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.

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.

Infrared Spectra of the 1-Methylvinoxide Radical and Anion Isolated in Solid Argon.

The 1-methylvinoxy radical (1-MVO) is an important intermediate in the combustion and tropospheric reaction of OH. However, the vibrational structures of this species and its anionic form, 1-methylvinoxide anion (1-MVO-), are not fully known. Thus, in this study, we obtained the infrared (IR) absorption spectra of 1-MVO and 1-MVO- trapped in a solid Ar matrix. 1-MVO- anions were produced by electron bombardment during matrix deposition of Ar containing a small amount of acetone. The anions were destroyed upon irradiation at 675, 365, and 160 nm, although the formation of 1-MVO was only observed upon irradiation at 675 nm. The assignment of the IR bands of 1-MVO- and 1-MVO was based on the expected chemistry upon photoexcitation and comparison of line wavenumbers, relative IR intensities, and D-isotopic shift ratios with those predicted at the B3LYP/aug-cc-pVTZ level of theory.

Direct IR Absorption Spectra of Propargyl Cation Isolated in Solid Argon.

The direct infrared (IR) absorption spectra of propargyl cations were recorded. These cations were generated via the electron bombardment of a propyne/Ar matrix sample during matrix deposition. Secondary photolysis with selected ultraviolet (UV) light was used for grouping the observed bands of various products. The band assignment of the propargyl cation in solid Ar was performed according by referring to the previous infrared photodissociation (IRPD) and velocity-map imaging photoelectron (VMI-PE) data, and via theoretical predictions of the anharmonic vibrational wavenumbers, band intensities, and deuterium-substituted isotopic ratios. Almost all the IR active bands with an observable intensity were recorded and the ν11 mode was reported for the first time.

UV absorption spectrum of allene radical cations in solid argon.

Electron bombardment during deposition of an Ar matrix containing a small proportion of allene generated allene cations. Further irradiation of the matrix sample at 385 nm destroyed the allene cations and formed propyne cations in solid Ar. Both cations were identified according to previously reported IR absorption bands. Using a similar technique, we recorded the ultraviolet absorption spectrum of allene cations in solid Ar. The vibrationally resolved progression recorded in the range of 266-237 nm with intervals of about 800 cm-1 was assigned to the A2E ← X2E transition of allene cations, and the broad continuum absorption recorded in the region of 229-214 nm was assigned to their B2A1 ← X2E transition. These assignments were made based on the observed photolytic behavior of the progressions and the vertical excitation energies and oscillator strengths calculated using time-dependent density functional theory.

Formation and identification of borane radical anions isolated in solid argon.

The infrared (IR) spectrum of borane(3) anions (BH3-) isolated in solid Ar was recorded; two vibrational modes were observed at 2259.4 and 606.6 cm-1, which were assigned to the BH2 stretching (ν3) and out-of-plane large-amplitude (ν2) modes, respectively. These anions were produced by the electron bombardment of an Ar matrix sample containing a small proportion of B2H6 and H2 during matrix deposition or by the photolysis of single-bridged-B2H5- in an Ar matrix with the selected ultraviolet light. The band positions, relative intensity ratios, isotopic splitting pattern, and isotopic shift ratios of the observed IR features of BH3- are generally in good agreement with those predicted by the B2PLYP/aug-cc-pVTZ method.

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.

Quantitative analysis of nitrogen defect N4 in diamond with photoluminescence excited in the 170-240 nm region.

Upon excitation at 170-240 nm, diamonds emit strong luminescence in wavelength range of 300-700 nm. The spectral features observed in the photoluminescence excitation (PLE) spectra show two vibrational progressions, A and B, related to nitrogen defects N2 and N4, respectively. We used PLE spectra excited in region 170-240 nm to identify the type of diamond and demonstrate quantitative analysis of the B center as a N4 nitrogen defect in diamonds; the least detectable concentration of the N4 nitrogen defect is about 13 ppb, and the sensitivity of PLE is about 30 times than that practicable with infrared absorption spectra.

Infrared absorption spectra of methylidene radicals in solid neon.

Infrared absorption lines of methylidene--(12)C(1)H, (13)C(1)H, and (12)C(2)H--dispersed in solid neon at 3 K, recorded after photolysis of methane precursors with vacuum-ultraviolet light at 121.6 nm, serve as signatures of these trapped radicals.

Vacuum-ultraviolet photolysis of methane at 3 K: synthesis of carbon clusters up to C20.

Samples of pure methane and of methane dispersed in solid neon at 3 K subjected to irradiation at wavelengths less than 165 nm with light from a synchrotron yielded varied products that were identified through their infrared absorption spectra, including CH3, C2H2, C2H3, C2H4, C2H6, C4H2, C4H4, C5H2, C8H2, CnH with n = 1-5, and carbon chains Cn with n = 3-20. The efficiency of photolysis of methane and the nature of the photoproducts depended on the concentration of methane and the wavelength selected for irradiation; an addition of H2 into solid neon enhanced the formation of long carbon chains.

Production of N3 upon photolysis of solid nitrogen at 3 K with synchrotron radiation.

The photodissociation of gaseous molecular nitrogen has been investigated intensively, but the corresponding knowledge in a solid phase is lacking. Irradiation of pure solid nitrogen at 3 K with vacuum-ultraviolet light from a synchrotron produced infrared absorption lines of product l-N3 at 1657.8 and 1652.6 cm(-1). The threshold wavelength to generate l-N3 was determined to be (143.7±1.8) nm, corresponding to an energy of (8.63±0.11) eV. Quantum-chemical calculations support the formation of l-N3 from the reaction N2 +N2, possibly through an activated complex l-N4 upon photoexcitation with energy above 8.63 eV. The results provide a possible application to an understanding of the nitrogen cycle in astronomical environments.

Identification of nitrogen defects in diamond with photoluminescence excited in the 160-240 nm region.

Photoluminescence (PL) spectra of natural diamond powders of type IaAB at 300 and 13 K were excited with synchrotron radiation in the wavelength range of 150-260 nm. The spectral features observed in the excitation spectra at 13 K show four vibrational progressions related to nitrogen defects in diamond: A, B, B', and N3. Progression A has a spacing of 1258 ± 40 cm(-1), associated with the N2 (or A) center; progression B has a spacing of 1181 ± 40 cm(-1) and progression B' has a spacing of 744 ± 40 cm(-1) related to the N4 (or B) center; and progression N3 has a spacing of 1417 ± 40 cm(-1) associated with the N3 center. The PL of these defects comprise continuous emission with two broad lines with maxima of ∼420 and 469 nm at 300 K. Upon excitation with light at wavelengths of <200 nm, the distinct zero-phonon lines of N3 and N4 centers in diamond at a temperature of 13 K become prominent at 416.0 and 491.2 nm, respectively. The vibrational progressions in the photoluminescence excitation (PLE) spectra of N2, N3, and N4 centers in diamond of type IaAB at 13 K are identified for the first time. We suggest the use of PL spectra excited in the region of 160-240 nm to analyze and identify the type of diamond.

Infrared absorption spectra of t-HNOH radicals generated on VUV irradiation of NO in solid hydrogen.

Photoproduct signature: Irradiation of solid hydrogen near 3 K containing NO with vacuum-UV light from synchrotron radiation yields new infrared absorption lines at 1241.7, 1063.6 and 726.2 cm(-1) (see figure). These new lines are assigned to vibrational modes of t-HNOH. This photoproduct is formed from electronically excited NO reacting with neighboring hydrogen in the solid sample.Irradiation of solid H(2) near 3 K containing NO with vacuum-ultraviolet light from a synchrotron yields new infrared absorption lines at 1241.7, 1063.6 and 726.2 cm(-1). The structures of four possible structural isomers: H(2)NO, t-HNOH, c-HNOH and NOH(2), their vibrational wavenumbers, IR intensities and D-isotopic shifts are calculated with density-functional theory according to B3LYP and PW91PW91/aug-cc-pVTZ methods. Based on the results of those calculations and of experiments with deuterium labeling, we assign the new lines to nu(4) (cis bending), nu(5) (N==O stretching) and nu(6) (out-of-plane deformation) modes, respectively, of t-HNOH. This photoproduct is formed through reaction of electronically excited NO with neighboring H(2) in the solid sample.

Infrared absorption spectra of vinyl radicals isolated in solid Ne.

Irradiation of samples of solid Ne near 3.0 K containing ethene (C(2)H(4)) with vacuum ultraviolet radiation at 120 nm from synchrotron yielded new spectral lines at 3141.0, 2953.6, 2911.5, 1357.4, 677.1, 895.3, and 857.0 cm(-1). These features are assigned to alpha-CH stretching (nu(1)), CH(2) antisymmetric stretching (nu(2)), CH(2) symmetric stretching (nu(3)), CH(2)-bending (nu(5)), HCCH cis bending (nu(7)), CH(2) out-of-plane bending (nu(8)), and alpha-CH out-of-plane bending (nu(9)) modes of C(2)H(3), respectively, based on results of (13)C- and D-isotopic experiments and quantum-chemical calculations. These calculations using density-functional theory (B3LYP and PW91PW91/aug-cc-pVTZ) predict vibrational wavenumbers, IR intensities, and isotopic ratios of vinyl radical that agree satisfactorily with our experimental results.