G. Zimmerer

Status report on luminescence investigations with synchrotron radiation at HASYLAB

Abstract The past development and the present technical parameters of the luminescence experimental stations HIGITI, SUPERLUMI, CLULU and VISUV at HASYLAB are described. The experimental possibilities and limitations are demonstrated with recent results from various fields including rare gas solids, rare gas clusters, matrix-isolated and dense gaseous systems. The future development of luminescence spectroscopy at HASYLAB with undulator—monochromator systems for primary excitation are briefly discussed.

Optical and luminescent properties of anisotropic tungstate crystals

Features of the reflectivity spectra in the fundamental absorption region were analysed for a series of tungstates. The contribution of electronic states of cations to the formation of the bottom of the conduction band and the top of the valence band is demonstrated. Its relation with the dominant mechanism of the energy transfer to the emission centres and the nature of these centres is discussed.

VUV spectroscopy of KYF4 crystals doped with Nd3+, Er3+ and Tm3+

Abstract Emission, excitation and absorption spectra as well as luminescence decay kinetics of KYF 4 crystals doped with Nd 3+ , Er 3+ and Tm 3+ were studied in the VUV spectral range. It is shown that all the crystals have intense VUV luminescence due to interconfiguration transitions originating from the 5d states of the rare earth (RE) ions and terminating on the 4f states. The UV luminescence caused by intraconfiguration 4f–4f transitions in the RE ions is also observed from the crystals doped with the Nd 3+ or the Er 3+ ions under 4f–5d excitation. The spin-allowed 5d–4f luminescence from the crystals doped with Er 3+ or Tm 3+ is extremely weak, i.e. there exists very efficient nonradiative relaxation from higher lying 5d states to the lowest 5d level responsible for spin-forbidden luminescence. This peculiarity of the studied crystals doped with Er 3+ or Tm 3+ is very attractive for developing VUV solid-state lasers based on the crystals of such a type.

Formation of KrCl* and ArCl* molecules and radiative lifetimes of their B states investigated with selective synchrotron radiation excitation

Abstract Monochromatic pulsed VUV excitation of Cl2 or inert gas atoms in Cl2 doped Kr and Ar leads to formation of KrCl* and ArCl*. The radiative lifetimes of the B states (KrCl* 19 ns, ArCl* 9 ns) and rate constants for excimer formation, quenching and collisional mixing of B and C states are given. The radiative lifetime and the quenching rate increase with vibrational excitation of the B state (KrCl*).

Optical properties of argon clusters in the VUV

Optical properties of argon clusters and microcrystals containing between 2 and 500 000 atoms have been studied in the VUV using fluorescence excitation spectroscopy with Synchrotron Radiation. Below the ionisation limit the fluorescence yield roughly corresponds to the absorption cross section. Surface and bulk excitations as well as Rydberg states have been observed. The evolution with cluster size shows strong variations of the band intensities whereas the energetic positions are only slightly shifted compared to the solid. It turns out that the Wannier excitons appear only in clusters if the radius of the cluster is approximately four times larger than the radius of the exciton.

Oscillatory structures in bound-free fluorescence spectra of Xe2, Kr2 and Ar2

Abstract Following state-selective pulsed synchrotron radiation excitation of Xe2, Kr2, and Ar2, oscillatory bound-free structures of the first continua were observed in time-resolved fluorescence spectra. The low-energy onsets are assigned to 0u+ → X0g+ transitions at the left turning point. Our results rule out earlier assignments of fluorescence features to the left turning point observed under e− or α-particle excitation of Kr and Ar. The shift of the oscillatory structures with excitation energy clearly supports our assignment.

Time and spectrally resolved fluorescence of Xe2 molecules excited with synchrotron radiation

Abstract Time and energy resolved fluorescence of Xe has been investigated using pulsed synchrotron radiation. The radiative lifetime of the relaxed Xe*2 (Iu) and its collisional mixing with Xe*2 (O+u) are measured. Rate constants for molecular formation are deduced. Different steps of the reaction kinetics are isolated by using various excitation wavelengths.

Recent results from ESD and PSD on rare gas solids with low energy electron and with synchrotron radiation excitation

Abstract Desorption of neutral atoms following selective excitonic excitation with low energy electrons or with synchrotron radiation underlines the crucial role of exciton–lattice interaction in the microscopic desorption mechanisms. All rare gas solids yield desorption of ground state atoms which stem from relaxation of the molecular-type trapped excitations after their radiative decay. In addition, solid Ne and Ar yield desorption of metastables, 3P1 and 1P1 atoms. The desorption of excited atoms is a consequence of the negative work function. It is closely related to exciton trapping in an atomic type configuration with cavity formation. The combination of various experimental techniques is needed to get a detailed insight into the excitonic desorption mechanisms.

VUV spectroscopy of wide band-gap crystals doped with rare earth ions

Abstract Spectral and kinetic properties of vacuum ultraviolet (VUV) luminescence from different fluoride crystals doped with Nd 3+ , Er 3+ or Tm 3+ are discussed. Concentration quenching of VUV luminescence, the impact mechanism of the energy transfer from the matrix to the emission centers as well as nonradiative relaxation processes in the rare earth ions have been detected and studied. The crystal properties are analyzed from the viewpoint of their possible applications as materials for fast VUV scintillators and VUV solid state lasers. Only some of Nd 3+ doped crystals possess simultaneously both high light output and relatively good timing properties of VUV luminescence and such crystals can be considered as promising materials for fast VUV scintillators. On the other hand, many of Er 3+ and Tm 3+ doped fluoride crystals are very attractive candidates as active media for tunable VUV solid state lasers with optical pumping.

Spectroscopic properties of Pr3+ luminescence in complex fluoride crystals

Abstract Spectroscopic properties of several complex fluoride crystals K 2 YF 5 , KYF 4 , LiKYF 5 , CsY 2 F 7 and CsGd 2 F 7 doped with Pr 3+ have been studied under synchrotron radiation and F 2 excimer laser excitation. The Pr 3+ doped K 2 YF 5 and LiKYF 5 crystals with only a single crystallographic site for Pr 3+ ions show typical properties as in the case of spectroscopic systems where the low edge of the 4f5d Pr 3+ configuration lies slightly below the 1 S 0 level of the 4f 2 Pr 3+ configuration. The Pr 3+ :KYF 4 crystal has the same properties caused with only the 5d–4f luminescence of Pr 3+ in spite of the fact that in this crystal there are six sites suitable for Pr 3+ ions. The luminescence spectrum of the Pr 3+ :CsY 2 F 7 crystal is determined by both the 5d–4f luminescence and the 4f–4f emission originating from the 1 S 0 Pr 3+ level, which is due to two groups of optical Pr 3+ centers with the energy levels of the 1 S 0 state just between the lowest 5d levels of Pr 3+ for these two kinds of centers.