Raman Scattering Excitation Profiles as a Probe of the Valence Electron Excitations of Molecules Adsorbed on Metal Surfaces.

In this thesis measurements of Raman scattering -excitation profile spectra for totally symmetric and non -totally symmetric vibration modes of free-base phthalocyanine molecules adsorbed on silver have been used as a modulation spectroscopic probe to obtain information about the energies, widths and vibronic character of the singlet ((pi) to (pi)*) valence electron excitations and about the character of the bonding of the adsorbate by the substrate. These studies were carried out using a cw dye laser pumped by an argon ion laser and a krypton ion laser as the excitation sources. The Raman signals were detected using a computer interfaced Raman spectrometer with a photomultiplier used in a photon counting mode. The data show that the Q(,x) and Q(,y) excitations are red-shifted relative to the corresponding excitations of isolated free-base phthalocyanine molecules, and broadened, and that the Q(,x) and Q(,y) energy separation is reduced by about a factor of two. On the other hand the frequencies of the various vibration modes are unshifted. The data indicate that the resonant Raman scattering at the Q(,x) and Q(,y) excitations is due primarily to a Herzberg-Teller mechanism that involves vibronic coupling of the nearly degenerate Q(,x) and Q(,y) states with one another and with higher singlet states. These results together with the absence of any sizable xz, yz and zz components of the contribution to the Raman tensor from the Qx and Qy excitations suggest that the (pi) electrons are not primarily involved in the bonding of the phthalocyanine to the silver substrate. The decrease in the Qx - Qy separation is tentatively attributed to a weakening of the hydrogen bonds at the central nitrogen atoms that may result from the bonding of the molecules by the substrate via the lone pair electrons of the central nitrogen atoms.