Absolute Absorption Cross Sections Research Articles

Ultraviolet Photolysis of HCHO: Absolute HCO Quantum Yields by Direct Detection of the HCO Radical Photoproduct

Absolute quantum yields for the radical (H + HCO) channel of HCHO photolysis, Phi(HCO), have been measured for the tropospherically relevant range of wavelengths (lambda) between 300 and 330 nm. The HCO photoproduct was directly detected by using a custom-built, combined ultra-violet (UV) absorption and cavity ring down (CRD) detection spectrometer. This instrument was previously employed for high-resolution (spectral resolution approximately 0.0035 nm) measurements of absorption cross-sections of HCHO, sigma(HCHO)(lambda), and relative HCO quantum yields. Absolute Phi(HCO) values were measured at seven wavelengths, lambda = 303.70, 305.13, 308.87, 314.31, 320.67, 325.59, and 329.51 nm, using an independent calibration technique based on the simultaneous UV photolysis of HCHO and Cl(2). These Phi(HCO) measurements display greater variability as a function of wavelength than the current NASA-JPL recommendations for Phi(HCO). The absolute Phi(HCO)(lambda) determinations and previously measured sigma(HCHO)(lambda) were used to scale an extensive set of relative HCO yield measurements. The outcome of this procedure is a full suite of data for the product of the absolute radical quantum yield and HCHO absorption cross-section, Phi(HCO)(lambda)sigma(HCHO)(lambda), at wavelengths from 302.6 to 331.0 nm with a wavelength resolution of 0.005 nm. This product of photochemical parameters is combined with high-resolution solar photon flux data to calculate the integrated photolysis rate of HCHO to the radical (H + HCO) channel, J(HCO). Comparison with the latest NASA-JPL recommendations, reported at 1 nm wavelength resolution, suggests an increased J(HCO) of 25% at 0 degrees solar zenith angle (SZA) increasing to 33% at high SZA (80 degrees). The differences in the calculated photolysis rate compared with the current HCHO data arise, in part, from the higher wavelength resolution of the current data set and highlight the importance of using high-resolution spectroscopic techniques to achieve a complete and accurate picture of HCHO photodissociation processes. All experimental Phi(HCO)(lambda)sigma(HCHO)(lambda) data are available for the wavelength range 302.6-331.0 nm (at 294 and 245 K and under 200 Torr of N(2) bath gas) as Supporting Information with wavelength resolutions of 0.005, 0.1, and 1.0 nm. Equivalent data sets of Phi(H(2)+CO)(lambda)sigma(HCHO)(lambda) for the molecular (H(2) + CO) photofragmentation channel, produced using the measured Phi(HCO)(lambda) sigma(HCHO)(tau) values, are also provided at 0.1 and 1.0 nm resolution.

Line strength and collisional broadening studies of hydrogen sulphide in the 1.58 μm region using diode laser spectroscopy

High resolution diode laser spectroscopy has been applied to the detection of hydrogen sulphide at ppm levels utilizing different transitions within the region of the ν1 + ν2 + ν3 and 2ν1 + ν2 combination bands around 1.58 µm. Suitable lines in this spectral region have been identified, and absolute absorption cross sections have been determined through single-pass absorption spectroscopy and confirmed in the Doppler linewidth regime using cav- ity enhanced absorption spectroscopy (CEAS). The desire for a sensitive system potentially applicable to H2S sens- ing at atmospheric pressure has led to an investigation on suitable transitions using wavelength modulation spec- troscopy (WMS). The set-up sensitivity has been calculated as 1.73 × 10 −8 cm −1 s 1/2 , and probing the strongest line at 1576.29 nm a minimum detectable concentration of 700 ppb under atmospheric conditions has been achieved. Further- more, pressure broadening coefficients for a variety of buffer gasses have been measured and correlated to the intermolec- ular potentials governing the collision process; the H2S-H2S dimer well depth is estimated to be 7.06 ± 0.09 kJ mol −1 .

Infrared band intensities of bromine nitrate, BrONO2

The integrated band intensities at 296K of gaseous bromine nitrate (BrONO2) in the spectral range 500–2000cm−1 have been measured using Fourier-transform absorption spectroscopy. The absorption spectra were calibrated to absolute absorption cross-sections using the integrated band strength of the ν3 band around 803cm−1 (2.70±0.50×10−17cmmolecule−1) published previously. In this way, integrated band intensities were determined for the first time for the ν1 band around 1709cm−1 (2.43±0.48×10−17cmmolecule−1), the ν2 band around 1286cm−1 (2.99±0.60×10−17cmmolecule−1), the weak ν4/ν8 bands between 720 and 760cm−1 (0.16±0.04×10−17cmmolecule−1) and the ν5 band around 559cm−1 (1.02±0.20×10−17cmmolecule−1). The results are compared with the infrared band strengths of chlorine nitrate and with ab initio calculations.

Electron emission from laser-heated fullerene dianions: Probing the repulsive Coulomb barrier

Electron detachment from fullerene dianions Cm2- (m=76,84) stored in a room temperature Penning trap was probed upon pulsed laser excitation at wavelengths of 355 and 532 nm. The fraction of Cm(2-) surviving trapping times exceeding tens of milliseconds under UHV conditions, as well as the fraction of singly charged anions Cm(-) generated were recorded as a function of the laser fluence. Analysis by means of Poisson statistics yields absolute absorption cross sections and the number of photons necessary to induce the detachment. The cross sections obtained are in good agreement with the literature values. By describing the electron detachment as a statistical unimolecular process, we deduce effective activation energies from the number of photons required. These energies are compared to the sum of the second electron affinity and the Coulomb barrier height as calculated from an electrostatic charging model.

Photophysical characterization of pyrromethene 597 laser dye in cross-linked silicon-containing organic copolymers

Samples of the dipyrromethene–BF 2 dye PM597 incorporated in copolymers of 3-trimethoxysilylpropyl 2-methylprop-2-enoate (TMSPMA, number of polymerizable CC double bonds: κ = 1) with 2-(2-methylprop-2-enoyloxy)ethyl 2-methylprop-2-enoate (EGDMA, κ = 2), [2-(hydroxymethyl)-3-prop-2-(prop-2-enoyloxymethyl)propyl] prop-2-enoate (PETA, κ = 3), and [3-prop-2-enoyloxy-2,2-bis(prop-2-2-enoyloxymethyl)propyl]prop-2-enoate (PETRA, κ = 4) are characterized. The fluorescence quantum distributions, fluorescence quantum yields, degrees of fluorescence polarization, and fluorescence lifetimes are measured. The radiative lifetimes are calculated from fluorescence lifetime and quantum yield. Absorption coefficient spectra are determined from transmission measurements. Absolute absorption cross-section spectra and dye concentrations are obtained by calibration to the radiative lifetimes and to saturable absorptions. Excited-state absorption cross-sections at 527 nm are determined by saturable absorption measurements. The photo-degradation is studied under cw laser excitation conditions and quantum yields of photo-degradation are extracted. The excited-state absorption cross-sections were found to be rather small, and the photo-stability turned out to be high (up to 3 million excitation cycles before degradation) making this class of dipyrromethene dye-doped polymers attractive active laser media. Structural and thermo-mechanical properties of the materials have been determined by Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetry, densitometry, and refractometry. They improve with increasing inter-crossing (copolymerization of TMSPMA with PETA and PETRA). The laser properties of the PM597 doped copolymers were evaluated by transverse pumping with 6 ns laser pulses at 532 nm. The best laser materials resulted to be the 7:3 and 9:1 TMSPMA-monomer copolymers.

In situ infrared aerosol spectroscopy for a variety of nerve agent simulants using flow-through photoacoustics

We present newly measured results of an ongoing experimental program established to measure optical cross sections in the mid- and long-wave infrared for a variety of chemically and biologically based aerosols. For this study we consider only chemically derived aerosols, and in particular, a group of chemical compounds often used as simulants for the detection of extremely toxic organophosphorus nerve agents. These materials include: diethyl methylphosphonate (DEMP), dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), and diethyl phthalate (DEP). As reported in a prior study [Appl. Opt. 44, 4001 (2005)], we combine two optical techniques well suited for aerosol spectroscopy [i.e., flow-through photoacoustics and Fourier transform infrared (FTIR) emission spectroscopy], to measure in situ the absolute extinction and absorption cross sections over a variety of wavelengths spanning the IR spectral region from 3 to 13 mum. Aerosol size distribution(s), particle number density, and dosimetric measurements are recorded simultaneously in order to present optical cross sections that are aerosol mass normalized, i.e., m(2)/gram. Photoacoustic results, conducted at a series of CO(2) laser lines, compare well with measured broadband FTIR spectral extinction. Both FTIR and photoacoustic data also compare well with Mie theory calculations based on measured size distributions and previously published complex indices of refraction.

Measurements of Line Strengths in the 2ν1 Band of the HO2 Radical Using Laser Photolysis/Continuous Wave Cavity Ring-Down Spectroscopy (cw-CRDS)

Absolute absorption cross sections of the absorption spectrum of the 2nu1 band of the HO2 radical in the near-IR region were measured by continuous wave cavity ring-down spectroscopy (cw-CRDS) coupled to laser photolysis in the wavelength range 6604-6696 cm(-1) with a resolution better than 0.003 cm(-1). Absolute absorption cross sections were obtained by measuring the decay of the HO2 self-reaction, and they are given for the 100 most intense lines. The most important absorption feature in this wavelength range was found at 6638.20 cm(-1), exhibiting an absorption cross section of sigma = 2.72 x 10(-19) cm2 at 50 Torr He. Using this absorption line, we obtain a detection limit for the HO2 radical at 50 Torr of 6.5 x 10(10) cm(-3).

Growth and optical characterization of Tm3+-doped LiNbO3

Single LiNbO3:Tm3+ crystals, with variable Tm3+ content, have been grown by using the Czochralski method. The incorporation of Tm3+ ions into LiNbO3 crystals has been determined by using X-ray fluorescence spectroscopy. At moderated doping levels the active ion incorporation is driven by a linear regime, with a segregation coefficient Keff=1.19±0.08. Saturation effects have been observed only for highly concentrated crystals. The absorption spectra of Tm3+-doped crystals indicate that the melt congruent composition is preserved within the crystals. The Tm3+ absolute absorption cross sections have been obtained.

Daytime tropospheric loss of hexanal and <I>trans</I>-2-hexenal: OH kinetics and UV photolysis

Abstract. The ultraviolet (λ=250–370 nm) photolysis and the OH-initiated oxidation of hexanal and trans-2-hexenal, which are relevant atmospheric processes, have been investigated at room temperature and as a function of temperature (T=263–353 K), respectively. This kinetic study as a function of temperature is reported here for the first time. Absolute absorption cross sections (σλ) were obtained using a recently built system operating in the UV region. The obtained σλ allowed the estimation of the photolysis rates (J) across the troposphere. Kinetic measurements of the gas-phase reaction of hydroxyl radicals (OH) with hexanal and trans-2-hexenal were performed by using the laser pulsed photolysis/laser-induced fluorescence technique. Rate coefficients kOH for both aldehydes were determined at temperatures between 263 and 353 K at 50 Torr in helium or argon bath gases. The temperature dependence of kOH for both aldehydes was found to be slightly negative. The tropospheric lifetime of hexanal and trans-2-hexenal due to the chemical removal by OH radicals has been estimated across the troposphere. The loss rate due to the OH chemical removal was compared with the estimated photolysis rates. Our results show that OH-reaction is the main loss process for these aldehydes in the troposphere, although photolysis is not negligible for hexanal.

Absorption and emission spectroscopic characterization of some azo dyes and a diamino-maleonitrile dye

A linear and nonlinear optical spectroscopic characterization is carried out on three azo dyes (Reactive orange 1, Reactive violet 8, and Acidproof purplish red), and on N-( p-hydroxybenzylidene)-diamino-maleonitrile. Fluorescence quantum distributions, fluorescence quantum yields, and fluorescence lifetimes are measured. The saturable absorption is studied by nonlinear transmission measurements with intense picosecond laser pulses. The ground-state absorption recovery is studied by picosecond time-resolved pump and probe measurements. Absolute ground-state absorption cross-sections, excited-state absorption cross-sections, and dye concentrations are extracted from saturable absorption studies. The azo dyes have fluorescence lifetimes and ground-state absorption recovery times of around 2 ps and their excited-state absorption cross-sections are small (measured at 527 nm) making them good mode-locking dyes for picosecond and femtosecond lasers. The investigated diamino-maleonitrile dye exhibits sub-picosecond fluorescence lifetime and slow ground-state absorption recovery (>1 ns).

On the photochemistry of IONO2 : absorption cross section (240–370 nm) and photolysis product yields at 248 nm

The absolute absorption cross section of IONO(2) was measured by the pulsed photolysis at 193 nm of a NO(2)/CF(3)I mixture, followed by time-resolved Fourier transform spectroscopy in the near-UV. The resulting cross section at a temperature of 296 K over the wavelength range from 240 to 370 nm is given by log(10)(sigma(IONO(2))/cm(2) molecule(-1)) = 170.4 - 3.773 lambda + 2.965 x 10(-2)lambda(2)- 1.139 x 10(-4)lambda(3) + 2.144 x 10(-7)lambda(4)- 1.587 x 10(-10)lambda(5), where lambda is in nm; the cross section, with 2sigma uncertainty, ranges from (6.5 +/- 1.9) x 10(-18) cm(2) at 240 nm to (5 +/- 3) x 10(-19) cm(2) at 350 nm, and is significantly lower than a previous measurement [J. C. Mössinger, D. M. Rowley and R. A. Cox, Atmos. Chem. Phys., 2002, 2, 227]. The photolysis quantum yields for IO and NO(3) production at 248 nm were measured using laser induced fluorescence of IO at 445 nm, and cavity ring-down spectroscopy of NO(3) at 662 nm, yielding phi(IO) </= 0.02 and phi(NO(3)) = 0.21 +/- 0.09. It is likely that photolysis to I + NO(3) is the only significant channel, as shown by accompanying quantum chemistry calculations. The low phi(NO(3)) is explained by the production of hot NO(3), most of which dissociates to NO(2) + O. In terms of atmospheric relevance, the noon photolysis frequency of J(IONO(2)) = (3.0 +/- 2.1) x 10(-3) s(-1) (40 degrees N, July) is fast enough to limit the effectiveness of IONO(2) as a daytime reservoir of iodine oxides, but the formation and subsequent photolysis of IONO(2) is very inefficient as an ozone-depleting cycle.

Effects of column density on I&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; spectroscopy and a determination of I&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; absorption cross section at 500 nm

Abstract. The use of ro-vibronic spectra of I2 in the region of 543 nm to 578 nm as reference spectra for atmospheric Differential Optical Absorption Spectroscopy is studied. It is shown that the retrieval of atmospheric column densities with Differential Optical Absorption Spectroscopy set-ups at FWHM at and above 1 nm depends critically on the column density, under which the used reference spectrum was recorded. Systematic overestimation of the comparatively low atmospheric column density of I2 of the order of 13% is possible. Under low pressure conditions relevant in laboratory studies, the systematic deviations may grow up to 45%. To avoid such effects with respect to field measurements, new reference spectra of I2 were determined under column density of the order of 1016 cm-2 close to that expected for an atmospheric measurement. Two typical configurations of Differential Optical Absorption Spectroscopy, which use grating spectrometers, were chosen for the spectroscopic set-up. One spectrum was recorded at similar resolution (0.25 nm FWHM) but finer binning (0.035 nm/pixel) than previously published data. For the other (0.59 nm FWHM, 0.154 nm/pixel) no previously published spectra exist. Wavelength calibration is accurate to ±0.04 nm and ±0.11 nm respectively. The absorption cross section for the recordings was determined under low column density with an accuracy of ±4% and ±3% respectively. The absolute absorption cross section of I2 at 500 nm (wavelength: in standard air) in the continuum absorption region was determined using a method independent of iodine vapour pressure. Obtained was σI2 (500 nm)=(2.186±0.021·10-18 cm2 in very good agreement with previously published results, but at 50% smaller uncertainty. From this and previously published results a weighted average of σI2(500 nm)=(2.191±0.02)·10-18 cm2 is determined.

Two-Photon Photosensitized Production of Singlet Oxygen: Optical and Optoacoustic Characterization of Absolute Two-Photon Absorption Cross Sections for Standard Sensitizers in Different Solvents

Singlet molecular oxygen, O2(a1Deltag), can be produced upon resonant two-photon excitation of a photosensitizer. In the present study, two molecules that have received recent attention in studies of nonlinear organic materials were characterized for use as standard two-photon sensitizers: 2,5-dicyano-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene, CNPhVB, and 2,5-dibromo-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene, BrPhVB. Absolute two-photon absorption cross sections, delta, were independently determined for these molecules using two techniques that have heretofore not been applied to this problem: an optical technique (time-resolved detection of O2(a1Deltag) phosphorescence) and a nonoptical technique (a time-resolved laser-induced optoacoustic experiment). For experiments performed in toluene, a solvent commonly used for such nonlinear optical studies, appreciable absorption by the solvent itself complicates the measurements. In cyclohexane, however, delta values could be obtained without the interfering effects of solvent absorption. On the basis of these results, we discuss key aspects of the respective techniques used to quantify values of delta. The information reported herein provides some explanation for the lack of consensus that is routinely observed in published values of delta, certainly for experiments performed in aromatic solvents such as toluene and benzene.

Photofragmentation of the Fluorene Cation: I. New Experimental Procedure Using Sequential Multiphoton Absorption

The hydrogen-loss channel, induced by sequential multiphoton absorption, of the vapor-phase fluorene cation was investigated using a supersonic molecular beam and a time-of-flight mass spectrometer. The fluorene cation was prepared by resonantly enhanced multiphoton ionization. The ultimate goal of this experiment is the determination of the evolution of the dissociation rate constant in a wide energy range. In this paper, we give a description of the original experimental procedure, show that the absorption process is non-Poissonian, and determine the absolute photon absorption cross section.

Photofragmentation of the Fluorene Cation: II. Determination of the H-Loss Energy-Dependent Rate Constant

The hydrogen-loss channel, induced by sequential multiphoton absorption, of the vapor-phase fluorene cation was investigated using a pulsed supersonic molecular beam, a time-of-flight mass spectrometer, and pulsed nanosecond lasers. Our new method leads to the determination of the absolute absorption cross section. Its attenuation with the number of absorbed photons has been approximated by means of statistical models. A model-free determination of the evolution of the dissociation rate constant in a relatively large energy range was obtained by solving the set of coupled differential kinetic equations numerically. Particular attention was focused on the data analysis techniques. The free fit of these rate constants is close to the photothermodissociation statistical model, but shows a discrepancy with the Rice and Ramsperger and Kassel model mainly at high energy. The resulting activation energy is in agreement with both that deduced from the ab initio calculations and that from the tight-binding energy potential surface model.

Photodetachment of fullerene monoanions and dianions in a Penning trap: Probes of delayed electron emission and associated activation barriers

Singly and doubly charged fullerene anions were introduced into gas phase by means of electrospray starting from a solution containing a very efficient electron donor compound. Following isolation and trapping, mass selected anions were irradiated with a ns-pulse from a Nd:YAG laser at the third harmonic ( hν = 3.5 eV) in order to study (delayed) photoemission. We present results on the laser-fluence dependence of photoemission for singly charged C 76 − and C 84 − as well as for doubly charged C 76 2−. Results are discussed in terms of statistical electron emission as opposed to direct detachment. They are further analyzed to give information on the number of photons necessary to induce delayed electron emission on the experimental time scale. The averaged absolute absorption cross-sections obtained were about 0.6–0.8 Å 2 for all species probed. The agreement between experimental data and statistical modelling shows that electronic excitations which rapidly relax to vibrationally hot ions provide the dominant contributions to the 355 nm absorption cross-sections. As a corollary, we show that direct single-electron detachment is a minor process for the singly charged fullerene anions having a cross-section <0.02 Å 2. For C 76 2−, electron emission follows an absorption law in which single-photon excitation is the leading contribution. From the data we deduce a Coulomb barrier height for C 76 2− of 1.20 eV.

Absolute measurement of molecular two-photon absorption cross-sections using a fluorescence saturation technique

We have developed a fluorescence saturation technique for accurate measurements of the absolute molecular two-photon absorption (TPA) cross-section of fluorescent dyes. We determine the TPA crosssection both from measurements at excitation intensities well below saturation onset (in the square power-law regime) and from data obtained near the onset of saturation. The two estimates have different sensitivities to potential sources of errors. Using the square power-law regime requires calibration of the overall collection efficiency of the detection channel, including the quantum yield of the dye. In the saturation regime, the two key requirements are a good knowledge of the excitation profile and an adequate model of the two-photon excitation transition. To fulfill the former requirement, we developed diagnostic tools to characterize the tightly focussed excitation beam. To satisfy the latter requirement, we included the correct polarization dependent averaging over molecular orientations in our model. We measured the TPA cross-section of Rhodamine B (RhB) and Rhodamine 6g (Rh6g) in methanol at 798 nm for linear and circular polarization. For RhB we observed excellent agreement between the TPA cross-section estimate < sigma2 > obtained from the square power-law regime and that obtained from the saturation regime, < sigma2 >(sat). For the case of linear polarization we found: < sigma2 > = 12 +/- 2 GM and < sigma2 >(sat) = 10.5 +/- 2 GM. For the case of circular polarization we obtained: < sigma2 > = 8.4+/-2 GM and < sigma2 >(sat) = 7.5+/-2 GM. The results obtained with linear polarization are in good agreement with previously published non-linear transmission data (delta= 2sigma = 20.4 GM at 800nm). For Rh6g the difference between < sigma2 > and < sigma2 >(sat) is larger, but still considerably smaller than the variance of sigma2 values found in the literature.

Spectroscopic studies of the I 2/O 3 photochemistry : Part 1: Determination of the absolute absorption cross sections of iodine oxides of atmospheric relevance

Multichannel time resolved absorption spectroscopy has been coupled with flash photolysis of mixtures of molecular iodine and ozone to study the spectra and determine absorption cross sections of iodine oxides. Simultaneously, the behaviour of the iodine atoms has been measured by atomic resonance spectroscopy. To separate overlapping molecular absorptions, multivariate analysis techniques have been used to yield the optical density versus time curve at an optimal wavelength for each individual molecular absorber. After the initial photolysis of I 2 and some O 3, it is assumed that the number of iodine atoms contained in the chemical system is invariant in time and that the individual optical densities of all the relevant species are observed. The solution of the resultant over-determined system of linear equations yields the absolute absorption cross sections of the iodine containing molecular species: ground state iodine monoxide, IO(X 2Π 3/2, ν″ = 0), vibrationally excited iodine monoxide, IO(X 2Π 3/2, ν″ > 0), ground state iodine dioxide, OIO( 2B 1), and the spectra of three other iodine oxides, some of which have been observed for the first time in this study. One of these oxides has been tentatively assigned to I 2O 2, and possible assignments of the other two have been discussed. The values of absolute absorption cross sections for selected vacuum wavelengths at 298 K were determined to be: σ IO(4←0)(427.2 nm) = (3.5 ± 0.3) × 10 −17 cm 2, σ IO(3←1)(459.3 nm) = (4.5 ± 0.5) × 10 −17 cm 2, σ IO(1←2)(484.9 nm) = (6.0 ± 0.5) × 10 −17 cm 2, σ OIO(0,5,1←0,0,0)(549.3 nm) = (1.3 ± 0.3) × 10 −17 cm 2 and for an up to now unidentified higher iodine oxide σ(356 nm) ≥ (7.8 ± 1.2) × 10 −19 cm 2 × I atom −1. The spectral resolution of the resultant absorption cross sections is 0.12 nm FWHM in case of IO and 0.35 nm FWHM in case of OIO. Previous determinations of these absorption cross sections have been reviewed.

UV absorption spectra of methyl-substituted hydroxy-cyclohexadienyl radicals in the gas phase

UV absorption spectra of five methyl-substituted hydroxy-cyclohexadienyl radicals, formed by the addition of the hydroxyl radical (OH) to toluene (methyl benzene), o-, m- and p-xylene (1,2-, 1,3- and 1,4-dimethyl benzene, respectively) and mesitylene (1,3,5-trimethylbenzene), have been determined at 298 K, 1 atm pressure (N 2 + O 2), and the corresponding absolute absorption cross-sections measured, using laser flash photolysis and time-resolved UV absorption detection. As observed for other cyclohexadienyl-type radicals, a strong absorption band is present in the 260–340 nm spectral region, with maximum cross-sections in the range (0.9–2.2) × 10 −17 cm 2 molecule −1. The shape of the band varies significantly from one radical to the next for the series of aromatic precursors investigated. The nature and yields of hydroxylated ring-retaining oxidation products, identified in previous studies of the OH-initiated oxidation of aromatic hydrocarbons, and the results of theoretical density functional theory (DFT) calculations indicate that one or more possible isomers of the various OH-adducts may contribute to the observed spectra. Isomers where the OH-group is ortho- (or both ortho- and ipso-) to a substituent methyl-group are likely to be the most abundant but other isomers may also be formed to a significant extent. Nonetheless, the present study provides absorption spectra of the adduct radicals formed from the gas phase addition of OH to the aromatic hydrocarbons considered, near room temperature and 1 atm pressure.

The absorption cross-section and photochemistry of OIO

The absolute absorption cross-section of OIO was measured from 558 to 578 nm by using cavity ring-down spectroscopy to measure the fraction of OIO removed following absorption of a laser pulse of known fluence. This procedure yields σ OIO = (1.51 ± 0.18) × 10 −17 cm 2 at 567.93 nm, which is one of the prominent vibrational band peaks in the OIO spectrum. The recovery of ground-state OIO after a few microseconds indicates that, after excitation from the ground 2B 1 to the (first) excited 2B 2 state, OIO undergoes rapid internal conversion onto high vibrational levels of the 2B 1 state, followed by quenching collisions with the bath gas. A detailed kinetic model is used to show that the OIO yield from the IO self reaction is 0.31 ± 0.10 at 40 Torr and 293 K. The rapid removal of OIO in the reactor is explained by the recombination of atomic I and OIO with a rate constant of (1.1 ± 0.3) × 10 −10 cm 3 molecule −1 s −1. Ab initio calculations combined with RRKM theory are used to show that this rate constant is consistent with the addition of the I atom to the central I, rather than either of the terminal O atoms. The unexpectedly fast disappearance of I atoms, and the corresponding formation of I 2, is explained by iodine oxides such as IO, OIO and I 2O 3 acting as chaperone molecules.