Vacuum Ultraviolet Region Research Articles

Tunable and nonlinear metamaterials for controlling circular polarization

Controlling polarization using metamaterials has been one of the research areas that attract immense attention. In particular, the symmetry of the structure plays an important role in controlling polarization-sensitive optical phenomena. Circular polarization control, which is used for important applications such as circular dichroism spectroscopy, requires designing the symmetry of a metamaterial with circular polarization eigenstates. In the linear response, a giant optical activity was observed in chiral metamaterials. It is possible to actively tune the magnitude and sign of polarization by external stimuli or deforming the chiral metamaterial. Furthermore, in the nonlinear optical response, the metamaterial having the rotational symmetry enables wavelength conversion of circularly polarized light and also controls the phase thereof. This article provides an overview of these previous achievements on the metamaterials for controlling circular polarization with isotropic linear response. The article also discusses the prospects of these technologies that will enable polarization control, not only in the visible region but also in the long-wavelength (terahertz) and extremely short-wavelength (vacuum ultraviolet and extreme ultraviolet) regions in combination with the two advanced technologies: high-order harmonic generation and microelectromechanical systems.

Numerical investigation of the electronic and optical properties of LiLuF4 vacuum ultraviolet material

The electronic band structure and optical properties of a perfect LiLuF4 crystal were calculated using density functional theory within the projector-augmented wave method and a hybrid exchange-correlation functional. LiLuF4 is found to have a direct band gap of 10.55 eV, which is attributed to the transition between the F 2p and Li 3s bands. The dominant absorption peak originates from transitions between the F 2p to Lu 6d states and therefore, efficient emission in the vacuum ultraviolet (VUV) region is expected from LiLuF4 by exciting into the Lu 6d states using 15.21 eV (82 nm) of pump energy. The LiLuF4 crystal is highly transparent with a transmission edge of 10.37 eV (119 nm). Moreover, it is optically isotropic down to the transmission edge. Optical isotropy and transparency are desirable in the development of optical components for VUV applications, whereas the direct band gap transition is essential for efficient VUV light emission.

Tunable third harmonic generation in the vacuum ultraviolet region using dielectric nanomembranes

Tunable coherent light sources operating in the vacuum ultraviolet (VUV) region in the 100–200-nm (6–12 eV) wavelength range have important spectroscopic applications in many research fields, including time-resolved angle-resolved photoemission spectroscopy. Recent advances in laser technology have enabled the upconversion of visible femtosecond lasers to the vacuum and extreme ultraviolet regions. However, the complexity of their experimental setups and the scarcity of bulk nonlinear crystals for VUV generation have hampered its widespread use. Here, we propose the use of a free-standing dielectric nanomembrane as a simple and practical method for tunable VUV generation. We demonstrate that third harmonic VUV light is generated with sufficient intensity for spectroscopic applications from commercially available SiO2 nanomembranes of submicron thicknesses under excitation with visible femtosecond laser pulses. The submicron thickness of the nanomembranes is optimal for maximizing VUV generation efficiency and prevents self-phase modulation and spectral broadening of the fundamental beam. The observed VUV photons are up to 107 photons per pulse at 157 nm with a 1-kHz repetition rate, corresponding to a conversion efficiency of 10−6. Moreover, the central VUV wavelength can be tuned in the 146–190-nm wavelength range by changing the fundamental wavelength. We also explore material and thickness dependence with experiments and calculations. The presented results suggest that dielectric nanomembranes can be used as practical nonlinear media for VUV spectroscopic applications.

VUV spectroscopy of lanthanide doped fluoride crystals K2YF5

K2Y0.99Ln0.01F5 (Ln = Ce, Eu and Sm) single crystals have been synthesized by the hydrothermal technology. The optical properties of Ce3+ and Eu3+ ions doped into K2YF5 matrix have been studied under excitation in the deep UV and vacuum UV (VUV) region. The obtained data from excitation spectra and Dorenbos models have been used to predict the energies of the f-d and charge transfer transitions as well as the energy levels location of divalent (Ln2+) and trivalent (Ln3+) lanthanide ions within the band gap of K2YF5. In particular, the f-d and charge-transfer transitions of Sm3+ in the luminescence excitation spectrum of K2YF5:Sm3+ have been identified. The host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes have been constructed for Ln2+ and Ln3+ ions doped K2YF5 crystal which were used for the analysis of possible applications of K2YF5 doped with Ln3+ ions as thermoluminescence materials.

Experimental determination of solidified lithium disilicate crystal bandgap energy using EELS and XPS

AbstractLithium disilicate (LS2) has been a crucial parent composition for glass‐ceramics since the 1950s because of its excellent chemical and physical durability. In addition, a wide range of electrical properties can be obtained by changing the composition and crystallinity. Bandgap energy is one of the critical electrical properties for designing new lithium silicate‐based materials. In this study, the bandgap energy of a synthesized LS2 crystal is evaluated using electron energy‐loss spectroscopy and X‐ray photoelectron spectroscopy. These two techniques unambiguously establish that the bandgap energy of LS2 is 7.7‐7.8 eV, which is in the vacuum ultraviolet region. This confirms the insulating nature of the LS2 crystal.

Photodissociation branching ratios of 13C16O and 12C18O in the vacuum ultraviolet region from 107 800 to 109 700 cm−1

In this study, we present C+ ion photofragment spectroscopic studies and photodissociation branching ratio measurements for two CO isotopologs, 13C16O and 12C18O, in the vacuum ultraviolet (VUV) region from 107 800 cm−1 (92.76 nm) to 109 700 cm−1 (91.16 nm) using a time-slice velocity-map imaging setup and a tunable VUV laser radiation source generated by the two-photon resonance-enhanced four-wave mixing technique. Several absorption bands of 12C16O in the above energy region are reinvestigated up to higher rotational levels compared with previous studies. The results are compared among 12C16O, 13C16O, and 12C18O on a state-by-state basis, and the photodissociation branching ratios for channels C(1D) + O(3P), and C(3P) + O(1D) are dramatically changed for most of the absorption bands due to the substitutions of 12C by 13C and 16O by 18O. The branching ratios of 13C16O and 12C18O are close to each other due to their similar reduced masses. The strong and selective isotope effects obtained here not only provide useful information for understanding the complicated predissociation dynamics of CO, but are also important for developing a comprehensive photochemical model for explaining the C and O isotope heterogeneities as observed in the Solar System.

Photodissociation dynamics of OCS near 128 nm: S(3PJ=2,1,0), S(1D2) and S(1S0) channels

Here we report the study of the photodissociation dynamics of carbonyl sulfide in the vacuum ultraviolet region using the time-sliced velocity map ion imaging technique. Images of S(3PJ=2,1,0), S(1D2) and S(1S0) products were measured at four photolysis wave-lengths of 129.32, 128.14, 126.99, and 126.08 nm, respectively. Four main dissociation channels: S(3PJ=2,1,0)+CO(X1Σ+), S(3PJ=2,1,0)+CO(A3Π), S(1D2)+CO(X1Σ+) and S(1S0)+CO(X1Σ+) channels, have been clearly observed and identified. Vibrational states of the CO co-products were partially resolved in the experimental images. From these images, the product total kinetic energy releases, the branching ratios and angular distributions of products have been derived. While the S(3PJ=2,1,0)+CO(A3Π) product channel is formed through the adiabatic dissociation process after the excitation to the (31Σ+) excited state, the results suggest that strong nonadiabatic coupling plays an important role in the formation of other three channels.

Luminescence Properties of Nd3+‐Doped AlF3‐Based Fluoride Glass in the Vacuum Ultraviolet Region

The development and characterization of the optical properties of a neodymium(III)‐doped 42.75AlF3–33.25CaF2–19BaF2–5YF3 (Nd3+‐doped BCA‐5Y) glass in the vacuum ultraviolet (VUV) region are reported. Photoluminescence peaks at 180 and 227 nm are observed upon excitation with the 157 nm emission of a F2 laser. The dominant 180 nm emission peak is assigned to the 4f25d–4f3 (4I9/2 level) transition in Nd3+. This emission peak exhibits a fast, single exponential luminescence with a 16 ns decay time. The strong luminescence intensity and the fast decay time allude to its prospects for scintillator applications.

Solid state synthesis and luminescent properties of bright blue-emitting Ba2P2O7:Eu2+ phosphor

Eu2+-activated Ba2P2O7 pyrophosphates were successfully synthesized by the solid-state reaction under reduced atmosphere and were characterized by X-ray diffraction (XRD) and Nuclear magnetic resonance and infrared (IR)/Raman spectroscopy. The luminescence properties of Ba2−xP2O7: xEu2+ (x = 0.01, 0.05) phosphors were investigated in ultraviolet (UV) and vacuum ultraviolet (VUV) regions. Under 131 or 320 nm excitation, the phosphors exhibit a blue emitting band ranging from 350 to 500 nm, which is provided corresponding to the 5d–4f transition of Eu2+. The as-synthesized phosphors present good thermal luminescence stability on temperature quenching and high color purity with the CIE coordinate (0.172, 0.031) for Ba2P2O7:1%Eu2+ phosphor. These results suggest that the Ba2P2O7:Eu2+ pyrophosphate might be a promising blue-emitting phosphor for near ultraviolet (n-UV) light-emitting diodes (LEDs).

Photodissociation Branching Ratios of 13C16O in the Vacuum Ultraviolet Region from 102,745 to 106,360 cm−1

Abstract Direct branching ratio measurements for 13C16O are reported for the three lowest dissociation channels that produce C(3P)+O(3P), C(1D)+O(3P), and C(3P)+O(1D) in the vacuum ultraviolet (VUV) region from 102,745 cm−1 (97.33 nm) to 106,360 cm−1 (94.02 nm) and covering six 1Σ+ and six 1Π states. A time-slice velocity-map ion imaging apparatus with a tunable VUV laser source that is generated by the two-photon resonance-enhanced four-wave mixing technique is used to make these measurements. The results show that the substitution of 12C by 13C dramatically changes the photodissociation branching ratios into channels that produce C and O atoms in the excited 1D state for most of the absorption bands in the titled energy range. This isotope effect strongly depends on the specific rovibronic quantum states of CO that are being excited. The branching ratio data from the present study for 13C16O may significantly impact existing photochemical models because of the higher reactivity of the 1D states of the C and O atoms. In addition to this isotope effect, the rotational dependence of the branching ratios to high J′ levels for several vibronic states has been determined. This provides useful information for unraveling the complicated predissociation dynamics of 13C16O.

Spin coating of TPB film on acrylic substrate and measurement of its wavelength shifting efficiency

Scintillation light from liquid noble gas in a neutrino or dark matter experiment lies typically within the vacuum ultraviolet (VUV) region and might be strongly absorbed by surrounding materials such as light guides or photomultiplier. Tetraphenyl butadiene (TPB) is a fluorescent material and acts as a wavelength shifter (WLS) which can turn the UV light to the visible light around a peak wavelength of 425 nm, enabling the light signals to be detected easily for physics study. Compared with a traditional TPB coating method using vapor deposition, we propose an alternative technique with a spin coating procedure in order to facilitate the development of neutrino and dark matter detectors. This article introduces how to fabricate the TPB film on acrylics using the spin coating method, reports measurement of sample film thickness and roughness, shows the reemission spectrum, and quantifies the wavelength shifting efficiency (WLSE).

Photodissociation branching ratios of 12C16O from 108000 cm−1 to 113200 cm-1 measured by two-color VUV-VUV laser pump-probe time-slice velocity-map ion imaging method: Observation of channels for producing O(1D)

The photoabsorption and photodissociation of carbon monoxide (CO) in the vacuum ultraviolet (VUV) region is one of the most important photochemical processes in the interstellar medium, thus it has attracted numerous experimental and theoretical studies. Here, we employed the two-color VUV-VUV laser pump-probe time-slice velocity-map ion imaging method to measure the relative branching ratios [C(3P0)+O(1D)]/ {[C(3P0)+O(3P)]+ [C(3P0)+O(1D)]} and [C(3P2)+O(1D)]/ {[C(3P2)+O(3P)]+[C(3P2)+O(1D)]} in the VUV photoexcitation energy range of 108000−113200 cm−1. Here, one tunable VUV laser beam is used to excite CO to specific rovibronic states, and a second independently tunable VUV laser beam is used to state-selectively ionize C(3P0) and C(3P2) for detection. State-selective photoionization through the 1VUV+1UV/visible resonance-enhanced multiphoton ionization scheme has greatly enhanced the detection sensitivity, which makes many new weak absorption bands observable in the current study. The branching ratio measurement shows that the spin-forbidden channels C(3P0)+O(1D) and C(3P2)+O(1D) only open at several discrete narrow energy windows. This might be caused by certain accidental resonance-enhanced spin-orbit interactions between the directly excited Rydberg states and valence states of triplet type which finally dissociate into the spin-forbidden channels.

Possibilities for the Application of Low-Temperature Argon Plasma in the Treatment of Postoperative and Long-Term Non-Healing Wounds

Aim. To review available information on the use of low-temperature argon plasma in the treatment of postoperative and long-term non-healing wounds. General findings. Low-temperature argon plasma is an ionised gas, one of the four classical aggregate states of matter. Its therapeutic effect is achieved by means of the gas-dynamic effect, i.e. an argon flow with a high heat content and wide-spectrum recombination radiation — from the vacuum ultraviolet region to the near infrared range, as well as by means of the pronounced catalytic properties of gaseous argon, which is important for a number of biochemical reactions. Low-temperature argon plasma has a pronounced antibacterial effect. In a number of studies, the wound healing effect of low-temperature argon plasma was demonstrated. Conclusion. The use of low-temperature argon plasma can reduce the time of wound healing, the titre of clinically significant microorganisms and the time spent by patients in hospital. In addition, the use of low-temperature argon plasma can improve patients’ quality of life in the postoperative period.

Formation of Halogen-bearing Species. II. Irradiation of Chloromethane in Carbon Monoxide Ice with VUV Light and Electrons

Abstract The synthesis of chlorine-bearing species in CO ice was studied by the irradiation of CH3Cl:CO ice at 10 K with vacuum-ultraviolet (VUV) light and energetic electrons. In contrast to the photochemical behavior of CH3F:CO ice, photolysis of CH3Cl:CO ice with Lyα or broadband VUV light afforded various products. This discrepancy was attributed to the abundant absorption bands of CH3Cl in the VUV region, particularly in the Lyα region. The Cl-bearing species including Cl2O, ClCO, C3Cl2, C3HCl, and HOOCl were characterized by observing their IR features. In contrast, electron bombardment of ice mixtures produced various carbon oxides and primary products, such as CH2Cl and HCO. In addition, the mechanism of energetic processes in electron bombardment was discussed.

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.

UV synchrotron radiation linear dichroism spectroscopy of the anti-psoriatic drug anthralin

Anthralin (1,8-dihydroxyanthrone, 1,8-dihydroxy-9(10H)-anthracenone), also known as dithranol and cignolin, is one of the most efficient drugs in the treatment of psoriasis and other skin diseases. The precise mode of biochemical action is not fully understood, but the activity of the drug is increased by the influence of UV radiation. In the present investigation, the UV absorption of anthralin is studied by synchrotron radiation linear dichroism (SRLD) spectroscopy on molecular samples partially aligned in stretched polyethylene, covering the near and vacuum UV regions with wavenumbers ranging from 23,000 to 58,000 cm–1(430–170 nm). The observed polarization spectra are well predicted by quantum chemical calculations using time-dependent density functional theory (TD–DFT). About a dozen spectral features are assigned to computed electronic transitions. The calculations support interpretation of the anomalous fluorescence of anthralin as a result of barrier-less excited state intramolecular proton transfer (ESIPT) to the tautomer 8,9-dihydroxy-1(10H)-anthracenone.

Synthesis and characterization of luminescent Ln3+ (Ln = Eu, Tb and Dy)-doped LiYF4 microcrystals produced by a facile microwave-assisted hydrothermal method

This paper reports microstructural and luminescent properties of lithium yttrium fluoride (LiYF4) doped with rare earth ions. Submicrometric crystals of LiYF4 were successfully produced using the microwave-assisted hydrothermal method, which allowed the formation of single crystalline phase at temperatures as low as 140 °C for a reaction time of 15 min. The structural study was carried out by X-ray diffraction (XRD), revealing single tetragonal phase. The electronic structure and microstructure formation of LiYF4 was investigated through the binding energy of the atoms, measured via X-ray photoemission spectroscopy (XPS) and scanning electron microscopy (SEM). The luminescent properties as scintillator were investigated through X-ray excited luminescence (XEL) and time-resolved luminescence. Spectroscopy studies were done via photoluminescence (PL) technique in the visible (VIS) and vacuum ultraviolet (VUV) regions, from ∼1.0 to 15.0 eV. PL excitation results showed that the band gap (Eg) of the LiYF4 is at around 11.0 (±0.1) eV. 4f-5d transitions were identified for Eu3+, Tb3+ and Dy3+. The experimental data were used to predict the energy levels of all divalent and trivalent lanthanide (Ln) ions relative to the valence and conduction bands of LiYF4 and to construct the host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes.

Photoluminescence response of acrylic (PMMA) and polytetrafluoroethylene (PTFE) to ultraviolet light

Some publications indicate that poly(methyl methacrylate) (PMMA) and polytetrafluoroethylene (PTFE) exhibit low levels of photoluminesence (fluorescence and/or phosphorescence) when irradiated with photons in the ultraviolet (UV) to visible range. PMMA (also known as acrylic) and PTFE are commonly used to contain the liquid argon (LAr) or xenon (LXe) target material in rare-event search experiments. LAr and LXe scintillate in the vacuum UV region, and the PMMA and PTFE can be directly illuminated by these photons. Photoluminescence from support materials could cause unexpected signals in these detectors. We investigate photoluminesence in the 400 nm to 550 nm region in response to excitation with UV light between 130 and 250 nm at levels relevant to rare-event search experiments. Measurements are done at room temperature and the signal intensity is time-integrated over several minutes. We tested PMMA and PTFE samples from the batches used in the DEAP-3600 and LUX experiments and observed no photoluminescence signal. We put limits on the efficiency of the plastics to shift UV photons to a wavelengths region of 400 nm–550 nm at 0.05%–0.35% relative to the wavelength shifting efficiency of tetraphenyl-butadiene.

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.

Strong and selective isotope effect in the vacuum ultraviolet photodissociation branching ratios of carbon monoxide

Rare isotope (13C, 17O and 18O) substitutions can substantially change absorption line positions, oscillator strengths and photodissociation rates of carbon monoxide (CO) in the vacuum ultraviolet (VUV) region, which has been well accounted for in recent photochemical models for understanding the large isotopic fractionation effects that are apparent in carbon and oxygen in the solar system and molecular clouds. Here, we demonstrate a strong isotope effect associated with the VUV photodissociation of CO by measuring the branching ratios of 12C16O and 13C16O in the Rydberg 4p(2), 5p(0) and 5s(0) complex region. The measurements show that the quantum yields of electronically excited C atoms in the photodissociation of 13C16O are dramatically different from those of 12C16O, revealing strong isotope effect. This isotope effect strongly depends on specific quantum states of CO being excited, which implies that such effect must be considered in the photochemical models on a state by state basis.