Ultra-fast Dissociation of CH3C(O)SH in the vicinity of the sulfur 2p threshold

M. F. ERBEN; M. GERONÉS; R. M. ROMANO; C. O. DELLA VÉDOVA

Campinas, Brasil

Workshop; IWP International Workshop on Photoionization; 2005

Laboratório Nacional de Luz Síncrotron

In 1986 Morin and Nenner reported the occurrence of a new mechanism for the dissociation of a core excited species [1]. The electron energy spectra of HBr were explained in terms of ionic states of a fragment, instead of ionic states of the parent molecule. Thus, although Auger decay of a core hole occurs on the femtosecond time scale, the dissociation may occur directly form the core hole excited state. In the frame of the core equivalent model, an atom with a core hole and a nuclear charge Z has the properties of the Z+1 atom or ion because of the loss of shielding by the missing core electron. In particular, the core equivalent model predicts that the valence properties of the Z+1 atom determine whether the core hole state will be bonding or dissociative. For instance, in the case of HBr, the excitation of a Br 3d electron to an antibondind orbital transform Br into Ar and the HBr bond becomes unstable. The repulsive potential surface imparts a high velocity to the H atom, which then escapes and the electronic relaxation of the core hole occurs in the bromine atom [2]. Such ultra-fast dissociation needs to be considered for light atoms, primarily hydrogen atoms that are bonded to atomic centers whose valence cannot tolerate an increase in the atomic number. It have been reported that SiH4 follows this fragmentation upon Si 2p excitation [3]. Ultra fast dissociation of the core hole excite state also was considered for methyl bromide. When the hydrogen atom is replaced by a methyl group, the time scale for dissociation is lengthened, and autoionization competes more effectively with the dissociation and direct dissociation of the core hole state become less important. Following these ideas, we are interested in the study of dissociation mechanisms in CH3C(O)SH, as excited at the S 2p edge. In order to achieve this goal, Total Ion Yield spectra of CH3C(O)SH was recorded at the S 2p in the edge by using the TGM (Toroidal Grating Monochromator) beamline available at the LNLS (Campinas) facilities. PEPICO spectra were obtained at resonant and out of resonance photon energies in order to discriminate the effect arising from resonant processes. Furthermore, triple coincidence PEPIPICO spectra were measured in order to understand the dynamic of the ionic fragmentation [4, 5]. From the core equivalent model, it is expected that the shallow core excitation of this molecule leads to the unstable specie CH3C(O)Cl*H. The presence of the acidic hydrogen atom bonded to the core excited atom should permit, in principle, that the effects originated from ultra fast fragmentation can be observed. Furthermore, similar studies were enfaced taken S-Methyl thioacetate, CH3C(O)SCH3 as the target compound. The presence of a heavier methyl group bonded to the sulfur atom should preclude the occurrence of the fast fragmentation following S 2p excitation.4 follows this fragmentation upon Si 2p excitation [3]. Ultra fast dissociation of the core hole excite state also was considered for methyl bromide. When the hydrogen atom is replaced by a methyl group, the time scale for dissociation is lengthened, and autoionization competes more effectively with the dissociation and direct dissociation of the core hole state become less important. Following these ideas, we are interested in the study of dissociation mechanisms in CH3C(O)SH, as excited at the S 2p edge. In order to achieve this goal, Total Ion Yield spectra of CH3C(O)SH was recorded at the S 2p in the edge by using the TGM (Toroidal Grating Monochromator) beamline available at the LNLS (Campinas) facilities. PEPICO spectra were obtained at resonant and out of resonance photon energies in order to discriminate the effect arising from resonant processes. Furthermore, triple coincidence PEPIPICO spectra were measured in order to understand the dynamic of the ionic fragmentation [4, 5]. From the core equivalent model, it is expected that the shallow core excitation of this molecule leads to the unstable specie CH3C(O)Cl*H. The presence of the acidic hydrogen atom bonded to the core excited atom should permit, in principle, that the effects originated from ultra fast fragmentation can be observed. Furthermore, similar studies were enfaced taken S-Methyl thioacetate, CH3C(O)SCH3 as the target compound. The presence of a heavier methyl group bonded to the sulfur atom should preclude the occurrence of the fast fragmentation following S 2p excitation. References: [1] C. Morin and I. Nenner, Phys. Rev. Lett. 56 (5), 1913 (1986). [2] D.M. Hanson, Adv. Chem. Phys. 77, 1 (1990). [3] G.G.B. de Souza, P. Morin, and I.Nenner, Phys. Rev. A 34 (6), 4770 (1986). [4] M.F. Erben, R.M. Romano, and C.O. Della Védova, J. Phys. Chem. A. 108 (18), 3938 (2004). [5] M.F. Erben, R.M. Romano, and C.O. Della Védova, J. Phys. Chem. A. 109 (2), 304 (2005).56 (5), 1913 (1986). [2] D.M. Hanson, Adv. Chem. Phys. 77, 1 (1990). [3] G.G.B. de Souza, P. Morin, and I.Nenner, Phys. Rev. A 34 (6), 4770 (1986). [4] M.F. Erben, R.M. Romano, and C.O. Della Védova, J. Phys. Chem. A. 108 (18), 3938 (2004). [5] M.F. Erben, R.M. Romano, and C.O. Della Védova, J. Phys. Chem. A. 109 (2), 304 (2005).77, 1 (1990). [3] G.G.B. de Souza, P. Morin, and I.Nenner, Phys. Rev. A 34 (6), 4770 (1986). [4] M.F. Erben, R.M. Romano, and C.O. Della Védova, J. Phys. Chem. A. 108 (18), 3938 (2004). [5] M.F. Erben, R.M. Romano, and C.O. Della Védova, J. Phys. Chem. A. 109 (2), 304 (2005).34 (6), 4770 (1986). [4] M.F. Erben, R.M. Romano, and C.O. Della Védova, J. Phys. Chem. A. 108 (18), 3938 (2004). [5] M.F. Erben, R.M. Romano, and C.O. Della Védova, J. Phys. Chem. A. 109 (2), 304 (2005).108 (18), 3938 (2004). [5] M.F. Erben, R.M. Romano, and C.O. Della Védova, J. Phys. Chem. A. 109 (2), 304 (2005).109 (2), 304 (2005). Acknowledgements: This work has been largely supported by the LNLS under proposals D05A-412. The authors are indebted to the Facultad de Ciencias Exactas (UNLP), CONICET, DAAD and CIC for financial support. The authors wish to thank Arnaldo Naves de Brito and his research group for fruitful discussions and generous collaboration during their stay in Campinas and the TGM beamlines staff for their assistance throughout the experiments.: This work has been largely supported by the LNLS under proposals D05A-412. The authors are indebted to the Facultad de Ciencias Exactas (UNLP), CONICET, DAAD and CIC for financial support. The authors wish to thank Arnaldo Naves de Brito and his research group for fruitful discussions and generous collaboration during their stay in Campinas and the TGM beamlines staff for their assistance throughout the experiments.