J. R. Grover

Dissociation energy of the benzenewater van der Waals complex

Abstract The dissociation energy of the complex C 6 H 6 ·H 2 O was determined by the threshold difference method. These complexes were produced in molecular beams via the expansion of a mixture of helium, and benzene and water vapours in the ratio of 96:3:1. The method requires the ionization potential of C 6 H 6 and the appearance potential of C 6 H + 6 from C 6 H 6 ·H 2 O, the difference between which is the dissociation energy. The necessary values are 9.246 and 9.344 ± 0.012 eV, respectively, from which D (C 6 H 6 ·H 2 O) = 0.098 ± 0.012 eV = 2.25 ± 2.25 ± 0.28 kcal mol −1 . The ionization potential of C 6 H 6 ·H 2 O is 9.170 ± 0.014 eV, which leads to the dissociation energy of the complex ion of D ([C 6 H 6 ·H 2 O] + ) = 4.0 ± 0.4 kcal mol −1 .

Inner‐valence shell photoionization of Ar2, Ar3, Kr2, and Kr3

The inner‐valence shell photoionization spectra (420<λ<500 A) of Ar2, Ar3, Kr2, and Kr3 have been obtained with a molecular beam apparatus using synchrotron radiation from the VUV storage ring at the National Synchrotron Light Source. The photoionization spectra exhibit regular series of window resonances which are closely associated with the ms→np Rydberg series in atomic Ar(m=3) and Kr(m=4). Vertical ionization potentials of the lowest ionic state in the molecular ms regions were obtained by series extrapolation and these were used to estimate the well depths in the Franck–Condon region and dissociation energies. Assignments of the observed Rydberg transitions were made by analogy with similar excitations in He2 and the group II and II B metal dimers. For Ar+2 and Kr+2, the observed structure is tentatively assigned to 1u (1Πu) and O+u (1Σu+) states which correlate with the ‘‘promoted’’ ndπg and ndσg series. Plausible assignments for the trimer spectra are also discussed.

Reactions of chlorine and bromine with hydrogen astatide

Angle‐differential cross sections are reported for the Reactions (2), Cl+Hat→HCl+At and (3), Br+Hat→HBr+At. Total reaction cross sections at 6.5 kcal/mole initial translational energy are 25.6±4.2 A2 for (2) and 1.48±0.34 A2 for (3). Mean energies in product translation are 7.2±0.6 kcal/mole for (2) and 1.0±0.2 kcal/mole for (3). The center‐of‐mass angular distribution of (2) is forward–backward peaked and almost symmetric about 90°. For (3), about as much product scatters into the forward hemisphere as into the backward. The measured cross sections for both (2) and (3) are close to the limiting value, which is calculated by correcting an assumed total cross section for inelastic plus reactive processes of 50 A2 for kinematic restrictions independent of spatial configuration and for the effects of electronic adiabaticity. This means that the smaller cross section for (3) is consistent with its smaller mean product translational energy. It also means that for the direction and position of either attack or retreat (or both) no limitations can be claimed if electronic adiabaticity is assumed, and only loose restrictions if such adiabaticity is relaxed. The data thus do not require a model that specifies attack and retreat at small impact parameters exclusively. Reactions of Hat with Cl and Br are broadly similar to the corresponding reactions of HI, although there are obvious differences of detail. The information obtained in this experiment demonstrates the feasibility of using reagents labeled with short‐lived nuclides for the study of reactive scattering in crossed molecular beams.

Photoionization of gas‐phase bromotrifluoromethane and its complexes with methanol: State dependence of intracluster reactions

Weakly bound complexes of CF3Br molecules or of CF3Br and CH3OH were prepared in supersonic jet expansions and studied by photoionization mass spectrometry. Argon was present in the jet expansion to promote cooling of the complexes. Measured ionization potentials are CF3Br, 11.404±0.014 eV; (CF3Br)2, 11.10±0.02 eV; (CF3Br⋅CH3OH), 10.76±0.05 eV. The appearance potential of CF+3 from CF3Br was found to be 11.56±0.02 eV, and evidence is presented that this value is adiabatic. From this appearance potential, ΔH0f0(CF+3)=86.6±1.1 kcal mol−1. The magnitude of the spin–orbit splitting in X 2E CF3Br+ into E1/2 and E3/2 states dominates any Jahn–Teller distortion. The effects of these phenomena on the stability of X 2E CF3Br+ are discussed. The following dissociation channels were observed: CF+3 from X 2E CF3Br+, CF2Br+ from A 2A1 CF3Br+, Br+ from B 2A2 CF3Br+, and CF+2 from D 2E CF3Br+. (CF3⋅CH3OH)+ is not produced from neutral dimers, but is created when a CF3Br moiety embedded in a trimer or larger cluste...

Collision energy dependence of the reactive scattering of Br with HAt

Laboratory angle‐differential cross sections were measured for the reaction Br+HAt→HBr+At at collision energies from 4.2 to 11.7 kcal/mol. Transformation of these data into the center‐of‐mass system revealed that the mean energy in product translation is extraordinarily small, rising from 1.5±0.2 to 3.3±0.6 kcal/mol. The ratio of products scattered forward to products scattered backward is about 1.0 to 1.5 at all collision energies studied, but more detailed angular information could not be obtained. The total reaction cross section is in the range of 1.5 to 2.0 A2 from 4.2 to 8.3 kcal/mol of collision energy, and rises to 2.9±0.34 A2 at 11.7 kcal/mol. (The uncertainties cited for the cross section values represent random errors only, systematic errors contribute an additional ±15%.) These results confirm the observations made earlier at a collision energy of 5.8 kcal/mol. Phase space calculations show that for collision energies greater than about 7 kcal/mol the total reaction cross section is approachin...

A photoionization study of the van der Waals molecule C2H4 · HCl

The dissociation energy of the C2H4 · HCl van der Waals complex was determined to be 3.18±0.73 kcal mol−1 by a dissociative photoionization technique. C2H4 · HCl was produced by free expansion of a 1:4 mixture of C2H4 in HCl and the clusters were ionized with tunable synchrotron radiation. The photoionization efficiency function of (C2H4 · HCl)+ from C2H4 · HCl was determined between 600 and 1,300 Å and the onset for (C2H4 · HCl)+ was established as 1,163±2 Å = 10.66±0.02 eV; these values give ΔHf0(C2H4 · HCl) = −10.7±0.7 kcal mol−1 and ΔHf0(C2H4·HCl+)=235.1±0.9 kcal mol−1. A complex ion dissociation energyD0(C2H4 · HCl+) = −0.3±0.9 kcal mol−1 was calculated from the results. The major features on the PIE curve for C2H4 · HCl+ can be analyzed in terms of the known energetic features of C2H4+ and HCl. An extended energy diagram for the C2H4 + HCl system is presented.

Non-statistical behavior in the dissociative photoionization of the 1,3-butadiene/sulfur dioxide van der Waals complex

Abstract Dissociative photoionization of the van der Waals complex of 1,3-butadiene and SO2 to yield the fragment ion C4H6-SO+ is reported. From

Experimental techniques for the study of nuclear Bragg scattering in systems containing 57Fe (invited)

Resonant filtration of synchrotron radiation has been proposed as a method of obtaining x rays with extremely narrow bandwidths (or very long coherence lengths). Resonant scattering of x rays and neutrons is of general interest, and displays several phenomena, which require novel instrumentation in order to be properly observed. We describe here some of the techniques which we have developed for the specific purpose of studying dynamical diffraction by the Mossbauer resonance in 57Fe.

Competitive production of weakly bound heterodimers in free jet expansions

Abstract The relative density of heterodimers in molecular beams formed from free jet expansions of gas mixtures is measured into the region where trimers and larger clusters dominate, using single-photon dissociative photoionization below the trimer threshold. Calculations based on simple kinetics and pseudoequilibrium models are consistent with the dimer beam intensities at low pressures, but predict intensities at pressures well above the intensity maxima that are far too small. Possible reasons for the continuation of substantial dimer intensities to high nozzle pressures are discussed.

Photoionization of C2F4/O2 complexes and C2F4 homoclusters

Single-photon ionization of the clusters C2F4⋅O2, (C2F4)2O2, (C2F4)2, and (C2F4)3 is reported for energies up to 28 eV. The ionization potentials found are IP(C2F4⋅O2)=9.879±0.011 eV, IP([C2F4]2O2)=9.776±0.016 eV, IP([C2F4]2)=9.826±0.017 eV, and IP([C2F4]3)=9.838±0.022 eV. In addition, we measured IP(C2F4)=10.117±0.004 eV, in excellent agreement with prior work. The appearance potential of C2F4+ from C2F4⋅O2 was found to be AP=10.253±0.012 eV, which, when combined with appropriate values from above gives the cluster dissociation energies D0(C2F4⋅O2)=0.136±0.013 eV (3.14±0.29 kcal mol−1), and D0([C2F4⋅O2]+)=0.374±0.016 eV (8.62±0.38 kcal mol−1). The former value helps to understand the remarkable solubility of oxygen in fluorocarbons. No evidence was seen for photoionization-induced reactions of O2 with C2F4. A surprisingly intense Rydberg progression converging to the excited state of C2F4+ near 16.0 eV can be identified in the parent C2F4, and persists strongly in all four of the clusters studied. The la...