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This paper presents measured fluorescence results for PMMA dye-coated 5 × 5 gold plasmonic nanoantenna arrays. We use numerical electromagnetic modeling to show how array size and element spacing can be used to control emitted beam shape and compare this with experimental data. The Friis formula from RF antenna theory is used to calculate the intensity enhancement produced by the array. A figure of merit is then developed, which accounts for the very small mode volume from which the array emission is occurring.

Bessel beam (BB) optical traps have become widely used to confine single and multiple aerosol particles across a broad range of sizes, from a few microns to < 200 nm in radius. The radiation pressure force exerted by the core of a single, zeroth-order BB incident on a particle can be balanced by a counter-propagating gas flow, allowing a single particle to be trapped indefinitely. The pseudo non-diffracting nature of BBs enables particles to be confined over macroscopic distances along the BB core propagation length; the position of the particle along this length can be finely controlled by variation of the BB laser power. This latter property is exploited to optimize the particle position at the center of the TEM00 mode of a high finesse optical cavity, allowing cavity ring-down spectroscopy (CRDS) to be performed on single aerosol particles and their optical extinction cross section, σext, measured. Further, the variation in the light from the illuminating BB elastically scattered by the particle is recorded as a function of scattering angle. Such intensity distributions are fitted to Lorenz-Mie theory to determine the particle radius. The trends in σext with particle radius are modelled using cavity standing wave Mie simulations and a particle’s varying refractive index with changing relative humidity is determined. We demonstrate σext measurements on individual sub-micrometer aerosol particles and determine the lowest limit in particle size that can be probed by this technique. The BB-CRDS method will play a key role in reducing the uncertainty associated with atmospheric aerosol radiative forcing, which remains among the largest uncertainties in climate modelling.

The 267 nm photodissociation dynamics of p-methylthiophenol (p-MePhSH) and p-methylthioanisole (p-MePhSMe) dissolved in CD3CN have been probed by subpicosecond time-resolved broadband infrared spectroscopy. Prompt (τ < 1 ps) S-H bond fission in p-MePhSH is confirmed by monitoring the time-evolution of the parent (S0) bleach and the transient absorption of the p-MePhS products. Vibrational relaxation of the latter occurs on a ∼8.5 ps time scale, and ∼40% of the total radical population undergoes geminate recombination over a ∼150 ps time scale, yielding (mainly) the p-MePhSH(S0) parent. S-Me bond fission following photoexcitation to the S1 state of p-MePhSMe occurs over a much longer timescale, with a rate that is very dependent on the degree of vibrational excitation within S1. The various findings are compared and contrasted with results from complementary gas-phase photofragmentation studies of both molecules, which are shown to provide a valuable starting point for describing the solution-phase dynamics.

Quasi-classical trajectory calculations on a newly constructed and full-dimensionality potential energy surface (PES) examine the dynamics of the reaction of Cl atoms with propene. The PES is an empirical valence bond (EVB) fit to high-level ab initio energies and incorporates deep potential energy wells for the 1-chloropropyl and 2-chloropropyl radicals, a direct H atom abstraction route to HCl + allyl radical (CH2CHCH2(•)) products (Δ(r)H(298K)(⊖) = −63.1 kJ mol(-1)), and a pathway connecting these regions. In total, 94 000 successful reactive trajectories were used to compute distributions of angular scattering and HCl vibrational and rotational level populations. These measures of the reaction dynamics agree satisfactorily with available experimental data. The dominant reaction pathway is direct abstraction of a hydrogen atom from the methyl group of propene occurring in under 500 fs. Less than 10% of trajectories follow an addition–elimination route via the two isomeric chloropropyl radicals. Large amplitude motions of the Cl about the propene molecular framework couple the addition intermediates to the direct abstraction pathway. The EVB method provides a good description of the complicated PES for the Cl + propene reaction despite fitting to a limited number of ab initio points, with the further advantage that dynamics specific to certain mechanisms can be studied in isolation by switching off coupling terms in the EVB matrix connecting different regions of the PES. ispartof: Journal of Physical Chemistry A vol:119 issue:36 pages:9452-9464 ispartof: location:United States status: published

We describe a parallelized linear-scaling computational framework developed to implement arbitrarily large multi-state empirical valence bond (MS-EVB) calculations within CHARMM and TINKER. Forces are obtained using the Hellmann-Feynman relationship, giving continuous gradients, and good energy conservation. Utilizing multi-dimensional Gaussian coupling elements fit to explicitly correlated coupled cluster theory, we built a 64-state MS-EVB model designed to study the F + CD3CN → DF + CD2CN reaction in CD3CN solvent (recently reported in Dunning et al. [Science 347(6221), 530 (2015)]). This approach allows us to build a reactive potential energy surface whose balanced accuracy and efficiency considerably surpass what we could achieve otherwise. We ran molecular dynamics simulations to examine a range of observables which follow in the wake of the reactive event: energy deposition in the nascent reaction products, vibrational relaxation rates of excited DF in CD3CN solvent, equilibrium power spectra of DF in CD3CN, and time dependent spectral shifts associated with relaxation of the nascent DF. Many of our results are in good agreement with time-resolved experimental observations, providing evidence for the accuracy of our MS-EVB framework in treating both the solute and solute/solvent interactions. The simulations provide additional insight into the dynamics at sub-picosecond time scales that are difficult to resolve experimentally. In particular, the simulations show that (immediately following deuterium abstraction) the nascent DF finds itself in a non-equilibrium regime in two different respects: (1) it is highly vibrationally excited, with ∼23 kcal mol(-1) localized in the stretch and (2) its post-reaction solvation environment, in which it is not yet hydrogen-bonded to CD3CN solvent molecules, is intermediate between the non-interacting gas-phase limit and the solution-phase equilibrium limit. Vibrational relaxation of the nascent DF results in a spectral blue shift, while relaxation of the post-reaction solvation environment results in a red shift. These two competing effects mean that the post-reaction relaxation profile is distinct from what is observed when Franck-Condon vibrational excitation of DF occurs within a microsolvation environment initially at equilibrium. Our conclusions, along with the theoretical and parallel software framework presented in this paper, should be more broadly applicable to a range of complex reactive systems. ispartof: Journal of Chemical Physics vol:143 issue:4 ispartof: location:United States status: published

Ultrafast deactivation pathways bestow photostability on nucleobases and hence preserve the structural integrity of DNA following absorption of ultraviolet (UV) radiation. One controversial recovery mechanism proposed to account for this photostability involves electron-driven proton transfer (EDPT) in Watson-Crick base pairs. The first direct observation is reported of the EDPT process after UV excitation of individual guanine-cytosine (G center dot C) Watson-Crick base pairs by ultrafast time-resolved UV/visible and mid-infrared spectroscopy. The formation of an intermediate biradical species (G[-H]center dot C[+H]) with a lifetime of 2.9 ps was tracked. The majority of these biradicals return to the original G center dot C Watson-Crick pairs, but up to 10% of the initially excited molecules instead form a stable photoproduct G*center dot C* that has undergone double hydrogen-atom transfer. The observation of these sequential EDPT mechanisms across intermolecular hydrogen bonds confirms an important and long debated pathway for the deactivation of photoexcited base pairs, with possible implications for the UV photochemistry of DNA.

A new experiment is presented for the measurement of single aerosol particle extinction efficiencies, Qext, combining cavity ring-down spectroscopy (CRDS, λ = 405 nm) with a Bessel beam trap (λ = 532 nm) in tandem with phase function (PF) measurements. This approach allows direct measurements of the changing optical cross sections of individual aerosol particles over indefinite time-frames facilitating some of the most comprehensive measurements of the optical properties of aerosol particles so far made. Using volatile 1,2,6-hexanetriol droplets, Qext is measured over a continuous radius range with the measured Qext envelope well described by fitted cavity standing wave (CSW) Mie simulations. These fits allow the refractive index at 405 nm to be determined. Measurements are also presented of Qext variation with RH for two hygroscopic aqueous inorganic systems ((NH4)2SO4 and NaNO3). For the PF and the CSW Mie simulations, the refractive index, nλ, is parameterised in terms of the particle radius. The radius and refractive index at 532 nm are determined from PFs, while the refractive index at 405 nm is determined by comparison of the measured Qext to CSW Mie simulations. The refractive indices determined at the shorter wavelength are larger than at the longer wavelength consistent with the expected dispersion behaviour. The measured values at 405 nm are compared to estimates from volume mixing and molar refraction mixing rules, with the latter giving superior agreement. In addition, the first single-particle Qext measurements for accumulation mode aerosol are presented for droplets with radii as small as ∼300 nm.

Mechanisms of bimolecular chemical reactions in solution are amenable to study on picosecond timescales, both by transient absorption spectroscopy and by computer simulation. The dynamics of exothermic reactions of CN radicals and of Cl and F atoms with organic solutes in commonly used solvents are contrasted with the corresponding dynamics in the gas phase. Many characteristics of the gas-phase reaction dynamics persist in solution, such as efficient energy release to specific vibrational modes of the products. However, additional complexities associated with the presence of the solvent are open to investigation. These features of liquid-phase reactions include the role of solvent-solute complexes, solvent caging, coupling of the product motions to the solvent bath, thermalization of internally excited reaction products, incipient hydrogen bond formation, and involvement of charge-separated states that arise from proton transfer.Mechanisms of bimolecular chemical reactions in solution are amenable to study on picosecond timescales, both by transient absorption spectroscopy and by computer simulation. The dynamics of exothermic reactions of CN radicals and of Cl and F atoms with organic solutes in commonly used solvents are contrasted with the corresponding dynamics in the gas phase. Many characteristics of the gas-phase reaction dynamics persist in solution, such as efficient energy release to specific vibrational modes of the products. However, additional complexities associated with the presence of the solvent are open to investigation. These features of liquid-phase reactions include the role of solvent-solute complexes, solvent caging, coupling of the product motions to the solvent bath, thermalization of internally excited reaction products, incipient hydrogen bond formation, and involvement of charge-separated states that arise from proton transferMechanisms of bimolecular chemical reactions in solution are amenable to study on picosecond timescales, both by transient absorption spectroscopy and by computer simulation. The dynamics of exothermic reactions of CN radicals and of Cl and F atoms with organic solutes in commonly used solvents are contrasted with the corresponding dynamics in the gas phase. Many characteristics of the gas-phase reaction dynamics persist in solution, such as efficient energy release to specific vibrational modes of the products. However, additional complexities associated with the presence of the solvent are open to investigation. These features of liquid-phase reactions include the role of solvent-solute complexes, solvent caging, coupling of the product motions to the solvent bath, thermalization of internally excited reaction products, incipient hydrogen bond formation, and involvement of charge-separated states that arise from proton transfer.

The HCl(v' = 0, J') rotational state distributions produced in the photoinitiated reactions of ground spin-orbit state Cl(2P3/2) atoms with the organic heterocycles oxirane (c-C2H4O), thiirane (c-C2H4S) and oxetane (c-C3H6O) have been measured using the technique of 2 + 1 resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. In contrast to previous studies of H-atom abstraction dynamics by Cl atoms with linear functionalized molecules, the nascent HCl(v' = 0, J') products of the reactions of the three-membered heterocycles oxirane and thiirane are formed rotationally cold, with mean rotational energies of (0.28 ± 0.02) kcal mol-1 and (0.31 ± 0.02) kcal mol-1, respectively. The reaction of Cl atoms with the four-membered heterocycle oxetane, however, results in more rotationally excited HCl(v' = 0, J') products for which = (0.78 ± 0.03) kcal mol-1. Ab initio calculations were performed at the G2//MP2/6-311G(d,p) level to characterize molecular complexes and transiti...

A fast Fourier transform (FFT) method for analysis of ring-down decays from a cavity ring-down (CRD) spectrometer has been tested and compared with alternative fitting methods. The ring-down times derived from the FFT method are obtained with a precision close to that of the Levenberg–Marquardt non-linear least-squares method, but the fitting algorithm is ~100 times faster, allowing real-time fitting of individual ring-down traces on a personal computer. Advantages of the FFT method are discussed, and the method is demonstrated for the measurement of NO2 partial pressures equivalent to mixing ratios of 150 pptv and above in laboratory air, using a CRD spectrometer based on an external cavity diode laser operating at wavelengths around 410 nm. The absorption by NO2 is distinguished from other cavity losses either by using synthetic (zero) air reference samples, or by diverting the sampled laboratory airflow through an NO2 chemical scrubber consisting of hydroxyapatite on a TiO2 substrate. Typical mixing ratios of NO2 in the laboratory air are ~25 ppbv.

Aqueous mixtures of photosensitive and inert surfactants have been prepared; photoreactions and changes in aggregation after irradiation have been characterized. The photosensitive component was a stilbene-containing gemini photosurfactant (E-SGP), and the inert surfactants were either DTAB (dodecyltrimethylammonium bromide) or one of two different gemini surfactants, 12-4-12 or 16-4-16 (butanediyl-1,4-bis(dodecyldimethylammonium bromide) or butanediyl-1,4-bis(hexadecyldimethylammonium bromide)). Small-angle neutron scattering (SANS) studies revealed that in general the initial nonirradiated mixed systems form vesicle-type aggregates (100-200 angstroms radius), in equilibrium with some smaller charged spheroidal or ellipsoidal micelles (approximately 20 angstroms radius). In all cases, UV irradiation resulted in disruption of these vesicles and the formation of charged micelles. 1H NMR showed that the main photoproduct is the cis, anti, cis dimer of E-SGP (ZEZ-DiSGP); hence, photochemically induced changes in the reactive SGP drive significant changes in the preferred aggregation structure. These results demonstrate the utility of photoactive surfactants in mixtures with inert analogues.

The rotational state distributions of nascent HCl( v ′ =0) products of the reactions between ground spin–orbit state Cl( 2 P 3/2 ) atoms and CH 4 and CH 3 X (X=F, Cl, Br, I) molecules are reported. Reactions were initiated by photolysis of molecular chlorine at 355 nm and the nascent HCl( v ′ =0) products were probed using 2 + 1 resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. In accord with previous measurements, the HCl( v ′ =0) products for the Cl + CH 4 reaction are rotationally very cold. A much greater degree of rotational excitation is observed in the HCl( v ′ =0) products of the reactions involving the methyl halides, which increases such that CH 3 I 3 Br 3 Cl 3 F, correlating with the magnitudes of the dipole moments of the radical fragment co-products. Ab initio calculations were performed at the G2//MP2/6-311G(d,p) level to characterize molecular complexes and transition states on the reaction pathways for all the reactions studied except Cl + CH 3 I. All proceed over barriers and have weakly bound complexes in the pre- and post-transition state regions of the potential energy surfaces. Comparisons are drawn between the dynamics of these reactions and those of Cl atoms with other organic molecules containing heteroatoms.

The dynamics of H-atom abstraction reactions of Cl atoms with CH 3 Cl and CH 3 Br have been studied using velocity map imaging to obtain center-of-mass frame differential cross sections for HCl formed in its vibrational ground state and rotational levels with J = 2-5. The HCl products are preferentially backward scattered in both reactions, with broad angular distributions of velocities that extend to sideways and forward scatter, and which show almost no variation with J. The products are formed with a wide range of kinetic energies, and the mean fractions of the total available energy becoming product translational energy are 0.25 and 0.23 for the respective reactions. The CH 2 Cl and CH 2 Br are formed with considerable internal excitation, estimated to correspond, on average, to fractions of the total available energy of 0.69 and 0.72, respectively. The scattering dynamics are interpreted with the aid of ab initio calculations of the minimum energy pathways for the two reactions, which exhibit low barriers late on the reaction coordinate.

The dynamics of Cl atom reactions with methane, ethane, and methanol have been studied by calculation of quasi-classical trajectories, with computation of potential energies and gradients only at the geometries through which the trajectories pass. Trajectories were started from the transition state, with 2 kcal mol(-1) of energy given to the mode with an imaginary frequency (representing the reaction coordinate at the transition state) and inclusion of zero-point energy in some or all of the remaining vibrational modes. The trajectories were propagated as far as separated products, with the majority of potential energy calculations performed at the HF/6-31G level of theory. The rotational quantum state population distributions of the HCl products from the reactions of Cl atoms with methane, ethane and methanol peaked at J'=1, 2, and 6, respectively. The calculations thereby exhibit somewhat greater rotational excitation than is found experimentally, but correctly describe the trend of increasing HCl product rotation for the three respective reactions. In agreement with previous observations, only 4% of the energy available to the products of the reaction of Cl atoms with methane was channeled into CH3 radical internal energy, and 1% into HCl rotation, with 92% ending up as translational energy. For the reaction of Cl atoms with ethane and with methanol, the corresponding values for radical internal energy, HCl rotation and product translation are 21, 3, and 78%, and 46, 13, and 42%, respectively. For the latter two reactions, the radical internal energy is mostly accounted for by rotational motion. The clear increase in rotational excitation of the HCl products from the Cl atom reaction with methanol is explained in terms of a dipole-dipole interaction between the departing polar fragments. A smaller set of more computationally expensive trajectory calculations using potentials and gradients from the MP2/6-311G(d,p) level of theory were performed for reactions of Cl atoms with methanol, and give results in better agreement with experimentally measured HCl rotational excitation, consistent with the model of dipole-induced product rotation because the MP2/6-311G(d,p) calculations give smaller dipole moments for both products than the HF/6-31G calculations. The calculated angles between the rotational angular momentum vectors and recoil velocities of the radical peak sharply at 90 degrees for the reactions of Cl atoms with ethane and methanol, but exhibit a much broader distribution for reaction with methane.

Absorption cross sections are reported for several rotationally resolved lines in the QQ-branch of the ν5 + ν9 combination vibrational band of ethene (ethylene) at wavelengths around 1.625 μm. The measurements were made using cavity ring-down spectroscopy with a near infra-red diode laser. In the strongest region of the band, a line-integrated absorption cross section for the QQ4(4) feature of 5.08 ± 0.38 × 10−22 cm2 molecule−1 cm−1 is obtained, with a peak cross section at line centre of 3.23 ± 0.03 × 10−20 cm2 molecule−1. The effects of addition of pressures of up to 200 Torr of air on spectral linewidths are analysed to obtain an air broadening coefficient (HWHM) of γ = 0.1060 ± 0.0011 cm−1 atm−1. In 1 atm of air, the line centre absorption cross sections in the QQ4 branch are reduced by a factor of ∼8 because of line broadening. The consequences of these data for direct monitoring of ethene in the atmosphere using sensors based on diode laser cavity ring-down spectroscopy are discussed.

Absorption spectroscopy measurements are reported of the absolute number density (3.0 ± 0.9 × 10 12 cm −3 ) and vibrational temperature (3000 ± 500 K) of C 2 (X 1 Σ g + ) radicals in a DC arc activated chemical vapour deposition reactor under the standard operating conditions (3.3% CH 4 in H 2 and 6.4 kW discharge) used to grow polycrystalline diamond films. The ratio of number densities of C 2 radicals in their ground (X 1 Σ g + ) and low-lying electronically excited (a 3 Π u ) states is close to that expected from a Boltzmann distribution at the measured temperature, despite the more rapid rates of reaction of C 2 (X) with H 2 and hydrocarbon molecules.

Continuous-wave cavity ring-down spectroscopy (CW-CRDS) with an external cavity diode laser has been used to make diagnostic measurements of acetylene absorption in hot-filament and DC-arc plasma jet reactors operating with 1% CH 4 in H 2 and ≤0.8% CH 4 /H 2 in excess Ar, respectively, and used for diamond chemical vapour deposition. The acetylene was widely distributed throughout the reactor, extending well beyond the regions of highest thermal activation as a result of diffusion processes. The average acetylene concentration was determined as (2.9±0.6)×10 13 molecules cm −3 in the hot filament reactor and (1.2±0.2)×10 14 molecules cm −3 in the DC arc jet reactor, accounting for ∼1.7% and 40% of the total C budget in the respective cases. The average acetylene gas temperature along the viewing column was 550±150 K in both environments. Variations in the concentration and temperature of acetylene with the gas feed composition and input power processing conditions for the two reactors are presented. The experiments demonstrate the effectiveness of near-IR diode laser CRDS as a probe of highly luminous environments.

Diode laser cavity ring-down spectroscopy (CRDS) has been used to measure rotational line intensities and air pressure broadening coefficients for the 3 ν 3 (N–N stretching) vibrational overtone band of N 2 O. The line intensity measurements are in quantitative agreement with previously reported Fourier transform spectroscopy results [J. Mol. Spectrosc. 197 (1999) 158]. The air pressure broadening coefficients increase from 0.086 ± 0.015 cm −1 atm −1 for the P (34) line to 0.139 ± 0.010 cm −1 atm −1 for the P (3) line, and are greater than values reported for other ro-vibrational bands of N 2 O observed at lower frequencies. The effects of pressure broadening on line-centre absorption cross-sections, and thus for the sensitivity of CRDS detection of N 2 O using narrow bandwidth near-IR lasers are quantified.

Experimental and theoretical studies of the behaviour of hydrocarbon species in d.c.-arc jet chemical vapour deposition reactors are reported, as a function of carbon source gas flow rate. CH(X) and C (a) radical number densities have been measured in 2 absorption (by cavity ring-down spectroscopy) and via their optical emission in an arc jet plume operating with a standard CH y 4 H yAr feedstock gas mixture. The C (a) radical number density is seen to exhibit a linear (or sub-linear) dependence on CH 22 4 flow rate, in accord with previous findings (J. Appl. Phys. 82 (1997) 2072) for both C (a) and C (X) radicals in a lower power 23 d.c.-arc jet. The present findings, together with the comprehensive set of earlier experimental data on gas velocity and gas temperature measurements (Diam. Relat. Mater. 7 (1998) 165; Plasma Sources Sci. Technol. 10 (2001) 595) have been used in developing a model of the plasma plume. The present calculations suggest that the observed high diamond growth rates ()50– 100 mmyh) are most probably related to atomic C, which is present at concentrations as high as 10 cm , though C species 14 y3 2

The detection of NO2 at sub part per billion by volume (ppbv) concentrations has been demonstrated using a cavity ring-down spectrometer with a blue diode laser operating at wavelengths close to 410 nm. Measurements at peaks in the absorption spectrum at wavelengths of 410.4955 and 410.3533 nm demonstrate a minimum detectable number density of NO2 of 9.8×109 molecules cm−3, which would correspond to 0.4 ppbv under atmospheric conditions in the lower troposphere. Additional experiments performed with a pulsed, tuneable dye laser system provide absorption cross-sections for NO2 over the restricted wavelength ranges of 408.5–410.5 and 435.0–435.5 nm, but at a resolution of ∼ 0.06 cm −1 that is higher than previously reported and close to the Doppler broadening limit at 295 K. These spectra show considerably more structure than is evident in published Fourier transform (FT) spectra obtained at lower resolution, but yield cross-sections that are in quantitative agreement with the FT measurements.

Deuterium fluoride gets born shivering Modern spectroscopic techniques can analyze collisions between gas phase molecules in exquisite detail, highlighting exactly which vibrations and rotations come into play. However, much chemistry of interest takes place in solution, where it's harder to tease out what happens. Dunning et al. applied infrared spectroscopy to study solution-phase formation of deuterium fluoride (DF) from F atoms, a longstanding test bed of gas phase dynamics. The DF product vibrated for a surprisingly long time before dissipating its energy to the surrounding solvent molecules. Science , this issue p. 530

The rotationally inelastic scattering of methyl radical with Ar and N2 is examined at collision energies of 330 ± 25 cm(-1) and 425 ± 50 cm(-1), respectively. Differential cross sections (DCSs) were measured for different final n' rotational levels (up to n' = 5) of the methyl radicals, averaged over k' sub-levels, using a crossed molecular beam machine with velocity map imaging. For Ar as a collision partner, we present a newly constructed ab initio potential energy surface and quantum mechanical scattering calculations of state-resolved DCSs. These computed DCSs agree well with the measurements. The DCSs for both Ar and N2 collision partners are strongly forward peaked for all spectroscopic lines measured. For scattering angles below 60°, the theoretical CD3-Ar DCSs show diffraction oscillations that become less pronounced as n' increases, but these oscillations are not resolved experimentally. Comparisons are drawn with our recently reported DCSs for scattering of methyl radicals with He atoms.

Criegee intermediates are important species formed during the ozonolysis of alkenes. Reaction of stabilized Criegee intermediates with various species like SO2 and NO2 may contribute significantly to tropospheric chemistry. In the laboratory, self-reaction can be an important loss pathway for Criegee intermediates and thus needs to be characterized to obtain accurate bimolecular reaction rate coefficients. Cavity ring-down spectroscopy was used to perform kinetic measurements for various reactions of CH2OO at 293 K and under low pressure (7 to 30 Torr) conditions. For the reaction CH2OO + CH2OO (8), a rate coefficient k8 = (7.35 ± 0.63) × 10(-11) cm(3) molecule(-1) s(-1) was derived from the measured CH2OO decay rates, using an absorption cross section value reported previously. A rate coefficient of k4 = (3.80 ± 0.04) × 10(-11) cm(3) molecule(-1) s(-1) was obtained for the CH2OO + SO2 (4) reaction. An upper limit for the unimolecular CH2OO loss rate coefficient of 11.6 ± 8.0 s(-1) was deduced from studies of reaction (4). SO2 catalysed CH2OO isomerization or intersystem crossing is proposed to occur with a rate coefficient of (3.53 ± 0.32) × 10(-11) cm(3) molecule(-1) s(-1).

We report a theoretical investigation of the CH4 + Cl hydrogen abstraction reaction in the framework of empirical valence bond (EVB) theory. The main purpose of this study is to benchmark the EVB method against previous experimental and theoretical work. Analytical potential energy surfaces for the reaction have been developed on which quasi-classical trajectory calculations were carried out. The surfaces agree well with ab initio calculations at stationary points along the reaction path and dynamically relevant regions outside the reaction path. The analysis of dynamical data obtained using the EVB method, such as vibrational, rotational, and angular distribution functions, shows that this method compares well to both experimental measurements and higher-level theoretical calculations, with the additional benefit of low computational cost. ispartof: Journal of Physical Chemistry A vol:119 issue:37 pages:9590-9598 ispartof: location:IL, Champaign status: published

Cavity ring-down spectroscopy has been used to measure the absorbances, concentrations and temperatures of C2(a3Πu v=0) and CH(X2Π v=0) radicals in a ⩽10 kW dc arc jet used for chemical vapor deposition (CVD) of polycrystalline diamond films and operated with CH4/H2/Ar gas mixtures. Gas temperatures derived from the intensities and widths of rotationally resolved C2 d3Πg−a3Πu spectral lines are 3300±200 K in the free plume, rising to ∼4800 K close to the substrate on which the diamond film is grown. For an input power of ∼6 kW, a 3.3% CH4/H2 ratio with excess Ar and a pressure of 50 Torr, the conditions typically employed for diamond film CVD, concentrations of C2(a) are between 6.0±0.2×1012 and 1.5±0.2×1013 cm−3 in the free plume at distances >5 mm from the substrate. These values are derived assuming a 1 cm column length as implied by spatially resolved studies of C2(d−a) optical emission; the spread reflects the decline in performance of the arc jet torch heads over time. The concentration of CH(X) in ...

The dynamics of ground state Cl(2P3/2) atom reactions with methanol, methanol-d1, ethanol, and dimethyl ether have been studied both experimentally and theoretically. The reactions were photoinitiated by 355 nm photolysis of Cl2 to produce monoenergetic Cl(2P3/2) atoms that react with ground electronic state organic molecules under single collision conditions. The rotational quantum state population distributions of the nascent HCl(ν′) products were probed by 2+1 resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. Nascent HCl(ν′=0) products from reaction of Cl atoms with methanol, methanol-d1 (CH3OD), ethanol, and dimethyl ether, at mean collision energies in the range of 5.6–6.7 kcal/mol, exhibit distributions of population over rotational levels that all peak at J′=3–5. The average rotational energies of the HCl(ν′=0) products for the respective reactions are 〈Erot〉=330±29, 300±24, 340±24, and 256±17 cm−1 (1σ uncertainties). Ab initio calculations were performed in order to ...

The photodissociation dynamics of jet-cooled BrCl molecules have been investigated at four different wavelengths in the range 425–485 nm by high-resolution velocity map ion imaging. Four images of the Cl(2P3/2) atomic fragments are recorded at each photolysis wavelength with the probe laser polarization, respectively, linearly aligned and vertical (i.e., perpendicular to the detection axis), right circularly polarized, horizontally linearly polarized (i.e., parallel to the detection axis) and left circularly polarized on successive laser shots, thereby ensuring automatic mutual self-normalization. Appropriate linear combinations of these images allow quantification of the angular momentum alignment of the Cl(2P3/2o) fragments [i.e., the correlation between their recoil velocity (v) and their electronic angular momentum (J)] in terms of the alignment anisotropy parameters s2, α2, η2, and γ2, and determination of the “alignment-free” recoil anisotropy parameter, β0, as a function of parent excitation wavelength. Both incoherent and coherent contributions to the alignment are identified, with both simultaneous parallel and perpendicular excitations to the B 3Π(0+) and C 1Π(1) states and excitations to the Ω=±1 components of the C state contributing to the latter. The deduced values of the alignment-free β parameters indicate (wavelength dependent) contributions from both parallel and perpendicular parent absorptions in this wavelength range. Such a conclusion accords with approximate deconvolutions of the parent absorption spectrum that are currently available, and with determinations of the orientation parameter γ1′ obtained by fitting the difference image obtained when using left and right circularly polarized radiation to probe the ground state Cl atoms arising in the 480.63 nm photodissociation of BrCl when the photolysis laser radiation is polarized linearly at 45° to the detection axis.

Resonance enhanced multiphoton ionization and cavity ring down spectroscopies have been used to provide spatially resolved measurements of relative H atom and CH3 radical number densities, and NH column densities, in a hot filament (HF) reactor designed for diamond chemical vapor deposition and here operating with a 1% CH4/n/H2 gas mixture—where n represents defined additions of N2 or NH3. Three-dimensional modeling of the H/C/N chemistry prevailing in such HF activated gas mixtures allows the relative number density measurements to be placed on an absolute scale. Experiment and theory both indicate that N2 is largely unreactive under the prevailing experimental conditions, but NH3 additions are shown to have a major effect on the gas phase chemistry and composition. Specifically, NH3 additions introduce an additional series of “H-shift” reactions of the form NHx+H⇌NHx−1+H2 which result in the formation of N atoms with calculated steady state number densities >1013 cm−3 in the case of 1% NH3 additions in ...

~Received 5 February 2002; accepted 19 March 2002! Orientation and alignment parameters have been computed from first principles for the photodissociation of the HF and DF diatomic molecules. The calculations are entirely ab initio and the break-up dynamics of the molecule is treated rigorously taking account of the electronically nonadiabatic dynamics on three coupled adiabatic electronic potential energy curves. The potential energy curves and spin‐orbit interactions, which have been previously reported @J. Chem. Phys. 113, 1870 ~2000!#, are computed using ab initio molecular electronic structure computer codes. These are then used to compute photofragmentation T matrix elements using a time-dependent quantum mechanical wave packet treatment and from these a complete set of anisotropy parameters with rank up to K53 is computed. The predicted vector correlations and alignment parameters are presented as a function of energy for HF and DF initially in both their ground and first excited vibrational states. The parameters predicted for the molecules which are initially in their excited vibrational states display a pronounced sharp energy dependence arising from the nodal structure of the initial vibrational wavefunction. The theoretical results are analyzed using a simple model of the dynamics and it is demonstrated how the magnitude and relative phases of the photofragmentation T matrix elements can be deduced from the experimentally measured alignment parameters. No experimental measurements have yet been made of alignment parameters for hydrogen halide diatomics and the present results provide the first predictions of these quantities which may be compared with future experimental observations. © 2002 American Institute of Physics. @DOI: 10.1063/1.1476937#

The use of continuous wave cavity ring-down spectroscopy (cw CRDS) is demonstrated for the detection of tropospheric methane. Spectra of the 2ν3 overtone band of methane at wavelengths near 1.65 μm have been recorded in both laboratory air and under reduced pressures, and show quantitative agreement with absorption intensities and pressure-broadened line widths reported in the HITRAN 96 database. Experiments demonstrate a minimum detectable absorption coefficient that is currently αmin = 1.5 × 10−8 cm−1, equivalent to a number density of CH4 at ambient temperature of 2.9 × 1011 molecules cm−3 in a low pressure environment where Doppler broadening dominates the spectral lineshapes. The effects of pressure broadening reduce the detection limit of CH4 in 1 atm of air to 1.3 × 1012 molecules cm−3, corresponding to 52 parts per billion by volume. This is well below the 1.72 parts per million by volume average methane concentration in the lower troposphere.

Rotationally inelastic scattering of ND3 with Ar is studied at mean collisionenergies of 410 and 310 cm−1. In the experimental component of the study, ND3 molecules are prepared by supersonic expansion and subsequent hexapole state selection in the ground electronic and vibrational levels and in the jk±=11− rotational level. A beam of state-selected ND3 molecules is crossed with a beam of Ar, and scattered ND3 molecules are detected in single final j′k′± quantum states using resonance enhanced multiphoton ionization spectroscopy. State-to-state differential cross sections for rotational-levelchanging collisions are obtained by velocity map imaging. The experimental measurements are compared with close-coupling quantum-mechanical scattering calculations performed using an ab initio potential energy surface. The computed DCSs agree well with the experimental measurements, confirming the high quality of the potential energy surface. The angular distributions are dominated by forward scattering for all measured final rotational and vibrational inversion symmetry states. This outcome is in contrast to our recent results for inelastic scattering of ND3 with He, where we observed significant amount of sideways and backward scattering for some final rotational levels of ND3. The differences between He and Ar collision partners are explained by differences in the potential energy surfaces that govern the scattering dynamics.

The photoisomerization dynamics of N-methyl-2-pyridone (NMP) dissolved in CH3CN have been interrogated by time-resolved electronic and vibrational absorption spectroscopy. Irradiation at two different wavelengths (330 or 267 nm) prepares NMP(S1) molecules with very different levels of vibrational excitation, which rapidly relax to low vibrational levels of the S1 state. Internal conversion with an associated time constant of 110(4) ps, leading to reformation of NMP(S0) molecules, is identified as the dominant (>90%) decay pathway. Much of the remaining fraction undergoes a photoinitiated rearrangement to yield two ketenes (revealed by their characteristic antisymmetric C═C═O stretching modes at 2110 and 2120 cm(-1)), which are in equilibrium. The rate of ketene formation is found to be pump-wavelength dependent, consistent with ab initio electronic structure calculations which predict a barrier on the S1 potential energy surface en route to a prefulvenic conical intersection, by which isomerization is deduced to occur. Two kinetic models-differentiated by whether product branching occurs in the S1 or S0 electronic states-are presented and used with equal success in the analysis of the experimental data, highlighting the difficulties associated with deducing unambiguous mechanistic information from kinetic data alone.

The heterocyclic ring-opening dynamics of thiophenone and furanone dissolved in CH3CN have been probed by ultrafast transient infrared spectroscopy. Following irradiation at 267 nm (thiophenone) or 225 nm (furanone), prompt (τ1 ps) ring-opening is confirmed by the appearance of a characteristic antisymmetric ketene stretching feature around 2150 cm(-1). The ring-opened product molecules are formed highly vibrationally excited, and cool subsequently on a ∼6.7 ps timescale. By monitoring the recovery of the parent (S0) bleach, it is found that ∼60% of the initially photoexcited thiophenone molecules reform the parent molecule, in stark contrast with the case in furanone where there is less than 10% parent bleach recovery. Complementary ab initio calculations of potential energy cuts along the S-C([double bond, length as m-dash]O) and O-C([double bond, length as m-dash]O) ring-opening coordinate reveals insights into the reaction mechanism, and the important role played by dissociative (n/π)σ* states in the UV-induced photochemistry of such heterocyclic systems.

A combination of ultrafast transient electronic absorption spectroscopy (TEAS) and transient vibrational absorption spectroscopy (TVAS) is used to investigate whether photoinduced N–H bond fission, mediated by a dissociative 1πσ(*) state, is active in aqueous adenine (Ade) at 266 and 220 nm. In order to isolate UV/visible and IR spectral signatures of the adeninyl radical (Ade[-H]), formed as a result of N–H bond fission, TEAS and TVAS are performed on Ade in D2O under basic conditions (pD = 12.5), which forms Ade[-H](-) anions via deprotonation at the N7 or N9 sites of Ade's 7H and 9H tautomers. At 220 nm we observe one-photon detachment of an electron from Ade[-H](-), which generates solvated electrons (eaq(-)) together with Ade[-H] radicals, with clear signatures in both TEAS and TVAS. Additional wavelength dependent TEAS measurements between 240–260 nm identify a threshold of 4.9 ± 0.1 eV (∼250 nm) for this photodetachment process in D2O. Analogous TEAS experiments on aqueous Ade at pD = 7.4 generate a similar photoproduct signal together with eaq(-) after excitation at 266 and 220 nm. These eaq(-) are born from ionization of Ade, together with Ade(+) cations, which are indistinguishable from Ade[-H] radicals in TEAS. Ade(+) and Ade[-H] are found to have different signatures in TVAS and we verify that the pD = 7.4 photoproduct signal observed in TEAS following 220 nm excitation is solely due to Ade(+) cations. Based on these observations, we conclude that: (i) N–H bond fission in aqueous Ade is inactive at wavelengths ≥220 nm; and (ii) if such a channel exists in aqueous solution, its threshold is strongly blue-shifted relative to the onset of the same process in gas phase 9H-Ade (≤233 nm). In addition, we extract excited state lifetimes and vibrational cooling dynamics for 9H-Ade and Ade[-H](-). In both cases, excited state lifetimes of500 fs are identified, while vibrational cooling occurs within a time frame of 4–5 ps. In contrast, 7H-Ade is confirmed to have a longer excited state lifetime of ∼5–10 ps through both TEAS and TVAS.

To resolve some of the significant uncertainties in the impact of aerosols on global climate, new tools are required to probe light scattering and absorption by aerosol particles. Ideally, such tools should allow direct measurements on individual particles over extended periods of time, providing data to better constrain the optical properties of aerosol, how they depend on the environmental conditions (relative humidity and temperature) and how they change with time. Here, we present a new technique using a combination of a Bessel beam to manipulate individual particles and cavity ringdown spectroscopy for ultrasensitive measurements of the optical extinction. We show that particles can be spatially separated along the propagation direction of a Bessel beam according to their size and refractive index when confined by a Bessel beam core and a counter-propagating gas flow, referred to as optical chromatography. The time-dependent position of a particle is shown to be a consequence of the differing size dependencies of the forces arising from Stokes drag and radiation pressure. We also show that particles captured in a Bessel beam can be moved in and out of an optical cavity formed by two highly reflective mirrors. The time constant for the ringdown in light coupled within the cavity can then be used to measure the optical cross-section of the individual particle with high accuracy. An individual particle can be captured indefinitely and its change in optical cross-section measured with change in environmental conditions.

Bimolecular reactions in the gas phase exhibit rich and varied dynamical behaviour, but whether a profound knowledge of the mechanisms of isolated reactive collisions can usefully inform our understanding of reactions in liquid solutions remains an open question. The fluctuating environment in a liquid may significantly alter the motions of the reacting particles and the flow of energy into the reaction products after a transition state has been crossed. Recent experimental and computational studies of exothermic reactions of CN radicals with organic molecules indicate that many features of the gas-phase dynamics are retained in solution. However, observed differences may also provide information on the ways in which a solvent modifies fundamental chemical mechanisms. This perspective examines progress in the use of time-resolved infra-red spectroscopy to study reaction dynamics in liquids, discusses how existing theories can guide the interpretation of experimental data, and suggests future challenges for this field of research. (C) 2014 AIP Publishing LLC.

The extinction cross-sections of individual, optically confined aerosol particles with radii of a micrometer or less can, in principle, be measured using cavity ring-down spectroscopy (CRDS). However, when the particle radius is comparable in magnitude to the wavelength of light stored in a high-finesse cavity, the phenomenological cross-section retrieved from a CRDS experiment depends on the location of the particle in the intracavity standing wave and differs from the Mie scattering cross-section for plane-wave irradiation. Using an evaporating 1,2,6-hexanetriol particle of initial radius ∼1.75 μm confined within the 4.5 μm diameter core of a Bessel beam, we demonstrate that the scatter in the retrieved extinction efficiency of a single particle is determined by its lateral motion, which spans a few wavelengths of the intracavity standing wave used for CRDS measurements. Fits of experimental measurements to Mie calculations, modified to account for the intracavity standing wave, allow precise retrieval of the refractive index of 1,2,6-hexanetriol particles (with relative humidity, RH < 10%) of 1.47824 ± 0.00072.

A single horizontally-propagating zeroth order Bessel laser beam with a counter-propagating gas flow was used to confine single fine-mode aerosol particles over extended periods of time, during which process measurements were performed. Particle sizes were measured by the analysis of the angular variation of light scattered at 532 nm by a particle in the Bessel beam, using either a probe beam at 405 nm or 633 nm. The vapour pressures of glycerol and 1,2,6-hexanetriol particles were determined to be 7.5 +/- 2.6 mPa and 0.20 +/- 0.02 mPa respectively. The lower volatility of hexanetriol allowed better definition of the trapping environment relative humidity profile over the measurement time period, thus higher precision measurements were obtained compared to those for glycerol. The size evolution of a hexanetriol particle, as well as its refractive index at wavelengths 532 nm and 405 nm, were determined by modelling its position along the Bessel beam propagation length while collecting phase functions with the 405 nm probe beam. Measurements of the hygroscopic growth of sodium chloride and ammonium sulfate have been performed on particles as small as 350 nm in radius, with growth curves well described by widely used equilibrium state models. These are the smallest particles for which single-particle hygroscopicity has been measured and represent the first measurements of hygroscopicity on fine mode and near-accumulation mode aerosols, the size regimes bearing the most atmospheric relevance in terms of loading, light extinction and scattering. Finally, the technique is contrasted with other single particle and ensemble methods, and limitations are assessed.

The photodissociation of jet-cooled IBr molecules has been investigated at numerous excitation wavelengths in the range 440–685 nm using a state-of-art ion imaging spectrometer operating under optimal conditions for velocity mapping. Image analysis provides precise threshold energies for the ground, I(2P3/2)+Br(2P3/2), and first excited [I(2P3/2)+Br(2P1/2)] dissociation asymptotes, the electronic branching into these two active product channels, and the recoil anisotropy of each set of products, as a function of excitation wavelength. Such experimental data have allowed mapping of the partial cross-sections for parallel (i.e., ΔΩ=0) and perpendicular (i.e., ΔΩ=±1) absorptions and thus deconvolution of the separately measured (room temperature) parent absorption spectrum into contributions associated with excitation to the A 3Π(1), B 3Π(0+) and 1Π(1) excited states of IBr. Such analyses of the continuous absorption spectrum of IBr, taken together with previous spectroscopic data for the bound levels suppor...

This paper extends the knowledge of highly excited Rydberg states of HBr by presenting detailed measurements of the 3 + 1 resonance enhanced multiphoton ionisation (REMPI) spectrum of H81Br molecules obtained following excitation in the wavelength range 366–316 nm, i.e. at energies up to the first ionisation energy. Spectra have been recorded using both linearly and circularly polarised laser light. 34 vibronic bands have been analyzed, of which 13 are identified as new Rydberg origins (′ = 0) and 5 as new members of vibrational progressions (′≠0) originating from the promotion of a π electron into a σ, π or δ Rydberg orbital and correlating with the ground X2Π state of the HBr+ ion core. Symmetry assignments in terms of either (Λ, S) or (Ω, ω) coupling schemes for the various identified excited states are offered, based on band contour simulation and quantum defect considerations. The main result of the present work is the observation of new states and bands involving vibrationally excited levels which were not accessible in previous one and two-colour spectra. In particular, the three-photon selection rules allow observation of the ′ = 0 and 1 levels of the L1Φ3 state arising from the dδ Rydberg manifold, as well as three other Ω = 3 states within the dπ and dδ series which are reported here for the first time.

Cavity ring-down spectra of the FCO radical, recorded over the wave number range 29 500–31 600 cm−1 reveal rotational structure of the electronically excited state for the first time. The spectra demonstrate the need for a complete re-assignment of the vibronic features: The rotationally resolved bands are successfully simulated as arising from c-type transitions from the ground X 2A′ state to the linear 2A″ component of the A 2Π state. The bands are attributed to two overlapping vibrational progressions: one progression involves excitation of the F–C–O bending mode (v3′), the other consists of a combination of v3′ and one quantum of the C–F stretch (v2′). Sharp rotational structure is only observed for sub-bands with K′=0; bands with K′>0 are diffuse, indicating rapid, rotation induced predissociation. Band origins, rotational constants for the excited state, and spectral linewidths have been derived from the K′=0–K″=1 sub-bands. All rotational lines are somewhat broadened and there is evidence of linew...

The electric-dipole forbidden a1Δg − X3 (0,0) band of gas-phase O2 has been studied in absorption at wavelengths around 1.27 μm using Fourier transform spectroscopy and a long-path absorption cell. Experiments were conducted at temperatures of 294, 243, and 200 K and at pressures in the range 140−750 Torr. Both pure O2 and a mixture of 21% O2/79% N2 were studied, and line widths, integrated line intensities, and integrated absorption intensities (AIs) for the (0,0) vibrational band were measured. Integrated AIs were found to be independent of temperature, pressure, and gas composition, and the recommended value for the vibrational band from the current study is Sint = 3.210(15) × 10-24 cm molecule-1 (1σ error) for pure 16O2, corresponding to an Einstein A-coefficient of A = 2.256(10) × 10-4 s-1. The effect of including other oxygen isotopomers is to increase the integrated AI value for this origin band to Sint = 3.226(15) × 10-24 cm molecule-1. Widths of individual spectroscopic lines decrease with increa...

An electronic absorption spectrum of FO has yet to be observed experimentally but as a guide for future spectroscopic studies of this radical, CASSCF/MRCI ab initio calculations have been performed to locate and identify the lowest energy valence and Rydberg states. No bound valence excited potentials were found save the a4Σ- state. The calculations provide estimates of the transition energies Te to the lowest Rydberg states and the wavelengths required for spectroscopic detection. Results for both the doublet and quartet states are presented, revealing the 3sσ, 3pπ, 3pσ, 3dδ, 4sσ, 3dπ, and 4dδ Rydberg orbitals. It is likely that the repulsive valence states 12Σ- and 12Δ predissociate the lowest Rydberg states of the same symmetry. Consequently, the 2Π Rydberg states, three of which are found here, are the most promising candidates for optical detection.

Experimental and theoretical methods have been applied to investigate the effect of internal parent excitation on the ultraviolet photodissociation dynamics of HCl (X 1Σ+) molecules. Jet-cooled H35Cl molecules within a time-of-flight mass spectrometer were prepared by infra-red absorption in the following quantum states: v=1, J=0 and J=5; v=2, J=0 and J=11; v=3, J=0 and J=7. The excited molecules were then photodissociated at λ∼235 nm and the Cl(2Pj) photofragments detected using (2+1) resonance enhanced multiphoton ionization. The results are presented as the fraction of total chlorine yield formed in the spin–orbit excited state, Cl(2P1/2). The experimental measurements are compared with the theoretical predictions from a time-dependent, quantum dynamical treatment of the photodissociation dynamics of HCl (v=1−3, J=0). These calculations involved wavepacket propagation using the ab initio potential energy curves and coupling elements previously reported by Alexander, Pouilly, and Duhoo [J. Chem. Phys. 9...

The spectroscopy and photochemistry of HI have been studied at excitation energies in the vicinity of H-atom Lyman–α (82259 cm−1) using the techniques of 1-photon vacuum UV photodissociation, resonance-enhanced multiphoton ionization (REMPI) spectroscopy and photofragment ion imaging. REMPI spectra, recorded using UV photons in the energy range 27350–27750 cm−1, exhibit complicated structure in channels corresponding to H+, I+ and HI+ ion formation. The former two channels require absorption of a minimum of 5 UV photons, whereas HI+ X 2Π formation requires 4 UV photons. The spectral features are assigned to resonances at the 3-photon energy involving six states with Ω = 0 and one state with Ω = 3. Ion images recorded in the same range of UV frequencies exhibit multiple velocity components for the H+ ionic fragment, and the excitation energy dependences of the ring radii reveal two mechanisms for H+ formation. The first route is 3 + 1 photon excitation to super-excited Rydberg states of neutral HI with [A 2Σ+] cores that autoionize to high vibrational levels (+15) of HI+ (X 2Π); subsequent 1-photon photolysis of these vibrationally excited ions makes H+. The second route is dissociation of 4-photon excited HI to H(n = 2) + I(2P3/2), followed by 1-photon ionization of the H(n = 2) atoms. Vacuum UV photolysis of HI at 121.6 nm combined with H-Rydberg atom photofragment translational spectroscopy reveals dissociation ia four different channels: H(n = 1) atoms are formed in association with I(5p5, 2P3/2), I(5p5, 2P1/2), I(5p4 6s1, 4P5/2) and I(5p4 6s1, 2P3/2). Anisotropy parameters for all these channels are positive but non-limiting, and photolysis propensities show a marked dependence on the initial rotational state of the HI. These observations are discussed in terms of excitation to predissociated Rydberg or ion-pair states identified in the REMPI spectra. A previously unreported Ω = 3 Rydberg state is identified ia HI+ formation following multiphoton absorption resonant at the 2 UV + 1 visible photon energy. A band with origin at 68326.18(73) cm−1, is assigned to excitation to the ′ = 0 level of a 3Δ3 state composed of a Rydberg dπ electron orbiting an HI+ X 2Π3/2 core.

The photodissociation of jet-cooled DCl molecules subsequent to excitation in the long-wavelength tail of the first UV absorption band (A1Π1←X1Σ+) has been investigated at five wavelengths in the range 200–220 nm. Ground state Cl(2P3/2) and spin–orbit excited Cl∗(2P1/2) photofragments were monitored using (2+1) resonance enhanced multiphoton ionization in a time-of-flight mass spectrometer. The product branching fractions are reported and compared with previous experimental results and high-level quantum mechanical calculations for HCl and DCl. A significant H/D isotope effect in the branching fractions is found at all the studied wavelengths, in quantitative agreement with recent theoretical predictions.

We demonstrate the application of cavity ring-down spectroscopy (CRDS) to the measurement of concentrations of IO radical and of reaction rate coefficients for the reaction systems, IO + IO and IO + NO, using the source reaction, O(3P) + CF3I. By monitoring IO radicals, we obtain the 295 K rate coefficients, k(O + CF3I → IO + CF3) = (5.8 ± 1.5) × 10-12 cm3 molec-1 s-1; k(IO + IO) = (1.0 ± 0.3) × 10-10 cm3 molec-1 s-1, and k(IO + NO) = (1.9 ± 0.5) × 10-11 cm3 molec-1 s-1 at the pressures of 1250 Pa (9.4 Torr) and 4000 Pa (30.1 Torr). For the IO A 2Π3/2 − X 2Π3/2 (2, 0) bandhead at 445.04 nm we have determined an absorption cross-section, σ = (7.3 ± 0.7) × 10-17 cm2. Error limits indicate the confidence of two standard deviations and propagate the uncertainty in the absorption cross-section.

Ab-initio calculations at the CASSCF and MRCI level have been performed on the valence doublet and quartet electronic states of the ClO radical in order to explore the predissociation dynamics of the A2ΠΩ state (Ω=1/2 or 3/2). The ab-initio calculations neglect spin–orbit coupling. Large basis sets with diffuse orbitals were employed to ensure the calculated X2Π and A2Π potentials match closely the available experimental data. In total, the potential energies of fifteen valence states were computed at a series of internuclear separations in order to map the potential energy curves. The X2Π, A2Π and a4Σ- potentials are bound; though the latter has yet to be observed experimentally. All the other states are repulsive, and the lowest ten cross the A2Π3/2 component at energies below the v′=6 vibrational level. Using arguments based on electronic configurations, candidate states for the predissociation of the A2ΠΩ state can be selected. Simulations of the predissociation were performed by Fermi golden rule calculations for each vibrational level, using the ab-initio repulsive potentials and RKR curves for the A2Π3/2 and A2Π1/2 states. The vibrational dependence of the predissociation rate in the A2Π3/2 component was used to derive the magnitude of the individual couplings with the repulsive potentials. Comparison between the lifetimes of the vibrational levels in the A2Π3/2 and A2Π1/2 components restricts the principal players in the predissociation to three repulsive states: 14ΣΩ+, 24ΣΩ- and 32ΠΩ. The possibility of coupling between the A2Π3/2 component and the 12Δ state is also suspected. A marked similarity is revealed for the variation of predissociation rates with v′ for the A2Π3/2 and A2Π1/2 components despite a spin–orbit state dependence of the absolute rates for individual v′ levels. A repulsive 22Σ+ state is identified which is likely to be involved in the UV photodissociation of ClO. Similar ab-initio calculations on FO have revealed no bound excited valence 2Π state correlating diabatically to the Cl(2P)+O(1D) dissociation limit.

The UV absorption of ClO at wavelengths between 285 and 320 nm has been investigated using cavity ring-down spectroscopy. This wavelength region spans the (0,0) to (7,0) bands of the A2Π3/2–X2Π3/2 and A2Π1/2–X2Π1/2 transitions. The previously unobserved A2Π3/2–X2Π3/2 (0,0) and (1,0) absorption bands have been recorded with rotational resolution, and spectra of the (2,0) to (6,0) bands of the A2Π1/2–X2Π1/2 transition are shown for the first time. Analysis of the spectra gives refined band origins and rotational constants for the A2Π v′ levels and reveals a strong v′ dependence in the linewidths of rotational features. The lifetimes of the A2Π3/2v′=0–2 and A2Π1/2v′=2–4 levels are revised from previous estimates, and the lifetimes of A2Π1/2v′=5 and 6 levels have been determined. The deduced predissociation rates for A2Π3/2v′=3–7 confirm earlier studies. The lifetimes of vibrational levels of the two spin–orbit components of the A2ΠΩ state are markedly different. No evidence of J′-dependence in the predissociation is found, in contrast to the corresponding A2Π3/2 state of the IO radical.