Molecular beam photoionization study of acetone and acetone-d6

Abstract

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).