ELSEVIER Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 9-17 JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena Invited paper Angular distributions of is photoelectrons from fixed-in-space molecules as a probe of shape resonances E. Shigemasa* Photon Factory, 1MSS, KEK, Oho 1-1, Tsukuba-shi, lbaraki-ken 305, Japan Abstract The angular distributions of ls photoelectrons from fixed-in-space molecules have been measured around the a* shape resonances of N2, CO2, and CO molecules, for the case of light polarization parallel to the molecular axis, when only a continuum channels contribute. On the basis of the quantitative analyses of the results, the natures of the a* shape resonances in these molecules are discussed. © 1998 Elsevier Science B.V. Keywords: Angular distributions; Fixed-in-space molecules; Shape resonances; K-shell photoionization 1. Introduction Molecular inner-shell photoabsorption spectra exhibit broad resonances above the ionization thresh- olds, in sharp contrast to the corresponding atomic spectra that show monotonically decreasing behavior [1]. These structures are so-called a* shape reso- nances [2]. Shape resonances are one of the intriguing phenomena associated with molecular inner-shell excitation spectra and the name originates from the concept that the excited state is stabilized against immediate decay by a hump, due to a centrifugal bar- rier effect in the continuum wave function, in the molecular potential. Shape resonances are also asso- ciated with particular high orbital angular momentum components, which are trapped only along the molecular axis [3,4]. Rather than using such a scatter- ing picture for the interpretation of the cr* shape reso- nance, one can explain the resonance in a molecular orbital picture. In this concept the shape resonance * Tel: +81-298-64-5594;fax: +81-298-64-2801; e-mail: sigemasa@kekvax.kek.jp) can be described as a transition into an antibonding virtual molecular orbital in the continuum [5]. By taking account of the similarity between the multiple scattering wave function and the contour maps utilized for picturing molecular orbitals, the link between the scattering and molecular orbital concepts is established most elegantly [6]. Angular distribution measurements of photoelec- trons can access dynamical information on the molecular photoionization processes, i.e. relative phases of alternative degenerate ionization channels [7], and therefore have been widely applied to inves- tigate the shape resonance phenomena [1]. However, owing to the random orientation of the molecular tar- gets, it is impossible to obtain the direct information of the molecular photoionization dynamics by usual gas phase photoelectron spectroscopy. By measuring the partial a and 7r channel cross-sections separately using angle-resolved photoion spectroscopy, the fact that the shape resonances appear only in the a chan- nels has recently been proved for several molecules [8-10]. In order to investigate further detailed natures of the shape resonances more directly, the visualization 0368-2048/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0368-2048(97)00125-4