Metastable fragmentation of photoionized styrene cluster ions: Implications for cluster ion structure

Styrene clusters are produced by supersonic expansion of a styrene/Ar mixture, photoionized with 193 or 248 nm light, and analyzed in a reflectron time-of-flight mass spectrometer. The styrene cluster ions Stn+, n ≤ 10, show an alternation in intensity that favors cluster ions containing an even number of molecules. Mass-selected cluster ions, n = 2–8, are allowed to undergo metastable fragmentation, and the fragment ions are separated from the parent ions in the reflectron. For n = 4–8, fragmentation proceeds exclusively by elimination of a series of styrene molecules. Loss of as many as five styrene molecules is observed for n = 6–8. For n = 4, the St+ fragment signal is very small, and for n = 3 it is nonexistent. A robust signal for St2+ indicates the major product channel in both cases. These results are interpreted in terms of formation of covalently bound styrene dimer ions within the ionized clusters, especially n = 3 and 4. The present fragmentation results are also discussed in relation to earlier work. Internal energy considerations show that there should be sufficient energy present to cause fragmentation of the structures proposed in the prior work, but no fragments corresponding to such fragmentation are observed. Thus the present work does not support the cluster ion structures proposed earlier, though it also does not completely rule them out.

Figure optionsDownload high-quality image (241 K)Download as PowerPoint slideHighlights
► Photoionized styrene cluster ions show intensity alternation in the mass spectrum.
► Metastable fragmentation shows almost exclusively evaporation of whole molecules.
► Fragment ion intensity patterns imply formation of a covalently bound dimer ion.
► Our results do not support but also do not disprove earlier structural predictions.