Vacuum Ultraviolet Emission Research Articles

Numerical investigation of vacuum ultra-violet emission in Ar/O2 inductively coupled plasmas

Abstract Controlling fluxes of vacuum ultraviolet (VUV) radiation is important in a number of industrial and biomedical applications of low pressure plasma sources because, depending on the process, VUV radiation may be desired, required to a certain degree, or unwanted. In this work, the emission of VUV radiation from O atoms is investigated in low-pressure Ar/O2 inductively coupled plasmas via numerical simulations. For this purpose, a self-consistent Ar/O2 plasma-chemical reaction scheme has been implemented in a zero dimensional plasma chemical kinetics model and is used to investigate VUV emission from excited O atoms (3s 5S 2 0 and 3s 3S 1 0 ) at 130 and 135 nm. The model is extensively compared with experimental measurements of absolute VUV emission intensities, electron densities and Ar excited state densities. In addition, oxygen VUV emission intensities are investigated as a function of pressure, Ar/O2 mixture, and power deposition and the dominant reaction pathways leading to oxygen VUV emission are identified and described. In general terms, absolute oxygen VUV emission intensities increase with power and oxygen fraction over the ranges investigated and peak emission intensities are found for pressures between 5–50 Pa. The emission is dominated by the 130 nm resonance line from the decay of the O(3s 3S 1 0 ) state to the ground state. Besides, at low pressure (0.3–1 Pa), the flux of oxygen VUV photons to surfaces is much lower than that of positive ions, whereas oxygen VUV fluxes dominate at higher pressure, ≳ 5–50 Pa depending on O2 fraction. Finally, oxygen atom fluxes to surfaces are, in general, larger than those of VUV photons for the parameter space investigated.

Development of Electron-Beam Induced Soft X-ray and Vacuum Ultraviolet Emission Spectrometer

Development of Electron-Beam Induced Soft X-ray and Vacuum Ultraviolet Emission Spectrometer

Vacuum Ultraviolet Emission from a Plasma of Continuous Electron Cyclotron Resonance Discharge

Vacuum Ultraviolet Emission from a Plasma of Continuous Electron Cyclotron Resonance Discharge

On the Prospects of BaF2 as a Fast Scintillator for Time-of-Flight Positron Emission Tomography Systems

Future time-of-flight positron emission tomography (TOF-PET) will be in need of ultrafast scintillation materials, with potential seen in cross-luminescence (CL). BaF2, for example, shows a sub-100-ps decay time with 300 photons produced per MeV. However, it poses challenges, such as medium radiation length, low photofraction, moderate light yield, and vacuum ultraviolet (VUV) emission around 200nm. Recent developments in UV-sensitive solid-state photodetectors (SiPMs) have the potential to establish CL for ultrafast TOF in PET. In this work, we aim to study BaF2, read out by these new VUV SiPMs, as a viable alternative to cerium-doped lutetium–yttrium oxyorthosilicate (LYSO:Ce). In order to investigate the prospects of BaF2 in TOF-PET, we compare the CTR of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2\times $ </tex-math></inline-formula> 20 mm 3 undoped and yttrium-doped BaF2 crystals, readout by Hamamatsu S13370–3050CN SiPMs with a maximum photon detection efficiency (PDE) of 14% at 200nm and air coupling, to LYSO:Ce:Ca crystals of the same size. With high-frequency readout electronics, we reached 233ps with undoped BaF2 and 213ps with BaF2:Y, while the performance of LYSO:Ce:Ca was 125ps (measured with HPK S14160–3050HS, PDE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 60$ </tex-math></inline-formula> % and glue coupling). Conducting measurements at different depth of interaction positions shows a pronounced impact on the CTR. Furthermore, we investigated the performance for systems of BaF2 with TOFPET2c ASIC measurements and Geant4 simulations for effective sensitivity comparisons to LYSO:Ce:Ca.

Modification of Porous Ultralow-k Film by Vacuum Ultraviolet Emission

Modification of spin-on-deposited porous PMO (periodic mesoporous organosilica) ultralow-k (ULK) SiCOH films (k = 2.33) containing both methyl terminal and methylene bridging groups by vacuum ultraviolet (VUV) emission from Xe plasma is studied. The temporal evolution of chemical composition, internal defects, and morphological properties (pore structure transformation) is studied by using Fourier transform infrared spectroscopy, in situ laser ellipsometry, spectroscopic ellipsometry, ellipsometric porosimetry (EP), positron-annihilation lifetime spectroscopy (PALS), and Doppler broadening positron-annihilation spectroscopy. Application of the different advanced diagnostics allows making conclusions on the dynamics of the chemical composition and pore structure. The time frame of the VUV exposure in the current investigation can be divided into two phases. During the first short phase, film loses almost all of its surface methyl and matrix bridging groups. An increase of material porosity due to removal of methyl groups with simultaneous matrix shrinkage is found by in situ ellipsometry. The removal of bridging bonds leads to an increase of matrix intrinsic porosity. Nevertheless, when the treated material is exposed to the ambient air, the sizes of micro- and mesopores and pores interconnectivity decrease with the VUV exposure time according to PAS and EP data. The last is the result of the additional film shrinkage caused by atmosphere exposure. During the second phase the increase of mesopore size is detected by both EP and PAS. The increase of mesopore size goes all the time as it is expected from in situ ellipsometry, but it is masked by the air exposure.

Role of excimer formation and induced photoemission on the Ar metastable kinetics in atmospheric pressure Ar–NH3 dielectric barrier discharges

Tunable diode laser absorption spectroscopy was used to record the space-and time-resolved number density of argon metastable atoms, Ar(1s3) (Paschen notation), in plane-to-plane dielectric barrier discharges (DBDs) operated in a Penning Ar–NH3 mixture at atmospheric pressure. In both low-frequency (LF 650 V, 50 kHz) discharges and dual LF–radiofrequency (RF 190 V, 5 MHz) discharges operated in α–γ mode, the density of Ar(1s3) revealed a single peak per half-period of the LF voltage, with rise and decay times in the sub-microsecond time scale. These results were compared to the predictions of a 1D fluid model based on continuity and momentum equations for electrons, argon ions (Ar+ and Ar2 +) and excited argon 1s atoms as well electron energy balance equation. Using the scheme commonly reported for Ar-based DBDs in the homogeneous regime, the Ar metastable kinetics exhibited much slower rise and decay times than the ones seen in the experiments. The model was improved by considering the fast creation of Ar2 * excimers through three-body reactions involving Ar(1s) atoms and the rapid loss of Ar2 * by vacuum ultraviolet light emission. In optically thin media for such photons, they can readily reach the dielectric barriers of the DBD electrodes and induce secondary electron emission. It is shown that Ar2 * and photoemission play a significant role not only on the Ar metastable kinetics, but also on the dominant ionization pathways and possible α–γ transition in dual frequency RF–LF discharges.

Overall and state-to-state quenching of atomic oxygen O(2p33p 3PJ) by collisions with He and N2

Electronically excited atomic oxygen O(2p33p3PJ) was prepared by ultraviolet (225.6 nm) two-photon excitation from the ground state 2p43P2. The 2p33p3PJ state transfers to the 2p33s3S state via the infrared transition at 845 nm and/or by collisions with ambient gases. The vacuum ultraviolet (VUV) emission at 130 nm via the transition 2p33s3S → 2p43PJ was detected, and the time-resolved VUV emission at varying pressures of He and N2 was recorded. The kinetic analysis made by the integrated profiles method has given the rate coefficients not only for the overall quenching of the 2p33p3PJ state but also for the state-to-state quenching 2p33p3PJ → 2p33s3S by He and N2.

High power vacuum ultraviolet source based on gasdynamic ECR plasma

We report experimental results of vacuum ultraviolet (VUV) emission from the plasma of an electron cyclotron resonance (ECR) discharge in hydrogen, sustained by millimeter-wavelength radiation of a gyrotron. The considered discharge is characterized with the high plasma density (1013 cm−3 order of magnitude) and, at the same time, the high electron average energy (10–300 eV), which makes it possible to significantly increase the efficiency of VUV re-emission of the energy deposited into the plasma by the microwave radiation. Experiments were performed with the plasma confined in a simple mirror trap and heated by pulsed gyrotron radiation of 37.5 GHz/100 kW under the ECR condition. The measured volumetric VUV emission power of Lyman-alpha line (122 10 nm) overlapping with the Werner band, Lyman band (160 10 nm), and molecular continuum (180 20 nm) reached 45, 25, and 55 W/cm3, respectively. The total absolute radiation power in these three ranges integrated over the plasma volume is estimated to be 22 kW, i.e., 22% of the incident microwave power, which matches theoretical predictions. The development of effective technological VUV sources of a kilowatt power level is expected to be the next step following the optimization of the conditions of the ECR hydrogen discharge sustained by powerful gyrotron radiation.

Vacuum Ultraviolet Emission Properties of Rocksalt‐Structured MgZnO Microcrystals Prepared on Quartz Glass Substrates

Rocksalt‐structured (RS) MgZnO crystals are grown on quartz glass substrates by the mist chemical vapor deposition (mist CVD) method. Cathodoluminescence (CL) emission properties are comparatively studied with those for the RS‐MgZnO epitaxial film prepared on MgO substrate to explore their potential for vacuum ultraviolet (VUV) emitter. In the experiment, the newly developed VUV spectroscopy system is employed, where the CL and optical transmittance spectra are simultaneously measured. Growths of RS‐MgZnO on quartz glass substrate indicate formation of microcrystals (MCs). The MCs dominantly exhibit a near‐band‐edge VUV emission at 6.20 eV (200 nm) at 6 K and 6.07 eV (204 nm) at 300 K. Moreover, the MCs show a noticeable higher energy shoulder at 6.6 eV (188 nm) at 6 K and 6.4 eV (194 nm) at 300 K. The results ensure potential of RS‐MgZnO crystals for active element in VUV emitter regardless of the substrate material.

Detection of atomic bromine (4p5 2PJ; J = 1/2, 3/2) by two-photon laser-induced vacuum ultraviolet emission

Two ultraviolet photons (250–282 nm) excited atomic bromine, Br(4p52PJ;J=1/2,3/2), to the terms built from the 4p45p electronic configuration. Through visible and infrared (VIS–IR) transitions and/or collisions with ambient gases, the terms transfer to the 2,4DJ and 2,4PJ states in the 4p44d and 4p45s electronic configurations. The vacuum ultraviolet (VUV) emission (125–163 nm) from the 4p44d and 4p45s states to the 4p52PJ state was detected. Twenty in 25 allowed two-photon transitions were observed; however, no VUV or VIS–IR emission from the other five transitions was detected. The findings differ from those by the previous reports on the two-photon resonance-enhanced ionization.

Predictive estimation of vacuum ultraviolet emission intensity in a low-pressure inductively coupled hydrogen plasma based on the branching ratio technique

Vacuum ultraviolet (VUV) emission has recently attracted attention in low pressure processing plasmas because of the possibility of high-energy photon damages on the substrates. To quantify the VUV induced damages during the plasma processes, it is need to use of a VUV spectrometer equipped with vacuum systems not readily available in industries. In this work, therefore, we report a simple method to estimate the VUV emission intensity of hydrogen plasmas utilizing a conventional visible spectrometer widely used in plasma processes. From the measurement of hydrogen emission spectra in the visible wavelength region, the VUV emission line (Lyman-β) was calculated using the branching ratio technique and enabled the estimation of Lyman-α emission intensity based on the Boltzmann relation with given plasma parameters. In addition, it was found that the method could also predict the VUV emission intensity for high density hydrogen plasma cases by considering the self-absorption effect by hydrogen atoms. • VUV emission intensity measurement and predictive estimation. • Calculation of VUV Lyman-α (121 nm) emission based on Branching ratio technique. • Correlation between predicted and measured VUV Lyman-α (121 nm) emission intensity. • Self-absorption effect of hydrogen atoms on VUV emission intensity estimation.

Evolution of vacuum ultraviolet emission in dual-frequency capacitively coupled plasmas

In plasma material processing, vacuum ultraviolet (VUV) emission is released from gas discharges, leading to undesirable results. Energetic VUV photons enable the creation of an electron-hole pair current when their energy is larger than the bandgap energy of the plasma-facing top layer during plasma material processing. For example, the high energy of VUV photons from helium (21.2 eV), argon (11.6 eV), and oxygen (13.6 eV) is sufficient to generate induced currents in SiO2 thin films. These feedstock gases are widely used in many procedures utilizing low-temperature industrial plasmas. Thus, the VUV emission evolution with both the power ratio between high (60 MHz) and low (2 MHz) frequencies and pulse duty ratio of the low-frequency radio frequency (rf) power in a dual-frequency capacitively coupled plasma, which is indispensable in modern plasma etching processes, was investigated. Both the power ratio between high and low frequencies and the pulse duty ratio changed the electron temperature, leading to evolution of the VUV emission intensity.

Line identification of boron and nitrogen emissions in extreme- and vacuum-ultraviolet wavelength ranges in the impurity powder dropping experiments of the Large Helical Device and its application to spectroscopic diagnostics

An impurity powder dropper was installed in the 21st campaign of the Large Helical Device experiment (Oct. 2019–Feb. 2020) under a collaboration between the National Institute for Fusion Science and the Princeton Plasma Physics Laboratory for the purposes of real-time wall conditioning and edge plasma control. In order to assess the effective injection of the impurity powders, spectroscopic diagnostics were applied to observe line emission from the injected impurity. Thus, extreme-ultraviolet (EUV) and vacuum-ultraviolet (VUV) emission spectra were analyzed to summarize observable impurity lines with B and BN powder injection. Emission lines released from B and N ions were identified in the EUV wavelength range of 5–300 Å measured using two grazing incidence flat-field EUV spectrometers and in the VUV wavelength range of 300–2400 Å measured using three normal incidence 20 cm VUV spectrometers. BI–BV and NIII–NVII emission lines were identified in the discharges with the B and BN powder injection, respectively. Useful B and N emission lines which have large intensities and are isolated from other lines were successfully identified as follows: BI (1825.89, 1826.40) Å (blended), BII 1362.46 Å, BIII (677.00, 677.14, 677.16) Å (blended), BIV 60.31 Å, BV 48.59 Å, NIII (989.79, 991.51, 991.58) Å (blended), NIV 765.15 Å, NV (209.27, 209.31) Å (blended), NVI 1896.80 Å, and NVII 24.78 Å. Applications of the line identifications to the advanced spectroscopic diagnostics were demonstrated, such as the vertical profile measurements for the BV and NVII lines using a space-resolved EUV spectrometer and the ion temperature measurement for the BII line using a normal incidence 3 m VUV spectrometer.

The role of photon self-absorption on the H (n = 2) density determination by means of VUV emission spectroscopy and TDLAS in low pressure plasmas

The H (n = 2) atomic density determination by means of vacuum ultra-violet (VUV) emission spectroscopy of the Lyman-α line in laboratory low pressure plasmas is strongly affected by self-absorption of emitted photons inside the plasma leading to an underestimation of this density. A correction of the densities obtained from VUV emission spectroscopy measurements is performed by using the escape factor method. The corrections applied can reach orders of magnitude even in low pressure plasmas. Assumptions on the spatial distribution of emitting and absorbing particles as well as on the corresponding line profiles have to be made. Consequently, additional measurements are performed, which raises the requirement of a benchmark of the applied correction procedure. In contrast, tunable diode laser absorption spectroscopy (TDLAS) on the Balmer-α line is a direct measurement of the H (n = 2) density and is thus suitable for a benchmark. For the ICP under consideration, H (n = 2) densities obtained via TDLAS are near 3 × 1015 m−3 whereas uncorrected VUV emission spectroscopy gives values in the range of roughly 1013 m−3 depending on pressure and applied RF power. The calculated escape factors are on the order of 2–8 × 10−3. An excellent agreement with TDLAS is observed by applying them to the results of the VUV emission spectroscopy.

Improvement of quantum efficiency through Gd3+ to Eu3+ energy transfer in YF3 phosphor.

This paper reports the energy transfer from Gd3+ to Eu3+ in YF3 and the consequent downconversion luminescence for the YF3 :Gd3+ , Eu3+ fluoride phosphor. The phosphor was synthesized using a soft chemical route, followed by a reactive atmosphere process. Because of the wide band gap in YF3 and the correct energy site for 8 S7/2 -6 GJ transitions of Gd3+ ions, fluoride YF3 doped with Gd3+ -Eu3+ were studied in their vacuum-ultraviolet (VUV) spectral regions. Powder X-ray diffraction (XRD) analysis showed the structural purity of YF3 . VUV excitation and emission properties were explored using a VUV synchrotron radiation beam line. Downconversion of energy from VUV (157 nm) to visible light with quantum efficiency c. 189% was seen. This YF3 :Gd3+ , Eu3+ phosphor would be an option for mercury-free fluorescence lamps.

Study of vacuum ultraviolet emission in helium and helium/nitrogen mixtures

Helium is frequently used as a working medium for the generation of plasmas and is capable of energetic photon emissions. These energetic photon emissions are often attributed to the formation of helium excimer and subsequent photon emission. When the plasma device is exposed to another gas, such as nitrogen, this energetic photon emission can cause photoionization and further ionization wave penetration into the additional gas. Often ignored are the helium resonance emissions that are assumed to be radiation trapped and therefore not pertinent to photoionization. Here, experimental evidence for the presence of helium atomic emission in a pulsed discharge at ten’s of Torr is shown. Simulations of a discharge in similar conditions agree with the experimental measurements. In this context, the role of atomic and molecular helium light emission on photoionization of molecular nitrogen in an ionization wave is studied using a kinetic modeling approach that accounts for radiation dynamics in a developing low-temperature plasma. Three different mixtures of helium at a total pressure of 250 Torr are studied in simulation. Photoionization of the nitrogen molecule by vacuum ultraviolet helium emission is used as the only seed source ahead of the ionization front. It is found that even though radiation trapped, the atomic helium emission lines are the significant source of photoionization of nitrogen. The significant effect of radiation trapped photon emission on ionization wave dynamics demonstrates the need to consider these radiation dynamics in plasma reactors where self-absorbed radiation is ignored.

Suppression of Fragmentation in Mass Spectrometry.

Suppressing fragmentation is a constant challenge in mass spectrometry because a molecular ion can readily be identified and provides information concerning the molecular weight of an analyte. Several techniques such as charge exchange chemical ionization (CECI) and vacuum ultraviolet emission ionization (VUVEI) have been developed to date for achieving this purpose. In this study, we report on the use of tunable ultraviolet (UV) and near-infrared (NIR) femtosecond (fs) lasers (35 fs) for the multiphoton ionization (MPI) of cis- and trans-4-methylcyclohexanols, the reference molecules that are currently used to examine fragmentation suppression. The results obtained here were compared with those obtained by CECI and VUVEI because they were reported as the best techniques for suppressing fragmentation. A molecular ion was strongly enhanced by carefully minimizing the excess energy in the ionic state using tunable UV and NIR fs-lasers. The ratio of the intensities for molecular and fragment ions, [M]+/[M-H2O]+, increased significantly (9.5-fold and 8.5-fold for cis- and trans-isomers, respectively, in UV fs-MPI) compared to the values obtained by CECI and VUVEI, respectively.

Investigations on the electric-dipole allowed 4f25d → 4f3 broadband emission of Nd3+-doped 20Al(PO3)3-80LiF glass for potential VUV scintillator application

We report the absorption and emission properties of neodymium (Nd3+)-doped APLF glasses [20Al(PO3)3-80LiF + x NdF3, x = 0.5–2.0 mol%] as potential vacuum ultraviolet (VUV) scintillator materials due to their interconfigurational 4f25d transitions which have not yet been investigated. The Nd3+-doped APLF glasses exhibit absorption and emission peaks from the VUV to the near-infrared (NIR) regions which correspond to the different interconfigurational 4f25d and intraconfigurational 4f3 transitions of Nd3+ ions. Detailed analysis of the absorption and emission spectra reveals that the Nd3+-doped glass has a disordered structure and low symmetry, as expected from amorphous materials. However, the most important feature of these glasses is their electric-dipole allowed 4f25d → 4f3 (4I9/2) broadband emissions, between two configurations of opposite parity, around 187 nm (VUV) whose ~ 5.0 ns decay times are faster than known Nd3+-doped scintillators. At room temperature (RT), the absorption edge located around 192 nm overlaps with this VUV emission indicating that self-absorption primarily limits the emission intensity. This overlap could potentially be minimized by working at low temperatures and doping with higher (>2.0 mol%) concentrations. Despite the presence of self-absorption, the fast emission decay times from the 4f25d excited state make the Nd3+-doped APLF glasses promising new VUV scintillator materials for high-counting-rate fast neutron detection.

Efficiency investigation of a negative hydrogen ion beam production with the use of the gasdynamic ECR plasma source

Negative hydrogen ion sources are of great demand in modern physics as injectors into accelerators and drivers for neutral beam injectors for fusion devices. It has been shown earlier that the use of the gasdynamic ECR discharge provides the opportunity to extract up to 80 mA/cm2 of negative ion current density. We studied experimentally the volumetric negative hydrogen ion production and vacuum ultraviolet emission in a gasdynamic ECR discharge. The high-density plasma was sustained by the pulsed 37 GHz / 100 kW gyrotron radiation in a magnetic configuration consisting of two consecutive simple mirror traps. The future prospects of the volumetric H− source based on the gasdynamic ECR discharge related to the transition from pulsed to continuous operating mode with the use of an improved magnetic confinement system are discussed. Numerical simulation of the negative hydrogen ion beam extraction at the continuous operating mode facility was performed. Optimal configuration of the extraction electrodes and the electron damping magnets was found.

Study of VUV Emission From Xe–Ne Microhollow Cathode Discharge in the Self-Pulsing Regime

This article by employing a zero-dimensional model investigates vacuum ultraviolet (VUV) emission from a xenon–neon microdischarge. The effects of the pressure, the input voltage, and the neon fraction are carefully studied. For specific input voltages, the microdischarge operates in an oscillatory regime which is called the self-pulsing regime. The self-pulsing frequency is mainly affected by the input voltage as well as the pressure. Increasing the input voltage enhances the average densities but has no effects on the peak densities. However, the pressure has significant effects on both densities. The main peaks of the VUV emission spectrum are seen at $\lambda =173$ nm and $\lambda =147$ nm which are attributed to xenon triplet-excited dimers and resonance-level excited atoms, respectively. Adding the neon gas enhances the emission at 147 nm through reducing the power loss in elastic collisions, but attenuates the emission at 173 nm. The results also show that steady-state densities of the major VUV emitters in the stationary regime are higher than the time average densities in the self-pulsing regime. Although, the lower power consumption of the self-pulsing regime has a more decisive effect such that the VUV emission efficiency in the self-pulsing regime is greater than that of the stationary regime.