The orbital control of stereochemistry is discussed with special reference to the Na (3p P-2) + H-2 collision. As seen by H-2, the p orbital of the electronically excited Na atom is like a quadrupole, which may or may not lock along the molecular axis. Quantum mechanically, variations in the alignment of the orbital represent changes in the electronic state of the system and so dynamical methods which allow for such interstate transitions must be used. A new, time dependent quantum mechanical method for propagating the wave function on several electronic states is used to study these interstate transitions. Particular attention is given to the question of orbital following. The computational method is fully quantum mechanical but it uses a basis set which takes full account of the classical motion on any given electronic state while the solution of the Schrodinger equation addresses the electronic-state-changing transitions. We pay specific attention to the orbital alignment for both cold and rotationally warm H-2 and for low and high impact parameters throughout the course of the collision. It is concluded that orbital locking is not necessarily instantaneous and can lag behind the faster nuclear motion, including the (fast) rotational motion of H-2.

The hypothetical four-center nitrogen exchange reaction of N-2 + N-2 is analyzed. We show that the three level crossings accompanying the least-motion nitrogen exchange reaction occur at different points along the reaction coordinate, leading to a mechanism requiring three `'singly forbidden'' reaction steps. Simple MO arguments show that the loss of one electron in N-2 + N-2(+) reduces the energy demand associated with the energetically dominating first and third level crossing,, suggesting that ionization of the reaction system lowers significantly the high activation barrier. This is supported by nonlocal density functional calculations on various N-4 and N-4(+) structures, which, however, also indicate that the barrier still remains at high energy: the tetraazacyclobutadiene intermediate involved in the neutral reaction is 166.7 kcal/mol higher in energy than N-2 + N-2; the corresponding radical cation is only 52.2 kcal/mol above N-2 + N-2(+). The DFT results also indicate that the N-2 + N-2(+) nitrogen exchange reaction, if it occurs at all, may also proceed via a competing mechanism involving a T-shaped transition state at 102.8 kcal/mol above N-2 + N-2(+). Suggestions for further experimental investigations emerge from this analysis.

A time-dependent quantum mechanical method for propagating the wave function on several electronic states is discussed for the polyatomic case and illustrated by the quenching collision of a Na (3p P-2) atom by H-2. The specification of method is governed by the need to have a clear physical interpretation of the results, by the recognition that the motion on a given electronic state can often (but not always) be well approximated by classical mechanics, and by the need for a computational procedure that is simple enough to handle polyatomic systems. These desiderata are realized by the spawning technique which is discussed in detail. One more feature of the method is that it allows for a smooth interface with the methodologies of quantum chemistry so that the electronic structure problem can be solved simultaneously with the time propagation of the nuclear dynamics. The method is derived from a variational principle and so can yield quantum mechanically numerically converged results. The parameters that govern the numerical accuracy of the method are explicitly discussed with special reference to their physical significance. The quenching of a Na (3p P-2) atom by H-2 due to a conical intersection of two potential energy surfaces is used as a computational example since it illustrates many of the features of the method. This collision is found to be sticky and exhibits many sequential nonadiabatic couplings, each of which is localized in time, where the quenching probability per traversal of the conical intersection region is small. However, the accumulated transfer of population to the ground state can be significant since the duration of the overall transfer is spread over many vibrational periods of H-2.

An efficient route to the site-selective reactivity of electronically excited states of multicentered molecules is discussed. In the first stage the migration of the electronic excitation occurs. This can operate over an extensive range without extensive draining of energy into the nuclear frame. Only in a second stage, once the optimal site has been reached, does the excess energy become available for bond breaking or isomerization at the new, optimal, site. This two-stage mechanism, where electronic excitation (or the charge) is the scout, avoids the pitfall of conventional large molecule kinetics. (In that view, known as the quasi equilibrium theory, the electronic excitation is first converted to nuclear modes. But then there are so many available vibrational states that the probability for the excitation energy to become localized at the necessary site, is too small and the resulting reaction rate is too slow.) By confining the site search to the electronic manifold, it becomes a highly efficient process. The recent novel experiments of Weinkauf et al. on (positive) charge migration and dissociation of peptide ions are suggested as an example of the considerations above where there is a facile migration of the positive charge followed by reactivity at the selected site. The peptide is modeled as beads on a chain. Interbead and intrabead coupling are discussed in terms of adiabatic and diabatic states. We find a multistep mechanism (unlike superexchange): a charge-directed reactivity (CDR) model. Such efficient ranging could also take place in other chain structures and suggests that there will be examples where electronic processes set the time scale for the chemical change.

Time evolution when many states are strongly coupled is approximated by a reduced description where nearly degenerate states are taken to be equally populated, on the average. This grouping, valid at longer times, can significantly reduce the dimension of the problem, thereby making tractable computations which include all nearly isoenergetic states. The validity of the approach is examined by comparison with exact results, The application illustrated is to high molecular Rydberg states where a large basis size is required for convergence because, at times of experimental interest for ZEKE spectroscopy, many zero-order states have been accessed.

The quantum dynamics of unimolecular dissociation are discussed for a typical situation where the number of energy rich but bound (zero order) states far exceeds the number of states with enough energy along the reaction coordinate in order to dissociate. It is shown that one can introduce a new basis of states, the `trapping' states, which diagonalize the effective Hamiltonian that governs the dynamics in the bound subspace. The properties of these states are discussed with special reference to the two distinct time regimes (prompt and delayed) for the dissociation that are expected for such a congested level structure.

ZEKE spectroscopy is based on delayed detection by pulsed field ionization. It is thereby possible to monitor the time evolution at a given excitation frequency. Moreover, by varying the depth of detection, one can harvest different Rydberg series. The qualitative features expected for such a spectrum are discussed. The quantitative theory required to compute spectra is outlined and applied to the realistic example of Na-2(+). The computed spectrum is found to very accurately exhibit two time scales, just as has been observed, with the shorter decay time being faster for lower Rydberg states. Extensive interseries coupling is noted.

We investigate the influence of an internal barrier on an exothermic adiabatic reaction model between diatomic ions and molecules. Reaction cross-sections are calculated from quasi-classical trajectories for different initial vibrational and rotational states of the reactants and for relative collision energies in the range from 0.01 to 3 eV. It is shown that the height of a late internal barrier strongly influences both the characteristics of the state-selected cross-sections and the energy distributions of the products. In contrast to complex formation in the entrance region according to the Langevin model our analysis emphasizes the role of the full potential energy surface for an understanding of the dynamics of ion-molecule reactions. (C) 1996 American Institute of Physics.

The effect of an electric field on the overall intensity of the ZEKE spectrum and on the lifetime is discussed for very long living states detected by a pulsed field ionization delayed by several microseconds or more. It is shown that the presence of a de electrical field can shorten the very long lifetimes and that it can also reduce the overall intensity of the very long living states, The decrease in the long lifetimes of the ZEKE states is complementary to the field-induced elongation of the shorter lifetimes. The discussion is based on quantum mechanical considerations and is illustrated by detailed computational studies, for a model problem, using an effective Hamiltonian formalism for an energy range just above the lowest ionization threshold, The model allows for coupling of different Rydberg series built on different excited states of the core where the continuum corresponds to the ground state. Predissociation is not allowed for in the model Hamiltonian. The trends are in accord with the observations of Held et al, as reported in the preceding paper, and the magnitude of the measured lifetimes of the ZEKE states (dozens of microseconds) are reproduced by the computations.

Molecular dynamics simulations of the dissociation of I-2 embedded in large Ar-n (n = 319, 553) clusters, which impact at high velocities (upsilon = 7-15 km s(-1)) on Pt surfaces, result in information on heterogeneous and homogeneous dissociation mechanisms. A broad distribution of dissociation lifetimes is exhibited, which can be attributed to prompt and retarded heterogeneous dissociation and to prompt, retarded and outbound homogeneous dissociation events. The propagation of a microshock wave within a large cluster can be interrogated by the homogeneous dissociation of a chemical probe, with the velocity of the propagation of the dissociation front being close to the cluster impact velocity.

Impact heating of cold molecular clusters moving at high velocities dissipates extreme amounts of energy (often more than several eV per atom) in very short times. Molecular dynamics simulations of larger rare gas clusters show that this excess energy is thermalized in 100 fs or less, depending on cluster size and impact velocity. Dissipation is also extensive for smaller clusters but these shatter before being fully thermalized. A simple analytical hard sphere model that recovers this behavior is discussed. The model attributes the ultrafast relaxation to the random orientation of the interatomic distance before the collision, A perfectly ordered army of atoms is indeed found not to relax. Such an array also allows for a dispersion-free propagation of a shock front. The route to equilibrium is therefore the efficient mixing in phase space caused by the velocity components after the collision having a random part. The implications for the maximum entropy description of cluster impact induced chemistry, for the production of electronically excited and ionic species and for electron emission are discussed.

Using exact matrix elements for the coupling, the effect of the anisotropy of the core on high molecular Rydberg states is studied by quantum dynamics. It is found that on the time scale which can be probed by zero kinetic energy spectroscopy there is extensive interseries mixing. In particular, the long decay times are due to the sojourn in Rydberg series which are not directly effectively coupled to the continuum. These are series built on higher rotationally excited states of the core and a dynamical bottleneck controls the coupling to the bound series directly coupled to the ionization continuum. The computations are carried out for realistic molecular parameters and in the presence of a weak external de field. The quadrupolar coupling is often more effective in interseries coupling than the dipolar anisotropy even though the latter has a far higher range. The external field exhibits the expected `'dilution'' or `'time stretching'' effect at short times (of the order of the Stark period) but enhances the interseries mixing at longer times. An incomplete I mixing is the origin of another dynamical bottleneck. The time evolution is described both by exact quantum propagation and by a reduced description where degenerate states (i.e., states which differ only in the magnetic quantum numbers) are taken to be equally populated, on the average. This grouping, valid at longer times, facilitates the quantal computations which include several series with the full complement of angular momentum states of the electron. Such computations are possible by taking advantage of the conservation of the (total projection) quantum number M. For higher values of IM the coupling to the continuum is very much hindered and the bound Rydberg series exhibit extreme stability. The paper concludes by an analysis of the three bottlenecks which can give rise to longer decays. (C) 1996 American Institute of Physics.

The unique amplitude obtained by an inversion of an observed Raman excitation profile is one of minimal phase. As the frequency varies, the phase of such an amplitude can span a range larger than 2 pi. Therefore such an inversion can yield an amplitude that accurately reproduces the input profile. It is possible to add to the minimal phase additional terms but such modifications of the phase of the Raman amplitude cannot change the corresponding Raman excitation profile. Hence such terms cannot be determined from the profile and introducing them does not constitute an inversion procedure.

The coupling of a Rydberg electron to the vibrational motion is discussed in the intermediate regime in which the orbital period is long on the scale of the vibrational motion but is still considerably faster than the rotation of the core. Two dimensionless variables characterize the dynamics: the ratio of time scales and the action exchanged between the electron and the core, per one revolution. The classical dynamics are reduced to a map which provides a realistic approximation in the limit when the action exchanged is larger than h. There are two distinguishable time regimes, that of prompt processes where the corresponding spectrum is so broad that individual Rydberg states cannot be resolved and a much slower process, where the electron revolves many times around the core before it ionizes. The overall spectrum is that of a Rydberg series, where the lines are broadened by (the delayed) vibrational autoionization superimposed on a broad background. The semiclassical dynamics is quantitatively more accurate in the typical situation when the action exchanged is comparable or smaller than h. Explicit analytical expressions are obtained for the width for vibrational autoionization including for the case when resonances are possible. The presence of resonances is evident in Rydberg lines which are broader. For low Rydberg states the present approach recovers the Herzberg-Jungen approximation in the weak coupling limit. (C) 1996 American Institute of Physics.

Ab initio dynamics on more than one electronic state is reported for the NaI system. This requires both a method for computing the electronic energy curves and their coupling and a matched method for propagating the equations of motion for the atoms. The long-range electron transfer (a `harpoon' process) requires a particularly accurate electronic computation and many configurations are employed in a novel method which combines the advantages of the molecular orbital and valence bond approaches. The computation is performed `on the fly' with the required electronic input being generated at each nuclear separation reached by the system.

We present the first application of first-principles molecular dynamics to a chemical process occurring on more than one electronic state. The example is the collisional chemi-ionization of NaI using a novel ab initio technique for the electronic states and a previously described full multiple spawning (FMS) classically motivated quantal method to describe the nuclear dynamics, The results for the dynamics are compared with fully exact quantal propagation, The FMS method which generates quantal amplitudes and inherently conserves normalization is shown to perform remarkably well for this heavy particle problem. The ab initio generated potentials and interstate couplings are compared with empirical potentials for NaI. Particular attention is given to the localized molecular orbital/generalized valence bond (LMO/GVB) method used for the electronic problem and to its interface with the equations of motion for the nuclei. The ability to incorporate atomic input (such as the ionization potential or the electron affinity) into the LMO/GVB method is emphasized. (C) 1996 American Institute of Physics.

A spectrum of final states can be produced in many ways; It can be a bound-bound process as in optical spectroscopy, a bound-continuum process as in photodissociation, or even a continuum-continuum transition as in a scattering experiment. A homogeneously broadened, resolved, spectrum exhibits variations in intensity which, while fully predictable by quantum dynamics, appear to the eye as `'fluctuations''. A phase space structure in which the statistical properties of such fluctuations can be examined is defined and discussed. In the classical limit, one recovers the results of `'surprisal analysis''. In the more general case, one obtains an extension to situations where quantal interference effects can be important.

The effect on differential and integral cross sections of varying the potential energy of the entrance channel to a polyatomic reaction is investigated theoretically. The rotating bond approximation (RBA) is used for the CH4 + OH –> CH3 + H2O reaction. The reaction exhibits peripheral dynamics, with a higher reactivity at higher impact parameters. Due to the significant potential barrier the scattering of the products is however sideways and not forward. It is found that varying the long-range isotropic terms in the potential has almost no effect on the opacity function or the cross sections. The addition of long range anisotropic terms induces rotational transitions of the OH reactant and thereby reduces the reactivity at higher impact parameters, resulting in backward scattering.

{{The shattering transition expected upon ultrafast heating has been observed in size selected (NH3)(n-1)NH4+ clusters