Volume 128, number 5,6 CHEMICALPHYSICS LhI 1CKb 0 Augurr zyxwvutsrqponmlk lYi5b PEKING IO~~~O~ IN BENZENE* zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Ar AND FLWOROBENZENE* Ar VAN DER WAALS MOLEXXJLES AND IN COLLISIONS OF BENZENE WITH METASTABLE Ar ATOMS E. RUHL, P. BISLING, B. BRUTSCHY Instirur fi& zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Fhy s~kalische Chemi~?, Ibee university Berlin, T~~tr~se 3, I)-ItW O Berlin 33, Fe&ral Rep&& of Germany K. BECKMANN, 0. LEISIN and H. MORGNER Fasts j2r Physikder University,Hewer-Her~r-Str~se 3, O-7800 Freiburrg. Federul ~ep~~~~of Germany Received 25 March 1986 The photoion efficiency curves of the van der Waals complexes benzene. Ar (J&z* Ar) and fluorobenzene~ Ar (Fb . Ax) ex- hibit sharp resonances, which correspond to excitation to the Ar 2P3f24s and 2P~/24s resonance states. The peaks are red- shifted relative to their ~ptoti~ values IBzsAr, AlC= -70 * 10 meV; Fb. Ar, AI3 = -40 f 10 meV). These fmdings 8re supported by electron spectroscvpy studies of the Perming ionization of benzene by state-selected me&&able Ar (%J, 90) atoms. Strong evidence is presented that Penning ionization is the process observed in both cases. i . Introduction Recently, considerable experimental activity has been directed towards the spectroscopic investigation of van der Waals (vdW) complexes as molecular sys- tems for modeling ~te~~lecul~ interaction from a microscopic point of view { zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1 ,2] , Aggregates are usual- ly produced in supersonic expansions and are ana- lysed by photoio~ation mass s~c~ornet~ (PI&IS) (see refs. [3,4] for recent reviews).*A growing amount of information has recently been provided by laser- induced multiphoton ionization or fluorescence spec- troscopy [4-61. Parallel to our work on laser-induced resonant two-photon ionization (R2PI) of fluoroben- zene clusters [7,8 ] we also studied the co~espond~g vdW complexes at the Berlin electron storage ring (BESSY) using synchrotron radiation 193.A recent publication by Kamke et al. [lo] reports the observa- tion of ~tramolecul~ Penning ionization (PI) of ben- zonitrile-rare gas complexes Bn*R(R=Ar,Kr)+hv -+ Bn*R* 7: (Bn+*R) t e- , Bn’+R+e-. 512 0) @I) The process was identified by the appearance of reso- nances in the photoion efficiency (PIE) curves of Bn and Bn+Ar, which arise from the optical excitation of the Ar %‘3,2 4s (1066.7 A) and 91~2 4s (1048.2 A) states f 111. The peaks were broad, blue-~ted and unresolved. The resonance in the PIE curve of Bn was interpreted in terms of Penning ionization of the Bn * Ar* complexes with successive fragmentation (II). In this work we report measurements, which, ~thou~ differing in detail, strongly support the ~te~retation of the latter authors. The data are compared with find- ings from Penning-ionization electron spectroscopy (PIES) of benzene in collisions with state-selected metastabie argon atoms Ar* (3P2, 3P& 2. Experimental The apparatus has been described [9]. In brief, the clusters are synthesized in a continuous supersonic ex- pansion by the seeded beam technique with argon as a seed gas. VUV light from BESSY, dispersed by a 1 m 0 009.2614~g6~$03..50 0 Blsevier Science Publishers B.V. (North- Holland Physics Publishing Division)