Investigation of paraexcitons in KI

Abstract

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...

Magnetic Field Splitting and Dispersion of the Lower 1S‐Exciton Polariton in KI and RbI

“…In addition to the r; transitions, which are one-photon allowed, a very narrow line of r; symmetry was excited by TPS in CsI [5]. In high magnetic fields transitions to paraexcitons were seen in KI [6] and RbI [4]. The rather small linewidth of about 0.2 meV allowed an accurate determination of g-values (8, ) of the paraexcitons and first results for the exchange splitting A,, [4].…”

“…In a magnetic field the paraexciton E , becomes dipole-allowed. It was detected in KI and RbI by the use of TPS [4,6]. The exchange energy A,, is calculated using the definition of Onodera and Toyozawa [20],…”

Temperature Dependence of Exciton Linewidth in CsI and CsBr

“…Recently Beerwerth and Frohlich [6] were able to detect the paraexciton in KI, which coincides also with the free-exciton luminescence as pointed out by Itoh [15].…”

“…Its splitting in a magnetic field is clearly resolved. Narrow lines of r; symmetry are seen also in K I [6] and RhI [7]. The simukaneous observation of broad and narrow exciton lines in alkali halides is not yet understood.…”

Phonon‐Assisted Two‐Photon Absorption in the Reststrahl Region of Excitons in Alkali Halides

“…The paraexcitons as pure triplet states can also not be excited by an electric dipole transition. However, by application of a magnetic field [8] or a uniaxial stress [9] these states can be detected by three-photon spectroscopy. Very recently Frohlich et al…”