Photoresponsive LB Films under Light Irradiation Observed by Time-Resolved XRR

Langmuir-Blodgett (LB) films containing azobenzene undergo reversible structural change under light irradiation because of trans-to-cis or cis-to-trans transformations of azobenzene molecules. Such films are a candidate for molecular machines. Time resolved measurements of specular X-ray reflectivity (XRR) curves were carried out for polymer specimens of azobenzene-containing polyvinil alcohol (6Az10-PVA) monolayer LB films on quartz substrates during light (365 nm or 436 nm) irradiation (1 mW/cm2). Measurements were performed with a time-resolution of 10 s using an X-ray reflectometer [1, 2], which can simultaneously measure the whole XRR curves with no need to rotate the specimen, detector or monochromator crystal. Profiles of XRR curves changed as a function of the elapsed time after initiation of the light irradiation reflecting the structural change of the film. Despite of a common belief that the photo-induced structural change occurs directly between the initial and final states, we found an evidence of the existence of the intermediate third structure. We also found that the time needed for changes in XRR curves was several times longer than for optical absorption spectroscopy (OAS) spectra reported with the same irradiation power. Details of such changes of XRR curves and structures of the film will be discussed and compared with the changes of OAS spectra. An XRR curve for the intermediate state of the 6Az10-PVA monolayer LB film specimen separated from the XRR curves measured under 365 nm light irradiation is shown in the figure together with curves for the initial and final states of the same specimen.

Gemstones and Salts as Light Emitters for Learning X-ray Detectors

The scintillation counter is a widely-used X-ray detector. It contains a scintillator as a luminescent material that converts X-rays into visible light, which is detected with a sensor. A well-known scintillator in the X-ray region is sodium iodide, NaI, an ionic crystal. Before use, it is important to understand how the detector works. For students, the material name and the chemical formula of the scintillator are not familiar, however. In addition, students cannot watch or touch the key element in the detector, because the scintillator is installed inside the housing. Many jewels emit visible light or change their colors under ultraviolet light irradiation. Under X-ray irradiation, the same jewels exhibit similar responses as well. If popular jewels instead of special ionic crystals were used as scintillators, students might show interest in these materials. We propose that photographs of beautiful, brightly shining gemstones and salts could be used as visual educational materials for students to learn the principles of X-ray detectors. Different gemstones and salts were irradiated by intense white synchrotron X-ray radiation at beamline NE7A1 of the PF-AR synchrotron radiation facility at KEK, Japan. Photographs of fluorescence and phosphorescence from the gemstones, and of color changes due to the irradiation, were taken with a remote controlled digital camera. It should be noted that the experimental setup of this study is an easily understood handmade X-ray detector. We will present photographs of exciting gemstones such as Fluorite from the US, Hackmanite from Afghanistan, Mangano Calcite from China, Ruby from Brazil, Selenite from Canada, and Black Opal from Australia. We also irradiated different kinds of colored Himalayan Rock Salt from India or Pakistan, shown in Fig. 1. We will explain basic concepts of X-ray detectors, such as photon counting, dead time, recording, and quantum efficiency, with these photographs.