Ultraviolet Wavelength Range Research Articles

Synthesis, optical properties, energy transfer, thermal behavior, and LED package of Eu3+ doped lutetium tungsten oxide phosphors

By using a solid-state reaction method, Eu3+ doped monoclinic phase of Lu2WO6, mixture of Lu2WO6 and Lu2W3O12, Lu2W3O12, and hexagonal phase of Lu6WO12 lutetium tungsten oxide phosphors were synthesized. The obtained phosphors were composed of micrometre or nanometre scale particles. The energy-dispersive spectra (EDS) illustrated that the W atom percent increased with the sequence of Lu6WO12, Lu2WO6, mixture of Lu2WO6 and Lu2W3O12, and Lu2W3O12. The diffuse reflection (DR) spectra presented the broad absorption bands in the ultraviolet wavelength range and the energy gap values were evaluated based on the DR spectra. The photoluminescence (PL) excitation and emission spectra illustrated that the Lu2WO6 presented the most intense PL intensities in the four phosphors and the optimal Eu3+ concentration was 20 mol% in Lu2WO6 phosphors. The 20 mol% Eu3+ doped Lu2WO6 phosphor presented good thermal stability and kept 85% intensity of 50 °C at 200 °C. The deduced lifetime of Eu3+5D0→7F2 transition was several hundred micro-minutes. The energy transfer processes and the energy transfer efficiency were analysed based on the energy level diagram and the PL emission spectra. The obtained red phosphor was packaged into 308 nm chips and red light-emitting diodes (LEDs) were obtained. The chromaticity coordinates and the color purity were measured to evaluate the packaged LED devices. The results suggest that the Eu3+ doped Lu2WO6 has good thermal stability and has potential applications in phosphor converted LEDs.

Flying Through Uncertainty

AbstractSpace weather is the main source of uncertainty in the position of all objects in low Earth orbit (LEO) below about 1,000 km. The main impact is strong variation in the neutral density of the thermosphere as it responds to radiative inputs from the Sun in the extreme ultraviolet wavelength range, energetic particle precipitation in the high‐latitude auroral zones, and global‐scale electrical currents generated during geomagnetic storms. Waves and instabilities from the lower atmosphere can also influence thermospheric density in complex ways. The variation in neutral density leads to variable drag forces on satellites flying through the thermosphere, which in turn causes orbital track changes. We currently lack the ability to accurately model and predict the neutral density changes in the thermosphere in response to space weather inputs. Operational empirical models of thermospheric density are inaccurate during space weather events, and mandate that LEO orbital tracks carry large “error ellipsoids” around all objects to account for positional uncertainty. This leads to many more “conjunction” warnings than necessary as large error ellipsoids are frequently calculated to intersect in orbit. As the LEO domain becomes more crowded with the advent of commercial “megaconstellations” we face a growing challenge to reduce orbital uncertainties by developing whole atmosphere models to enable timely and accurate forecasts of thermospheric conditions. We recommend that researchers, forecasters, and policy makers coordinate to ensure that space weather research and forecasting is tightly integrated into upcoming changes to the operational Space Traffic Management system.

Ultraviolet light-emitting diode arrays using Ga-doped ZnO as current spreading layer

In this article, the authors investigate the fabrication and characterization of the 4 × 4 GaN-based ultraviolet (UV) light-emitting diode (LED) array with a pixel size of 200 × 200 μm2 and a pitch of 200 μm. For comparison, the single broad-area LED with a size of 400 × 400 μm2 is also fabricated from the same epitaxial wafer as a reference. The Ga-doped ZnO (GZO) film is used as a current spreading layer onto the ultraviolet LEDs to enhance the light output power. The GZO film with a thickness of 100 nm was deposited by atomic layer deposition (ALD) and has an electron concentration of ∼8 × 1020 cm−3, a low resistivity of ∼4 × 10−4 Ω cm, and a high transmittance of ∼84% in the near UV wavelength range. The ALD-GZO film was also used as the internal wires to connect the pixels of an LED array. The LEDs have a peak wavelength of 370 nm with a slight redshift at higher injection currents. The 4 × 4 LED array has a better performance in the maximum light output power density of 7.25 W/cm2 at 240 mA and a highly linear light output power to injection current, while the broad-area LED only exhibits the maximum light output power density of 1.34 W/cm2 at 70 mA. The single pixel of a 4 × 4 UV LED array reveals a maximum light output power density of 7.25 W/cm2 at 240 mA and a wider divergence angle of 137°, which are better than the broad-area LED of 1.34 W/cm2 at 70 mA and 128°, respectively. Finally, the authors show the demonstration of photos of an 8 × 8 LED array.

Ab initio calculations of optoelectronic properties of antimony sulfide nano-thin film for solar cell applications

Antimony sulfide (Sb2S3) micro thin-film have been received great interest as an absorbing layer for solar cell technology. In this study, to explore its further potential, electronic and optical properties of Sb2S3 simulated nano-thin film are investigated by the first-principles approach. To do so, the highly accurate full-potential linearized augmented plane wave (FP-LAPW) method framed within density functional theory (DFT) as implemented in the WIEN2k package is employed. The films are simulated in the [0 0 1] direction using the supercell method with a vacuum along z-direction so that slab and periodic images can be treated independently. From our calculations, indirect band gap energy values of Sb2S3 for various slabs are found to be 0.568, 0.596 and 0.609 eV for 1, 2 and 4 slabs respectively. Moreover, optical properties comprising of real and imaginary parts of the complex dielectric function, absorption coefficient, refractive index are also investigated to understand the optical behavior of the obtained simulated Sb2S3 thin films. From the analysis of their optical properties, it is clearly seen that Sb2S3 thin films have good values for optical absorption parameters in the visible and ultraviolet wavelength range, showing the aptness of antimony sulphide thins films for versatile optoelectronic applications as a base material.

Transparent Conductive Electrodes of β-Ga₂O₃/Ag/β-Ga₂O₃ Multilayer for Ultraviolet Emitters.

We investigated the optical and electrical properties of a β-Ga₂O₃/Ag/β-Ga₂O₃ multilayer transparent conductive electrode deposited on an α-Al2O₃ (0001) substrate. For the deposition of a continuous Ag layer, we preliminarily performed anultraviolet-ozone pretreatment of the Ga₂O₃ bottom layer. To obtain a stable β-phase of Ga₂O₃, the β-Ga₂O₃/Ag/β-Ga₂O₃ multilayer was annealed at 700 °C under N₂ atmosphere. The transmittance and sheet resistance of the β-Ga₂O₃/Ag/β-Ga₂O₃ multilayer were critically affected by the surface morphology and thickness of the Ag interlayer. The multilayer with optimized thicknesses (β-Ga₂O₃ top layer: 30 nm; Ag interlayer: 12 nm; β-Ga₂O₃ bottom layer: 60 nm) exhibited a resistance of 8.48 Ωsq-1, an average optical transmittance of 87.16% in the ultraviolet wavelength range from 300 to 350 nm, and a figure of merit of 29.81 × 10-3 Ω-1.

Synthesis and characterization of vanadium doped Bi4Ti3O12 as photocatalyst material

One of interesting properties of Bi4Ti3O12 is photocatalyst. The band gap of Bi4Ti3O12 is 2.9 eV as results only work at ultraviolet wavelength range. Metal doped to Bi4Ti3O12 can reduce band gap energy. In this work, Bi4Ti3O12 was doped vanadium (Bi4Ti3-xVxO12 with x= 0.05; 0.1; 0.15; 0.2) using solid state reaction method. X-ray diffraction data show that Bi4Ti3-xVxO12 (x= 0.05; 0.1; 0.15; 0.2) were successfully formed with space group B2cb, however there is small impurities of VO2 phase. Scanning electron microscopy (SEM) images showed that the particle of all obtained Bi4Ti3-xVxO12 (x= 0.05; 0.1; 0.15; 0.2) have shape platelet-like. Diffuse Reflectance Spectroscopy data showed that the energy band gap of Bi4Ti3-xVxO12 decreased by increasing vanadium dopant.

Study of iron impurity behaviour in the ADITYA tokamak

Iron (Fe) impurity behaviour in ADITYA tokamak plasma has been studied using Fe spectral line emissions in the vacuum ultra violet (VUV) wavelength range. A VUV survey spectrometer has been utilized to record spectral lines at 28.41 nm from Fe14+ , and 33.54 nm and 36.08 nm from Fe15+ . It has been observed that the intensities of the Fe emissions decrease with an increase in plasma electron density. The observed emission and the intensity ratio of Fe14+ and Fe15+ ions from two discharges having relatively low and high plasma density are modelled using an impurity transport code. It is found that the observed data could be modelled using the same ratio of the convective velocity to the diffusion coefficient D profile, but with two different Fe concentrations. The ratio varies from the value of − 0.22 m−1 at the plasma normalized radius to a maximum value of − 0.35 m−1 at . The obtained diffusion coefficient value at the plasma core region is explained in terms of neo-classical transport, indicating that Fe impurity transport follows the same behaviour in the core plasma of the ADITYA tokamak.

Effect of Supporting Electrolyte on the Surface Corrosion and Anodic Oxidation Performance of Graphite Electrode

This study presents a comparative investigation of supporting electrolyte effect on the surface corrosion and anodic oxidation performance of graphite electrode. The effect of electrolyte on the electrochemical behavior is investigated in solution containing 0.05 mol L−1 sodium salt, using current-overpotential curves and Tafel lines. The surface morphology and composition of the graphite anodes before and after electrolysis are characterized by scanning electron microscopy, X-ray diffraction, and Raman spectra. In aqueous solution, graphite anode is positive polarized, functionalizes the surface, and releases soluble organics. The light-absorbing corrosion products of graphite anode are recorded by a UV-Vis spectrophotometer in the ultraviolet wavelength range. Batch experiments on atrazine oxidation show that NaCl is more efficient than Na2SO4 and NaNO3 to promote the combustion of organics due to the mediated oxidation of the electrogenerated active chlorine species. The oxidation process is confirmed to employ a non-radical mechanism in which the organics are degraded at the active sites of graphite anode.

Highly sensitive photo-detectors for the ultra-violet wavelength range based on a dielectric stack and a silicon on insulator substrate

We propose and demonstrate planar metal-insulator-semiconductor-metal photodetectors fabricated on a silicon-on-insulator substrate with an n-type silicon device layer. The gate insulator comprises a double layer dielectric stack of SiO2 and HfO2. Detectors with different electrode geometries were characterized in a wide wavelength range: from 245 nm to 880 nm. A responsivity of 1.77 A/W was achieved at 405 nm independent of the illumination intensity, while at 245 nm, the responsivity was found to be nonlinear and at an intensity of 8 μW/cm2, it reached a record value of 30.5 A/W. Local fringing electric fields across asymmetric metal-insulator-semiconductor and metal-semiconductor junctions, stemming from the insulator stack and a reduction of the effective barrier height under illumination, are assumed to cause the high responsivity at wavelengths longer than 365 nm. The super linear rise in responsivity for wavelengths shorter than 285 nm is due to deep trap states which are charged by the injected electrons.

Lidar observations of the structure and evolution of aerosol clusters in the DELICAT project

The main goal of the DELICAT (DEmonstration of LIdar based CAT detection) project was to check the possibility of early detection of clear air turbulence regions by recording the intensity of reverse molecular scattering by means of a specially designed lidar mounted on the aircraft, operating in the ultraviolet range of wavelengths. Flight tests were conducted from July 17 to August 12, 2013 over the western part of Europe and the adjacent maritime territories. Although measurements in a clear sky were preferred, in most cases aerosol clusters of different power were present along the flight paths. Aerosol clusters should be considered as a source of interference, but their observations also provide an opportunity to obtain information on the properties of the aerosol and the spatial-temporal evolution of clusters. In this paper, we give statistics of observations of aerosol clusters in flights, describe the diversity of their types and trace the influence of flight parameters on the observed characteristics of clusters.

Investigation of the thermo stability of aluminum thin-film filters with protective MoSi2 cap layers.

Aluminum thin-film spectral filters are widely used in telescopes for space observations of the Sun in the extreme ultraviolet range of wavelengths. The main purpose of film filters is to block radiation in the UV, visible, and near-IR spectral ranges. In connection with the development of new projects for the observation of the Sun from close distances, the thermal stability of the entrance film filter is an important characteristic. In this paper, the thermal stability of Al films with 2.5nm thick MoSi2 protective cap layers has been studied. MoSi2 was chosen as a coating material because MoSi2 caps effectively protect the Al film from oxidation and simultaneously increase the mechanical strength of the Al film. Vacuum annealing of MoSi2/Al/MoSi2 films was carried out at temperatures up to 300°C. It has been demonstrated that at an annealing temperature of more than 200°C for 24h, a noticeable decrease in the blocking properties of the MoSi2/Al/MoSi2 film is observed in the visible wavelength range, which is caused by the appearance of semi-transparent crystalline silicon dendritic structures that are tens of micrometers in size in the film. In the annealed area of the film specimen, the intermetallic Al12Mo phase was detected by electron diffraction structure analysis, indicating a possible reason for the appearance of silicon atoms needed for dendrite growth as a result of the chemical interaction of Al and MoSi2. Due to the requirements for a high degree of visible radiation blocking (106 to 107 times), the appearance of even one dendritic structure significantly reduces the blocking properties of the film filter and is, therefore, not permissible. Based on the measurement of the transmission of MoSi2-2.5 nm/Al-72 nm/MoSi2-2.5 nm films at 633nm for isothermal annealing at 200°C-250°C, the activation energy for the formation of dendritic structures (E=1.55±0.1 eV) was measured and the maximum permissible film temperature (130°C) at which dendritic structures did not appear during a 5-year mission was predicted.

Evaluation of tungsten influx rate and study of edge tungsten behavior based on the observation of EUV line emissions from W6+ ions in HL-2A

The tungsten spectrum in the extreme ultraviolet (EUV) wavelength range has been investigated in tokamak HL-2A to find line emissions from low-ionized tungsten ions which can be used for tungsten transport study in the plasma edge. For this purpose, tungsten is injected in the HL-2A plasma using the laser blow-off (LBO) technique with a newly developed double-layer tungsten target. The total number of injected tungsten particles can be accurately controlled by adjusting the distance from the focal lens to the tungsten target. A quasi-continuum spectrum called an unresolved transition array (UTA), consisting of many line emissions from several ionization stages of tungsten ions, is observed at around 50 Å as a typical indication of the tungsten spectrum. In the present HL-2A experiment the quasi-continuum of tungsten is mainly composed of line emissions from W14+ to W23+ ions. Analyzing the observed tungsten spectrum, two isolated line emissions of W VII (5p–5d: 216.219 Å) and W VII (5p–5d: 261.387 Å) from W6+ ions are successfully identified for the first time in tokamaks. The intensity ratio and the wavelengths of the two lines determined in the present experiment show good agreement with the intensity ratio calculated with a collisional-radiative model developed for the W6+ ion and a previous result by Sugar and Kaufman (1975 Phys. Rev. A 12 994), respectively. On the other hand, the W VII line emission occasionally reappears after the LBO injection, suggesting a re-entry of the tungsten deposited onto divertor plates and a movement of the strike point during the discharge to a certain area of the divertor plate where tungsten is deposited. Based on the W VII emission observed during the re-entry event of tungsten ions, an influx of the W6+ ion, , to the edge plasma is evaluated using the inverse photon efficiency calculated with the ADAS code and using an average-configuration option. As a result, it is found that is typically within the range (0.1–2.0) × 1013 cm−2 s−1. A brief discussion is also made on the influx evaluated in the present study.

Nondestructive Evaluation of Solid Pharmaceutical Products Using Wavelet Transformations and Multispectral Data

Abstract The nondestructive analysis of a solid pharmaceutical product (SPP) is essential to evaluating the quality without destroying the product. This analysis may be performed using various signal processing techniques on multispectral data. Based on this analysis, the SPPs may be classified as defective or nondefective. In this research, we have used multispectral data and applied wavelet transformations in conjunction with various machine learning techniques for the classification. The SPPs (categorized as defective) are exposed to three different environmental factors (humidity, temperature, and moisture) over different time periods, and the variations in multispectral data are analyzed to judge the effects of these factors on the classification of the SPPs. The results show that the spectra extracted from only the ultraviolet (UV) wavelength range are more suitable for the classification of defective and nondefective SPPs. Furthermore, results also describe that the K-nearest neighbors classifier or ensemble of classifiers is a more appropriate classifier.

Photonic crystals with a continuous, Gaussian-type surface profile for near-perfect light trapping

We show a unique design of teepee-like photonic crystal (TP-PC) structure that possesses a true gradient, Gaussian-type surface profile for smooth and accurate index matching between air and silicon for near-perfect light trapping. Such funnel-like, inverse-conical topography is capable of achieving near-zero optical reflection and near-unity solar absorption with excellent angular response over the entire visible light wavelength range. The fabrication only requires standard microelectronics reactive-ion etching (RIE) process. We demonstrate how various process parameters, such as RIE gas mixture ratio, RIE power, thickness of silicon dioxide (SiO2) coatings, and lattice constant of the photonic crystal, can impact the details of the “Gaussian” profile and further improve the optical performance of the TP-PC structure at broad-λ, broad-θ, especially in the ultraviolet (UV) wavelength range. Our finite-difference time-domain (FDTD) simulation of the TP-PC structure reveals existence of multiple absorption resonances in the 800- to1100-nm wavelength range. Poynting vector plots show that such strong absorption enhancements at the resonant frequencies are due to long-lifetime photonic modes arising from parallel-to-interface refraction of the incoming sunlight and formation of vortex-like energy flow pattern inside the TP-PC structure. Our design will lead the way for future development of ultrathin, high-efficiency c-Si solar photovoltaics.

Ultraviolet spectral reflectance of carbonaceous materials

A number of planetary spacecraft missions have carried instruments with sensors covering the ultraviolet (UV) wavelength range. However, there exists a general lack of relevant UV reflectance laboratory data to compare against these planetary surface remote sensing observations in order to make confident material identifications. To address this need, we have systematically analyzed reflectance spectra of carbonaceous materials in the 200–500 nm spectral range, and found spectral-compositional-structural relationships that suggest this wavelength region could distinguish between otherwise difficult-to-identify carbon phases. In particular (and by analogy with the infrared spectral region), large changes over short wavelength intervals in the refractive indices associated with the trigonal sp2π–π* transition of carbon can lead to Fresnel peaks and Christiansen-like features in reflectance. Previous studies extending to shorter wavelengths also show that anomalous dispersion caused by the σ–σ* transition associated with both the trigonal sp2 and tetrahedral sp3 sites causes these features below λ = 200 nm. The peak wavelength positions and shapes of π–π* and σ–σ* features contain information on sp3/sp2, structure, crystallinity, and powder grain size. A brief comparison with existing observational data indicates that the carbon fraction of the surface of Mercury is likely amorphous and submicroscopic, as is that on the surface of the martian satellites Phobos and Deimos, and possibly comet 67P/Churyumov–Gerasimenko, while further coordinated observations and laboratory experiments should refine these feature assignments and compositional hypotheses. The new laboratory diffuse reflectance data reported here provide an important new resource for interpreting UV reflectance measurements from planetary surfaces throughout the solar system, and confirm that the UV can be rich in important spectral information.

Compact Deep UV System at 222.5 nm Based on Frequency Doubling of GaN Laser Diode Emission

Laser light sources emitting in the deep ultraviolet wavelength range between 210 and 230 nm are of great interest for spectroscopic applications. Here, a compact DUV diode laser system emitting at a wavelength of 222.5 nm is presented. The system is based on frequency doubling of the laser radiation from a micro-integrated GaN external cavity diode laser module (μECDL) emitting at 445 nm. The μECDL has an optical pump power of 1.4 W with an emission bandwidth of 35 pm. Narrowband laser radiation in continouos wave operation with an output power of 160 μW at 222.5 nm is generated in a single-pass frequency doubling stage with a β-BaB 2 O 4 crystal. The results are suitable to address applications such as spectroscopic investigations of biological samples. The presented concept of a compact and efficient deep ultraviolet laser light source enables the realization of portable systems for which a small footprint and a low power consumption is essential.

Retrieval Accuracy of HCHO Vertical Column Density from Ground-Based Direct-Sun Measurement and First HCHO Column Measurement Using Pandora

In the present study, we investigate the effects of signal to noise (SNR), slit function (FWHM), and aerosol optical depth (AOD) on the accuracy of formaldehyde (HCHO) vertical column density (HCHOVCD) using the ground-based direct-sun synthetic radiance based on differential optical absorption spectroscopy (DOAS). We found that the effect of SNR on HCHO retrieval accuracy is larger than those of FWHM and AOD. When SNR = 650 (1300), FWHM = 0.6, and AOD = 0.2, the absolute percentage difference (APD) between the true HCHOVCD values and those retrieved ranges from 54 (30%) to 5% (1%) for the HCHOVCD of 5.0 × 1015 and 1.1 × 1017 molecules cm−2, respectively. Interestingly, the maximum AOD effect on the HCHO accuracy was found for the HCHOVCD of 3.0 × 1016 molecules cm−2. In addition, we carried out the first ground-based direct-sun measurements in the ultraviolet (UV) wavelength range to retrieve the HCHOVCD using Pandora in Seoul. The HCHOVCD was low at 12:00 p.m. local time (LT) in all seasons, whereas it was high in the morning (10:00 a.m. LT) and late afternoon (4:00 p.m. LT), except in winter. The maximum HCHOVCD values were 2.68 × 1016, 3.19 × 1016, 2.00 × 1016, and 1.63 × 1016 molecules cm−2 at 10:00 a.m. LT in spring, 10:00 a.m. LT in summer, 1:00 p.m. LT in autumn, and 9:00 a.m. LT in winter, respectively. The minimum values of Pandora HCHOVCD were 1.63 × 1016, 2.23 × 1016, 1.26 × 1016, and 0.82 × 1016 molecules cm−2 at around 1:45 p.m. LT in spring, summer, autumn, and winter, respectively. This seasonal pattern of high values in summer and low values in winter implies that photo-oxidation plays an important role in HCHO production. The correlation coefficient (R) between the monthly HCHOVCD values from Pandora and those from the Ozone Monitoring Instrument (OMI) is 0.61, and the slope is 1.25.

Low-emittance thermionic-gun-based injector for a compact free-electron laser

A low-emittance thermionic-gun-based injector was developed for the x-ray free-electron laser (XFEL) facility known as the SPring-8 angstrom compact free-electron laser (SACLA). The thermionic-gun-based system has the advantages of maintainability, reliability, and stability over a photocathode radio-frequency (rf) gun because of its robust thermionic cathode. The basic performance of the injector prototype was confirmed at the SPring-8 compact self-amplified spontaneous emission source (SCSS) test accelerator, where stable FEL generation in an extreme ultraviolet wavelength range was demonstrated. The essential XFEL innovation is the achievement of a constant beam peak current of 3--4 kA, which is 10 times higher than that generated by the SCSS test accelerator, while maintaining a normalized-slice emittance below 1 mm mrad. Thus, the following five modifications were applied to the SACLA injector: (i) a nonlinear energy chirp correction; (ii) the optimization of the rf acceleration frequency; (iii) rf system stabilization; (iv) nondestructive beam monitoring; and (v) a geomagnetic field correction. The SACLA injector successfully achieved the target beam performance, which shows that a thermionic-gun-based injector is applicable to an XFEL accelerator system. This paper gives an overview of the SACLA injector and describes the physical and technical details, together with the electron beam performance obtained in the beam commissioning.

Measurements of density dependent intensity ratios of extreme ultraviolet line emission from Fe X, XI, and XII

We report high-resolution density dependent intensity ratio measurements for middle charge states of iron in the extreme ultraviolet (EUV) spectral wavelength range of 160−200 A. The measurements were performed at the Tokyo EBIT laboratory by employing a flat-field grazing incidence spectrometer installed on a low energy compact electron beam ion trap. The intensity ratios for several line pairs stemming from Fe X, Fe XI and Fe XII were extracted from spectra collected at the electron beam energies of 340 and 400 eV by varying the beam current between 7.5 and 12 mA at each energy. In addition, the effective electron densities were obtained experimentally by imaging the electron beam profile and ion cloud size with a pinhole camera and visible spectrometer, respectively. In this paper, the experimental results are compared with previous data from the literature and with the present calculations performed using a collisional-radiative model. Our experimental results show a rather good agreement with the calculations and previous reported results.

Low-Temperature Experimental and Theoretical Rate Constants for the O(1D) + H2 Reaction

In the present joint experimental and theoretical study, we report thermal rate constants for the O(1D) + H2 reaction within the 50-300 K temperature range. Experimental kinetics measurements were performed using a continuous supersonic flow reactor coupled with pulsed laser photolysis for O(1D) production and pulsed laser-induced fluorescence in the vacuum ultraviolet wavelength range (VUV LIF) for O(1D) detection. Theoretical rate constants were obtained using the ring polymer molecular dynamics (RPMD) approach over the two lowest potential energy surfaces 11A' and 11A″, which possess barrierless and thermally activated energy profiles, respectively. Both the experimental and theoretical rate constants exhibit a weak temperature dependence. The theoretical results show the dominant role of the 11A' ground state and that contribution of the 11A″ excited state to the total thermal rate decreases dramatically at lower temperature. Agreement between the experimental and theoretical results is good, and the discrepancy does not exceed 25%. It is argued that these differences are likely to be due to nonadiabatic couplings between the 11A' and 21A' surfaces.