Collisional and photoinitiated reaction dynamics in the ground electronic state of Ca–HCl (original) (raw)
Related papers
Photodissociation of isocyanic acid, HNCO, was studied with high-level ab initio methods. Geometry optimizations of stationary points and surface crossing seams were performed with the complete active space self-consistent-field ͑CASSCF͒ method, and the energetics were re-evaluated with single-point second-order multireference perturbation theory ͑CASPT2͒. The three product channels that participate in the photodissociation process are †1 ‡ HN(X 3 ⌺ Ϫ )ϩCO at 86.0 ͑calculated 79.6͒ kcal/mol, †2 ‡ HϩNCO(X 2 ⌸) at 109.7 ͑108.7͒ kcal/mol, and †3 ‡ HN(a 1 ⌬) ϩCO at 122.2 ͑120.8͒ kcal/mol. The four electronic states, S 0 , S 1 , T 1 , and T 2 , that interconnect these channels were studied in detail. S 1 exhibits dissociation barriers to both, channel †2 ‡ and †3 ‡, whose respective reverse heights are 11.3 and 1.2 kcal/mol, in good agreement with experiment as well as previous theoretical works. The two triplets, T 1 and T 2 , show barriers of similar heights for HN bond fission, while S 0 has no barriers to either channel. Various key isomerization transition states as well as numerous minima on the seam of surface crossings ͑MSX's͒ were also found. At photoexcitation energies near channel †3 ‡ threshold, products to channel †3 ‡ are likely to be formed via S 1˜ † 3 ‡ ͑if enough energy in excitation͒ and S 1˜S 0˜ † 3 ‡. Channel †2 ‡ can be formed via S 1˜S 0˜ † 2 ‡; ͑HN-mode quanta͒ϩS 1˜T 1˜ † 2 ‡; S 1˜T 2˜ † 2 ‡; S 1˜T 2˜T 1˜ † 2 ‡, and channel †1 ‡ via S 1˜S 0˜T 1˜ † 1 ‡, S 1˜T 1˜ † 1 ‡ and S 1˜T 2˜T 1˜ † 1 ‡. At higher photoexcitation energies the S 1˜ † 3 ‡ pathway is expected to be dominant while S 1˜ † 2 ‡, with the higher activation energy, is expected to drop rapidly. Also addressed are such important issues as the impact of a vibrationally excited HN mode on a channel †2 ‡ yield, and the band origin of the S 1 -S 0 excitation spectrum.
Photodissociation processes in the HCl molecule
Journal of Chemical Physics, 1982
Various ab initio methods have been employed for the study of photodissociation processes in the HCI molecule. Potential curves for selected singlet and triplet states and dipole transition moments between singlet states have been calculated. The transition moments vary significantly with internuclear distance for all states studied. The lifetime of the B 1,2' + state is predicted to be 3 ns. The calculations show that photodissociation of Hel occurs by absorption into the repulsive A III state and by absorption into the bound C III state, followed by predissociation. The theoretical photodissociation cross sections for the A III state and oscillator strengths for the C I II state are in good agreement with experimental data. The contributions from other excited states are investigated. The photodissociation rate of HCl in diffuse interstellar clouds is computed.
Reactivity enhanced by under-barrier tunneling and resonances: the F+H2→HF+H reaction
Chemical Physics Letters, 2003
Accurate quantum mechanical rate constants (all contributing partial waves, fine energy grid Boltzmann averaging) for the title reaction are obtained by the hyperquantization algorithm, covering the range from above room temperature down to the cold regime (few K). The good agreement with available experiments down to $200 K, obtained by blending ab initio description of the transition state and molecular beam scattering experimental characterization of the entrance channel, establishes the reliability of the approach to describe deviations from Arrhenius behavior at those low temperatures where quantum mechanics can induce specific selectivity in chemical reactivity Ó
The Journal of Chemical Physics, 1994
This study uses emission spectroscopy of H,S at excitation energies near 200 nm to probe the dissociation dynamics from a conical intersection in the Franck-Condon region to the H+SH product exit channel. Photoexcitation accesses these coupled surfaces near the transition state region of the lower adiabat, a potential surface for the excited state H+SH--+HS+H reaction. Excitation wavelengths from 199-203 nm tune through the first of the resonances in the absorption spectrum assigned to recurrences in the motion along the symmetric stretch orthogonal to the reaction coordinate and also access energies just above and at the conical intersection. We disperse the emission from the dissociating molecules at each of five excitation wavelengths in this region to probe several features of the reaction dynamics on the coupled potential energy surfaces. The resulting emission spectra cover the range of final vibrational eigenstates from 500 to 11 000 cm-' above the initial ground vibrational state for all five excitation wavelengths, and go out to 16 500 cm-' for the 199 and 201 nm excitation wavelengths. The resulting spectra, when considered in conjunction with recent scattering calculations by Heumann and Schinke on ab initio potential energy surfaces for this system, evidence a progression of emission features to low vibrational eigenstates in the SH stretch that result from coupling of the nuclear motion from the bound to the dissociative region of the potential energy surfaces. This emission, into local mode eigenstates such as OO+l, 1 l+O, 1 l+l, 21+0, 21f 1, evidences the antisymmetric dissociative motion and bending induced near the conical intersection, and dominates the spectrum at excitation wavelengths only near 200 nm. We analyze the excitation wavelength dependence of these features and also of the n0 +O progression for ns4, which reflect the exit channel dynamics. The excitation wavelength dependence shows that while the emission spectra do not reveal any dynamics unique to scattering states that access a symmetric stretch resonance in the Franck-Condon region, they do reveal the energy location of and the dynamics at the conical intersection. A reanalysis of other workers' measurements of the SH product vibrational state distribution shows that u =0 products are strongly favored at excitation wavelengths near the conical intersection. 5652
The Journal of Chemical Physics, 2002
We have measured and theoretically analyzed a photodissociation spectrum of the CH ϩ molecular ion in which most observed energy levels lie within the fine-structure splitting of the C ϩ fragment and predissociate, and where the observed irregular line shapes and dipole-forbidden transitions indicate that nonadiabatic interactions lead to multichannel dynamics. The molecules were prepared in low rotational levels JЉϭ0 -9 of the vibrational ground state X 1 ⌺ ϩ (vЉϭ0) by storing a CH ϩ beam at 7.1 MeV in the heavy-ion storage ring TSR for up to 30 s, which was sufficient for the ions to rovibrationally thermalize to room temperature by spontaneous infrared emission. The internally cold molecules were irradiated with a dye laser at photon energies between 31 600-33 400 cm Ϫ1 , and the resulting C ϩ fragments were counted with a particle detector. The photodissociation cross section displays the numerous Feshbach resonances between the two C ϩ fine-structure states predicted by theory for low rotation. The data are analyzed in two steps. First, from the overall structure of the spectrum, by identifying branches, and by a Le Roy-Bernstein analysis of level spacings we determine the dissociation energy D 0 ϭ(32 946.7Ϯ1.1) cm Ϫ1 ͑with respect to the lower fine-structure limit͒ and assign the strongest features to the vibrational levels vЈϭ11-14 of the dipole-allowed A 1 ⌸ state. The majority of the 66 observed resonances cannot be assigned in this way. Therefore, in a second step, the complete spectrum is simulated with a close-coupling model, starting from recent ab initio Born-Oppenheimer potentials. For the long-range induction, dispersion and exchange energies, we propose an analytical expression and derive the C 6 coefficients. After a systematic variation of just the vibrational defects of the four Born-Oppenheimer potentials involved, the close-coupling model yields a quantitative fit to the measured cross section in all detail, and is used to assign most of the remaining features to the dipole-forbidden a 3 ⌸ state (vЈϭ17-20), and some to the weakly bound c 3 ⌺ ϩ state (vЈ ϭ0 -2). The model potentials, which reproduce the spectrum and compactly represent the spectroscopic data, should help to predict more accurately C ϩ ϩH scattering in the interstellar medium.
The Journal of Chemical Physics, 2002
Time-dependent and time-independent quantum scattering methods are used to investigate state-to-state inelastic and reactive collision dynamics for a three-dimensional ͑3D͒ atomϩtriatom model of ClϩH 2 O→HClϩOH. The results elucidate the role of ͑i͒ intramolecular vibrational energy transfer and ͑ii͒ vibrational nonadiabaticity on the time scale of a reactive encounter in systems with nearly degenerate stretching ''local modes.'' Adiabatic two-dimensional ͑2D͒ vibrational eigenfunctions ͓ n (r 1 ,r 2 ,R)͔ and eigenvalues ͓E n (R)͔ are first obtained in OH bond coordinates (r 1 ,r 2) as a function of Cl-H 2 O center-of-mass separation ͑R͒, which then provides the requisite adiabatic potential energy curves and nonadiabatic coupling matrix elements for full 3D quantum wave packet propagation. Inspection of these 2D vibrational eigenfunctions indicates that near degeneracy between H 2 O symmetric ͉01 ϩ ͘ and antisymmetric ͉01 Ϫ ͘ states is systematically lifted as R decreases, causing vibrational energy to flow into local-mode OH excitations pointing either toward ͑''proximal''͒ or away from ͑''distal''͒ the approaching Cl atom, respectively. This suggests a simple yet powerful physical model for mode-specific reactive scattering dynamics, the predictions of which are confirmed by full 3D quantum wave packet calculations over a range of collision velocities.
Chemical Physics Letters, 1986
Theoretical calculations of the low-energy CH+ photodissociation spectrum display extremely complicated structure due to strong non-adiabatic couplings which are present when molecules dissociate to open-shell atoms. An analysis of the spectrum, using methods which will be explained elsewhere, leads to general principles for describing the resonances. These principles should be of aid in assigning experimental CHC photodissociation spectra and those of other systems with strong non-adiabatic interactions.
The Journal of Chemical Physics, 2012
The dynamics of the C( 3 P)+OH(X 2 ) → CO(a 3 )+H( 2 S) on its second excited potential energy surface, 1 4 A , have been investigated in detail by means of an accurate quantum mechanical (QM) time-dependent wave packet (TDWP) approach. Reaction probabilities for values of the total angular momentum J up to 50 are calculated and integral cross sections for a collision energy range which extends up to 0.1 eV are shown. The comparison with quasi-classical trajectory (QCT) and statistical methods reveals the important role played by the double well structure existing in the potential energy surface. The TDWP differential cross sections exhibit a forward-backward symmetry which could be interpreted as indicative of a complex-forming mechanism governing the dynamics of the process. The QM statistical method employed in this study, however, is not capable to reproduce the main features of the possible insertion nature in the reactive collision. The ability to stop individual trajectories selectively at specific locations inside the potential energy surface makes the QCT version of the statistical approach a better option to understand the overall dynamics of the process.
The Journal of Chemical Physics, 2003
A strong enhancement of absorption to the lowest 2 A 1 state is observed for vibrationally excited chloromethyl radicals. It is demonstrated that this enhancement is due to a significant increase in both electronic and vibrational Franck-Condon factors. Electronic structure calculations of potential energy surfaces ͑PESs͒ and transition dipole moments for the ground and the two lowest excited states of A 1 symmetry, the 1 2 A 1 valence and 2 2 A 1 Rydberg states, reveal the origin of this effect. The shelflike shape of the 1 2 A 1 PES in the Franck-Condon region and the strong dependence of the electronic transition dipole moment on C-Cl distance are responsible for the enhancement. Analysis of the shape of the electron density distribution demonstrates that Rydberg-valence interaction in the two lowest excited states causes the changes in the shape of PESs and transition dipoles with C-Cl distance.