Excitation of the A-band low-lying electronic states in the methyl halides, CH(3)I, CH(3)Br, CH(3)Cl, and CH(3)F, has been investigated for the (n-->sigma*) transitions, using electron energy loss spectroscopy (EELS) in the range of 3.5-7.5 eV. For the methyl halides, CH(3)I, CH(3)Br, and CH(3)Cl, three components of the Q complex ((3)Q(1), (3)Q(0), and (1)Q(1)) were directly observed, with the exception of methyl fluoride, in the optically forbidden EELS experimental conditions of this investigation. The effect of electronic-state curve crossing emerged in the transition probabilities for the (3)Q(0) and (1)Q(1) states, with spin-orbit splitting observed and quantified against results from recent ab initio studies.
