Dankward Schmid

Large-sample effects in superfluorescence of O-2 centers in KCl

Abstract Superfluorescence is the spontaneous collective emission of coherent light from an ensemble of two-level systems which are initially all in the excited state. In this contribution we attempt to distinguish this phenomenon from standard laser activity as well as to indicate common features of both processes. We report on experiments with O-2 centers in KCl and emphasize a number of novel experimental observations such as multicolor superfluorescence, transverse mode structure and superfluorescence-brightened laser activity.

Frenkel excitons in : excitation energy transfer and exciton coherence

A review is presented about sensitized luminescence and luminescence lineshape studies on pure and doped sodium nitrite which have led to detailed knowledge about the lowest excited states of this system. Both the lowest triplet state and the first excited singlet state are Frenkel excitons, states that can migrate through the crystal. This migration is appropriately described by a one-dimensional energy transfer process for the triplet and, surprisingly, also for the singlet state. The exciton states themselves show an evolution in time that reveal the transition from coherent to incoherent behaviour with increasing temperature. In combination with theoretical investigations a consistent picture of the exciton states in sodium nitrite is achieved.

Slow thermalization of singlet excitons in NaNO2 crystals

Abstract We have investigated the luminescence of NaNO 2 crystals and its dependence on the excitation conditions. Using a broad band light source the luminescence lines resulting from transitions into intramolecular vibrational states (“vibronic lines”) are asymmetric. Comparing these line shapes for different states involved we conclude that the asymmetry is caused by the dispersion of the excitons and by that of the vibrational state of the electronic ground state. Exciting the crystal with a narrow band laser we observed at low temperatures a strong dependence of the fluorescence line shapes on the excitation energy. For temperatures above 15 K the “broad band-excitation line shape” was observed independent of the excitation energy. This behaviour leads to the conclusion that in the case of selective excitation the exciton band is not populated uniformly due to the conservation of quasi-momentum and that the decay of excitons is faster than the thermalization within the exciton band at T = 4 K. At temperatures above T ≈ 15 K the thermalization is possibly accelerated by multi-phonon scattering processes and becomes the faster component.

Vibronically induced intersystem crossing in pentacene in p-terphenyl and benzoic acid crystals

Abstract The temperature behaviour of pentacene fluorescence of p-terphenyl: pentacene and benzoic acid: pentacene is investigated by site-selectively measuring the fluorescence quantum yield Φ F of pentacene in the four sites (O 1 , O 2 , O 3 , O 4 ) in p-terphenyl (ptp) and in the single site (O 0 ) in benzoic acid (bza) varying the temperature from 4.2 to 140 K. A theoretical analysis of the experimental data yields that the temperature dependences of Φ ptp F (O 1 ), Φ ptp F (O 2 ) and Φ bza F (O 0 ) arise from a vibronically induced intersystem crossing due to an out-of-plane vibrational mode of pentacene.

Bound multiphonon complexes in NaNO2

Abstract Indications of the two-phonon quasi-bound state and bound multiphonon complexes in NaNO 2 are observed in high resolution spectra of the triplet and singlet luminescence and of the multiple-order Raman scattering caused by the ν 2 vibration of NO 2 − . The anharmonicity constant and the width of the 1 ν 2 phonon branch are estimated to be A =0.8±0.5 cm −1 and 2 W =1.7±0.5 cm −1 , respectively.

FEMTOSECOND PROBING OF EXCITON RELAXATION AND TRANSPORT DYNAMICS IN POLYBITHIOPHENE

The relaxation and transport dynamics of singlet excitons in 100 nm thick, electrochemically prepared polybithiophene films were investigated by monitoring the time evolution of photoinduced bleaching of the S0–S1 absorption and photoinduced absorption (S1–Sn) with femtosecond resolved transient absorption spectroscopy. The decay dynamics of both photoinduced bleaching and photoinduced absorption in the spectral range from 500 to 800 nm are observed to be independent of the pump pulse wavelength and can be fitted by a double exponential, the time constants of which are τ1=120±20 fs and τ2=2±0.3 ps. The fast decay process with ∼τ1=120 fs was ascribed to the transport dynamics of the initially generated free excitons migrating over parallel aligned polymer segments to structural defects acting as traps. The competing process, occurring predominantly in structurally disordered regions, is structural relaxation of the initially generated free excitons to self-trapped excitons decaying with a lifetime of about...

Singlet excitation energy transfer in tetracene doped p-terphenyl single crystals

Abstract Fluorescence quantum yields of host and guest molecules in tetracene doped p -terphenyl crystals were measured for different guest concentrations c G between 1.2 × 10 −6 and 1.3 × 10 −4 mol/mol at temperatures ranging from 2 to 300 K. In the low-temperature region (2–60 K) the ratio of the quantum yields from guest to host molecules decreases with rising temperature, where the slope of the decrease depends significantly on c G . On the other hand, for temperatures higher than 60 K the ratio of quantum yields increases with rising temperature and this temperature behaviour is independent of c G . These different temperature characteristics of the quantum yield ratio in relation to the guest concentration are interpreted in terms of singlet excitation energy migration processes on the host molecules. The discussion is based on a rationalization which postulates a temperature inhibited exciton motion at temperatures below 60 K and dopant concentrations below 10 −4 mol/mol.

Nonlinear optical spectroscopy and microscopy of potential-sensitive hemicyanine dyes in membranes

Abstract A nonlinear optical spectroscopy method is presented, which allows for precise recording of membrane potentials. The signal intensity of second harmonic generation (SHG) mediated by hemicyanine dye molecules embedded in the membrane of oxidized cholesterol was used as sensitive probe to membrane potential changes.

Spectroscopic studies of singlet exciton-thermalization in NaNO2 single crystals

Abstract The relaxation of singlet excitons in NaNO 2 crystals upon excitation has been investigated by means of fluorescence excitation spectroscopy and fluorescence line narrowing techniques. The comparison of the observed evolution of the exciton states in time with former data concerning the transfer of the excitation energy through the crystal indicates a coherent behaviour of the singlet excitons at low temperatures.

Fluorescence excitation spectra of phonon-induced guest sites in p-terphenyl: Tetracene crystals

Abstract The multiplet structure in the optical spectra of tetracene in p-terphenyl single crystals and their differences in absorption and emission spectra were reinvestigated by monitoring high-resolution fluorescence excitation spectra between 2 K and 4.2 K. These experiments confirm that the substructure superimposed on the phonon sidebands in the absorption spectra is due to localized or pseudolocalized phonon modes which are induced by the presence of guest molecules in the excited state. Moreover it could be shown that these modes contribute in a very specific way to the ‘phonon-induced’ formation of relaxed excited-state guest sites (O3 and O4) which manifest themselves only in the fluorescence spectra in addition to the excited states of the guest molecules observed both in absorption and emission (O1 and O2).