Molecular beam photoionization studies of molecules and their van der Waals clusters

Ono, Yoshi
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A new photoionization apparatus incorporating a supersonic^molecular beam, a quadrupole mass spectrometer and a 3-meter^vacuum ultraviolet monochromator has been used to study the^nitric oxide, carbon disulfide, carbonyl sulfide and acetylene^molecules along with their van der Waals clusters. The improved^ion intensity and higher resolution of this apparatus over earlier^similar instruments has allowed us to obtain new and more^accurate information regarding these simple molecules and,^moreover, has demonstrated this technique to be an excellent^method of studying clusters. Using a wavelength resolution of^0.14 (ANGSTROM) (FWHM), photoionization efficiency curves were obtained^for NO, CS(,2), OCS, and C(,2)H(,2) in the respective threshold regions and^ionization energy determinations accurate to within 3 meV were^made. The autoionization structures resolved have led to new^interpretations and assignments to Rydberg levels. Photoionization^efficiency data for the clusters were obtained under lower resolution,^typically 1.4 (ANGSTROM) (FWHM) and led to binding energy determinations of^the cluster ions. Other thermochemical values were deducted from^fragment appearance energies and sharp rises in ionization^efficiencies. From the (CS(,2))(,2) fragment curves, relative reaction^probabilities for the formation of the various product channels as a^function of the Rydberg level n were determined for CS(,2)('*)(V,n) (.) CS(,2)^and the branching ratios were measured for the reaction CS(,2)('*)(V,n) +^CS(,2). The shifts observed in the autoionization peaks of the triatomic^molecules CS(,2) and OCS due to clustering was investigated and a^model is proposed which accounts for these characteristics.^Ionization of the acetylene dimer and trimer resulted in clusters^which could rearrange to form ions of a more stable configuration.Comparisons made between the fragmentation patterns of (C(,2)H(,2))(,3)and the C(,6)H(,6) isomers benzene, 2,4-hexadiyne and 1,3-hexadiynealong with the fragment appearance energies strongly support theconclusion that the acetylene trimer ions rearrange to some commonprecursors as other C(,6)H(,6)('+) isomers prior to dissociation. Energydependence measurements of the acetylene dimer fragmentation;processs yielding C(,4)H(,3)('+) and C(,4)H(,2)('+) were found to be consistent with predictions based on the quasi-equilibrium theory;*USDOE Report IS-T-1009. This work was performed under Contract W-7405-Eng-82 with the U.S. Department of Energy.