Physical Review A 61 (2000) 042505 (14 pages)

Experimental and theoretical study of generalized oscillator strengths
for C 1s and O 1s excitation in CO2

I. G. Eustatiu, T. Tyliszczak, A. P. Hitchcock
Department of Chemistry, McMaster University, Hamilton, ON, L8S 4M1 CANADA

C.C. Turci, A. B. Rocha and C. E. Bielschowsky
Instituto de Química, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, 21910 Brazil

Electron energy loss spectra of CO2 in the region of C 1s and O 1s excitation have been recorded over a wide range of momentum transfer (K), (2 a.u.-2 < K2 < 70 a.u.-2).  The dipole forbidden transition to the (C 1s sg-1, sg*) 1Sg+ state in CO2 is detected for the first time.  A detailed analysis, with careful consideration of minimization of systematic experimental errors, has been used to convert the measured relative cross sections to absolute, momentum transfer dependent, generalized oscillator strength (GOS) profiles for all resolved C 1s and O 1s transitions of CO2.  Theoretical results for the GOS, computed within the first Born approximation, were obtained with ab-initio configuration interaction wave functions for the C 1s transitions and with ab-initio generalized multi structural wave functions for the O 1s transitions.  These wave functions include relaxation, correlation and hole localization effects.  Theory predicts large quadrupole contributions to the GOS for O 1s excitations. In addition the computed GOS for O 1s -> nss and nps Rydberg states clearly show oscillations arising from interference between localized core excitations. Overall there is good agreement between the experimental and theoretical results indicating that the first Born approximation holds to a surprisingly large momentum transfer for the core excitations studied.

PACS number(s): 33.70.Ca, 34.80.Gs, 33.20.-t, 31.15.-p