Journal of Electron Spectroscopy and Related Phenomena 107 (2000) 273–282 www.elsevier.nl / locate / elspec Study of outer valence orbitals of carbon dioxide by (e, 2e) spectroscopy * X.J. Chen , G. Ouyang, C.C. Jia, L.L. Peng, C.K. Xu, S.X. Tian, K.Z. Xu Laboratory of Atomic and Molecular Physics, Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230027, PR China Received 9 December 1999; received in revised form 4 April 2000; accepted 6 April 2000 Abstract By using symmetric non-coplanar electron momentum spectroscopy at impact energy of 1200 eV plus binding energy, valence shell binding energy spectra of carbon dioxide have been obtained. The experimental momentum profiles of four outer valence orbitals: 1p ,1p ,3s ,4s are compared with theoretical profiles calculated by Hartree–Fock and density g u u g functional theory (DFT-B3LYP) methods using both 6-31111G* and double-zeta basis sets. The experimental results are in good agreement with the calculated results. A minimum at p ¯ 1.2 a.u. for the 3s orbital is found. The experimental u momentum profiles generally agree better with the profiles calculated by the DFT method than those by the HF method. The position and momentum space density maps for four outer valence orbitals of CO are also presented.  2000 Elsevier 2 Science B.V. All rights reserved. Keywords: Carbon dioxide; (e, 2e); Momentum distribution; Binding energy; Density functional theory; Hartree–Fock 1. Introduction macromolecules [4] and in material science [5] is alluring. These advantages in the molecular study of Electron momentum spectroscopy (EMS) is a so many fields are due to the fact that EMS can powerful tool for investigating binding energy spec- directly obtain electron distribution of valence orbi- tra (BES) and momentum distributions (MDs) of tals in selected samples. valence orbitals in atoms and molecules [1,2]. The Carbon dioxide, a typically linear molecule, is of binary (e, 2e) spectroscopy has become increasingly considerable fundamental as well as applied interest. important in providing detailed information on MDs Before this work, the EMS of carbon dioxide had of valence orbitals. The experimental results can help been measured three times [6–8]. The measurement us evaluate all kinds of models offered by different by Giardini-Guidoni et al. only obtained the momen- theories [2] and to understand the relation between tum distribution of 1p because of low energy g electron density distribution and chemical reactivity resolution (2.6 eV FWHM) and the orbital momen- [3]. The prospect of EMS application in biochemical tum distributions were obscured by the Mott scatter- ing cross section because of the coplanar geometry employed. The non-coplanar measurements by Cook *Corresponding author. Fax: 186-551-360-1164. E-mail address: xjun@ustc.edu.cn (X.J. Chen) and Brion used higher energy resolution (1.3 eV 0368-2048 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0368-2048(00)00173-0