To better quantify the rates at which key trace gases interact with sea-salt aerosols, the kinetics of uptake of HOBr, HNO3, O3, and NO2 by deliquescent NaCl aerosols at 75% relative humidity (RH) and room temperature have been studied using an aerosol kinetics flow tube technique. Results for HOBr indicate that the uptake coefficient (γ) is larger than 0.2 for highly acidic aerosols at pH 0.3 and for aerosols that have been buffered to pH 7.2 using a 0.25 M NaH2PO4/Na2HPO4 buffer. For unbuffered NaCl aerosols, the HOBr uptake coefficient due to reaction is less than 1.5 × 10-3. For HNO3, the uptake coefficient on unbuffered, NaCl aerosols is greater than 0.2, being driven by the very high solubility of HNO3 in aqueous salt solutions. Both NO2 and O3 show low reactivity on pH neutral aerosols with upper limits to the uptake coefficients of 10-4. With acidic aerosols, slight O3 loss occurs either on the walls of the flow tube or on the aerosols, giving rise to Cl2. These experiments are the first reported kinetics studies of the loss of HOBr, HNO3, and O3 on aqueous NaCl solutions, and they imply that gas-phase diffusion, and not reaction kinetics, determines the mass-transfer rates of gas-phase HNO3 and HOBr to marine aerosols in the boundary layer. Also, the HOBr results support modeling studies which have proposed that HOBr uptake initiates autocatalytic release of bromide from sea-salt aerosols.

This paper presents the first experimental investigation under collisionless conditions of the competing photodissociation channels of methylamine excited in the first ultraviolet absorption band. Measurement of the nascent photofragments' velocity distributions and preliminary measurements of some photofragments' angular distributions evidence four significant dissociation channels at 222 nm: N-H, C-N, and C-H bond fission and H2 elimination. The data, taken on photofragments from both methylamine and methylamine-d2, elucidate the mechanism for each competing reaction. Measurement of the emission spectrum of methylamine excited at 222 nm gives complementary information, evidencing a progression in the amino wag (or inversion) and combination bands with one quantum in the methyl (umbrella) deformation or with two quanta in the amino torsion vibration. The emission spectrum reflects the forces in the Franck-Condon region which move the molecule toward a ciscoid geometry. The photofragment kinetic energy distributions measured for CH3ND2 show that hydrogen elimination occurs via a four-center transition state to produce HD and partitions considerable energy to relative product translation. The reaction coordinates for N-H and C-N fission are analyzed in comparison to that for ammonia dissociation from the A state and with reference to ab initio calculations of cuts along the excited state potential energy surface of methylamine which show these reactions traverse a small barrier in the excited state from a Rydbergkalence avoided crossing and then encounter a conical intersection in the exit channel. The measured kinetic energy distribution of the C-N bond fission photofragments indicates that the NH2 (NDz) product is formed in the A 2A1 state; the C-N fission reactive trajectories thus remain on the upper adiabat as they traverse the conical intersection. The mechanism for C-H bond fission is less clear; most of the kinetic energy distribution indicates the reaction evolves on a potential energy surface with no barrier to the reverse reaction, consistent with dissociation along the excited state surface or upon internal conversion to the ground state, but some of the distribution reflects more substantial partitioning to relative translation, indicating that some molecules may dissociate via a repulsive triplet surface. In general, the photofragment angular distributions were anisotropic, but the measured p -0.4 f 0.4 for C-N bond fission indicates dissociation is not instantaneous on the time scale of molecular rotation. We end with analyzing why in methylamine three other primary dissociation channels effectively compete with N-H fission while in CH30H and CH3SH primarily 0-H and S-H fission, respectively, dominate.

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A detailed study of the interaction of HOBr and HCl in cold sulfuric acid solutions has been performed using a coated-wall flow tube coupled to an electron-impact mass spectrometer. The liquid-phase bimolecular rate constants, measured over a temperature range from 213 to 238 K and in solutions from 59.7 to 70.1 wt % composition, show a strong positive dependence on both acid composition and temperature. The solubility of HOBr has also been measured in these solutions by analyzing its time-dependent uptake. Henry's Law constants (H) determined from the measured values of HD1/2 and the liquid-phase diffusion coefficient (D) are independent of acid composition over the above range of solution compositions. The values of H demonstrate a clear Clausius−Clapeyron temperature dependence, with a heat of solution of −9 ± 1 kcal/mol. When the atmospheric importance of these data is assessed, two conclusions are reached. In the stratosphere, under aerosol conditions observed soon after the Mt. Pinatubo volcanic eruption, the rates of HCl activation via the HOBr/HCl heterogeneous reaction are comparable with the rate of activation via gas-phase reaction with OH at relatively warm temperatures (205−220 K), where other HCl-activating heterogeneous reactions occur slowly. In the high Arctic boundary layer, it is possible that significant HCl activation could occur when elevated levels of photochemically active bromine are present.

Due to the implication of iodine chemistry in stratospheric ozone depletion, an accurate atmospheric lifetime of methyl iodide has recently become of interest. To calculate this lifetime, a reliable temperature-dependent UV photodissociation cross section, in the region that overlaps with available solar light, is vital. Unfortunately, measurement of this cross section is complicated by the fact that, at typical laboratory pressures, methyl iodide readily forms dimers whose ultraviolet absorption differs from that of the monomer and that dimer formation is also temperature-dependent. We use a combination of theory and experiment to separate the changes in the absorption due to the temperature dependence of dimer formation from the narrowing of the absorption band that results from rotational and vibrational cooling of isolated methyl iodide molecules. Calculation of the predicted absorption cross section shows that the valence band absorption spectrum narrows only slightly upon cooling from 25 to −73 °C (200 K). Absorption spectra were also measured experimentally at a range of pressures from 0.1 to 2.4 Torr and a range of temperatures from −22 to 100 °C. The temperature-dependent cross section measured at 0.1 Torr agrees well with the calculated temperature dependence. The spectra at higher pressures show strong pressure as well as temperature dependence. This pressure dependence allowed us to constrain the temperature-dependent equilibrium constant for dimer formation.

These experiments on bromopropionyl chloride investigate a system in which the barrier to C-Br fission on the lowest lA" potential energy surface is formed from a weakly avoided electronic configuration crossing, so that nonadiabatic recrossing of the barrier to C-Br fission dramatically reduces the branching to C-Br fission. The results, when compared with earlier branching ratio measurements on bromoacetyl chloride, show that the additional intervening CH 2 spacer in bromopropionyl chloride reduces the splitting between the adiabatic potential energy surfaces at the barrier to C-Br fission, further suppressing C-Br fission by over an order of magnitude. The experiment measures the photofragment velocity and angular distributions from the 248 nm photodissociation of Br(CH 2 hCOCI, determining the branching ratio between the competing primary C-Br and C-CI fission pathways and detecting a minor C-C bond fission pathway. While the primary C-CI:C-Br fission branching ratio is 1:2, the distribution of relative kinetic energies imparted to the C-Br fission fragments show that essentially no C-Br fission results from promoting the molecule to the lowest lA" potential energy surface via the l[n(O),rr*(C=O)] transition; C-Br fission only results from an overlapping electronic transition. The results differ markedly from the predictions of statistical transition state theories which rely on the Born-Oppenheimer approximation. While such models predict that, given comparable preexponential factors, the reaction pathway with the lowest energetic barrier on the lA" surface, C-Br fission, should dominate, the experimental measurements show C-CI bond fission dominates by a ratio ofC-CI:C-Br= 1.0: <0.05 upon excitation of the l[n(O),1T*(C=O)] transition. We compare this result to earlier work on bromoacetyl chloride, which evidences a less dramatic reduction in the C-Br fission pathway (C-CI:C-Br = 1.0:0.4) upon excitation of the same transition. We discuss a model in which increasing the distance between the C-Br and C=O chromophores decreases the electronic configuration interaction matrix elements which mix and split the In(O)rr*(C=O) and np(Br)a*(C-Br) configurations at the barrier to C-Br bond fission in bromopropionyl chloride. The smaller splitting between the adiabats at the barrier to C-Br fission increases the probability of nonadiabatic recrossing of the barrier, nearly completely suppressing C-Br bond fission in bromopropionyl chloride. Preliminary ab initio calculations of the adiabatic barrier heights and the electronic configuration interaction matrix elements which split the adiabats at the barrier to C-Br and C-CI fission in both bromopropionyl chloride and bromoacetyl chloride support the interpretation of the experimental results. We end by identifying a class of reactions, those allowed by overall electronic symmetry but Woodward-Hoffmann forbidden, in which nonadiabatic recrossing of the reaction barrier should markedly reduce the rate constant, both for ground state and excited state surfaces.

These experiments investigate the competition between CC and C-Br bond fission in bromoacetone excited in the l[n(O),1T*(C=O)] absorption, elucidating the role of molecular conformation in influencing the probability of adiabatically traversing the conical intersection along the CC fission reaction coordinate. In the first part of the paper, measurement of the photofragment velocity and angular distributions with a crossed laser-molecular beam time-offlight technique identifies the primary photofragmentation channels at 308 nm. The time-offlight spectra evidence two dissociation channels, C-Br fission and fission of one of the two CC bonds, BrH 2 C-COCH 3 • The distribution of relative kinetic energies imparted to the C-Br fission and CC fission fragments show dissociation is not occurring via internal conversion to the ground electronic state and allow us to identify these channels in the closely related systems of bromoacetyl-and bromopropionyl chloride. In the second part of the work we focus on the marked conformation dependence to the branching between CC fission and C-Br fission. Photofragment angular distribution measurements show that C-Br fission occurs primarily from the minor, anti, conformer, giving a f3 of 0.8, so CC fission must dominate the competition in the gauche conformer. Noting that the dynamics of these two bond fission pathways are expected to be strongly influenced by nonadiabatic recrossing of the reaction barriers, we investigate the possible mechanisms for the conformation dependence of the nonadiabatic recrossing with low-level ab initio electronic structure calculations on the C-Br reaction coordinate and qualitative consideration of the conical intersection along the C

CH3SH ultraviolet absorption cross sections in the region 192.5-309.5 nm and photodecomposition at 222 and 193 nm and 296 K A crossed lasermolecular beam study of the one and two photon dissociation dynamics of ferrocene at 193 and 248 nm These experiments use molecular photodissociation of CH 3 SH to probe the dynamics and the influence of nonadiabatic coupling in the transition state region of the CH 3 +SH--+CH 3 S+H reaction. Photoexcitation at 222 and 248 nm in the first of two absorption bands accesses the lower of the two coupled potential energy surfaces near the saddle point of the excited state reaction coordinate. Measurement of the resulting photofragments' velocities and angular distributions determine the branching between the CH 3 +SH and the CH 3 S+H exit channels. At all wavelengths within the first absorption band, we observe preferential fission of the stronger S-H bond over the weaker C-S bond. Fission of the C-S bond occurs only to a small degree at 222 nm and is not observable at 248 nm. Comparison with our earlier data at 193 nm, corresponding to excitation to the upper bound adiabat which is nonadiabatically coupled to the lower dissociative surface reached at 222 nm, shows that the branching ratio between C-S bond fission and S-H bond fission is a factor of eight larger at 193 nm. To probe the forces in the Franck-Condon region, we also measure the photoemission spectrum from dissociating CH 3 SH excited at 222 nm and compare it to the previous measurement at 193 nm. The 222 nm spectrum evidences emission into the S-H stretch and methyl stretch vibrations but not into C-S stretching modes, consistent with the dominance of S-H fission on the lower adiabat, while the 193 nm emission spectrum, reassigned here, has only a progression in the C-S stretch. The comparison of the spectra suggests a model in which stretching along the C-S coordinate on the bound upper state occurs as the amplitude couples nonadiabatically to the lower dissociative surface, allowing the molecule to access the region near the saddle point on the lower surface at extended C-S bond lengths. This results in better overlap with the C-S fission exit channel and thus an increased branching to C-S bond fission over that observed upon direct excitation to the lower dissociative surface at 222 nm. To further advance the experimental conclusions, we present collaborative calculations of the potential energy surfaces using the effective valence-shell Hamiltonian method developed by Freed and co-workers.