M. Washio

Picosecond time-resolved fluorescence studies of poly(N-vinylcarbazole) using a pulse-radiolysis technique

Abstract The formation processes of two distinct excimers of poly(N-vinylcarbazole)(PVCZ) have been directly observed by using a picosecond single-pulse radiolysis system with 10 ps time resolution. The primary processes of radiation-induced excitation in the polymer solutions have been studied. A so-called second excimer (λmax of fluorescence = 375 nm) of PVCZ is almost completely formed immediately after a 10 ps electron pulse. Most of the sandwich-type excimer (λmax of fluorescence = 420 nm) is formed in several nanoseconds. Most of the second excimer was confirmed to be formed directly in preexisting sites. Most of the sandwich-type excimer was confirmed to be formed through structural change of the excited carbazolyl and the ground-state carbazolyl chromophore to the sandwich conformation. Both the concentration of the preexisting trap sites and the efficiency of energy migration along the polymer chain before relaxation or solvation of the excited state are considered to be most important in the formation of the second excimer. Most of the primary excited states of PVCZ in irradiated toluene solutions of PVCZ are formed immediately after the 10 ps electron pulse. These very fast formation processes of excited states of solute molecules agree with the recent experimental results observed in irradiated hydrocarbon solutions of aromatic molecules in the picosecond time domain.

Pulse radiolysis of polystyrene and benzene in cyclohexane, chloroform and carbon tetrachloride

Abstract Pulse radiolysis studies on pure carbon tetrachloride, pure chloroform, and their solutions containing polystyrene and benzene, and on polystyrene in cyclohexane solutions were carried out by using the pico- and nanosecond pulse radiolysis system. First direct observation of geminate processes in non polar liquid systems such as pure carbon tetrachloride and pure chloroform were made. In the radiolysis of pure carbon tetrachloride, it was observed that a transient species at 480 nm in formed by the geminate ion recombination process. In the case of chloroform, two absorption bands were observed at 330 nm and around 550 nm. The 550 nm band has not been clearly identified, but it can be considered that the absorption may be cationic species and the decay is related to the geminate recombination process. Pulse radiolysis of the polystyrene solution in cyclohexane showed the excimer absorption with lifetime of 20 ns. In the pulse radiolysis of polystyrene and benzene solutions in both carbon tetrachloride and chloroform, the charge transfer complex between polystyrene (or benzene) and Cl was observed in both solutions. In the pulse radiolysis of benzene in their solutions, the charge transfer complex between benzene and Cl was observed in both solutions. The lifetime of the charge transfer complexes between polystyrene and Cl is shorter than that of the charge transfer complexes between benzene and Cl, in both solutions. This implies that the disappearance of these species is due to proton transfer process.

Intramolecular excimer formation of oligostyrenes from dimer to tridecamer: The measurements of rate constants for excimer formation, singlet energy migration, and relaxation of internal rotation

The emission properties of styrene oligomers from dimer to tridecamer have been investigated by picosecond pulse radiolysis at room temperature. The oligomers are precisely fractionated and are considered to be models of atactic polystyrene. The rate constant for excimer formation kDM of the order of 108 s−1, increases with increasing number of monomer units n for 2≤n≤8, then levels off at the value for polystyrene. The rate constant for singlet energy migration between adjacent chromophores in polystyrene was determined to be 3×1010 s−1 (∼30 ps). An average time of 7.2 ns was found to be necessary for two adjacent chromophores to assume the excimer conformation (g−t or tg− in a racemo dyad; tt in a meso dyad) from their initial equilibrium distribution of conformations. The present result is an indication that one may investigate the dynamics of internal rotation by photophysical techniques. Moreover, from a comparison of the results of photostationary measurements with those of transient methods, it is ...

Picosecond pulse radiolysis studies on geminate ion recombination in saturated hydrocarbon

Abstract The geminate recombination kinetics of the excess electron and the electron hole are discussed, based on time-resolved data on picosecond and nanosecond time scales. The recombination times of the excess electron and the electron hole are evaluated to be 3 ps for cyclohexane on the basis of the comparison between the experimental and the calculated results. The spin correlation decay of the geminate ion pairs and the triplet state formation before the spin correlation loss have also been discussed. The rapidly decaying species with very broad absorption spectra, which are similar to the absorption spectra of the cation radicals of saturated hydrocarbons, have been observed in neat saturated hydrocarbons in the sub-nanosecond and a few nanosecond time regions. The identification of the rapidly decaying species were not definitely made but those species are tentatively assigned to the excited states and/or the tail of the geminate cation radicals of saturated hydrocarbons.

Pulse radiolysis with picosecond time resolution

Abstract A new absorption spectroscopy system with a time resolution of 20 picoseconds (ps) is described. Two parallel linacs which can accelerate single bunched beams (picosecond single beams) are used in the system. One linac of the two is used as an irradiation source and the other is used as aCˇerenkov light source. The time interval between the irradiation source and theCˇerenkov light source is changed by delaying the source beam for theCˇerenkov light. The new system was named “Twin Linac Pulse Radiolysis System.” This details of the twin-linac pulse-radiolysis system and some of the experimental results are described.

Picosecond single electron pulse for pulse radiolysis studies

Abstract The S-band linac which can produce a picosecond single electron beam, has been operated since 1977. Several improvements of the linac have increased the reproducibility of picosecond single beam operation, the stability of average current of the output beams and the intensity of picosecond single beams. The application of the linac has been also investigated. Emission and absorption spectroscopy systems with a response of sub-nanosecond have been developed.

Red fluorescence from the merocyanine form of spirobenzopyran

Abstract A brilliant red emission observed for the colored form of 1,3,3-trimethyl-6′-nitrospiro[indoline-2,2′-[2H]benzopyran] both in solution and in polymer films was ascertained to be a fluorescence from its merocyanine form. The fluorescence lifetimes were measured to be 1.8 to 3.6 ns in polycarbonate and other polymer films and 230 ps in ethanol solution by picosecond laser flash and pulse radiolysis spectroscopies.

Picosecond pulse radiolysis and laser flash photolysis studies on polymer degradation of polystyrene and poly-α-methylstyrene

Abstract Recently early events in photolysis of polystyrene (PSt) solutions in chloroform and in 1,4-dioxane have been made clear by using a nanosecond laser flash photolysis system (Nd-YAG: 265 nm) at Hahn-Meitner-Institut [1]. Early events in radiolysis of PSt solution have also been studied by using a picosecond pulse radiolysis system at Univ. of Tokyo [2]. In the present paper, the difference and the similarity between photolysis and radiolysis of PSt and poly-α-methylstyrene (P-α-MeSt) are discussed. Important results are as follows; 1. (1) In both cases of radiolysis and photolysis, strong absorptions of intramolecular excimer have been observed for PSt ( λ max = 530 nm) and P-α-MeSt ( λ max = 520 nm) solutions in dioxane and cyclohexane, in addition to the monomer and excimer fluorescence of these polymers. In these solutions, those polymers are hardly decomposed. 2. (2) In both radiolysis and photolysis, charge-transfer complexes (PSt +δ , Cl −δ or P-α-MeSt +δ , Cl −δ ) have been observed for PSt and P-α-MeSt solutions in chloroform (CHCl 3 ) and in carbon tetrachloride (CCl 4 ). In these solutions, polymers are effectively decomposed. The charge-transfer complexes are precursors of macroradicals. 3. (3) The charge-transfer complexes are mainly produced from ion pairs (PST + …Cl − or P-α-MeSt + …Cl − ) in the case of radiolysis and from exciplexes of excited polymer molecules with solvent molecules in the case of photolysis. 4. (4) Since the presence of oxygen is prerequisite to main chain cleavage of PSt in CHCl 3 and in CCl 4 , it is important to note that oxygen is not involved in initial stages of the main process of macroradical formation.

Compact soft x-ray source using Thomson scattering

A compact soft x-ray source using Thomson scattering, enabled by the combination of a picosecond laser and an electron rf gun, was developed aiming at biological studies such as those using an x-ray microscope. The x-ray source included both a photoinjector system and a picosecond laser system with a tabletop size of 2×2m2. An infrared laser beam (λ0=1047nm) was obtained from an all-solid-state mode-locked Nd:YLF laser system and injected into the photocathode of an accelerator system. A 4.2MeV electron beam was generated from a laser-driven photocathode rf gun system. The residual laser beam was amplified up to about 4.2mJ/pulse using a flash-lamp-pumped laser amplifier. Upon collision of the electron beam with the amplified laser beam, 300eV soft x rays were generated by Thomson backscattering. The stable interaction between the two beams was achieved using the same seed laser pulse for irradiating the photocathode and the scattering process with laser photons.

Pulse radiolyses of anthraquinone and anthraquinone–triethylamine in acetonitrile and toluene at room temperature

Nanosecond pulse radiolysis of anthraquinone (AQ) in several solvents has been performed at room temperature, and the following results are obtained: (1) In acetonitrile (CH3CN), the formation of triplet AQ and a free-radical anion (AQ˙–) of AQ is observed. The former is produced by energy transfer from an excited neutral of CH3CN which may be produced via the geminate recombination of a radical cation and a radical anion of CH3CN in a spur, while the latter is produced by electron transfer from anionic species such as a solvated electron, a monomeric and/or a dimeric radical anion of CH3CN. In CH3CN–triethylamine (TEA), both free AQ˙– and triplet AQ mentioned above are also produced; however, the latter reacts with TEA, giving rise to the formation of free AQ˙–(from the second triplet state of AQ) and an exciplex of the lowest triplet state of AQ with ground-state TEA. This exciplex decomposes to free AQ˙– and the radical cation of TEA. (2) In toluene, only triplet AQ is produced by energy transfer from triplet toluene to AQ, and, in the presence of TEA, the formation of the triplet exciplex of AQ–TEA is observed. On a microsecond timescale, however, this exciplex changes to a contact ion pair followed by proton transfer, generating anthrasemiquinone radical and triethylamine radical in accordance with the result of photolysis.