Volume 112, Issue 24, 22 June 2000
 COMMUNICATIONS


Detection and characterization of alkyl peroxy radicals using cavity ringdown spectroscopy
View Description Hide DescriptionCavity ringdown spectra of the electronic transition in the IR are reported for the methyl and ethyl peroxy radicals. Analysis of partially resolved rotational structure for the origin band of the transition provides information about both the and states of An estimate for the absorption cross section is determined from the CRDS absorption and the rate of radical–radical recombination.

Ultrafast excitedstate proton transfer and subsequent coherent skeletal motion of 2(2^{′}hydroxyphenyl)benzothiazole
View Description Hide DescriptionThe excitedstate intramolecular proton transfer of 2(2^{′}hydroxyphenyl)benzothiazole was investigated with 30 fs temporal resolution. The proton takes 60 fs to arrive at its equilibrium position and the electron distribution changes on the same time scale. The molecule cannot adjust to the new structure of the Hchelate ring equally fast and starts to strongly vibrate in four lowfrequency skeletal modes. This coherent wave packet motion is the cause of the experimentally observed strong signal oscillations that are only weakly damped despite the solvent environment.

Probing the nature of surface intersection by ab initio calculations of the nonadiabatic coupling matrix elements: A conical intersection due to bending motion in
View Description Hide DescriptionNonadiabatic coupling matrix elements between the and electronic states of the radical are computed using ab initio full valence active space CASSCF method. The lineintegral technique is then applied to study possible geometric phase effects. The results indicate the existence of a unique conical intersection due to CCH bending between the and states at the linear configuration in the vicinity of Å and Å. The lineintegral calculations with ab initiononadiabatic coupling terms confirm that when a path encircles the conical intersection, the line integral always produces the value π for the topological (Berry) phase and when a path encircles the two (symmetrical) conical interactions or none of them, the line integral produces the value of zero for the topological phase.

Evidence of triplet ethylene produced from photodissociation of ethylene sulfide
View Description Hide DescriptionTunable synchrotron radiation has been used to probe the dissociation dynamics of ethylene sulfide, providing selective determination of the translational energy distributions of both excited and groundstate sulfur atoms, with momentummatching to the ethylene cofragments. The results suggest the presence of a channel giving in conjunction with triplet ethylene and allow for the first experimental measure of the energy of the latter species near the equilibrium geometry, in which the two methylene groups occupy perpendicular planes.

A computer simulation study of the static structure and dynamic properties of liquid using Girifalco's potential
View Description Hide DescriptionUsing Girifalco's potential to describe the interaction between two molecules, we performed computer simulations to analyze the static structure and dynamic properties of liquid and calculated its diffusion constant and shear viscosity. Our dynamical calculations predict that this yetunobserved liquid does not support collective phenomena.
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 ARTICLES

 Theoretical Methods and Algorithms

Thermodynamic consistency of reaction mechanisms and null cycles
View Description Hide DescriptionAt equilibrium, the relationship between the equilibrium constant and the rate constants of an elementary reaction is a consequence of the principle of detailed balancing. Out of equilibrium, it remains valid for most elementary reactions. In general, it is not valid for complex reactions out of equilibrium but its validity for the elementary steps of the mechanism may have important consequences for the interpretation of the experimental results. A related criterion of thermodynamic consistency of reaction mechanisms is the absence of null cycles defined as sets of different reactions taking place under given conditions and leading to no net stoichiometric change.

A numerical study on the performance of the multiconfiguration timedependent Hartree method for density operators
View Description Hide DescriptionA numerical study based on the multiconfiguration timedependent Hartree (MCTDH) method for the propagation of density operators is presented. Within the MCTDH framework, there exist two types of expansions of the density operator which employ different kinds of socalled singleparticle density operators. The latter may either represent Hermitian operators (type I), or else ketbra products of socalled singleparticle functions (type II). The performance of these two types of representations is tested on three models for closed and open system dynamics. The open dynamics is induced for each system by Lindbladtype dissipation operators. We find that the MCTDH representation of type I is most efficient if the coupling between the degrees of freedom is weak, but if the temperature of the initial state and/or the strength of the dissipation is moderate. On the other hand, for strong coupling between the degrees of freedom, but for lower temperatures and for weak dissipation type II is more efficient. Furthermore, considering the open dynamics of the systems both types of MCTDH density operators can be very efficiently used to calculate absorption spectra. The Lindbladtype dissipation operator is shown, however, to capture only partially the effects of a real environment.

Quantum wave packet dynamics with trajectories: Implementation with distributed approximating functionals
View Description Hide DescriptionThe quantum trajectory method (QTM) was recently developed to solve the hydrodynamicequations of motion in the Lagrangian, movingwiththefluid, picture. In this approach, trajectories are integrated for N fluid elements (particles) moving under the influence of both the force from the potential surface and from the quantum potential. In this study, distributed approximating functionals (DAFs) are used on a uniform grid to compute the necessary derivatives in the equations of motion. Transformations between the physical grid where the particle coordinates are defined and the uniform grid are handled through a Jacobian, which is also computed using DAFs. A difficult problem associated with computing derivatives on finite grids is the edge problem. This is handled effectively by using DAFs within a least squares approach to extrapolate from the known function region into the neighboring regions. The QTM–DAF is then applied to wave packet transmission through a onedimensional Eckart potential. Emphasis is placed upon computation of the transmitted density and wave function. A problem that develops when part of the wave packet reflects back into the reactant region is avoided in this study by introducing a potential ramp to sweep the reflected particles away from the barrier region.

Auxiliary basis sets for gridfree density functional theory
View Description Hide DescriptionDensity functional theory(DFT) has gained popularity because it can frequently give accurate energies and geometries. The evaluation of DFT integrals in a fully analytical manner is generally impossible; thus, most implementations use numerical quadrature over grid points. The gridfree approaches were developed as a viable alternative based upon the resolution of the identity (RI). Of particular concern is the convergence of the RI with respect to basis set in the gridfree approach. Conventional atomic basis sets are inadequate for fitting the RI, particularly for gradient corrected functionals [J. Chem. Phys. 108, 9959 (1998)]. The focus of this work is on implementation of and selection of auxiliary basis sets. Auxiliary basis sets of varying sizes are studied and those with sufficient flexibility are found to adequately represent the RI.

Basis set convergence of correlated calculations on He, and
View Description Hide DescriptionThe convergence behavior of the total and correlation energies of He, and with the increase of basis quality in the correlationconsistent basis sets, ccpVXZ and was studied to search for a proper extrapolation scheme to predict the accurate complete basis set (CBS) limits at the MP2, CCSD, and CCSD(T) level. The functional form employed for extrapolation is a simple polynomial including inverse cubic power and higherorder terms of the cardinal number in the correlationconsistent basis set as well as exponential function. It is found that a simple extrapolation of two successive correlationconsistent basis set energies (total or correlation energies) using for MP2 and for CCSD and CCSD(T) level] gives in general the most reliable (and accurate in case of total energy) estimates to the CBS limit energies. It is also shown that the choice of proper basis set, which can represent the electronic motions in the fragment and complex equally well, appears necessary for reliable estimate of the relative energies such as the binding energy of the complex. From the extrapolation of augccpV5Z and augccpV6Z energies with we obtained 21.3(21.4), 28.4(29.0) and 33.2(33.8) microhartrees as the CBS limit binding energy of at the internuclear separation of 5.6 a.u. at the MP2, CCSD, and CCSD(T) level, respectively, with the values in parentheses representing the exact CBS limit binding energies.
 Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Tunneling and decay dynamics of outer well states in hydrogen
View Description Hide DescriptionWe present an experimental study of the decay dynamics of rovibrational states confined in the outer minimum of the potential of the hydrogen molecule. Specific quantum states are populated using a twostep excitation process involving a pulsed extreme ultraviolet laser source. Decay products are selectively probed with additional laser pulses, using the timing of these pulses to determine lifetimes. The competition between dissociation and ionization, occurring at short internuclear distance after tunneling through the potential barrier, is addressed. Observed lifetimes and decay channels of levels are consistent with a semiclassical description of tunneling of the nuclear motion through the potential barrier towards small internuclear distance. For the HD isotopomer breaking of the symmetry is found to strongly affect the decay dynamics.

HCl photodissociation on argon clusters: Effects of sequential solvation and librational preexcitation
View Description Hide DescriptionPhotolysis of the HCl molecule surface solvated on clusters with 2 to 12 argon atoms is investigated by means of quantum molecular dynamics simulations. Two basic questions are addressed: (i) How does the cage effect change upon increasing the size of the cluster, and (ii) how can caging be influenced by an infrared (IR) excitation of HCl hindered rotation (libration) prior to ultraviolet (UV)photolysis. The efficiency of caging is discussed in terms of measurable quantities. In the time domain, temporary populations of the trapped hydrogen atom are monitored, while in the energy domain shortlived vibrational resonances are observed as a fine structure in the hydrogen kinetic energy distribution. While caging is negligible for the smallest clusters, it becomes more efficient upon increasing the cluster size, and for 12 solvent atoms the cage effect is already very strong. Finally, it is shown that while in the ground state the hydrogen atom points essentially toward the rare gas cluster, in excited librational states hydrogen is directed mostly away from argon atoms. As a consequence, caging of the photodissociating hydrogen atom in the case of a surface solvated HCl molecule can be efficiently “turned off” by librational preexcitation.

High resolution pulsed field ionization–photoelectron study of in the energy range of 13.6–14.7 eV
View Description Hide DescriptionThe vacuum ultraviolet pulsed field ionization–photoelectron (PFI–PE) spectra for have been measured in the energy range of 13.6–14.7 eV, revealing complex vibronic structures for the ground state. Many vibronic bands for which were not resolved in previous photoelectron studies, are identified in the present measurement based on comparison with available optical data and theoretical predictions. As observed in the HeIphotoelectron spectrum of the PFI–PE spectrum is dominated by the symmetry allowed (symmetric stretch) vibrational progression for However, PFI–PE vibronic bands due to excitation of the symmetry disallowed (bending) and (asymmetric stretch) modes with both odd quanta, together with the symmetry allowed even quanta excitations, are clearly discernible. The simulation of rotational contours resolved in PFI–PE vibronic bands associated with excitation to the vibrational levels has yielded accurate ionizationenergies for the formation of these vibronic states from

Timedependent Hartree approaches for the study of intramolecular dynamics in dimer systems
View Description Hide DescriptionWe apply and the timedependent Hartree (TDH) method to the study of intramolecular dynamics in dimer systems. The HCl dimer is chosen as test case. Model calculations are performed on reduced dimensional representation of this system namely two, three, and fourdimensional ones. We assess the validity of different implementations of the TDH method including the account of direct correlations between coordinate pairs, and mixed quantumclassical and quantumGaussian wave packets treatments. The latter yields very good results compared to the fully quantal treatment.

Vibrationally excited states of Intramolecular vibrational redistribution and photodissociation dynamics
View Description Hide DescriptionUtilizing vibrationally mediated photodissociation of jetcooled enabled monitoring of the second and third overtones of the methyl in the ground electronic state. The excited molecules are photodissociated by ∼235 or 243.135 nm photons that further tag and isotopes or H photofragments via mass selective resonantly enhanced multiphoton ionization. The vibrational spectra are characterized by a multiple peak structure related to C–H stretches and to Fermi resonating levels involving the deformation. The cooling in the expansion reduces the rotational and vibrational congestion and affords a determination of the splittings and the upper limits for homogeneous broadening of the transitions. The highestfrequency peak of exhibits an additional splitting, related to coupling of the mixed stretch–deformation states with other modes of the molecule. The yield of all three photofragments increases as a result of preexcitation, demonstrating that the energy is not preserved in the excited bond but rather flows to the C–Cl bond. The initial vibrational state preparation not only enhances C–Cl and C–H bond cleavage but also affects the branching ratio, as compared to the nearly isoenergetic onephoton 193 nm photolysis of vibrationless ground state implying that it alters the photodissociation dynamics.

Ab initio study of the electronspin magnetic moments (gfactors) of and
View Description Hide DescriptionThe electronspin gshifts of the title radicals are calculated using secondorder perturbation theory, a Breit–Pauli Hamiltonian, and multireference configuration interaction (MRCI) wave functions. The isotropic values are −830 for for and −7595 for in ppm, with For the ground states of and they are −315, −290, and −7770 ppm, respectively. Experimental values in Ar matrices, 3900 ppm for and 3535 ppm for do not agree with calculated results. However, those predicted for the πtype excited state of and (3415 and 3635 ppm, with large parallel components of 9185 ppm for and 10 350 ppm for are in good agreement with the measured data. Thus, it appears that and were experimentally prepared, and their ESR spectra measured, in their lowest excited state For the state of is about 9820 ppm. The latter value is mainly determined by a large shift of about 28 000 ppm.

Fourier transform emission spectrum of the HCSi radical, transition
View Description Hide DescriptionA Fourier transformemission spectrum of the HCSi, transition was observed by means of a Schülertype discharge tube in the 9000–14 000 cm^{−1}spectral region. Three bands whose origins appeared at 12 934.406, 11 766.721, and 10 752.430 cm^{−1} were rotationally analyzed and assigned, respectively, to the (100)–(000), (000)–(000), and (000)–(100) bands. The new ab initio calculated spin–orbit constant agrees well with the experimental value. Weak rotational perturbations in the (100) level of the upper electronic state are likely due to highly excited vibrational levels of the ground state, whereas the “anomalous” increase of the spin–rotation constant in the (100) level of the ground state relative to the (000) level was shown to result of a Fermi interaction with the (020) Renner–Teller components. A comparison was made between the rotational constants of the (000) ground state level with those obtained in concomitant works.

Resonance enhanced multiphoton ionization of the hydrogen halides: Rotational structure and anomalies in Rydberg and ionpair states of HCl and HBr
View Description Hide Description(2+1) resonance enhanced multiphoton ionization spectra have been recorded, simulated, and used to derive energies of rovibrational levels in the and states for HCl and and HBr and Spectroscopic parameters derived for the F states compare nicely with those derived by others using conventional analysis methods. Clear evidence for near resonance interactions between the F and the states is seen for the first time, both in HCl and HBr. Shape of curves for rotational level energy spacings versus rotational quantum numbers are found to depend characteristically on the nature of offresonance interactions observed between the E and the states. Model calculations for state interactions, based on perturbation theory, are performed for HCl. These prove to be useful to interpret observed perturbations, both qualitatively and quantitatively. Interaction strengths are evaluated for F to and E to state interactions. Variations observed in the intensity ratios of O and S line series to Q line series in vibrational bands of the state for HCl are discussed and mechanisms of twophoton excitation processes are proposed.

Electronimpact ionization of the chlorine molecule
View Description Hide DescriptionRelative partial ionization cross sections for the formation of and from molecular chlorine have been recorded as a function of the ionizing electron energy. In these measurements particular attention has been paid to the efficient collection of fragment ions with high translational energies and the minimization of any massdependent discrimination effects. The cross sections show that at electron energies above the double ionization threshold the yield of fragment ions can be comparable with the ion yield of nondissociative ionization. Further analysis shows that at electron energies above 50 eV the yield of fragment ions from multiple ionization is comparable with the yield of fragment ions from single ionization:dissociative multiple ionization contributes 14% of the ion yield at 50 eV electron energy and 26% at 100 eV. The decay of by heterolytic cleavage to form is a result of approximately 5% of the dissociative double ionization events. This heterolytic process has a threshold of Electronimpact induced triple ionization to form longlived ions has been detected for the first time. This nondissociative triple ionization process makes up approximately 2% of the triple ionization events and triple ionization is responsible for approximately 2% of the ion yield above 100 eV. The threshold for dissociative triple ionization is determined to be a value in good agreement with a trication precursor state energy derived from the kinetic energy release for the fragmentation of to and which provides the first experimental estimate of the triple ionizationenergy of molecular chlorine.

Vibrational mode and collision energy effects on proton transfer in phenol cation–methylamine collisions
View Description Hide DescriptionMassanalyzed threshold ionization has been used to prepare vibrationally stateselected phenol cations, that were then reacted with methylamine at collision energies ranging from 0.1 to 2 eV. Integral cross sections and product recoil velocity distributions are reported. Ab initio calculations of stationary points on the surface and RRKM (Rice–Ramsperger–Kassel–Marcus) analysis of complex lifetimes are also presented for comparison. The only reaction observed over the entire energy range is exoergic proton transfer (PT). For groundstate reactants, the PT cross section is reduced by increasing collision energy, such that the reaction efficiency declines from ∼71% at low to ∼50% at 2 eV. Excitation of either or vibrations inhibits reaction over the entire collision energy range, with the effect being somewhat modespecific and increasing with increasing At low both vibrational and collision energy inhibit reaction with similar efficiency. Collision energy effects diminish at high while vibration continues to have a strong effect. Product ion velocity distributions are approximately forward–backward symmetric at but are backward peaked at high energies. Mechanistic implications of these results are discussed.