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Isoprene is the most abundant alkene in the atmosphere. Ozonolysis of isoprene produces three very reactive carbonyl oxides (Criegee intermediates), including formaldehyde oxide, methyl vinyl ketone oxide (MVKO, CH3(C2H3)COO), and methacrolein oxide. The latter two Criegee intermediates are resonance-stabilized due to the vinyl group. Recently, the electronic spectrum of thermalized MVKO has been reported [Caravan, et al., Proc. Natl. Acad. Sci. U. S. A., 2020, 117, 9733]. In this work, we utilized this strong UV/visible absorption to investigate the reaction kinetics of MVKO with SO2 under a wide pressure range of 4 to 700 Torr. We followed a new method [Barber, et al., J. Am. Chem. Soc., 2018, 140, 10866], in which MVKO is produced through the reaction of a resonance-stabilized iodoalkene radical with O2. The experimental data are consistent with a kinetic model that the reaction goes through an adduct of CH3(C2H3)CIOO, similar to the cases of H/alkyl substituted Criegee intermediates. However, different from the H/alkyl adducts, which are stable over the time scales of typical kinetic experiments, this vinyl adduct CH3(C2H3)CIOO is less stable and decomposes to MVKO + I at a time scale of 10-3 s (faster at higher temperature), consistent with the results of quantum chemistry calculations and the fact that the resonance stabilization is disrupted at the adduct structure. The adduct decomposition is the major pathway that forms MVKO for pressures higher than 50 Torr. In addition, temperature dependence has been investigated for 278-319 K. The experimental activation energy of the adduct decomposition was measured to be 12.7 ± 0.3 kcal mol-1, consistent with the calculated dissociation energy of the adduct to MVKO + I (14 kcal mol-1). Furthermore, the temperature dependent rate coefficient of MVKO + SO2 reaction has been measured to be kSO2 = (4.0 ± 0.6) × 10-11 cm3 s-1 at 4-700 Torr and 298 K with a negative activation energy of -3.7 ± 0.4 kcal mol-1.

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.

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.

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.

The kinetics of CH$_2$OO reaction with ozone has been studied by monitoring CH$_2$OO with time-resolved infrared (IR) absorption spectroscopy, which utilized fast chirped IR pulse train from a quantum cascade laser [J. Chem. Phys. 146, 244302 (2017)]. CH$_2$OO was prepared by photolyzing a gas mixture of CH$_2$I$_2$/O$_2$/O$_3$ at 352 nm; the photolysis wavelength was chosen to minimize the photodissociation of O$_3$. The measured rate coefficient at 298 K and 30 Torr is (6.7$\pm$0.5)x10$^{-14}$ cm$^3$sec$^{-1}$, independent of pressure from 30 to 100 Torr. The result indicates that previous ab initio calculations either underestimated or overestimated this reaction rate by one order of magnitude or more. The result also implies that in laboratory studies of ozonolysis of alkenes, the reaction of Criegee intermediate with ozone may play a role. However, this reaction would not compete with other CH$_2$OO sinks in the atmosphere.

The absolute absorption cross sections of (CH 3 ) 2 COO under a jet-cooled condition were measured via laser depletion to be (1.32 ± 0.10) × 10 −17 cm 2 molecule −1 at 308 nm and (9.6 ± 0.8) × 10 −18 cm 2 molecule −1 at 352 nm. The peak UV cross section is estimated to be (1.75 ± 0.14) × 10 −17 cm 2 molecule −1 at 330 nm, according to the UV spectrum of (CH 3 ) 2 COO (Huang et al., 2015) scaled to the absolute cross section at 308 nm.

In this study, we performed ab initio calculations and obtained the bimolecular rate coefficients for the CH2OO/CH3CHOO reactions with H2O/(H2O)2. The energies were calculated with QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) and the partition functions were estimated with anharmonic vibrational corrections by using the second order perturbation theory. Furthermore, we directly measured the rate of the CH2OO reaction with water vapor at high temperatures (348 and 358 K) to reveal the contribution of the water monomer in the CH2OO decay kinetics. We found that the theoretical rate coefficients reproduce the experimental results of CH2OO for a wide range of temperatures. For anti- (syn-) CH3CHOO, we obtained theoretical rate coefficients of 1.60 × 10(-11) (2.56 × 10(-14)) and 3.40 × 10(-14) (1.98 × 10(-19)) cm(3) s(-1) for water dimer and monomer reactions at room temperature. From the detailed analysis of the quantum chemistry and approximations for the thermochemistry calculations, we conclude that our calculated values would be within a factor of 3 of the correct values. Furthermore, at [H2O] = 1 × 10(17) to 5 × 10(17) cm(-3), we estimate that the effective first-order rate coefficients for CH2OO, anti- and syn-CH3CHOO reactions with water vapor will be ∼10(3), ∼10(4), and ∼10(1) s(-1), respectively. Thereby, for Criegee intermediates with a hydrogen atom on the same side as the terminal oxygen atom, the reaction with water vapor will likely dominate the removal processes of these CIs in the atmosphere.

SO2 scavenging and self-reaction of CH2OO were utilized for the decay of CH2OO to extract the absorption spectrum of CH2OO under bulk conditions. Absolute absorption cross sections of CH2OO at 308.4 and 351.8 nm were obtained from laser-depletion measurements in a jet-cooled molecular beam. The peak cross section is (1.23 ± 0.18) × 10(-17) cm(2) at 340 nm.

The reactions of F 2 with isomers of butene were investigated with crossed-beam reactive scattering and ab initio computations. Only one product channel F + C 4 H 8 F was observed in the experiments; the HF + C 4 H 7 F channel was searched but not found. The reaction path of F 2 + C 4 H 8 → F + C 4 H 8 F was studied theoretically. Very low barriers of less than 1 kcal/mol were found for the F 2 reactions with cis -2-butene, trans -2-butene, and isobutene, which all contain two methyl substitution groups. This theoretical prediction, yet to be confirmed experimentally, suggests that double methyl substituted C C double bonds can react with F 2 almost without a barrier.

Using a crossed-molecular-beam apparatus and a synchrotron as a tunable vacuum-ultraviolet light source, we investigated oxysilanes formed in the reaction of atomic oxygen with silane. We measured the time-of-flight spectra, photoionization yields, and branching ratios of dissociative ionization of reaction products H 3 SiO/H 2 SiOH, H 2 SiO/HSiOH, HSiO/SiOH and SiO as a function of photoionization energy. The observed ionization thresholds correspond to isomeric products H 2 SiOH, HSiOH, and SiOH. The H 2 -loss channel has a branching ratio greater than the H-loss channel for the dissociative ionization of H 3 SiO/H 2 SiOH, whereas this situation is reversed for H 2 SiO/HSiOH, which we interpret according to the enthalpies of reactions.

The title reaction was investigated under crossed-beam conditions at collisional energies ranging from about 0.4 to 7.5 kcal/mol. Product velocity distributions were measured by a time-sliced, velocity-map imaging technique to explicitly account for the density-to-flux transformation factors. Both the state-resolved, pair-correlated excitation functions and vibrational branching ratios are presented for the two isotopic product channels. An intriguing resonance tunneling mechanism occurring near the reaction threshold for the HF+CD3 product channel is surmized, which echoes the reactive resonances found previously for the F+HD--HF+D reaction and more recently for the F+CH4 reaction.

Contains fulltext : 60528.pdf (Publisher’s version ) (Open Access) The velocity map ion imaging technique was applied to measure the reaction excitation function for the first time. It was found that the "raw" excitation function was significantly distorted by the density-to-flux transformation of the title reaction. Through a systematic investigation, possible reasons for such a dramatic effect are outlined. In addition, the state-resolved, pair-correlated excitation functions and branching ratios are presented. Effects of imperfect time slicing in the time-sliced velocity imaging technique in general are also discussed. (C) 2004 American Institute of Physics.

The Photodissociation dynamics of CO 2 has been reinvestigated using the photofragment translational spectroscopic technique based on direct vacuum ultraviolet (VUV) ionization using synchrotron radiation. The product translational energy spectrum as well as the product angular anisotropy have been measured for both the CO and O products. The experimental results indicate that the O( 3 P) + CO channel has an anisotropy parameter of 1.25, while that for the O( 1 D) + CO channel depends on the translational energy dependent.

The title reaction was investigated at three different collision energies in a pulsed, crossed-beam apparatus. The (2+1) REMPI spectra of the CHD2 products revealed, in addition to the anticipated 4mn vibronic bands, a hitherto unobserved feature. The new feature was shown and assigned to the 311 band. A time-sliced ion velocity imaging technique was applied to map out the coincident DF attributes of the two product states 42 and 31, whose energy levels lie nearly degenerate. Remarkably similar results were found for the two states in every aspect at all three collision energies. A simple model of Fermi-coupled states was proposed to rationalize this, at first sight, surprising finding. Implications to collisional processes which involve mixed molecular basis states in general are outlined. Possible quantum interference phenomenon is suggested.

The 18O(1D)+44CO2 oxygen isotope exchange reaction has been studied using a crossed molecular beam apparatus at a collision energy of 7.7 kcal/mol. Two different reaction channels are observed, both proceeding via a relatively long-lived CO3 complex. Electronic quenching of O(1D) is the major channel, accounting for 68% of all isotope exchange, while 32% occurs without quenching. These results are the first experimental evidence that isotope exchange can occur through a long-lived CO3 intermediate without subsequent crossing to the triplet surface, and could prove important for atmospheric models of oxygen isotope exchange between CO2 and O3.

The title reaction was investigated in a crossed-beam experiment. A (2+1) resonance-enhanced multiphon ionization technique was used to interrogate the internal-state distributions of the CD3 product at three different collision energies. Only the ν2 (umbrella) mode excitation was observed. Its distribution changes from a monotonically declined distribution at low energy to a slightly inverted one at higher collision energy. Although the rotational excitations of CD3 were small, a strong preference for K=0 was found, indicative of the dominance of the tumbling rotation motion of the CD3 product. The vibration-resolved excitation functions were also measured for ν2=0–3. A reaction barrier of 0.5 kcal/mol was deduced.

Photodissociation of methyl chloride at 157.6 nm has been investigated using a molecular beam apparatus with a universal detector, which is based on VUV photoionization by synchrotron radiation. Three dissociation channels have been observed: Cl+CH3, H+CH2Cl and HCl+CH2. Product translational energy distributions were determined for all observed dissociation channels. An intriguing HCl elimination channel was observed, which consists of two distinctive micro channels: one is CH 2 ( X 3 B 1 )+ HCl , the other is CH 2 ( a 1 A 1 )+ HCl . The fraction of the available energy released as product translational energy was determined to be 0.75 for CH 2 ( X 3 B 1 )+ HCl and 0.38 for CH 2 ( a 1 A 1 )+ HCl . Other product channels such as H2+CHCl were not found.

Photodissociation dynamics of cyclopropane at 157 nm has been investigated using a new molecular beam apparatus based on vacuum ultraviolet ionization by synchrotron radiation. Four distinct dissociation pathways have been observed. The observed channels are the H formation process, the H2 formation process, the CH2+C2H4 formation process, and the CH3+C2H3 formation process. Experimental results indicate that the H atom products are possibly produced via a synchronous, concerted 2H elimination process, while other channels are all binary dissociation processes. Product kinetic energy distribution of each dissociation channel has been determined from simulating the experimental time of flight spectra. Relative branching ratios for all observed dissociation channels were also estimated based on all detected products.

Photodissociation of vinylfluoride (H2CCFH) at 157 nm has been investigated using the photofragment translational spectroscopy technique. Five dissociation pathways have been clearly observed: H2CCF/HCCHF+H, HCCF+2H, HCCH+H+F, HCCH/H2CC:+HF, and HCCF/HFCC:+H2. Molecular HF elimination and the HCCH+H+F triple dissociation channel are found to be equally important, while other channels are all minor. The product kinetic energy distributions for all dissociation channels have been determined. Branching ratios for different channels have also been estimated.

The region 1273-1290 cm$^{-1}$ of the $��$4 fundamental of the simplest Criegee intermediate, CH$_2$OO, has been measured using a quantum cascade laser transient absorption spectrometer, which offers greater sensitivity and spectral resolution (< 0.004 cm$^{-1}$) than previous works based on thermal light sources. Gas phase CH$_2$OO was generated from the reaction of CH$_2$I + O2 at 298 K and 4 Torr. Analysis of the absorption spectrum has provided precise values for the vibrational frequency and the rotational constants, with fitting errors of a few MHz. The determined ratios of the rotational constants, A'/A" = 0.9986, B'/B" = 0.9974 and C'/C" = 1.0010, and the relative intensities of the a- and b-type transitions, 90:10, are in good agreement with literature values from a theoretical calculation using the MULTIMODE approach, based on a high-level ab initio potential energy surface. The low-K (= Ka) lines can be fitted extremely well, but rotational perturbations by other vibrational modes disrupt the structure for K = 4 and K $\geq$ 6. Not only the spectral resolution but also the detection sensitivity of CH$_2$OO IR transitions has been greatly improved in this work, allowing for unambiguous monitoring of CH$_2$OO in kinetic studies at low concentrations.

The UV spectrum of CH3CHOO was measured by transient absorption in a flow cell at 295 K. The absolute absorption cross sections of CH3CHOO were measured by laser depletion in a molecular beam to be (1.06 ± 0.09) × 10(-17) cm(2) molecule(-1) at 308 nm and (9.7 ± 0.6) × 10(-18) cm(2) molecule(-1) at 352 nm. After scaling the UV spectrum of CH3CHOO to the absolute cross section at 308 nm, the peak UV cross section is (1.27 ± 0.11) × 10(-17) cm(2) molecule(-1) at 328 nm. Compared to the simplest Criegee intermediate CH2OO, the UV absorption band of CH3CHOO is similar in intensity but blue shifted by 14 nm, resulting in a 20% slower photolysis rate estimated for CH3CHOO in the atmosphere.

Photodissociation dynamics of benzene under 193 nm and 157 nm photon excitation have been studied using the molecular beam photofragmentation spectroscopic technique. By detecting the H atom products produced away from the photolysis region, the lifetime of the excited benzene molecule as well as the kinetic energy distribution of the H atom elimination process can be determined simultaneously. Experimental results show that the H atom elimination from benzene occurs on the ground electronic state in which the excess energy is likely fully randomized.

The O(1D)+NH3 reaction has been investigated using the improved universal crossed molecular beam technique. Two different reaction channels, OH+NH2 and NHOH/NH2O+H, have been observed. OH formation is the dominant process, while the H formation process is minor. Different product angular distributions are also observed for these two product channels, indicating that the two reaction channels proceed with significantly different dynamics.

The O(1D)+C2H6 reaction has been reinvestigated using the improved universal crossed molecular beam technique. Four different reaction channels, OH+C2H5, CH3+H2COH/H3CO, H+H4C2OH/H5C2O and H2+H3C2OH/H4C2O, are observed. Among these reaction channels, the CH3 formation channel is the dominant pathway and the OH formation is also a significant process, while the H and H2 formation processes are minor. Different product angular distributions are also observed for these channels, indicating that the reaction product formation in these channels proceeds with distinctive reaction dynamics.

The O(1D)+C3H5 reaction has been investigated using the universal crossed molecular beam method. A number of reaction pathways have been observed. One of the most interesting channels is the H2CO/HCOH+C2H4 process, in which H2CO products are clearly identified. Experimental results indicate that the H2CO products are likely produced through a long-lived complex formation process, for which insertion of O(1D) into the C–C bond should be responsible.

The O(1D)+D2-->OD+D reaction at two different collisional energies, 2.0 and 3.2 kcal/mol, is investigated by using the H(D) Rydberg "tagging" time-of-flight technique. Experimental results in this study indicate that the OD product is clearly more backward scattered relative to the O(1D) beam direction at the collisional energy of 3.2 kcal/mol. The extra backward scattered OD products are mostly in the highly vibrationally excited states (upsilon = 4,5,6), which is typical of a collinear abstraction mechanism. The experimental results in this work also provide a good testing bed for quantitative theoretical investigations of this benchmark system.

Photodissociation dynamics of H2O at 121.6 nm have been studied using the H atom Rydberg “tagging” time-of-flight technique and by quasiclassical trajectory (QCT) calculations. Product kinetic energy distributions and angular distributions have been measured. From these distributions, rovibronic distributions of the OH radical product as well as the state resolved angular anisotropy parameters were determined. The dissociation energy D00(H–OH) is determined to be 41151±5 cm−1. Two clear alternations in the OH(X,v=0) rotational distribution have been observed, with each alternation corresponding to an oscillation in the anisotropy distribution. These oscillations had been attributed to the dynamical interference between the two conical intersection pathways. Further theoretical modeling in this work strongly supports this argument. Very highly vibrationally excited OH(X) products (up to v=9) have also been observed. These are ascribed to interconversion of H–O–H bending (H–H vibration) and O–H vibration in...

The atomic and molecular hydrogen elimination processes from ethylene have been studied using a molecular beam apparatus. Site and isotope effects on the molecular hydrogen elimination from ethylene have been clearly observed from the photodissociation of ethylene at 157 nm. Experimental results show that there are three different types of molecular elimination processes: 1,1 elimination, 1,2-cis elimination, and 1,2-trans elimination. Significant differences have been detected between 1,1 elimination and 1,2 eliminations in their kinetic energy distributions. Noticeable difference is also found between 1,2-cis elimination and 1,2-trans elimination for molecular deuterium elimination. Branching ratios for atomic and molecular hydrogen elimination processes have also been determined for ethylene and its isotopomers. Isotope and site effects on the branching ratios of different molecular elimination channels have been observed. The experimental results are also compared with recent theoretical studies.

The O(1D,3P)+CH3F reaction has been investigated using universal crossed molecular beam methods. The following reaction channels: HF+H2CO, H+CHFOH/CFH2O, and H2+CFOH/CHFO, have been observed in a single set of experiments. Time-of-flight spectra have been measured for products from these channels. Product angular distributions have been determined for these product channels, indicating that the dominant dynamical pathway for the reaction is the insertion mechanism. Relative branching ratios of the observed reaction channels have also been estimated. Experimental results in this work indicate that the HF formation channel is the dominant reaction pathway. This study provides a rather complete dynamical picture of the multiple channel O+CH3F reaction. It is believed that the O(1D) reaction is playing a major role in this experiment since it is energetically more favorable.

Theoretical estimates of reactive cross sections for O(1D)+H2(X,v=0,j)→OH(X)+H(2S), with H2 rotational quantum numbers j=0 and 1, are obtained for a range of collision energies, Ecol. Crossed molecular beam measurements are also used to infer the ratio, r1,0, of the j=1 and 0 cross sections at Ecol=0.056 eV. The theory indicates that the 1 1A′ potential surface is the most important one. However, the 2 1A′ and 1 1A″ surfaces can also contribute. Adiabatic dynamics on the 1 1A″ surface, particularly at Ecol above its 0.1 eV barrier to reaction plays a role. The 2 1A′ surface, while not correlating with ground electronic state products, can still lead to products via nonadiabatic interactions with the 1 1A′ surface. Many quantum dynamics and quasiclassical classical trajectory calculations are carried out. Accurate, ab initio based potential energy surfaces are employed. Quantum cross sections are based on helicity decoupled wave packet calculations for several values of total angular momentum. Nonadiabatic...

In this report, the O+SiH4 reaction is investigated using crossed molecular beam techniques. Multiple pathways have been observed for this reaction. Angular resolved time-of-flight spectra have been measured for all reaction products in a single set of experiments. Different product angular distributions were measured for various product channels, indicating that multiple reaction pathways with distinctive dynamics are present. An intriguing triple product channel, SiO+2H2, has also been clearly identified. However, the role of O(3P) vs O(1D) in the reactions remains to be clarified in further experimental investigations. This study provides a good example of multiple dynamical pathways in a complicated reaction, which opens enormous opportunities in investigating the dynamics of chemical reactions that are important in combustion and atmospheric chemistry.

Rydberg “tagging” time-of-flight (TOF) techniques has been applied to the investigation of the O(1D)+HD→OD+H reaction with very high translational energy resolution and sensitivity. High resolution TOF spectra for the H atom product at different laboratory angles have been measured for the first time, making the determination of the OD product quantum state distributions at different scattering angles possible. Preliminary experimental results show that different vibrationally excited OD products at low rotational levels have strikingly different angular distributions, indicating that intriguing dynamics are involved in this reaction. The experimental results in this work also provide a solid test ground for quantitative theoretical investigations of this benchmark system for insertion mechanism.

Photodissociation of propane at 157 nm excitation has been investigated using the photofragment translational spectroscopic (PTS) technique. Three dissociation channels have been experimentally observed: the atomic hydrogen (H) elimination, the molecular hydrogen (H2) elimination, and the methyl radical (CH3) elimination. Product translational energy distributions have been measured and relative branching ratios have also been estimated, which indicate that all these three processes are significant. The site effects on the H and H2 elimination have been quantified by investigating the photodissociation of the four propane isotopomers: CH3CD2CH3, CD3CH2CD3, CH2DCH2CH2D, and CD3CD2CD3. Experimental results indicate that the yield of the H elimination from the two terminal CH3 groups (terminal-H elimination) is larger than that from the internal CH2 group (internal-H elimination). H2 elimination from the internal carbon atom of propane (2,2-H2 elimination) is the dominant process while H2 elimination channel...

Photodissociation of propyne at 157 nm has been investigated using photofragment translational spectroscopy. Detailed investigation of various photofragments from the deuterated compounds CD3CCH and CH3CCD, as well as the unlabeled propyne provides a uniquely clear picture of an inherently complex process. Hydrogen atom elimination processes from both the CH3 group and the C≡C–H group have been clearly observed. H atom elimination from the methyl group appears to be a single dynamical process, while ethynyl H elimination shows two distinctive dynamical pathways with a ratio of 0.30 (fast): 0.43 (slow). The relative contribution of the atomic hydrogen elimination from the two different sites was determined to be 0.73 (ethynyl): 0.27 (methyl). Molecular hydrogen elimination processes have also been observed, but with a much smaller yield compared to the atomic hydrogen elimination (1:9.6). Comparison of the H2 HD and D2 photoproducts from various deuterated propyne molecules shows that the molecular hydroge...

Photodissociation of the H2S molecule at 157.6 nm was studied experimentally using the Rydberg tagging technique. Translational energy distributions of the H-atom product from the H2S photodissociation were measured, and the SH(X 2Π)+H(2S) channel was found to be the dominant dissociation process. Spin-orbit and rovibrational state distributions were also obtained for the SH product, which was found to be both vibrationally and rotationally excited. An intriguing bimodal rotational distribution in the lowest two vibrational states, v=0 and 1, has been clearly observed for the SH product, indicating that there are two distinctive dissociation mechanisms involved in the photodissociation of H2S at 157 nm excitation.

Site effects on the molecular hydrogen elimination from propane at 157 nm excitation have been studied using the photofragment translational spectroscopic technique. Experimental results indicate that H2 elimination from the internal carbon of propane (2,2-elimination) is predominant while eliminations from the terminal carbon (1,1- and 1,3-elimination) and the vicinal carbons (1,2-elimination) are minor. The translational energy distributions obtained for these processes also show that the dynamics of H2 eliminations from different sites are significantly different. Relative branching ratios of the atomic hydrogen (H) and the molecular hydrogen (H2) elimination processes were also determined.

Photofragment translational spectra at m/e=1(H), 2(H2,D), 3(HD), and 4(D2) have been obtained for CH3OH, CH3OD, and CD3OH at 157 nm excitation. Analysis of the time-of-flight spectra reveals two different atomic H loss channels: hydroxyl H elimination, and methyl H elimination. While the hydroxyl H elimination seems to be a single fast process, the methyl H loss exhibits clearly two significantly different mechanisms: one fast and one slow. Experimental results also show two molecular hydrogen elimination channels: three-center elimination from the methyl group, which displays two different micropathways, and four-center elimination involving hydrogen atoms on both the C and O sites. The relative branching of the atomic versus molecular hydrogen elimination channels was found to be 1:0.15. These results present a uniquely clear picture of methanol photodissociation at 157 nm, and thus provide an excellent case for quantitative theoretical investigations.

Photodissociation dynamics of H2O at 121.6 nm through the B 1A1′ state have been studied using the high-resolution H atom Rydberg tagging technique. Experimental evidences show two different dissociation pathways to form the ground OH (X,v=0) products: dissociation through the B–X conical intersection, and dissociation through B–A Coriolis coupling. By preparing the H2O molecules at higher rotational temperatures, dissociation through the B–A Coriolis coupling pathway can be enhanced.

Photodissociation of 1,1-difluoroethylene (F2CCH2) at 157 nm has been investigated using photofragment translational spectroscopy. Five dissociation channels have been experimentally observed; molecular HF elimination, H atom elimination, molecular hydrogen (H2) elimination, F atom elimination, and double bond breaking. Molecular HF elimination and H atom elimination channels are found to be the two major dissociation pathways in photodissociation of F2CCH2 at 157 nm excitation. Molecular hydrogen (H2) elimination and double bond cleavage are also significant, while F atom elimination is a minor process. Product translational energy distributions for all dissociation channels have also been measured. All translational energy releases are peaked at energies away from zero, indicating that the dissociation of F2CCH2 at 157 nm excitation most likely occurs with exit barriers on the ground electronic potential surface through internal conversion from the initially excited electronic state. Branching ratios an...

Photodissociation of trifluoroethylene (F2CCFH) at 157 nm has been investigated using photofragment translational spectroscopy. Four dissociation channels have been experimentally observed: molecular HF elimination, H atom elimination, F atom elimination, and double bond breaking. Double bond breaking is found to be the most important channel, while molecular HF elimination and H atom elimination are found to be significant. Contribution from F atom elimination is minor. Product translational energy distributions for all dissociation channels have been determined. The translational energy distributions for all four dissociation channels are peaked away from zero energy. This is quite similar to that of 1,1-difluoroethylene. Branching ratios and averaged energy partitions for all dissociation channels have also been estimated.

The O(1D)+CH4 reaction has been investigated using a new universal crossed molecular beam apparatus. Hydrogen atom elimination channel has been experimentally observed in this reaction. The pattern of dissociative ionization of the reaction products (from hydrogen loss channel) was compared with that of the methoxy (CH3O) radical produced from the photodissociation of CH3OH at 193 nm which has almost the same total energy deposition as the bimolecular reaction O(1D)+CH4. The experimental results suggest that the main hydrogen loss channel in the O(1D)+CH4 reaction should be CH2OH (hydroxymethyl)+H, while the CH3O (methoxy)+H channel is at most a minor reaction channel. This study provides an excellent experimental example of different dynamical behaviors exhibited in the unimolecular and bimolecular reactions of an essentially same chemical system (excited CH3OH) since the O(1D)+CH4 reaction likely occurs through the insertion mechanism.

Site and isotope effects on the molecular hydrogen elimination from ethylene have been studied from the photodissociation of ethylene at 157 nm excitation. Experimental results show that there are three different types of molecular elimination processes: 1,1 elimination, 1,2-cis elimination, and 1,2-trans elimination. These elimination processes show significantly different translational energy distributions. Isotope effect on the dynamics of these molecular hydrogen elimination processes has been also investigated carefully.

In this work, photodissociation of O2 at 157 nm has been studied using the photofragmentation translational spectroscopic technique. Two product channels O2+hν→O(1D)+O(3P), O2+hν→O(3P)+O(3P) have been observed. The relative yields and anisotropy parameters of both channels are determined. Anisotropy mixing of dissociation resulting from a perpendicular excitation and a parallel-type excitation has been observed in the dissociation channel O2+hν→O(3P)+O(3P). The observed results can be used to look at the detailed dynamical processes of the O2 dissociation through the Schumann–Runge band.

Photodissociation of OClO at 157.6 nm excitation has been investigated using the photofragment translational spectroscopic technique. Two distinctive chemical dissociation channels have been observed; one is the binary dissociation process, OClO+hν→ClO+O; the other one is the triple dissociation process, OClO+hν→Cl+O+O. The branching ratio of the binary dissociation channel to the triple dissociation channel is determined to be 0.59:0.41. Bimodal vibrational distribution of the ClO product has been observed for the OClO→ClO(X 2Π)+O(3P,1D) channel, implying that two distinctive dissociation routes possibly exist in the binary dissociation process. The bimodal distribution is likely caused by the two dissociation pathways from two excited electronic states: the D(2A1) and E(2B1) states of OClO. These arguments are further supported by the results of the anisotropy parameter measurements for the binary dissociation channels. Experimental results also show that the OClO+hν→ClO(X 2Π)+O(1S) and OClO+hν→ClO(A 2Π)+O(3P) channels might also exist in addition to the ClO(X 2Π)+O(3P,1D) channel. In the triple dissociation process, experimental results show that the main product channel is the OClO+hν→Cl(2P)+O(1D)+O(3P) channel, while the OClO+hν→Cl(2P)+O(3P)+O(3P) channel is the minor one. The branching ratio of these two channels is determined to be 0.89:0.11. From the modeling of the time of flight spectra of the O atom product, it is believed that the triple dissociation process of OClO is a simultaneous process within the time scale of one rotation period. Two-photon dissociative ionization process OClO+hν→Cl++O2+e− has also been observed.

The crossed molecular beam technique has been utilized to investigate a large number of elementary reactions. However, most of the studied reactions involve atoms or radicals; reactions between two stable molecular reactants are, in fact, seldom studied with the crossed molecular beam method. In this perspective, reactions between two stable molecules are reviewed and discussed. With crossed molecular beams and vacuum UV photoionization, the nascent products have been unambiguously identified. Consistent pictures of the reaction paths have been constructed based on the experimental data and ab initio calculations. Furthermore, there are intriguing features about the reaction barriers. The F(2) + organosulfur reactions are barrierless, demonstrating the first examples of such interactions between two closed-shell reactants. The barrier of F(2) + alkene reaction decreases with more methyl substitution groups at the C=C double bond, yet the absolute barrier heights from experiment and theory disagree with each other by ~2 kcal mol(-1), leaving an issue to be resolved in the future.

This study utilized a mass-resolved detection of ClOOCl to determine its photodissociation cross section, which is the product of the absorption cross section and dissociation quantum yield. An effusive molecular beam of ClOOCl was generated and its photodissociation probability was determined through measuring the decrease in the ClOOCl beam intensity upon laser irradiation. By comparing with a reference molecule, the absolute cross sections of ClOOCl were obtained without knowing its absolute concentration. The determined cross section of ClOOCl at 248.4 nm is (8.85+/-0.42)x10(-18) cm(2) at 200 K, significantly larger than previously reported values. The temperature dependence of the cross section was investigated at 248.4 nm in the range of 160-260 K; only a very small and negative temperature effect was observed. Because 248.4 nm is very close to the peak of the UV absorption band of ClOOCl, this work provides a new calibration point for normalizing relative absorption spectra of ClOOCl. In this work, the photodissociation cross section at 266 nm and 200 K was also reported to be (4.13+/-0.21)x10(-18) cm(2).

These experiments study the preparation of and product channels resulting from OCH(2)CHCH(2), 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-OCH(2)CHCH(2) radicals; these undergo a facile ring opening to the OCH(2)CHCH(2) 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 kcalmol. To elucidate the product channels resulting from the OCH(2)CHCH(2) 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 C(3)H(4)O (acrolein)+H, C(2)H(4)+HCO (formyl radical), and H(2)CO (formaldehyde)+C(2)H(3). A small signal from C(2)H(2)O (ketene) product is also detected. The measured velocity distributions and relative signal intensities at me=27, 28, and 29 at two photoionization energies show that the most exothermic product channel, C(2)H(5)+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.

The reaction between O((1)D) and C(6)H(6) (or C(6)D(6)) was investigated with crossed-molecular-beam reactive scattering and time-resolved Fourier-transform infrared spectroscopy. From the crossed-molecular-beam experiments, four product channels were identified. The major channel is the formation of three fragments CO+C(5)H(5)+H; the channels for formation of C(5)H(6)+CO and C(6)H(5)O+H from O((1)D)+C(6)H(6) and OD+C(6)D(5) from O((1)D)+C(6)D(6) are minor. The angular distributions for the formation of CO and H indicate a mechanism involving a long-lived collision complex. Rotationally resolved infrared emission spectra of CO (1or=upsilonor=6) and OH (1or=upsilonor=3) were recorded with a step-scan Fourier-transform spectrometer. At the earliest applicable period (0-5 mus), CO shows a rotational distribution corresponding to a temperature of approximately 1480 K for upsilon=1 and 920-700 K for upsilon=2-6, indicating possible involvement of two reaction channels; the vibrational distribution of CO corresponds to a temperature of approximately 5800 K. OH shows a rotational distribution corresponding to a temperature of approximately 650 K for upsilon=1-3 and a vibrational temperature of approximately 4830 K. The branching ratio of [CO]/[OH]=2.1+/-0.4 for O((1)D)+C(6)H(6) and [CO]/[OD]2.9 for O((1)D)+C(6)D(6) is consistent with the expectation for an abstraction reaction. The mechanism of the reaction may be understood from considering the energetics of the intermediate species and transition states calculated at the G2M(CC5) level of theory for the O((1)D)+C(6)H(6) reaction. The experimentally observed branching ratios and deuterium isotope effect are consistent with those predicted from calculations.

The F(2)+CH(3)SCH(3) reaction was studied with crossed molecular beam techniques and high level ab initio calculations. Significant reactivity was observed even at low collision energies, consistent with the negligible barrier height obtained from the ab initio calculations. All experimental findings are consistent with a weakly bound reaction intermediate of F-F-S(CH(3))(2) structure, which possesses a special type of three-center four-electron bonding. Analogous intermediates can also explain the reactions of F(2) with CH(3)SH and CH(3)SSCH(3).