Vacuum Ultraviolet Spectral Region Research Articles

Radiative emissions from charge exchange processes in collisions of 0.7–10.0 keV He[formula omitted] with N[formula omitted] and O2 molecules

We present an experimental study of the dissociative excitation in the collision of helium ions with nitrogen and oxygen molecules for collision energy of 0.7–10.0 keV. Absolute emission cross sections are measured and reported for the most pronounced nitrogen and oxygen atomic and ionic lines in vacuum ultraviolet (80–130nm) and visible (380–670nm) spectral regions. Remarkable similarities of the processes realized in He++N2 and He++O2 collision systems are observed. We present polarization measurements for He++N2 collision system.The emission of excited dissociative products was detected using an improved high-resolution optical spectroscopy method. This method incorporates the retarding potential method and a high resolution electrostatic energy analyzer to precisely measure the energy of incident particles and the energy of dispersion. The improvement in the optical sensitivity allows us to measure the cross section on the order of 10−19 cm2 or lower.

Optical solitons in hollow-core fibres

I review the historical observation and subsequent research on optical soliton dynamics in gas-filled hollow-core optical fibres. I include both large-core hollow capillary fibres, and hollow-core photonic-crystal or microstructured fibres with smaller cores, in particular photonic bandgap and antiresonant guiding fibres. I discuss how the optical guidance properties of these different fibre structures influence the soliton dynamics that can be obtained. The dynamics I consider include: soliton propagation at peak power levels ranging from the megawatt to terawatt level, and pulse energies from sub-microjoule to millijoule range; pulse self-compression, leading to sub-cycle and sub-femtosecond pulse duration; soliton self-frequency shifting due to both the Raman effect, and the influence of photoionisation and plasma formation; and resonant dispersive wave emission, leading to the generation of tuneable few-femtosecond pulses across the vacuum and deep ultraviolet, visible, and near-infrared spectral regions.

Emission of Shock-Heated Air in the Vacuum Ultraviolet Spectral Region

Emission of Shock-Heated Air in the Vacuum Ultraviolet Spectral Region

Vacuum ultraviolet photoluminescence of NaMgF3:Sm and NaMgF3:Sm,Ce: energy levels of the lanthanides in NaMgF3:Ln compounds

The luminescence properties of NaMgF3:Sm and NaMgF3:Ce,Sm were studied in the vacuum ultraviolet spectral region. Excitation bands corresponding to the charge transfer processes F− → Sm3+, O2− → Sm3+, and O2− → Ce3+, and the energy transfer processes Ce3+ → Sm3+ and O2− → Sm3+, were observed. The energies of the Sm3+ charge transfer transitions and the crystal field split Ce3+ 4f 05d 1 transitions were used to construct a complete host referred binding energy diagram for the series of lanthanide-doped NaMgF3:Ln compounds. We demonstrate that the optical and luminescence properties predicted by the binding energy diagram are in good agreement with those predicted by the binding energy diagram constructed via the alternative impurity-informed method, and all available experimental data regarding the NaMgF3:Ln compounds. We demonstrate that NaMgF3:Ln compounds are model systems for the study of charge trapping phenomena and divalent lanthanide luminescence. Ultimately, we validate that the impurity-informed method can be used to establish the energy levels of lanthanides in fluoride systems.

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.

Zinc gallate spinel dielectric function, band-to-band transitions, and Γ-point effective mass parameters

We determine the dielectric function of the emerging ultrawide bandgap semiconductor ZnGa2O4 from the near-infrared (0.75 eV) into the vacuum ultraviolet (8.5 eV) spectral regions using spectroscopic ellipsometry on high quality single crystal substrates. We perform density functional theory calculations and discuss the band structure and the Brillouin zone Γ-point band-to-band transition energies, their transition matrix elements, and effective band mass parameters. We find an isotropic effective mass parameter (0.24 me) at the bottom of the Γ-point conduction band, which equals the lowest valence band effective mass parameter at the top of the highly anisotropic and degenerate valence band (0.24 me). Our calculated band structure indicates the spinel ZnGa2O4 is indirect, with the lowest direct transition at the Γ-point. We analyze the measured dielectric function using critical-point line shape functions for a three-dimensional, M0-type van Hove singularity, and we determine the direct bandgap with an energy of 5.27(3) eV. In our model, we also consider contributions from Wannier–Mott type excitons with an effective Rydberg energy of 14.8 meV. We determine the near-infrared index of refraction from extrapolation (1.91) in very good agreement with results from recent infrared ellipsometry measurements (ε∞=1.94) [M. Stokey, Appl. Phys. Lett. 117, 052104 (2020)].

Sulfur Detection in Soil by Laser Induced Breakdown Spectroscopy Assisted by Multivariate Analysis.

Laser-induced breakdown spectroscopy (LIBS) is used for the detection and determination of sulfur content in some organic soil samples. The most suitable sulfur spectral lines for such tasks were found to occur in the vacuum ultraviolet (VUV) spectral region and they were used for the construction of calibration curves. For the analysis, both univariate and multivariate statistical models were employed. The results obtained by the different analysis techniques are evaluated and compared. The present study demonstrates both the applicability and efficiency of LIBS for fast sulfur detection in soil matrices when aided by multivariate analysis methods improving the accuracy and extending the potential use of LIBS in such applications.

Short-wavelength probes in time-resolved photoelectron spectroscopy: an extended view of the excited state dynamics in acetylacetone.

Femtosecond pulses of light in the vacuum ultraviolet (VUV) spectral region permit extended observation of non-adiabatic dynamics in gas-phase molecules. When used as a probe in time-resolved photoelectron spectroscopy, such pulses project deeply into the ionization continuum and allow the evolution of excited state population to be monitored across multiple potential energy surfaces. When compared with longer-wavelength probes, this often provides a more complete view along the reaction coordinate(s) connecting photoreactants to photoproducts. Here we report the use of 160 nm VUV light to interrogate the excited state dynamics operating in acetylacetone following 267 nm excitation. Multiple non-adiabatic processes (internal conversion and intersystem crossing) were observed on timescales ranging from a few femtoseconds to hundreds of picoseconds. Our quantitative results are in excellent agreement with earlier studies that individually sampled smaller sub-sections of the total reaction coordinate. Furthermore, we also observe additional dynamical signatures not previously reported elsewhere. Overall, our findings provide a good illustration of the need to use short-wavelength VUV probes to obtain the most comprehensive picture possible in photoionization-based studies of photochemical dynamics.

Band-to-band transitions and critical points in the near-infrared to vacuum ultraviolet dielectric functions of single crystal urania and thoria

Band-to-band transition energy parameters for single-crystal actinide samples of uranium oxide and thorium oxide were determined and compared using spectroscopic ellipsometry and critical-point dielectric function analyses. Spectroscopic ellipsometry measurements from the near-infrared to the vacuum ultraviolet spectral region were used to determine the dielectric functions of uranium oxide and thorium oxide. The critical-point structure is similar between UO2 and ThO2 but strongly blue shifted for ThO2. We find bandgap energies of 2.1 eV and 5.4 eV for UO2 and ThO2, respectively.

Ultrafast Molecular Spectroscopy Using a Hollow-Core Photonic Crystal Fiber Light Source.

We demonstrate, for the first time, the application of rare-gas-filled hollow-core photonic crystal fibers (HC-PCFs) as tunable ultraviolet light sources in femtosecond pump-probe spectroscopy. A critical requirement here is excellent output stability over extended periods of data acquisition, and we show this can be readily achieved. The time-resolved photoelectron imaging technique reveals nonadiabatic dynamical processes operating on three distinct time scales in the styrene molecule following excitation over the 242-258 nm region. These include ultrafast (<100 fs) internal conversion between the S2(ππ*) and S1(ππ*) electronic states and subsequent intramolecular vibrational energy redistribution within S1(ππ*). Compact, cost-effective, and highly efficient benchtop HC-PCF sources have huge potential to open up many exciting new avenues for ultrafast spectroscopy in the ultraviolet and vacuum ultraviolet spectral regions. We anticipate that our initial validation of this approach will generate important impetus in this area.

Drude and Kukharskii mobility of doped semiconductors extracted from Fourier-transform infrared ellipsometry spectra

The factorized plasmon-phonon polariton description of the infrared dielectric function is generalized to include an additional factor to account for the effects of interband electronic transitions. This new formalism is superior to the usual Drude–Lorentz summation of independent oscillators, especially in materials with large transverse-longitudinal optical phonon splittings, multiple infrared-active phonon modes, or high concentrations of free carriers, if a broadband description of the dielectric function from the far-infrared to the vacuum-ultraviolet spectral region is desired. After a careful comparison of both approaches, the factorized description is applied to the dielectric function of undoped and doped semiconductors (GaAs, GaSb, and InAs) and metal oxides from 0.03 to 9.0 eV. Specifically, the authors find that both descriptions of the far-infrared dielectric function yield the same carrier density and mobility, at least for a single species of carriers. To achieve valid results for moderately high doping concentrations, measurements to lower energies would be helpful.

Acetylacetone photodynamics at a seeded free-electron laser

The first steps in photochemical processes, such as photosynthesis or animal vision, involve changes in electronic and geometric structure on extremely short time scales. Time-resolved photoelectron spectroscopy is a natural way to measure such changes, but has been hindered hitherto by limitations of available pulsed light sources in the vacuum-ultraviolet and soft X-ray spectral region, which have insufficient resolution in time and energy simultaneously. The unique combination of intensity, energy resolution, and femtosecond pulse duration of the FERMI-seeded free-electron laser can now provide exceptionally detailed information on photoexcitation–deexcitation and fragmentation in pump-probe experiments on the 50-femtosecond time scale. For the prototypical system acetylacetone we report here electron spectra measured as a function of time delay with enough spectral and time resolution to follow several photoexcited species through well-characterized individual steps, interpreted using state-of-the-art static and dynamics calculations. These results open the way for investigations of photochemical processes in unprecedented detail.

EUV polarimetry for thin film and surface characterization and EUV phase retarder reflector development.

The knowledge and the manipulation of light polarization state in the vacuum ultraviolet and extreme ultraviolet (EUV) spectral regions play a crucial role from materials science analysis to optical component improvements. In this paper, we present an EUV spectroscopic ellipsometer facility for polarimetry in the 90-160 nm spectral range. A single layer aluminum mirror to be used as a quarter wave retarder has been fully characterized by deriving the optical and structural properties from the amplitude component and phase difference δ measurements. The system can be suitable to investigate the properties of thin films and optical coatings and optics in the EUV region.

Ultraviolet discharges from a radio-frequency system for potential biological/chemical applications

In this work, we describe a new electrode-less radio-frequency (RF) excitation technique for generating excimers in the vacuum ultraviolet (VUV) and ultraviolet (UV) spectral regions for potential biological/chemical applications. Spectra data of Xe, XeI, and KrI generated by this new technique are presented. Optical efficiency of the lamp system ranges from 3% to 6% for KrI, 7% to 13% for XeI, and 15% to 20% for Xe. Also, results of irradiating E-coli with XeI discharge from this lamp system is presented to show one of the promising applications of such electrode-less apparatus. This new RF lamp system offers an interesting addition to the already existing technologies for generating VUV and UV light for various biological, physical, and chemical processes especially those requiring large area for high productivity.

The Cu II Spectrum

New wavelength measurements in the vacuum ultraviolet (VUV), ultraviolet and visible spectral regions have been combined with available literature data to refine and extend the description of the spectrum of singly ionized copper (Cu II). In the VUV region, we measured 401 lines using a concave grating spectrograph and photographic plates. In the UV and visible regions, we measured 276 lines using a Fourier-transform spectrometer. These new measurements were combined with previously unpublished data from the thesis of Ross, with accurate VUV grating measurements of Kaufman and Ward, and with less accurate older measurements of Shenstone to construct a comprehensive list of ≈2440 observed lines, from which we derived a revised set of 379 optimized energy levels, complemented with 89 additional levels obtained using series formulas. Among the 379 experimental levels, 29 are new. Intensities of all lines observed in different experiments have been reduced to the same uniform scale by using newly calculated transition probabilities (A-values). We combined our calculations with published measured and calculated A-values to provide a set of 555 critically evaluated transition probabilities with estimated uncertainties, 162 of which are less than 20%.

Luminescence in undoped CaYAl3O7 produced via the Pechini method

The luminescent properties of CaYAl3O7 (CYAM) were carried out in undoped powder samples produced via Pechini method. The thermal analysis combined with XRD measurements showed that 1000°C/4h is the best condition for the CYAM synthesis. Morphological analysis showed submicrometric particles with irregular shape. Photoluminescence (PL) emission and excitation spectra were studied with excitation in the ultraviolet and vacuum ultraviolet spectral region using synchrotron radiation. The CYAM band gap energy was measured and found to be around 6.8eV. Low intense emission band around 4.4eV, upon excitation at 6.5eV, was assigned to the self-trapped excitons (STE) that are generated during the VUV excitation. Three main emission bands peaking at 2.57, 2.94 and 3.23eV are responsible for most of the PL emission and radioluminescence (RL) emission in CYAM. Time-resolved measurements under pulsed 4.7eV light showed that the PL decay curve is composed by 3 main processes with very short lifetime, smaller than 30ns, and a small amount of long time constant process. Time-resolved RL measurements exciting with 4056eV photons, above the Ca K edge, showed that the decay curve is composed by two processes with time constants around 27 and 130ns and a small component due to a long decay constant bigger. All this features allowed the identification of the main emissions centres at 2.94 and 3.23eV to the F+ centres, while the emission centre at 2.57eV is due to the F type centre.

VUV study of electron impact dissociative excitation of thymine

Dissociative excitation of thymine following electron impact was studied in the energy range up to 430 eV. Emissions in the vacuum ultra-violet spectral region below 150 nm were studied and found to be dominated by the hydrogen Lyman series. Emission cross section data reveal that Lyman-α excitation displays a broad maximum at an electron impact energy of 160 eV. The probability of extracting other excited atoms from the parent molecule is found to be insignificant. Possible excitation and dissociation mechanisms in the parent molecule are discussed.

Spectral Properties and Energy Transfer of a Potential Solar Energy Converter

The energy transfer between Ce3+ and Eu2+ has been investigated in the host Ca3Sc2Si3O12 (CSS), prepared by a modified sol–gel method. Excitation and emission measurements from the near-infrared to the vacuum ultraviolet spectral regions have been performed upon CSS, Ce3+-doped CSS, Eu2+-doped CSS and Ce3+, Eu2+-co-doped CSS, at various concentrations, including experiments at temperatures range of 15–460 K. The energy transfer efficiency from Ce3+ to Eu2+ can approach 90%, and the Ce3+ donor decay curves for different Eu2+ acceptor concentrations in the codoped system were fitted by the Inokuti–Hirayama method, indicating that it is energy transfer induced by electric dipole interaction. The use of the Ce3+, Eu2+ couple in the CSS host as a wideband harvester with an emission profile tailored to the response of the silicon solar cell in solar energy conversion suffers from two main drawbacks relating to valence instability and emission quenching of Eu2+. Possible solutions are suggested.

Self-absorption coefficients of sulfur emission lines

A theoretical approach is developed to calculate the mean probability of photon capture for the resonance lines emitted by a neutral sulfur atom in the vacuum ultraviolet spectral region in the case of a thermal plasma mixture. The calculations are to estimate the self-absorption effect in a quantitative way assuming a Voigt line profile. The variation of the probability of photon capture with the temperature, the SF6proportions in SF6-N2thermal plasma mixture, and the size of the plasma are considered. The results obtained may be of practical importance in plasma modeling and plasma diagnostics. They may also be applied to find out self-absorption-free emission lines appropriate for laser-induced breakdown spectroscopy experiments used for analytical purposes because of their simplicity. Influence of three parameters (temperature, plasma size, and SF6proportion) on the probability of photon capture has been studied using a Voigt line profile.

Optical functions and time-resolved luminescence of lithium hydride single crystals upon far-ultraviolet excitation

Low-temperature reflection spectra of lithium hydride (LiH) single crystals cleaved in ultrahigh vacuum (3×10−10Torr, T=10K), were recorded using synchrotron radiation in vacuum ultraviolet spectral region. Based on the obtained experimental data, the optical functions of LiH in the energy range from 3.7 to 35eV were analyzed using the Kramers–Krönig relations. Time-resolved photoluminescence excitation spectra were studied in detail for the near edge free exciton-phonon luminescence at 4.67eV and photoluminescence at 2.4eV due to the Bi3+ impurity centers. The effect of multiplication of electronic excitations due to inelastic scattering of hot photoelectrons and hot photoholes was revealed at photon energies above 15eV (more than 3Eg). It was found that the radiative lifetime for free excitons in LiH at 4.67eV is less than 1ns as low temperatures as at 10K. The interpretation of the electronic band structure of lithium hydride in the ultraviolet and vacuum ultraviolet spectral regions were carried out on the basis of the present experimental results with the involvement of the available band structure calculations.