Taiji Kitagawa

Two-photon ionization of pyrenesulfonate and formation of its dianion in the water pool of H2O/AOT/isooctane reversed micellar system

Abstract Transient absorption spectra of sodium pyrenesulfonate (NaPSA) after laser excitation at 351 nm were observed in H 2 O/AOT/isooctane reversed micellar system as well as in aqueous solution. Two-photon ionization of PSA, monitored by detecting the transient absorptions at 455 and 700 nm attributable to the zwitterion and the solvated electron, respectively, was found to hardly occur in the small reversed micelle. The transient absorption at 490 nm has been attributed to the dianion of PSA in which the electron attaches to an unexcited PSA. This peak was observed only in the largest water pool with H 2 O/AOT=50. These results are discussed in terms of the location and the number of PSA solubilized in the reversed micelle.

Mass spectrometric studies on hydrogen-bonded clusters produced via supersonic expansions. characteristic ions for detection of (H2O)n, (CH3OH)n and (C2H5OH)n (n ⩽ 4)

Abstract Mass spectra of (H 2 O) n , (CH 3 OH) n and (C 2 H 5 OH) n ( n ⩽ 4) were measured by varying the source conditions for the supersonic expansions. The dependences have revealed that (H 2 O) + 2 , (CH 3 OH) + 2 , m/z 77 ion from ethanol dimer and (M 2 H - 18) + ions (M = CH 3 OH, C 2 H 5 OH) from the timers are useful for detection of the corresponding neutral clusters without significant contamination by fragmentation from other clusters.

Mass spectrometric studies on hydrogen-bonded clusters produced via supersonic expansions. I. Mass spectra of acetic acid dimer. Ionization and fragmentation efficiencies of the dimer relative to the monomer

Abstract Mass spectra have been measured using 70 eV electrons in a molecular beam involving acetic acid monomer and the dimer. Analysis of the spectra as a function of the source conditions for the adiabatic expansions of a neat gas allowed determination of the branching ratio of the dimer for the major fragment ions (Fi+) relative to the protonated ion (MH+), αFiD/αMHD, as well as that of the monomer, αFiM/αMM. The results obtained were: Fi+ (m/z): 61 (MH+), 60 (M+), 45, 43, 29, 15, 105 [(M·CO2H)+]; αFiD/αMHD: 1.0, 0.01, 0.08, 0.63, 0.03, 0.14, 0.016; αFiM/αMM: 0.023, 1.0, 1.48, 1.60, 0.25, 0.77, 0.0. Several features in the fragmentations of the dimer ion characterized by large values of αFiD/αMHD for Fi+ (m/z) 61 and 43 and small values for 60 and 45 are discussed in terms of their relevant ion/molecule reactions. The relative efficiency of αMHD/αMM is determined by mass spectrometric measurements in conjunction with the measurements of beam intensities. With this ratio (5.64±0.6) and the value of αMD/αMHD, the monomer-dimer ratio can be estimated in a beam of acetic acid vapor.

Mass spectrometric studies on hydrogen-bonded clusters produced via supersonic expansions. V. Mass spectra of acetic acid trimer and monomer—dimer—trimer distribution

Mass spectra of the acetic acid trimer involving the fragmentation to the monomer fragment ions were obtained by investigating the dependence of the intensities of characteristic ions of the dimer (m/z 105 ion) and the trimer (m/z 103 and 121 ions) on the stagnation conditions for producing supersonic beams of the acid. The monomer—dimer—trimer distributions in the beams were estimated on the basis of the mass spectra of the three species and the relation between the overall ionization efficiencies of the trimer and monomer. Nearly pure beams of the dimer (ca. 85%) and of a 1:1 mixture of the dimer and trimer (ca 90%) can be produced under appropriate expansion conditions. This feature in the cluster formation of acetic acid is discussed on the basis of the structural models of these small clusters reported previously.

Mass spectrometric studies on hydrogen-bonded clusters produced via supersonic expansions. IV. Temperature-dependent fragmentations in the electron-impact ionization of acetic acid dimer and trimer

Abstract Molecular beams containing acetic acid dimer and trimer as major species were produced via supersonic expansions. Four ions at m/z 105, 61, 103, and 121 generated by electron-impact ionization of the dimer and trimer were investigated as a function of the stagnation conditions and the ion injection energy, by which the cluster temperatures and the times of flight of these ions passing through a quadrupole mass analyser (t105, etc.) were varied, respectively. As a result, the temperature-dependent fragmentations of the dimer to the m/z 105 ion and of the trimer to the m/z 121 ion were found. This can be described in terms of the dependence of the lifetimes of these ions on the temperatures of the parent clusters. The temperature-dependent lifetime of the m/z 105 ion was estimated to be of the order of 10−4 s, comparable with t105. Possible structures of these ions and their energetics are discussed.

Multiphoton ionization of benzene at 193 nm: laser power dependence and isotope effect

Abstract Laser power dependence of multiphoton ionization (MPI) of benzene- d 6 as well as benzene- h 6 was observed at 193 nm with an ArF excimer laser. The dependences were analysed by the rate equation model. It was found that two-photon ionization occurs mainly from the energy state(s) internally converted from the excited 1 B 1u state. Furthermore it is very likely that one of the dominant absorbing species of the fourth photon is the C 4 D + 4 ion formed via a three-photon process. A remarkably different isotope effect and similar laser power dependences of the C 4 D + 4 and C 4 D + 3 ions suggest that the C 4 D + 3 ion is produced from the metastable C 4 D + 4 ion. Some experimental evidences are given that neutral products largely contribute to the MPI of benzene at 193 nm.

Laser photolysis of pyrenesulfonate and pyrenetetrasulfonate via two-photon ionization in aqueous and reverse micellar solutions

Abstract We investigated the laser photolysis of tetrasodium 1,3,6,8-pyrene-tetrasulfonate (Na 4 PS 4 ) and sodium 1-pyrenesulfonate (NaPS) in aqueous and reverse micellar solutions. The photoproducts as well as their yields were found to strongly depend on the reaction parameters such as pH, dissolved gases and the size of water pool. The primary reaction in aqueous solution was commonly presented by laser-induced formation of the cation radicals followed by hydroxylation of them. In the case of PS 4 4− , pyranine was efficiently and highly selectively formed, possibly via desulfonation of the cation radical, P + S 4 4− , followed by the hydroxylation. On the other hand, from PS − , hydroxypyrenesulfonate (PSOH) was initially formed in alkaline solution and desulfonation occurred secondarily, leading to the formation of hydoxypyrene. These photoreactions were markedly suppressed within a small water pool of the reverse micelle. As the size of water pool increased similar photochemical reactions occurred depending on the dissolved gases. The observed micellar effects could be explained based on the pH of water pool and the nature of the cation radicals in it.

Mass spectrometric studies on hydrogen-bonded clusters produced via supersonic expansions: Fragmentations to the monomer fragment ions and estimate of cluster size distribution of (CH3OH)n, (n ⩽ 4)

Abstract Mass spectra of (CH 3 OH) n ( n ⩽ 4) produced by supersonic expansions of the neat gas were measured together with the beam intensity. Several relations between the observed intensities were used to determine the branching ratio for fragmentation to the monomer fragment ions from the dimer and the efficiencies relative to the monomer. From these values cluster size distributions can be estimated in beams.

Ab initio MO computation of ionization potentials of small benzenoid hydrocarbons and their –SO3Na, –SO3H, and –SO−3 substituted compounds

Abstract STO-3G calculations on benzene, naphthalene, phenanthrene, anthracene, pyrene and their –SO3Na, –SO3H, and –SO−3 substituted compounds were carried out. The calculated values for ionization potential of –SO3Na substituted compounds are close to those of the corresponding hydrocarbons with benzene being the only exception. On the contrary, the calculated ionization potentials of –SO−3 substituted compounds are very small in comparison with those of the corresponding neutral compounds. The highest occupied molecular orbital (HOMO) of sodium pyrenesulfonate comprises almost pπ atomic orbitals of conjugated hydrocarbon skeleton and is similar to that of pyrene. The HOMO of pyrenesulfonate anion is quite different from that of pyrene. Ionization potential of sodium pyrenesulfonate in water was discussed based on these results.