Dynamics of the CH(A2Delta) product from the reaction of C2H with O2 studied by Fourier transform visible spectroscopy

The reaction C(2)H + O(2) --> CH(A(2)Delta) + CO(2) is investigated using Fourier transform visible emission spectroscopy. C(2)H radicals, produced by 193 nm photolysis of C(2)H(2), react with O(2) molecules at low total pressures to produce electronically excited CH(A(2)Delta). Observation of the CH(A(2)Delta-X(2)Pi) electronic emission to infer nascent rotational and vibrational CH(A(2)Delta) distributions provides information about energy partitioning in the CH(A(2)Delta) fragment during the reaction. The rotational and vibrational populations of the CH(A(2)Delta) product are determined by fitting the rotationally resolved experimental spectra with simulated spectra. The CH(A(2)Delta) product is found to be rotationally and vibrationally excited with T(rot) congruent with 1150 K and T(vib) congruent with 1900 K. The mechanism for this reaction proceeds through one of two five-atom intermediates and requires a crossing between electronic potential surfaces. The rotational excitation suggests a bent geometry for the final intermediate of this reaction before dissociation to products, and the vibrational excitation involves an elongation of the C-H bond from the compressed transition state to the final CH(A) state.     

B(2)A(')-X(2)A(') detection of vibrationally excited HCO produced by the O((3)P)+C(2)H(4) reaction

The distribution of rotational and vibrational energy in HCO produced by the O((3)P)+C(2)H(4) reaction has been measured using laser-induced fluorescence detection via the B(2)A(')-X(2)A(') transition. Over a detection wavelength range of 248-290 nm, our experiments have shown that HCO is formed in both the ground state and in at least six vibrationally excited states with up to two quanta of energy in the C-O stretch and the bending mode. Dispersed fluorescence experiments were conducted to positively assign all of the HCO vibrational bands. The experiments confirmed that many bands, including the B(000)-X(000) band, are affected by overlap with other HCO bands. Spectral modeling was used to separate the contributions of overlapping HCO B-X bands and to determine a nascent HCO rotational temperature of approximately 600 K, corresponding to approximately 6% of the total energy from the O((3)P)+C(2)H(4) reaction. HCO vibrational distributions were determined for two different average collision energies and were fit with vibrational temperatures of 1850+/-80 K and 2000+/-100 K, corresponding to approximately 15% of the total energy. The observed Boltzmann distribution of vibrational energy in HCO indicates that HCO and CH(3) are formed by the dissociation of an energized intermediate complex.     

State-resolved dynamics of the CH(A2Delta) channels from single and multiple photon dissociation of bromoform in the 10-20 eV energy range

Single photon dissociation of bromoform using synchrotron radiation has been investigated by Fourier transform visible fluorescence spectroscopy (FTVIS). The photodissociation of bromoform in the 12-18 eV energy range results in several products, among which are the CH(A2Delta) and CH(B2Sigma) radicals. Vibrational and rotational state distributions of the CH(A2Delta) are determined from their fluorescence spectra. From the threshold photon energy above which emission from the CH(A2Delta) radicals is observed, the most likely process leading to CH(A) formation is CHBr3 --> CH + 3Br rather than CHBr3 --> CH + Br + Br2. The rotational Boltzmann temperatures in the CH(A --> X) emission spectra for v' = 0 and v' = 1 range between 1570 and 3650 K, depending on the excitation photon energy. From the high rotational excitation, the results suggest that the mechanism for the loss of three bromine atoms is most likely sequential. A small negative emission anisotropy of the CH(A) radicals [(Ipar - Iper)/(Ipar + 2Iper) = -0.024 +/- 0.005] is constant across the action spectrum; a small net absorption dipole of CHBr3 in the vacuum ultraviolet is parallel to the 3-fold symmetry axis of the CHBr3 molecule. The state distributions of the CH(A2Delta) radicals from multiphoton dissociation of bromoform using the 266 nm output (three photons) of a femtosecond laser (Boltzmann temperatures: T(v'=0)(rot)= 4250 +/- 300 K; T(v'=1)(rot)= 3100 +/- 550 K) are compared to those from the single photon dissociation results (Boltzmann temperatures: T(v'=0)(rot)= 3650 +/- 150 K; T(v'=1)(rot)= 2400 +/- 200 K) at the same total excitation energy under collision free conditions. The analysis of the CH(A) rotational populations shows hotter rotational populations for the femtosecond experiment, also suggesting sequential dissociation of the bromoform in the femtosecond experiment. The duration of the femtosecond laser pulse is approximately 180 fs, setting a limit on the time scales for the multiple dissociations.     

CH3NO2与HE(23S)、Ne(3P0.2)的解离激发反应

利用分子束和化学发光技术，在单次碰撞条件下，首次研究了亚稳态原子He（23S）、Ne（3P0．2）与CH3NO2的解离激发反应，探测到反应的激发态产物（CH（A）、CH（B）、CH（C）的化学发光，在He（23S）／CH3NO2反应中同时探测到H（Balmer）的发射．利用He（23S）＋N2→N2＋（B）＋He＋e－作参考反应，测定了反应He（23S）／CH3NO2产生的CH的A－X，B－X，C－X以及H原子的发射速率常数．利用化学发光光谱的计算机模拟，求得了激发态产物CH（A）的初生态振动布居和转动温度．结合相空间理论对解离过程CH（A）的形成通道进行了讨论，认为CH（A）的形成是经由中间体CH3*的二体解离过程．     

State-resolved dynamics of the CN(B2Sigma+) and CH(A2Delta) excited products resulting from the VUV photodissociation of CH3CN

Fourier transform visible spectroscopy, in conjunction with VUV photons produced by a synchrotron, is employed to investigate the photodissociation of CH3CN. Emission is observed from both the CN(B2Sigma+-X2Sigma+) and CH(A2Delta-X2Pi) transitions; only the former is observed in spectra recorded at 10.2 and 11.5 eV, whereas both are detected in the 16 eV spectrum. The rotational and vibrational temperatures of both the CN(B2Sigma+) and CH(A2Delta) radical products are derived using a combination of spectral simulations and Boltzmann plots. The CN(B2Sigma+) fragment displays a bimodal rotational distribution in all cases. Trot(CN(B2Sigma+)) ranges from 375 to 600 K at lower K' and from 1840 to 7700 K at higher K' depending on the photon energy used. Surprisal analyses indicate clear bimodal rotational distributions, suggesting CN(B2Sigma+) is formed via either linear or bent transition states, respectively, depending on the extent of rotational excitation in this fragment. CH(A2Delta) has a single rotational distribution when produced at 16 eV, which results in Trot(CH(A2Delta))=4895+/-140 K in v'=0 and 2590+/-110 K in v'=1. From thermodynamic calculations, it is evident that CH(A2Delta) is produced along with CN(X2Sigma+)+H2. These products can be formed by a two step mechanism (via excited CH3* and ground state CN(X2Sigma+)) or a process similar to the "roaming" atom mechanism; the data obtained here are insufficient to definitively conclude whether either pathway occurs. A comparison of the CH(A2Delta) and CN(B2Sigma+) rotational distributions produced by 16 eV photons allows the ratio between the two excited fragments at this energy to be determined. An expression that considers the rovibrational populations of both band systems results in a CH(A2Delta):CN(B2Sigma+) ratio of (1.2+/-0.1):1 at 16 eV, thereby indicating that production of CH(A2Delta) is significant at 16 eV.     

Vibrational distributions of the CO(v) products of the C2H2 + O(3P) and HCCO + O(3P) reactions studied by FTIR emission

The C2H2 + O(3P) and HCCO + O(3P) reactions are investigated using Fourier transform infrared (FTIR) emission spectroscopy. The O(3P) radicals are produced by 193 nm photolysis of an SO2 precursor or microwave discharge in O2. The HCCO radical is either formed in the first step of the C2H2 + O(3P) reaction or by 193 nm photodissociation of ethyl ethynyl ether. Vibrationally excited CO and CO2 products are observed. The microwave discharge experiment [C2H2 + O(3P)] shows a bimodal distribution of the CO(v) product, which is due to the sequential C2H2 + O(3P) and HCCO + O(3P) reactions. The vibrational distribution of CO(v) from the HCCO + O(3P) reaction also shows its own bimodal shape. The vibrational distribution of CO(v) from C2H2 + O(3P) can be characterized by a Boltzmann plot with a vibrational temperature of approximately 2400 +/- 100 K, in agreement with previous results. The CO distribution from the HCCO + O(3P) reaction, when studied under conditions to minimize other processes, shows very little contamination from other reactions, and the distribution can be characterized by a linear combination of Boltzmann plots with two vibrational temperatures: 2320 +/- 40 and 10 300 +/- 600 K. From the experimental results and previous theoretical work, the bimodal CO(v) distribution for the HCCO + O(3P) reaction suggests a sequential dissociation process of the HC(O)CO++ --> CO + HCO; HCO --> H + CO.     

Product spin-orbit state resolved dynamics of the H+H2O and H+D2O abstraction reactions

The product state-resolved dynamics of the reactions H+H(2)O/D(2)O-->OH/OD((2)Pi(Omega);v',N',f )+H(2)/HD have been explored at center-of-mass collision energies around 1.2, 1.4, and 2.5 eV. The experiments employ pulsed laser photolysis coupled with polarized Doppler-resolved laser induced fluorescence detection of the OH/OD radical products. The populations in the OH spin-orbit states at a collision energy of 1.2 eV have been determined for the H+H(2)O reaction, and for low rotational levels they are shown to deviate from the statistical limit. For the H+D(2)O reaction at the highest collision energy studied the OD((2)Pi(3/2),v'=0,N'=1,A') angular distributions show scattering over a wide range of angles with a preference towards the forward direction. The kinetic energy release distributions obtained at 2.5 eV also indicate that the HD coproducts are born with significantly more internal excitation than at 1.4 eV. The OD((2)Pi(3/2),v'=0,N'=1,A') angular and kinetic energy release distributions are almost identical to those of their spin-orbit excited OD((2)Pi(1/2),v'=0,N'=1,A') counterpart. The data are compared with previous experimental measurements at similar collision energies, and with the results of previously published quasiclassical trajectory and quantum mechanical calculations employing the most recently developed potential energy surface. Product OH/OD spin-orbit effects in the reaction are discussed with reference to simple models.     

Ab initio investigations of the radical-radical reaction of O(3P) + C3H3

We present ab initio calculations of the reaction of ground-state atomic oxygen [O((3)P)] with a propargyl (C(3)H(3)) radical based on the application of the density-functional method and the complete basis-set model. It has been predicted that the barrierless addition of O((3)P) to C(3)H(3) on the lowest doublet potential-energy surface produces several energy-rich intermediates, which undergo subsequent isomerization and decomposition steps to generate various exothermic reaction products: C(2)H(3)+CO, C(3)H(2)O+H, C(3)H(2)+OH, C(2)H(2)+CHO, C(2)H(2)O+CH, C(2)HO+CH(2), and CH(2)O+C(2)H. The respective reaction pathways are examined extensively with the aid of statistical Rice-Ramsperger-Kassel-Marcus calculations, suggesting that the primary reaction channel is the formation of propynal (CHCCHO)+H. For the minor C(3)H(2)+OH channel, which has been reported in recent gas-phase crossed-beam experiments [H. Lee et al., J. Chem. Phys. 119, 9337 (2003); 120, 2215 (2004)], a comparison on the basis of prior statistical calculations is made with the nascent rotational state distributions of the OH products to elucidate the mechanistic and dynamic characteristics at the molecular level.     

Imaging the state-specific vibrational predissociation of the C2H2-NH3 hydrogen-bonded dimer

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia-acetylene dimer were studied following excitation in the asymmetric CH stretch. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the asymmetric CH stretch fundamental, ammonia fragments were detected by 2 + 1 REMPI via the B1E' <-- X1A1' and C'1A1' <-- X1A1' transitions. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational levels of ammonia with one or two quanta in the symmetric bend (nu2 umbrella mode) and were converted to rotational-state distributions of the acetylene co-fragment. The latter is always generated with one or two quanta of bending excitation. All the distributions could be fit well when using a dimer dissociation energy of D0 = 900 +/- 10 cm(-1). Only channels with maximum translational energy <150 cm(-1) are observed. The rotational excitation in the ammonia fragments is modest and can be fit by temperatures of 150 +/- 50 and 50 +/- 20 K for 1nu2 and 2nu2, respectively. The rotational distributions in the acetylene co-fragment pair-correlated with specific rovibrational states of ammonia appear statistical as well. The vibrational-state distributions, however, show distinct state specificity among channels with low translational energy release. The predominant channel is NH3(1nu2) + C2H2(2nu4 or 1nu4 + 1nu5), where nu4 and nu5 are the trans- and cis-bend vibrations of acetylene, respectively. A second observed channel, with much lower population, is NH3(2nu2) + C2H2(1nu4). No products are generated in which the ammonia is in the vibrational ground state or the asymmetric bend (1nu4) state, nor is acetylene ever generated in the ground vibrational state or with CC stretch excitation. The angular momentum (AM) model of McCaffery and Marsh is used to estimate impact parameters in the internal collisions that give rise to the observed rotational distributions. These calculations show that dissociation takes place from bent geometries, which can also explain the propensity to excite fragment bending levels. The low recoil velocities associated with the observed channels facilitate energy exchange in the exit channel, which results in statistical-like fragment rotational distributions.     

亚稳态He(23S)、Ar(3P0.2)原子与C2H3分子传能反应

自由基CN、CH、H在燃烧化学、大气化学、天体发光、环境污染等方面占有极为重要的地位，对于这些自由基发光及形成动力学机理的探讨，无疑是重要的．近年来，人们利用亚稳态惰性原子与膨化物碰撞传能，探讨了CN（AB－＋X）的化学发光［‘一、发现亚稳态的Ar（‘几，。）原子与H     

A study of the radical-radical reaction dynamics of O(3P)+t-C4H9 --> OH+iso-C4H8

The radical-radical reaction dynamics of ground-state atomic oxygen [O(3P)] with t-butyl radicals (t-C4H9) in the gas phase were investigated using high-resolution laser spectroscopy in a crossed-beam configuration, together with ab initio theoretical calculations. The radical reactants, O(3P) and t-C4H9, were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of the precursor, azo-t-butane, respectively. A new exothermic channel, O(3P)+t-C4H9 --> OH+iso-C4H8, was identified and the nascent rovibrational distributions of the OH (X 2Pi: upsilon" = 0,1,2) products were examined. The population analyses for the two spin-orbit states of F1(2Pi32) and F2(2Pi12) showed that the upsilon" = 0 level is described by a bimodal feature composed of low- and high-N" rotational components, whereas the upsilon" = 1 and 2 levels exhibit unimodal distributions. No noticeable spin-orbit or Lambda-doublet propensities were observed in any vibrational state. The partitioning ratio of the vibrational populations (Pupsilon") with respect to the low-N" components of the upsilon" = 0 level was estimated to be P0:P1:P2 = 1:1.17+/-0.24:1.40+/-0.11, indicating that the nascent internal distributions are highly excited. On the basis of the comparison of the experimental results with the statistical theory, the reaction mechanism at the molecular level can be described in terms of two competing dynamic pathways: the major, direct abstraction process leading to the inversion of the vibrational populations, and the minor, short-lived addition-complex process responsible for the hot rotational distributions. After considering the reaction exothermicity, the barrier height, and the number of intermediates along the addition reaction pathways on the lowest doublet potential energy surface, the formation of CH3COCH3(acetone)+CH3 was predicted to be dominant in the addition mechanism.     

Rotational state-resolved reaction cross section in the reactions of state-selected CH with NO and with O2

A pure and highly intense state-selected pulsed supersonic CH(X (2)Pi) radical beam source was developed by use of the C((1)D)+H(2) reaction with the combination of the state selection and purification by an electrostatic hexapole field. Under the beam-cell condition, the elementary reactions of CH+NO and CH+O(2) were studied by using this state-selected CH beam. NH(A (3)Pi) [and NCO(A (2)Sigma(+))] formations and OH(A (2)Sigma(+)) formation were directly identified in the elementary reaction of CH+NO and CH+O(2), respectively. For the CH+NO reaction, the relative branching ratio sigma(NCO*)sigma(NH) of NCO(A (2)Sigma(+)) formation to NH(A (3)Pi) formation was determined to be 0.35+/-0.15. The state-selected reaction cross sections were determined for each rotational state of CH. In the CH+NO reaction, a remarkable rotational state dependence of the reactive cross section was revealed, while the CH+O(2) reaction showed little rotational state dependence.     

Quantum and classical studies of the O(3P) + H2(v = 0-3,j = 0) --> OH + H reaction using benchmark potential surfaces

We present results of time-dependent quantum mechanics (TDQM) and quasiclassical trajectory (QCT) studies of the excitation function for O(3P) + H2(v = 0-3,j = 0) --> OH + H from threshold to 30 kcal/mol collision energy using benchmark potential energy surfaces [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)]. For H2(v = 0) there is excellent agreement between quantum and classical results. The TDQM results show that the reactive threshold drops from 10 kcal/mol for v = 0 to 6 for v = 1, 5 for v = 2 and 4 for v = 3, suggesting a much slower increase in rate constant with vibrational excitation above v = 1 than below. For H2(v > 0), the classical results are larger than the quantum results by a factor approximately 2 near threshold, but the agreement monotonically improves until they are within approximately 10% near 30 kcal/mol collision energy. We believe these differences arise from stronger vibrational adiabaticity in the quantum dynamics, an effect examined before for this system at lower energies. We have also computed QCT OH(v',j') state-resolved cross sections and angular distributions. The QCT state-resolved OH(v') cross sections peak at the same vibrational quantum number as the H2 reagent. The OH rotational distributions are also quite hot and tend to cluster around high rotational quantum numbers. However, the dynamics seem to dictate a cutoff in the energy going into OH rotation indicating an angular momentum constraint. The state-resolved OH distributions were fit to probability functions based on conventional information theory extended to include an energy gap law for product vibrations.     

He(23S)与含氢卤代甲烷传能反应中产生的CH(A2Δ,ν'=0)的新生态转动分布

The excited fragments CH(A), CH(B), CH(C) etc. were produced by the bombardment of He(2~3S) with CH_nX_(4-n)(X=Cl, Br, I). Based on the CH(A-X) emission spectra, the rotational populations can be interpretated in terms of Boltzmann distribution. The effective rotational temperature for CH(A,v'=0) produced from various methyl halides are nearly the some. Experiments under various pressure and calculation of collisional dynamics showed that the distributions are approximate to the nascent distributions.     

Dynamics of the gas-liquid interfacial reaction of O(1D) with a liquid hydrocarbon

The dynamics of the gas-liquid interfacial reaction of the first electronically excited state of the oxygen atom, O((1)D), with the surface of a liquid hydrocarbon, squalane (C(30)H(62); 2,6,10,15,19,23-hexamethyltetracosane) has been studied experimentally. Translationally hot O((1)D) atoms were generated by 193 nm photolysis of a low pressure (nominally 1 mTorr) of N(2)O a short distance (mean = 6 mm) above a continually refreshed liquid squalane surface. Nascent OH (X(2)Π, v' = 0) reaction products were detected by laser-induced fluorescence (LIF) on the OH A(2)Σ(+)-X(2)Π (1,0) band at the same distance above the surface. The speed distribution of the recoiling OH was characterized by measuring the appearance profiles as a function of photolysis-probe delay for selected rotational levels, N'. The rotational (and, partially, fine-structure) state distributions were also measured by recording LIF excitation spectra at selected photolysis-probe delays. The OH v' = 0 rotational distribution is bimodal and can be empirically decomposed into near thermal (~300 K) and much hotter (~6000 K) Boltzmann-temperature components. There is a strong positive correlation between rotational excitation and translation energy. However, the colder rotational component still represents a significant fraction (~30%) of the fastest products, which have substantially superthermal speeds. We estimate an approximate upper limit of 3% for the quantum yield of OH per O((1)D) atom that collides with the surface. By comparison with established mechanisms for the corresponding reactions in the gas phase, we conclude that the rotationally and translationally hot products are formed via a nonstatistical insertion mechanism. The rotationally cold but translationally hot component is most likely produced by direct abstraction. Secondary collisions at the liquid surface of products of either of the previous two mechanisms are most likely responsible for the rotationally and translationally cold products. We do not think it likely, a priori, that they could be produced in the observed significant yield via a statistical insertion mechanism for a molecule the size of squalane embedded in a surrounding liquid surface.     

A gas-phase crossed-beam study of OH produced in the radical-radical reaction of O((3)P) with iso-propyl radical (CH3)2CH

The gas-phase radical-radical reaction dynamics of ground-state atomic oxygen [O((3)P)] with iso-propyl radicals, (CH(3))(2)CH, were investigated by applying a combination of high-resolution laser-induced fluorescence spectroscopy in a crossed-beam configuration and ab initio calculations. The nascent distributions of OH (X(2)Π: υ' = 0) from the major reaction channel O((3)P) + (CH(3))(2)CH → C(3)H(6) (propene) + OH showed substantial internal excitations with a bimodal feature of low- and high-N' components with neither spin-orbit nor Λ-doublet propensities. Unlike previous kinetic results, proposed to proceed only through the direct H-atom abstraction process, on the basis of the population analysis and comparison with the statistical theory, the title reaction can be described in terms of two competing mechanisms at the molecular level: direct abstraction process and indirect short-lived addition-complex-forming process with a ratio of 1.25?:?1.     

Molecular elimination in photolysis of o- and p-fluorotoluene at 193 nm: Internal energy of HF determined with time-resolved Fourier transform spectroscopy

Following the photodissociation of o-fluorotoluene [o-C(6)H(4)(CH(3))F] at 193 nm, rotationally resolved emission spectra of HF(1< or =v< or =4) in the spectral region of 2800-4000 cm(-1) are detected with a step-scan Fourier transform spectrometer. HF(v< or =4) shows nearly Boltzmann-type rotational distributions corresponding to a temperature approximately 1080 K; a short extrapolation from data in the period of 0.5-4.5 mus leads to a nascent rotational temperature of 1130+/-100 K with an average rotational energy of 9+/-2 kJ mol(-1). The observed vibrational distribution of (v=1):(v=2):(v=3)=67.6: 23.2: 9.2 corresponds to a vibrational temperature of 5330+/-270 K. An average vibrational energy of 25+/-(3) (12) kJ mol(-1) is derived based on the observed population of HF(1< or =v< or =3) and estimates of the population of HF (v=0 and 4) by extrapolation. Experiments performed on p-fluorotoluene [p-C(6)H(4)(CH(3))F] yielded similar results with an average rotational energy of 9+/-2 kJ mol(-1) and vibrational energy of 26+/-(3) (12) kJ mol(-1) for HF. The observed distributions of internal energy of HF in both cases are consistent with that expected for four-center elimination. A modified impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts satisfactorily the rotational excitation of HF. An observed vibrational energy of HF produced from fluorotoluene slightly smaller than that from fluorobenzene might indicate the involvement of seven-membered-ring isomers upon photolysis.     

He(2~3S)与含氢卤代甲烷传能反应中产生的CH(A~2Δ,ν′=0)的新生态转动分布

流动余辉技术在基元反应动力学研究中已得到了广泛应用。当亚稳态稀有气体原子与某些分子发生传能反应时,母体分子解离产生一些较小的激发态碎片。通过测量碎片的发射光谱可以获得产物内能分布以及解离过程的动力学信息。使用该技术,Someda和Roychowdhury分别研究了He(2~3S)与NH_3和PF_3的反应,获得了NH(A,C)和PF(A)的内能分布规律,并讨论了解离反应的机理。     

CH(A2Delta) Formation in hydrocarbon combustion: the temperature dependence of the rate constant of the reaction C2H + O2--> CH(A2Delta) + CO2

The temperature dependence of the rate constant of the chemiluminescence reaction C2H + O2 --> CH(A) + CO2, k1e, has been experimentally determined over the temperature range 316-837 K using pulsed laser photolysis techniques. The rate constant was found to have a pronounced positive temperature dependence given by k1e(T) = AT(4.4) exp(1150 +/- 150/T), where A = 1 x 10(-27) cm(3) s(-1). The preexponential factor for k1e, A, which is known only to within an order of magnitude, is based on a revised expression for the rate constant for the C2H + O(3P) --> CH(A) + CO reaction, k2b, of (1.0 +/- 0.5) x 10(-11) exp(-230 K/T) cm3 s(-1) [Devriendt, K.; Van Look, H.; Ceursters, B.; Peeters, J. Chem. Phys. Lett. 1996, 261, 450] and a k2b/k1e determination of this work of 1200 +/- 500 at 295 K. Using the temperature dependence of the rate constant k1e(T)/k1e(300 K), which is much more accurately and precisely determined than is A, we predict an increase in k(1e) of a factor 60 +/- 16 between 300 and 1500 K. The ratio of rate constants k2b/k1e is predicted to change from 1200 +/- 500 at 295 K to 40 +/- 25 at 1500 K. These results suggest that the reaction C2H + O2 --> CH(A) + CO2 contributes significantly to CH(A-->X) chemiluminescence in hot flames and especially under fuel-lean conditions where it probably dominates the reaction C2H + O(3P) --> CH(A) + CO.     

Reaction dynamics of Ca(4s3d 1D2)+CH4-->CaH(X 2Sigma+)+CH3: reaction pathway and energy disposal for the CaH product

The reaction pathway for Ca(4s3d 1D2)+CH4-->CaH(X 2Sigma+)+CH3 has been investigated by using a pump-probe technique in combination with potential-energy surface (PES) calculations. The nascent product distributions of CaH have been characterized with Boltzmann rotational temperatures of 1013+/-102 and 834+/-70 K for the v=0 and 1 levels, respectively, and a Boltzmann vibrational temperature of 1313+/-173 K. The rotational and vibrational energy partitions in CaH have been estimated to be 461+/-45 and 252+/-15 cm(-1), respectively. According to the PES calculations, the pathway favors an insertion mechanism. Ca(3 1D2) approaches CH4 along an attractive potential surface in a C2v (or Cs) symmetry and then the collision complex undergoes nonadiabatic transition to the reactive ground-state surface. An Arrhenius plot shows a potential-energy requirement of 2695+/-149 cm(-1), which accounts for the endothermicity of 2930 cm(-1) for the reaction scheme. The Ca-C bond distance in the transition state structure is short enough to allow for tight orbital overlap between CaH and CH3. The strong coupling between the moieties renders the energy transfer sufficient from CaH into the CH3 radical. As compared to the Ca(4 1P1) reaction, the dissociation lifetime of the intermediate complex with less excess energy is prolonged so as to cause much less vibrational energy disposal into CaH.