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The temperature dependence of the linewidth of the lowest two r; excitons in Csl and in CsBr and of a very narrow I ? ; exciton in CsI is reported. All spectra are measured by three-photon spectroscopy. The analysis of the experimental results allows conclusions about the decay mechanism of excitons in alkali halides. The temperature dependent broadening is quantitatively described by taking into account exciton-phonon interaction.Die Temperaturabhiingigkeit der Linienbreiten der beiden niederenergetischen r;-Exzitonen in CsI und CsBr und eines sehr schmalen r;-Exzitons in CsI wird untersucht. Alle Spektren werden mit der Methode der Dreiphotonenspektroskopie ausgemessen. Die Auswertung der experimentellen Resultate erlaubt Ruckschliisse auf den Zerfdlsmechanismus der Exzitonen in Alkalihalogeniden.Unter Einbeziehung der Exziton-Phonon-Wechselwirkung kann die temperaturabhiingige Linienverbreiterung quantitativ beschrieben werden.

The lowest paraexciton resonance in KI is measured for the first time. The narrow exciton line at 5.8279 eV is split into four components in a magnetic field. A g value of 1.35 is deduced from the Zeeman splitting of the paraexciton. In addition, a reliable value for the exchange splitting (6 45 meV) is gained from the three-photon spectra, which is much lower than that obtained by one-photon spectroscopy.Though the first excitonic transitions were observed in alkali halides' more than fifty years ago, direct determination of fundamental electronic parameters such as exchange interaction and g values of valence and conduction band has not been possible up to now. Due to the rather small exciton radius the exchange interaction has to be taken into account, at least for the analysis of the lowest exciton transitions. A similar situation is found in molecular crystals, where Frenkel-type excitons are also expected. The alkali halides are regarded as model substances for the study of Frenkel excitons. A complete understanding of the excitonic properties of these crystals of high symmetry (Oh ) will certainly help in the understanding of more complicated structures like molecular crystals of lower symmetry, where the influence of anisotropy on important electronic parameters like exchange interaction and g values cannot be neglected anymore. Onodera and Toyozawa have discussed the importance of exchange interaction in the case of alkali halides in detail. The exchange interaction splits the pure triplet states (F=2) from the singlet-triplet mixed states (F=1). Because of the large linewidth of one-photon transitions (about 50 meV in KI) there is no chance to resolve transitions to the paraexciton (F=2 transition) by application of high magnetic fields. Nevertheless, there is fine structure observed in one-photon spectra ' which is attributed to phonon sidebands of the paraexciton. Petroff, Pinchaux, Chekroun, and Balkanski deduce from fine structure in the reflection spectrum of KI, an exchange splitting of 6, =43 meV. More recently O'Connel-Bronin interprets LO-phonon-assisted luminescence in the framework of light and heavy excitons. This point is certainly important in one-photon spectra, since rather large K values are involved in the resonance region, which leads to mixing of heavy and light excitons, as discussed by Fishman for semiconductors.The determination of g values causes similar problems. Because of the large linewidth of the I 4 exciton (F=l), a splitting, which is expected to be less than 1 meV at about 5 T, cannot be resolved directly. Ahrenkiel and Teegarden determine an effective g value of about 0.9 from magnetoreflectance measurements for the first exciton in KI. More recently Onaka and Kita and Onaka and Iwamoto' have measured magnetic circular dichroism in thin films. For KI they cite effective g values between 0.57 and 1.3, which even depend on the thickness of the evaporated films.In this Rapid Communication, we report three-photon spectra of KI in magnetic fields up to 9 T. Contrary to one-ph...

Three-photon magnetoabsorption measurements in fields up to 9 T are presented for the first time. A narrow exciton line of Ff symmetry is resolved in Csl at 5.8076 eV. A Rvalue of 1.17 is deduced from the Zeeman splitting of this exciton. At high magnetic fields an exciton of Fj" symmetry is detected. The analysis of the field-dependent mixing of the Ff" and Ff excitons yields a zero-field shift of the Ty exciton to lower energies of 0.54 meV and an effective g value for the admixture of 1.20. 42.65.Ft, 71.70.Ej Three-photon spectroscopy was recently introduced by Beerwerth and Frohlich^ as a powerful technique to study exciton polaritons in crystals with inversion symmetry. The authors measured resonances on the upper polariton branch and even the longitudinal exciton of the one-photon-allowed exciton of Ff symmetry in several alkali halides. Besides this one-and three-photon-allowed F^ exciton there are excitons of Ff, Ff, TJ, and Ff symmetry, which are only allowed in three-photon absorption, as can be easily derived with the use of the tables of Koster et al? In this Letter we present the first demonstration of such a one-photon-forbidden transition. Again the alkali halides are chosen, since they show strong odd-parity exciton transitions. Among the alkali halides the cesium halides have attracted much interest recently, since application of hydrostatic pressure causes a drastic red shift of the band gap.^ There are even predictions of metallization of these ionic crystals at superhigh pressures.-^' "^ Kuznetsov et al? showed that in Csl, excitons derived from the lowest ^conduction band (Fg" symmetry) shift to lower energy by 0.25 eV under hydrostatic pressure of 120 MPa. We were able to resolve a new, very narrow exciton line of Ff symmetry in Csl, which is assigned to the same t/conduction band. The width of this line is only 0.2 meV at 4.2 K. This small linewidth makes possible, for the first time, the direct observation of the Zeeman splitting of an exciton transition in alkali halides. In onephoton absorption, ^values of the F^ exciton can only be determined by magnetoreflectance^ or magnetoabsorption dichroism,^ because the linewidth of about 30-50 meV is much larger than the expected Zeeman splitting (about 1 meV at 9 T).For our measurements we used ultrapure single crystals of Csl which were cut for measurements in three orientations ([001], [110], and [111]). The crystals were oriented by use of stress-induced birefringence as described by Maier.^ The magnetic fields for the measurements in the Faraday configuration were produced by a superconducting magnet at 4.2 K; fields up to 9 T were available. As a light source we used a frequency-doubled Nd-doped yttriumaluminum-garnet laser (Quanta Ray model DCR-2 A) to pump a tunable dye laser (Lambda Physik model FL 2002). Three-photon absorption was detected via the luminescence of the self-trapped exciton at 290 nm.In Fig. 1 we present a three-photon spectrum of Csl and the one-photon data of Teegarden and Baldini.^ The three rather broad lines corr...