Edward A. Walters

Molecular beam photoionization study of acetone and acetone‐d6

High resolution photoionization efficiency curves have been obtained for CH3COCH3+ and CD3COCD3+ using supersonic molecular beam sampling. As a result of adiabatic cooling during the nozzle expansion, sufficient concentrations of (CH3COCH3)2, (CD3COCD3)2, (CH3COCH3)3, and (CH3COCH3)4 were formed to permit the study of their photoion yield curves as well. Appearance potential curves have been determined for CH3CO+, CD3CO+, and (CH3COCH3) ⋅CH3CO+ fragments. The measured ionization potentials of acetone and acetone‐d6 monomers are 9.694±0.006 and 9.695±0.006 eV, respectively. Transitions to higher vibrational levels in CH3COCH3+ are seen at 320, 695, and 930−1370 cm−1 above threshold. The effect of perdeutero substitution is to reduce these frequencies to 260 and 660–1100 cm−1. Appearance potentials of CH3CO+ and CD3CO+ fragments are observed at 10.52±0.02 and 10.56±0.02 eV, respectively. The measured ionization energies for (CH3COCH3)n, n=1–4, are found to decrease linearly as a function of 1/n. Observed io...

Molecular beam photoionization of (H2S)n, n = 1–7

The photoionization thresholds of polymers of hydrogen sulfide (H2S)n were measured for n = 1–7. They are, for increasing values of n, 10.449±0.006, 9.74±0.01, 9.63±0.01, 9.61±0.01, 9.58±0.01, 9.50±0.02, and 9.63±0.03 eV, respectively. The sequence of ionization thresholds with increasing n does not correlate well with a linear relationship to 1/n as do other nonmetal clusters reported previously. We find a binding energy for (H2S)2+ of 0.737±0.012 eV. This value diminishes for each successive cluster to n = 5. At n = 6 the incremental binding is almost as large as for trimer formation while formation of n = 7 is endoergic, that is, (H2S)7+ is unstable with respect to (H2S)6++H2S.

Complexes of oxygen with benzene and hexafluorobenzene

The complexes of C6H6⋅O2, (C6H6)2O2, and C6F6⋅O2 were studied by photoionization using synchrotron radiation. Dissociation energies were measured to be D0(C6H6⋅O2)=1.6±0.3 kcal mol−1, D0[(C6H6⋅O2)+]=3.4±0.3 kcal mol−1, D0(C6F6⋅O2)=2.1±0.4 kcal mol−1, and D0[(C6F6⋅O2)+]=3.2±0.4 kcal mol−1. We calculate from the above that D298(C6H6⋅O2)=0.4±0.4 kcal mol−1, verifying that the benzene–oxygen interaction is only a ‘‘contact’’ at room temperature. The dissociation energies of the heterodimer ions are much smaller than those of the homodimer ions of their constituents [viz. 15, 11, and 7 kcal mol−1 for (C6H6)+2, (O2)+2, and (C6F6)+2. ] Sharp onsets were observed for C6H6⋅O2→(C6H6⋅O2)+ and C6F6⋅O2→(C6F6⋅O2)+, at 9.172±0.004 and 9.856±0.003 eV, respectively, measurements made possible by autoionization in the threshold region. Surprisingly, the heterodimer ion (C6H6⋅O2)+ is formed essentially entirely from neutral C6H6⋅O2, with no contribution from fragmentation of larger clusters. Production of C6H6O+ and C6F6O+ ...

Simulation of chemical reaction initiation through high velocity collisions of NO clusters with a surface

Some computational results have been obtained for a system of diatomic molecules clustered together and driven to impact on a surface at sufficient energy to induce an observable quantity of chemical reactions. The diatomic molecules were modeled to be energetically similar to nitric oxide, NO, which is a detonable material when in the condensed phase. The system was intended to simulate an experiment devised to examine the initiation phase of a detonation of liquid NO stimulated by impact with a high‐speed flyer plate. Classical trajectories were computed for six different cluster sizes, from 4 molecules to 50, and the clusters were directed into a wall at five different impact speeds ranging from 3.0 to 11.8 km s−1. The interatomic forces used for the computations were based on a modification of an empirical potential suggested by Tersoff. The characteristics of the products (O2, N2, NO, and N and O atoms) are examined, as well as the dynamic features of the collisions of the clusters with the wall. The...

Photoionization of carbon disulfide monomers and dimers in a supersonic molecular beam

Photoionization efficiency curves have been obtained for CS2+ and (CS2)2+ using molecular beam sampling. The monomer was studied between 1005–1240 A using an optical resolution of about 1.5 A (12 meV at 1200 A). The observed ionization potential of CS2(2π3/2 ground state) is 10.070±0.006 eV. A small number of steplike fine structure features are seen in the vicinity of threshold. This structure is identified with transitions to the 2π1/2 spin state (10.125±0.006 eV) and a few low energy vibrational levels of the ion. These features and the intense autoionization structure exhibited by this species are discussed. The photoion yield curve of (CS2)2+ was measured over the wavelength range 1000–1295 A at an energy resolution of about 30 meV. Ionization is first observed at approximately 9.63 eV. The onset is very weak and the photoionization efficiency rises gradually with increasing photon energy. From the measured monomer and dimer ionization thresholds, a lower limit to the dimer ion binding energy is calc...

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...

Calculations on species relevant to the photoionization of the van der Waals molecule (H2S)2

Ab initio geometry optimizations are performed on the neutral and cation of H2S and (H2S)2. The neutral dimer is a weakly bound complex possessing only shallow minima in the potential energy surface, but the dimer cation has a deeper minimum where a proton has been transferred to give a complex of H3S+ and HS. Dimer binding energies, ionization energies, and appearance potentials are calculated. Intermonomer distances can be quite sensitive to basis set and correlation improvements on the HF/6‐31G* level of theory. The potential energy surfaces, however, are flat enough so that the energetic consequences are not large. A simple procedure for estimating energy changes involving ionization is suggested; this method separates an adiabatic energy difference into a vertical ionization followed by a geometric relaxation.

Molecular beam photoionization and fragmentation of D2S, (H2S)2, (D2S)2, and H2S⋅H2O

A molecular beam photoionization study of (H2S)2 and (D2S)2, generated in the supersonic expansion of H2S and d2S through a nozzle is reported. The ionization potentials of H2S and D2S are measured. No isotope shift is observed in the ionization potential. (AIP)

The A3Π(1) → X1Σ(0+) emission spectrum of iodine monofluoride: Analyses of the vibrational and rotational structure☆

Abstract Iodine monofluoride A 3 Π(1) → X 1 Σ(0 + ) emission has been observed in the reaction of I 2 with F 2 at temperatures ranging from 300 to 800 K. Two previously unreported v ′ progressions are identified, and vibrational molecular constants are reported for the IF 3 Π(1) state using bandheads in progressions based on 0 ≤ v ′ ≤ 15 and 0 ≤ v ″ ≤ 5. The rotational structure has also been recorded and analyzed for nine vibrational bands. The triplet P -, Q -, and R -branch features are resolved for the first time which unambiguously confirms the Ω = 1 assignment of the emitting electronic state. Rotational molecular constants have been determined, and RKR potential energy functions are used to calculate Franck-Condon factors for the IF( A → X ) system.