of nitrogen and hydrogen radicals observed by vacuum ultraviolet absorption spectroscopy were 1.5 × 1011 and 1.0 × 1011 cm−3, respectively. Once the device current reached 0.25 A or more, the surface became cloudy. The cause of this cloudiness was investigated by X-ray diffraction and cross-sectional bright-field scanning transmission electron microscopy and was found to be a deposition layer around 50 nm thick of a mixed crystal containing the ε phase Fe23N and γ' phase Fe4N and the formation of a structure with a period of around 300 nm at the surface. Although deposition of Fe23N and Fe4N also occurred at the surface in samples without cloudiness, the deposition layer was only around 10 nm thick, and no periodic structure was formed. A chromium oxide-rich layer was found directly under the Fe23N and Fe4N deposition layer by scanning transmission electron microscopy with electron energy-loss spectroscopy. This chromium oxide-rich layer likely occurred due to selective etching of iron oxide and chromium oxide. In ASP nitriding method, the sputtering action of charged particles of N2+ and N+ was experimentally shown to play an important role in nitriding and surface morphology. Furthermore, it was discovered that there was no compound layer several micrometers thick as would form in the conventional nitriding process.

Evaluation and application of gas chromatography - vacuum ultraviolet spectroscopy for drug- and explosive precursors and examination of non-negative matrix factorization for deconvolution.

Since the introduction of a benchtop vacuum ultraviolet (VUV) absorption spectroscope with an increased wavelength range towards to the high energetic ultraviolet radiation, gas chromatography coupled to VUV has been proven a powerful tool in several fields of application such as petroleomics, permanent gas analytic, pesticide analytic and many more. In this study, the potential of GC-VUV for investigations was examined, focusing on drug- and explosive precursors as well as chemical warfare simulants. The ability of VUV absorption spectra to differentiate isomers is presented, among others for nitroaromatics. In addition, the limit of detection for target compounds was determined to 0.7 ng absolute on column. Furthermore, non-negative matrix factorization (NMF) was successfully implemented as alternative deconvolution approach and evaluated for the deconvolution of unknown substances. In comparison, the spectral library-based deconvolution was applied to a standard mixture and a simulated case study. The results reveal that the NMF is a useful additional tool for deconvolution because, unlike library-based deconvolution, it allows to investigate unknown substances as well.

Electron-Induced Reactions in 3-Bromopyruvic Acid.

3-Bromopyruvic acid (3BP) is a potential anti-cancer drug, the action of which on cellular metabolism is not yet entirely clear. The presence of a bromine atom suggests that it is also reactive towards low-energy electrons, which are produced in large quantities during tumour radiation therapy. Detailed knowledge of the interaction of 3BP with secondary electrons is a prerequisite to gain a complete picture of the effects of 3BP in different forms of cancer therapy. Herein, dissociative electron attachment (DEA) to 3BP in the gas phase has been studied both experimentally by using a crossed-beam setup and theoretically through scattering and quantum chemical calculations. These results are complemented by a vacuum ultraviolet absorption spectrum. The main fragmentation channel is the formation of Br- close to 0 eV and within several resonant features at 1.9 and 3-8 eV. At low electron energies, Br- formation proceeds through σ* and π* shape resonances, and at higher energies through core-excited resonances. It is found that the electron-capture cross-section is clearly increased compared with that of non-brominated pyruvic acid, but, at the same time, fragmentation reactions through DEA are significantly altered as well. The 3BP transient negative ion is subject to a lower number of fragmentation reactions than those of pyruvic acid, which indicates that 3BP could indeed act by modifying the electron-transport chains within oxidative phosphorylation. It could also act as a radio-sensitiser.

Systematic diagnostics of the electrical, optical, and physicochemical characteristics of low-temperature atmospheric-pressure helium plasma sources

The performance characteristics of two plasma sources, based on different types of discharge, were assessed. Three primary aspects of these sources were examined. These were electrical parameters (voltage and current flowing through a target), plasma parameters (gas temperature and electron density, determined using optical emission spectroscopy), and levels of gas-phase oxygen atoms (measured by vacuum ultraviolet absorption spectroscopy) and liquid-phase ▪OH radicals (generated by plasma treatment of water and detected using an electron spin resonance spin-trapping technique). As a result, there were few significant differences such as electron density, oxygen atom density, and gas temperature between the two plasma sources. However, the time-averaged electrical currents flowing to the target and the amount of liquid-phase ▪OH radicals showed a large difference, especially when the plume made contact with the target. Thus, many points of similarity but some differences in the two devices were found by the benchmarking study. These shall contribute to our understanding of the mechanisms for each consequence in medical applications.

Vacuum ultraviolet excited state dynamics of the smallest ring, cyclopropane. I. A reinterpretation of the electronic spectrum and the effect of intensity borrowing.

Cyclopropane, the smallest organic ring compound, exhibits complex spectroscopy and excited state dynamics. In Paper I, we reinterpret the vacuum ultraviolet (VUV) electronic absorption spectrum of cyclopropane via ab initio computation. The first two bands in the VUV spectrum are simulated using wavepacket propagations employing the multiconfigurational time-dependent Hartee method and a newly parameterized linear vibronic coupling Hamiltonian. The parameters of the model Hamiltonian are obtained directly from high level multireference configuration interaction calculations. An analysis of the results, with an emphasis on previously neglected vibronic coupling effects, reveals that these vibronic coupling terms must be included in order to account for strong intensity borrowing effects. This treatment dramatically changes the assignment of much of the VUV spectrum, with intensity borrowing by the optically dark (σ3p x /3p y ) and (σ3p x /3p y ) states from the E'(σ3p x /3p y ) state being found to give rise to almost all the spectral intensities below 8 eV. This is in stark contrast to previous studies, which attributed the first two bands to transitions to the E'(σ3p x /3p y ) state. This highlights the limitations of assigning spectral features based solely on calculated electronic excitation energies and oscillator strengths. Furthermore, we address the significant but infrequently discussed difficulties involved in determining the electronic character of a wavepacket produced in the pump step of ultrafast pump-probe experiments for systems exhibiting strong vibronic coupling.

Mechanism of asymmetric etched profiles in trimming process

In order to improve asymmetric etched profiles at the wafer edge region, the atomic oxygen density distributions and the degree of the asymmetric etched profiles in the trimming process were evaluated with a plasma process system employing vacuum ultraviolet absorption spectroscopy. The non-uniform atomic oxygen distribution was found to create the asymmetric etched profile having a larger trimming amount at the higher atomic oxygen density side compared to that at the opposite side. There is a strong relationship between the degree of asymmetric etched profiles and the gradient of atomic oxygen density distributions. Furthermore, formulation of the degree of the asymmetric etched profiles revealed that a shorter mean free path under higher gas pressure conditions could improve the asymmetric etched profiles. It was experimentally confirmed that the degree of the asymmetric etched profiles was improved by around 70% when the gas pressure was increased from 2.7 to 10.8 Pa.

VUV Absorption Spectra of Gas-Phase Quinoline in the 3.5-10.7 eV Photon Energy Range.

The absorption spectrum of quinoline was measured in the gas phase between 3.5 and 10.7 eV using a synchrotron photon source. A large number of sharp and broad spectral features were observed, some of which have plasmon-type collective π-electron modes contributing to their intensities. Eight valence electronic transitions were assigned, considerably extending the number of π-π* transitions previously observed mainly in solution. The principal factor in solution red-shifts is found to be the Lorentz-Lorenz polarizability parameter. Rydberg bands, observed for the first time, are analyzed into eight different series, converging to the D0 ground and two excited electronic states, namely, D3 and D4, of the quinoline cation. The R1 series limit is 8.628 eV for the first ionization energy of quinoline, a value more precise than previously published. This value, combined with cation electronic transition data, provides precise energies, respectively, 10.623 and 11.355 eV, for the D3 and D4 states. The valence transition assignments are based on density functional theory (DFT) calculations as well as on earlier Pariser-Parr-Pople (P-P-P) self-consistent field linear combination of atomic orbitals molecular orbital results. The relative quality of the P-P-P and DFT data is discussed. Both are far from spectroscopic accuracy concerning electronic excited states but were nevertheless useful for our assignments. Our time-dependent DFT calculations of quinoline are excellent for its ground-state properties such as geometry, rotational constants, dipole moment, and vibrational frequencies, which agree well with experimental observations. Vibrational components of the valence and Rydberg transitions mainly involve C-H bend and C═C and C═N stretch modes. Astrophysical applications of the vacuum UV absorption of quinoline are briefly discussed.

The valence and Rydberg states of difluoromethane: A combined experimental vacuum ultraviolet spectrum absorption and theoretical study by ab initio configuration interaction and density functional computations.

The vacuum ultraviolet (VUV) spectrum for CH2F2 from a new synchrotron study has been combined with earlier data and subjected to detailed scrutiny. The onset of absorption, band I and also band IV, is resolved into broad vibrational peaks, which contrast with the continuous absorption previously claimed. A new theoretical analysis, using a combination of time dependent density functional theory (TDDFT) calculations and complete active space self-consistent field, leads to a major new interpretation. Adiabatic excitation energies (AEEs) and vertical excitation energies, evaluated by these methods, are used to interpret the spectra in unprecedented detail using theoretical vibronic analysis. This includes both Franck-Condon (FC) and Herzberg-Teller (HT) effects on cold and hot bands. These results lead to the re-assignment of several known excited states and the identification of new ones. The lowest calculated AEE sequence for singlet states is 11B1 ∼ 11A2 < 21B1 < 11A1 < 21A1 < 11B2 < 31A1 < 31B1. These, together with calculated higher energy states, give a satisfactory account of the principal maxima observed in the VUV spectrum. Basis sets up to quadruple zeta valence with extensive polarization are used. The diffuse functions within this type of basis generate both valence and low-lying Rydberg excited states. The optimum position for the site of further diffuse functions in the calculations of Rydberg states is shown to lie on the H-atoms. The routine choice on the F-atoms is shown to be inadequate for both CHF3 and CH2F2. The lowest excitation energy region has mixed valence and Rydberg character. TDDFT calculations show that the unusual structure of the onset arises from the near degeneracy of 11B1 and 11A2 valence states, which mix in symmetric and antisymmetric combinations. The absence of fluorescence in the 10.8-11 eV region contrasts with strong absorption. This is interpreted by the 21B1 and 11A1 states where no fluorescence is calculated for these two states, which are only active in absorption. The nature of the two states, 11B1 and 21B1, is fundamentally different, but both are complex owing to the presence of FC and HT effects occurring in different ways. The two most intense bands, close to 12.5 and 15.5 eV, contain valence states as expected; the onset of the 15.5 eV band shows a set of vibrational peaks, but the vibration frequency does not correspond to any of the photoelectron spectral (PES) structure and is clearly valence in nature. The routine use of PES footprints to detect Rydberg states in VUV spectra is shown to be inadequate. The combined effects of FC and HT in the VUV spectral bands lead to additional vibrations when compared with the PES.

Luminescence of defect clusters in the disordered anion sublattice of CaF2 crystals

The fast quenching of CaF2 crystals preheated up to 1000 °C allows to stabilize the high-temperature disordering of anion sublattice typical for the superionic state of the crystal. Such procedure results in the formation of structured ultraviolet to vacuum ultraviolet (UV–VUV) absorption with a lengthy long-wavelength tail stretching into the visible spectral range. Such absorption in the quenched crystals cannot be related to any local center and testifies to the presence of nano-sized complexes of displaced anions, the electron structure of which is drastically changed as compared with a regular lattice. Possibly, these clusters are frozen dynamic clusters that were discovered earlier in the superionic state with quasi-elastic diffuse neutron-scattering technique. Four luminescence bands in near UV and visible spectral range correspond to four low-energy absorption bands. Unlike absorption bands that are of zone nature, the luminescence occurs from the localized state that arises due to strong electron-vibration coupling in the clusters. Absorption and luminescence states are divided by a potential barrier that is responsible for the spacing between corresponding excitation and emission bands as well as the temperature quenching of luminescence. The luminescence study reveals the features of the electronic structure of clusters within preheated/quenched CaF2 crystals. The quenching method allows to study the high-temperature superionic state of crystals with fluorite structure using a wide set of experimental techniques.

Vacuum Ultraviolet Spectroscopy and Mass Spectrometry: A Tandem Detection Approach for Improved Identification of Gas Chromatography-Eluting Compounds.

For wide class characterizations of volatile organic compounds (VOCs), conventional gas chromatography mass spectrometry (GC-MS)-based techniques are utilized. These GC-MS-based chemical identification approaches typically rely on library searches against ion fragmentation patterns of known compounds. Although MS library searches can often provide correct chemical identities, erroneous chemical assignments of structurally similar unknown compounds are also possible. Other detection systems, such as absorption spectrometers, have been used for VOC analysis and can provide complementary absorption data. Here, we demonstrate the analytical advantages of coupling vacuum ultraviolet (VUV) absorption spectroscopy and MS in tandem for the improved characterization of structurally similar VOCs. We also discuss technical considerations and limitations of coupling a VUV spectrometer to a quadrupole mass spectrometer. Moreover, we show that combining the isomer selectivity of VUV spectroscopy, as a nondestructive analyte detection approach, with the mass selectivity of MS in a VUV-MS detection system improves characterization of GC-eluting compounds. Utilizing GC/VUV-MS data, we demonstrate that orthogonal VUV and MS library searches improve identification of VOCs present in complex mixtures such as a mixed standard sample, a commercial perfume product, and an essential oil sample.

Measurement of the vacuum-ultraviolet absorption spectrum of low-k dielectrics using X-ray reflectivity

During plasma processing, low-k dielectrics are exposed to high levels of vacuum ultraviolet (VUV) radiation that can cause severe damage to dielectric materials. The degree and nature of VUV-induced damage depend on the VUV photon energies and fluence. In this work, we examine the VUV-absorption spectrum of low-k organosilicate glass using specular X-ray reflectivity (XRR). Low-k SiCOH films were exposed to synchrotron VUV radiation with energies ranging from 7 to 21 eV, and the density vs. depth profile of the VUV-irradiated films was extracted from fitting the XRR experimental data. The results show that the depth of the VUV-induced damage layer is a function of the photon energy. Between 7 and 11 eV, the depth of the damaged layer decreases sharply from 110 nm to 60 nm and then gradually increases to 85 nm at 21 eV. The maximum VUV absorption in low-k films occurs between 11 and 15 eV. The depth of the damaged layer was found to increase with film porosity.

VUV-absorption cross section of carbon dioxide from 150 to 800 K and applications to warm exoplanetary atmospheres

Context. Most exoplanets detected so far have atmospheric temperatures significantly higher than 300 K. Often close to their star, they receive an intense UV photons flux that triggers important photodissociation processes. The temperature dependency of vacuum ultraviolet (VUV) absorption cross sections are poorly known, leading to an undefined uncertainty in atmospheric models. Similarly, data measured at low temperatures similar to those of the high atmosphere of Mars, Venus, and Titan are often lacking. Aims. Our aim is to quantify the temperature dependency of the VUV absorption cross sections of important molecules in planetary atmospheres. We want to provide high-resolution data at temperatures prevailing in these media, and a simple parameterisation of the absorption in order to simplify its use in photochemical models. This study focuses on carbon dioxide (CO2). Methods. We performed experimental measurements of CO2 absorption cross sections with synchrotron radiation for the wavelength range (115–200 nm). For longer wavelengths (195–230 nm), we used a deuterium lamp and a 1.5 m Jobin-Yvon spectrometer. We used these data in our one-dimensional (1D) thermo-photochemical model in order to study their impact on the predicted atmospheric compositions. Results. The VUV absorption cross section of CO2 increases with the temperature. The absorption we measured at 150 K seems to be close to the absorption of CO2 in the fundamental ground state. The absorption cross section can be separated into two parts: a continuum and a fine structure superimposed on the continuum. The variation in the continuum of absorption can be represented by the sum of three Gaussian functions. Using data at high temperature in thermo-photochemical models significantly modifies the abundance and the photodissociation rates of many species in addition to CO2, such as methane and ammonia. These deviations have an impact on synthetic transmission spectra, leading to variations of up to 5 ppm. Conclusions. We present a full set of high-resolution (Δλ = 0.03 nm) absorption cross sections of CO2 from 115 to 230 nm for temperatures ranging from 150 to 800 K. A parameterisation allows us to calculate the continuum of absorption in this wavelength range. Extrapolation at higher temperature has not been validated experimentally and therefore should be used with caution. Similar studies on other major species are necessary to improve our understanding of planetary atmospheres.

Chemical kinetics in an atmospheric pressure helium plasma containing humidity.

Atmospheric pressure plasmas are sources of biologically active oxygen and nitrogen species, which makes them potentially suitable for the use as biomedical devices. Here, experiments and simulations are combined to investigate the formation of the key reactive oxygen species, atomic oxygen (O) and hydroxyl radicals (OH), in a radio-frequency driven atmospheric pressure plasma jet operated in humidified helium. Vacuum ultra-violet high-resolution Fourier-transform absorption spectroscopy and ultra-violet broad-band absorption spectroscopy are used to measure absolute densities of O and OH. These densities increase with increasing H2O content in the feed gas, and approach saturation values at higher admixtures on the order of 3 × 1014 cm-3 for OH and 3 × 1013 cm-3 for O. Experimental results are used to benchmark densities obtained from zero-dimensional plasma chemical kinetics simulations, which reveal the dominant formation pathways. At low humidity content, O is formed from OH+ by proton transfer to H2O, which also initiates the formation of large cluster ions. At higher humidity content, O is created by reactions between OH radicals, and lost by recombination with OH. OH is produced mainly from H2O+ by proton transfer to H2O and by electron impact dissociation of H2O. It is lost by reactions with other OH molecules to form either H2O + O or H2O2. Formation pathways change as a function of humidity content and position in the plasma channel. The understanding of the chemical kinetics of O and OH gained in this work will help in the development of plasma tailoring strategies to optimise their densities in applications.

Detection of cannabinoids and cannabinoid metabolites using gas chromatography with vacuum ultraviolet spectroscopy

AbstractRelatively new technology vacuum ultraviolet absorption spectroscopic (120–240 nm) detection for gas chromatography, was evaluated and shown to successfully differentiate cannabinoids, their metabolites, and derivatives thereof. The possibility to deconvolute co‐eluting peaks due to additivity of overlapping absorbance spectra of the analytes was also demonstrated. This feature allows the use of faster temperature ramps, faster analysis time, and reduces the need to baseline resolve all analytes, without sacrificing sensitivity. The combination of vacuum ultraviolet spectroscopy with gas chromatography allows for the analysis of cannabinoids that have similar structures and molecular weight, and whose chromatographic separation can be difficult to obtain. The sensitivity of the approach (low ppm; low ng on‐column) is currently sufficient for tracking cannabinoids in plant material, but significant sample enrichment would be needed to target cannabinoids and metabolites in biological samples.

Photoabsorption and damage of OSG low‐k films by VUV emission at 140–160 nm

Vacuum Ultraviolet (VUV) absorption and damage of porous OSG low‐k dielectrics in the wavelength range 140–160 nm was measured. The measurements were done for two OSG films with different porosity. VUV absorption by OSG dielectrics very slowly decreases with increasing wavelength compared to SiO2 which absorption sharply drops reaching the absorption edge. As the analysis has shown the absorption cross‐sections reduced to the Si atom density of these films are very close that indicates the same mechanisms of absorption and damage. The possible absorption and damage mechanisms are briefly discussed.

Controlled production of atomic oxygen and nitrogen in a pulsed radio-frequency atmospheric-pressure plasma

Radio-frequency driven atmospheric pressure plasmas are efficient sources for the production of reactive species at ambient pressure and close to room temperature. Pulsing the radio-frequency power input provides additional control over species production and gas temperature. Here, we demonstrate the controlled production of highly reactive atomic oxygen and nitrogen in a pulsed radio-frequency ( MHz) atmospheric-pressure plasma, operated with a small % air-like admixture (/ at ) through variations in the duty cycle. Absolute densities of atomic oxygen and nitrogen are determined through vacuum-ultraviolet absorption spectroscopy using the DESIRS beamline at the SOLEIL synchrotron coupled with a high resolution Fourier-transform spectrometer. The neutral-gas temperature is measured using nitrogen molecular optical emission spectroscopy. For a fixed applied-voltage amplitude (234 V), varying the pulse duty cycle from 10% to 100% at a fixed 10 kHz pulse frequency enables us to regulate the densities of atomic oxygen and nitrogen over the ranges of – and – , respectively. The corresponding 11 K increase in the neutral-gas temperature with increased duty cycle, up to a maximum of K, is relatively small. This additional degree of control, achieved through regulation of the pulse duty cycle and time-averaged power, could be of particular interest for prospective biomedical applications.