OH Rotational Excitation Research Articles

Velocity map imaging of OH radical products from IR activated (CH3)2COO Criegee intermediates.

The unimolecular dissociation dynamics of the dimethyl-substituted Criegee intermediate (CH3)2COO is examined experimentally using velocity map imaging to ascertain the translational and internal energy distributions of the OH and H2CC(CH3)O radical products. The energy profile of key features along the reaction coordinate is also evaluated theoretically. Unimolecular decay of (CH3)2COO is initiated by vibrational activation in the CH stretch overtone region and the resultant OH X(2)Π3/2 (v = 0) products are state-selectively ionized and imaged. Analysis reveals an isotropic spatial distribution, indicative of a 3 ps lower limit for the timescale of dissociation, and a broad and unstructured total kinetic energy release distribution. The energy released to products is partitioned principally as internal excitation of the H2CC(CH3)O fragments with modest translational excitation of the fragments and a small degree of OH rotational excitation. The total kinetic energy release distribution observed for (CH3)2COO is compared with that predicted for statistical partitioning over product quantum states, and contrasted with recent experimental and quasi-classical trajectory results for syn-CH3CHOO [N. M. Kidwell et al., Nat. Chem. 8, 509 (2016)].

UV photochemistry of peroxyformic acid (HC(O)OOH): an experimental and computational study investigating 355 nm photolysis.

The photochemistry of peroxyformic acid (PFA), a molecule of atmospheric interest exhibiting internal hydrogen bonding, is examined by exciting the molecule at 355 nm and detecting the nascent OH fragments using laser-induced fluorescence. The OH radicals are found to be formed in their ground electronic state with the vast majority of available energy appearing in fragment translation. The OH fragments are vibrationally cold (v" = 0) with only modest rotational excitation. The average rotational energy is determined to be 0.35 kcal/mol. Further, the degree of OH rotational excitation from PFA is found to be significantly less than that arising from the dissociation of H2O2 as well as other hydroperoxides over the same wavelength. Ab initio calculation at the EOM-CCSD level is used to investigate the first few electronic excited states of PFA. Differences in the computed torsional potential between PFA and H2O2 help rationalize the observed variation in their respective OH fragment rotational excitation. The calculations also establish that the electronic excited state of PFA accessed in the near UV is of (1)A" symmetry and involves a σ*(O-O) ← n(O) excitation. Additionally, the UV absorption cross section of PFA at 355 and 282 nm is estimated by comparing the yield of OH from PFA at these wavelengths to that from hydrogen peroxide for which the absorption cross sections is known.

Electronic quenching of OH AΣ+2 radicals in single collision events with molecular hydrogen: Quantum state distribution of the OH XΠ2 products

We report a combined experimental and theoretical investigation of the nonreactive quenching channel resulting from electronic quenching of OH A 2Sigma+ by molecular hydrogen. The experiments utilize a pump-probe scheme to determine the OH X 2Pi population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH A 2Sigma+ (nu'=0, N'=0), which has a significantly reduced fluorescence lifetime due to quenching by H2. The probe laser monitors the OH X 2Pi (nu", N") population via laser-induced fluorescence on various A-X transitions under single collision conditions. The experiments reveal a high degree of rotational excitation (N") of the quenched OH X 2Pi products observed in nu"=1 and 2 as well as a pronounced propensity for quenching into the Pi(A') Lambda-doublet level. These experiments have been supplemented by extensive multireference, configuration-interaction calculations aimed at exploring the topology of the relevant potential energy surfaces. Electronic quenching of OH A 2Sigma+ by H2 proceeds through conical intersections between two potentials of A' reflection symmetry (in planar geometry) that correlate with the electronically excited A 2Sigma+ and ground X 2Pi states of OH. The conical intersections occur in high-symmetry geometries, in which the O side of OH points toward H2. Corroborating and extending earlier work of Hoffman and Yarkony [J. Chem. Phys. 113, 10091 (2000)], these calculations reveal a steep gradient away from the OH-H2 conical intersection as a function of both the OH orientation and interfragment distance. The former will give rise to a high degree of OH rotational excitation, as observed for the quenched OH X 2Pi products.

Decay dynamics of the vibrationally activated OH–CO reactant complex

The decay dynamics of the OH–CO reactant complex have been examined following infrared excitation in the OH overtone region using various IR pump–UV probe methods. The time scale for overall decay of the OH–CO (2vOH) complex has been bracketed between 0.19 and 5 ns through linewidth and direct time-domain measurements. The inelastically scattered OH (v=1) fragments exhibit a bimodal internal energy distribution, which reveals that vibrational predissociation proceeds through two pathways. The dominant inelastic decay channel involves vibrational energy transfer from OH to CO with little excess energy remaining for rotational excitation of the OH fragment, while a slower secondary channel releases most of the excess energy as OH rotational excitation. Intermolecular bending excitation of the OH–CO complex through combination bands results in increased rotational excitation of the OH fragments. The most probable OH product states display a strong lambda-doublet preference indicating that the singly occupied pπ orbital of OH is aligned perpendicular to the OH rotation plane following vibrational predissociation of the complex. These product states also minimize the translational recoil of the fragments and maximize the rotational angular momentum of the OH fragment. Abrupt cutoffs in the OH (v=1) fragment internal energy distributions are utilized to determine an upper limit for the ground state binding energy of OH–CO, D0⩽410 cm−1, which is in good accord with ab initio predictions. Finally, a comparison of infrared band intensities obtained using action and depletion detection methods suggests that geared bend and H-atom bend excitation may promote reactive decay of the OH–CO reactant complex.

Variational transition state theory and quasiclassical trajectory studies of the H2+OH→H+H2O reaction and some isotopic variants

Variational transition state theory (VTST) methods and quasiclassical trajectories (QCT) have been used to study the dynamics of the OH+H2 reaction, along with the isotopic counterparts OD+H2, OH+HD, OD+H2, OD+D2, and the reverse H+H2O→H2+OH reaction. Two new global analytical potential energy surfaces (PES) for H3O are employed, Wu, Schatz, Lendvay, Fang, Harding (WSLFH) and Ochoa, Clary (OC), both of which are based on high quality electronic structure calculations. Extensive comparisons with earlier results based on the Walch, Dunning, Schatz, Elgersma (WDSE) PES are also presented. The WSLFH PES surface, in combination with our best VTST estimate (ICVT/μOMT), yields rate constants for OH+H2 in quantitative agreement with experiment, while the OC PES yields somewhat less accurate results. The agreement with the OH+D2 experimental rate constants is less quantitative, but the WSLFH PES rate constant agrees with experiment to within a factor of 2 at all temperatures for which there are measurements. The OH+HD, OD+H2, and OD+D2 WSLFH PES rate constants calculated at the ICVT/μOMT level are in very good agreement with the less detailed experimental information that is available for these isotopes. The two surfaces give comparable predictions for the reverse H+H2O reaction at high temperatures, with deviations of less than 30%. This good agreement is maintained by the WSLFH PES at room temperature, while the OC PES predicts rate constants one order of magnitude larger than experiment. The QCT excitation functions for OH+H2, OH+D2, and OH+HD are well below experiment for both potentials, as was the case for earlier accurate quantum mechanical calculations that employed the WDSE PES. The WSLFH PES improves the agreement with the experimental vibrational state selected rate constants for the OH+H2 reaction compared to the WDSE PES. OC is also less accurate and presents antithreshold behavior for H2(v=1)+OH. H2 and OH rotational excitation have opposing effects: while rotation in H2 promotes reactivity, OH rotation impedes it. This impeding effect applies likewise to HD for high rotational excitation, explaining the selectivity toward HOH+D products in the OH+HD reaction.

Effects of reagent rotation on the dynamics of the H2+OH reaction: A full dimension quantum study

We have extended the time-dependent wave packet method to calculate cross sections and rate constants for rotationally excited initial states by using the centrifugal sudden (CS) approximation. A detailed study of the effects of rotational excitation of reagents on the title reaction on the WDSE PES has been carried out. It is found that (a) OH rotational excitation very mildly enhances the total cross section, (b) H2 rotational excitation quite substantially reduce the cross section, and (c) simultaneous OH and H2 rotational excitation has a largely uncorrelated effect. As a result, we found that the thermal rate constant can be obtained fairly accurately by only taking into account the effect of H2 rotation. A model calculation by changing the mass of an O atom reveals that the weak dependence of the cross section on OH rotation is not because the O atom is left relatively stationary by OH rotation. We speculate that it may be a general feature for the diatom-diatom reaction that the nonreactive diatom acts as a spectator not only vibrationally but also rotationally. It was also found that the “J-shifting” approximation works quite well for the reaction. On the other hand, the effect of K on the dynamics is found to be much stronger and more complicated than the J effect, making the “K-shifting” approximation not good for the reaction.

Laser-induced fluorescence diagnostics of a propane/air flame with a manganese fuel additive

Laser-induced fluorescence (LIF) has been used to investigate the effects of MMT, a managanese-containing fuel additive, on a premixed propane/air flame burning at 40 torr. The emphasis was placed on the chemistry of formation of prompt nitric oxide. No measurable effect due to the MMT on the gas-phase chemistry of this flame was observed. LIF profiles as a function of burner height were made for OH, H, O, CH, NO, CO, Mn, MnO, and temperature using OH rotational excitation scans. Photochemical interferences were observed when exciting O, H, CO, and NO deep in the ultraviolet (205–230 nm), due either to laser-generated trace species or concomitant LIF from other species. Low laser power is required to avoid these problems. The interferences were significantly more severe in this propane flame than seen previously for a methane flame under similar conditions, most certainly due to absorption and photochemical activity of pyrolytic and partially oxidized fragmentary hydrocarbons with two or more carbons. This suggests caution when applying laser diagnostics (including excimer lasers) to flames with complex hydrocarbon fuels. Mn and MnO LIF detection strategies were developed on an atmospheric pressure burner and applied to this low pressure flame. The Mn LIF signals are strong but those from MnO are of low intensity and in a region of strong LIF from C 2 molecules.

Quantum scattering calculations on the CH4+OH→CH3+H2O reaction

Quantum scattering calculations on the CH4+OH→CH3+H2O reaction have been performed at thermal energies. The rotating bond approximation is used, treating CH3 as a pseudoatom. The OH rotation and a reactive C–H stretch of CH4 are treated explicitly as well as the bending motion and one OH local stretch vibration of H2O. Two potential energy surfaces are used. Both have accurate reactant and product rovibrational energy levels for the modes explicitly treated in the scattering calculations and incorporate the zero point energy of the other modes. They have correct bond dissociation energies and transition state geometries in reasonable accord with ab initio data. Mode selectivity is found: reactants in the ground rovibrational states produce ground state H2O, and vibrationally excited CH4 produces vibrationally excited H2O. Reactant OH rotational excitation decreases the reaction cross sections. Rate constants are obtained using an adiabatic approach to account for all degrees of freedom not explicitly treated in the scattering calculations. Large contributions due to tunneling are observed. The rate constants are in quite good agreement with previous theoretical and experimental work.

LIF measurements in methane/air flames of radicals important in prompt-NO formation

Laser-induced fluorescence measurements have been made of the OH, CH, and NO radicals in slighly rich methane/air flames burning at 30, 70, and 120 torr. Absolute NO and OH concentrations were determined using separate calibration experiments. Temperature profiles were deduced from OH rotational excitation scans, and fluorescence quenching rates for NO and CH were determined. Predicted profiles of the concentrations of these radical species as a function of height above the burner were obtained from a computer model of the flame, and comparison made with experiment. Good agreement is achieved for the relative concentration profiles, the absolute NO concentration, and concentration ratios between 30 and 70 torr, although slight disagreement with the CH profile indicates there remain some unknown aspects of the flame chemistry.

Decay dynamics of H 2O( 1B 1): full characterization of OH product state distribution

H 2O was excited to the lowest electronic excited state, 1B 1, at 157.6 nm and the OH product state distribution was completely analyzed by special laser-induced-fluorescence (LIF) experiments probing the fragment distribution by the transitions (υ′ = 0, 1←υ″). The OH rotational excitation is relatively low and can be described by a temperature parameter of ≈ 500 K independently of the OH vibrational excitation. This is in accordance with former measurements as well as with theoretical calculations. No selective population of the electronic fine-structure levels was observed which is in agreement with the expectations for a room-temperature experiment. The observed magnitude of the vibrational excitation ( P(υ″ = 0): P(υ″ = 1): P(υ″ = 2): P(υ″ = 3): P(υ″ = 4) = 59.2:33.1:6.1:1.4:0.2) is smaller than the one calculated. The calculations predicted the υ″ = 6 level to be populated, whereas in the experiment, transitions probing the υ″ = 5 state were not observed.

Photodissociation dynamics of HCOOH(Ã)

The technique of Doppler-resolved, polarised LIF spectroscopy has been employed in a study of the energy and momenta disposal in the OH fragments generated via photodissociation of HCOOH (Ã 1A″) at 225 nm. The majority of the excess energy resides in fragment translation (≈ 78%) and internal excitation of the HCO fragment (≈ 12%). An analysis of the Doppler-resolved profiles reveals that OH rotational excitation (for N″ ≈ 6) is generated primarily by out-of-plane C-OH torsional motion in the excited parent molecule. The results are compared with those for the OH fragments generated via photodissociation of the iso-electronic molecule HONO(Ã 1A″).

Photodissociation dynamics of H 2O 2 at 193 nm: An example of the rotational reflection principle

The photodissociation of H 2O 2 at 193 nm is examined using classical mechanics and ab initio potential energy surfaces for the two lowest excited states (Ã, 1A and B̃ 1B). In accordance with recent experimental studies we find that OH rotational excitation originates primarily from the dependence of the excited-state potentials on the OH-OH torsional angle. Scalar properties (rotational state distributions) as well as vector properties ( v, j correlation) are compared with experimental results and good agreement is achieved. The rotational state distributions provide nice examples of the rotational reflection principle.

H+H2O reaction dynamics: state distribution for the OH product

A method has been developed to study the dynamics of high barrier reactions by combining hot-atom production by laser photolysis and fast-product detection by laser induced fluorescence. The nascent OH rotational, vibrational and fine structure state distributions produced in the endothermic reaction\(H + H_2 O \to \underline {OH(K,\upsilon ,f)} + H_2\) have been measured with hot hydrogen atoms from the photodissociation of HBr at 193 nm. OH rotational excitation is low and corresponds to a partitioning of only around 3% of the total reaction energy to OH rotation. OH could only be observed in the vibrational ground state, i.e.σ(v=1)/σ(v=0)≦0.1. The OH spin doublets are produced statistically, but the partitioning in the λ-doublets is very non-statistical with a strong preference for the π+-component (σ(π+)/σ(π−)=3.2±1.0) demonstrating the exit channel of the reaction to be preferentially planar. By measuring reactant and product densities at short times, an absolute total cross-section of 0.24±0.1[A2] has been obtained.

Hot atom-laser induced fluorescence experiments on the reaction of H(2S) with O2

A method has been developed to study the dynamics of reactions with substantial threshold energies by combining hot atom production by laser photolysis and fast product detection by laser induced fluorescence. The nascent OH rotational, vibrational, and fine structure state distributions produced in the endothermic reaction H+O2→OH(K,v, f )+O have been measured with hot hydrogen atoms from the photodissociation of HBr at 193 nm. OH rotational excitation is high and corresponds to the phase space statistical distribution over the measured range of rotational states. Also OH vibrational excitation is substantial. The OH spin doublets are produced statistically, but the partitioning in the λ doublets is very nonstatistical with a strong preference for the π+ component demonstrating the reaction to occur essentially in a plane at high collision energies. By measuring reactant and product densities at short times, an absolute total cross section of 0.42±0.40.2 (Å2) has been obtained.

Hot-atom—laser-induced-fluorescence experiments on the reaction of H(2S) with CO2

The nascent rotational and fine-structure state distributions in OH(υ = 0) produced in the endothermic reaction H + CO2 → OH + CO have been measured with hot hydrogen atoms from the photodissociation of HBr at 193 nm. Considerable OH rotational excitation broadly peaked at aroundK = 11 is observed. The OH spin doublets are produced statistically (±10%). The partitioning between the λ doublets is non-statistical with a strong preference for the π+ component (σ(π+)/σ(π−) = 3.0 ± 1.0), consistent with a planar HOCO intermediate whose OCOH bond breaks in that plane.

The chemical dynamics of hydrogen atom abstraction from unsaturated hydrocarbons by O(3P)

By molecular beam-laser induced fluorescence experiments, we have probed the nascent internal state distribution and dynamic energy thresholds of OH formed by the abstraction of hydrogen atoms from alkyl substituted olefins and alkynes. Low thresholds are obtained, especially for olefins, suggesting that this is an important mechanism in high temperature combustion. OH rotational excitation is small and implies a collinear transition state similar to that observed previously for O(3P) reaction with saturated hydrocarbons. OH vibrational excitation is also low for the olefins and suggests that considerable internal excitation of the allyl or methyl allyl radicals occur in the reactions.