Resonance Lamp Research Articles

Optical Excitation and Trapping of ^{81}Kr.

We have realized optical excitation, trapping, and detection of the radioisotope ^{81}Kr with an isotopic abundance of 0.9ppt. The 124nm light needed for the production of metastable atoms is generated by a resonant discharge lamp. Photon transport through the optically thick krypton gas inside the lamp is simulated and optimized to enhance both brightness and resonance. We achieve a state-of-the-art ^{81}Kr loading rate of 1800 atoms/h, which can be further scaled up by adding more lamps. The all-optical approach overcomes the limitations on precision and sample size of radiokrypton dating, enabling new applications in the earth sciences, particularly for dating of polar ice cores.

Boosting Efficient Ammonia Synthesis over Atomically Dispersed Co-Based Catalyst via the Modulation of Geometric and Electronic Structures

Boosting Efficient Ammonia Synthesis over Atomically Dispersed Co-Based Catalyst via the Modulation of Geometric and Electronic Structures

“EXPERIMENTAL STUDY OF PHOTOELECTRONIC SPECTRA OF NIOBIUM-HAFNIUM-ZIRCONIUM ALLOY”

Crystals of niobium and its alloys obtained by low-energy implantation of hafnium and zirconium ions have been studied in a multifunctional experimental setup. Distribution profiles of implanted hafnium and zirconium atoms, energy distributions of photoelectrons N (E), and spectral dependences of the quantum yield of photoelectron emission before and before and after heating of the niobium-hafnium-zirconium alloy have been investigated. The contribution of surface states and bands formed by hafnium and zirconium atoms to photoelectron emission in the photon energy range of 8–10 eV is analyzed. The experimental results are discussed on the basis of theoretical calculations of the surface, photoelectron emission in which the dominant factors are indirect optical transitions. The experiments were carried out under ultrahigh vacuum conditions using photoelectron and Auger electron spectroscopy. Krypton and xenon resonance lamps were used as radiation sources.

Rate Constants for Reactions of HCCO and HCCCO Radicals with O2 over the Temperature Range 243-423 K.

A pulsed laser photolysis-photoionization mass spectrometer system has been employed to measure the rate constants of HCCO + O2 and HCCCO + O2 over the temperature range 243-423 K in 1.2-8.4 Torr of He or N2. Radicals of HCCO and HCCCO were produced by 193 nm ArF laser photolysis of ethyl ethynyl ether and methyl propiolate, respectively. HCCO was photoionized by a Kr resonance lamp with a CaF2 window (10.03 eV), and HCCCO was ionized by a Xe lamp with a sapphire window (8.44 eV). Both ions were detected as parent ions in a quadrupole mass spectrometer. From analysis of the time profiles of the ion signals for various O2 concentrations, the overall rate constants at 298 K are represented by the values k2 = (6.3 ± 1.0) × 10-13 for HCCO + O2 and k5 = (5.7 ± 0.6) × 10-12 for HCCCO + O2 in the units cm3 molecule-1 s-1. The rate coefficients for the two reactions can be described by k2(T) = (1.5-0.7+1.5) × 10-12 exp[-(225 ± 220)/T] and k5(T) = (1.8-0.9+1.9) × 10-12 exp[(343 ± 228)/T] in the units cm3 molecule-1 s-1 over the temperature range 243-423 K.

Kinetics of the Reaction of CH3O2 Radicals with OH Studied over the 292-526 K Temperature Range.

Reaction of methyl peroxy radicals with hydroxyl radicals, CH3O2 + OH → CH3O + HO2 (1a) and CH3O2 + OH → CH2OO + H2O (1b) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 292-526 K temperature range and pressure 1 bar (bath gas He). Hydroxyl radicals were generated in the reaction of electronically excited oxygen atoms O((1)D), produced in the photolysis of N2O at 193.3 nm, with H2O. Methyl peroxy radicals were generated in the reaction of methyl radicals, CH3, produced in the photolysis of acetone at 193.3 nm, and subsequent reaction of CH3 with O2. Temporal profiles of OH were monitored via transient absorption of light from a DC discharge H2O/Ar low-pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light was determined by accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The overall rate constant of the reaction is k1a+1b = (8.4 ± 1.7) × 10(-11)(T/298 K)(-0.81) cm(3) molecule(-1) s(-1) (292-526 K). The branching ratio of channel 1b at 298 K is less than 5%.

Reaction CH3 + OH Studied over the 294–714 K Temperature and 1–100 bar Pressure Ranges

Reaction of methyl radicals with hydroxyl radicals, CH(3) + OH → products (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 294-714 K temperature and 1-100 bar pressure ranges (bath gas He). Methyl radicals were produced by photolysis of acetone at 193.3 nm. Hydroxyl radicals were generated in reaction of electronically excited oxygen atoms O((1)D), produced in the photolysis of N(2)O at 193.3 nm, with H(2)O. Temporal profiles of CH(3) were recorded via absorption at 216.4 nm using xenon arc lamp and a spectrograph; OH radicals were monitored via transient absorption of light from a dc discharge H(2)O/Ar low pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light inside the reactor was determined by an accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The results of this study indicate that the rate constant of reaction 1 is pressure independent within the studied pressure and temperature ranges and has slight negative temperature dependence, k(1) = (1.20 ± 0.20) × 10(-10)(T/300)(-0.49) cm(3) molecule(-1) s(-1).

Reaction OH + OH Studied over the 298–834 K Temperature and 1 - 100 bar Pressure Ranges

Self-reaction of hydroxyl radicals, OH + OH → H(2)O + O (1a) and OH + OH → H(2)O(2) (1b), was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 298-834 K temperature and 1-100 bar pressure ranges (bath gas He). A heatable high-pressure flow reactor was employed. Hydroxyl radicals were prepared using reaction of electronically excited oxygen atoms, O((1)D), produced in photolysis of N(2)O at 193 nm, with H(2)O. The temporal behavior of OH radicals was monitored via transient absorption of light from a dc discharge in H(2)O/Ar low-pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light was determined by accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The results of this study combined with the literature data indicate that the rate constant of reaction 1a, associated with the pressure independent component, decreases with temperature within the temperature range 298-414 K and increases above 555 K. The pressure dependent rate constant for (1b) was parametrized using the Troe expression as k(1b,inf) = (2.4 ± 0.6) × 10(-11)(T/300)(-0.5) cm(3) molecule(-1) s(-1), k(1b,0) = [He] (9.0 ± 2.2) × 10(-31)(T/300)(-3.5±0.5) cm(3) molecule(-1) s(-1), F(c) = 0.37.

Shock Tube and Theoretical Studies on the Thermal Decomposition of Propane: Evidence for a Roaming Radical Channel

The thermal decomposition of propane has been studied using both shock tube experiments and ab initio transition state theory-based master equation calculations. Dissociation rate constants for propane have been measured at high temperatures behind reflected shock waves using high-sensitivity H-ARAS detection and CH(3) optical absorption. The two major dissociation channels at high temperature are C(3)H(8) → CH(3) + C(2)H(5) (eq 1a) and C(3)H(8) → CH(4) + C(2)H(4) (eq 1b). Ultra high-sensitivity ARAS detection of H-atoms produced from the decomposition of the product, C(2)H(5), in (1a), allowed measurements of both the total decomposition rate constants, k(total), and the branching to radical products, k(1a)/k(total). Theoretical analyses indicate that the molecular products are formed exclusively through the roaming radical mechanism and that radical products are formed exclusively through channel 1a. The experiments were performed over the temperature range 1417-1819 K and gave a minor contribution of (10 ± 8%) due to roaming. A multipass CH(3) absorption diagnostic using a Zn resonance lamp was also developed and characterized in this work using the thermal decomposition of CH(3)I as a reference reaction. The measured rate constants for CH(3)I decomposition agreed with earlier determinations from this laboratory that were based on I-atom ARAS measurements. This CH(3) diagnostic was then used to detect radicals from channel 1a allowing lower temperature (1202-1543 K) measurements of k(1a) to be determined. Variable reaction coordinate-transition state theory was used to predict the high pressure limits for channel (1a) and other bond fission reactions in C(3)H(8). Conventional transition state theory calculations were also used to estimate rate constants for other tight transition state processes. These calculations predict a negligible contribution (<1%) from all other bond fission and tight transition state processes, indicating that the bond fission channel (1a) and the roaming channel (1b) are indeed the only active channels at the temperature and pressure ranges of the present experiments. The predicted reaction exo- and endothermicities are in excellent agreement with the current version of the Active Thermochemical Tables. Master equation calculations incorporating these transition state theory results yield predictions for the temperature and pressure dependence of the dissociation rate constants for channel 1a. The final theoretical results reliably reproduce the measured dissociation rate constants that are reported here and in the literature. The experimental data are well reproduced over the 500-2500 K and 1 × 10(-4) to 100 bar range (errors of ∼15% or less) by the following Troe parameters for Ar as the bath gas: k(∞) = 1.55 × 10(24)T(-2.034) exp(-45 490/T) s(-1), k(0) = 7.92 × 10(53)T(-16.67) exp(-50 380/T) cm(3) s(-1), and F(c) = 0.190 exp(-T/3091) + 0.810 exp(-T/128) + exp(-8829/T).

Efficiency of photodesorption of Rb atoms collected on polymer organic film in vapor-cell

The efficiency of photodesorption of Rb atoms previously collected on polymer organic film has been studied in detail. This study was carried out in a Pyrex glass cell of which the inner surface was covered with (poly)dimethylsiloxane (PDMS) film and illuminated by a powerful flash lamp. The desorption dynamic of the Rb atoms density in the cell caused by the illumination and percentage of desorbed atoms was studied by using of Rb resonance lamp and free running diode laser as sources of probing light. It was determined that 85 percent collected chemical active Rb atoms and stored during 16 s in the closed cell, 75 percent in the pumped cell can be desorbed by single flash of the lamp. The number of stored atoms decays with a characteristic time of 60 min in isolated cell and with a time 12.4 min in a pumped cell. We believe that this efficient method of collection and fast realization of atoms or molecules could be used for enhancement of sensitivity of existed sensors for the trace detection of various elements (including toxic or radioactive ones) which is important to environmental applications, medicine or in geology. The effect might help to construct an efficient light-driven atomic source for a magneto-optical trap in a case of extremely low vapor density or very weak flux of atoms, such as artificial radioactive alkali atoms.

Classical ultraviolet photoelectron spectroscopy of polymers

Although X-ray photoelectron spectroscopy of polymers was well established by Clark and coworkers in the 1970s, ultraviolet photoelectron spectroscopy of polymer films, was developed later. Previous to the 1970s, the first attempts to use ultraviolet light on polymer films took the form of appearance potential (valence band edge) measurements. Only some years later could the full valence band region of thin polymer films, including insulating polymers, semiconducting polymers and electrically conducting polymers. The development of what might be termed “classical ultraviolet photoelectron spectroscopy” of polymer films may be loosely based upon a variety of issues, including adapting thin polymer film technology to ultra high vacuum studies, the widespread use of helium resonance lamps for studies of solid surfaces, the combined advent of practical and sufficient theoretical–computational methods. The advent of, and the use of, easily available synchrotron radiation for multi-photon spectroscopies, nominally in the area of the near UV, is not included in the term “classical”. At the same time, electrically conducting polymers were discovered, leading to applications of the corresponding semiconducting polymers, which added technologically driven emphasis to this development of ultraviolet photoelectron spectroscopy for polymer materials. This paper traces a limited number of highlights in the evolution of ultraviolet photoelectron spectroscopy of polymers, from the 1970s through to 2008. Also, since this issue is dedicated to Prof. Kazuhiko Seki, who has been a friend and competitor for over two decades, the author relies on some of Prof. Seki's earlier research, unpublished, on who-did-what-first. Prof. Seki's own contributions to the field, however, are discussed in other articles in this issue.

Digital Control of a Low-Frequency Square-Wave Electronic Ballast With Resonant Ignition

This paper proposes a two-stage low-frequency square-wave (LFSW) electronic ballast with digital control. The first stage of the ballast is a power factor correction (PFC) stage, and the second is a full-bridge (FB) converter used for both lamp ignition and LFSW drive. As a novelty for LFSW ballasts, ignition is achieved without an additional igniter circuit by operating the FB during start-up as a high-frequency resonant inverter. After ignition, the converter operates as an LFSW inverter to avoid exciting acoustic resonances by controlling the FB as a buck converter and regulating alternately positive or negative current to the lamp. Lamp power is regulated by adjusting the average current supplied by the PFC stage. Another contribution of this paper is to utilize digital control as a simple solution to achieve multimode control, including resonant lamp ignition, LFSW transitions, and lamp current and power regulation.

Ultrahigh-Resolution Vacuum Ultraviolet Light Source System for Extremely Low Energy Photoelectron Spectroscopy

We propose an efficient monochromatization method for the new ultrahigh-resolution Ar, Kr, and Xe light sources to be used for extremely low energy photoelectron spectroscopy (ELEPES). By fully utilizing the short-wavelength transmittance limits known as the Urbach tail of LiF, CaF2, and sapphire, we can extract only the longest wavelength lines of the resonance lamp with Ar, Kr, and Xe gases by using LiF heated up to ∼50 °C as well as CaF2 and sapphire at room temperature. The full width at half maximum of the longest wavelength Xe I resonance line is found to be narrower than 500 µeV. Using these light sources, we can perform ELEPES in laboratories at three photon energies with ultrahigh energy resolution and high efficiency.

Ultraviolet Photoelectron Spectroscopy of Polymers

During the past three decades, ultraviolet photoelectron spectroscopy of polymer films has evolved from a sort of appearance-potential (valence band edge) measurement, into a tool for studying the full valence band region of thin polymer films, including insulating polymers, semiconducting polymers and electrically conducting polymers. Progress may be loosely divided into several categories: (A) the melding of thin polymer film technology with ultra high vacuum technology and the widespread use of helium resonance lamps for studies of solid surfaces; (B) the combined approach of measurements and appropriate theoretical-computational methods; and (C) the advent of synchrotron radiation resulting in multi-photon spectroscopies, nominally in the area of the near UV. A coincident discovery of electrically conducting polymers, and especially the evolution of applications of semiconducting polymers, added technologically driven emphasis to this development of UPS for polymer materials. This contribution traces a limited number of highlights in the evolution of UPS of polymers, from the ‘70's through to 2005.

Hydrophobic recovery of VUV/NH3 modified polyolefin surfaces: Comparison with plasma treatments in nitrogen

Film samples of two very pure polyolefins (low density polyethylene, LDPE and biaxially oriented polypropylene, BOPP) were surface-modified by two different methods, namely vacuum ultraviolet (VUV) irradiation with a Kr resonant lamp in low-pressure NH3 gas, and atmospheric pressure glow discharge (APGD) plasma treatment in pure N2 gas. Samples were then stored in air and the time-dependence of surface properties (the surface energy and chemical composition) was monitored using several complementary surface-sensitive techniques: contact angle goniometry (CAG), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). We show that the main mechanism responsible for hydrophobic recovery is the motion of polymer chains and chain segments, which governs an apparent “loss” of functional groups, within the first monolayers of the surface (∼1nm). Finally, comparing BOPP samples modified by both techniques, we show that aging can be reduced by crosslinking near the surface, as illustrated by depth-sensing nano-indentation measurements.

Hydrophobic Recovery of Vacuum Ultraviolet Irradiated Polyolefin Surfaces

AbstractSummary: Film samples of low‐density polyethylene (LDPE) and biaxially oriented poly(propylene) (BOPP) were surface modified by vacuum ultraviolet (VUV) irradiation using a Kr resonant lamp at λ = 123.6 nm in low‐pressure ammonia gas, and were then stored in air. The time‐dependence of the surface properties was monitored using several complementary surface‐sensitive techniques such as contact angle goniometry (CAG), X‐ray photoelectron spectroscopy (XPS), and time‐of‐flight secondary ion mass spectroscopy (ToF‐SIMS), which allows one to determine the surface energy, and chemical composition at different depths. The relative importance of four possible mechanisms involved in surface hydrophobic recovery is discussed, and we show that in our particular case the main mechanism is rotational and/or translational motion of polymer chains and chain segments. This restructuring determines the observed “loss” of functional groups, which occurs within the first few monolayers of the surface (∼1 nm), as shown by the ToF‐SIMS results, and which leads to the observed decrease in the surface energy. In the deeper surface regions (∼10 nm) long‐lived radicals react with oxygen and water vapor upon exposure to the atmosphere, leading to an increase in the concentration of bound oxygen, as observed by XPS. Finally, CAG measurements show that the hydrophobic recovery is reversible and can be significantly reduced by cross‐linking near the surface, as illustrated by depth sensing nano‐indentation measurements on BOPP surfaces.Surface energy, γs, values as a function of storage time for VUV/NH3‐treated a) LDPE (Ed = 14.1 J · cm−2), and b) BOPP samples (Ed = 28.2 J · cm−2). Open symbols pertain to rinsed samples.magnified imageSurface energy, γs, values as a function of storage time for VUV/NH3‐treated a) LDPE (Ed = 14.1 J · cm−2), and b) BOPP samples (Ed = 28.2 J · cm−2). Open symbols pertain to rinsed samples.

Reliable and reproducible determination of work function and ionization potentials of layers and surfaces relevant to organic light emitting diodes

Ultra violet photoelectron spectroscopy has been used in most of the research related to determination of the energy levels of the materials in multilayer devices, such as organic light emitting diodes, to measure the Fermi and HOMO levels of conductors and organic materials, respectively. The variation in the values reported in the literature often remains unexplained despite apparently similar experimental conditions. We report (i) on the methods that we have found are required to achieve reproducible and stable values of work function (WF) and ionization potentials (IP) of ITO and common organic layers, and (ii) on the origin of the variations. Not unexpectedly, we have found that hydrocarbon contamination arising from vacuum pumps (e.g. oil diffusion pumps) can cause significant changes in WF or IP. Of greater possible significance is the observation that continuous exposure to ultraviolet light can enhance these effects and can change the work function of an interface much faster than would occur in the absence of UV light. The major effect probably arises from effects of the secondary electrons produced by the 21.22 eV photons of the He resonance lamp. By dealing with these problems, we have been able to measure stable values for WF and IP over extended periods of time ranging from hours to days.

Reliable and reproducible determination of work function and ionization potentials of layers and surfaces relevant to organic light emitting diodes

Ultra violet photoelectron spectroscopy has been used in most of the research related to determination of the energy levels of the materials in multilayer devices, such as organic light emitting diodes, to measure the Fermi and HOMO levels of conductors and organic materials, respectively. The variation in the values reported in the literature often remains unexplained despite apparently similar experimental conditions. We report (i) on the methods that we have found are required to achieve reproducible and stable values of work function (WF) and ionization potentials (IP) of ITO and common organic layers, and (ii) on the origin of the variations. Not unexpectedly, we have found that hydrocarbon contamination arising from vacuum pumps (e.g. oil diffusion pumps) can cause significant changes in WF or IP. Of greater possible significance is the observation that continuous exposure to ultraviolet light can enhance these effects and can change the work function of an interface much faster than would occur in the absence of UV light. The major effect probably arises from effects of the secondary electrons produced by the 21.22 eV photons of the He resonance lamp. By dealing with these problems, we have been able to measure stable values for WF and IP over extended periods of time ranging from hours to days.

Rate Constants, 1100 ≤ T ≤ 2000 K, for H + NO2 → OH + NO Using Two Shock Tube Techniques: Comparison of Theory to Experiment

Rate constants for the reaction H + NO2 → OH + NO have been measured over the temperature range 1100−2000 K in reflected shock wave experiments using two different methods of analysis. In both methods, the source of H-atoms is from ethyl radical decomposition in which the radicals are formed essentially instantaneously from the thermal decomposition of C2H5I. The first method uses atomic resonance absorption spectrometry (ARAS) to follow the temporal behavior of H-atoms. Experiments were performed under such low [C2H5I]0 that the title reaction could be chemically isolated, and the decay of H-atoms was strictly first-order. The results from these experiments can be summarized as k = (1.4 ± 0.3) × 10-10 cm3 molecule-1 s-1 for 1100 ≤ T ≤ 1650 K. The second method utilizes a multipass optical system for observing the product radical, OH. A resonance lamp was used as the absorption source. Because this is the first OH-radical kinetics investigation from this laboratory, extensive calibration was required. This procedure resulted in a modified Beer's law description of the curve-of-growth, which could subsequently be used to convert absorption data to OH-radical profiles. Rate constants by this method required chemical simulation, and the final result can be summarized as k = (1.8 ± 0.2) × 10-10 cm3 molecule-1 s-1 for 1250 ≤ T ≤ 2000 K. Because the results from the two methods statistically overlap, they can be combined giving k = (1.64 ± 0.30) × 10-10 cm3 molecule-1 s-1 for 1100 ≤ T ≤ 2000 K. The present results are compared to earlier work at lower temperatures, and the combined database yields the temperature dependence over the large range, 195−2000 K. The combined results can be summarized as k = (1.47 ± 0.26) × 10-10 cm3 molecule-1 s-1 for 195 ≤ T ≤ 2000 K. The reaction is subsequently considered theoretically using ab initio electronic structure calculations combined with modern dynamical theory to rationalize the thermal rate behavior.

Observation of wavelength‐sensitive mass‐independent sulfur isotope effects during SO2 photolysis: Implications for the early atmosphere

Mass‐independent isotopic signatures for δ33S, δ34S, and δ36S produced in the photolysis of sulfur dioxide exhibit a strong wavelength dependence. Photolysis experiments with three light sources (ArF excimer laser (193 nm), mercury resonance lamp (184.9 and 253.7 nm), and KrF excimer laser (248 nm)) are presented. Products of sulfur dioxide photolysis undertaken with 193‐nm radiation exhibit characteristics that are similar to sulfur multiple‐isotope data for terrestrial sedimentary rock samples older than 2450 Ma (reported by Farquhar et al. [2000a]), while photolysis experiments undertaken with radiation at other wavelengths (longer than 220 nm and at 184.9 nm) exhibit different characteristics. The spectral window between 190 and 220 nm falls between the Schumann‐Runge bands of oxygen and the Hartley bands of ozone, and its absorption is therefore more sensitive to changes in altitude and atmospheric oxygen content than neighboring wavelengths. These two observations are used to suggest a link between sulfur dioxide photolysis at 193 nm and sulfur isotope anomalies in Archean rocks. This hypothesis includes the suggestion that UV wavelengths shorter than 200 nm penetrated deep in the Earth's atmosphere during the Archean. Potential implications of this hypothesis for the chemistry, composition, and UV absorption of the atmosphere are explored. We also explore the implications of these observations for documentation of bacterial sulfur metabolisms early in Earth's history.

Airborne vacuum ultraviolet resonance fluorescence instrument for in situ measurement of CO

An airborne instrument for fast‐response, high‐precision measurement of tropospheric carbon monoxide (CO) was developed using a vacuum ultraviolet (VUV) resonance fluorescence technique. The excitation radiation is obtained by a DC discharge CO resonance lamp combined with an optical filter for the CO fourth positive band emission around 150 nm. The optical filter consists of a VUV monochromator and a crystalline quartz window (&lt;147‐nm cutoff). The crystalline quartz window ensures a sharp discrimination against wavelengths below 135.7 nm that yield a positive interference from water vapor. Laboratory tests showed that the optical system achieved a precision of 1.1 parts per billion by volume (ppbv) at a CO concentration of 100 ppbv for a 1‐s integration period, and the flow system provided a response time (1/e time constant) of ∼2 s. The aircraft measurement campaign Biomass Burning and Lightning Experiment‐phase B (BIBLE‐B) was conducted between August and September 1999 over the western Pacific and Australia. The flight data obtained during this campaign were used to demonstrate the high precision and fast response of the instrument. An intercomparison of the VUV CO measurement and a gas chromatographic CO measurement was conducted during BIBLE‐B. Overall, these two independent measurements showed good agreement, within the experimental uncertainties.