R.Rotomskis, V.Vaicaitis, D.Frolov, S.Bagdonas,
Laser Research Centre, Vilnius University, Sauletekio 9, 2040 Vilnius Lithuania

• Introduction
• Materials and Methods
• Results
• Discusion
• References

Meso-tetra(4-sulfonato-phenyl)porphine (TPPS4) was obtained from Porphyrin Products (Lugan, UT, USA). Spectral grade solvents were used for all measurements, which were carried out at room temperature. Stock solutions of sensitizer (10-3 M) were prepared by dissolving crude TPPS4 in distilled water and adjusting pH to 7.5 with 0.1 M NaOH. Further dilution was carried out directly before the measurements. The pH of TPPS4 solutions was varied by adding small amounts of 0.1 M HCl.

Steady state absorption spectra of TPPS₄ were recorded by the Ocean Optics, PC 1000 fibre optic built-in spectrometer. Quartz cuvettes of 10 mm width were used. Steady state emission measurements were carried out on a Perkin Elmer spectrofluorometer LS50B.

A picosecond pump and probe spectrometer based on Nd:YAG³⁺ laser and optical parametric oscillators was used to measure the time-resolved kinetics of absorption changes at the selected wavelength. The fundamental beam after single pulse selection was triplicated by glass plates placed beyond each amplifier. The most intensive pulse was used for pumping radiation. Two KDP crystals parametric laser excited by second harmonic of fundamental radiation produced continuously tuneable (800—1400 nm) probing light. The frequency doubling of output radiation gives the possibility of tuning the wavelength in the visible region up to 400 nm. Each of the less intense beams of fundamental radiation was used to form independent probing channels. One of them included heavy water cell to generate picosecond continuum as probing light. Another probing channel was based on temperature tunable LiNbO₃ crystal parametric oscillator with frequency doubling. It was applied for accurate kinetic measurements at desirable wavelength within 400-2000 nm spectral range. As a result, a sensitivity of > 10^-3 optical density units was obtained, the sample absorption being around one unit of optical density units. The time resolution of the spectrophotometer was up to 10 ps. Picosecond transient absorption and kinetics studies were performed basically as described previously [17]. Samples in a 1, 2, 3 and even 5 mm path length cells were excited with a 15 ps pulse at 532 nm (0.1� 0.3 mJ) and probed at various delay times with a weak 25 ps broad-band (picosecond continuum in D₂O) and/or parametric oscillator pulses at 590 nm.

Fluorescence kinetic spectroscopy equipment is shown in Fig.2 A Kerr-lens mode-locked titanium:sapphire laser with a central wavelength of 800 nm, an average power of ~200mW and a pulse duration of about 140 fs was used as excitation source. The second harmonic of the 800 nm wavelength was generated in a BBO crystal of type I (oo-e). The experimental setup consists of two monochromators, one of which is used for tuning of the excitation wavelength and the other for tuning of the emission wavelength. The slits were set to 0.5 mm for both monochromators, this accords to a spectral width of ~2nm.

A Hamamatsu micro-channel plate photomultiplier with an instrument response of 35 ps at FWHM was employed for counting of fluorescence photons. A time-correlated single photon counting unit and software (Becker & Hickl, SPC-430) were used for acquisition of fluorescence kinetics. The data were analysed using a fluorescence decay data analysis software (FluoFit from Picoquant GmbH), which allows deconvolution of measured data with the instrument response function. The time resolution that we attained with this system was ca. 20 ps.

The excitation intensity was kept low so that the probability of detecting of one fluorescence photon per excitation pulse was between 0,01 – 0,1.

Fig.2. Fluorescence kinetic spectroscopy equipment.

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