Gold nanoparticle films with different particles sizes (35 ( 5, 15 ( 1 nm) and agglomerated states were prepared on glass substrates, by salting-out the colloidal solutions with NaClO 4 or NaOH. An ethanol solution of rhodamine 6G was cast on the gold nanoparticle film and the surface-enhanced Raman scattering (SERS) spectrum was measured. We found that the intensity of the SERS signal was higher for the film with larger particles and that the film prepared from NaClO 4 gave higher SERS signals than that prepared from NaOH. Considerable coalescence of larger gold nanoparticles was quite effective for obtaining larger SERS signals.The SERS enhancement factor was estimated to be ∼10 8 for the film prepared from NaClO 4 .

We found a new method for the preparation of gold nanorod and nanosphere films. The nanoparticle films could be formed at the liquid-liquid interface between aqueous and hexane solutions and were almost monolayers. Surface-enhanced Raman scattering (SERS) spectra of rhodamine 6G under the nonresonance condition were measured using these films as SERS substrates. It is experimentally demonstrated that nanorods with a higher aspect ratio are more effective in SERS enhancement.

Photochromism of indolinospironaphthooxazine (SNO) microcrystalline powder was investigated by steady state and time-resolved diffuse reflectance spectroscopy using a femtosecond laser as a light source. When laser intensity is weak, the photoinduced ring-opening reaction in a picosecond time region and subsequent thermal ring-closure one in a nanosecond scale take place in the crystal, not leading to permanent photocoloration of the powder. The same sample shows photochromism upon intense femtosecond laser excitation. The absorption spectra after laser excitation are similar to those of the photocolored form in solution. The spectral shape is independent of the laser fluence, whereas the yield increases nonlinearly with the fluence. To reveal the photocoloration mechanism, we examined the excitation fluence dependence of the transient absorption spectra and elucidated the photocoloration behavior by femtosecond double pulse excitation varying the delay of the two pulses. These experimental results show that cooperative interactions between the excited states and short-lived intermediates are important to generate a long-lived photomerocyanine. Femtosecond induced photocoloration will be discussed in terms of transient and local lattice deformations, and a cooperative photochemical reaction model is proposed.

Photocoloration of microcrystalline powder of spirooxazine and spiropyran compounds induced by femtosecond laser excitation was investigated using time-resolved and steady state diffuse reflectance spectroscopy. The intermediate products were observed in a ns time region, and assigned to non-planar open forms. On the other hand, photocoloration can occur even in the crystalline phase by intense femtosecond pulse excitation, and the yield of colored forms increased nonlinearly with laser fluence. The mechanism of photochromic reaction induced by high-density excitation is discussed on the basis of excitation fluence and shot number dependences of photocoloration, thermal bleaching of colored forms, and transient absorption spectra. It is considered that free volume around molecules to change the molecular conformation is generated due to the local deformed crystal lattice following the densely formed excited states and intermediate products. y Dedicated to Professor Frank Wilkinson on the occasion of his retirement.

No abstract