Nascent Rovibrational Distributions Research Articles

A combined crossed-beam and ab initio study of the atom–radical reaction dynamics of O(3P) + C2H5→ C2H4 + OH: analysis of nascent internal state distributions of the OH product

The oxidation reaction dynamics of ethyl radicals (C(2)H(5)) in the gas phase are investigated by applying a combination of high-resolution laser induced fluorescence spectroscopy in a crossed-beam configuration and ab initio theoretical calculations. The supersonic atomic oxygen (O((3)P)) and ethyl (C(2)H(5)) reactants are produced by photodissociation of NO(2) and supersonic flash pyrolysis of a synthesized precursor (azoethane), respectively. An exothermic channel leading to the C(2)H(5) + OH (X(2)Pi: upsilon'' = 0, 1) products is identified. The nascent rovibrational state distributions of the OH product show substantial bimodal internal excitations consisting of low- and high-N'' components with neither spin-orbit nor Lambda-doublet propensities in the ground and first excited vibrational states. The averaged vibrational population (P(upsilon'')), partitioning with respect to the low-N'' components of the upsilon'' = 0 level, shows a comparable population ratio of P(0)ratioP(1) = 1 ratio 1.06. On the basis of comparison between the population analyses using ab initio and prior statistical calculations, the title atom-radical reactive scattering processes are governed by dynamic characteristics. The reaction mechanism can be rationalized by two competing mechanisms: abstraction versus addition. The major low N''-components can be described in terms of the direct abstraction process responsible for the comparable vibrational populations, while the minor but hot rotational distribution of the high N''-components implies that some fraction of radical reactants is sampled to proceed through the short-lived addition-complex forming process.

Radical–radical reaction dynamics: A combined crossed-beam and theoretical study

We present an overview of our recent studies of the gas-phase reaction dynamics of O(3P) with a series of hydrocarbon radicals [allyl (C3H5), propargyl (C3H3), t-butyl (t-C4H9)] as prototypal radical–radical oxidation reactions. High-resolution laser spectroscopy in a crossed-beam configuration was applied to examine the nascent rovibrational state distributions and Doppler profiles of the reactive scattering products. The analyses of the product energy and population distributions demonstrated the existence of unusual dynamic characteristics and competition between the addition and abstraction reaction mechanisms at the molecular level. These features, which are in sharp contrast with those of the oxidation reactions of closed-shell hydrocarbon molecules, are discussed with the aid of ab initio and quantum statistical calculations. Contents PAGE 1. Introduction 614 2. Experiment 615 2.1. Characterization of the molecular beam in the supersonic flash pyrolysis source 616 2.2. Nascent product state distributions from reactive scattering processes 617 3. Ab initio calculations 619 4. Experimental and statistical analysis 620 4.1. Nascent internal state distributions of OH products 620 4.2. Translational energy release 621 4.3. Statistical analysis 621 5. Radical–radical reaction dynamics 623 5.1. Characteristics of the molecular beam in the supersonic flash pyrolysis source 623 5.2. Reaction dynamics of O(3P) with C3H5 626 5.2.1. Ab initio potential energy surface: addition vs. abstraction 626 5.2.2. O(3P) + C3H5 → C3H4 + OH 630 5.2.3. O(3P) + C3H5 → C3H4O + H 635 5.3. Reaction dynamics of O(3P) with C3H3 637 5.3.1. Ab initio potential energy surface 637 5.3.2. O(3P) + C3H3 → C3H2 + OH 640 5.3.3. O(3P) + C3H3 → C3H2O + H 643 5.4. Reaction dynamics of O(3P) with t-C4H9 644 5.4.1. Ab initio potential energy surface 644 5.4.2. O(3P) + t-C4H9 → iso-C4H8 + OH 646 6. Summary 646 Acknowledgments 648 References 649 Supplementary Materials 651

A study of the radical-radical reaction dynamics of O(P3)+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.

Atom-radical reaction dynamics of O(3P)+C3H5→C3H4+OH: Nascent rovibrational state distributions of product OH

The reaction dynamics of ground-state atomic oxygen [O(3P)] with allyl radicals (C3H5) has been investigated by applying a combination of crossed beams and laser induced fluorescence techniques. The reactants O(3P) and C3H5 were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of precursor allyl iodide, respectively. A new exothermic channel of O(3P)+C3H5→C3H4+OH was observed and the nascent internal state distributions of the product OH (X 2Π:υ″=0,1) showed substantial bimodal internal excitations of the low- and high-N″ components without Λ-doublet and spin–orbit propensities in the ground and first excited vibrational states. With the aid of the CBS-QB3 level of ab initio theory and Rice–Ramsperger–Kassel–Marcus calculations, it is predicted that on the lowest doublet potential energy surface the major reaction channel of O(3P) with C3H5 is the formation of acrolein (CH2CHCHO)+H, which is consistent with the previous bulk kinetic experiments performed by Gutman et al. [J. Phys. Chem. 94, 3652 (1990)]. The counterpart C3H4 of the probed OH product in the title reaction is calculated to be allene after taking into account the factors of reaction enthalpy, barrier height and the number of intermediates involved along the reaction pathway. On the basis of population analyses and comparison with prior calculations, the statistical picture is not suitable to describe the reactive atom-radical scattering processes, and the dynamics of the title reaction is believed to proceed through two competing dynamical pathways. The major low N″-components with significant vibrational excitation may be described by the direct abstraction process, while the minor but extraordinarily hot rotational distribution of high N″-components implies that some fraction of reactants is sampled to proceed through the indirect short-lived addition-complex forming process.

Quantum state-resolved reactive scattering of F+CH4→HF(v,J)+CH3: Nascent HF(v,J) product state distributions

State-to-state reactive scattering of F+CH4→HF(v,J)+CH3 is studied using crossed supersonic jets and high-resolution (Δν≈0.0001 cm−1) IR laser direct absorption techniques. Rovibrational state-resolved HF column-integrated absorption profiles are obtained under single collision conditions and converted to populations via appropriate density-to-flux transformation. Nascent rovibrational distributions in each HF(v,J) state are reported. Summed over all product rotational levels, the nascent vibrational quantum state populations for HF(v) [(v=3) 0.106(3); (v=2) 0.667(14); (v=1) 0.189(27); (v=0) 0.038(78); 2σ error bars] are in agreement with previous flow cell studies by Setser, Heydtmann, and co-workers [Chem. Phys. 94, 109 (1985)]. At the rotational state level, however, the current studies indicate nascent distributions for HF(v,J) that are significantly hotter than previously reported, ostensibly due to reduced collisional relaxation effects under supersonic jet conditions. Final HF rotational states from F+CH4 are observed near the maximum energetically accessible J values in both the v=2 and v=3 vibrational manifolds, which provides experimental support for a bent F–H–C transition state structure.

Photofragmentation of ammonia at 193.3 nm: Bimodal rotational distributions and vibrational excitation of NH2(Ã)

Time-resolved Fourier transform infrared emission spectroscopy is used to measure the nascent rovibrational distribution of low-lying electronically excited NH2(Ã 2A1) produced in the 193.3 nm photolysis of room-temperature and jet-cooled ammonia. Emission is observed predominantly from NH2(Ã) states with rotational motion about the a-axis and without bending excitation, υ2′=0. A bimodal N′=Ka′ rotational state population distribution is observed with up to Ka′=7 in υ2′=0 and with maxima at Ka′=5 and Ka′=1. We suggest that the bimodal rotational distribution may result from the competition between planar and bent geometries during dissociation. Weaker emission from NH2(Ã) with bending excitation, υ2′=1 and 2, is detected; the υ2′=1, N′=Ka′ rotational state population distribution spans from Ka′=0 to the energetic limit of Ka′=4. The vibrational energy partitioning for the formation of NH2(Ã,υ2′=0):NH2(Ã,υ2′=1) is 3:1 and 2:1 in the room-temperature and jet-cooled conditions, respectively. An upper limit of the NH2(Ã,υ2′=2) population is ∼10% of the total NH2(Ã) photofragments. Emission from rotational states with N′>Ka′ (molecules with rotational excitation about the b/c-axes) is also observed. Under jet-cooled conditions the NH2(Ã) b/c-axes rotational temperature of ∼120 K is higher than that expected from the rotationally cold parent species and is attributed to a mapping of the zero-point bending motion in the ν4 H–N–H scissors bending coordinate of the NH3(Ã) predissociative state onto the NH2(Ã,υ2′,N′,Ka′)+H photofragments.

Nascent rovibrational distribution of NO+(A 1Π) produced from charge-transfer reaction of He2+ with NO at thermal energy

The NO+(A 1Π-X 1Σ+) emission resulting from the He2+/NO charge-transfer reaction at thermal energy has been observed in a He flowing afterglow. The vibrational and rotational distributions of NO+(A) were determined from a spectral simulation. The average vibrational and rotational energies deposited into NO+(A) were determined to be 0.22±0.02 and 0.10±0.1 eV, respectively. The vibrational population of NO+(A) decreases rapidly for v′=0–2 and becomes flat for v′=3,4, indicating that the vibrational distribution is bimodal. The bimodal vibrational distribution was explained as due to either two different entrance channels or two different dynamics (Demkov or Landau–Zener type). The rotational distributions were expressed by single Boltzmann temperatures of 1170±100 K.

Nascent rovibrational distributions of CO(d 3Δi,e 3Σ−,a′ 3Σ+) produced in the dissociative recombination of CO2+ with electrons

The d 3Δi–a 3Πr, e 3Σ−–a 3Πr, and a′ 3Σ+–a 3Πr transitions of CO resulting from the dissociative recombination of CO2+(X̃ 2Πg:0,0,0) with electrons have been observed from the He afterglow reaction of CO2. The formation rate constants of CO(d), CO(e), and CO(a′) were estimated to be 1.6×10−7, 3.3×10−9, and 2.4×10−7 cm3 s−1, respectively. The vibrational and rotational distributions of CO(d:v′=0–6,e:v′=2,3,a′=3–11) were determined. Most of available excess energies (91%∼98%) were deposited into the vibrational energy of CO(d,e,a′) and the relative translational energies of the products, indicating that CO(d,e,a′) were produced by direct curve crossings between the entrance e−/CO2+(X̃ 2Πg:0,0,0) potential and repulsive CO(d,e,a′)+O(3P) potentials with linear geometries. The vibrational distributions of CO(d) and CO(a′) slightly shifted to lower states than those in photodissociation at a similar excitation energy. A simple statistical model was unable to explain the observed vibrational distributions obtained by dissociative recombination.

Dissociative Excitation of CH4 via Triplet Superexcited State by Collisions with the Metastable Ne(3P0,2) Atoms in a Neon Flowing Afterglow

Abstract The CH(A-X, B-X, C-X) emission systems have been observed in the energy transfer reaction of the metastable Ne(3P0,2) atoms with CH4 in a neon flowing afterglow. The nascent rovibrational distributions of CH(A,B) were determined by a spectral simulation: N0:N1:N2 = 100(T0 = 3500 ± 300K):58 ± 3(T1 = 2800 ± 200K):9 ± 1(T2 = 2400 ± 300K) for CH(A) and N0 = 100 (T0 = 3000 ± 300K) for CH(B).

The study of E- E energy transfer between CO(a 3Π, ν′) and NO(X 2Π, ν″ = 0) in beams

The formation of NO(A,ν) and NO(B,ν) was investigated from the reaction CO(a, ν′) + NO(X, ν″ = 0) in a beam-beam configuration. The nascent rovibrational distributions of NO(A, ν = 0–2) and NO(B, ν = 0–2) were determined from NO(A → X) and NO(B → X) emissions by means of spectral simulation. The branching rate of NO(A) to NO(B) was determined to be γ/ β = 1.3 ± 0.3 by using the total emission intensities of NO(A) and NO(B). Experiments also show that the vibrational distribution of NO(A) is sensitive to the vibrational distribution of CO(a,ν′). An electron exchange mechanism is proposed for the reaction.

Nascent rovibrational distribution of CO(A 1Π) produced in the recombination of CO+2 with electrons

The dissociative electron–ion recombination processes of CO+2(X̃ 2Πg:0,0,0) has been studied by observing the CO(A 1Π–X 1Σ+) emission in the He and Ar afterglows. It was found that the CO(A:v′=0–2) states are formed in the dissociative recombination of CO+2(X̃:0,0,0) with electrons at thermal energy. The rovibrational distribution of CO(A) was N0:N1:N2=100:(T0=1000±100 K), 58±4(T1=700±50 K), and 9±2 (T2=400±100 K). The average fractions of total energy channeled into vibration and rotation of CO(A) and relative translation of the products were determined to be 〈fv〉=22%±2%, 〈fr〉=20%±2%, and 〈ft〉=58%±4%. The observed rovibrational distributions were in disagreement with statistical prior distributions, indicating that the reaction dynamics is not governed by the statistical theory. A comparison of the present results with the previous photodissociation data suggested that the CO(A:v′=0,1) states are formed through predissociation of near-resonant intermediate CO2** states coupled with a bent valence state, while the CO(A:v′=2) state is produced through predissociation of CO2** states just above the CO+2(X̃:0,0,0) state. The low CO(A:v′=2) population can be explained by the energetic constraint for thermal electrons plus CO+2(X̃:0,0,0) and/or a competition between predissociation and autoionization of CO2** states just above the CO+2(X̃:0,0,0) energy.

Dissociative excitation of CH3NO2 and CH3OH by collisions with metastable helium in the beam experiment

Dissociative excitation of CH3NO2 and CH3OH by collisions with He (22S) has been studied by observing CH (A2Δ-X2Πr B2Σ-X2Πr, C2Σ+-X2Πr), OH(A2Σ-X2Π) and H(Balmer) emissions in the beam apparatus. The emission rate constants were determined. The nascent rovibrational distributions of CH(A, v'= 0–2) were obtained by means of computer simulation. The dissociative process and the formation mechanism of CH(A) and H∗ were studied. A two-body dissociation process through intermediate CH∗3 for CH(A) was suggested.

Energy transfer reactions of the He(3 2S) atom with methane and chloromethane molecules under beam conditions

Energy transfer reactions of metastable He (2 3S) with methane and chloromethane molecules were investigated by measuring the emission spectra of the electronically excited fragments under beam conditions. The formation rate constants of the fragments CH(A 2Δ), CH(B 2∑ −), CH(C 2∑ +), H * and CCl(A 2Δ) for the reactions He(2 3S) + CH n Cl 4- n ( n = 1−4) have been determined. The nascent rovibrational distributions of CH(A 2Δ, v′ = 0−2) and CH(B 2∑ −, v′ = 0) have been obtained. The experimental results show that the rotational distribution of CH(A 2Δ, v′ = 0) from the reaction of He(2 3S) with methane is different from that of He (2 3S) with chloromethanes. In the former case, the rotational distribution can be expressed by a single Boltzmann temperature, while in the latter it is not a complete equilibrium distribution and may be expressed by a double Boltzmann distribution. A discussion of the reaction mechanism based on statistical theory is presented.

Rovibrational distributions of CH(A 2Δ,B 2Σ−) produced in energy-transfer reactions from Ar(3P2), Kr(3P2), and Xe(3P2) atoms to CH3 radical

Energy-transfer reactions from Ar(3P2), Kr(3P2), and Xe(3P2) to CH3 radical have been studied by observing emission spectra from excited fragments in the flowing afterglow. CH3 radicals were generated by the F+CH4 reaction. The CH(A 2Δ–X 2Πr:v′=0−2) and CH(B 2Σ−–X 2Πr:v′=0) emission systems were observed in the Ar(3P2) reaction, while only CH(A–X:v′=0,1) emission system was found in the Kr(3P2) and Xe(3P2) reactions. The nascent rovibrational distributions of CH(A:v′=0–2) were N0:N1:N2 =100(T0 =3400±400 K):28±5(T1 =1700±400 K):4±1(T2 =700±300 K) in the Ar(3P2) reaction and 100(T0 =1000±250 K):<5(T1 <800 K):0 in the Kr(3P2) and Xe(3P2) reactions. The rotational distribution of CH(B:v′=0) in the Ar(3P2) reaction was reproduced by a single Boltzmann temperature of 2800±300 K. The average fractions of total available energies channeled into vibration and rotation of CH(A,B) were less than 15% for all cases, suggesting that most of the available energies was deposited as relative translational energy of products and/or rovibrational energy of H2. The observed rovibrational distributions of CH(A) were colder than those predicted from statistical theories including and excluding the conservation of total angular momentum. The best agreement between the observed and statistical distributions was obtained for the mechanism giving CH(A,B) in two-body dissociation steps by assuming that 78–92% of the total available energy is released as kinetic energy in the first step, Rg(3P2)+CH3→CH*3+Rg, then the rest remains in the precursor CH*3 state as an internal energy.

Nascent Rovibrational Distributions of CO(d3Δi, a′3Σ+) Produced by Excitation-Transfer Reactions from Xe(3P2) to 12CO and 13CO at Thermal Energy

The excitation-transfer reactions from Xe(3P2) to 12CO and 13CO at 39 meVcm have been studied using a beam apparatus. The near-resonant endoergic d3Δi(v′=6, 7) and a′3Σ+(v′=11) levels of CO were selectively excited for both isotopes. The nascent rovibrational distributions of CO(d, a′) were determined from a spectral simulation. The rotational distributions were expressed by Boltzmann temperatures of 300-650 K for all levels. The low rotational excitation was explained by the fact that curve crossings between the entrance and exit potentials occur in the attractive region of the exit potentials. The spin-orbit state selectivity was estimated for the population of the 12CO(d3Δ1,2,3:v′=6) sublevels.

Optical Spectroscopic Studies on Penning Ionization and Charge-Transfer Reactions at Thermal Energy by Using Flowing Afterglow

The flowing afterglow apparatus has been coupled with a low pressure chamber by a small orifice for optical spectroscopic studies on Penning ioniza­ tion and charge-transfer reactions under single collision conditions. Nascent rovibrational distributions are determined from a spectral simulation. As selected examples, detailed results are reported about non-Franck-Condon type He(2 3 S)/02 Penning ionization and near-resonant He+ /H20, D20 charge-transfer reactions. A brief summary is also given of our optical spectroscopic studies by using the conventional flowing afterglow apparatus. The flowing afterglow (FA) method was developed in 1963 by Ferguson and his co-workers of NOAA Laboratories in Boulder, Colorado, for the study of ion-molecule reactions in the earth's atmosphere.o The main emphasis of their studies was placed upon measurements of formation rate constants and branching ratios of ionic products by using a mass spectrometer. More than a few thousands kinetic data at (near) thermal energy were compiled in the reported tables. 2 ) In our laboratory, the FA coupled with a DV and visible emission detection system has been used for studying the following ionization processes in collisions of metastable atoms (M*) and ions (M+) with a molecule (AB): M*+AB~ABH+M+e-, (Penning ionization) (1) ~ A H + B + M + e-, (Dissociative Penning ionization) (2)

Dissociative excitation of SiH 4 by collisions with krypton active species

The dissociative excitation of SiH 4 by collisions with metastable Kr( 3P 2) atoms and (Kr +) * ions has been studied using flowing afterglow and beam apparatus. The emission rate constants of SiH(A 2Δ) and Si * produced from the Kr( 3P 2)/SiH 4 reaction were determined to be (1.0±0.3)×10 −11 and (0.0069±0.0021)×10 −11 cm 3 s −1, respectively. The nascent rovibrational distribution of SiH(A) was estimated from a spectral simulation: N 0: N 1: N 2 = 100 ( T 0 = 1400 K):40 ( T 1 = 1100 K):5 ( T 2 = 800 K). When (Kr +) * ions were involved in the discharge flow, strong Si * lines resulting from electron-ion recombination processes were observed.

Violet emission of cyanogen produced in the reaction of argon(3P0,2) with cyanogen bromide. 1. Nascent vibrational and rotational distributions of CN(B2.SIGMA.+)

The CN(B 2 Σ + -X 2 Σ + ) emission spectrum, produced in the dissociative excitation reaction of BrCN with Ar( 3 P 0,2 ) at thermal collision energy, was observed by using the flowing afterglow method. The effect of collisional relaxation of the excited electronic states of CN by the ground-state Ar atoms was minimized by lowering the ambient argon pressure to less than 10 mTorr. The «nascent» rovibrational distribution of CN(B 2 Σ + ) was determined by spectral simulation. A surprisal analysis of the nascent rovibrational distribution of CN(B 2 Σ + ) indicated that the dominant pathway of the reaction of BrCN with Ar( 3 P 0,2 ) was energy transfer, instead of argon complex formation of ion-pair exciplex formation

Dissociative excitation of SiH 4 by collisions with metastable argon atoms

The excitation transfer from Ar* to SiH 4 has been studied by observing UV and visible emission in a flowing afterglow and beam apparatus. The emission rates of Sill (A 2Δ) and Si* atoms have been determined. From the nascent rovibrational distribution of SiH(A), the fractions of the available energy deposited into vibration and rotation of SiH(A) were estimated to be 2.9% and 4.4%, respectively.

Energy partitioning in Ar++O2 collisions at low energies: Analysis of product states by laser-induced predissociation

Combining the versatility of a guided ion beam (GIB) apparatus with a tunable dye laser system, we have studied in detail the ion–molecule reaction Ar++O2→O+2+Ar at collision energies ranging from 0.04 to 3 eV center of mass (c.m.). The results include integral cross sections and product angular distributions. The extracted kinetic energy distributions provide medium resolution information about the energy partitioning, and are indicative of a significant change of the reaction mechanism between 0.05 and 0.5 eV collision energy. Nascent rovibrational state distributions of metastable O+2(a 4Π) products are obtained from 0.5 to 1.4 eV by photofragmentation. In contrast to what is generally expected from charge transfer processes, a preference for excitation of high rotational states at the expense of vibrational states has been observed. The results are discussed qualitatively on the basis of a diatomics-in-molecules (DIM) surface.