The field of cluster research can trace its origins back to the mid-nineteenth century when early studies of colloids, aerosols, and nucleation phenomena were reported. The field underwent a resurgence of interest several decades ago when well-defined clusters were observed in supersonic expansions that could be investigated using mass spectrometers. The advent of the laser provided a new dimension, enabling detailed spectroscopic observations through the probing of systems of varying size and degree of solvation. Modern interest derives from recognition that interrogating clusters provides a way of studying the energetics and dynamics of intermediate states of matter as cluster systems evolve from the gas toward the condensed state. Herein, we endeavor to highlight some of the significant advances which have been made during the past several decades that have led to a nearly explosive growth of interest in the field of cluster science. Finally, we conclude that the field will continue to expand through i...

Clusters: Structure, Energetics, and Dynamics of Intermediate States of Matter

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).

Molecular beam photoionization study of acetone and acetone‐d6

The (pre)dissociation of acetone and acetone clusters after excitation to states corresponding to upper {S1,T1} and 3s Rydberg states of the acetone monomer are investigated through femtosecond pump–probe experiments coupled with molecular beam time‐of‐flight mass spectrometry techniques. Upon excitation to either state, [(CH3)2CO]n* dissociates rapidly. Acetyl fragments, [(CH3)2CO]n−1CH3CO+ may arise from ionization of an excited species formed by (pre)dissociation of intact precursors or by dissociation after the intact species has been ionized. The method employed to separate these two channels is discussed herein; the resulting transients are fit to a kinetic model to elucidate intermediate lifetimes and dissociation mechanisms. The present experiments establish that a stepwise dissociation mechanism is operative upon excitation to the 3s Rydberg state for the acetone monomer and dimer, with their corresponding acetyl fragments having lifetimes on the order of picoseconds. Larger cluster species, [(CH...

Further direct evidence for stepwise dissociation of acetone and acetone clusters

The dissociation of acetone in an intense (10(13)-10(14) W/cm(2)) femtosecond laser field has been investigated. The stepwise nature of the dissociation has been verified by analyzing the time-of-flight mass spectroscopic patterns at different laser intensities. The stepwise nature was interpreted using a quasi-diatomic field-assisted dissociation model and the QCT calculations. Both the experimental identification and the theoretical predictions show that if the laser intensity is gradually increased, the sequence of the primary dissociation is C-C, C-O, and C-H bonds in the acetone ion. The preference of breaking the C-C and C-O bond over the H-elimination of the C-H bond is also the case in the secondary dissociation.

K. A. G. MacNeil

Papers

Ion-molecule reactions in gaseous acetone