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Large amounts of volatile organic compounds (VOCs) are emitted into the atmosphere from both human and natural sources. A significant portion of VOCs would be oxidized via their reactions with atmospheric oxidants like OH, NO3, ozone, etc. The products of the oxidation reactions are often of low volatility and may condense to form secondary organic aerosols (SOA). To study the effect of VOC oxidation in aerosol formation, we are building an oxidation flow reactor system, which consists of (1) a 22‐l aluminum chamber, (2) an ozone source with an ozone detector, (3) a UV‐C (254 nm) lamp, (4) a photoionization detector to measure the effective VOC concentration, (5) various flow/concentration controlling apparatuses, and (6) a scanning mobility particle sizer to monitor the generated particles. Under the conditions of high UV and ozone levels, the oxidation process can be speeded up by orders of magnitude in this reactor. We hope to use this reactor: (i) to learn the "potential" mass of SOA that can be formed from a given VOC source like a traffic or industry site; (ii) to trace back the SOA source by utilizing the shortened reaction times; (iii) to learn the trends from VOC to SOA. [ABSTRACT FROM AUTHOR]

The collision free photochemistry of methyl azide [CH.sub.3][N.sub.3] at 193 nm is examined. Findings reveal a maximum translational energy of 60 percent of the available energy.

We demonstrate a method to determine the absolute value of the photodissociation cross section of a molecule of interest in a cell without knowing its absolute concentration. The target molecule can be either stable or unstable species. Proof‐of‐principle experiments have been demonstrated for the UV cross sections of CH2I2 and NO2. The relative concentrations of the target molecule were measured by monitoring its time‐resolved UV–visible absorption spectra with a high‐speed CMOS‐camera spectrometer before and after its photodissociation by a pulsed 248‐ or 352‐nm excimer laser beam. The absolute value of the photodissociation cross section can be deduced from the used laser fluence. The inhomogeneity of the photolysis laser beam has been found to be the major source of systematic error. However, this effect can be corrected by performing a calibration experiment on a molecule of which the photodissociation cross section is known. In some cases, post‐photolysis reactions may affect the measurements. If the kinetics of the post‐photolysis reactions is known, it is feasible to estimate their effect and retrieve the correct value of the photodissociation cross section. To our best knowledge, this is the first demonstration of such a method. [ABSTRACT FROM AUTHOR]

The dynamics of the O(¹D) + Xe electronic quenching reaction was investigated in a crossed beam experiment at four collision energies. Marked large-scale oscillations in the differential cross sections were observed for the inelastic scattering products, O(³P) and Xe. The shape and relative phases of the oscillatory structure depend strongly on collision energy. Comparison of the experimental results with time-independent scattering calculations shows qualitatively that this behavior is caused by Stueckelberg interferences, for which the quantum phases of the multiple reaction pathways accessible during electronic quenching constructively and destructively interfere. [ABSTRACT FROM AUTHOR]

This study initially characterizes the primary photodissociation processes of epichlorohydrin, c-(H2COCH)CH2Cl. The three dominant photoproduct channels analyzed are c-(H2COCH)CH2+Cl, c-(H2COCH)+CH2Cl, and C3H4O+HCl. In the second channel, the c-(H2COCH) photofission product is a higher energy intermediate on C2H3O global potential energy surface and has a small isomerization barrier to vinoxy. The resulting highly vibrationally excited vinoxy radicals likely dissociate to give the observed signal at the mass corresponding to ketene, H2CCO. The final primary photodissociation pathway HCl+C3H4O evidences a recoil kinetic energy distribution similar to that of four-center HCl elimination in chlorinated alkenes, so is assigned to production of c-(H2COC)=CH2; the epoxide product is formed with enough vibrational energy to isomerize to acrolein and dissociate. The paper then analyzes the dynamics of the C3H5O radical produced from C–Cl bond photofission. When the epoxide radical photoproduct undergoes facile ring opening, it is the radical intermediate formed in the O(3P)+allyl bimolecular reaction when the O atom adds to an end C atom. We focus on the HCO+C2H4 and H2CO+C2H3 product channels from this radical intermediate in this report. Analysis of the velocity distribution of the momentum-matched signals from the HCO+C2H4 products at m/e=29 and 28 shows that the dissociation of the radical intermediate imparts a high relative kinetic energy, peaking near 20 kcal/mol, between the products. Similarly, the energy imparted to relative kinetic energy in the H2CO+C2H3 product channel of the O(3P)+allyl radical intermediate also peaks at high-recoil kinetic energies, near 18 kcal/mol. The strongly forward-backward peaked angular distributions and the high kinetic energy release result from tangential recoil during the dissociation of highly rotationally excited nascent radicals formed photolytically in this experiment. The data also reveal substantial branching to an HCCH+H3CO product channel. We present a detailed statistical prediction for the dissociation of the radical intermediate on the C3H5O potential energy surface calculated with coupled cluster theory, accounting for the rotational and vibrational energy imparted to the radical intermediate and the resulting competition between the H+acrolein, HCO+C2H4, and H2CO+C2H3 product channels. We compare the results of the theoretical prediction with our measured branching ratios. We also report photoionization efficiency (PIE) curves extending from 9.25 to 12.75 eV for the signal from the HCO+C2H4 and H2CO+C2H3 product channels. Using the C2H4 bandwidth-averaged absolute photoionization cross section at 11.27 eV and our measured relative photoion signals of C2H4 and HCO yields a value of 11.6+1/-3 Mb for the photoionization cross section of HCO at 11.27 eV. This determination puts the PIE curve of HCO measured here on an absolute scale, allowing us to report the absolute photoionization efficiency of HCO over the entire range of photoionization energies. [ABSTRACT FROM AUTHOR]

These experiments study the preparation of and product channels resulting from OCH2CHCH2, a key radical intermediate in the O+allyl bimolecular reaction. The data include velocity map imaging and molecular beam scattering results to probe the photolytic generation of the radical intermediate and the subsequent pathways by which the radicals access the energetically allowed product channels of the bimolecular reaction. The photodissociation of epichlorohydrin at 193.3 nm produces chlorine atoms and c-OCH2CHCH2 radicals; these undergo a facile ring opening to the OCH2CHCH2 radical intermediate. State-selective resonance-enhanced multiphoton ionization (REMPI) detection resolves the velocity distributions of ground and spin-orbit excited state chlorine independently, allowing for a more accurate determination of the internal energy distribution of the nascent radicals. We obtain good agreement detecting the velocity distributions of the Cl atoms with REMPI, vacuum ultraviolet (VUV) photoionization at 13.8 eV, and electron bombardment ionization; all show a bimodal distribution of recoil kinetic energies. The dominant high recoil kinetic energy feature peaks near 33 kcal/mol. To elucidate the product channels resulting from the OCH2CHCH2 radical intermediate, the crossed laser-molecular beam experiment uses VUV photoionization and detects the velocity distribution of the possible products. The data identify the three dominant product channels as C3H4O (acrolein)+H, C2H4+HCO (formyl radical), and H2CO (formaldehyde)+C2H3. A small signal from C2H2O (ketene) product is also detected. The measured velocity distributions and relative signal intensities at m/e=27, 28, and 29 at two photoionization energies show that the most exothermic product channel, C2H5+CO, does not contribute significantly to the product branching. The higher internal energy onset of the acrolein+H product channel is consistent with the relative barriers en route to each of these product channels calculated at the CCSD(T)/aug-cc-pVQZ level of theory, although a clean determination of the barrier energy to H+acrolein is precluded by the substantial partitioning into rotational energy during the photolytic production of the nascent radicals. We compare the measured branching fraction to the H+acrolein product channel with a statistical prediction based on the calculated transition states. [ABSTRACT FROM AUTHOR]

Accurate thermochemical data of small molecules are invaluable to the progress of every aspect of chemistry, especially in the atmosphere, combustion and industry. In this work, photofragmentation translational spectroscopy and 1st principles electronic structure theory reveal the literature value of the heat of formation of chlorine-isocyanate to be in error by more than 40 kcal/mol. We report a revised experimental value for D0(Cl–NCO)=51±3 kcal/mol which leads to a ΔHf (ClNCO)=8.5±3 kcal/mol. High level ab initio (CCSD(T)) electronic structure calculations extrapolated to the complete basis set limit give D0(Cl–NCO)=56.3 kcal/mol, in good agreement with experiment. In light of the present results, the destabilization of azides relative to isoelectronic isocyanates has been evaluated empirically for three pairs of related molecules. It is found to be 90–110 kcal/mol, and has been attributed mainly to the weakening of the N–NN bond relative to the N–CO bond. Electronic structure calculations employing decomposition analysis suggest that, compared to homopolar N2, the +δCO-δ π polarity provides better orbital interaction (charge transfer) and electrostatic attraction and results in a closer encounter and larger stabilization between the fragments and that this is the origin of isoelectronic destabilization of azides relative to the isocyanates. [ABSTRACT FROM AUTHOR]

We present results of near-threshold photoionization of N3 photofragments produced by laser photodissociation of ClN3 at 248 nm. The time of flight of recoiling N3 is used to resolve two photochemical channels producing N3, which exhibit different translational energy release. The two forms of N3 resolved in this way exhibit different photoionization thresholds, consistent with their assignment to linear (X 2Πg) and cyclic N3. This result agrees with the existing theoretical calculations of excited and ionic states of N3 and strengthens previous experimental results which suggested that the ClN3 photolysis produces a cyclic form of N3. [ABSTRACT FROM AUTHOR]

We present results of near-threshold photoionization of N3 photofragments produced by laser photodissociation of ClN3 at 248 nm. The time of flight of recoiling N3 is used to resolve two photochemical channels producing N3, which exhibit different translational energy release. The two forms of N3 resolved in this way exhibit different photoionization thresholds, consistent with their assignment to linear (X 2Πg) and cyclic N3. This result agrees with the existing theoretical calculations of excited and ionic states of N3 and strengthens previous experimental results which suggested that the ClN3 photolysis produces a cyclic form of N3. [ABSTRACT FROM AUTHOR]

The reaction of the simplest Criegee intermediate, CH2OO, with ammonia and water vapor has been investigated at 278–308 K and under 100–760 Torr by monitoring the strong UV absorption of CH2OO. We found that the observed decay rate of CH2OO becomes much larger when ammonia and water vapor are both present; the combinational effect of ammonia and water vapor is significantly greater than the sum of their individual contributions, revealing a strong synergic effect. The kinetic data are consistent with a termolecular process of CH2OO + NH3 + H2O reaction, of which the reaction rate coefficient was determined to be kNH3+H2O = (8.2 ± 1.2) × 10−31 cm6 s−1 at 298 K with a negative activation energy, Ea = −8.0 ± 0.8 kcal mol−1 [kNH3+H2O(T) = 1.04 × 10−36 exp(4047/T)]. Quantum chemistry calculation (at the QCISD(T)/aug-cc-pVTZ//B3LYP/6-311+G(2d,2p) level) found a low-energy reaction pathway, on which water accepts a hydrogen atom (or proton) from ammonia and releases another hydrogen atom to the terminal oxygen of CH2OO. The predicted products are H2NCH2OOH and a new H2O molecule, indicating water catalysis. This reaction is very fast and probably barrierless, which poses a theoretical challenge to modeling the related kinetics. [ABSTRACT FROM AUTHOR]

Carbonyl oxides, also known as Criegee intermediates, are generated from ozonolysis of unsaturated hydrocarbons in the atmosphere. Alcohols are often used as a scavenger of the Criegee intermediates in laboratory studies. In this work, the reaction kinetics of CH3CHOO with methanol vapor was investigated at various temperatures, pressures, and isotopic substitutions using time-resolved UV absorption spectroscopy. The observed rate coefficients of the reaction of anti-CH3CHOO with methanol show a linear dependence on [CH3OH]. The bimolecular rate coefficient was determined to be k1Ha = (4.8 ± 0.5) × 10−12 cm3 s−1 at 298 K and 250 Torr with a negative activation energy Ea = −2.8 ± 0.3 kcal mol−1 for T = 288–315 K [k(T) = A exp(−Ea/RT)]. For the reaction of syn-CH3CHOO with methanol vapor, the observed rate coefficients show a quadratic dependence on [CH3OH], indicating that two methanol molecules participate in the reaction. The termolecular rate coefficient was determined to be k2Hs = (8.0 ± 1.0) × 10−32 cm6 s−1 at 298 K and 250 Torr with a strong negative temperature dependence (Ea = −13.2 ± 0.3 kcal mol−1) at 273–323 K. No significant pressure effect was observed at 250–760 Torr. A kinetic isotope effect, k2Hs/k2Ds = 2.5, was observed by changing CH3OH to CH3OD. Quantum chemistry and transition state theory calculations suggest that the observed isotope effect is mainly attributed to the changes of the vibrational zero-point energies and partition functions while tunneling plays a very minor role. The reaction of syn-CH3CHOO with one CH3OH molecule was not observed in the studied concentration range. [ABSTRACT FROM AUTHOR]

The kinetics of the reaction of the simplest Criegee intermediate (CH2OO) with ammonia has been measured under pseudo-first-order conditions with two different experimental methods. We investigated the rate coefficients at 283, 298, 308, and 318 K at a pressure of 50 Torr using an OH laser-induced fluorescence (LIF) method. Weak temperature dependence of the rate coefficient was observed, which is consistent with the theoretical activation energy of −0.53 kcal mol−1 predicted by quantum chemistry calculation at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level. At 298 K, the rate coefficient at 50 Torr from the OH LIF experiment was (5.64 ± 0.56) × 10−14 cm3 molecule−1 s−1 while at 100 Torr we obtained a slightly larger value of (8.1 ± 1.0) × 10−14 cm3 molecule−1 s−1 using the UV transient absorption method. These experimental values are within the theoretical error bars of the present as well as previous theoretical results. Our experimental results confirmed the previous conclusion that ammonia is negligible in the consumption of CH2OO in the atmosphere. We also note that CH2OO may compete with OH in the oxidation of ammonia under certain circumstances, such as at night-time, high altitude and winter time. [ABSTRACT FROM AUTHOR]