Overtone dissociation of peroxynitric acid (HO2NO2): absorption cross sections and photolysis products

Band strengths for the second (3nuOH) and third (4nuOH) overtones of the OH stretch vibration of peroxynitric acid, HO2NO2 (PNA) in the gas-phase were measured using Cavity Ring-Down Spectroscopy (CRDS). Both OH overtone transitions show diffuse smoothly varying symmetrical absorption profiles without observable rotational structure. Integrated band strengths (base e) at 296 K were determined to be S(3nuOH) = (5.7 +/- 1.1) x 10(-20) and S(4nuOH) = (4.9 +/- 0.9) x 10(-21) cm(2) molecule(-1) cm(-1) with peak cross sections of (8.8 +/- 1.7) x 10(-22) and (7.0 +/- 1.3) x 10(-23) cm(2) molecule(-1) at 10086.0 +/- 0.2 cm(-1) and 13095.8 +/- 0.4 cm(-1), respectively, using PNA concentrations measured on line by Fourier-transform infrared and ultraviolet absorption spectroscopy. The quoted uncertainties are 2sigma (95% confidence level) and include estimated systematic errors in the measurements. OH overtone spectra measured at lower temperature, 231 K, showed a narrowing of the 3nuOH band along with an increase in its peak absorption cross section, but no change in S(3nuOH) to within the precision of the measurement (+/-9%). Measurement of a PNA action spectrum showed that HO2 is produced from second overtone photodissociation. The action spectrum agreed with the CRDS absorption spectra. The PNA cross sections determined in this work for 3nuOH and 4nuOH will increase calculated atmospheric photolysis rates of PNA slightly.     

Near infrared photochemistry of pyruvic acid in aqueous solution

Recent experimental and theoretical results have suggested that organic acids such as pyruvic acid, can be photolyzed in the ground electronic state by the excitation of the OH stretch vibrational overtone. These overtones absorb in the near-infrared and visible regions of the spectrum where the solar photons are plentiful and could provide a reaction pathway for the organic acids and alcohols that are abundant in the earth's atmosphere. In this paper the overtone initiated photochemistry of aqueous pyruvic acid is investigated by monitoring the evolution of carbon dioxide. In these experiments CO(2) is being produced by excitation in the near-infrared, between 850 nm and ～1150 nm (11,765-8696 cm(-1)), where the second OH vibrational overtone (Δν = 3) of pyruvic acid is expected to absorb. These findings show not only that the overtone initiated photochemical decarboxylation reaction occurs but also that in the aqueous phase it occurs at a lower energy than was predicted for the overtone initiated reaction of pyruvic acid in the gas phase (13,380 cm(-1)). A quantum yield of (3.5 ± 1.0) × 10(-4) is estimated, suggesting that although this process does occur, it does so with a very low efficiency.     

Measurement of absolute absorption cross sections for nitrous acid (HONO) in the near-infrared region by the continuous wave cavity ring-down spectroscopy (cw-CRDS) technique coupled to laser photolysis

Absolute absorption cross sections for selected lines of the OH stretch overtone 2ν(1) of the cis-isomer of nitrous acid HONO have been measured in the range 6623.6-6645.6 cm(-1) using the continuous wave cavity ring-down spectroscopy (cw-CRDS) technique. HONO has been generated by two different, complementary methods: in the first method, HONO has been produced by pulsed photolysis of H(2)O(2)/NO mixture at 248 nm, and in the second method HONO has been produced in a continuous manner by flowing humidified N(2) over 5.2 M HCl and 0.5 M NaNO(2) solutions. Laser photolysis synchronized with the cw-CRDS technique has been used to measure the absorption spectrum of HONO produced in the first method, and a simple cw-CRDS technique has been used in the second method. The first method, very time-consuming, allows for an absolute calibration of the absorption spectrum by comparison with the well-known HO(2) absorption cross section, while the second method is much faster and leads to a better signal-to-noise ratio. The strongest line in this wavelength range has been found at 6642.51 cm(-1) with σ = (5.8 ± 2.2) × 10(-21) cm(2).     

Atmospheric chemistry of 2,3-pentanedione: photolysis and reaction with OH radicals

The kinetics of the overall reaction between OH radicals and 2,3-pentanedione (1) were studied using both direct and relative kinetic methods at laboratory temperature. The low pressure fast discharge flow experiments coupled with resonance fluorescence detection of OH provided the direct rate coefficient of (2.25 ± 0.44) × 10(-12) cm(3) molecule(-1) s(-1). The relative-rate experiments were carried out both in a collapsible Teflon chamber and a Pyrex reactor in two laboratories using different reference reactions to provide the rate coefficients of 1.95 ± 0.27, 1.95 ± 0.34, and 2.06 ± 0.34, all given in 10(-12) cm(3) molecule(-1) s(-1). The recommended value is the nonweighted average of the four determinations: k(1) (300 K) = (2.09 ± 0.38) × 10(-12) cm(3) molecule(-1) s(-1), given with 2σ accuracy. Absorption cross sections for 2,3-pentanedione were determined: the spectrum is characterized by two wide absorption bands between 220 and 450 nm. Pulsed laser photolysis at 351 nm was used and the depletion of 2,3-pentanedione (2) was measured by GC to determine the photolysis quantum yield of Φ(2) = 0.11 ± 0.02(2σ) at 300 K and 1000 mbar synthetic air. An upper limit was estimated for the effective quantum yield of 2,3-pentanedione applying fluorescent lamps with peak wavelength of 312 nm. Relationships between molecular structure and OH reactivity, as well as the atmospheric fate of 2,3-pentanedione, have been discussed.     

High-accuracy measurements of OH(•) reaction rate constants and IR and UV absorption spectra: ethanol and partially fluorinated ethyl alcohols

Rate constants for the gas phase reactions of OH(?) radicals with ethanol and three fluorinated ethyl alcohols, CH(3)CH(2)OH (k(0)), CH(2)FCH(2)OH (k(1)), CHF(2)CH(2)OH (k(2)), and CF(3)CH(2)OH (k(3)) were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220 to 370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions over the indicated temperature intervals: k(0)(220-370 K) = 5.98 × 10(-13)(T/298)(1.99) exp(+515/T) cm(3) molecule(-1) s(-1), k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1) [for atmospheric modeling purposes, k(0)(T) is essentially temperature-independent below room temperature, k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1)], k(1)(230-370 K) = 3.47 × 10(-14)(T/298)(4.49) exp(+977/T) cm(3) molecule(-1) s(-1), k(2)(220-370 K) = 3.87 × 10(-14)(T/298)(4.25) exp(+578/T) cm(3) molecule(-1) s(-1), and k(3)(220-370 K) = 2.48 × 10(-14)(T/298)(4.03) exp(+418/T) cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH(?) were estimated to be 4, 16, 62, and 171 days, respectively, under the assumption of a well-mixed atmosphere. UV absorption cross sections of all four ethanols were measured between 160 and 215 nm. The IR absorption cross sections of the three fluorinated ethanols were measured between 400 and 1900 cm(-1), and their global warming potentials were estimated.     

Growth,Structure,and Spectroscopic Characteristics of the Nd^3＋：LiGd（WO4）2 Crystal

The Nd3+:LiGd(WO4) 2 crystal with dimensions of 25mm×28mm×16mm was grown by the top-seeded solution growth method from the 60 mol% Li2W2O7 flux. LiGd(WO4) 2 crystallizes in the tetragonal system with space group I41/a(C4h6) and cell parameters: a = 5.1986 and c = 11.2652 . The hardness is about 5.0 Mohs' scale. The specific heat is 0.40 J·g-1·K-1 at 50 oC. The thermal expansion coefficients for a-and c-axes are 1.314×10-5 and 2.052×10-5 K-1,respectively. The room-temperature polarized absorption and emission spectra and the fluorescence decay curve was measured. The parameters of oscillator strengths,the spontaneous transition probabilities,the fluorescence branching ratios,the radiative lifetimes,and the emission cross sections have been investigated based on Judd-Ofelt theory and Füchtbauer-Ladenburg method. The absorption cross-section is 5.19×10-20 cm2 at 805 nm for π-polarization and its line width is 15 nm;the emission cross section is 1.726×10-19 cm2 at 1060.5 nm for π-polarization. The fluorescence and radiative lifetimes are 86 and 158 μs,respectively. The fluorescence quantum efficiency is 54.43%.     

Rotationally resolved absorption cross sections of formaldehyde in the 28100-28500 cm(-1) (351-356 nm) spectral region: implications for in situ LIF measurements

The rotationally resolved ultraviolet absorption cross sections for the 2(0)(0)4(1)(0) vibrational band of the A(1)A(2)-X(1)A(1) electronic transition of formaldehyde (HCHO) at an apodized resolution of 0.027 cm(-1) (approximately 0.0003 nm at 352 nm) over the spectral range 28100-28500 cm(-1) (351-356 nm) at 298 and 220 K, using Fourier transform spectroscopy, are first reported here. Accurate rotationally resolved cross sections are important for the development of in situ HCHO laser-induced fluorescence (LIF) instruments and for atmospheric monitoring. Pressure dependence of the cross sections between 75 and 400 Torr at 298 K was explored, and an average pressure broadening coefficient in dry air of 1.8 x 10(-4) cm(-1) Torr(-1) for several isolated lines is reported. Gaseous HCHO was quantitatively introduced into a flow cell by evaporating micron-sized droplets of HCHO solution, using a novel microinjector technique. The condensed-phase concentrations of HCHO were determined by iodometric titrations to an accuracy of <1%. Accuracy of the measured absorption cross sections is estimated to be better than +/-5%. Integrated and differential cross sections over the entire band at low resolution (approximately 1 cm(-1)) obtained with our calibration technique are in excellent agreement with previous measurements. A maximum differential cross section of 5.7 x 10(-19) cm(2) molecule(-1) was observed at high resolution-almost an order of magnitude greater than any previously reported data at low resolution.     

Laboratory studies of CHF2CF2CH2OH and CF3CF2CH2OH: UV and IR absorption cross sections and OH rate coefficients between 263 and 358 K

Fluorinated alcohols, such as 2,2,3,3-tetrafluoropropanol (TFPO, CHF(2)CF(2)CH(2)OH) and 2,2,3,3,3-pentafluoropropanol (PFPO, CF(3)CF(2)CH(2)OH), can be potential replacements of hydrofluorocarbons with large global warming potentials, GWPs. IR absorption cross sections for TFPO and PFPO were determined between 4000 and 500 cm(-1) at 298 K. Integrated absorption cross sections (S(int), base e) in the 4000-600 cm(-1) range are (1.92 ± 0.34) × 10(-16) cm(2) molecule(-1) cm(-1) and (2.05 ± 0.50) × 10(-16) cm(2) molecule(-1) cm(-1) for TFPO and PFPO, respectively. Uncertainties are at a 95% confidence level. Ultraviolet absorption spectra were also recorded between 195 and 360 nm at 298 K. In the actinic region (λ > 290 nm), an upper limit of 10(-23) cm(2) molecule(-1) for the absorption cross sections (σ(λ)) was reported. Photolysis in the troposphere is therefore expected to be a negligible loss for these fluoropropanols. In addition, absolute rate coefficients for the reaction of OH radicals with CHF(2)CF(2)CH(2)OH (k(1)) and CF(3)CF(2)CH(2)OH (k(2)) were determined as a function of temperature (T = 263-358 K) by the pulsed laser photolysis/laser induced fluorescence (PLP-LIF) technique. At room temperature, the average values obtained were k(1) = (1.85 ± 0.07) × 10(-13) cm(3) molecule(-1) s(-1) and k(2) = (1.19 ± 0.03) × 10(-13) cm(3) molecule(-1) s(-1). The observed temperature dependence of k(1)(T) and k(2)(T) is described by the following expressions: (1.35 ± 0.23) × 10(-12) exp{-(605 ± 54)/T} and (1.36 ± 0.19) × 10(-12) exp{-(730 ± 43)/T} cm(3) molecule(-1) s(-1), respectively. Since photolysis of TFPO and PFPO in the actinic region is negligible, the tropospheric lifetime (τ) of these species can be approximated by the lifetime due to the homogeneous reaction with OH radicals. Global values of τ(OH) were estimated to be of 3 and 4 months for TFPO and PFPO, respectively. GWPs relative to CO(2) at a time horizon of 500 years were calculated to be 8 and 12 for TFPO and PFPO, respectively. Despite the higher GWP relative to CO(2), these species are not expected to significantly contribute to the greenhouse effect in the next decades since they are short-lived species and will not accumulate in the troposphere even as their emissions grow up.     

Calculated electronic transitions in sulfuric acid and implications for its photodissociation in the atmosphere

We have calculated electronic transitions for sulfuric acid in the ultraviolet region using a hierarchy of coupled cluster response functions and correlation consistent basis sets. Our calculations indicate that the lowest energy singlet transition occurs at 8.42 eV with an oscillator strength of 0.01. The lowest energy triplet state occurs at 8.24 eV. Thus, the cross section of sulfuric acid in the actinic region is likely to be very small and smaller than the upper limit put on this cross section by previous experimental investigations. We estimate the cross section of sulfuric acid in the atmospherically relevant Lyman-alpha region ( approximately 10.2 eV) to be approximately 6 x 10 (-17) cm (2) molecule (-1), a value approximately 30 times larger than the speculative value used in previous atmospheric simulations. We have calculated the J values for photodissociation of sulfuric acid with absorption of visible, UV, and Lyman-alpha radiation, at altitudes between 30 and 100 km. We find that the dominant photodissociation mechanism of sulfuric acid below 70 km is absorption in the visible region by OH stretching overtone transitions, whereas above 70 km, absorption of Lyman-alpha radiation by high energy Rydberg excited states is the favored mechanism. The low lying electronic transitions of sulfuric acid in the UV region do not contribute significantly to its dissociation at any altitude.     

Kinetic study of the daytime atmospheric fate of (z)-3-hexenal

Rate coefficients for three daytime atmospheric reactions of (Z)-3-hexenal (3HA)-photolysis (J(1)), reaction with OH radicals (k(2)), and reaction with ozone (k(3))-were measured at 760 Torr and 298 K using a 6 m(3) photochemical reaction chamber. The UV absorption cross sections (σ(3HA)(λ)) were obtained in the wavelength range 240-350 nm. The photodissociation rate of 3HA relative to that of NO(2) was measured by a solar simulator at 760 Torr and was determined to be J(1)/J(NO2) = (4.7 ± 0.4) × 10(-3). Using the obtained σ(3HA)(λ) and J(1)/J(NO2), the effective photodissociation quantum yield was calculated to be Φ(3HA) = 0.25 ± 0.06. The rate coefficient for the reaction with OH radicals was measured by the relative rate method with three reference compounds and was determined to be k(2) = (6.9 ± 0.9) × 10(-11) cm(3) molecule(-1) s(-1). The rate coefficient for the reaction with ozone was measured by an absolute method and was determined to be k(3) = (3.5 ± 0.2) × 10(-17) cm(3) molecule(-1) s(-1). Using the obtained rate coefficients, the daytime atmospheric lifetime of 3HA was estimated.     

异丙醇的O-H伸缩振动泛频光谱和分子构像

使用高灵敏的光腔衰荡光谱(CavityRingDownSpectroscopy)技术测出了异丙醇的O-H伸缩v=4、5振动泛频光谱,每个振动能级都有三个吸收峰,被归属为分子构像的O-H伸缩泛频吸收.给出了光腔衰荡光谱的振动泛频吸收的谱带强度公式,并求得分子不同构像在不同振动能级的O-H伸缩泛频吸收的谱带强度;同时利用局域模理论,求得分子各O-H伸缩局域模振子的机械频率(X1)、非谐性(X2)以及解离能(D).用密度泛函(DFT)B3LYP/6-31+G*理论方法优化了分子的各种可能构像,验证了分子存在反式(trans)和偏转(gauche)两种稳定构像,计算的分子的O-H伸缩频率及构像稳定性同实验结果是一致的.     

Laboratory kinetic and mechanistic studies on the OH-initiated oxidation of acetone in aqueous solution

The OH-initiated oxidation of acetone in aqueous solution is investigated because of its potential implications in atmospheric chemistry. The UV-spectrum of the transient acetonylperoxy radical was measured. Two characteristic absorption bands of the acetonylperoxy radical spectrum are found in the 220-400 nm wavelength region. The rate constant for the recombination reaction of the acetonylperoxy radical was determined as a function of temperature for the first time in aqueous solution with k(rec,298?K) = (7.3 ± 1.3) × 10(8) M(-1) s(-1), E(A) = 4.5 ± 3.3 kJ mol(-1), and A = (4.7 ± 2.7) × 10(9) M(-1) s(-1). Furthermore, kinetic investigations of the OH-initiated oxidation of methylglyoxal and pyruvic acid were performed with the following results: for methylglyoxal, k(second) = (6.2 ± 0.2) × 10(8) M(-1) s(-1), E(A) = 12 ± 2 kJ mol(-1), and A = (7.8 ± 0.2) × 10(9) M(-1) s(-1); for pyruvic acid (pH = 0), k(second) = (3.2 ± 0.6) × 10(8) M(-1) s(-1), E(A) = 15 ± 5 kJ mol(-1), and A?= (1.1 ± 0.1) × 10(11) M(-1) s(-1); for pyruvate (pH = 6), k(second) = (7.1 ± 2.4) × 10(8) M(-1) s(-1), E(A) = 25 ± 19 kJ mol(-1), and A = (1.5 ± 0.4) × 10(13) M(-1) s(-1). Quantitative product studies were done as a function of the number of laser photolysis pulses for acetone and its oxidation products methylglyoxal, hydroxyacetone, pyruvic acid, acetic acid, and oxalic acid. After the recombination reaction of acetonylperoxy radicals, there are two possible decomposition reactions where the primary products methylglyoxal and hydroxyacetone are formed. From product analysis after a single photolysis laser shot, the ratio of the main product-forming reactions was determined as (A) 30% and (B) 56% for the methylglyoxal formation via channel A to yield two molecules of methylglyoxal and channel B to yield one molecule of methylglyoxal and one molecule of hydroxyacetone. The remaining product can be ascribed to channel C, the radical-retaining channel forming alkoxy radicals with a yield of 14%. Pyruvic acid and acetic acid were found to be the major intermediates estimated with concentrations in the same order of magnitude and a similar time profile, indicating that acetic acid is also a possible oxidation product of methylglyoxal.     

A kinetic study of Mg+ and Mg-containing ions reacting with O3, O2, N2, CO2, N2O and H2O: implications for magnesium ion chemistry in the upper atmosphere

Reactions between Mg(+) and O(3), O(2), N(2), CO(2) and N(2)O were studied using the pulsed laser photo-dissociation at 193 nm of Mg(C(5)H(7)O(2))(2) vapour, followed by time-resolved laser-induced fluorescence of Mg(+) at 279.6 nm (Mg(+)(3(2)P(3/2)-3(2)S(1/2))). The rate coefficient for the reaction Mg(+) + O(3) is at the Langevin capture rate coefficient and independent of temperature, k(190-340 K) = (1.17 ± 0.19) × 10(-9) cm(3) molecule(-1) s(-1) (1σ error). The reaction MgO(+) + O(3) is also fast, k(295 K) = (8.5 ± 1.5) × 10(-10) cm(3) molecule(-1) s(-1), and produces Mg(+) + 2O(2) with a branching ratio of (0.35 ± 0.21), the major channel forming MgO(2)(+) + O(2). Rate data for Mg(+) recombination reactions yielded the following low-pressure limiting rate coefficients: k(Mg(+) + N(2)) = 2.7 × 10(-31) (T/300 K)(-1.88); k(Mg(+) + O(2)) = 4.1 × 10(-31) (T/300 K)(-1.65); k(Mg(+) + CO(2)) = 7.3 × 10(-30) (T/300 K)(-1.59); k(Mg(+) + N(2)O) = 1.9 × 10(-30) (T/300 K)(-2.51) cm(6) molecule(-2) s(-1), with 1σ errors of ±15%. Reactions involving molecular Mg-containing ions were then studied at 295 K by the pulsed laser ablation of a magnesite target in a fast flow tube, with mass spectrometric detection. Rate coefficients for the following ligand-switching reactions were measured: k(Mg(+)·CO(2) + H(2)O → Mg(+)·H(2)O + CO(2)) = (5.1 ± 0.9) × 10(-11); k(MgO(2)(+) + H(2)O → Mg(+)·H(2)O + O(2)) = (1.9 ± 0.6) × 10(-11); k(Mg(+)·N(2) + O(2)→ Mg(+)·O(2) + N(2)) = (3.5 ± 1.5) × 10(-12) cm(3) molecule(-1) s(-1). Low-pressure limiting rate coefficients were obtained for the following recombination reactions in He: k(MgO(2)(+) + O(2)) = 9.0 × 10(-30) (T/300 K)(-3.80); k(Mg(+)·CO(2) + CO(2)) = 2.3 × 10(-29) (T/300 K)(-5.08); k(Mg(+)·H(2)O + H(2)O) = 3.0 × 10(-28) (T/300 K)(-3.96); k(MgO(2)(+) + N(2)) = 4.7 × 10(-30) (T/300 K)(-3.75); k(MgO(2)(+) + CO(2)) = 6.6 × 10(-29) (T/300 K)(-4.18); k(Mg(+)·H(2)O + O(2)) = 1.2 × 10(-27) (T/300 K)(-4.13) cm(6) molecule(-2) s(-1). The implications of these results for magnesium ion chemistry in the atmosphere are discussed.     

High accuracy measurements of OH reaction rate constants and IR absorption spectra: substituted 2-propanols

Rate constants for the gas phase reactions of OH radicals with 2-propanol and three fluorine substituted 2-propanols, (CH(3))(2)CHOH (k(0)), (CF(3))(2)CHOH (k(1)), (CF(3))(2)C(OH)CH(3) (k(2)), and (CF(3))(3)COH (k(3)), were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220-370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions: k(0)(T) = 1.46 × 10(-11) exp{-883/T} + 1.30 × 10(-12) exp{+371/T} cm(3) molecule(-1) s(-1); k(1)(T) = 1.19 × 10(-12) exp{-1207/T} + 7.85 × 10(-16) exp{+502/T } cm(3) molecule(-1) s(-1); k(2)(T) = 1.68 × 10(-12) exp{-1718/T} + 7.32 × 10(-16) exp{+371/T} cm(3) molecule(-1) s(-1); k(3)(T) = 3.0 × 10(-20) × (T/298)(11.3) exp{+3060/T} cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH were estimated to be 2.4 days and 1.9, 6.3, and 46 years, respectively. UV absorption cross sections were measured between 160 and 200 nm. The IR absorption cross sections of the three fluorinated compounds were measured between 450 and 1900 cm(-1), and their global warming potentials were estimated.     

FT-IR, FT-Raman, NMR and UV-vis spectra, vibrational assignments and DFT calculations of 4-butyl benzoic acid

The solid phase FTIR and FT-Raman spectra of 4-butyl benzoic acid (4-BBA) have been recorded in the regions 400-4000 and 50-4000cm(-1), respectively. The spectra were interpreted in terms of fundamentals modes, combination and overtone bands. The structure of the molecule was optimized and the structural characteristics were determined by density functional theory (DFT) using B3LYP method with 6-311++G(d,p) as basis set. The vibrational frequencies were calculated for monomer and dimer by DFT method and were compared with the experimental frequencies, which yield good agreement between observed and calculated frequencies. The infrared and Raman spectra were also predicted from the calculated intensities. (13)C and (1)H NMR spectra were recorded and (13)C and (1)H nuclear magnetic resonance chemical shifts of the molecule were calculated using the gauge independent atomic orbital (GIAO) method. UV-visible spectrum of the compound was recorded in the region 200-400nm and the electronic properties HOMO and LUMO energies were measured by time-dependent TD-DFT approach. The geometric parameters, energies, harmonic vibrational frequencies, IR intensities, Raman intensities, chemical shifts and absorption wavelengths were compared with the available experimental data of the molecule.     

Growth and Spectroscopic Properties of Nd3＋：Sr3Gd2（BO3）4 Crystal

<正>This paper reports the growth,X-ray diffraction and spectroscopy of Nd~(3+):Sr_3Gd_2(BO_3)_4 crystal.A Nd~(3+):Sr_3Gd_2(BO_3)_4 crystal with dimensions ofφ20×45 mm~3 has been grown by the Czochralski method.Nd~(3+):Sr_3Gd_2(BO_3)_4 crystal belongs to the orthorhombic system,space group Pnma(D_(2h))with a=0.7401,b=1.604 and c=0.8755 nm.The absorption and emission spectra of Nd~(3+):Sr_3Gd_2(BO_3)_4 were investigated.The absorption cross sectionσ_a is 3.11×10~(-20)cm~2 at 808 nm. The absorption transition at 808 nm has an FWHM of 14 nm.The luminescence lifetimeτ_f is 51.7 us.The emission cross sectionσ_e at 1064 nm wavelength is 1.09×10~(-19)cm~2.     

XUV free-electron laser desorption of NO from graphite (0001)

We report results of femtosecond laser induced desorption of NO from highly oriented pyrolytic graphite using XUV photon energies of hν = 38 eV and hν = 57 eV. Femtosecond pulses with a pulse energy of up to 40 μJ and about 30 fs duration generated at FLASH are applied. The desorbed molecules are detected with rovibrational state selectivity by (1 + 1) REMPI in the A(2)Σ(+) ← X(2)Π γ-bands around λ = 226 nm. A nonlinear desorption yield of neutral NO is observed with an exponent of m = 1.4 ± 0.2. At a fluence of about 4 mJ/cm(2) a desorption cross section of σ(1) = (1.1 ± 0.4) × 10(-17) cm(2) is observed, accompanied with a lower one of σ(2) = (2.6 ± 0.3) × 10(-19) cm(2) observable at higher total fluence. A nonthermal rovibrational population distribution is observed with an average rotational energy of = 38.6 meV (311 cm(-1)), a vibrational energy of = 136 meV (1097 cm(-1)) and an electronic energy of = 3.9 meV (31 cm(-1)).     

Spectra and integrated band intensities of the low order OH stretching overtones in peroxyformic acid: an atmospheric molecule with prototypical intramolecular hydrogen bonding

The gas phase spectra of several vibrational bands of peroxyformic acid (PFA), an atmospheric molecule exhibiting intramolecular hydrogen bonding, are presented. In the fundamental region, Fourier transform infrared (FT-IR) spectroscopy is used to probe the C-O, O-H and C-H stretching vibrations, while in the region of the first and second OH-stretching overtones (2ν(OH) and 3ν(OH)) photoacoustic spectroscopy is used. Integrated absorption cross sections for the PFA vibrational bands are determined by comparing their respective peak areas with that for the OH-stretching bands of n-propanol for which the absorption cross section is known. The measured integrated intensities of the OH stretching bands are then compared with a local mode model using a one-dimensional dipole moment function in conjunction with the OH stretching potential computed at both the MP2/aug-cc-pVDZ and CCSD(T)/aug-cc-pVDZ levels. The data allow us to investigate changes in the OH stretch band position and intensity as a function of overtone order arising from the influence of hydrogen bonding. Furthermore, calculations at the MP2/aug-cc-pVDZ level show that there are three stable conformers of PFA with relative energies of 0, 13.54, and 13.76 kJ/mol, respectively. In the room temperature spectra, however, we see evidence for transitions from only the lowest energy conformer. The geometrical parameters and vibrational frequencies of the most stable conformer are presented.     

Photolysis, OH reactivity and ozone reactivity of a proxy for isoprene-derived hydroperoxyenals (HPALDs)

The C(5)-hydroperoxyenals (C(5)-HPALDs) are a newly-recognized class of multi-functional hydrocarbons produced during the hydroxyl radical (OH)-initiated oxidation of isoprene. Recent theoretical calculations suggest that fast photolysis of these compounds may be an important OH source in high-isoprene, low-NO regions. We report experimental constraints for key parameters of photolysis, OH reaction and ozone reaction of these compounds as derived from a closely-related, custom-synthesized C(6)-HPALD. The photolysis quantum yield is 1.0 ± 0.4 over the range 300-400 nm, assuming an absorption cross section equal to the average of those measured for several analogous enals. The yield of OH from photolysis was determined as 1.0 ± 0.8. The OH reaction rate constant is (5.1 ± 1.8) × 10(-11) cm(3) molecule(-1) s(-1) at 296 K. The ozone reaction rate constant is (1.2 ± 0.2) × 10(-18) cm(3) molecule(-1) s(-1) at 296 K. These results are consistent with previous first-principles estimates, though the nature and fate of secondary oxidation products remains uncertain. Incorporation of C(5)-HPALD chemistry with the above parameters in a 0-D box model, along with experimentally-constrained rates for C(5)-HPALD production from isomerization of first-generation isoprene hydroxyperoxy radicals, is found to enhance modeled OH concentrations by 5-16% relative to the traditional isoprene oxidation mechanism for the chemical regimes of recent observational studies in rural and remote regions. This enhancement in OH will increase if C(5)-HPALD photo-oxidation products also photolyze to yield additional OH or if the C(5)-HPALD production rate is faster than has been observed.     

Two-photon excited fluorescence of nitrogen-vacancy centers in proton-irradiated type Ib diamond

Two-photon fluorescence spectroscopy of negatively charged nitrogen-vacancy [(N-V)-] centers in type Ib diamond single crystals have been studied with a picosecond (7.5 ps) mode-locked Nd:YVO(4) laser operating at 1064 nm. The (N-V)- centers were produced by radiation damage of diamond using a 3 MeV proton beam, followed by thermal annealing at 800 degrees C. Prior to the irradiation treatment, infrared spectroscopy of the C-N vibrational modes at 1344 cm(-1) suggested a nitrogen content of 109 +/- 10 ppm. Irradiation and annealing of the specimen led to the emergence of a new absorption band peaking at approximately 560 nm. From a measurement of the integrated absorption intensity of the sharp zero-phonon line (637 nm) at liquid nitrogen temperature, we determined a (N-V)- density of (4.5 +/- 1.1) x 10(18) centers/cm3 (or 25 +/- 6 ppm) for the substrate irradiated at a dose of 1 x 1016) H(+)/cm(2). Such a high defect density allowed us to observe two-photon excited fluorescence and measure the corresponding fluorescence decay time. No significant difference in the spectral feature and fluorescence lifetime was observed between one-photon and two-photon excitations. Assuming that the fluorescence quantum yields are the same for both processes, a two-photon absorption cross section of sigma(TPA) = (0.45 +/- 0.23) x 10(-50) cm(4).s/photon at 1064 nm was determined for the (N-V)- center based on its one-photon absorption cross section of sigma(OPA) = (3.1 +/- 0.8) x 10(-17) cm2 at 532 nm. The material is highly photostable and shows no sign of photobleaching even under continuous two-photon excitation at a peak power density of 3 GW/cm(2) for 5 min.