Elementary photoprocesses in benzene clusters

Abstract

This article reports the use of the resonant two-photon ionization technique to selectively excite a molecular ion within a cluster and observe the dynamical outcome. We have excited clusters containing up to 14 benzene molecules to energies of 10.00 or 12.84 eV and measured the probability that an initially formed C6H6+ attacks a neighboring benzene unit of the cluster according to the vapor-phase reaction C6H6+ + C6H6 -* C7H7+ + C5H5, AH = 0.63 eV. The C5H5 radical is expelled from the cluster. At either energy excitation proceeds through an XH1 2 vibrational level of benzene (X = 6 or 8) but in the latter case the benzene cation is also produced electronically excited. Accordingly, at low-energy excitation the above pathway is entirely absent, while the 12.84-eV excitation leads to reaction with a probability increasing with cluster size, as predicted by solvation models. This result makes it appear quite likely that >12-eV excitation of condensed benzene will lead to transient tropylium ion centers for conduction electrons, accompanied by variable trapping of C5H5.

Kinetic and thermochemical study of the silyl + hydrogen bromide .dblharw. silane + bromine atom and silyl + hydrogen iodide .dblharw. silane + iodine atom equilibria

“…The heat of formation of SiH3 continues to have an uncertainty of «8 kJ mol-1 on the basis of recent determinations. [5][6][7][8] The lack of kinetic information is due largely to experimental factors. SiH3 is difficult to produce directly under controlled conditions, and the radical, until recently, has also been difficult to detect at the low concentrations required for quantitative chemical kinetic studies.…”

Chemical reactivity promoted by optical excitation transfer in mixed clusters

“…Certainly with regard to the dominant fragmentation route of the bromomethyl benzene ion, there is close agreement between the electron [23] and photon impact data [24]. The first two ions are to be expected given the efficiency with which 210 nm radiation can ionize benzene clusters [14][15][16]; however, the ion at m/z 168 has no analogue in the mass spectra of either pure benzene cluster ions [20,21] or C6HsCH2Br + (for the purposes of comparison in this case we would expect the ion CTH + (m/z 168 -C6H6) to be present in the mass spectrum of gaseous bromomethyl benzene, which it is not [23]). Clearly, the formation of C~3H~2 is associated with the interaction of bromomethyl benzene with a larger (C6H6).…”

“…In contrast, figure 1 (b) shows the effect of introducing a small amount of benzene (1 ml) into the first reservoir; the result is quite dramatic, with the appearance of just a single ion at m/z 91. Although it is known that the photoionization of benzene clusters produces C7H ~" (m/z 91) via an internal ion/molecule reaction [20,21], the latter is normally accompanied by a series Downloaded by [McMaster University] at 09:08 18 November 2014 of very intense benzene monomer, dimer, and trimer ion signals, which is not the case in figure l(b). A more likely candidate for the origin of the m/z 91 ion, is the bromomethyl benzene ion, which is known to have a very low critical barrier (~ 0.2 eV) to the loss of bromine [22].…”