Vacuum Ultraviolet Region Research Articles

Two-dimensional Sc2O3 monolayer with tunable ultrawide bandgap for solar-blind ultraviolet photodetector

A novel 2D scandia (Sc2O3) monolayer was identified using an evolutionary algorithm-based crystal structure prediction method. The Sc2O3 monolayer exhibits a P3‾m1 symmetry and excellent energetic, thermal, dynamical, and mechanical stability, as well as good mechanical flexibility. The monolayer possesses an ultrawide indirect band gap of 6.246 eV. The monolayer is transparent in the visible light zone, while the large exciton effect leads to significant absorption in the solar-blind and vacuum ultraviolet regions. The band gap of this monolayer can be reduced monotonically by external biaxial tensile strain, resulting in the absorption spectrum covering the entire solar blind spectral region when the load reaches 5.0 %. Additionally, the monolayer has an ultra-high in-plane dielectric constant of approximately 50. The superior stability, flexibility, and strain-tunable electronic and optical properties, as well as the ultra-high dielectric constant, suggest its potential application in a solar-blind photodetector in harsh environments.

Pressure-controlled luminescence in fast-response barium fluoride crystals

Cross-luminescence (CL) in a barium fluoride (BaF2) scintillator arising from the recombination of a valence band electron and a core band hole results in a fast picosecond decay time. However, the CL emission wavelength in the vacuum ultraviolet region is difficult to detect, and intrinsically intense and slow nanosecond self-trapped exciton (STE) luminescence occurs. Herein, we report a redshift in the CL emission wavelength with high-pressure application. The wavelength of the CL emission shifted from 221 nm to 240 nm when 5.0 GPa was applied via a sapphire anvil cell. Increasing the pressure decreases the core-valence bandgap due to the downward expansion of the valence band, resulting in a decrease in the valence band minimum. The onset of a phase transition from a cubic crystal structure to an orthorhombic crystal structure at 3.7 GPa inhibited the recombination of conduction band electrons and self-trapped holes, leading to the disappearance of the STE emission. Manipulating the band structure of BaF2 by high-pressure application enables control of its luminescence emission, providing a pathway toward solving the problems inherent in this leading fast-response scintillator.

Strongly Rotationally Dependent Lifetimes of C2 in the 23Σg- State: Implications for Predissociation in the Vacuum Ultraviolet Region.

The radiative and photodissociative properties of the dicarbon molecule, C2, in high-lying electronic states are of utmost importance for modeling the photochemical processes that occur in various astronomical environments. Despite extensive spectroscopic studies in the last two centuries, the photodissociation properties of C2 are still largely unknown, particularly for quantum states in the vacuum ultraviolet (VUV) region. Here, the lifetimes of C2 for each individual rovibrational level in the recently identified 23Σg- state are measured for the first time using a VUV-pump-UV-probe photoionization scheme. The lifetimes are found to be strongly dependent on the rotational and vibrational quantum levels in the 23Σg- state. The strongly rotationally dependent lifetimes observed here indicate that the 23Σg- state may mainly undergo a predissociation process through couplings with nearby repulsive electronic states. The current observation could have important applications in modeling the interstellar medium and cometary comae.

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.

Validation of the VUV-reflective coating for next-generation liquid xenon detectors

Coating detector materials with films highly reflective in the vacuum ultraviolet region improves sensitivity of the next-generation rare-event detectors that use liquid xenon. In this work, we investigate the MgF2-Al-MgF2 coating designed to achieve high reflectance at 175 nm, the mean wavelength of liquid xenon (LXe) scintillation. The coating was applied to an unpolished, passivated copper substrate mimicking a realistic detector component of the proposed nEXO experiment, as well as to two unpassivated substrates with “high” and “average” levels of polishing. After confirming the composition and morphology of the thin-film coating using TEM and EDS, the samples underwent reflectance measurements in LXe and gaseous nitrogen (GN2). Measurements in LXe exposed the coated samples to -100°C for several hours. No peeling of the coatings was observed after several thermal cycles. Polishing is found to strongly correlate with the measured specular reflectance (R spec). In particular, 5.8(5)% specular spike reflectance in LXe was measured for the realistic sample at 20° of incidence, while the values for similar angles of incidence on the high and average polish samples are 62.3(1.3)% and 27.4(7)%, respectively. At large angles (66°–75°), the R spec in LXe for the three samples increases to 23(5)%, 80(8)%, and 84(18)%, respectively. The R spec at around 45° was measured in both GN2 and LXe for average polish sample and shows a reasonable agreement. Importantly, the total reflectance of the samples is comparable and estimated to be 92(8)%, 85(8)%, and 83(8)% in GN2 for the realistic, average, and high polish samples, respectively. This is considered satisfactory for the next-generation LXe experiments that could benefit from using reflective films, such as nEXO and DARWIN, thus validating the design of the coating.

Experimental setup for the measurement of optical properties in the vacuum ultraviolet region

Abstract This work presents an experimental setup designed for measuring optical properties in the vacuum ultraviolet (VUV) region. Originally developed for the characterization of ICARUS photomultiplier tubes, the setup has been recently employed to measure the conversion efficiency of para-Terphenyl coatings, acting as wavelength shifters, as a function of thickness. The VUV region is of particular interest in particle physics experiments utilizing cryogenic noble gases as scintillating media. The setup includes a deuterium light source, a mirror system, an intensity monitor, and a sample chamber to host sample and photodetector. An example measurement of para-Terphenyl conversion efficiency — as wavelength shifter — with varying thickness is presented, demonstrating the versatility and effectiveness of the presented system.

Improved Adhesion of Direct Copper Seed Layer by Medium-Vacuum Sputtering Using Vacuum Ultraviolet Light

A sputtering method is used to form the seed layer for copper electric plating. In general, copper sputtering has weak adhesion to resin, so titanium sputter is combined to increase the adhesion strength. However, etching in the lithography process requires two types of processes, titanium and copper metal. Adhesion strength was improved by performing vacuum ultraviolet (VUV) treatment as a pretreatment for medium-vacuum sputtering. We discovered the relationship between the hydroxyl groups on the resin surface and the adhesion strength by the chemical modification XPS method. Furthermore, by XPS analysis of the peeled copper interface, the adhesion mechanism between the resin and copper due to VUV irradiation was estimated. We evaluated the absorption properties in the vacuum ultraviolet region of a thinly polished glass epoxy resin. We investigated the behavior of functional groups at the interface and considered the effect of vacuum ultraviolet light in the depth direction.

Spectroscopic Study of a New Electronic Band System 33Δg-a3Πu of C2.

As one of the most important diatomic molecules in the universe, the spectroscopic characterizations of C2 have attracted wide attention in various fields, such as interstellar chemistry, planetary atmospheric chemistry, and combustion. In recent years, a systematic spectroscopic study of C2 in the vacuum ultraviolet (VUV) region has been carried out in our laboratory by using the (1VUV+1'UV) resonance-enhanced multiphoton ionization method based on the combination of a tunable VUV laser source and a time-of-flight mass spectrometer. Two new electronic transition band systems have been reported, following the pioneering work of Herzberg and co-workers in 1969. In the current study, a total of 18 vibronic transition bands of C2 from the lower a3Πu state are experimentally observed in the VUV photon energy range 72000-81000 cm-1, and 6 new upper vibronic levels of 3Δg symmetry are identified, which are assigned as the v' = 0-5 vibrational levels of the 33Δg state of C2. The term energy Te of the 33Δg state is determined to be in the range of 78425-78475 cm-1 (9.724-9.730 eV) with respect to the ground X1Σg+ state, and the molecular constants such as vibrational and rotational constants are also determined, which are in reasonable agreement with those predicted by high-level ab initio theoretical calculations. Irregular vibrational energy level spacings in the 33Δg state are observed, which is tentatively attributed to the strong perturbations between the 33Δg and 23Δg states, as previously predicted by theory.

First-Principle Prediction of a Ground State Crystal Properties of a One-dimensional Nanotube-like Structure on Bismuth

First-principle calculations and density functional theory (DFT) have been combined to comparatively investigate the band structure, phonon spectrum, and optical and elastic properties of one-dimensional nanotube-like Bi. Our calculation reveals that Bi exhibits metallic properties, based on the valence band top (VBT) that lies above the conduction band bottom (CBB), and this is known as a negative bandgap. The optical bandgap is found to be very small, with a value of 257[Formula: see text]meV. The absorption coefficient is observed to be very high, up to [Formula: see text][Formula: see text]cm[Formula: see text]. This quantity falls within the vacuum ultraviolet (VUV) region. According to our reflectivity data, up to 82% of light has been reflected, which could be suitable for optical coating. Our elastic calculations further suggest the Bi-material should be brittle and covalent. Our data provide a deep understanding of various properties of Bismuth and could be useful to other theoretical and experimentalists.

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.

Laminar-type gratings overcoated with carbon-based materials to enhance analytical sensitivity of flat-field emission spectrograph in the VUV region.

Laminar-type spherical diffraction gratings overcoated with carbon-based materials were designed, fabricated, and evaluated for the purpose of enhancing the analytical sensitivity of the flat-field spectrograph in a vacuum ultraviolet region of 35-110 eV. As the design benchmark for numerical calculations, diffraction efficiency (DE) and spectral flux, which are defined by the product of the DE and numerical aperture and correlate with the analytical sensitivity of the spectrograph, were used. To simplify the feasibility study on the overcoating effects, we assumed a laminar-type grating having a grating constant of 1/1000 mm and coated with a Au layer of 30.0 nm thickness and an incidence angle of 84.0°. The optimized groove depth and duty ratio were 30.0 nm and 0.3, respectively. In addition, the optimum thicknesses of the overcoating layer were 44, 46, 24, and 30 nm for B4C, C, diamond-like-carbon, and SiC, respectively. Based on these results, we have fabricated a varied-line-spacing holographic grating overcoated with B4C with a thickness of 47 nm. For the experimental evaluation, we used the light source of Mg-L and Al-L emissions excited by the electron beam generated from an electron microscope, an objective flat-field spectrograph, and a CCD imaging detector. The experimental results showed that the spectrograph employing a new grating overcoated with the B4C layer indicated almost the same spectral resolution and 2.9-4.2 times higher analytical sensitivity compared with those obtained with a previously designed Au-coated grating having a grating constant of 1/1200 mm and used at an incidence of 86.0°.

Origin of Homochirality in Amino Acids Induced by Lyman-α Irradiation in the Early Stage of the Milky Way.

The enantiomeric excess (ee) of l-form amino acids found in the Murchison meteorite poses some issues about the cosmic origin of their chirality. Circular dichroism (CD) spectra of amino acids in the far-ultraviolet (FUV) at around 6.8 eV (182 nm) indicate that the circularly polarized light can induce ee through photochemical reactions. Here, we resort to ab initio calculations to extract the CD spectra up to the vacuum-ultraviolet (VUV) region (∼11 eV), and we propose a novel equation to compute the ee applicable to a wider range of light frequency than what is available to date. This allows us to show that the strength of the induced ee (|ee|) in the 10 eV VUV region is comparable to the one in the 6.8 eV FUV region. This feature is common for some key amino acids (alanine, 2-aminobutyric acid, and valine). In space, intense Lyman-α (Lyα) light of 10.2 eV is emitted from star forming regions. This study provides a theoretical basis that Lyα emitter from an early starburst in the Milky Way plays a crucial role in initiating the ee of amino acids.

Vacuum Ultraviolet Spectroscopic Properties in Gd3+-Eu3+ Ions Pair Doped MYF4 (M = Li, Na, and K)

LiYF4, NaYF4, and KYF4 doped with Gd3+-Eu3+ ion pairs synthesised by soft route (wet) chemical technique tracked by Reactive Atmosphere Process (RAP). The phase purity of every synthesized material is verified using the X-ray Diffraction (XRD) method. The photoluminescence excitation (PLE) and photoluminescence emission (PL) properties are examined in ultra-violet (UV) and vacuum ultraviolet (VUV) regions. A new type of photon harvesting (cutting) process is studied in the synthesized phosphors. The Quantum Cutting (QC) is observed only in host LiYF4 doped with Gd3+-Eu3+ Phosphor.

Consideration of Improved Adhesion of Direct Copper Seed Layer by Medium-Vacuum Sputtering Using Vacuum Ultraviolet Light

A sputtering method is used to form the seed layer for copper electric plating. In general, copper spatter has weak adhesion to resin, so titanium spatter is combined to increase the adhesion strength. However, etching in the lithography process requires two types of processes, titanium and copper metal. We have reported the improvement of adhesion of copper sputter to glass epoxy resin by medium vacuum sputtering equipment in 242nd ECS meeting. Adhesion strength was improved by performing vacuum ultraviolet treatment as a pretreatment for medium-vacuum sputtering. We discovered the relationship between the hydroxyl groups on the resin surface and the adhesion force by the chemical modification XPS method. Furthermore, by XPS analysis of the peeled copper interface, the adhesion mechanism between the resin and copper due to UV irradiation was estimated. We evaluated the absorption properties in the vacuum ultraviolet region of a thinly processed glass epoxy resin. We investigated the behavior of functional groups at the interface and considered the effect of vacuum ultraviolet rays in the depth direction.

Synthesis and VUV Spectral Properties of Red-Emitting K<sub>2</sub>Gd<sub>0.97</sub>Zr (PO<sub>4</sub>)<sub>3</sub>:3mol%Eu<sup>3+</sup> Phosphor via Down-Conversion

K2Gd0.97Zr (PO4)3:3mol% Eu3+ were prepared by the solid-state diffusion method and under a vacuum ultra-violet (VUV) region, photoluminescence properties of the phosphor were examined. The phenomenon of visible quantum harvesting via DC was observed in synthesized phosphor for the Gd3+–Eu3+ pair. In the process of quantum harvesting, the two visible light photons are emitted per absorbed VUV photon. This phenomenon occurred upon the 190 nm excitation of Gd3+ at the 6GJ level through a two-step energy transfer from Gd3+ to Eu3+ by cross-relaxation and sequential transfer of the remaining excitation energy. The consequences are that the quantum efficiency of the K2Gd0.97Zr (PO4)3:3mol% Eu3+ phosphor in the energy transfer process from Gd3+ to Eu3+ could reach 233%. The quantum efficiency goes to more than 200% (i.e., 233%) at just low concentration Eu3+ ions (i.e. 3% only). This novelty was found in the prepared phosphor. Therefore, the phosphor K2Gd0.97Zr (PO4)3:3mol% Eu3+ was a potential quantum harvester and potential candidate for mercury free fluorescent lamp and plasma display panel.

First-principles calculations to investigate the dielectric and optical anisotropy in two-dimensional monolayer calcium and magnesium difluorides in the vacuum ultraviolet

Anisotropic dielectric and optical properties of two-dimensional (2D) calcium and magnesium difluorides were investigated in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum (EM) using the first principles density functional theory (DFT). The anisotropy between the in-plane and out-of-plane directions shows that these materials are uniaxial, exhibiting optical and dielectric anisotropy. The optical functions of these anisotropic materials-optical absorption, photoconductivity, refractive index, reflection and extinction coefficients, and electron energy loss (EEL) spectra-are calculated in the framework of DFT. The low refractive index values and relatively small extinction coefficient make these materials alternative low-index 2D materials for the long wavelengths in the VUV region of the EM spectrum. The reflection and transmission spectra indicate the antireflective property of these materials. The calculated EEL function shows less energy loss of fast-traveling electrons in the material's medium. The maxima in the EEL spectrum are the main feature of plasma oscillations. The dissipation in the incident light radiation energy propagating through the dielectric medium is estimated with the dielectric loss tangent (tanδ). The magnesium difluoride is identified as a less dielectric loss medium than calcium difluoride in the VUV region. The present results suggest that these 2D materials are promising in low refractive index, high reflective, and antireflective coating materials in optoelectronic device applications. Also, electronic studies revealed that these are excellent materials for gate insulators in field-effect transistors based on 2D electronic materials.

Consideration of Improved Adhesion of Direct Copper Seed Layer by Medium-Vacuum Sputtering Using Vacuum Ultraviolet Light

A sputtering method is used to form the seed layer for copper electric plating. In general, copper sputtering has weak adhesion to resin, so titanium sputter is combined to increase the adhesion strength. However, etching in the lithography process requires two types of processes, titanium and copper metal. Adhesion strength was improved by performing vacuum ultraviolet (VUV) treatment as a pretreatment for medium-vacuum sputtering. We discovered the relationship between the hydroxyl groups on the resin surface and the adhesion strength by the chemical modification XPS method. Furthermore, by XPS analysis of the peeled copper interface, the adhesion mechanism between the resin and copper due to VUV irradiation was estimated. We evaluated the absorption properties in the vacuum ultraviolet region of a thinly polished glass epoxy resin. We investigated the behavior of functional groups at the interface and considered the effect of vacuum ultraviolet light in the depth direction.

Observation of the electronic band system 23Σg--a3Πu of C2 in the vacuum ultraviolet region.

A systematic spectroscopic study of the dicarbon molecule C2 has important applications in various research fields, such as astrochemistry and combustion. In the short vacuum ultraviolet (VUV) wavelength region, recent theoretical calculations have predicted many absorption band systems of C2, but only few of them have been verified experimentally yet. In this work, we employed a tunable VUV laser radiation source based on the two-photon resonance-enhanced four-wave mixing method and a time-of-flight mass spectrometer to investigate the absorption bands of C2 in the VUV range of 64 000-66 000cm-1. The electronic transition 23Σg-(v')-a3Πu(v″) of C2 has been observed and identified experimentally for the first time. The term value Te for the 23Σg- state is determined to be 66 389.9 ± 0.5cm-1 above the ground state X1Σg+, and the vibrational and rotational constants are also determined. The experimentally measured spectroscopic parameters in this study are in excellent agreement with the theoretical results based on high-level abinitio calculations.

Investigating the Rydberg states and photodissociation dynamics of Kr2using velocity map imaging

Although the krypton dimer, Kr2, is a prototypical weakly bound van der Waals complex in its ground electronic state, the excited states of Kr2exhibit many and various interactions. Resonance-enhanced multiphoton ionization and velocity map imaging provide an avenue to explore the electronic structure and photofragmentation dynamics of Kr2in the vacuum ultraviolet region. By monitoring photofragmentation product channels and recoil anisotropy distributions, one can gain insights into excited state symmetries and lifetimes, which can aid in interpreting spectroscopy. Here, we study the spectroscopy and photodissociation dynamics of Kr2in the 91 000–94 500 cm−1region. We record and assign photofragment product channels for 32 vibronic bands corresponding to seven electronic transitions. We also identify two previously unobserved vibronic band systems, which we assign as the Kr2 5p [5/2]3 [Formula: see text] and 5p [3/2]2[Formula: see text] transitions. Velocity map images show photofragments arising from predissociation of the neutral Rydberg states and from dissociative photoionization at the n > 2 photon level. We also observe variation in branching ratios between the predissociative and dissociative photoionization channels depending on the lifetime of the intermediate; longer lived states favour higher dissociative photoionization yields and exhibit β6anisotropy parameters for predissociation product channels, indicating “rotational smearing” of the product angular distributions.

Verdet constant dispersion of magnesium fluoride for deep-ultraviolet and vacuum-ultraviolet Faraday rotators.

The Verdet constant dispersion in magnesium fluoride (MgF2) crystals was evaluated over a wavelength range of 190-300 nm. The Verdet constant was found to be 38.7 rad/(T·m) at a wavelength of 193 nm. These results were fitted using the diamagnetic dispersion model and the classical Becquerel formula. The fitted results can be used for the designing of suitable Faraday rotators at various wavelengths. These results indicate the possibility of using MgF2 as Faraday rotators not only in deep-ultraviolet regions, but also in vacuum-ultraviolet regions owing to its large bandgap.