Investigation of Photoinduced Processes in Water-Soluble Cationic Ni-Porphyrin by the Resonance Raman Scattering Method

By the resonance Raman scattering spectroscopy (RRS) method, photoinduced processes involving cationic Ni-5,10,15,20-tetrakis(4-N-methylapyridiniumyl)porphyrin (Ni(TMpy-P4)) in a phosphate water buffer have been investigated. Excitation into different regions of the Soret band at wavelengths of 397, 423, 441.6, and 457.9 nm permitted separate investigation of the behavior of the 4- and 6-coordinated forms of Ni(TMpy-P4). It has been found that Ni(TMpy-P4) is characterized by two photoinduced processes: the deactivation of initially 4-coordinated Ni(TMpy-P4) is followed by the addition to it, as axial ligands, of two water molecules from the environment to form a nonequilibrium 6-coordinated form Ni(TMpy-P4)(H₂O)₂ in the ground state, whereas in the process of deactivation of the excited 6-coordinated complex there occurs a dissociation of the axial water ligands to form a nonequilibrium 4-coordinated complex Ni(TMpy-P4) in the ground state. Some of the 4-coordinated molecules of Ni(TMpy-P4) are deactivated through the excited B _1g ^*-state without undergoing extracoordination. Quantitative analysis of the RRS spectra using the global fitting method has shown that in the course of deactivation 33% of photoexcited 6-coordinated molecules of Ni(TMpy-P4)(H₂O)₂ retain their axial ligands, relaxing through the dark long-lived intermediate state of the 6-coordinated form.     

Electronically driven structure changes of Si captured by femtosecond electron diffraction

The excitation of a high density of carriers in semiconductors can induce an order-to-disorder phase transition due to changes in the potential-energy landscape of the lattice. We report the first direct resolution of the structural details of this phenomenon in freestanding films of polycrystalline and (001)-oriented crystalline Si, using 200-fs electron pulses. At excitation levels greater than approximately 6% of the valence electron density, the crystalline structure of the lattice is lost in <500 fs, a time scale indicative of an electronically driven phase transition. We find that the relaxation process along the modified potential is not inertial but rather involves multiple scattering towards the disordered state.     

The crystal structure of Rb2CdCl4 in para- and ferroelastic phases

Abstract

The crystal structure of Rb₂CdCl₄ has been studied by X-ray diffraction at 295 and 160 K in the initial phase D ¹⁷ _4h as well as at 105 K in the ferroelastic phase. It was found that the phase transition D ¹⁷ _4h ? D ¹⁰ _2h takes place instead of D ¹⁷ _4h ? D ¹⁸ _2h as proposed earlier. The first of the transitions corresponds to unequal and the second to equal Φ-tilts of CdCl₆-octahedra around the a and b axes of the tetragonal unit cell.     

Role of coherence in interaction of opticalradiation with macromolecules

Based on analysis of the properties of macromolecules in a coherent optical radiation field and taking into account experimentally established data on the specificity of the interaction between laser radiation and biomolecules (the dependence of the efficacy of the interaction on the coherence length, the presence of an effect in a region of the spectrum far away from the absorption band), we propose a mechanism for wave interaction of coherent optical radiation with macromolecules, and we construct a very simple mathematical model for such interaction. Using the mathematical model, we calculate the dependence of the vibrational energy of the macromolecule in a coherent radiation field on the coherence time and the intramolecular relaxation rate. We show that the increase in the vibrational energy of the macromolecules strongly depends on the radiation coherence length. When exposed to incoherent radiation, the vibrational energy of the biomolecules remains practically constant, while when exposed to laser radiation (coherence length ≈3 cm), the vibrational energy of the atoms increases by 2–4 orders of magnitude, leading to a change in the conformation of the biomolecules and the activity of enzymes. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 225–231, March–April, 2006.     

A resonance-reman-scattering multifrequency spectrometer for investigating excited states of molecules and photoinduced high-rate processes

A resonance-Raman-scattering multifrequency laser spectrometer operating in a wide spectral range (355–750 nm) and making possible the recording of both stationary Raman spectra and spectra with a time resolution of up to 100 psec in the time interval of 0–50 nsec has been developed. The spectrometer has been used with advantage for the study of the excited states of molecules of metalloporphyrins in solutions and processes of interaction of model metalloporphyrins with DNA and DNA-modeling polynucleotides. To whom correspondence should be addressed. B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus 68, F. Skorina Ave., Minsk, 210072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 726–733, September–October, 1999.     

Picosecond dynamics and mechanism of the interaction of a photoexcited Cu(II)-porphyrin with a DNA-modeling polynucleotide

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 2, pp. 110–121, March–April, 1995.