河野 淳也

Colloidal gold nanoparticles having an average diameter of 8 nm were prepared by laser ablation at 1064 nm of a gold metal plate in an aqueous solution of sodium dodecyl sulfate (SDS) and were subjected to fragmentation under irradiation of a pulsed laser at 532 nm. Gold clusters exhibiting no significant optical absorption in the visible wavelength region were produced in a solution, together with nanoparticles. UV-visible optical absorption spectroscopy revealed that the gold clusters grow gradually through attachment to the nanoparticles and through mutual aggregation. The growth processes depend crucially on the concentration of SDS in the aqueous solution of nanoparticles.

Colloidal gold nanoparticles with a broad size distribution were prepared by laser ablation of a gold metal plate in an aqueous solution of sodium dodecyl sulfate (SDS) and were fragmented under irradiation of a 532 nm laser at different SDS concentrations and laser fluences. Gold nanoparticles with a desired average size (1.7-5.5 nm in diameter) were prepared by tuning properly the surfactant concentration and the laser fluence. The concentration of SDS was found to be higher than a critical micelle concentration so as to gain a significant size reduction and to obtain stable final products. Laser ablation in combination with the laser-induced size control provides a versatile full physical preparation method of size-selected gold nanoparticles.

A liquid beam technique allows us to prepare a clean liquid surface in a vacuum. We describe experimental studies on structures and dynamics of molecules and clusters on clean liquid surfaces on the basis of the liquid beam technique in combination with high sensitivity laser-, photoelectron-spectroscopic and mass spectrometric techniques. In particular, we refer to characterization of a continuous liquid flow in the vacuum (liquid beam), and findings on solvation structures and reactions on various solution surfaces by use of liquid beam-multiphoton ionization-mass spectrometry.

Silver nanoparticles were produced by laser ablation of a metal silver plate in aqueous solutions of surfactants, CnH2n+1SO4Na (n = 8, 10, 12, 16). The nanoparticles thus produced were characterized by electron microscopy and UV-visible absorption spectroscopy. The abundances of the nanoparticles before and after centrifugation were measured as a function of the surfactant concentration. The concentration dependence of the abundance implies that the surfactant coverage and the charge state on the nanoparticle surface are closely related to the stability of the nanoparticles in the solutions. The nanoparticles tend to be aggregated when the coverage is less than unity, while they are very stable when the surface is covered with a double layer of the surfactant molecules.

Resorcinol molecules and those solvated with solvent water molecules were isolated in the gas phase from a liquid beam of an aqueous solution of resorcinol by resonant vibrational excitation of solvent water molecules under IR-laser irradiation. The spatial distribution of the ejected species at various delay times from the IR-laser irradiation indicates that two different isolation mechanisms operate: One dominates in a time range shorter than similar to 1 mu s (early-time domain), and the other in a time range longer than similar to 1 ys (late-time domain). A time-dependent measurement of the liquid-beam profile by optical diffraction shows that the beam has a smooth surface in the early-time domain, whereas in the late-time domain the surface roughness overweighs the wavelength of the illumination laser. (C) 2000 Elsevier Science B.V. All rights reserved.

A continuous liquid flow of a calcium chloride (CaCl2) solution in ethanol (EtOH) in a vacuum (a liquid beam) was irradiated with a 266 nm laser, and ions ejected from the surface following multiphoton ionization via the CTTS (charge transfer to solvent) band of Cl- were observed by a time-of-flight mass spectrometer. A variety of core ions (Ca+, CaOEt+, CaOH+, CaCl+, Hi, etc.) are formed by reactions involving Ca2+, solvated electrons, and solvent molecules after the CTTS excitation by the laser irradiation and are ejected into vacuum with several accompanying alcohol molecules. The proposed mechanism is verified by the change of the ion intensity with introduction of an electron scavenger, CHCl3, in the solution. The cluster ion, Ca+(EtOH)(m), remains intact for m < 3, while it dissociates into CaOEt+(EtOH)(m-1) for m greater than or equal to 3. This size-dependent dissociation is simply explained by the energetics.

An aniline solution in l-propanol, or a pure ethanol Liquid, was introduced into vacuum as a continuous liquid flow (liquid beam) and was irradiated with a 266-nm laser. Ions were produced in the liquid beam by multiphoton absorption and partially ejected into the vacuum. The abundance of ions remaining inside the liquid beam and that ejected in the gas phase were measured simultaneously by using an inductive detector and a time-of-flight mass spectrometer, respectively, as a function of irradiation-laser power. The result is explained by the Coulomb ejection model for the ion ejection from the liquid surface. The essential feature changes slightly with the rate of the ion diffusion with respect to that of the ion ejection.

Solute resorcinol molecules were isolated from a liquid beam of an aqueous resorcinol solution by employing selective vibrational excitation of solvent molecules in the solution under irradiation of an infrared (IR) laser. Resorcinol molecules and their clusters with solvent water molecules isolated in the gas phase were ionized by multiphoton excitation under irradiation of an ultraviolet (UV) laser, and ions thus produced were detected by mass spectrometry. The dependence, of the ion intensities on the delay time from the IR to the UV excitation indicates that the neutral species are isolated in the gas phase through two different processes; one has a short characteristic time and the other a. long characteristic time.

A liquid beam (continuous liquid now in a vacuum) of a calcium iodide (CaI2) solution in ethanol (EtOH) was irradiated with a 220 nm laser, and ions ejected from the surface following multiphoton ionization were observed by a time-of-flight mass spectlometer. Cluster ions, CaOEt+(EtOH)(m) (m = 2-9), CaI+(EtOH)(m) (rn = 1-7). and H+(EtOH)(m) (m = 3-5) were found to be produced. The measurement of the velocities of the product cluster ions indicates that two mechanisms an operative: ions formed on the liquid surface: are ejected with accompanying solvent molecules, and ions generated by Coulomb explosion of a divalent cluster ion, Ca2+(EtOH)(m), are repelled with gaining a sizable translational energy.

Methanol, ethanol, or 1-propanol was introduced into vacuum as a continuous liquid flow (liquid beam) and was subjected to nonresonant multiphoton ionization under irradiation of a 270 nm laser. Ions ejected into vacuum were mass-analyzed by means of time-of-flight mass spectrometry, where the ions were extracted by a static electric field, or a pulsed electric field with a delay time of similar to 1.6 mu s with respect to a pulse laser. Ions, H+(ROH)(n) (n greater than or equal to 1), were dominantly produced by ion-molecule reactions in the liquid beam. On the other hand, H+(CH3)(2)O was interpreted to be produced by unimolecular dissociation of H+(CH3OH)(2) in the gas phase, and its rate constant was estimated to be (5 +/- 3) x 10(4) s(-1). This small rate constant suggests that the internal energy of the H+(CH3OH)(2) is dissipated efficiently into the liquid, so that the rate constant is much smaller than that for the same process in H+(CH3OH)(2) produced by ionization of a gas phase cluster. In addition, the fragment ions, H+ and C+, having similar kinetic energies of similar to 8 eV are considered to be produced by a multiphoton process.

An alcohol solution of benzoic acid, C6H5COOH, was introduced into a vacuum as a continuous liquid flow (liquid beam) and irradiated with a laser beam at a wavelength of 272 nm; the alcohols used were ethanol and propanol. Ions produced by multiphoton excitation in the liquid beam and ejected from it were analyzed by time-of-flight mass spectrometry. The mass spectra of the ions produced from benzoic acid in these different alcohol solutions led us to conclude that a protonated benzoic acid, C6H5C(OH)(2)(+), produced by laser irradiation reacts with an alcohol molecule, ROH, in the solution and that C6H5C(OH)(2)(ROH)(+) and C6H5C(OH)(OR)(+) are produced. Some of the product ions are regarded as reaction intermediates of ester formation from benzoic acid and the alcohols.

An alcohol solution of phenyl ketone C(6)H(5)COR(1), where R(1) is CH3, C2H5, H, Or C6H5, was introduced into a vacuum as a continuous liquid flow (liquid beam) and irradiated with a laser beam at wavelengths of 250, 280, and 355 nm. Ions produced by multiphoton ionization in the liquid beam and ejected from it were analyzed by time-of-flight mass spectrometry. The mass spectra of the ions produced from various alcohol solutions of phenyl ketones at different excitation wavelengths indicate that a protonated phenyl ketone ion, C6H5C(OH)R(I)(+), produced by laser irradiation reacts with an alcohol molecule, R(2)OH, in the solution and C6H5C(OR(2))R(I)(+) is produced. This reaction corresponds to the initial process for acetal formation from a phenyl ketone and an alcohol, and these product ions are identified to be its reaction intermediates. When an alcohol solution of benzophenone, C6H5COC6H5, was used, a pinacol ion, (C6H5)(2)C(OH)C(OH)(C6H5)(2)(+) was produced in addition to C6H5C(OR(2))C6H5+. The appearance of the pinacol ion suggests the presence of benzophenone dimers in the vicinity of the liquid surface.

A simple technique of preparing a continuous laminar liquid flow in vacuum (liquid beam) was developed and combined with multiphoton ionization and a time-of-flight mass spectrometer. This technique was applied to the study on resonance photoionization of an aniline (AN)-propanol (PrOH) solution (0.1-0.3 M). Binary cluster ions of aniline and propanol, AN(+)(PrOH)(n) (n greater than or equal to 1), and protonated propanol cluster ions, H+(PrOH)(n) (n greater than or equal to 1), were observed as product ions in the gas phase. The relative intensities of AN(+)PrOH and those of H+(PrOH)(2) were measured as functions of the excitation laser power and the concentration of aniline in the propanol solution. The dependences of the ion intensities on the laser power and the AN concentration are explained in terms of a Coulomb ejection model, where the ions are ejected from the surface by Coulomb repulsion exerted from neighboring ions. It is also concluded that H+(PrOH), is produced by a proton transfer reaction from an aniline ion to solvent molecules in the solution.

An anisole-ethanol solution was introduced into vacuum as a continuous liquid flow (liquid beam), and the molecules in the liquid beam were ionized by laser two-photon ionization. Ions ejected from the liquid beam were extracted by applying a pulsed electric field for the measurement of time-of-flight mass spectra of the ions. The intensities and peak profiles of the ions were measured by varying the delay time from the laser ionization to the pulse extraction of the ions at different laser powers. All the ions observed have almost the same velocity (approximate to 700 m s(-1)) and need approximate to 1 mu s to leave the liquid beam after laser irradiation. This finding implies that each photoion forming a solvation structure with almost the same number of solvent molecules is expelled from the liquid surface by Coulomb ejection and is dissociated into a cluster ion outside the influence of the Coulomb potential. The rate constants for ion ejection were determined.