Vacuum Ultraviolet Light Source Research Articles

Detection of the keto-enol tautomerization in acetaldehyde, acetone, cyclohexanone, and methyl vinyl ketone with a novel VUV light source

The discovery of enols in combustion environments and our atmosphere has garnered increasing attention to the many unanswered questions surrounding enol chemistry. The scarcity of experimental data concerning these enols renders combustion and atmospheric models with a lack of constraining parameters, leading to varying computational predictions. Experimental detection is difficult because mass spectrometry, a powerful tool for probing a wide variety of species, cannot distinguish between enols and their thermodynamically favorable ketone isomers. A solution to this ambiguity is to use tunable vacuum ultraviolet (VUV) light from a synchrotron to identify the presence of the enol by its lower ionization energy compared to the isomer. We present a tabletop-scale VUV light source that implements highly cascaded harmonic generation, a new regime of cascaded nonlinear optics, to provide a set of spectral lines spaced by 1.2 eV. We demonstrate that the variety of photon energies available allows us to detect the keto-enol tautomerization of four aldehydes and ketones. By combining this novel VUV light source with an established microreactor, we first revisit the formation of vinyl alcohol from acetaldehyde and confirm that the observed isomerization is indeed unimolecular. Secondly, we observe the thermal tautomerization of acetone to propen-2-ol for the first time. Finally, we observe the thermal tautomerization of cyclohexanone to 1-cyclohexenol and methyl vinyl ketone to 2-hydroxybutadiene, where the results are in good agreement with those reported at a synchrotron. Our measurements can be used to constrain models, inform future experimental studies of enol reactivity, and potentially enhance current understanding of combustion and environmental chemistry.

Vacuum Ultraviolet-Enhanced Oxidation—A Route to the Atomic Layer Etching of Palladium Metal

Low-temperature, plasma-free atomic layer etching (ALE) of Pd0 is explored. A vacuum ultraviolet (VUV) light source (115 < λ < 400 nm) is used in conjunction with a controlled O2 gas exposure to pr...

Generating tunable vacuum ultraviolet coherent light sources without phase matching

Researchers generate tunable vacuum ultraviolet coherent light sources using dielectric nanomembranes and observe light intensities suitable for spectroscopic applications.

Photoemission from Free Particles and Droplets.

Intriguing properties of photoemission from free, unsupported particles and droplets were predicted nearly 50 years ago, though experiments were a technical challenge. The last few decades have seen a surge of research in the field, due to advances in aerosol technology (generation, characterization, and transfer into vacuum), the development of photoelectron imaging spectrometers, and advances in vacuum ultraviolet and ultrafast light sources. Particles and droplets offer several advantages for photoemission studies. For example, photoemission spectra are dependent on the particle's size, shape, and composition, providing a wealth of information that allows for the retrieval of genuine electronic properties of condensed phase. In this review, with a focus on submicrometer-sized, dielectric particles and droplets, we explain the utility of photoemission from such systems, summarize several applications from the literature, and present some thoughts on future research directions.

Role of nuclear symmetry in below-threshold harmonic generation of molecules

Below-threshold harmonic generation (BTHG) is one of few potential methods for the generation of novel vacuum-ultraviolet (VUV) light sources. However, most previous studies have focused on the atomic BTHG; a comprehensive understanding of the molecular BTHG is valuable for the advancement of this field. Here, we present an ab initio study of the role of nuclear symmetry on BTHG of both homonuclear and heteronuclear diatomic molecules. We reveal very distinctive electronic dynamical behaviors in molecular BTHG by identifying subtle multiple-channel contributions. In particular, it is found that the channels associated with the electron-nuclear coupling from light nuclear core to heavy nuclear core of the asymmetric molecules play an important role in the constructive interference of the harmonic emission, leading to a significant enhancement of the BTHG. Our results suggest that asymmetric molecular systems can be used for substantial improvement of the molecular BTHG-based VUV light sources.

A Vacuum Ultraviolet Photoionization Time-of-Flight Mass Spectrometer for Investigation of Free Radical Reaction

Abstract A home-made vacuum ultraviolet (VUV) photoionization linear time-of-flight mass spectrometer (VUVPI-LTOFMS) was utilized to study atmospheric radical reactions. A commercial krypton discharge lamp with photon energy at 10.6 eV was installed as VUV light source and combined with a cage-shape photoionization source to efficiently extract and focus ions. A limit of detection (LOD) of 0.03 μg/L (S/N = 3) for benzene was achieved. A microwave discharge fast flow tube was employed as chemical reactor to produce free radicals and initiate their atmospheric reactions. The concentration of methyl radical was measured with its self-reaction kinetics and the available rate constant. As a representative example, the reaction of the methyl radical with nitric oxide was studied. Both the stable species and radicals involved in the reaction were probed, and the reaction rate constant at room temperature and pressure of 300 Pa was measured to be k(CH3 + NO) = 1.2 × 10−12 cm3/(molecule·s).

Novel design of a personnel safety system for Hefei Light Source-II

The Hefei Light Source-II (HLS-II) is a vacuum ultraviolet synchrotron light source. The personnel safety system (PSS), which is a personnel access control system, is a crucial part of the HLS-II, as it protects the staff and users at HLS-II from radiation damages. The prior version of HLS-II PSS was based on an access control system called SiPass. This lacked the personnel management function. Meanwhile, as the prior PSS is a turn-key system, it was not effective for sharing information. To overcome these drawbacks, the novel design of PSS for HLS-II is proposed based on the Siemens redundant programmable logic controller under the Experimental Physics and Industrial Control System. The proposed PSS consists of a safety interlock system, access control system, and a radiation monitoring system. The safety interlock system is used to define the interlock logic. The access control system is designed to restrict the access of staff and users at HLS-II, and to provide a personnel management function. The radiation monitoring system is used to monitor the radiation dose rate in both the light source and the surrounding areas. This paper details the architecture and the specific design of the novel PSS. The off-line test results demonstrate that the proposed system has achieved the design objectives.

Quantum state-to-state vacuum ultraviolet photodissociation dynamics of small molecules

The present review focused on selected, recent experimental progress of photodissociation dynamics of small molecules covering the vacuum ultraviolet (VUV) range from 6 eV to 20 eV. These advancements come about due to the available laser based VUV light sources, along with the developments of advanced experimental techniques, including the velocity-map imaging (VMI), H-atom Rydberg tagging time-of-flight (HRTOF) techniques, as well as the two-color tunable VUV-VUV laser pump-probe detection method. The applications of these experimental techniques have allowed VUV photodissociation studies of many diatomic and triatomic molecules to quantum state-to-state in detail. To highlight the recent accomplishments, we have summarized the results on several important molecular species, including H2 (D2, HD), CO, N2, NO, O2, H2O (D2O, HOD), CO2, and N2O. The detailed VUV photodissociation studies of these molecules are of astrochemical and atmospheric relevance. Since molecular photodissociation initiated by VUV excitation is complex and is often governed by multiple electronic potential energy surfaces, the unraveling of the complex dissociation dynamics requires state-to-state cross section measurements. The newly constructed Dalian Coherent Light Source (DCLS), which is capable of generating coherent VUV radiation with unprecedented brightness in the range of 50–150 nm, promises to propel the photodissociation experiment to the next level.

Photolysis of Indigo Carmine solution by planar vacuum-ultraviolet (147 nm) light source

Advanced oxidation processes (AOPs) are an attractive method to decompose dye-containing wastewaters, because they avoid issues of secondary pollution. In particular, a vacuum-ultraviolet (VUV) process is the simplest method, because an oxidation accelerator or a catalyst is not required. Conventional VUV sources with wavelengths of 185 nm or 172 nm have been used. We predicted that a shorter wavelength VUV process would have a higher ability to decompose dyes in wastewater. We developed a new planar light source that could emit a resonance line at 147 nm and a broad molecular line at 172 nm. The irradiance was 8.7 mW/cm2 at a distance of 20 mm from the emission surface to the UV power meter. We then conducted photocatalytic experiments of an Indigo Carmine solution at 3.33 × 10−4 mol/L to confirm the decomposition abilities of the developed light source, an excimer lamp of 172 nm and a Hg lamp of 254 nm. From the HPLC results, changes in Indigo Carmine concentration with the developed light source were equivalent to those with the excimer lamp. However, the residual ratio of total organic carbon (TOC) with the developed light source was lower than those with the other lamps. Therefore, a wavelength of 147 nm is superior to the conventional wavelength of 172 nm for Indigo Carmine decomposition. In addition, the developed light source emits VUV from only one side, which is a flat emitting surface. Consequently, the developed light source would have reduced manufacturing and maintenance costs compared to current VUV processing equipment.

DUV/VUV All-Solid-State Lasers: Twenty Years of Progress and the Future

We briefly review the main methods for generating deep ultraviolet (DUV) or vacuum ultraviolet (VUV) light sources, and describe in detail our progress in the design and development of precise and practical DUV all-solid-state lasers based on second harmonic generation. Their incorporation into various kinds of advanced analytic instruments is also discussed. Already, many significant results have been obtained and new phenomena discovered with these state-of-the-art tools. In the last part, we look at possible future developments in the field of DUV/VUV all-solid-state lasers and their potential applications in an even broader range of research fields.

Premixed flame chemistry of a gasoline primary reference fuel surrogate

Investigating the combustion chemistry of gasoline surrogate fuels promises to improve detailed reaction mechanisms used for simulating their combustion. In this work, the combustion chemistry of one of the simplest, but most frequently used gasoline surrogates – primary reference fuel 84 (PRF 84, 84 vol% iso-octane and 16 vol% n-heptane), has been examined in a stoichiometric premixed laminar flame. Time-of-flight mass spectrometry coupled with a vacuum ultraviolet (VUV) synchrotron light source for species photoionization was used. Reactants, major end-products, stable intermediates, free radicals, and isomeric species were detected and quantified. Numerical simulations were conducted using a detailed chemical kinetic model with the most recently available high temperature sub-mechanisms for iso-octane and heptane, built on the top of an updated pentane isomers model and AramcoMech 2.0 (C0C4) base chemistry. A detailed interpretation of the major differences between the mechanistic pathways of both fuel components is given. A comparison between the experimental and numerical results is depicted and rate of production and sensitivity analyses are shown for the species with considerable disagreement between the experimental and numerical findings.

Time-resolved photoelectron imaging with a femtosecond vacuum-ultraviolet light source: Dynamics in the A∼/B∼- and F∼-bands of SO2

Time-resolved photoelectron imaging is demonstrated using the third harmonic of a 400-nm femtosecond laser pulse as the ionization source. The resulting 133-nm pulses are combined with 266-nm pulses to study the excited-state dynamics in the A∼/B∼- and F∼-band regions of SO2. The photoelectron signal from the molecules excited to the A∼/B∼-band does not decay for at least several picoseconds, reflecting the population of bound states. The temporal variation of the photoelectron angular distribution (PAD) reflects the creation of a rotational wave packet in the excited state. In contrast, the photoelectron signal from molecules excited to the F∼-band decays with a time constant of 80 fs. This time constant is attributed to the motion of the excited-state wave packet out of the ionization window. The observed time-dependent PADs are consistent with the F∼ band corresponding to a Rydberg state of dominant s character. These results establish low-order harmonic generation as a promising tool for time-resolved photoelectron imaging of the excited-state dynamics of molecules, simultaneously giving access to low-lying electronic states, as well as Rydberg states, and avoiding the ionization of unexcited molecules.

Photoinduced intermolecular dynamics and subsequent fragmentation in VUV-ionized acetamide clusters.

Photofragmentation of small gas-phase acetamide clusters (CH3CONH2)n (n ≤ 10) produced by a supersonic expansion source has been studied using time-of-flight ion mass spectroscopy combined with tunable vacuum-ultraviolet (VUV) synchrotron radiation. Fragmentation channels of acetamide clusters under VUV photoionization resulting in protonated and ammoniated clusters formation were identified with the discussion about the preceding intramolecular rearrangements. Acetamide-2,2,2-d3 clusters were also studied in an experiment with a gas discharge lamp as a VUV light source; comparison with the main experiment gave insights into the mechanism of formation of protonated acetamide clusters, indicating that proton transfer from amino group plays a dominant role in that process. Geometry of the acetamide dimer was discussed and the most stable arrangement was concluded to be achieved when subunits of the dimer are connected via two N-H⋯O -C hydrogen bonds. Also, the influence of the photon energy on the stability of the clusters and their fragmentation channels has been examined.

Photocurrents obtained from a triple planar probe sensor for in situ extreme ultraviolet (EUV) radiation measurements on low Earth orbiting CubeSats

Purpose In the Earth’s upper atmosphere, damage to satellite electronics is caused by exposure to extreme ultraviolet (EUV) radiation. One particular region where this type of radiation occurs is the South Atlantic Magnetic Anomaly region. As a result, there is a need to design and develop a sensor which could be used to investigate the flux and energy levels of radiation in this region. To do so, the aim of this study is to characterise the sensor and its electric response to typical EUV radiation levels based on the photoelectric effect principle. Design/methodology/approach For this purpose, a copper plate planar sensor prototype with dimensions that fit on the sides of a one-unit (1U) CubeSat was constructed. The sensor prototype was placed in a vacuum chamber and was subjected to continuous radiation from a vacuum ultraviolet deuterium light source at test facilities available in the Western Cape region (South Africa). Subsequently, the terminal voltage of the sensor was measured and compared with theory. Findings The measured time-averaged terminal voltages indicate the generation of photocurrents of the order of 1 μA, which is consistent with theory. Originality/value Conclusively, these results validate the measurement approach and operation of the sensor, which can be used to design a 1U CubeSat sensor that measures EUV radiation in low Earth orbit.

Vacuum ultraviolet excitation luminescence spectroscopy of few-layered MoS2

We report on vacuum ultraviolet (VUV) excited photoluminescence (PL) spectra emitted from a chemical vapor deposited MoS2 few-layered film. The excitation spectrum was recorded by monitoring intensities of PL spectra at ~1.9 eV. A strong wide excitation band peaking at 7 eV was found in the excitation. The PL excitation band is most intensive at liquid helium temperature and completely quenched at 100 K. Through first-principles calculations of photoabsorption in MoS2, the excitation was explicated and attributed to transitions of electrons from p- and d- type states in the valence band to the d- and p-type states in the conduction band. The obtained photon-in/photon-out results clarify the excitation and emission behavior of the low dimensional MoS2 when interacting with the VUV light sources.

(Invited) Vacuum Ultraviolet Photochemical Atomic Layer Deposition of Alumina and Titania Films

Conventional atomic layer deposition (ALD) is a thermo-chemical process where co-reagents are sequentially pulsed in cycles onto a heated substrate. As an alternative to substrate heating, various forms of other “non-thermal” ALD processes are being investigated. Herein, the photochemical atomic layer deposition of Al2O3 and TiO2 thin films at 60°C is reported using a shuttered vacuum ultraviolet light source to excite molecular oxygen as a co-reagent with the metal precursors. The growth mechanisms using trimethyl aluminium and titanium tetraisopropoxide precursors, are investigated using in-situ quartz crystal microbalance and post-deposition ellipsometric measurements. The photochemical ALD Al2O3 films exhibit different capacitance equivalent thicknesses for irradiated and masked regions respectively, even after post-deposition annealing. The photochemical ALD titania films are amorphous and when incorporated into Pt / TiO2 / Pt metal - insulator - metal structures, the titania exhibits a resistive switching behavior.

(Invited) Vacuum Ultraviolet Photochemical Atomic Layer Deposition of Alumina and Titania Films

Conventional atomic layer deposition (ALD) is a thermo-chemical process where co-reagents are sequentially pulsed in cycles onto a heated substrate. As an alternative to substrate heating, various forms of other “energy – enhanced” ALD processes are being investigated. Herein, the photochemical atomic layer deposition of Al2O3 and TiO2thin films at 60°C is reported using a shuttered vacuum ultraviolet (VUV) light source to excite molecular oxygen as a co-reagent with the metal precursors. The growth mechanisms using trimethyl aluminium (TMA) and titanium isopropoxide precursors, are investigated using in-situ quartz crystal microbalance (QCM) and post-deposition ellipsometric measurements. The figure below illustrates the mass gain / loss processes during a series of ALD cycles of TMA with O2/ UV excitation from a VUV lamp with peak emissions at 125 nm (9.9 eV) and 160 nm (7.7 eV). Further, the figure shows the growth per cycle (GPC) derived from the QCM data and also separately from single wavelength (633 nm) ellipsometry measurements made on silicon (100) samples exposed to the same growth conditions. Both data sets indicate that as the VUV illumination time is extended beyond 5 s to 10 s the overall mass gain per cycle saturates and becomes approximately constant, which is consistent with other self limiting ALD processes. The photochemical process is exploited to achieve area-selective film deposition. Selective area deposition was achieved by projecting the VUV light through a metalized magnesium fluoride photolithographic mask. The selectivity of deposition is a function of the UV exposure time and the mask – substrate separation. The photochemical atomic layer deposition of TiO2 thin films from titanium isopropoxide exhibits similar growth characteristics to the Al2O3 photochemical process, however subtle differences in the growth mechanisms are attributed to the different reactions of the excited oxygen species (O( 3P)) with the Al-CH3 and Ti-OCH(CH3)2 bonds. Figure 1

OPTIMIZATION OF A SOLAR SIMULATOR FOR PLANETARY-PHOTOCHEMICAL STUDIES

Low-temperature microwave-powered plasma based on hydrogen and hydrogen with noble gas mixtures are widely used as a continuous vacuum ultraviolet (VUV) source in laboratory experiments carried out to mimic the photochemistry in astrophysical environments. In this work, we present a study dedicated to optimizing such sources in terms of mono-chromaticity at Lyα (H(Lyα) line at 121.6 nm ∼ 10.2 eV) and high spectral irradiance. We report the influence on the emission spectrum of a wide range of experimental conditions including gas composition (pure H2, pure He, and H2/He mixture), gas pressure, flow rates, and microwave power. The absolute spectral irradiance delivered by this VUV light source has been measured. With a microwave input power of 100 W, the best conditions for producing a quasi-monochromatic source are a 1% H2/He gas mixture at a total pressure of 5 mbar and a flow rate of 2 sccm. By changing the microwave input power from 30 to 120 W, H(Lyα) increases by more than one order of magnitude. A comparison between the current measurements and the solar VUV spectral irradiance is reported over 115–170 nm.

Multiplexed Photoionization Mass Spectrometry Investigation of the O((3)P) + Propyne Reaction.

The reaction of O((3)P) + propyne (C3H4) was investigated at 298 K and 4 Torr using time-resolved multiplexed photoionization mass spectrometry and a synchrotron-generated tunable vacuum ultraviolet light source. The time-resolved mass spectra of the observed products suggest five major channels under our conditions: C2H3 + HCO, CH3 + HCCO, H + CH3CCO, C2H4 + CO, and C2H2 + H2 + CO. The relative branching ratios for these channels were found to be 1.00, (0.35 ± 0.11), (0.18 ± 0.10), (0.73 ± 0.27), and (1.31 ± 0.62). In addition, we observed signals consistent with minor production of C3H3 + OH and H2 + CH2CCO, although we cannot conclusively assign them as direct product channels from O((3)P) + propyne. The direct abstraction mechanism plays only a minor role (≤1%), and we estimate that O((3)P) addition to the central carbon of propyne accounts for 10% of products, with addition to the terminal carbon accounting for the remaining 89%. The isotopologues observed in experiments using d1-propyne (CH3CCD) and analysis of product branching in light of previously computed stationary points on the singlet and triplet potential energy surfaces (PESs) relevant to O((3)P) + propyne suggest that, under our conditions, (84 ± 14)% of the observed product channels from O((3)P) + propyne result from intersystem crossing from the initial triplet PES to the lower-lying singlet PES.

Efficient and tunable high-order harmonic light sources for photoelectron spectroscopy at surfaces

With the recent progress in high-order harmonic generation (HHG) using femtosecond lasers, laboratory photoelectron spectroscopy with an ultrafast, widely tunable vacuum-ultraviolet light source has become available. Despite the well-established technique of HHG-based photoemission experiments at kilohertz repetition rates, the efficiency of these setups can be intrinsically limited by the space-charge effects. Here we present recent developments of compact HHG light sources for photoelectron spectroscopy at high repetition rates up to megahertz, and examples for angle-resolved photoemission experiments are demonstrated.