PHYSICAL REVIEW A VOLUME 51, NUMBER 1 JANUARY 1995 High-resolution study of the Xe 4d5fz'. 4dsfz branching ratio A. Ausmees, * S. J. Osborne, R. Moberg, and S. Svensson Department of Physics, University of Uppsala, Box 580, S 751-21 Uppsala, Sweden S. Aksela, O.-P. Sairanen, A. Kivimaki, A. Naves de Brito, E. Nommiste, J. Jauhiainen, and H. Aksela Department of Physics, University of Oulu, FIN 905-70 Oulu, Finland and Finnish Synchrotron Radiation Facility at MAX LA-B, Uniuersity of Lund, Sweden (Received 20 3une 1994) The Xe 4d5y&. 4d3gz branching ratio has been determined in the photon energy range 80 — 250 ev using high-resolution electron spectroscopy and undulator radiation at the new Finnish beamline at the MAX I storage ring. The results were found to be in a satisfactory agreement with the calculated literature values obtained with the relativistic random-phase approximation. A few discrepancies between the theoretical predictions and our experimental results indicate additional correlation efFects not accounted for in theory. The branching ratio is now shown to have a maximum of about 1. 6 at around 230 eV. Lifetime widths of 114(4) and 121(4) meV are obtained for the 4dsyq and 4d5gq core hole states, respectively. PACS number(s): 32.80.Fb, 32.80.Hd Photoionization of the Xe 4d subshell has been of con- tinuous interest to both experimentalists and theoreti- cians over the past decades [1]. The 4d photoionization gives the major contribution to the total absorption cross section in a wide photon-energy region, starting &om the broad shape resonance that has a maximum around 100 eV, up to 3d ionization threshold around 700 eV. Above a threshold photon energy, the 4d core ionization process can be accompanied by a simultaneous transition of an- other electron either to a higher unoccupied orbital or to a continuum state, borrowing intensity from the 4d sin- gle ionization channel. Recently, a quantitative partition of the total cross section into its components has been achieved in the region of the shape resonance [2 — 6]. The partial cross sections and the angular-distribution asym- metry parameters of the photoelectrons have been deter- mined with reasonable consistency across a large photon energy range [2 — 4, 7, 8]. Several sophisticated theoretical approaches [9 — 14] have given a fairly reasonable, though not excellent, agreement with the experimental findings. Even though present theory accounts for electron corre- lation at least to some extent, better consideration of the correlation effects is necessary. A different situation holds for the experiments where the 4d5~q and 4d3gq spin-orbit split components are well resolved, relatively few data are available within a limited photon energy range. From the experimental point of view, the requirements on both intensity and resolution are stringent, which points towards the use of undula- tor radiation sources in conjunction with high-resolution monochromators and electron spectrometers. The Xe 4ds~~. . 4ds~q branching ratio (BR) as a function of photon energy has earlier been determined between 4d thresh- olds and 150 eV [7, 15 — 17]. The only exception was Ref. *Permanent address: Institute of Physics, Estonian Academy of Sciences, Riia 142, EE-2400 Tartu, Estonia. ~Present address: Department of Physics, University of Brasilia, 70910 Brasilia DF, Brazil. [16], where the BR for a few photon energies above 150 eV was determined using x-ray line sources. Because the measurements were not performed at the magic an- gle, the accuracy of the BR values of Ref. [16] might suffer from angular-dependence effects. The rather mod- erate intensity and instrumental resolution in the earlier measurements made it difFicult to determine the BR val- ues with high accuracy. The experiments [7, 15 — 17] have shown that the 4dsy~. 4d3y~ BR values strongly deviate &om the statistical value of 1. 5 in the whole photon en- ergy region studied, leaving open the question of where the BR reaches its statistical value. In this report, we present the Xe 4d5yq. . 4d3yq branch- ing ratios at photon energies between 80 and 250 eV. The results are based on high-resolution gas-phase photoelec- tron spectra measured using monochromatized undulator radiation. The main purpose of the present study is to determine accurately the 4d BR values and to extend considerably the photon energy range compared to the previous studies. The experimental values of the BR will be compared with available theoretical results. The experiments have been carried out at the Finnish beamline (BL 51) [18] at the MAX I storage ring in Lund, Sweden. Synchrotron radiation in the photon energy range of 60 to 600 eV obtained from a short period un- dulator is monochromatized by a modified Zeiss SX-700 plane grating monochromator [19]. The degree of lin- ear polarization of undulator radiation in the case of the lower-order harmonics has been shown to be very high [5, 20]. The gas-phase experiment is isolated from the ul- trahigh vacuum of the monochromator by a permanent differential pumping section. This is equipped with a toroidal refocusing mirror that produces a small spot 1 mm in diameter at the source point in the sample com- partment. The electron spectrometer SES-144 [21] is of the truncated hemispherical type with a mean radius of 144 mrn. The analyzer is combined with a four element retarding or accelerating electron lens that focuses elec- trons onto the entrance slit of the analyzer. Electrons are retarded or accelerated to a constant pass energy. In the present study, pass energies of 10, 20, and 50 eV were 1050-2947/95/51(1)/855(4)/$06. 00 51 855 1995 The American Physical Society