Rovibrational Distributions Research Articles

A combined experimental and theoretical study of the reaction between methylglyoxal and OH/OD radical: OH regeneration

Experimental studies have been conducted to determine the rate coefficient and mechanism of the reaction between methylglyoxal (CH(3)COCHO, MGLY) and the OH radical over a wide range of temperatures (233-500 K) and pressures (5-300 Torr). The rate coefficient is pressure independent with the following temperature dependence: k(3)(T) = (1.83 +/- 0.48) x 10(-12) exp((560 +/- 70)/T) cm(3) molecule(-1) s(-1) (95% uncertainties). Addition of O(2) to the system leads to recycling of OH. The mechanism was investigated by varying the experimental conditions ([O(2)], [MGLY], temperature and pressure), and by modelling based on a G3X potential energy surface, rovibrational prior distribution calculations and master equation RRKM calculations. The mechanism can be described as follows: Addition of oxygen to the system shows that process (4) is fast and that CH(3)COCO completely dissociates. The acetyl radical formed from reaction (4) reacts with oxygen to regenerate OH radicals (5a). However, a significant fraction of acetyl radical formed by reaction (R4) is sufficiently energised to dissociate further to CH(3) + CO (R4b). Little or no pressure quenching of reaction (R4b) was observed. The rate coefficient for OD + MGLY was measured as k(9)(T) = (9.4 +/- 2.4) x 10(-13) exp((780 +/- 70)/T) cm(3) molecule(-1) s(-1) over the temperature range 233-500 K. The reaction shows a noticeable inverse (k(H)/k(D) < 1) kinetic isotope effect below room temperature and a slight normal kinetic isotope effect (k(H)/k(D) > 1) at high temperature. The potential atmospheric implications of this work are discussed.

The mechanism of H-bond rupture: the vibrational pre-dissociation of C2H2–HCl and C2H2–DCl

Pair correlated fragment rovibrational distributions are presented following vibrational predissociation of the C2H2-DCl van der Waals dimer initiated by excitation of the asymmetric (asym) C-H stretch. The only observed fragmentation pathways are DCl (v= 0; j= 6-9)+ C2H2(nu2= 1; j= 1-5). These and previously reported data on the related C2H2-HCl species are analysed using the angular momentum (AM) method. Calculations accurately reproduce fragment rovibrational distributions following dissociation of the C2H2-HCl dimer initiated either by excitation of the asym C-H stretch or via the HCl stretch, and those from C2H2-DCl initiated via asym C-H stretch excitation. The calculations demonstrate that the dimer is bent at the moment of dissociation. Several geometries are found that lead to H-bond breakage via a clearly identified set of fragment quantum states. The results suggest a hierarchy in the disposal of excess energy and angular momentum between fragment vibration, rotation and recoil. Deposition of the largest portion of energy into a C2H2 vibrational state sets an upper limit on HCl rotation, which then determines the energy and AM remaining for C2H2 rotation and fragment recoil. Acceptor C2H2 vibrational modes follow a previously noted propensity, implying that the dissociating impulse must be able to induce appropriate nuclear motions both in the acceptor vibration and in rotation of the C2H2 fragment.

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

Correlated vH2 and jCO product states from formaldehyde photodissociation: Dynamics of molecular elimination

A detailed study of the photoinduced molecular elimination pathway of formaldehyde on the ground state surface was carried out using high-resolution dc slice ion imaging. Detailed correlated H(2) rovibrational and CO rotational product quantum state distributions were measured by imaging spectroscopically selected CO velocity distributions following photodissociation at energies from approximately 1800 to approximately 4100 cm(-1) above the barrier to molecular elimination. Excitation to the 2(1)4(1), 2(1)4(3), 2(2)4(1), 2(2)4(3), and 2(3)4(1) bands of H(2)CO are reported here. The dependence of the product rovibrational distributions on excitation energy are discussed in light of a dynamical model which has been formulated to describe the strong product state correlations observed.

A Photoelectron and TPEPICO Investigation of the Acetone Radical Cation

The valence shell photoelectron spectrum, threshold photoelectron spectrum, and threshold photoelectron photoion coincidence (TPEPICO) mass spectra of acetone have been measured using synchrotron radiation. New vibrational progressions have been observed and assigned in the X 2B2 state photoelectron bands of acetone-h6 and acetone-d6, and the influence of resonant autoionization on the threshold electron yield has been investigated. The dissociation thresholds for fragment ions up to 31 eV have been measured and compared to previous values. In addition, kinetic modeling of the threshold region for CH3* and CH4 loss leads to new values of 78 +/- 2 kJ mol(-1) and 75 +/- 2 kJ mol(-1), respectively, for the 0 K activation energies for these two processes. The result for the methyl loss channel is in reasonable agreement with, but slightly lower than, that of 83 +/- 1 kJ mol(-1) derived in a recent TPEPICO study by Fogleman et al. The modeling accounts for both low-energy dissociation channels at two different ion residence times in the mass spectrometer. Moreover, the effects of the ro-vibrational population distribution, the electron transmission efficiency, and the monochromator band-pass are included. The present activation energies yield a Delta(f)H298 for CH3CO+ of 655 +/- 3 kJ mol(-1), which is 4 kJ mol(-1) lower than that reported by Fogleman et al. The present Delta(f)H298 for CH3CO+ can be combined with the Delta(f)H298 for CH2CO (-47.5 +/- 1.6 kJ mol(-1)) and H+ (1530 kJ mol(-1)) to yield a 298 K proton affinity for ketene of 828 +/- 4 kJ mol(-1), in good agreement with the value (825 kJ mol(-1)) calculated at the G2 level of theory. The measured activation energy for CH4 loss leads to a Delta(f)H298 (CH2CO+*) of 873 +/- 3 kJ mol(-1).

The rovibrational distribution of H2 and HD formed on a graphite surface at 15–50 K

The rotational distributions of H2 and HD formed on a highly oriented pyrolitic graphite surface at temperatures of 15-50 K have been measured using laser spectroscopy. The population of the rovibrational levels nu=1, J=0-4 and nu=2, J=0-4 has been observed and the average rotational temperatures of the nascent H2 and HD molecules have been determined. We find that the average rotational temperature of the newly formed molecules is much higher than the surface temperature on which they have formed. We compare our results with other recent experimental data and theoretical calculations.

Theoretical study of hydrogen dissociative adsorption on the Cu(110) surface

We have calculated the six-dimensional (6D) potential energy surface for H2 in front of a frozen Cu(110) surface using density functional theory for 22 H2-surface configurations and the corrugation reducing procedure to interpolate between them. We carry out classical trajectory calculations on the dissociative adsorption process and find excellent agreement with measurements. We find that it is of prominent importance to account for the rovibrational state distribution in the incident H2 beam. A straightforward analysis leads to the conclusion that the motion along the surface does not play an appreciable role in the dynamics whereas the dynamical role of molecular rotation is crucial. The latter fact precludes any interpretation of dissociation in terms of a static concept such as "barrier distributions."

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.

A quasi-classical trajectory study of the product energy partition and rovibrational distribution for the H + CD 4 reaction

We have performed a state-to-state dynamics study of the H + CD 4 gas-phase abstraction reaction using quasi-classical trajectory calculations on a potential energy surface (PES-2002) previously developed by our group, for collision energies in the range 0.7–2.0 eV. Most of the available energy appears as product translational energy, with the HD product being vibrationally and rotationally cold, in agreement with experiment. The CD 3 co-product appears with a noticeable internal energy, which has been neither experimentally nor theoretically reported. The results show the capacity of the PES-2002 surface to reproduce the experimental tendency.

Imaging study of vibrational predissociation of the HCl–acetylene dimer: pair-correlated distributions

The state-to-state predissociation dynamics of the HCl-acetylene dimer were studied following excitation in the asymmetric C-H (asym-CH) stretch and the HCl stretch. Velocity map imaging (VMI) and resonance enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Different vibrational predissociation mechanisms were observed for the two excited vibrational levels. Following excitation in the of the asym-CH stretch fundamental, HCl fragments in upsilon = 0 and j = 4-7 were observed and no HCl in upsilon = 1 was detected. The fragments' center-of-mass (c.m.) translational energy distributions were derived from images of HCl (j = 4-7), and were converted to rotational state distributions of the acetylene co-fragment by assuming that acetylene is generated with one quantum of C-C stretch (nu(2)) excitation. The acetylene pair-correlated rotational state distributions agree with the predictions of the statistical phase space theory, restricted to acetylene fragments in 1nu(2). It is concluded that the predissociation mechanism is dominated by the initial coupling of the asym-CH vibration to a combination of C-C stretch and bending modes in the acetylene moiety. Vibrational energy redistribution (IVR) between acetylene bending and the intermolecular dimer modes leads to predissociation that preserves the C-C stretch excitation in the acetylene product while distributing the rest of the available energy statistically. The predissociation mechanism following excitation in the Q band of the dimer's HCl stretch fundamental was quite different. HCl (upsilon = 0) rotational states up to j = 8 were observed. The rovibrational state distributions in the acetylene co-fragment derived from HCl (j = 6-8) images were non-statistical with one or two quanta in acetylene bending vibrational excitation. From the observation that all the HCl(j) translational energy distributions were similar, it is proposed that there exists a constraint on conversion of linear to angular momentum during predissociation. A dimer dissociation energy of D(0) = 700 +/- 10 cm(-1) was derived.

Quasi-classical trajectories study of Ne79Br2(B) vibrational predissociation

A full-dimensional quasi-classical trajectories study on the vibrational predissociation (VP) of the Ne79Br2(B) complex is presented. Following the most recent experiments, the Br2(B) vibrational levels v'=16-29 were explored. The total angular momentum, J, was taken to be zero, and a semiclassical Franck-Condon model to compute initial conditions from quantum distributions was employed. Predissociation lifetimes were extracted from Ne79Br2 population decay by using two different exponential laws. Predicted lifetimes are in excellent agreement with the last experimental results [J. A. Cabrera, C. R. Bieler, B. C. Olbricht, W. E. van der Veer and K. C. Janda, J. Chem. Phys., 2005, 123, 054311]. The Br2 fragment ro-vibrational distributions resulting from the VP of the molecule were obtained from the statistics of classical magnitudes using the standard binning procedure. Computed rotational distributions (for the Deltav'=-1, -2 channels) are also in very good agreement with the experimental results [M. Nejad-Sattari and T. A. Stephenson, J. Chem. Phys., 1997, 106 5454]. The influence of two quantum effects-the closing of the Deltav'=-1 dissociation channel and the intramolecular vibrational relaxation (IVR) mechanism-on the agreement with experimental rotational distributions, is discussed. Due to the classical character of our calculations and the binning procedure we used, the agreement of computed vibrational distributions with experimental and quantum theoretical is qualitative. For instance, for v'=28-for which the Deltav'=-1 channel is experimentally found to be closed-the Deltav'=-2 channel becomes statistically more significant. A discussion on the viability of similar quasi-classical methods to model the VP dynamics of analogous clusters is presented.

Energy partitioning in the femtosecond-laser-induced associativeD2desorption from Ru(0001)

Energy transfer to different degrees of freedom during the femtosecond-laser-induced recombinative desorption of ${\mathrm{D}}_{2}$ from a deuterium-covered Ru(0001) surface $({\mathrm{D}}_{\mathrm{ads}}+{\mathrm{D}}_{\mathrm{ads}}∕\mathrm{Ru}\ensuremath{\rightarrow}{\mathrm{D}}_{2,\text{gas}}+\mathrm{Ru})$ has been investigated. $(1+{1}^{\ensuremath{'}})$-resonance-enhanced multiphoton photoionization (REMPI) and time-of-flight (TOF) measurements are utilized to provide information on the internal and external (translational) energy content, respectively. Rovibrational population distributions of the reaction product are detected via various $B\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{u}^{+}\ensuremath{\leftarrow}X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+}$ Lyman bands using tunable vacuum ultraviolet laser radiation in the resonant excitation step. Rotational quantum state populations in the vibrational ground state and the first excited state are measured yielding average rotational energies of $⟨{E}_{\mathrm{rot}}⟩∕{k}_{\mathrm{B}}=800$ and $1500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively, for an absorbed laser fluence $⟨F⟩$ of $85\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$. In addition, a mean vibrational energy of the desorbing molecules is extracted which amounts to $⟨{E}_{\mathrm{vib}}⟩∕{k}_{\mathrm{B}}=1200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Extensive TOF measurements enable complete energy balancing with $⟨{E}_{\mathrm{trans}}⟩∕2{k}_{\mathrm{B}}=2500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $⟨F⟩=85\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$ and underline the nonthermal and unequal energy partitioning between the different degrees of freedom within the reaction product. The effects of multidimensional electronic friction between substrate and adsorbate layer and peculiarities of the potential energy landscape governing the ${\mathrm{D}}_{2}$ recombination are discussed.

New experimental data on the K 2[formula omitted] state analyzed with the multi-parameter approach

New observations of the 39K 2 a 3 Σ u + state are reported. The results include ro-vibrational term values and intensity distributions of the transitions from excited triplet states to this lowest triplet state. The new and previous experimental data are analyzed to obtain the multi-parameter [modified Lennard-Jones] description of the a 3 Σ u + state potential function in a range including both attractive and repulsive limbs. The same description is applied to the K 2 3 3Π g and 4 3 Σ g + states.

Potential surfaces and dynamics of the O(P3)+H2O(XA111)→OH(XΠ2)+OH(XΠ2) reaction

We present global potential energy surfaces for the three lowest triplet states in O(P3)+H2O(XA11) collisions and present results of classical dynamics calculations on the O(P3)+H2O(XA11)→OH(XΠ2)+OH(XΠ2) reaction using these surfaces. The surfaces are spline-based fits of ∼20000 fixed geometry ab initio calculations at the complete-active-space self-consistent field+second-order perturbation theory (CASSCF+MP2) level with a O(4s3p2d1f)/H(3s2p) one electron basis set. Computed rate constants compare well to measurements in the 1000–2500 K range using these surfaces. We also compute the total, rovibrationally resolved, and differential angular cross sections at fixed collision velocities from near threshold at ∼4kms−1 (16.9kcalmol−1 collision energy) to 11kms−1 (122.5kcalmol−1 collision energy), and we compare these computed cross sections to available space-based and laboratory data. A major finding of the present work is that above ∼40kcalmol−1 collision energy rovibrationally excited OH(XΠ2) products are a significant and perhaps dominant contributor to the observed 1–5 μ spectral emission from O(P3)+H2O(XA11) collisions. Another important result is that OH(XΠ2) products are formed in two distinct rovibrational distributions. The “active” OH products are formed with the reagent O atom, and their rovibrational distributions are extremely hot. The remaining “spectator” OH is relatively rovibrationally cold. For the active OH, rotational energy is dominant at all collision velocities, but the opposite holds for the spectator OH. Summed over both OH products, below ∼50kcalmol−1 collision energy, vibration dominates the OH internal energy, and above ∼50kcalmol−1 rotation is greater than vibrational energy. As the collision energy increases, energy is diverted from vibration to mostly translational energy. We note that the present fitted surfaces can also be used to investigate direct collisional excitation of H2O(XA11) by O(P3) and also OH(XΠ2)+OH(XΠ2) collisions.

Diagnostics of Rovibrational Distribution of H2 in Low Temperature Plasmas by Fulcher-α band Spectroscopy - on the Reaction Rates and Transition Probabilities

A novel fitting procedure is proposed for a better determination of H2 rovibrational distribution from the Fulcher-α band spectroscopy. We have recalculated the transition probabilities and the results show that they deviate from Franck-Condon approximation especially for the non-diagonal transitions. We also calculated the complete sets of vibrationally resolved cross sections for electron impact d3Πu - X3Σg transition based on the semi-classical Gryzinski theory. An example of experimental study confirms that current approach provides a tool for a better diagnostics of H2 rovibrational distribution in electronic ground state.

Experimental study of negative ion profiles in H2-MAR plasmas in divertor simulator MAP-II

The negative hydrogen ion density profile is measured in hydrogen molecular assisted recombination (MAR) plasmas in the steady-state linear divertor/edge plasma simulator material and plasma (MAP)-II device. Fulcher band spectroscopy is also performed for the measurement of the ro-vibrational distribution of hydrogen molecules and the degree of dissociation. We have developed negative ion diagnostics using a combined scheme of an ‘eclipse’ laser photo-detachment system and a double probe. The former is to avoid probe surface ablation phenomena while the latter is to avoid the anomaly in the Langmuir probe characteristics in recombining plasmas, which leads to an overestimation of Te. Negative hydrogen ions are detected in the peripheral regions of the hot plasma column while vibrationally excited molecules exist in the plasma core. Based on the rate equation, the measured negative ion density is compared with the results of model calculations.

Retrieving physical conditions from interstellar H2 emission lines: a non linear fitting technique

The importance of the formation pumping on the rovibrational level distribution of H2 is revisited. A detailed comparison of different formation mechanisms on dust surfaces and in the gas-phase is carried out, and it is found that dust surface formation dominates the formation pumping. It is shown that formation pumping in a cold gas can mimic the excitation conditions found in hotter regions. The presence of a significant fraction of molecular hydrogen in very high rotational levels observed towards OMC-1 Peak 1 is explained by means of pumping by newly formed H2 molecules and does not require high temperature gas.

High rotational excitation of molecular hydrogen in plasmas

Highly rotationally excited H 2 molecules are observed in a hydrogen plasma. The rotational distribution, which is measured up to J = 17 in the vibrational state v = 2 is highly non-thermal. Whereas the lower rotational levels ( J < 5) are distributed at the ambient temperature, the high rotational levels ( J > 7) are distributed at a much higher temperature, close to the apparent vibrational temperature. The ro-vibrational distribution is compared to distributions observed in other plasma sources and all of the rotational distributions show strikingly similar non-thermal distributions. Of all the possible production mechanisms surface processes are strongly suggested to be responsible for the observed high rotational excitation.

Rovibrational product state distribution for inelastic H+D2 collisions.

Experimental measurements of rovibrational product state distributions for the inelastic scattering process H + D2(nu=0,j)-->H + D2(nu' = 1,2,j') are presented and compared with the results of quasiclassical and quantum mechanical calculations. Agreement between theory and experiment is almost quantitative. Two subtle trends are found: the relative amount of energy in product rotational excitation decreases slightly with increasing collision energy and increases slightly with increasing product vibrational excitation. These trends are the reverse of what has been found for reactive scattering in which the opposite trends are much more pronounced.

Rovibrational distributions of HF in the photodissociation of vinyl fluoride at 193 nm: a direct MP2 quasiclassical trajectory study.

Quasiclassical trajectory calculations were performed to calculate rovibrational distributions of the nascent HF fragment in the photodissociation of vinyl fluoride at 193 nm. The trajectories were initiated at the transition states of the four-center (4C) and three-center (3C) HF elimination channels, using a microcanonical, quasiclassical normal-mode sampling. In general, the calculated distributions are in reasonably good agreement with experiment. In particular, the trajectory distributions show bimodal character, although not as pronounced as that observed experimentally. The calculations predict that the 3C and 4C distributions are rather similar to each other, which suggests that the low-J and high-J components of the rotational distributions cannot be specifically assigned to each of these channels.