The reflectivity of freshly cleaved single crystals of RbCl, RbBr, and RbI and of an evaporated RbF film was measured at room temperature for photon energies between 10 and 30 eV using synchrotron radiation. With a resolution of 2 A over the whole spectral range considerable new spectral features were observed in the region where high energy valence band transitions occur and electrons from the first core level, the Rb+ 4p level, are excited. For all Rb halides excitation of the Rb+ 4p level starts at about 16 eV with an exciton multiplet which can be explained as being due to spin-orbit splitting of the core level and to critical points at Γ and X in the conduction band. The energies of the multiplet are compared to transition energies of the 4p electrons of the free Rb+ ion. The complex dielectric constant, the absorption coefficient, and the energy loss function are derived from the reflectivity by means of a dispersion analysis. The results are compared with absorption and electron energy loss experiments on evaporated polycrystalline films.



Das Reflektionsvermogen von frisch gespaltenen RbCl-, RbBr- und RbJ-Einkristallen und einer RbF-Aufdampfschicht wurden bei Zimmertemperatur mit Synchrotronstrahlung fur Photonenenergien zwischen 10 und 30 eV gemessen. Bei einer Auflosung von 2 A uber den ganzen Spektralbereich konnten zahlreiche neue elektronische Anregungen vom Valenzband und dem ersten Rumpfniveau (Rb+4p) gefunden werden. Die Ubergange vom Rb+4p-Niveau beginnen fur alle Rb-Halogenide bei etwa 16 eV mit einem Exzitonen-multiplett, das durch Spin-Bahn-Aufspaltung des Rumpfniveaus und kritische Punkte bei Γ und X im Leitungsband erklart werden kann. Die Energien des Multipletts werden mit Anregungsenergien der 4p-Elektronen des freien Rb+-Ions verglichen. Aus dem Reflektionsvermogen wurden mittels Dispersionsrelation die komplexe Dielektrizitatskonstante, der Absorptionskoeffizient und die Energieverlustfunktion berechnet. Die Ergebnisse werden mit Messungen der Absorption und des Energieverlustes von Elektronen an aufgedampften polykristallinen Schichten verglichen.

Dielectric Properties of the Rubidium Halide Crystals in the Extreme Ultraviolet up to 30 eV

The excitation of the Cs-5p core level in Cs halides which starts a t about 13 eV was studied by measuring the reflection spectra of evaporated films between 12.5 and 26 eV at room temperature and a t 30 K. Synchrotron radiation was used as a light source. In comparison to the spectra at room temperature new remarkable fine structure was found at 30 K. The very complex spectra are discussed with regard to band calculations including excitons associated with critical points. I n a complementary view the spectra are compared with transitions of the free Cs+ ion. A sideband at the high energy part of the most prominent exciton was observed for every Cs halide. It is, presumably, caused by excitonphonon interaction.



Die Anregung des Cs-5p Rumpfniveaus in Cs-Halogeniden, die bei etwa 13 eV beginnt, wurde mit Hilfe von Messungen der Reflexion aufgedampfter Schichten zwischen 12,5 und 26 eV bei Raumtemperatur und bei 30 K untersucht. Synchrotronstrahlung diente als Lichtquelle. Im Vergleich zu den Spektren bei Raumtemperatur findet man bei 30 K neue bemerkenswerte Feinstruktur. Die stark strukturierten Spektren werden an Hand von Bandstrukturberechnungen unter Einschlus von Exzitonen an kritischen Punkten diskutiert. I n komplementiirer Betrachtungsweise werden sie mit den Ubergiingen des freien Cs+ Ions verglichen. Fur alle Cs-Halogenide wurde ein Seitenband auf der hochenergetischen Flanke des starksten Exzitons beobachtet. Es ist moglicherweise auf Exziton–Phonon- Wechselwirkung zuriickzufuhren.

Excitation of the Cs-5p core level in cesium halides at 30 K

A complete survey of the vacuum ultraviolet and soft X-ray spectroscopy of solids should include not only reflection and transmission spectroscopy, but X-ray and ultraviolet photoemission spectroscopy, electron energy loss spectroscopy, several other spectroscopies, and the theoretical bases for their interpretation, for all deal with the same microscopic phenomena. This chapter will be limited, of necessity, to absorption, reflection, luminescence, and modulation spectroscopy of solids in the 6–300 eV region. (The dividing line between soft X-ray and vacuum ultraviolet is not well-defined and will shift several times during this chapter). There is an enormous literature to cover, for conventional discharge sources have been in use for many years, and improved versions are still widely used. Synchrotron radiation has a number of advantages over them, and its use has led to the discovery of new kinds of spectral features. For example, the high intensity and temporal stability of synchrotron radiation permit high-resolution and modulation spectroscopy to be carried out almost routinely, revealing previously unresolved structure, and the polarization allows work on anisotropic materials. Because synchrotron radiation has a continuous spectrum over several decades of energy the search for spectral features can be carried out thoroughly. This chapter describes work done almost exclusively with synchrotron radiation, although some of it could have been carried out with conventional sources. In fact, some has been so carried out, often earlier, but space does not permit adequate referencing of much of such work.

Solid-State Spectroscopy

Optical Properties of Ionic Insulators

The coefficients (c vdW) for the dipole-dipole van der Waals interactions between two ions in the isoelectronic alkali halide crystals are estimated within Mayer's method from the optical spectra. The Kramers-Kronig analyses of the recent optical and reflection measurements, obtained mainly by the extensive use of synchrotron radiation, lead to the determination of the optical constants and hence to the “experimental” values of the matrix elements of the dipole moment operator between the ground and the excited electronic states. These matrix elements appear in the expression of the c vdW coefficients from second order perturbation theory in the asymptotic expansion. The adopted procedure is applied to the rare gas crystals for the first time. The reported values of the electronic polarizabilities and the c vdW of Kr indicate the expected agreement with the corresponding gas phase values. The effect of the full local field corrections, appropriate for these insulators, on the c vdW coefficients i...

M. Skibowski

Papers

Excitation of the Cs-5p core level in Cs halides