A. Suisalu

Spectroscopic study of nanocrystalline TiO2 thin films grown by atomic layer deposition

Abstract Photoluminescence characteristics of nanocrystalline TiO 2 films grown by atomic layer deposition were studied. In dependence of growth conditions the films consisted of anatase, rutile and TiO 2 -II phases. A broad band and two sharp peaks were observed in the photoluminescence spectra measured under continuous-wave Ar + laser excitation at temperatures 5–165 K. At 5 K the maximum of the broad band was at 2.24 eV in the anatase films, and at 2.37–2.40 eV in the rutile and TiO 2 -II films. The intensity of sharp lines peaking at 3.31 and 3.37 eV depended on the crystal structure of the films and increased significantly after X-ray irradiation. The temperature dependence and decay times of different emission bands were also investigated. The data obtained allowed a defect-trapped-exciton interpretation of the sharp peaks although the free-exciton origin of the 3.31 eV peak could still be argued. The broad-band emission at 2.24–2.40 eV was obviously due to self-trapped exciton recombination.

Self-trapped exciton emission in crystalline anatase

A comparative spectroscopic investigation of self-trapped exciton (STE) emission in anatase single crystals and thin films was carried out at 5 K. In the single crystals, a strongly polarized STE emission with an electrical vector perpendicular to the c-axis was observed. Influence of annealing on the spectral characterictics of samples was tested.

Spectral narrowing of self-trapped exciton emission in anatase thin films

Self-trapped excition (STE) emission of TiO2 (anatase) thin films grown by atomic layer deposition technique on single crystal α-Al2O3 (0 1 2) substrates were studied at temperatures 5–120 K. An in-plane preferential orientation of anatase crystallites was detected in the films by using the dependence of the emission polarisation on the observation angle. The STE emission recorded in the direction perpendicular to the film surface had a wide spectrum with the maximum at approximately 2.3 eV. The spectrum recorded from a cleaved edge in the direction nearly parallel to the film surface, showed two sharp peaks (spectral width as low as 0.03 eV) on the top of the broad STE emission band. The beam divergence determined at the wavelengths of these peaks was 6–10°. The effect was interpreted as a constructive interference of the emission leaving the film at the angle that is close to the angle of total internal reflection.

Spectral Hole Burning in Doped Organic Crystals and Polymer Glasses under Hydrostatic Pressure

We give an overview of results obtained in our isobaric studies of spectral holes in chlorin-doped glassy polystyrene, frozen n-octane and crystalline p-terphenyl under various fixed pressures up to 8.4 kbar at 4.2–18 K. Some results on low-pressure (<0.1 kbar) tuning of spectral holes at 1.5–4.2 K in benzophenone, biphenyl and frozen n-octane doped with chlorin and in frozen n-hexane doped with dimethyl-s-tetrazine are also described along with a theoretical work on the pressure effects in such non-isobaric studies. The most interesting effect, observed in our high-pressure isobaric experiments and explained theoretically, is the pressure-induced narrowing of spectral holes in glassy polystyrene, caused mainly by reduction in the number of soft localized modes in a glass under pressure.

Anomalous hole spectra of chlorin-doped low-temperature glasses

Abstract Spectral holes have been measured for chlorin dopant educt and photo-product states in six disordered systems: in glasses of o-terphenyl, benzophenone, polystyrene, ethanol, and triethylamine and on the surface of Al2O3 powder at 5 K. In contrast to the conventional picture of zero-phonon holes and phonon sideholes observable in the educt state, no zero-phonon hole is observable in the photoproduct state in glasses yet the absorption spectra exhibit a peculiarity resonant with the burning frequency (“Λ-spectra”). A more complicated superpositional picture has been observed for Al2O3 powder The low-temperature hole spectra of an impurity in a 1D elastic lattice have been calculated and demonstrated to match qualitatively well the experimental observations. Physical relevance of such a model is discussed.

Energy transfer in ethane-bisporphyrin dimers studied by fluorescence line narrowing and spectral hole burning

Abstract The quasi-line fluorescence excitation spectrum of 1,2-bis (2,3,7,8,12,13,17,18-octaethyl-21 H ,23 H -porphino) ethane at 4.8 K consists of two subbands with the splitting mean value of 51cm −1 , that are ascribed to the donor and the acceptor half of the homodimer. The donors fluorescence is quenched by an efficient energy transfer to the acceptor. The energy transfer rate of 10 11 s −1 , determined by spectral hole burning, has been compared with the calculated value and a conclusion of nonconsistency with the Forster energy transfer mechanism has been drawn.

PRESSURE-INDUCED DYNAMICS IN SOLID N-ALKANES AS PROBED BY OPTICAL SPECTROSCOPY

The dependence of frequency, width, and area of spectral holes on pressure were measured at 1.6 K in the pressure range up to 2.5 MPa for dimethyl-s-tetrazine (DMST) doped n-hexane (Shpol’skii system), and as reference systems, for DMST-doped durene (“hard” molecular crystal) and ethanol:methanol glass. For the Shpol’skii system, in addition the inhomogenous fluorescence spectra were measured for normal and high (200 MPa) pressures. The main observations were the following: (i) spectral holes in the Shpol’skii system exhibit very large pressure-induced broadening (up to 65 GHz/MPa) depending essentially on the prehistory (freezing pressure) and exceeding the corresponding values for durene (by far) and glass; (ii) spectral holes in the Shpol’skii system exhibit strong, and to a large extent, reversible, area reduction with applied pressure; and (iii) the inhomogeneous fluorescence lines show quite a moderate (as compared to holes) pressure broadening of about several GHz/MPa. The results for the Shpol’ski...

Anomalous hole spectra in glasses: A case for 1D dynamics?

Abstract Holes in absorption spectra have been measured for chlorin dopant educt and photoproduct states in several molecular and polymeric glasses. In contrast to the well-known picture of zero-phonon holes and phonon sideholes observable in educt state, no zero-phonon hole is observable in the photoproduct state in glasses, yet the absorption spectra exhibit a peculiarity resonant with the burning frequency (‘A-spectra’). The low-temperature vibronic spectra of an impurity in 1D elastic lattice have been calculated for free and anchored lattices. Calculated spectra match qualitatively well the experimental observations. The 1D models are discussed in view of structural models of glasses assuming existence of 1D inherent structural defects (disclinations). Applicability of Frenkel- Kontorova model (FKM) as a generalisation of 1D lattice models to the dynamics of molecules in the core region of disclinations is discussed. Possible connection of the experimental findings to the transition by breaking of analyticity in FKM is analysed.

Pressure effects on relaxation in a polymer glass: A persistent spectral hole burning study

The influence of hydrostatic pressure in the range of some kilobars on low-temperature (T < 20 K) relaxation in a polymer (polystyrene) glass after optical excitation of a probe chromophore in it is studied using two different kinds of spectral hole burning experiments—under isothermal-isobaric and in temperature cycling conditions. In the first case, the temperature dependence of the hole width reflects the dynamics of interaction of the electronic transition in a probe molecule with soft localized vibrational modes and with two-level systems, whereas, in the second case, the observed residual hole broadening after the temperature cycle arises from activated (overbarrier) transitions in almost symmetric double-well soft potentials. It is shown that both these processes are essentially suppressed by the applied hydrostatic pressure (the hole width in the first case and its increment in the second case are both reduced about twofold at 5 kbar). An extension of the soft potential model for glasses is proposed explaining in a coherent manner both effects. Its essential points are the presence in the potential of an extra term linear in pressure and the soft coordinate and an assumption about asymmetric distribution of the cubic anharmonicity parameter ξ in the potential.

Energy Selection Is Not Correlated in the Qx and Qy Bands of a Mg-Porphyrin Embedded in a Protein

The Qx-Qy splitting observed in the fluorescence excitation spectra of Mg-mesoporphyrin-IX substituted horseradish peroxidase (MgMP-HRP) and of its complex with naphthohydroxamic acid (NHA) was studied by spectral hole burning techniques. The width of a hole directly burnt in the Qy band and that of a satellite hole indirectly produced in Qy as a result of hole burning in Qx was compared. We also studied the dependence of the satellite hole in the Qy band on the burning frequency used in the Qx band. Both the directly and indirectly burnt holes were very broad in the (higher energy) Qy band. The width of the satellite hole in the Qy band was equal to the entire width of the inhomogeneously broadened band, independently from the position of hole burning in Qx. This is indicative of a clear lack of correlation between the electronic transition energies of the Qx and Qy bands. A photoproduct was produced by laser irradiation of the MgMP-HRP/NHA complex and was identified as a species with lowered Q-splitting. Conversion of the photoproduct could be achieved by thermal activation measured in temperature-cycling experiments, with a characteristic temperature of 25 K. We attribute the phototransformation to a conformational change of MgMP.