J. Electron Spectroscopy and Related Phenomena 123 (2002) 303–314

Generalized oscillator strengths for C 1s excitation of acetylene and ethylene

A.P. Hitchcock , S. Johnston , T. Tyliszczak
Department of Chemistry , McMaster University , Hamilton , Ont ., Canada L 8S 4M

C.C. Turci , M. Barbatti , A.B. Rocha ,C.E. Bielschowsky
Instituto de Quýmica , Universidade Federal do Rio de Janeiro, Rio de Janeiro , 21910- 900, RJ , Brazil

Received 2 December 2001; received in revised form 8 January 2002; accepted 23 January 2002

Dedicated to Professor C.E. Brion on the occasion of his 65th birthday

The generalized oscillator strength profiles for discrete C 1s excited states of C2H2 and C2H4 have been derived from angle-dependent inelastic electron scattering cross-sections measured with 1300 eV final electron energy. The measured GOS profiles for the strong C 1s-> p* transition in each species are compared to theoretical calculations computed within the first Born approximation, using ab-initio generalized multi structural wave functions. These wave functions include relaxation, correlation and hole localization effects. Theory predicts large quadrupole contributions to the p* GOS of each species, analogous to those previously reported for computed GOS profiles for O 1s-> p* excitation of CO . We find good agreement 2 between experiment and theory as to the shape of the p* GOS but, when the relative GOS extracted from the experimental 2 data is normalized to the optical oscillator strength at K 50, the magnitude is in better agreement with the GOS computed for only the dipole channel than for the sum of the dipole and quadrupole channels.

2002 Elsevier Science B.V. All rights reserved.

Keywords : Generalized oscillator strength; Electron scattering; EELS; Quadrupole transitions