ELECTRON-IMPACT OVERTONE EXCITATION VIA THE (2)SIGMA(U) RESONANCE IN N-2 FILMS PHYSISORBED ON AG(110) AT 15 K

The growth and structure of solid nitrogen films on Ag(110) at 15 K wa s investigated by means of high resolution electron energy loss spectr oscopy (EELS), thermal desorption spectroscopy (TDS), and low energy e lectron diffraction (LEED). The N-2 film exhibits a parallelogram stru cture in the monolayer. For the bilayer and multilayers an incoherent hexagonal structure-has been found. The dynamics of the resonant vibra tional excitation of the N-2 molecule via the (2) Sigma(u) resonance i s studied in detail both by the EELS method (as a function of the surf ace coverage) and theoretically. This allows a discussion of the effec t of the adsorption on the vibrational excitation process and, in part icular, on the overtone excitation. The (2) Sigma(u) resonance excitat ion process is found to be almost not influenced by the adsorption in strong contrast with the case of the low energy ((IIg)-I-2) N-2 resona nce. The differences between these two processes are discussed. (C) 19 98 American Institute of Physics.