Far-UV absorption spectra of SiH2 and dibridged Si2H2 isolated in solid argon.

We employ electron bombardment during the deposition of an Ar matrix containing a small proportion of SiH4 to generate various silicon hydrides. Subsequently, the irradiation of a matrix sample at 365 nm decomposes SiH2 and dibridged Si2H2 in solid Ar, which we identify through infrared spectroscopy. We further recorded the corresponding ultraviolet absorption spectra at each experimental stage. An intense band observed in the range of 170-203 nm is largely destroyed upon 365-nm photolysis, which is assigned to the C1B2 â† X1A1 transition of SiH2. Moreover, a moderate band observed in the region of 217-236 nm is reduced slightly, which is assigned to the 31B2 â† X1A1 transition of dibridged Si2H2. These assignments are made based on the observed photolytic behavior, and the prediction of the vertical excitation energies with the corresponding oscillator strengths by using time-dependent density functional theory and equation-of-motion coupled cluster theory.

Formation of Para-H2O by Vacuum-UV Photolysis of O2 in Solid Hydrogen: Implication for Astrochemistry.

The observation that the ortho to para ratio (OPR) of interstellar H2O is smaller than 3 is an important yet unresolved subject in astronomy. We irradiated O2 embedded in solid H2 at 3 K with vacuum-ultraviolet (VUV) light and observed IR lines associated with para-H2O (denoted as pH2O) and nonrotating H2O-(oH2)n (where oH2 denotes ortho-H2) but no lines associated with ortho-H2O (denoted as oH2O). After maintaining the matrix in darkness for ∼30 h, the amount of pH2O decreased, accompanied by an increase in H2O-(oH2)n via diffusion of oH2. After that, the continuous nuclear-spin conversion from oH2 to para-H2 (denoted as pH2) in solid H2 over time resulted in the conversion of nonrotating H2O-(oH2)n to rotating pH2O in solid pH2. The observation of the formation and conversion of pH2O in our experiment suggests a plausible route in which VUV irradiation of O2 and H2 adsorbed on grain surfaces might be responsible for the smaller OPR of interstellar H2O.

IR absorption spectra of aniline cation, anilino radical, and phenylnitrene isolated in solid argon.

Electron bombardment of aniline (PhNH2) in an Ar matrix mainly generated the aniline cation (PhNH2+), anilino (PhNH) and phenyl (Ph) radicals, and phenylnitrene (PhN). Further irradiation of the electron-bombarded matrix sample at 365 nm depleted PhNH2+ and PhN, and resulted in the formation of PhNH2, PhNH, and Ph. In separate experiments, irradiation of the PhNH2/Ar matrix samples at 265 or 160 nm mainly generated PhNH and Ph radicals, but without the formation of PhNH2+ and PhN. According to the observed photochemical behaviors, quantum-chemically predicted harmonic vibrational wavenumbers of each species, and the information reported in previous photodissociation studies, we unambiguously characterized the IR features of the aromatic species. The information of the vibrational fundamentals of PhNH is new and the formation mechanism is discussed.

Far-UV spectroscopy of mono- and multilayer hexagonal boron nitrides.

Hexagonal boron nitrides (hBNs) have a very high luminescence efficiency and are promising materials for deep-UV emitters. Although intense deep-UV emissions have been recorded in various forms of hBN excited by photons or energetic electrons, information on the electronic structure of the conduction band has been derived mainly from theoretical works. Therefore, there is a lack of high-resolution absorption data in the far-UV region. In this study, the far-UV absorption spectra of chemical-vapor-deposition-grown mono- and multilayer hBNs were recorded at 10 and 298 K. In addition to the previously reported band at 6.10 eV, two absorption bands at 6.82 and 8.86 eV were observed for the first time in thin-film hBN. Furthermore, excitation of the hBN thin film samples with 6.89-eV photons revealed intense emission peaks at 6.10 (mono) and 5.98 (multi) eV with a bandwidth of ∼0.7 eV. Comparing the absorption and photoluminescence data, we believe that both direct and indirect transitions occur in the radiative processes.

Possible detection of hydrazine on Saturn's moon Rhea.

We present the first analysis of far-ultraviolet reflectance spectra of regions on Rhea's leading and trailing hemispheres collected by the Cassini Ultraviolet Imaging Spectrograph during targeted flybys. In particular, we aim to explain the unidentified broad absorption feature centred near 184 nm. We have used laboratory measurements of the UV spectroscopy of a set of candidate molecules and found a good fit to Rhea's spectra with both hydrazine monohydrate and several chlorine-containing molecules. Given the radiation-dominated chemistry on the surface of icy satellites embedded within their planets' magnetospheres, hydrazine monohydrate is argued to be the most plausible candidate for explaining the absorption feature at 184 nm. Hydrazine was also used as a propellant in Cassini's thrusters, but the thrusters were not used during icy satellite flybys and thus the signal is believed to not arise from spacecraft fuel. We discuss how hydrazine monohydrate may be chemically produced on icy surfaces.

IR absorption spectra of hexafluorobenzene anions and pentafluorophenyl radicals in solid argon.

Hexafluorobenzene anions (HFB-) and pentafluorophenyl radicals (PFP) were generated by the electron bombardment of a HFB/Ar sample during matrix deposition. Further irradiation of the matrix sample at 365 nm detached the electron from HFB- and produced its neutral counterpart HFB in solid Ar. Secondary photolysis of the matrix sample at 160 nm destroyed HFB and generated HFB- and PFP. In a separate experiment, the photolysis of the HFB/Ar matrix at 160 nm produced PFP, Dewar-C6F6, and various neutral fluorocarbon species, but without the production of HFB-. The infrared (IR) lines of HFB- and PFP were assigned on the basis of the observed photochemical behaviors, as well as by comparing the predictions of the vibrational wavenumbers with the corresponding IR intensities and the relative stabilities of the related species predicted via the B3LYP/aug-cc-pVTZ theory.

Formation and IR spectrum of monobridged Si2H4 isolated in solid argon.

The infrared (IR) spectrum of monobridged Si2H4 (denoted as mbr-Si2H4) isolated in solid Ar was recorded, and a set of lines (in the major matrix site) observed at 858.3 cm-1, 971.5 cm-1, 999.2 cm-1, 1572.7 cm-1, 2017.7 cm-1, 2150.4 cm-1, and 2158.4 cm-1 were characterized. The species was produced by the electron bombardment of an Ar matrix sample containing a small proportion of SiH4 during matrix deposition. Upon photolysis of the matrix samples using 365 nm and 160 nm light, the content of mbr-Si2H4 increased. The band positions, relative intensity ratios, and D-isotopic shift ratios of the observed IR features are generally in good agreement with those predicted by the B3LYP/aug-cc-pVTZ method. In addition, the photochemistry of the observed products was discussed.

Nitrogen-Vacancy Centers in Diamond for High-Performance Detection of Vacuum Ultraviolet, Extreme Ultraviolet, and X-rays.

Fluorescent nanodiamonds (FNDs) containing nitrogen-vacancy (NV) centers as built-in fluorophores exhibit a nearly constant emission profile over 550-750 nm upon excitation by vacuum-ultraviolet (VUV), extreme ultraviolet (EUV), and X-radiations from a synchrotron source over the energy (wavelength) range of 6.2-1450 eV (0.86-200 nm). The photoluminescence (PL) quantum yield of FNDs increases steadily with the increasing excitation energy, attaining a value as great as 1700% at 700 eV (1.77 nm). Notably, the yield curve is continuous, having no gap in the VUV to X-ray region. In addition, no significant PL intensity decreases were observed for hours. Applying the FND sensor to measure the absorption cross-sections of gaseous O2 over 110-200 nm and comparing the measurements with the sodium-salicylate scintillator, we obtained results in agreement with each other within 5%. The superb photostability and broad applicability of FNDs offer a promising solution for the long-standing problem of lacking a robust and reliable detector for VUV, EUV, and X-radiations.

Mid-infrared spectra of silane dispersed in solid neon.

We report mid-infrared spectra of silane dispersed in solid neon at relative concentrations 1:1000 and 1:5000, recorded with spectral resolution 0.15 cm-1. Apart from major lines associated with internal vibrational motions of 28SiH4, 29SiH4 and 30SiH4 in fundamental modes ν3 and ν4, several weak accompanying lines in each region become discernible at the resolution of our experimental measurements, and are tentatively associated with librational motions of silane molecules in the solid neon lattice. The wavenumbers associated with a few overtone and combination modes are also presented.

Thermal reaction and luminescence of long-lived N 2D in N2 ice.

Photochemistry of an N2 ice and thermal reaction of the irradiated sample were studied with vacuum-ultraviolet (VUV) light from a synchrotron. Concurrent detection of infrared absorption and visible emission spectra provide evidence for the generation of energetic products N (2D) and N (2P) atoms, N2 (A) molecule and linear-N3 (l-N3) radical after excitation of icy N2 at 121.6 nm. Irradiation at 190 nm is shown to be an effective way to eliminate the l-N3 radical. After the photolysis and photoelimination of the l-N3, we initiate synthesis of l-N3 via the thermal ramping of the sample in temperature range 3.5 to 20 K. In addition, the emission from the N (2D) atom was observed during the thermal ramping process. These behaviors indicate that a long-lived N (2Dlong) atom is generated in the VUV-photolyzed N2 ice. A comparison of the variations of the visible emission of N (2D) and the infrared absorption of l-N3 with time indicates that the long-lived N (2Dlong) dominated the thermal synthesis of l-N3 The results have enhanced suggestion and understanding of the conversion for nitrogen species in cold astrophysical environments with VUV irradiation.

Electronic and Vibrational Absorption Spectra of NH2 in Solid Ne.

Irradiation of ammonia dispersed in solid neon near 4 K with tunable far-ultraviolet light from a synchrotron yielded amidogen, NH2, and imidogen, NH, radicals as products. The electronic absorption spectra of amidogen radicals in isotopic variants NH2, NHD, and ND2 were recorded in the visible and near-ultraviolet regions after photolysis of NH3 and ND3. The infrared absorption lines of NH2 associated with vibration-rotational levels of vibrational modes ν1 at 3234.3 (00,0-10,1), 3244.9 (00,0-11,1), and 3249.3 cm-1 (00,0-11,0), and ν2 at 1498.7 (10,1-11,1), 1509.5 (11,0-10,1), 1516.5 (00,0-10,1), 1528.6 (00,0-11,1), and 1533.7 cm-1 (00,0-11,0) were unambiguously identified according to the results of experiments with deuterium isotopes. The 00,0-00,0 lines of ν1 and ν2 for NH2 were derived to be at 3213.5 and 1494.6 cm-1 in solid neon.

Thresholds of photolysis of O2 and of formation of O3 from O2 dispersed in solid neon.

Irradiation of O2 dispersed in solid Ne with ultraviolet light produced infrared absorption lines of O3 and emission lines from atomic O (1D2 → 3P1,2), molecular O2 (A' 3Δu → X 3Σg) and radical OH (A 2Σ+ → X 2ΠI) in the visible and near-ultraviolet regions. The threshold wavelength for the formation of O3 was determined to be 200 ± 4 nm, corresponding to energy 6.20 ± 0.12 eV, which is hence the threshold for dissociation of O2. The thresholds of emission from excited O (1D2), O2 (A' 3Δu) and OH (A 2Σ+) were all observed to be 200 ± 4 nm, the same as for the formation of O3 in this photochemical system. The results indicate that, once O3 was generated, it was readily photolyzed to produce the long-lived atom O (1D2). Further reactions of O (1D2) with O3 produced excited O2 (A' 3Δu); reaction with water yielded radical OH (A 2Σ+). These results enhance our understanding of the evolution of the transformation of oxygen and open a window for the understanding of complicated processes in the solid phase.

Photolysis of O2 dispersed in solid neon with far-ultraviolet radiation.

Irradiation at 173 or 143 nm of samples of 16O2 or 18O2 in solid Ne near 4 K produced many new spectral lines in absorption and emission from the mid-infrared to the near-ultraviolet regions. The major product was ozone, O3, that was identified with its mid-infrared and near-ultraviolet absorption lines. Oxygen atoms were formed on photolysis of O2 and stored in solid neon until the temperature of a sample was increased to 9 K, which enabled their migration and combination to form O3 and likely also O2. O2 in five excited states and O in two excited states detected through the emission spectra indicate that complicated processes occurred in solid Ne after far-ultraviolet excitation. For the transition 1D2 → 3P1,2 of O, the lifetime was determined to be 5.87 ± 0.10 s; the lifetime of the upper state of an unidentified transition associated with an emission feature at 701.7 nm was determined to be 2.34 ± 0.07 s.

Far-UV-Excited Luminescence of Nitrogen-Vacancy Centers: Evidence for Diamonds in Space.

The nitrogen-vacancy (NV) centers in diamond are among the most thoroughly investigated defects in solid-state matter; however, our understanding of their properties upon far-UV excitation of the host matrix is limited. This knowledge is crucial for the identification of NV as the carrier of extended red emission (ERE) bands detected in a wide range of astrophysical environments. Herein, we report a study on the photoluminescence spectra of NV-containing nanodiamonds excited with synchrotron radiation over the wavelength range of 125-350 nm. We observed, for the first time, an emission at 520-850 nm with a quantum yield greater than 20 %. Our results share multiple similarities with the ERE phenomena, suggesting that nanodiamonds are a common component of dust in space.

Identification of cyc-B3H3 with Three Bridging B-H-B Bonds in a Six-Membered Ring.

Irradiation of samples of diborane(6), B2H6 and B2D6, separately and together, dispersed in solid neon near 4 K with tunable far-ultraviolet light from a synchrotron yielded new infrared absorption lines that are assigned to several carriers. Besides H, B, BH, BH2, BH3, B2, B2H2, and B2H4, previously identified, a further species is assigned on the basis of quantum-chemical calculations of vibrational wavenumbers and intensities to be cyc-B3H3 (D 3h , singlet state) in several isotopic variants, which feature three bridging B-H-B bonds in a six-membered ring.

Ultraviolet and Infrared Spectra of Diboron in Solid Neon at 4 K.

Apart from products H, B, BH, BH2 and BH3 identified from their emission spectra in the UV/Vis region, photolysis of diborane(6) dispersed in solid neon at 4 K with far-ultraviolet light from a synchrotron led to observation of absorption line (0,0) of the electronic transition A 3 Σu- ←X 3 Σg- of B2 at 326.39 nm. Absorption lines (1,0) of 11 B2 , 11 B10 B and 10 B2 were recorded at 316.63, 316.40 and 316.15 nm, respectively. ΔG1/2 of state A 3 Σu- for 11 B2 , 11 B10 B and 10 B2 in solid neon are accordingly derived to be 945, 968 and 993 cm-1 , respectively. Weak lines (0,1) of 11 B2 at 29586 cm-1 and of 11 B10 B at 29560 cm-1 , corresponding to 1042±30 and 1068±30 cm-1 for vibrational modes in the electronic ground state, were recorded in emission. An absorption line recorded at 1066.5±0.5 cm-1 in infrared spectra after photolysis of either B2 H6 in Ne or B2 D6 with D2 in Ne is thus attributed to 11 B10 B.

Infrared and Ultraviolet Spectra of Diborane(6): B2H6 and B2D6.

We recorded absorption spectra of diborane(6), B2H6 and B2D6, dispersed in solid neon near 4 K in both mid-infrared and ultraviolet regions. For gaseous B2H6 from 105 to 300 nm, we report quantitative absolute cross sections; for solid B2H6 and for B2H6 dispersed in solid neon, we measured ultraviolet absorbance with relative intensities over a wide range. To assign the mid-infrared spectra to specific isotopic variants, we applied the abundance of (11)B and (10)B in natural proportions; we undertook quantum-chemical calculations of wavenumbers associated with anharmonic vibrational modes and the intensities of the harmonic vibrational modes. To aid an interpretation of the ultraviolet spectra, we calculated the energies of electronically excited singlet and triplet states and oscillator strengths for electronic transitions from the electronic ground state.

Analysis of Nickel Defect in Diamond with Photoluminescence upon Excitation near 200 nm.

Photoluminescent (PL) spectra of synthetic diamond powders at temperatures between 10 and 300 K were excited with synchrotron radiation in the wavelength range 125-375 nm. Prominent spectral PL features were detected at 484.6 and 489.0 nm (2.559 and 2.535 eV), associated with nickel defect. During our measurement of PL excitation (PLE) spectra of Ni defect in diamond, we observed a distinct PLE line at 215 nm for the first time. We thereby suggest the use of UV-PL spectra excited in the region 200-220 nm to analyze and to identify nickel defect in diamonds.

Identification of diborane(4) with bridging B-H-B bonds.

The irradiation of diborane(6) dispersed in solid neon at 3 K with tunable far-ultraviolet light from a synchrotron yielded a set of IR absorption lines, the pattern of which implies a carrier containing two boron atoms. According to isotope effects and quantum-chemical calculations, we identified this new species as diborane(4), B2H4, possessing two bridging B-H-B bonds. Our work thus establishes a new prototype, diborane(4), for bridging B-H-B bonds in molecular structures.