13 March 1998 Ž . Chemical Physics Letters 285 1998 71–76 Quasi two-step dissociation effects observed in the core excited OCS molecule K. Yoshiki Franzen a , P. Erman a , P.A. Hatherly b , A. Karawajczyk a , E. Rachlew a , ´ M. Stankiewicz c a Department of Physics I, The Royal Institute of Technology, S-10044 Stockholm, Sweden b J.J. Thomson Physical Laboratory, UniÕersity of Reading, Whiteknights, Reading RG6 6AF, UK c Instytut Fizyki Mariana Smoluchowskiego, Uniwersytet Jagiellonski, Reymonta 4, 30-059 Krakow, Poland ˜ ´ Received 10 November 1997 Abstract Studies of the decay dynamics following carbon 1s and sulfur 2p electron excitations of the OCS molecule using the photoelectron–photoion–photoion coincidence method and a synchrotron radiation source are presented. A Monte Carlo code is utilized to interpret the data obtained, which together with previous measurements including branching ratios, kinetic energy distributions and ionic b-parameters give new information on the dynamics of three-body dissociation processes. The results indicate that the instantaneous and sequential dissociation models should be replaced by quasi two-step models. q 1998 Elsevier Science B.V. 1. Introduction The decay dynamics of multiply ionized triatomic w x molecules has received much attention 1–4 . A fun- damental question addresses whether the fragmenta- wx tion occurs instantaneously or sequentially 2 . De- tailed information may be obtained by using the photoelectron–photoion–photoion coincidence Ž . wx wx PEPIPICO method 5 as shown by Eland 2 for wx several tri-atomic molecules. Masuoka 3 utilized this technique in synchrotron radiation experiments and it became obvious that the dissociation mecha- nisms of the OCS molecule did not depend on the excitation energy in the valence region. However, studies of core excited tri-atomic molecules have indicated several effects not exhibited in the valence excitation region. For example, core excited N O, 2 CO and OCS have exhibited ionic branching ratio 2 w x dependence on the site of the excited electron 6–10 . In the case of OCS high-resolution measurements w x have also revealed branching ratio 10,11 , ionic w x emission asymmetry 12 and kinetic energy distribu- w x tion 10 variations, depending upon the state to which the particular core electron was promoted. This indicates that the dissociation dynamics of core and valence excited polyatomic molecules may be quite different. In the present Letter we study the dissociation dynamics of the core excited OCS molecule using the PEPIPICO technique. To interpret some of the data we have used a Monte Carlo simulation code. By combining these data with previous results including 0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0009-2614 97 01463-2