Experimental and theoretical studies on different ionic states of ethylthio CH3CH2S radical

A continuous flowing ethylthio CH3CH2S radical is produced in situ by pyrol ysis of CH3CH2SSCH2CH3 at 242(+/- 0.5) degrees C in a double heater inlet s ystem on a double chamber UPS machine-II which was built specifically to de tect transient species. By combining HeI photoelectron spectroscopic (PES) measurement with ab initio GAUSSIAN2 (G2) calculations on different ionic s tates of the CH3CH2S radical, we have concluded that the CH3CH2S radical ha s C-s symmetry and the ground state of the CH3CH2S+ cation is the (3)A" sta te which comes from removal of an electron of the SHOMO 13a' orbital for th e CH3CH2S radical. The PES peak at the lowest ionization energy 9.08 eV wit h vibrational structure 700 +/- 60 cm(-1) and corresponding with the G2 com puted ionization energy 9.123 eV is designated to the (3)A" ionic state. Th e removal of an electron of the HOMO 4a" for the CH3CH2S radical leads to a very sharp peak at 10.31 eV, corresponding to ionization CH3CH2S+ ((1)A')< -- CH3CH2S(X (2)A") and the G2 computed ionization energy 10.322 eV. The (1 )A" ionic state caused by removal of the electron of the SHOMO 13a' corresp onds to the PES peak at 10.50 eV and the G2 computed ionization energy 10.4 48 eV. The assignment of the PES bands of the CH3CH2S radical in the high i onization energy region (> 12.00 eV) is also supported by the G2 calculatio n. Both PES experiment and G2 calculations provide an evidence of existence of different ionic states for the CH3CH2S radical for the first time. (C) 2000 American Institute of Physics. [S0021- 9606(00)01926-7].