1. Introduction U.S.A. 3 ISIS, STFC, Rutherford Appleton Laboratory, Didcot, OX110QX, UK

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1 Proc. 2nd Int. Symp. Science at J-PARC Unlocking the Mysteries of Life, Matter and the Universe Measurement of Transient Photo-induced Changes in Thin Films at J-PARC Time-resolved Neutron Reflectivity Measurements of Silver Photo-diffusion into Ge-chalcogenide Films Yoshifumi SAKAGUCHI*, Hidehito ASAOKA 1, Yuki UOZUMI 1, Yukinobu KAWAKITA 1, Takayoshi ITO, Masato KUBOTA 1, Dai YAMAZAKI 1, Kazuhiko SOYAMA 1, Mahesh AILAVAJHALA 2, Kasandra WOLF 2, Maria MITKOVA 2, and Maximilian W. A. SKODA 3 Comprehensive Research Organization for Science and Society (CROSS), Tokai , Japan 1 Japan Atomic Energy Agency, Tokai , Japan 2 Department of Electrical and Computer Engineering, Boise State University, Boise, ID U.S.A. 3 ISIS, STFC, Rutherford Appleton Laboratory, Didcot, OX1QX, UK y_sakaguchi@cross.or.jp (Received November 8, 214) We report neutron reflectivity measurements of silver photo-diffusion into Ge-chalcogenide films as an example of the time-resolved measurement of photo-induced changes in thin films recently measured at J-PARC. Details of time-resolved time-of-flight spectra and neutron reflectivity profiles are presented. KEYWORDS: transient measurement, neutron reflectivity, silver photo-diffusion, chalcogenide films 1. Introduction Study on photo-induced changes for specific light-sensitive materials is fascinating from both academic and application points of view because it is one of the frontiers in materials science including concepts of electron-excitation and non-equilibrium state, and the materials can be controlled spatially by illuminated area from distant place [1]. With the aid of significant development of light source equipment such as femtosecond ultra-short pulsed laser systems, attractive photo-induced phenomena have been found in many materials in these days. In the leading-edge field, it is quite natural to be interested in finding the structural origin of the photo-induced changes. Although neutron scattering technique has always a difficulty in beam intensity, it is highly expected to be utilized in structural determination for magnetic materials, hydrogenated materials, and X-ray sensitive materials. At the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC), the proton beam intensity is now 3kW, and it is planned to go to 1MW in The Physical Society of Japan

2 323-2 the near future, which can provide more intensive neutrons at least by more than 3 times of the present beam. Also, all instruments in the MLF adopt event recording data acquisition system, with which the data are arbitrary sliced once the data set are acquired. These points give us big advantages to perform time-resolved measurements. Among the neutron scattering instruments in the MLF, a neutron reflectometer is one of the suitable instruments for time-resolved measurement because the intensity at total reflection region is the same as that of the direct beam and, at least, it is easier to obtain a reflectivity signal around the critical momentum transfer even with short time interval, compared to other signals like a powder diffraction peak or an inelastic peak. At the MLF, there are two reflectometers and both of them adopted time-of-flight technique, in which both incident and reflected angles are fixed. This is suitable for probing time-evolutionary changes in thin films. In this paper, we demonstrate our recent results of time-resolved neutron reflectivity for silver photo-diffusion into Ge-S films performed on BL17 (SHARAKU), including experimental and analytical procedures, as one of the examples of transient measurements for photo-induced changes at J-PARC. 2. Experimental 2.1 Experimental setup Fig.1 shows the experimental setup for the time-resolved neutron reflectivity measurements on BL17 (SHARAKU) at the MLF [2] while the silver photo-diffusion is taken place. The sample was fixed at the centre of the sample stage, which is at 155mm from the moderator. The angle resolution of the incident neutron beam was defined by the two slits which are located at 12mm and 15mm from the moderator. There are other 6 slits to minimize parasitic scattering. A 3 He tube point detector was used to detect reflected neutrons from the sample. White light from a 3W xenon lamp (MAX-33, ASAHI Spectra, Co., Ltd.) was used as an excitation light source. A quartz rod lens produces a uniform squared shaped light beam. The uniformity is important to evaluate the thickness of the reaction layer by the light exposure. Usually, we use a light beam with the area of 27mm x 27mm, setting the rod lens with a 12mm-distance from the (a) (b) Fig. 1. Experimental setup (a) Block diagram. (b) Photograph around the sample.

3 (a.u.) surface of the film and the surface of the lens. The light beam power is about 15mW/cm 2. The area of neutron beams is set as 25mm x 25mm at the sample position, to cover all neutron-irradiated area with visible-light illuminated area. The shutter of the light is controlled by a computer in the operation room. The shutter opening time is recorded by reading the clock in the room. The shutter opening status can be monitored by a webcam in the beamline hutch. 2.2 Data reduction At the MLF, neutron reflectivity profiles are obtained by time-of-flight (TOF) technique, fixing both incident and reflected angles. Since the angles are fixed during the experiment, transient changes can be detected by time-evolutionary changes in the TOF spectrum. This is the advantage of this technique in pursuing transient phenomenon and it is very important to select appropriate time-width for data acquisition considering required time-resolution (which depends on the reaction rate) and S/N ratio. At the MLF, neutron data are acquired using an event recording system in which every detected neutron is tagged with (1) neutron pulse number, (2) time taken from a facility-wide standard clock, and (3) TOF. The TOF spectrum is obtained by making a histogram where the start, end and interval (bin) of the TOF are designated. The clock time region can also be specified. Using the data reduction system, arbitrarily time sliced spectra can be obtained from the full recorded data set. Fig.2 shows the TOF spectrum of the direct beam with a bin width of, as an example of the histogram. At the MLF, a neutron pulse is with 25Hz (every msec.). On BL17 (SHARAKU), a flame overlap is carefully eliminated using 3 disk choppers, adjusting their phases [2]. The TOF window is also variable by adjusting the phases. In the figure, the phases were adjusted to obtain the TOF spectrum starting from t=msec. (2.2Å), eliminating faster neutrons than msec. In the spectrum, there are neutrons in -5msec. These are the neutrons in the second frame, which is actually in the time of -45msec. From detailed experiments and analysis, we confirmed that the neutron data in the second frame are also available. 3. Results and Discussion 3.1 Transient TOF spectra during visible light exposure So far, we have measured transient neutron reflectivity for Ag/Ge-S and Ag/Ge-Se system [3, 4]. Here, we demonstrate a good example of showing a drastic change during visible light exposure. Fig. 3 shows the time evolution of the TOF spectrum of Ag 5Å/ Ge 2 S 8 15Å/ Si substrate [3, 4]. For comparison, the TOF spectrum measured before visible light exposure with min. acquisition time is inserted in each figure. As clearly seen in Fig.3(a), the spectrum changes drastically in the first min. Thanks to the I Fig. 2. The TOF spectrum of the direct beam. Bin width: 3

4 (a.u.) (a.u.) (a) (b).5 (c) 2 min 1 min I 5 min (d) (e) (f).5.6 I sec event recording data acquisition system, we could decompose the data into much shorter ones. The merit of this system is that the time can be chosen arbitrary and we can try to see the time evolution with different ways once the full set of the data are acquired. Using the system, it would be possible to find unexpected fast changes, which were hardly observed as long as we use traditional data acquisition system, in which neutrons are just accumulated. At present, the S/N ratio for short time interval, such as 2 and sec., was not good enough. However, it is good to follow a basic future in the time-evolutionary TOF spectrum. For instance, the TOF spectrum in the total reflectivity region (35-msec.) changes too fast to observe gradual deviation in the spectrum, by 3sec. interval. But a gradual change in the region can be observed in the sec. interval spectrum even though the data are scattered. 3.2 Neutron reflectivity and analytical method sec Fig. 3. Time evolution of the TOF spectrum of Ag 5Å/Ge 2 S 8 15Å/Si substrate during visible light exposure. (a) min. interval. (b) 2min. interval. (c) 1 min. interval. (d) 3sec. interval, (e) 2sec. interval, (f) sec. interval. sec

5 min (a) -1-2 min (b) -1-1 min (c) R sec. -1 (d) -1-2 sec. (e) -1 - sec. (f) R Fig. 4. Time evolution of neutron reflectivity of Ag 5Å/ Ge 2 S 8 15Å/Si substrate during the visible light exposure. (a) min. interval (b) 2min. interval (c) 1min. interval (d) 3sec. interval (e) 2sec. interval (f) sec. interval Neutron reflectivity, R, is obtained by the TOF spectrum using the relationship, R = I / I, where I is the intensity of the reflected beam and I is the intensity of the incidence beam as function of the TOF. Fig.4 shows the time evolution of the neutron reflectivity profiles of Ag 5Å/ Ge 2 S 8 15Å/Si substrate film while it is exposed to the visible light. The neutron reflectivity profiles with 2minutes interval can be followed down to -4. However, the reflectivity region where one can determine experimentally becomes narrower as the interval gets to be shorter. When the interval is sec., a reliable reflectivity region is limited to -1. However, we can follow the changes within the region. For photo-induced dynamics studies, it is very important to reveal the transient nature experimentally with minimized assumptions. For this purpose, we measured and determined neutron reflectivity profiles for the initial and final status (before and after light exposure) with different angles. This way can narrowed down the models to the plausible ones. Furthermore, we applied Fourier transformation technique, which is one of the model-free technique used in X-ray/neutron reflectivity analysis [5]. This method is quite useful to determine the film structure such as number of layers and

6 323-6 relative changes in the thickness during the exposure [3]. According to our experience, a general model fitting was quite difficult. Therefore, we performed model fitting assuming the results obtained from the Fourier transformation technique, and could obtain fitting parameters [4]. Using the analytical methods, it was found that the silver photo-diffusion took place at the interface whose position was essentially fixed. This contradicts the previous model stating the progression of the diffusion front [3, 4]. 3.3 Further studies on transient photo-induced changes Finally, we remark on usefulness of a linear detector to observe specific surface photo-induced changes [4]. While we performed neutron reflectivity measurement of Ge Se 6 15Å/ Ag 5Å/ Si substrate on INTER in ISIS (U. K.), we observed enormous changes in the reflectivity profiles after about 3minutes exposure. The neutron reflectivity at the total reflectivity region decreased down to and the film was supposed to change abnormally. At that time, we used a linear detector and found that off-specular scattering was generated due to a macroscopic surface roughness. The experiment has shown that a linear detector is useful for further studies on transient photo-induced changes, related to macroscopic surface roughness. 4. Summary We have demonstrated that the neutron reflectometer in J-PARC is utilized in the photo-induced kinetics studies. Especially, the event recording data acquisition system is quite useful to find unknown fast kinetics. We also remarked on usefulness of a linear detector for further studies on photo-induced changes through the experiment on INTER at ISIS. Acknowledgment This work was supported by JSPS Grant-in-Aid for Scientific Research (C) Grant Number and by Defense Threat Reduction Agency under Grant HDTRA R. The neutron reflectivity measurements were performed on BL17 in J-PARC MLF under Project No. 212A68, 213A23, 213B159, and 214A141, and on INTER in ISIS under Project No. RB We would like to thank G. Foran (CROSS) for valuable discussions and M. Takeda (JAEA), M. Mizusawa, N. Miyata, K. Akutsu, and S. Kasai (CROSS) for technical support. References [1] For example, Special topics: Photo-Induced Phase Transitions and their Dynamics, J. Phys. Soc. Jpn. 75 (26) [2] M. Takeda, D. Yamazaki, K. Soyama, et al.: Chi. J. Phys. 5 (212) 161. [3] Y. Sakaguchi, H. Asaoka, Y. Uozumi, et al.: Can. J. Phys. 92 (214) 654. [4] Y. Sakaguchi, H. Asaoka, Y. Uozumi, et al.: submitted to J. Phys. Conf. Ser. [5] F. Bridou and B. Pardo: J. X-ray Sci. Tech. 4 (1994) 2, K. Sakurai, M. Mizusawa, and M. Ishii: Trans. Mat. Res. Soc. Jpn. 33 (28) 523