[Show abstract][Hide abstract] ABSTRACT: The dynamics of the reaction O((1)D) + HCl → ClO + H, OH + Cl has been investigated in detail by means of a time-dependent wave packet (TDWP) method in comparison with quasiclassical trajectory (QCT) and statistical approaches on the ground potential energy surface by Martínez et al. [Phys. Chem. Chem. Phys., 2000, 2, 589]. Fully coupled quantum mechanical (QM) reaction probabilities for high values of the total angular momentum (J≤ 50) are reported for the first time. At the low collision energy regime (E(c)≤ 0.4 eV) the TDWP probabilities are well reproduced by the QCT and statistical results for the ClO forming product channel, but for the OH + Cl arrangement, only QCT probabilities are found to agree with the QM values. The good accordance found between the rigorous statistical models and the dynamical QM and QCT calculations for the O + HCl → ClO + H process underpins the assumption that the reaction pathway leading to ClO is predominantly governed by a complex-forming mechanism. In addition, to further test the statistical character of this reaction channel, the laboratory angular distribution and time-of-flight spectra obtained in a crossed molecular beam study by Balucani et al. [Chem. Phys. Lett. 1991, 180, 34] at a collision energy as high as 0.53 eV have been simulated using the state resolved differential cross section obtained with the statistical approaches yielding a satisfactory agreement with the experimental results. For the other channel, O + HCl → OH + Cl, noticeable differences between the statistical results and those found with the QCT calculation suggest that the dynamics of the reaction are controlled by a direct mechanism. The comparison between the QCT and QM-TDWP results in the whole range of collision energies lends credence to the QCT description of the dynamics of this reaction.
Full-text · Article · Mar 2011 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The dynamics of the O((1)D) + HCl(v = 0, j = 0) --> Cl + OH reaction at a 0.26 eV collision energy has been investigated by means of a quasiclassical trajectory (QCT) and statistical quantum and quasiclassical methods. State-resolved cross sections and Cl atom velocity distributions have been calculated on two different potential energy surfaces (PESs): the H2 surface (Martinez et al. Phys. Chem. Chem. Phys. 2000, 2, 589) and the latest surface by Peterson, Bowman, and co-workers (PSB2) (J. Chem. Phys. 2000, 113, 6186). The comparison with recent experimental results reveals that the PSB2 PES manages to describe correctly differential cross sections and the velocity distributions of the departing Cl atom. The calculations on the H2 PES seem to overestimate the OH scattering in the forward direction and the fraction of Cl at high recoil velocities. Although the comparison of the corresponding angular distributions is not bad, significant deviations with a statistical description are found, thus ruling out a complex-forming mechanism as the dominant reaction pathway. However, for the ClO + H product channel, the QCT and statistical predictions are found to be in good agreement.
Full-text · Article · Dec 2009 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: In a crossed molecular-beam study we have measured angular and time-of-flight distributions of the product LiF from the reaction Li+HF(υ = 0)→LiF+H at various collision energies ranging from 97 to 363 meV for three markedly different rotational state distributions of HF obtained at nozzle temperatures close to 315, 510, and 850 K. Particularly, for the low and intermediate collision energies we observe significant effects of the varying j-state populations on the shape of the product angular distributions. At 315 K an additional feature appears in the angular distributions which is interpreted as being due to scattering from HF dimers. The experimental data are compared with simulations of the monomer reaction based on extensive quasiclassical trajectory calculations on a new state-of-the-art ab initio potential energy surface. We find an overall good agreement between the theoretical simulations and the experimental data for the title reaction, especially at the highest HF nozzle temperature.
No preview · Article · Jun 2005 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The technique of resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry (REMPI-TOF MS) has been used to study the photodissociation of fully deuterated dimethyl sulfide (CD3SCD3) following excitation at several wavelengths within the first absorption band (215−229 nm). Analysis of measured time-of-flight profiles of the nascent CD3 products indicates a strongly anisotropic photodissociation, with approximately 70−80% of the available energy appearing as fragment recoil translation. A hybrid statistical−impulsive dissociation model predicts a photolysis wavelength dependence of the translational energy release that compares favorably with experiment, supporting the suggestion (Manaa, M. R.; Yarkony, D. R. J. Am. Chem. Soc. 1994, 116, 11444) that both the 11A‘ ‘ and 21A‘ ‘ excited states play an important role in the photodissociation process. An analysis of nascent CD3 (μ, v, J) vector correlations, where the fragments are assumed to recoil axially along the CD3S−CD3 bond, adequately accounts for the observed probe laser polarization dependence of velocity-selected REMPI spectra. The weak observed rotational excitation of the methyl fragments originates from zero-point vibrational motion of the dissociating parent molecule.
No preview · Article · May 2004 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: The rotational relaxation of N2 in collisions with He has been investigated in free jets with a combination of resonance-enhanced-multiphoton ionization spectroscopy and time-of-flight techniques. From the measured data, a global cross-section for rotational relaxation has been derived. The cross section grows from a value of at 100 K to at 5–15 K. For the weakest supersonic expansions investigated, a breakdown of the translational equilibrium between He and N2 has been observed, that could be well accounted for by an isentropic jet model and classical collision cross-sections.
Full-text · Article · Jan 2003 · Chemical Physics Letters
[Show abstract][Hide abstract] ABSTRACT: The low temperature (< 100 K) relaxation of N-2 in self-collisions and in collisions with He and Ne has been experimentally studied with resonance-enhanced-multiphoton-ionization and time-of-flight techniques. The following average values of the rotational relaxation cross sections, a,, have been derived from the measurements: sigma(r2)(N2-N2) (T = 5 60 K) approximate to 35 Angstrom(2); sigma(r)(N2-He) (T=5-80K) approximate to 15 Angstrom(2) and sigma(r)(N2-Ne) (T=5-60K) approximate to 40 Angstrom(2). The cross section trend sigma(r)(N2-N2) approximate to sigma(r)(N2-Ne) > sigma(r)(N2-He) reported in the literature for T = 300 K is confirmed below 100 K, but the absolute values are larger in the low temperature range.
[Show abstract][Hide abstract] ABSTRACT: The rotational relaxation of nitrogen molecules in collisions with neon has been studied in supersonic expansions. N-2 rotational temperatures have been determined from resonance-enhanced-multiphoton-ionization (REMPI) spectra performed in a series of supersonic molecular beams of N-2 diluted in Ne. Terminal flow velocities and translational temperatures for the two expansion partners have been obtained from mass-selected dine-of-flight measurements. From the measured data, the approximate isentropic behavior of the expansion could be verified and the thermal cross section for N-2/Ne rotational relaxation at very low temperatures could be derived. The cross section increases from a value of approximate to 35 Angstrom (2) at 5 K, goes through a maximum of about 55 Angstrom (2) between 10 and 20 K, and then decreases smoothly to approximate to 25 Angstrom (2) for T = 60 K. The relaxation cross section for N-2/Ne collisions is larger than that for N-2/N-2 collisions below roughly 20 K and smaller above this temperature.
Full-text · Article · Jul 2001 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: The photodissociation of deuterated dimethyl sulfide (CD3SCD3) has been studied in the first absorption band at a laser wavelength of 229 nm. The resonance-enhanced multiphoton ionization time-of-flight technique has been employed to determine the recoil energy distribution and the anisotropy parameter β and to study the rovibrational state population of the nascent CD3. The observed value β = −0.9 ± 0.1 (perpendicular dipole transition) corroborates the C2v 1B1 (or Cs 1A‘ ‘) symmetry of the excited state accessed at 229 nm, as assigned previously by different groups. For dissociation yielding vibrationless CD3, the center-of-mass translational energy is, on average, 80% of the maximum energy available. CD3(v=0) fragments are produced with a rotational distribution showing a maximum at N‘ ‘ = 9−11. Evidence for activity of the ν1 symmetric stretching mode of CD3 is also observed. These results are compared with those obtained for deuterated methyl iodide (ICD3).
No preview · Article · Nov 2000 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: The photodissociation of dimethyl sulfide (CH3SCH3) at 229nm has been studied employing a combination of velocity map imaging and time-of-flight resonance-enhanced multiphoton ionization techniques to detect the CH3 products. Translational energy and recoil angle distributions as well as rotational state populations have been determined for the CH3 photofragments formed in the ground vibrational state. The electronic excitation of CH3SCH3 to the first absorption band is found to produce fast CH3S+CH3 (v=0) recoiling products with a negative spatial anisotropy parameter of β=−0.85±0.05. The CH3 (v=0) products are rotationally cold, the rotational distribution peaking at N′′=3–4.
No preview · Article · Jul 2000 · Chemical Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Laboratory
angular distributions (LAB ADs) have been measured for the Li+HF (v=1, j=1) reaction in a crossed
molecular beam experiment at the collision energies 0.231 eV and 0.416 eV and compared with the results
of extensive quasi-classical trajectory (QCT) calculations performed on the most recent ab initio potential energy
surface (PES) for this system. The calculations also include the collision energy dependence of the integral and
differential cross sections in the range 0.025–0.5 eV (2.4–48.2 kJ mol−1). In particular, the total QCT integral
reactive cross sections have been found to be in very good agreement with recent quantum mechanical (QM)
calculations carried out on the same PES by Lara et al. (J. Chem. Phys., 1998, 109, 9391). In addition, the triple
scattering angle–recoil velocity differential cross section has been calculated in order to simulate
the experimental LAB AD. An excellent concordance between both sets of data has been found, indicating
that the reaction
with HF in
v=1 can be very well described
by QCT calculations on the mentioned PES.
Preview · Article · Feb 2000 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The photodissociation of CH3–S–CH3 following the lowest dipole-allowed transition 11B1(9a1←3b1)←X1A1 has been studied. The CH3 photofragment has been detected by 2+1 resonance-enhanced multiphoton ionization through the 3pz2A2′′←X2A2′′ and 4pz2A2′′←X2A2′′ Rydberg transitions. The vibronic spectrum corresponding to the ν2 `umbrella' mode of CH3 has been measured and fragments with ν2=0 to ν2=4 quanta of vibrational excitation have been detected. From the analysis of the 4pz rotationally resolved 000 band of CH3, rotational temperatures TN=325±75 K and TK=125±50 K have been obtained for the ground vibrational state of the nascent CH3 fragment.
No preview · Article · Sep 1999 · Chemical Physics Letters
[Show abstract][Hide abstract] ABSTRACT: The H+HCl and H+DCl reactions have been investigated with quasiclassical trajectory calculations performed on the ab initio G3 potential energy surface of Allison et al. [J. Phys. Chem. 100 (1996) 13575]. The cross-sections for the abstraction and exchange channels have been calculated for collision energies from threshold to 2.4 eV. The role of initial rotation and vibration on reactivity is addressed and the results are compared to experimental measurements and to previous theoretical calculations.
Full-text · Article · Jun 1999 · Chemical Physics Letters
[Show abstract][Hide abstract] ABSTRACT: The rotational relaxation of molecular nitrogen has been investigated down to temperatures of about 5 K with a combination of resonance-enhanced multiphoton ionization and supersonic beam time-of-flight techniques. The average rotational relaxation cross section obtained shows a maximum value of 50−60 Å2 at 20−30 K. For lower temperatures this cross section decreases and reaches a value smaller than 30 Å2 at T ≈ 5 K. For temperatures above 30 K, the cross section decreases slowly as the temperature grows and converges approximately to the determinations from other non-jet techniques and theoretical estimates available for T > 80 K. The results are compared to previous measurements from other groups using different methods, and general good agreement is found. However, we observe a significant discrepancy with some of the data from electron-beam-induced fluorescence that yield much larger cross sections for temperatures lower than 30 K.
Full-text · Article · Feb 1999 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: Quasi-classical trajectory (QCT) calculations for the Li+HF(v=0,j)→LiF+H reaction have been performed on a recent ab initio potential energy surface (PES). Integral and differential cross-sections, as well as angle–velocity polar maps, have been calculated at the collision energies and initial rotational states of HF(v=0,j=0–3) relevant to the experiment of Becker et al. (J. Chem. Phys. 73 (1980) 2833). With these theoretical results, the laboratory angular distributions (LAB-AD) have been simulated and compared with experiment. The main features of the experimental LAB-AD and energy-dependent cross-section are qualitatively reproduced. In addition, the QCT total reaction cross-section as a function of the collision energy is compared with an approximate quantum mechanical calculation on the same PES.
No preview · Article · Jan 1999 · Chemical Physics Letters
[Show abstract][Hide abstract] ABSTRACT: The reaction has been studied by measuring its chemiluminescence at an average collision energy of ET = 0.16 eV under beam-gas conditions. Absolute values of the reaction cross-section for both chemiluminescence channels (e.g., A and B electronic states) were found to be . In addition, the product polarization of the chemiluminescence emission was also measured, indicating a strong product rotational angular momentum alignment. The results are discussed in the light of a recent stereodynamical model for these, heavy + heavy-light, kinematically constrained reactions.
No preview · Article · Jun 1995 · Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The View the MathML source reaction has been studied by measuring its chemiluminescence at an average collision energy of ET = 0.16 eV under beam-gas conditions. Absolute values of the reaction cross-section for both chemiluminescence channels (e.g., A and B electronic states) were found to be View the MathML source. In addition, the product polarization of the chemiluminescence emission was also measured, indicating a strong product rotational angular momentum alignment. The results are discussed in the light of a recent stereodynamical model for these, heavy + heavy-light, kinematically constrained reactions.
[Show abstract][Hide abstract] ABSTRACT: The reaction cross-section, σ, of the title reaction has been found, by using chemiluminescence emission, to be (1 ± 0.6)× 10–3Å2. This value appears to be the smallest so far measured for a bimolecular reaction.
No preview · Article · Dec 1994 · Journal of the Chemical Society Faraday Transactions
[Show abstract][Hide abstract] ABSTRACT: Absolute values of the reaction cross section, σR, for the title reaction family have been measured by chemiluminescence emission under beam-gas conditions. The σR values diminish as the radical group R increases in size, showing an important overall steric effect. These results are discussed in the light of simple models for reaction stereodynamics.
No preview · Article · Dec 1994 · Chemical Physics Letters