Temperature-dependent Absorption Cross Sections Research Articles

Mid-IR absorption spectra of C1-C4 alkyl acetates at high temperatures

We present measurements of the temperature-dependent absorption cross-section of C1-C4 alkyl acetates (methyl acetate, ethyl acetate, n-propyl acetate, n‑butyl acetate) covering their strongest infrared absorption band near 1250 cm−1. The methodology employed here consists of a broadband rapid-tuning MIRcat-QT™ laser in conjunction with a shock tube. The spectral measurements were performed over the wavelength range of 7.52 – 8.62 μm (1160 – 1330 cm−1), temperatures of 580 – 980 K, and pressures near 1.3 bar. The reported IR spectra are the first experimental measurements of the high-temperature absorption spectra of these species, and the measured spectra showed strong temperature dependence. We provide herein absorption cross-section correlations for a wide range of spectra and a wide range of temperatures. Rapid measurements of alkyl acetate in transient environments may be performed in a fixed-wavelength mode by choosing a suitable wavelength. Based on our analysis, we propose a wavelength of 1221.27 cm−1 which exhibited minimal temperature and pressure dependence over the range of investigated conditions. We utilized this strategy for measuring methyl acetate concentration during its high-temperature pyrolysis.

High-temperature mid-IR absorption and reaction kinetics of 2-methyl-1,3-dioxolane: An experimental and theoretical study

This work reports the mid-IR spectroscopy and reaction kinetics of 2-methyl-1,3-dioxolane (2M13DO). We carried out spectroscopic measurements to deduce temperature-dependent absorption cross-sections of 2M13DO over a broad wavelength range of 8.4–10.5 μm (950–1190 cm−1). For these measurements, we employed a rapidly tuning MIRcat-QT™ laser that can be operated either at a fixed wavelength or scanned mode over wide wavelength regions. By operating the laser at a fixed wavelength, we monitored the decay of 2M13DO behind reflected shock waves over T5 = 1050–1400 K and P5 = 0.7 and 2.6 bar. Our measured concentration time-histories of 2M13DO allowed us to directly extract the overall rate coefficients for the unimolecular decomposition of 2M13DO using the first-order rate law. We did not observe any pressure dependence in the measured rate coefficients, indicating that the reaction is close to the high-pressure limit. By employing the W1U composite method, we explored the important pyrolysis reaction pathways of 2M13DO in the reactive potential energy surface. Three important reaction channels, namely, 2M13DO → CH2CHOCH2CH2OH (IM1), 2M13DO → 2CH3CHO (P3), 2M13DO → CH3 + 1,3-dioxolan-2-yl (P4) were identified. Below 700 K, IM1 forming channel is dominant, whereas CH3CHO formation is dominant under our experimental conditions. Above 1500 K, the radical forming channel (CH3+P4) takes over other channels. At higher temperatures, the contribution of the radical forming channel continually increases, accounting for ∼ 99% at 2000 K. We used the stochastic RRKM-ME model to predict the pressure and temperature dependence of the rate coefficients, k(T, P), and time-resolved species profiles. Our theory showed excellent agreement with the measured rate coefficients. These are the first direct determination of the rate coefficients of the unimolecular decomposition of 2M13DO.

A mid-IR laser diagnostic for HCN detection

Hydrogen cyanide (HCN) is a major source of prompt-NOx formation especially in fuel-bound nitrogen systems. To date, there is still a significant disagreement between experimental data and theoretical predictions of the rate coefficients of combustion reactions involving HCN as a prompt-NOx precursor. Accurate modeling of NOx formation would greatly benefit from a diagnostic capable of performing high-fidelity measurements of HCN formation/consumption time-histories. In this study, a laser diagnostic is developed for sensitive and selective HCN sensing by probing its most intense absorption feature in the mid-infrared (MIR). The diagnostic is based on difference-frequency generation (DFG) between a CO2 gas laser and an external-cavity quantum cascade laser in a nonlinear orientation-patterned gallium arsenide crystal which results in a DFG laser tunable over 11.56 − 15 µm. HCN measurements were carried out at the peak of the Q-branch of its strong ν2 vibrational band near 14 µm. Pressure dependence of the absorption cross-section was investigated at room temperature over the pressure range of 0.07 − 1.07 bar. Temperature-dependent absorption cross-section measurements were conducted behind reflected shock waves over the temperature range of 850 − 3000 K. The diagnostic was demonstrated in reactive experiments in a shock tube where HCN mole fraction time-histories were measured during the thermal decomposition of isoxazole (C3H3NO) and the first-order rate coefficients of C3H3NO → HCN + CH2CO reaction were determined.

High-temperature mid-IR absorption spectra and reaction kinetics of 1,3-dioxolane

Cyclic saturated ethers are proposed as alternative fuels for future sustainability. In particular, dioxolanes have been identified as promising biofuel candidates. However, pyrolysis and oxidation of dioxolanes are not well understood. In this work, we measured the temperature-dependent absorption cross-section of 1,3-dioxolane (13DO) in the mid-IR region over 8.4 – 10.5 μm (950 –1190 cm−1). We applied the spectroscopic knowledge to investigate the temperature and pressure dependence of the rate coefficients of the unimolecular decomposition of 13DO over the temperature range of 1000–1400 K and pressures near 0.7 and 2.4 bar. We employed a rapidly tuning MIRcat-QT™ laser in conjugation with a shock tube to carry out high-temperature spectroscopic and pyrolysis measurements. Measured IR spectra of 13DO exhibited a strong temperature dependence. For the kinetic study, we employed the vibrational feature of 13DO near 1120.5 cm−1 to monitor the decay of 13DO quantitatively. The measured rate coefficients did not show any discernible pressure dependence. This indicates that the rate coefficients of the unimolecular decomposition of 13DO are close to the high-pressure limit under the present experimental conditions. In addition, we rationalized the temperature and pressure dependence of the reaction by employing electronic structure/RRKM-ME calculations. Our theoretical model accurately captured the experimental trends of the rate coefficients of the unimolecular decomposition of 13DO.

Investigation of the kinetics of conjugated diolefins using UV absorption spectroscopy

Conjugated diolefins are not only crucial intermediates in larger hydrocarbon pyrolysis and oxidation, but also key species in the formation and growth of polycyclic aromatic hydrocarbons (PAHs). In this work, we employed a sensitive UV laser diagnostic to measure absorption cross-sections and decomposition rates of three conjugated diolefins, namely 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), and 2,3-dimethyl-1,3-butadiene. The single-pass UV absorption diagnostic achieved a ppm-level detection limit between the wavelengths of 212.5 and 220.5 nm. The use of dilute conditions (119 – 500 ppm fuel in argon) enabled nearly isothermal measurements despite reaction enthalpy. Temperature-dependent absorption cross-sections were measured from room temperature to 1850 K and pressures ranging 0.75 – 1.50 bar in a shock tube. Decomposition of the molecules was observed at temperatures above ∼ 1350 K, and all three molecules exhibited similar activation energy. Around 1800 K, 2,3-dimethyl-1,3-butadiene decomposed twice as fast as isoprene and 4 times faster than 1,3-butadiene. Our measured overall decomposition rate coefficients are given as (unit of s − 1, ± 20% uncertainty):k1(1,3−butadiene)=9.65×109e(−24,338KT)(1411−1823K)k2(isoprene)=1.86×1010e(−24,341KT)(1464−1829K)k3(2,3−dimethyl−1,3−butadiene)=8.64×1010e(−25,845KT)(1401−1822K)1,3-Butadiene decomposition rate coefficients agree well with previous measurement at similar pressures. To our knowledge, this work reports first measurements of the decomposition rate coefficients of isoprene and 2,3-dimethyl-1,3-butadiene. As an additional application of the current UV diagnostic, we measured 1,3-butadiene decay time-histories during fuel-lean oxidation and compared our data with the predictions of AramcoMech 3.0. We updated the model with our measured 1,3-butadiene decomposition rate coefficients, which significantly improved the model prediction of fuel oxidation.

Temperature-dependence of the near-UV absorption of water vapor in the 290–350nm range

Water vapor plays a critical role in the energy balance of the Earth's atmosphere. But there are limited experimental measurements of single photon water vapor absorption spectra and absorption strengths in the near-UV region. We determined the temperature-dependent absorption cross-sections of water vapor at 1 nm increments in the 290–350 nm range by using cavity ring-down spectroscopy. Water vapor exhibited structured absorption bands in this region, with cross-sections ranging from 1.4 × 10−25 to 6.0 × 10−25 cm2/molecule at 283 K; 1.6 × 10−25 to 8.9 × 10−25 cm2/molecule at 313 K; and 1.9 × 10−25 to 1.0 × 10−24 cm2/molecule at 333 K. A radiative transfer model was used to evaluate atmospheric impacts of water vapor near-UV absorption using laboratory temperature-dependent cross-section data.

Temperature-dependent absorption cross section measurements for propene, 1-butene, cis-/trans-2-butene, isobutene and 1,3-butadiene in the spectral region 8.4–11.7 µm

We present a comprehensive study on the temperature-dependent mid-infrared absorption spectra between 8.4 µm and 11.7 µm for a complete list of C3-C4 alkenes, namely propene, 1-butene, cis-/trans-2-butene and isobutene, as well as 1,3-butadiene, from room temperature to roughly 1000 K. The methodology deploys a rapid-tuning, broad-scan external-cavity quantum-cascade laser (EC-QCL) in a shock tube and can provide quantitative absorption information at a rate in excess of 30 cm−1/ms at spectral intervals between 0.35–0.6 cm−1. The experimental procedure was validated with room-temperature, nitrogen-diluted, static spectra measurements for propene, 1-butene, isobutene, and 1,3-butadiene, showing good agreement with existing spectroscopic databases. Spectral measurements were extended to higher temperatures with shock-heated test gas mixtures diluted by argon, at experimental conditions between 600–1000 K and 1.5–2.5 atm. The resulting spectra show strong temperature dependence and represent the first experimental collection of broadband high-temperature absorption spectra for these species. The measured high-temperature spectra are in reasonable agreement with predictions from fixed-wavelength absorption cross section correlations in the literature. The data presented are archived in the Stanford ShockGas-IR database and provide useful expansions of the existing knowledge on molecular absorption in the mid-infrared at elevated temperatures.

Quantitative measurements of broad-band mid-infrared absorption spectra of formaldehyde, acetaldehyde, and acetone at combustion-relevant temperatures near 5.7 µm

Temperature-dependent absorption cross sections of formaldehyde, acetaldehyde, and acetone are presented for their CO stretching bands near 5.7 µm. The measurement methodology employs a broad-scan, rapid-tuning EC-QCL in conjunction with shock tube facilities and can provide quantitative spectroscopic information with an average spectral resolution of 0.6 cm−1 at a tuning rate in excess of 30 cm−1/ms. The absorption spectrum of formaldehyde in the ν2 vibrational band is reported at 998 K and 3.8 atm, showing reasonable agreement with spectral simulations based on HITRAN 2016. The residuals are ascribed to the emergence of hotbands and high-J rovibrational transitions at elevated temperatures. The first high-temperature absorption cross section measurements of acetaldehyde and acetone are reported near atmospheric pressure at combustion-related temperatures up to 1000 K, with strong temperature broadening observed in the measured spectra. The results presented are archived in the Stanford ShockGas-IR database and are expected to be useful in future spectroscopic studies.

Reinvestigation of the spectroscopy of the [formula omitted] transition of the NCN radical at high temperature: Application to quantitative NCN measurement in flames

The initiation reaction of the prompt-NO mechanism in flames (CH+N2=NCN+H) involves the NCN radical. This radical is a very minor species reaching only a few hundreds of ppb in mole fraction in hydrocarbon/air flames, which makes the experimental determination of its absolute concentration profile challenging. The aim of this study is (1) to show that unambiguous assignment of absorption spectral lines of the NCN radical can be obtained in selected flames and (2) to provide the temperature-dependent NCN absorption cross sections needed to determine the absolute NCN concentration. In this work, NCN spectroscopy of the A–X band is fully revisited using theoretical spectroscopic calculations. Assessment of the spectroscopic constants is based on comparison of the theoretical spectrum, simulated with the pgopher program, with an extended LIF spectrum obtained in a low-pressure acetylene/oxygen/nitrogen flame. From the calculated electronic transition moments and the partition function, the temperature-dependent absorption cross section of NCN σ(T) has been calculated at the most intense bandhead wavelength (329.13nm). Cross-section temperature dependence is in very good agreement with that experimentally obtained by Dammeier and Friedrichs [J. Phys. Chem. A 114 (2010) 12963] using NCN3 pyrolysis in a shock tube followed by single-pass NCN absorption measurement, though their values differ by a factor of 2.6. The first NCN absorption spectrum between 329.0 and 329.3nm obtained by CRDS at high temperature in a flame is provided here, from which the bandhead is clearly identified. Occurrence of possible interferences has been examined. Finally, peak mole fractions obtained by CRDS are given in three low-pressure premixed flames by reconsidering previous published data and adding a new datum. The experimental absorption cross section required for this determination is derived from the theoretical one, taking into account the laser bandwidth. The accuracy of the NCN measurements is discussed and highlights the benefit of coupling CRDS and LIF.

Temperature dependent absorption cross-sections of O2–O2 collision pairs between 340 and 630 nm and at atmospherically relevant pressure

The collisions between two oxygen molecules give rise to O4 absorption in the Earth atmosphere. O4 absorption is relevant to atmospheric transmission and Earth's radiation budget. O4 is further used as a reference gas in Differential Optical Absorption Spectroscopy (DOAS) applications to infer properties of clouds and aerosols. The O4 absorption cross section spectrum of bands centered at 343, 360, 380, 446, 477, 532, 577 and 630 nm is investigated in dry air and oxygen as a function of temperature (203-295 K), and at 820 mbar pressure. We characterize the temperature dependent O4 line shape and provide high precision O4 absorption cross section reference spectra that are suitable for atmospheric O4 measurements. The peak absorption cross-section is found to increase at lower temperatures due to a corresponding narrowing of the spectral band width, while the integrated cross-section remains constant (within <3%, the uncertainty of our measurements). The enthalpy of formation is determined to be ΔH(250) = -0.12 ± 0.12 kJ mol(-1), which is essentially zero, and supports previous assignments of O4 as collision induced absorption (CIA). At 203 K, van der Waals complexes (O(2-dimer)) contribute less than 0.14% to the O4 absorption in air. We conclude that O(2-dimer) is not observable in the Earth atmosphere, and as a consequence the atmospheric O4 distribution is for all practical means and purposes independent of temperature, and can be predicted with an accuracy of better than 10(-3) from knowledge of the oxygen concentration profile.

Temperature-dependent absorption cross-section measurements of 1-butene (1-C4H8) in VUV and IR

Vacuum ultraviolet (VUV) and infrared (IR) absorption cross-section measurements of 1-butene (1-C4H8; CH2=CHCH2CH3; Butylene) are reported over the temperature range of 296–529K. The VUV measurements are performed between 115 and 205nm using synchrotron radiation as a tunable VUV light source. Fourier Transform Infrared (FTIR) spectroscopy is employed to measure absorption cross-section and band strengths in the IR region between 1.54 and 25μm (∼6500–400cm−1). The measured room-temperature VUV and IR absorption cross-sections are compared with available literature data and are found to be in good agreement. The oscillator strength for the electronic transition (A1A′→X1A′) around 150–205nm is determined to be 0.32±0.01.The gas temperature has a strong effect on both VUV and IR spectra. Measurements made in the VUV region show that the peak value of the band cross-section decreases and the background continuum increases with increasing gas temperature. This behavior is due to a change in the rotational and vibrational population distribution of 1-butene molecule. Similar changes in rotational population are observed in the IR spectra. Moreover, variation of the IR spectra with temperature is used to measure the enthalpy difference between syn and skew conformations of 1-butene and is found to be 0.24±0.03kcal/mol, which is in excellent agreement with values reported in the literature. The measurements reported in this work will provide the much-needed spectroscopic information for the development of high-temperature quantitative diagnostics in combustion applications and validation of atmospheric chemistry models of extra-solar planets.

Kinetics and Mechanisms of the Allyl + Allyl and Allyl + Propargyl Recombination Reactions

The kinetics and mechanisms of the self-reaction of allyl radicals and the cross-reaction between allyl and propargyl radicals were studied both experimentally and theoretically. The experiments were carried out over the temperature range 295-800 K and the pressure range 20-200 Torr (maintained by He or N(2)). The allyl and propargyl radicals were generated by the pulsed laser photolysis of respective precursors, 1,5-hexadiene and propargyl chloride, and were probed by using a cavity ring-down spectroscopy technique. The temperature-dependent absorption cross sections of the radicals were measured relative to that of the HCO radical. The rate constants have been determined to be k(C(3)H(5) + C(3)H(5)) = 1.40 × 10(-8)T(-0.933) exp(-225/T) cm(3) molecule(-1) s(-1) (Δ log(10)k = ± 0.088) and k(C(3)H(5) + C(3)H(3)) = 1.71 × 10(-7)T(-1.182) exp(-255/T) cm(3) molecule(-1) s(-1) (Δ log(10)k = ± 0.069) with 2σ uncertainty limits. The potential energy surfaces for both reactions were calculated with the CBS-QB3 and CASPT2 quantum chemical methods, and the product channels have been investigated by the steady-state master equation analyses based on the Rice-Ramsperger-Kassel-Marcus theory. The results indicated that the reaction between allyl and propargyl radicals produces five-membered ring compounds in combustion conditions, while the formations of the cyclic species are unlikely in the self-reaction of allyl radicals. The temperature- and pressure-dependent rate constant expressions for the important reaction pathways are presented for kinetic modeling.

Revised ultraviolet absorption cross sections of H 2CO for the HITRAN database

A revised set of temperature-dependent absorption cross sections for ultraviolet (UV) measurements of formaldehyde (H 2CO) has been derived from two existing sets of laboratory cross sections, one using a Fourier transform spectrometer (FTS), and one using a grating instrument. This is conducted to satisfy the recommendation of the HITRAN Advisory Committee to provide a dataset with the spectral resolution and wavelength calibration of Fourier transform spectrometer measurements with the better intensity calibration that the grating measurements obtained. The re-scaled cross sections are now in the HITRAN database, and are recommended for use in atmospheric measurements and modeling, including photolysis calculations.

A brief introduction and some background to the article JQSRT 1998;60:1025–31 and its companion

This note provides a short introduction, some background information and relevant history related to reprint of the article “Atmospheric remote-sensing reference data from GOME: Part 1. Temperature-dependent absorption cross-sections of NO 2 in the 231–794 nm range” (JQSRT 1998;60:1025–31). The article and its companion “Atmospheric remote-sensing reference data from GOME: 2. Temperature-dependent absorption cross sections of O 3 in the 231–794 nm range” (JQSRT 1999;61:509–17) provided timely and unique data sets of the absorption cross sections of Nitrogen dioxide, NO 2, and ozone, O 3. The reported absorption cross sections have a spectral resolution sufficient to distinguish the electronic vibrational and rotational features of NO 2 and O 3 and were made in the temperature range of relevance to the earth's atmosphere, viz. 200–300 K, over a wide spectral range. These species are two of the most important trace gases in earth's atmosphere, and play key roles in determining the chemistry and dynamics of the earth's atmosphere and the conditions at the earth's surface experienced by the biosphere. Whilst providing valuable information about the electronic states of NO 2 and O 3, these spectra were determined primarily for their exploitation in the retrieval of trace gases from measurements of solar back scattered radiation by ground based and space borne instrumentation. These yield the local and global amounts and distributions of NO 2 and O 3, thereby constraining our knowledge about atmospheric chemistry and dynamics and the impact of pollution from the local to the global scale. The measurement, publication and use of these spectral sets are among many important milestones in the development of atmospheric remote sensing.

Two-wavelength mid-IR absorption diagnostic for simultaneous measurement of temperature and hydrocarbon fuel concentration

A two-wavelength mid-IR laser is used for time-resolved absorption-based measurements of temperature and n-heptane concentration in shock-heated gases. The novel difference-frequency-generation laser provides tunable mid-IR light from nonlinear conversion of near-IR light, enabling access to the strong hydrocarbon absorption bands between 3.3 and 3.5μm associated with the CH stretching vibration. This laser was modified to alternate between two mid-IR wavelengths at 200kHz, providing 5μs time resolution for simultaneous monitoring of temperature and concentration in reactive flows and combustion systems. Temperature-dependent absorption spectra of n-heptane are first measured in a cell from 298 to 773K using an FTIR spectrometer. These spectra are used to select candidate pairs of wavelengths with good sensitivity to temperature and concentration and to provide accurate temperature-dependent absorption cross-sections at the selected wavelengths. Laser absorption measurements of shock-heated n-heptane are then used to extend the cross-section data to 1300K and to investigate the sensor accuracy and noise characteristics. The temperature and concentration inferred from the measurements are compared to known post-shock conditions, with a 4.5% RMS deviation from the calculated temperature and 1.7% RMS deviation from the calculated concentration. Finally, at high temperatures, the sensor is used to monitor decomposition of n-heptane, illustrating the potential of this diagnostic for hydrocarbon kinetics experiments in shock tubes. This new sensor concept should prove useful for simultaneous, time-resolved temperature and hydrocarbon concentration measurements in a variety of combustion and propulsion applications.

Analysis of solid-state saturable absorbers with temperature dependent absorption cross-sections

We describe a comprehensive model which can be used in the continuous-wave (cw) analysis of solid-state transition metal-doped saturable absorbers, which, in addition to the commonly observed temperature dependence of the refractive index and fluorescence lifetime, also exhibit a temperature dependent absorption cross-section. By using a rate equation analysis, two coupled differential equations are derived for the beam power and the q-parameter of the pump beam. To test the validity of the model, we analyzed the cw and pulsed saturation data taken with Cr 4+:forsterite crystals which are subject to these three thermal effects at 1064 nm. Data for electric field parallel to the crystal b-axis ( E ‖ b ) and c-axis ( E ‖ c ) were analyzed. Results indicate that when thermal loading effects are taken into account, better agreement is obtained between the cross-section results of the cw and pulsed saturation data. In particular, inclusion of thermal effects reduced the fractional deviation between the average cw and pulsed cross-section values from 23% to 2% and from 34% to 29% for the E ‖ c and E ‖ b cases, respectively. In the E ‖ b case, the average absorption cross-section and the normalized strength of excited-state absorption were further determined to be 5.98 × 10 −19 cm 2 and 0.45, respectively. The presented model can be readily extended to analyze other thermal effects or other media with different energy-level schemes.

Temperature- and composition-dependent mid-infrared absorption spectrum of gas-phase gasoline: Model and measurements

A spectroscopic model is developed to predict the composition- and temperature-dependent absorption spectrum of gasoline vapor in the mid-IR region of the C–H stretching vibrations near 3.5 μm. Data from a detailed compositional analysis of a representative gasoline sample are used to select and weight the absorption spectra of individual hydrocarbon species, providing class-averaged spectra for five hydrocarbon classes: the normal and branched alkanes, olefins, aromatics, and oxygenates. The absorption spectra for 21 other samples of gasoline are then predicted by weighting these class-averaged spectra according to the volume fractions of olefins and aromatics, and the mass fraction of oxygenates in the gasoline, determined by standard chemical analysis (ASTM D1319 for olefins and aromatics, ASTM D4815 for oxygenates), and the volume fraction contributions from normal and branched alkanes estimated according to fuel grade. The absorption spectra predicted by the model are compared to FTIR-measured absorption spectra of these 21 gasoline samples at 50 and 450 °C. While the measured cross-section at selected wavelengths varies by a factor of 2 for this set of gasoline samples, the data agree well with the model predictions, exhibiting only a 6.8% RMS deviation. This deviation is reduced to 5.5% by excluding four samples of aged gasoline. The good agreement between model and measurements over a large temperature range illustrates the utility of this model to predict the temperature-dependent absorption cross-sections of gasoline in the C–H vibrational absorption band using two commonly available class analyses of gasoline samples.

Temperature- and pressure-dependent absorption cross sections of gaseous hydrocarbons at 3.39 µm

The pressure- and temperature-dependent absorption cross sections of several neat hydrocarbons and multi-component fuels are measured using a 3.39 µm helium–neon laser. Absorption cross section measurements are reported for methane, ethylene, propane, n-heptane, iso-octane, n-decane, n-dodecane, JP-10, gasoline and jet-A with an estimated uncertainty of less than 3.5%. The experimental conditions range from 298 to 673 K and from 500 to 2000 Torr with nitrogen as the bath gas. An apparatus is designed to facilitate these measurements, and specific care is taken to ensure the compositional accuracy of the hydrocarbon/N2 mixtures. The absorption cross sections of the smallest hydrocarbons, methane and ethylene, vary with temperature and pressure. The cross sections of larger hydrocarbons show negligible dependence on pressure and only a weak dependence on temperature. The reported data increase the range of conditions and the number of hydrocarbons for which cross section measurements are available at the HeNe laser wavelength.

Trifluoro methyl peroxynitrate (CF3OONO2): Temperature dependence of the UV absorption spectrum and atmospheric implications

The ultraviolet absorption spectrum of gas phase CF3OONO2 has been measured over the wavelength range 200–340 nm at 233–300 K. Absorption cross-sections at wavelengths of 290–340 nm were found to increase significantly with increasing temperature. The UV spectra of CF3C(O)Cl and CF3C(O)F were measured and were consistent with previous work [Rattigan et al., 1993. Temperature-dependent absorption cross-sections of CF3COCl, CF3COF, CH3COF, CCl3CHO and CF3COOH. Journal of Photochemistry and Photobiology A: Chemistry 73, 1–9]. Implications for the atmospheric chemistry of CF3OONO2 are discussed.

Temperature-dependent absorption cross-sections in the thermal infrared bands of SF 5CF 3

Absorption cross-sections have been measured at five temperatures between 213 and 323 K in the infrared bands of SF 5CF 3. The spectra were recorded at a resolution of 0.112 cm −1 using a commercial Fourier transform infrared spectrometer and a 20 cm temperature-controlled sample cell. Samples of SF 5CF 3 were pressurized with high-purity nitrogen to a total pressure of 1013.3 hPa (760 Torr) . Six or more spectra with varying SF 5CF 3 column amounts were analyzed at each temperature. The full spectral range of the measurements was 520– 6500 cm −1 , with only weak bands observed beyond 1400 cm −1 . Absorption of thermal radiation in the 8– 12 μm atmospheric window region being important for climate change, we report here the integrated cross-sections of the significant absorption bands in that spectral region. Our results closely match room temperature values reported previously. Only small variation of the integrated absorption cross-sections with temperature was found. Our results confirm the accuracy of the previous measurements, which find SF 5CF 3 important for global climate change on a per molecule basis. Absorption cross-sections derived from a single, near Doppler-limited spectrum recorded at room temperature do not show any rotational fine structure in the 700– 950 cm −1 region.