Structural characterization of (C(2)H(2))(1-6) (+) cluster ions by vibrational predissociation spectroscopy

Vibrational predissociation spectra are reported for the cationic acetylene clusters, (C(2)H(2))(n) (+), n=1-6, in the region of the C-H stretching fundamentals. For n=1 and 2, predissociation could only be observed for the Ar-tagged clusters. These were prepared by charge-transfer collisions of Ar(k) (+) with C(2)H(2) to create C(2)H(2) (+)Ar(m) clusters, which were then converted into larger members of the (C(2)H(2))(n) (+)Ar series by sequential addition of acetylene molecules. The (C(2)H(2))(2) (+)Ar spectrum indicates that this species is predominantly present as the cyclobutadiene cation. Although mobility measurements on the electron-impact-generated (C(2)H(2))(3) (+) ion indicated that it primarily occurs as the benzene cation, [P. O. Momoh, J. Am. Chem. Soc. 128, 12408 (2006)] photofragmentation of (C(2)H(2))(3) (+)Ar in the C-H stretching region is dominated by the loss of C(2)H(2) in addition to the weakly bound Ar atom. This suggests that the dominant n=3 species formed by sequential addition of C(2)H(2) is based on a covalently bound C(4)H(4) (+) core ion. Interestingly, the spectrum of this core C(4)H(4) (+) species is different from that found for the cyclobutadiene cation, displaying instead a new band pattern that is retained in the higher (C(2)H(2))(3-6) (+) clusters. Multiple isomers are clearly involved, as yet another pattern of bands is recovered when the (C(2)H(2))(3) (+)Ar action spectrum is recorded in the (minor) Ar loss fragmentation channel. One of these features does appear in the location of the single band characteristic of the Ar-tagged benzene cation reported earlier [Phys. Chem. Chem. Phys. 4, 24 (2002)], supporting a scenario where the benzene cation is one of the isomers present. We then compare the Ar predissociation results with (C(2)H(2))(n) (+) spectra obtained when the ions are prepared by electron impact ionization of neutral acetylene clusters. The photofragmentation behavior and vibrational spectra indicate that the dominant species formed in this way also occur with a covalently bound C(4)H(4) (+) core. There are absorptions, however, which are consistent with a minor contribution from (C(2)H(2))(n) (+) clusters based on the benzene cation.