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 ionization thresholds for (CH3COCH3)2, (CH3COCH3)3, and (CH3COCH3)4 are 9.26±0.03, 9.10±0.03, and 9.02±0.03 eV, respectively. Within experimental resolution, the ionization potentials of (CH3COCH3)2 and (CD3COCD3)2 are identical. The appearance potential of the process (CH3COCH3)2 → (CH3COCH3) ⋅CH3CO++CH3+e− is found to be 10.08±0.05 eV. By consideration of appropriate thermodynamic cycles, a lower bound for the acetone dimer ion binding energy is calculated to be 0.538 eV (12.4 kcal/mole) and the desolvation energy of (CH3COCH3) ⋅CH3CO+ is estimated to be 0.544 eV (12.5 kcal/mole).
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 X̃ 2E CF3Br+ into E1/2 and E3/2 states dominates any Jahn–Teller distortion. The effects of these phenomena on the stability of X̃ 2E CF3Br+ are discussed. The following dissociation channels were observed: CF+3 from X̃ 2E CF3Br+, CF2Br+ from à 2A1 CF3Br+, Br+ from B̃ 2A2 CF3Br+, and CF+2 from D̃ 2E CF3Br+. (CF3⋅CH3OH)+ is not produced from neutral dimers, but is created when a CF3Br moiety embedded in a trimer or larger cluster is ionized to à 2A1 CF3Br+. In contrast, (Br⋅CH3OH)+ does come from dimers as well as larger clusters. The cross sections for the production of (CF3Br)+2 and (CF3Br⋅CH3OH)+ from trimers or larger progenitors are enhanced when intracluster CF3Br is ionized to CF3Br+ in the B̃ or higher electronic states. Possible reasons for these state-specific reactions are given. Finally, a new method is presented for determining neutral cluster distributions in a molecular beam, and the method is shown to be in quantitative agreement with the near-threshold technique previously published by Grover et al. [J. Phys. Chem. 95, 6473 (1991)].
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