James M. Farrar

Intermolecular Potentials from Crossed Beam Differential Elastic Scattering Measurements. V. The Attractive Well of He2

The van der Waals well of the helium pair potential has been studied by crossed molecular beam elastic scattering measurements. Low energy helium beams were produced by specially designed beam sources cooled with liquid hydrogen. Only five partial waves were found to contribute significantly to the differential cross section. A multiparameter potential function is used to construct the interatomic potential from experimental results. The potential parameters describing the location and depth of the attractive well are given by e=11.0± 0.2oK, rm=2.963± 0.01 A . The potential does not contain a bound state of zero angular momentum. Comparisons with other He2 potentials based on bulk properties and ab initio calculations are presented; the temperature dependence of the second virial coefficient is also discussed for the proposed potential.

The question of energy randomization in the decomposition of chemically activated C2H4F

The question of whether energy randomization occurs in the decomposition of chemically activated C2H4F is further examined with the crossed molecular beam technique by measuring the collision energy dependence of the C2H3F product recoil energy distributions for the reaction F+C2H4. The symmetric angular distributions of product molecules indicate that the lifetime of C2H4F remains longer than a rotational period even at the highest collision energy studied. The effect of the exit channel barrier becomes less important at higher collision energies since the fraction of the total energy associated with the barrier becomes smaller as the collision energy increases. The experimental recoil velocity distributions yield average product translational energies which remain almost constant at ∼50% of the total available energy: These results are shown to be inconsistent with phase space theory and the recent ’’Tight Transition State’’ theory of Marcus. Our experiments and the infrared chemiluminescence experiment...

Attractive well of He–He from 3He–4He differential elastic scattering measurements

The elastic differential cross section for 3He–4He was measured at a relative collision energy of 0.799×10−14 erg, approximately five times the well depth. The data are fitted to a multiparameter potential form with e/k=10.57 °K and rm=2.97 A. Comparisons with recent experimental and theoretical helium potentials are made. No evidence for a significant isotope effect in the 3He–4He and 4He–4He interactomic potentials is found in this work.

Intermolecular Potentials from Crossed Beam Differential Elastic Scattering Measurements. VII. para‐H2+para‐H2

The differential elastic scattering cross section for para‐H2+para‐H2 has been measured at a collision energy of 2.71×10−14 erg. Oscillations due to nuclear symmetry were observed in the angular distribution. The data were fit to a spherically symmetric multiparameter potential form giving a well depth e /k= 34.8°K, rm= 3.49 A. This potential is closer to those obtained from the total elastic scattering cross section by Gengenbach and Toennies and from differential cross sections by Dondi, Valbusa, and Scoles than to those derived from bulk properties and early valence bond calculations. The position of the bound vibrational level has been computed and is found 2.0 cm−1 below the continuum.

Neon interatomic potentials from scattering data and crystalline properties

Abstract Interatomic potentials for Ne 2 derived from recent differential elastic scattering cross section measurements and properties of solid neon are pres Cross sections and second virial coefficients are calculated with the proposed potentials and comparisons with experiment are made. The Ne 2 potential derived from scattering data only is shown to provide a good fit to the solid state data.

Absorption spectra of size‐selected solvated metal cations: Electronic states, symmetries, and orbitals in Sr+(NH3)1,2 and Sr+(H2O)1,2

This paper presents photodissociation spectra for the solvated metal ion clusters Sr+(NH3)1,2 and Sr+(H2O)1,2 from 420 to 740 nm in the visible region of the spectrum. The spectra have a banded structure, corresponding to transitions from ground electronic states based upon the 2S configuration of the Sr+ ion to excited states based primarily upon p‐orbitals of the excited Sr+. The photodissociation cross sections are large, ∼10−17–10−16 cm2. For the same solvent, spectral band positions are only weakly dependent upon the degree of solvation. We show that a dramatic reduction in intensity of the second excited state band in the Sr+(NH3)2 spectrum suggests that this state has strong atomic ion d‐orbital parentage and that the molecule is centrosymmetric. Photodissociation of the H2O solvated species proceeds through three excited electronic states corresponding principally to three different orientations of the metal p orbitals with respect to solvent symmetry axis. Absorption band positions for Sr+(H2O)2 ...

Crossed molecular beam study of F+CH3I

The reaction F+CH3I→IF+CH3 has been studied at two relative collision energies of 2.6 and 14.1 kcal mole−1 using the crossed molecular beam technique. Angular and velocity distributions of the reactively scattered IF products have been measured. At the lower collision energy the distributions are consistent with reaction occurring through a complex for which the lifetime is many rotational periods. At the higher collision energy, the complex lives only a fraction of a rotational period. The energy distributions measured for these systems are shown to be fairly consistent with RRKM and phase space statistical models; in order to make a more reliable statistical calculation on this system, the stability of the intermediate complex, CH3IF, has been determined in independent measurements of the threshold for the reaction F2+CH3I→CH3IF+F.

ArHCl interaction potential from differential elastic scattering cross section measurements

Abstract The crossed molecular beam technique has been used to determine the ArHCl spherically symmetric potential. Analysis of the data obtained at a relative collision energy of 12.4 × 10−14 erg (1.78 kcal mole−1) yields a potential with ϵ/k = 191°K, rm = 4.0 A. These results are shown to be in agreement with other experimental conclusions on the ArHCl van der Waals complex as well as with transport phenomena.

Size‐dependent photodissociation cross sections for Sr+(NH3)n, n=3–6: Rydberg state formation and electron transfer

We report photodissociation cross sections for the mass‐selected cluster ions Sr+(NH3)n, with n in the range from 3 to 6. The cross sections exhibit large redshifts that increase monotonically with increasing cluster size. For the n=3 cluster, the absorption cross section peaks near 700 nm, while the n=6 cluster spectrum appears to reach a maximum at 1.5 μm, in the near infrared region of the spectrum. A spectral moment analysis of the cross sections shows that <r2≳ for the ground state electronic radial distribution increases by an order of magnitude over the cluster size range from 1 to 6 solvent molecules. In conjunction with path integral Monte Carlo calculations carried out by Martyna and Klein, we argue that the increase in <r2≳ is diagnostic of the increasing Rydberg character of cluster states that correlate with 5s and 5p atomic orbitals on Sr+. An analysis of the charge transfer to solvent process as described by a thermodynamic Born cycle suggests that the rapid stabilization of solvated ion‐pa...

Crossed beam studies of low energy proton transfer reactions: H2+(Ar,H) HAr+ from 0.4 to 7.8 eV (c.m.)

We present a crossed beam study of the proton transfer reaction H2+ (Ar,H) HAr+ over an extended energy range. At the lowest collision energies (∼0.4 eV), the reaction appears to proceed with substantial interaction among all three atoms, while the dynamics become ’’direct’’ in the higher energy regime. Collision induced dissociation of the HAr+ product is observed for collision energies above ∼2 eV, but products with internal energy in excess of the HAr+ dissociation limit are observed in all cases. These observations are rationalized in terms of the metastability of HAr+ products with rotational energy in excess of the dissociation limit. A surprisal analysis on the product translational energy distributions is also presented.