Ultraviolet Absorption Cross Sections Research Articles

Measurements of molecular absorption spectra with the SCIAMACHY pre-flight model: instrument characterization and reference data for atmospheric remote-sensing in the 230–2380 nm region

Using the scanning imaging absorption spectrometer for atmospheric chartography (SCIAMACHY) pre-flight model satellite spectrometer, gas-phase absorption spectra of the most important atmospheric trace gases (O 3, NO 2, SO 2, O 2, OClO, H 2CO, H 2O, CO, CO 2, CH 4, and N 2O) have been measured in the 230–2380 nm range at medium spectral resolution and at several temperatures between 203 and 293 K. The spectra show high signal-to-noise ratio (between 200 up to a few thousands), high baseline stability (better than 10 −2) and an accurate wavelength calibration (better than 0.01 nm) and were scaled to absolute absorption cross-sections using previously published data. The results are important as reference data for atmospheric remote-sensing and physical chemistry. Amongst other results, the first measurements of the Wulf bands of O 3 up to their origin above 1000 nm were made at five different temperatures between 203 and 293 K, the first UV-Vis absorption cross-sections of NO 2 in gas-phase equilibrium at 203 K were recorded, and the ultraviolet absorption cross-sections of SO 2 were measured at five different temperatures between 203 and 296 K. In addition, the molecular absorption spectra were used to improve the wavelength calibration of the SCIAMACHY spectrometer and to characterize the instrumental line shape (ILS) and straylight properties of the instrument. It is demonstrated that laboratory measurements of molecular trace gas absorption spectra prior to launch are important for satellite instrument characterization and to validate and improve the spectroscopic database.

Ultraviolet absorption spectrum and cross-sections of ethynyl (C 2H) radicals

This work reports the ultraviolet absorption spectrum and cross-sections of ethynyl (C 2H) radicals in the wavelength range 235–260 nm, determined at T=298 K and at a total pressure of about 20 kPa (150 Torr). Ethynyl radicals were produced from the single photon 193 nm excimer laser photolysis of dilute mixtures of C 2HCF 3 or C 2H 2 in He. Gas chromatographic/mass spectroscopic analysis of the photolyzed samples showed diacetylene (C 4H 2) as the major stable product. Addition of methane in the photolysis mixtures resulted in reduction of diacetylene and production of methylacetylene and ethane, products of the reaction of ethynyl radicals with methane. Survey of the ultraviolet spectral region, employing time-resolved UV-absorption spectroscopy, resulted in detection of a transient absorption centered at about 243 nm. The spectra obtained from the 193-nm photolysis of both C 2HCF 3/He and C 2H 2/He radical precursors were nearly identical, suggesting that the absorption feature can be attributed to the ethynyl radicals. The observed ultraviolet spectrum exhibits a relatively broad absorption feature with some structures and an absorption peak at about 243.5 nm. The absorption cross-sections for ethynyl radicals have been determined in this work for the first time. The C 2H cross-section, at the maximum absorption, at 243.5 nm is (7.5±0.9)×10 −19 cm 2 molecule −1 . The stated uncertainty includes the random and systematic measurement errors. The UV absorption feature, detected in this work, can be assigned to the transitions from the ground electronic state (X 2 Σ +) and also possibly from coupled ground X 2 Σ + and lowest electronic (A 2 Π) states to the electronic state B (or 3 2A ′ ) of C 2H. This assignment is based on previously reported high level ab initio molecular orbital calculations and results of recent laser-induced fluorescence studies of the ethynyl radical.

Experimental and theoretical studies on vacuum ultraviolet absorption cross sections and photodissociation of CH3OH, CH3OD, CD3OH, and CD3OD

Absorption cross sections of CH3OH, CH3OD, CD3OH, and CD3OD are measured in a 107–220 nm spectral region using synchrotron radiation. Spectra of improved quality for four deuterated isotopomers, coupled with extensive calculations on low-lying excited states of methanol using time-dependent density functional theory with a large cc-pV5Z basis set, enable us to improve assignments of observed spectral features and to better understand the nature of these electronic transitions. Energies and oscillator strengths of all transitions predicted with calculations are consistent with experimental results. Observed isotopic shifts clearly indicate that absorption features in the 163–220 nm region (transition 1 1A″–X 1A′) are associated mainly with breaking of the O–H bond, consistent with theoretical predictions. In the 151–163 nm region (transition 2 1A″–X 1A′), observed small vibrational spacings (806 cm−1 for CH3OH) associated with the C–O stretching mode can be rationalized with a broad double-well-like potential-energy curve resulting from avoided crossing of Rydberg states 11A″(2a″→3s) and 2 1A″(2a″→3p); with isotopic data, further vibrational progressions are identified. Absorption lines in the 140–151 nm region with regular vibrational spacing (∼1046 cm−1 for CH3OH), likely associated with the CH2 twisting mode, are assigned to nearly degenerate transitions 3 1A″–X 1A′ and 3 1A′–X 1A′; the 3 1A″ and 3 1A′ states are associated with excitations 2a″→3p′ and 2a″→3p″, respectively. Progressions associated with the torsional mode of the excited state are observed for the first time. For wavelengths smaller than 140 nm, Rydberg transitions with n⩾3 are tentatively assigned in accord with their quantum defects that are identical for all isotopomers in each Rydberg series.

Atmospheric chemistry of small organic peroxy radicals

Global atmospheric models play a key role in international assessments of the human impact on global climate and air pollution. To increase the accuracy and facilitate comparison of results from such models, it is essential they contain up‐to‐date chemical mechanisms. To this end, we present an evaluation of the atmospheric chemistry of the four most abundant organic peroxy radicals: CH3O2, C2H5O2, CH3C(O)O2, and CH3C(O)CH2O2. The literature data for the atmospheric reactions of these radicals are evaluated. In addition, the ultraviolet absorption cross sections for the above radicals and for HO2 have been evaluated. The absorption spectra were fitted to an analytical formula, which enabled published spectra to be screened objectively. Published kinetic and product data were reinterpreted, or in some case reanalyzed, using the new cross sections, leading to a self‐consistent set of kinetic, mechanistic, and spectroscopic data. Product studies were also evaluated. A set of peroxy radical reaction rate coefficients and products are recommended for use in atmospheric modeling. A three‐dimensional global chemical transport model (the Intermediate Model for the Global Evolution of Species, IMAGES) was run using both previously recommended rate coefficients and the current set to highlight the sensitivity of key atmospheric trace species to the peroxy radical chemistry used in the model.

Determination of the ultra violet absorption cross section of hexyl-ketohydroperoxides in solution in acetonitrile

Three hexyl-ketohydroperoxides, produced at 300°C by isomerization reactions in oxygen of hexyl-peroxy C 6H 13O 2 radicals (arising from n-hexane), were isolated through micro-preparative GC separation into two fractions: 2-hexanone-5-hydroperoxide, and mixture 2-hexanone-4-hydroperoxide/3-hexanone-5-hydroperoxide. Absorption cross sections versus UV wavelength (190–350 nm) were determined for these hexyl-ketohydroperoxides in solution in acetonitrile. Their absorption at wavelengths higher than 290 nm (being of tropospheric interest) is linked to the ketonic group and seems to be amplified by the peroxidic group. Photochemical data of alkyl-ketohydroperoxides might be introduced in tropospheric chemistry in order to modell urban atmospheric pollution.

Gas-phase absorption cross sections of 24 monocyclic aromatic hydrocarbons in the UV and IR spectral ranges

Absorption cross sections of 24 volatile and non-volatile derivatives of benzene in the ultraviolet (UV) and the infrared (IR) regions of the electromagnetic spectrum have been determined using a 1080 l quartz cell. For the UV a 0.5 m Czerny-Turner spectrometer coupled with a photodiode array detector (spectral resolution 0.15 nm) was used. IR spectra were recorded with an FT-IR spectrometer (Bruker IFS-88, spectral resolution 1 cm -1). Absolute absorption cross sections and the instrument function are given for the UV, while for the IR, absorption cross sections and integrated band intensities are reported. The study focused primarily on the atmospherically relevant methylated benzenes (benzene, toluene, o-xylene, m-xylene, p-xylene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, ethylbenzene, styrene) and their ring retaining oxidation products (benzaldehyde, o-tolualdehyde, m-tolualdehyde, p-tolualdehyde, phenol, o-cresol, m-cresol, p-cresol, 2,3-dimethylphenol, 2,4-dimethylphenol, 2,5-dimethylphenol, 2,6-dimethylphenol, 3,4-dimethylphenol, 3,5-dimethylphenol, 2,4,6-trimethylphenol and ( E,Z)- and ( E,E)-2,4-hexadienedial). The UV absorption cross sections reported here can be used for the evaluation of DOAS spectra (Differential Optical Absorption Spectroscopy) for measurements of the above compounds in the atmosphere and in reaction chambers, while the IR absorption cross sections will primarily be useful in laboratory studies on atmospheric chemistry, where FT-IR spectrometry is an important tool.

Ultraviolet absorption spectrum and cross-sections of vinyl (C 2H 3) radical in the 225–238 nm region

The room-temperature gas-phase ultraviolet absorption spectrum and cross-sections of vinyl (C 2H 3) radicals have been determined in the spectral range 225–238 nm, employing cavity ring-down absorption spectroscopy. Vinyl radicals in these experiments were produced from the 193 nm excimer laser photolysis of methyl vinyl ketone (CH 3COC 2H 3) and vinyl bromide (C 2H 3Br). The spectra obtained from the two systems were nearly identical. The observed spectrum exhibits a relatively broad and featureless absorption with a cross-section of 5.3×10 −18 cm 2 molecule −1 at 230 nm. A combined uncertainty of ∼25% for cross-section values has been assessed. The electronic transitions in the vinyl radical have been calculated by ab initio quantum chemical methods at the CIS, EOM-CCSD, CASSCF and CASPT2 levels of theories which assign the new observed band to the highly allowed and `in plane' π *(2a″) ← π(1a″) transition.

UV Absorption Cross Sections, Laser Photodissociation Product Quantum Yields, and Reactions of H Atoms with Methylhydrazines at 298 K

The gas-phase photochemistry of the diamines monomethylhydrazine (CH3NHNH2) and unsymmetrical dimethylhydrazine ((CH3)2NNH2) has been studied in the ultraviolet (UV) at 298 K. Relative UV absorbance spectra of dilute mixtures of the diamine in He buffer gas were recorded in the region 140−310 nm with an instrument resolution of 0.25 nm. These were converted to absolute absorption cross section profiles by scaling the data with absolute 253.65-nm cross section values of σ253.65 = (8.022 ± 0.481) × 10-20 and (35.883 ± 2.152) × 10-20 cm2 molecule-1 for CH3NHNH2 and (CH3)2NNH2, respectively, which we also determined in this work. Both compounds exhibit continuous diffuse absorption bands in the UV that correspond to electronic transitions to dissociative singlet excited states. Laser photodissociation product measurements revealed the absolute H atom primary quantum yields to be Φ248 = 1.07 ± 0.02, Φ222 = 0.99 ± 0.01, and Φ193 = 0.94 ± 0.07 for CH3NHNH2 photolysis at 248, 222, and 193 nm, respectively. The corresponding values for (CH3)2NNH2 photolysis were Φ248 = 0.97 ± 0.06, Φ222 = 0.91 ± 0.06, and Φ193 = 0.90 ± 0.04. Kinetic measurements gave the absolute second-order H atom rate coefficients to be k = (7.60 ± 1.14) × 10-13 and (13.48 ± 2.02) × 10-13 cm3 molecule-1 s-1 respectively for reactions with CH3NHNH2 and (CH3)2NNH2 at 298 K and in 26.0 Torr of He. Trends in the UV absorption cross section profiles, photodissociation product distributions, and the H atom reactivities in the homologous diamine series (CH3)2NNH2, CH3NHNH2, and N2H4 are discussed.

Ultraviolet absorption and cross sections of propargyl (C 3H 3) radicals in the 230–300 nm region

The ultraviolet absorption spectrum and cross sections of propargyl (C 3H 3) radicals have been, for the first time, determined in the gas phase in the spectral range of 230–300 nm. The propargyl radicals were produced from 193 nm excimer laser photolysis of C 3H 3Cl, 248 nm photolysis of C 3H 3Br and from reaction of Cl-atoms with propargyl chloride. The spectra obtained from the three systems were nearly identical. The ultraviolet spectrum for the propargyl radicals exhibits a relatively broad absorption with a maximum cross section of 1.2 × 10 −17 cm 2 molecule −1 at 242 nm. The electronic transitions in propargyl radical have also been calculated by ab initio quantum chemical methods at the CASSCF, CASPT2, CIS, and EOM_CCSD levels of theory and assign the new observed band to the in plane π ∗( b 2)←π( b 2) allowed transition. In addition, we report here the absorption spectra and cross sections for propargyl chloride and propargyl bromide, precursors of propargyl radicals, in the spectral range of 160–230 nm.

Gas-phase ultraviolet absorption cross-sections and atmospheric lifetimes of several C 2C 5 alkyl nitrates

Gas-phase ultraviolet absorption cross-sections of ethyl nitrate, 1-propyl nitrate, 2-propyl nitrate, 2-methyl, 1-propyl nitrate, 1-butyl nitrate and 1-pentyl nitrate have been measured over the wavelength range 220–340 nm using a dual-beam, diode array spectrometer. Each alkyl nitrate spectrum appears to be the sum of at least two Gaussian-shaped absorptions with an intense π−π ∗ band extending from 190–240 nm having a shoulder between 250–340 nm due to a π−π ∗ system. The absorption cross-sections recorded for ethyl nitrate, 1-propyl nitrate, 2-propyl nitrate and 1-butyl nitrate are within 10% of previous data: those of 2-methyl, 1-propyl nitrate and 1-pentyl nitrate have been measured for the first time. For ethyl nitrate, absorption cross-sections between 280–340 nm in the tail of the near-ultraviolet band declined with decreasing temperature from 298-233 K. At two-dimensional numerical model of tropospheric chemistry was used to calculate atmospheric lifetimes with respect to photodissociation and OH radical reaction that are markedly dependent on season, latitude and altitude. Relatively long, surface level atmospheric lifetimes of several days to weeks confirm that the C 2C 5 alkyl nitrates may act as temporary reservoirs for NO x and suggest that they may also constitute a significant fraction of total reactive odd-nitrogen, NO y , particularly during winter at northern hemisphere high latitudes.

Comparison of the ultraviolet absorption cross section of buckminsterfullerene in the gas phase and in hexane solution

Five previously reported measurements of the gas-phase absorption cross section for in the ultraviolet region are reviewed in the light of recent improvements in the determination of vapour pressure. An improved gas-phase absorption cross section at is presented and compared with a cross section derived from absorbance measurements in hexane solution by applying the (refractive index-dependent) Chako correction. The agreement between the measured gas-phase absorption cross section and that derived from the solution phase absorbences is very good. The highest energy absorption band (band G, 209 nm) is suppressed in intensity and shifted lower in energy more than the other two bands, E (254 nm) and G (326 nm).

Temperature dependence of the gas and liquid phase ultraviolet absorption cross sections of HCFC‐123 (CF3CHCl2) and HCFC‐142b (CH3CF2Cl)

The absorption cross sections for HCFC‐123 (CF3CHCl2) and HCFC‐142b (CH3CF2Cl) have been measured in the gas and liquid phases over the temperature range of about 220–330 K. The liquid phase results were converted into effective gas phase cross sections using a wavelength shift procedure, thereby extending the gas phase cross sections to longer wavelengths. The results are compared with other available data and lend increased confidence in atmospheric lifetime calculations for these important industrial alternatives to the fully halogenated chlorofluorocarbons.

Temperature dependent ultraviolet absorption cross sections of propylene, methylacetylene and vinylacetylene

Abstract The temperature dependence of the UV absorption cross sections of propylene (CH3CHCH2) and methylacetylene (CH3CCH) from 160 to 200 nm and vinylacetylene (CH2CHCCH) from 160 to 240 nm have been measured in the gas phase, for temperatures ranging from about 220 to 330 K. The propylene spectrum, exhibits a broad absorption band with a maximum at 172.0 nm with some structure to the short wavelength side of the broad band. At the absorption peaks a small increase (about 5%) in the cross sections has been observed with decreasing temperature from 333 to 233 K. At the long wavelength tail of the absorption band the cross sections are found to decrease with decreasing temperature. The absorption spectrum of methylacetylene in the region of 160 to 200 nm consists of a broad continuum with a maximum at 172.4 nm. The cross section values, for methylacetylene, near the absorption peak are found to remain nearly independent of temperature. However, at the long wavelength side of the absorption band the cross sections increase with increasing temperature. The spectrum of vinylacetylene consists of a very strong and structured absorption in the 160 to 170 nm region and strong diffuse bands between around 190 and 230 nm. The cross section values at the absorption peaks, determined at low temperatures, are significantly higher than those at higher temperatures. A sharp absorption feature at 168.2 nm shows the most pronounced temperature effect, with an increase in cross section from 35.86 × 10−18 to 63.54 × 10−18 cm2 for a temperature change of 333 to 223 K.

Ultraviolet spectrum of CF3OH: Upper limits to the absorption cross sections

The ultraviolet spectra of gaseous and liquid samples of the species CF3OH have been investigated in the laboratory. Upper limits to the absorption cross sections in the 190–300 nm wavelength range were determined. The species CF3OH is produced in the stratosphere by hydrogen abstraction reactions involving the CF3O radical; the upper limits to the ultraviolet absorption cross sections established in this work imply that photolysis of CF3OH in the stratosphere is too slow to regenerate CF3O radicals at rates significant for ozone depletion.

Temperature dependence of the ultraviolet absorption cross section of CF3I

Abstract The ultraviolet absorption cross section of CF3I has been measured in the gas phase over the wavelength range 160–240 nm for temperatures 240, 295 and 355 K and over the wavelength range 240–350 nm for temperatures ranging from 218 to 333 K. Two intense and sharp absorption bands centered at about 160 and 171 nm and a broad and relatively strong band centered at about 267.5 nm were observed. The absorption band centered at 171 nm shows significant vibrational structure. The cross sections, at absorption peaks, between 160 and 175 nm, increased by increasing temperature. At the near-UV maximum of 267.5 nm, the absorption increased with decreasing temperature, but decreased at the lowest temperature. At wavelengths greater than about 280 nm, the absorption cross section decreased with temperature. The absorption cross sections were found to all fit a triple Gaussian expression for λ > 250 nm.

The ultraviolet absorption cross sections of CH3I temperature dependent gas and liquid phase measurements

The absorption cross section for methyl iodide (CH3I) have been measured in the gas phase from 160 to 335 nm and in the liquid phase from 330 to 400 nm over the temperature range 223–333 K. The liquid phase results were converted into effective gas phase cross sections using a wavelength shift procedure described and verified in earlier works. In the extremely strong discrete absorption system between 160 and about 210 nm the absorption cross sections decrease with decreasing temperature. In the continuous absorption region from about 210 to 270 nm the cross sections increase with decreasing temperature with the change being most pronounced near the peak at 258.5 nm. At wavelengths longer than about 270 nm the across sections once again decrease with decreasing temperature, with more pronounced changes occurring in the long wavelength tail of the absorption band. The use of these temperature dependent absorption cross sections should lead to improved accuracy of and confidence in atmospheric lifetime calculations for CH3I.

Ultraviolet absorption spectra of some Br-containing haloalkanes

The ultraviolet absorption cross sections were measured for CF3Br, CF2ClBr, CF2Br-CF2Br, CF2Br2, CHF2Br, CHFBr-CF3, CH2Br-CF3, CHClBr-CF3 in the wavelength range 190–320 nm at 295 K. The photolysis is concluded to be the minor atmospheric sink for CHF2Br, CHFBr-CF3, CH2Br-CF3, CHClBr-CF3.

Temperature dependent ultraviolet absorption cross sections of 1,3-butadiene and butadiyne

The temperature dependence of the ultraviolet absorption cross sections of 1,3-butadiene, from 160 to 240 nm, and butadiyne (diacetylene) from 160 to 260 nm have been measured, in the gas phase, for temperatures ranging from about 220 to 330 K. The absorption spectrum of 1,3-butadiene shows several sharp absorption features between 160 and 190 nm and strong diffuse bands between 190 and 220 nm. The absorption cross sections, at absorption peaks, increased significantly with decreasing temperature, particularly in the range 185 to 215 nm. At about 220 nm, a cross over point, the absorption cross section remained unchanged with temperature variations while at wavelengths greater than 220 nm the cross section values decreased with decreasing temperature. A sharp absorption feature at 186.6 nm shows the most pronounced relative temperature effect with an increase in the cross section from 42.55 × 10−18 to 78.70 × 10−18 cm2 for a temperature change of 333 to 218 K. The spectrum for butadiyne consists of a very strong absorption at 164.6 nm with a cross section of4.40 × 10−16 cm2, at 295 K, followed by the complex absorption features between about 200 to 250 nm. The cross section values at absorption peaks determined at low temperatures are significantly higher than those at higher temperatures.

Scattered light and accuracy of the cross‐section measurements of weak absorptions: Gas and liquid phase UV absorption cross sections of CH3CFCl2

Ultraviolet absorption cross sections of CH3CFCl2 (HCFC‐141b) were determined in the gas phase (190–260 nm) and liquid phase (230–260 mm) at 298 K. The liquid phase absorption cross sections were then converted into accurate gas phase values using a previously described procedure. It has been demonstrated that scattered light from the shorter‐wavelength region (as little as several parts per thousand) can seriously compromise the absorption cross‐section measurement, particularly at longer wavelengths where cross sections are low, and can be a source of discrepancies in the cross sections of weakly absorbing halocarbons reported in the literature. A modeling procedure was developed to assess the effect of scattered light on the measured absorption cross section in our experiments, thereby permitting appropriate corrections to be made on the experimental values. Modeled and experimental results were found to be in good agreement. Experimental results from this study were compared with other available determinations and provide accurate input for calculating the atmospheric lifetime of HCFC‐141b.

CH3SH ultraviolet absorption cross sections in the region 192.5–309.5 nm and photodecomposition at 222 and 193 nm and 296 K

The absolute ultraviolet absorption cross sections of methyl mercaptan (CH3SH) have been determined in the wavelength region 192.5–309.5 nm at 296 K. The uv absorbance, A, in a column of slowly flowing standard CH3SH/helium gas mixtures, was monitored in long Pyrex absorption cells of path length, l, at several different pressures at the ambient cell temperature using a diode-array spectrometer with an instrumental resolution ranging from 0.25 to 1.10 nm. The absorption cross sections, σ=A/l[CH3SH], were calculated using the Beer–Lambert law from the known partial pressure of CH3SH in the gas mixture. The primary quantum yields of H atom produced in the 222 and 193 nm laser photolysis of CH3SH have been determined to be 0.95±0.10 and 0.49±0.08 relative to 248 nm CH3SH photolysis and 193 nm HBr photolysis, respectively, at 296 K. The H atoms were directly monitored in the photolysis by resonance fluorescence detection of H(2S).