Unimolecular Fragmentation Kinetics and Binding Energies of Mass Selected Benzene Cluster Ions

Abstract

In this work the unimolecular decay of cluster ions is investigated in order to deduce energetic and structural information on clusters as a function of size. The dissociation behavior of benzene cluster ions (C6H6)+n with n = 4 to n = 200 is observed after multiphoton ionization of a neutral benzene cluster beam within a reflectron mass spectrometer. With a pulsed deflection technique of undesired ions the metastable decay of mass‐selected cluster sizes within the drift region was observed. The metastable decay efficiency is monotonically increasing in this mass range but pronounced anomalies are found at n = 14, 20, 24, and 27. Using an evaporative ensemble ansatz and an RRKM model with a phase space restricted to the intermolecular cluster modes we were able to determine the dissociation energies for clusters up to n = 23 from a fit of the calculated metastable decay ratios to the measured ones. For this fit the size dependent sequential evaporation of two benzene molecules is important. The dissociation energy is largest for the dimer cation and found to decrease for clusters up to n = 5. For larger clusters it begins to increase with a step‐like behavior after n = 14 and n = 20. This is interpreted in terms of shell filling achieved for icosahedral structures about a center dimer cation.

Electronic Structures and Photoevaporation Dynamics of Benzene Cluster Ions

“…40,41 In this size range, the dissociation energy is smallest for (C 6 H 6 ) 5 + (≈0.12 eV) and increases with increasing cluster size except for steplike drops after n ) 14 and n ) 20. From the number of photoejected molecules, Beck and Hecht reported the values of 0.37-0.34 eV as upper limits to the average binding energies for n ) 7-15.…”

Observation of a1/tDecay Law for Hot Clusters and Molecules in a Storage Ring

“…On the other hand, the function t 21 is not invariant under time displacements like the exponential but refers to an absolute zero, the point in time when the ensemble was created. An integrated form of this decay law has been derived by Klots [5] and used on several occasions to account for metastable decay in mass spectrometric studies of clusters (see, for example, [6][7][8][9]). …”

“…(9) decreases nearly exponentially, with a lifetime t given by Eq. (8) with T e equal to the limiting value of T m . An example is the decay of hot fullerenes.…”

Photodissociation spectroscopy of benzene cluster ions in ultraviolet and infrared regions: Static and dynamic behavior of positive charge in cluster ions

“…The photon energy is only 0.36 eV (2900 cm -1 ) although the binding energy of (C 6 H 6 ) 2 + is 0.67 eV. 9 If the absorption occurs in the vibrational ground state, at least two photons are required to dissociate (C 6 H 6 ) 2…”