The effect of a strong infrared laser field on the collision between two rare gas atoms is examined as a model of intermolecular motion under such conditions. After examination of the classical collision dynamics, three novel classes of collisions are identified, all of which result from interaction with the angular potential well. The signatures of these “collision mechanisms” in the classical deflection function and differential cross-section are determined. The generality of the results and the feasibility of using a modified crossed molecular beam experiment to observe these laser-induced effects are discussed.

We describe the preparation of diatomic trications using intense femtosecond laser pulses, and discuss the feasibility of using femtoseond pump-probe techniques to measure the vibrational spectrum of Cl-2(3+). Initial attempts to observe the vibrational spectra of Cl-2(3+) were unsuccessful. Possible refinements to the experiment are guided by calculations of the electronic states, transition moments, and field-dressed potential curves for Cl-2(3+). Solution of the time-dependent Schrodinger equation; using these theoretical data as input allow an accurate simulation of pump-probe experiments and their time-delay signals. Optimization of the experimental parameters via the simulation suggests an improved approach to obtaining the spectra of trications, with special emphasis on the unusual aspects of these systems.

Multiple protonation of a stable species is investigated as an effective way of forming light, highly charged systems. As a test of these ideas, we report the structures, frequencies and thermochemistry of the metastable species B2H33+ at the G2 level of theory with additional calculations at the QCISD(T)/6-311G(d,p) level. The lowest energy isomer is a linear, singly bridged species while the second lowest isomer is triply bridged. In examining the thermochemistry related to the fragmentation of B2H33+, we report results for most of the mono-, di- and triprotonated forms of B, B2 and B2+. Suprisingly, we note a positive proton affinity for B2H+.

Highly charged molecular ions are generated in Coulomb explosion experiments involving multielectron dissociative ionization, but little is known about the precise mechanisms involved in their formation. To help improve the understanding of such experiments, potential energy curves are calculated in this paper for diatomic chlorine (Cl-2) and its ions Cl-2(n+), where n=1,2,3,4,6,8,10. Bound vibrational states an obtained in three low-lying electronic states for Cl-2(2+) and one state for Cl-2(3+). Vertical excitation energies are given for stepwise excitations up to Cl-2(10+). For all the ions examined there is a significant energy defect (Delta) from the corresponding Coulomb potential, in one case reaching magnitudes of over 20 eV. We analyze the origin of these energy defects in terms of residual chemical bonding, and discuss the contribution of strongly bonding configurations at short internuclear distance; Finally, we present a simple physical model which describes the qualitative behavior of Delta(R,Q). [S1050-2947(99)01606-6].

Highly charged molecular ions are generated in Coulomb explosion experiments involving multielectron dissociative ionization, but little is known about the precise mechanisms involved in their formation. To help improve the understanding of such experiments, potential energy curves are calculated in this paper for diatomic chlorine (Cl 2 ) and its ions Cl 2 n 1 , where n 5 1,2,3,4,6,8,10. Bound vibrational states are obtained in three low-lying electronic states for Cl 22 1 and one state for Cl 23 1 . Vertical excitation energies are given for stepwise excitations up to Cl 2 10 1 . For all the ions examined there is a significant energy defect ( D ) from the corresponding Coulomb potential, in one case reaching magnitudes of over 20 eV. We analyze the origin of these energy defects in terms of residual chemical bonding, and discuss the contribution of strongly bonding configurations at short internuclear distance. Finally, we present a simple physical model which describes the qualitative behavior of D ( R , Q ). @ S1050-2947 ~ 99 ! 01606-6 # PACS number ~ s ! : 42.50.Hz, 33.80.Gj, p, 33.40.

Strong laser fields exert forces comparable to, or exceeding, the forces which bind atoms to molecules. We exploit this physics to achieve selectivity in the dissociation of triatomic cations, using a simple, intuitive approach. We propose a class of molecules for which a strong bond can be preferentially dissociated over a weak bond using a strong infrared pulse with very simple pulse parameters. Selectivity is demonstrated for a model system of the type ABC(+). (C) 1998 Elsevier Science B.V.

We show that ultrashort pulse strong field multiphoton ionization efficiently produces metastable highly charged molecules such as metastable diatomic trications I-2(3+), Br-2(3+), and Cl-2(3+). The efficiency of stable trication production decreases rapidly with increasing pulse duration. Weak pre- or post-pulse irradiation also prevents efficient production or survival of trications, We propose strong-field femtosecond Raman spectroscopy to determine trication vibrational frequencies.

This Letter reports an accurate dipole moment surface for collinear H3. Using this surface, we examine the H + H2 reaction synamics in infrared laser fields. IR laser light interacting with the system via the dipole moment (A.E. Orel and W.H. Miller, Chem. Phys. Lett. 57 (1978) 362) has no effect on the system at moderate intensities (e.g. 1012 W/cm2), and the interaction only becomes significant at higher intensities where other processes become important. However, the shape of the collinear reaction probability obtained by Orel and Miller using an assumed dipole surface shows remarkable agreement with the results obtained in this more quantitative study.

This Letter reports a study on the effect of the Stark shift for exchange reactions in intense infrared laser fields. It is shown for the H + H-2 exchange reaction that such fields reduce the potential barrier, thus enhancing the reaction rate at low temperatures. Another interesting effect, caused by the change in molecular polarizability as reactants approach, is the creation of (laser-induced) potential minima along the reaction path.

We examine the role of the dipole moment induced by an intense nonresonant infrared laser field on exchange reactions of the type {ital A}+{ital BC}{r_arrow}{ital AB}+{ital C}. This is compared to previous work which included the effect of the permanent dipole moment and its variation along the reaction coordinate. The formalism for laser-molecule interaction is developed for the cases where the reciprocal laser frequency is comparable to or much shorter than the time required for the system to cross the transition state. It is predicted that the induced dipole moment will both lower the electronic barrier to reaction and also create bound states along the reaction path. Results of classical trajectory calculations are presented for the collinear H+H{sub 2}{r_arrow}H{sub 2}+H reaction, using {ital ab} {ital initio} dipole moment and polarizability surfaces. It is found that the collisional energy threshold for reaction is lowered significantly, and that the effects of the induced dipole moment dominate over those of the permanent dipole moment. A time-dependent analysis of the reaction shows that the fluctuating barrier can occasionally be very low when the transition state is approached, allowing trajectories to be reactive with very low collision energies. {copyright} {ital 1996 The American Physical Society.}

Summary form only given. Use of the Stark shift in intense infrared laser fields to trap and align molecules has been proposed recently. For the ground state of a molecule the Stark shift in a low frequency laser field is negative. It increases in absolute value when the molecule approaches the center of the focal spot and peaks for molecular orientation parallel to the polarization of the electric field. We study the effect of Stark shift on the interaction of trapped molecules, in particular on the exchange reactions.