I. Pelant

Transmission study of picosecond photocarrier dynamics in free-standing porous silicon

Abstract Pump and probe technique has been used to study time-resolved differential absorbance of free-standing porous silicon at room temperature. The second (532 nm) harmonics of a mode-locked Nd 3+ :YAG picosecond laser served as pump pulses and the fundamental frequency (1.06 μm) were used as probe pulses. Photocarrier dynamics shows a fast decay in time domain of hundred picoseconds. A model of bimolecular radiative recombination is suggested, the values of bimolecular radiative recombination coefficient B and excited carrier cross section σ have been found to be B = 4 × 10 −10 cm 3 s −1 and σ = 9 × 10 −18 cm 2 , very close to the values reported for direct gap semiconductors.

Site-selection and Shpolskii spectroscopy of model photosynthetic systems

Low temperature (T= 10 K) site‐selection fluorescence and excitation spectra of both isolated pheophorbide‐a molecules (I PHEO) and pheophorbide covalently bonded to the synthetic l‐lysyl‐l‐alanyl‐l.‐alanine polypeptide (B PHEO) were measured in dimethylformamide (DMF) matrices. The fluorescence spectra display sharp vibronic lines at different wavenumbers for different dye laser excitation wavelengths, superimposed on a broad band background. Spectral analysis provides frequencies of normal vibrations (FNV) and site distribution functions (SDF). The FNV and SDF of I PHEO in DMF have been found to show good agreement with other chlorophyll‐like molecules. A covalent binding between the polypeptide and PHEO molecules produces only slight changes in FNV but significant broadenings and shifts to lower energies in SDF. The fluorescence spectra of I PHEO in a typical Shpolskii matrix (n‐octane) were also measured and found to exhibit sharp non‐shifting vibronic lines for all wavelengths and kinds of excitation. A general model explaining both site‐selection and Shpolskii spectroscopy is presented.

Low-temperature fluorescence and excitation spectra of chlorophyll-a in normal alkanes

Excitation and flourescence spectra of chlorophyll-a were studied in matrices of normal alkanes (C8H18, C12H26, C16H34) at low temperature (5–30 K). Narrow band excitation of fluorescence using a pulsed tunable dye laser was used for the site selection to eliminate ambiguities due to impurities and multiple sites. Fluorescence and excitation spectra show 0-0 quasi-lines on band background. The spectra and the frequencies of normal vibrations in the first excited electronic state are compared to those of chlorophyll in other solvents.

Luminescence of direct-and indirect-gap electron-hole plasma in TlBr

Abstract Luminescence of TlBr crystals highly excited by a nanosecond pulsed-dye laser (3.4 eV) at the bath temperature ∼ 8.5 K was studied. Two emission lines labeled A (∼ 2.98 eV) and B (∼ 2.62 eV) were found, which show typical behavior of the electron-hole plasma recombination radiation. The A-line is assigned to the recombination of e-h pairs in the direct gap ( X + 6 − X - 3 ) and the B-line to the simultaneous recombination in the indirect gap ( X + 6 − R - 6 ). Condensation of carriers into an electron-hole liquid was not observed.

Photoluminescence assessment of B, P, and Al in Si wafers: The problem of sample heating by a laser beam

A line shape analysis of free‐exciton low‐temperature (4.2 K) photoluminescence spectra is applied to monitor the heating of Si samples due to an exciting Ar+‐ion laser beam. The heating is studied in dependence on the laser beam intensity. The temperature as a line shape fitting parameter can be established within ±0.15 K. Continuous minor increase of the sample temperature is observed up to an intensity of ∼200 W cm−2, followed by an abrupt temperature jump up to tens of K. Possible inaccuracies, resulting from the heating, in photoluminescent quantitative determination of boron, phosphorus, and aluminum are discussed.

Low temperature laser fluorescence and excitation spectra of free base phthalocyanine in normal alkanes matrices (Shpolskii effect)

Excitation and fluorescence spectra of free base phthalocyanine were studied in matrices of normal alkanes (C8H18, C12H26, C16H34) at low temperatures (5 to 30 K). Narrow band excitation of fluorescence using a pulsed tunable dye laser was used for site selection to eliminate ambiguities due to impurities and multiple sites. Fluorescence and excitation spectra show quasiline structure. Multiplicity of 0-0 transitions depends on the chain length of normal alkanes. The spectra and frequencies of normal vibrations are compared with those of phthalocyanine in other n-alkanes (C9H20, C10H22) and noble gas matrices (Ar, Ne) under different excitation conditions.

Site selection spectroscopy of chlorophyll-a

Abstract Site selection fluorescence and excitation spectra of chlorophyll-a in n-alkanes (octane, dodecane, hexadecane, eicosane) and polystyrene matrices were measured at low temperatures (5-30 K), using a pulsed dye laser tuned in the range 605 nm - 660 nm. The vibrational analysis of fluorescence quasilines gives very similar results for chlorophyll-a in matrices with different molecular chain lengths.

Luminescence of excitonic molecules in AgBr

Abstract Luminescence spectra of AgBr at the temperature range 5–50 K and under pulse excitation intensity up to ∼ 0.7 MW cm −2 are investigated. Intensity and temperature dependences of biexciton emission line (2.67 eV) yield thermodynamic determination of biexciton binding energy in agreement with the spectroscopic value (6.7 meV). The behaviour of biexciton emission in the investigated temperature range is explained by the concept of “biexciton pocket”. Some questions of the electron-hole liquid recombination radiation (2.60 eV) are discussed as well.

Note on coexistence of free and self-trapped excitons in AgCl

Abstract An attempt has been made to apply time-resolved photoluminescence spectroscopy in the ns time scale at T = 2–4.2 K to study exciton self-trapping and accompanying relaxation processes in AgCl crystals. No emission line attributable to the free exciton decay has been found. Two novel features, the transient broadening and a shoulder at 3 eV were observed on the well known broad emission band peaked at 2.5 eV. The broadening is interpreted as hot luminescence of the self-trapped exciton whereas the shoulder is assumed to be due to a fusion of a free exciton with a self trapped one.

Direct-indirect gap intervalley scattering of electrons in TlBr electron-hole plasma

Abstract Temperature dependence of the electron-hole plasma (EHP) radiative recombination in TlBr crystals was measured in the bath temperature range 8.5–56 K. Strong phonon-assisted competition due to intervalley electron scattering between direct and indirect gaps EHP components was found. A kinetic model is presented which explains the main features of the experiment.