Nội dung

Nghiên cứu khoa học công nghệ SPEED DETERMINATION OF ATOMIC GAS UNDER A SUPERSONIC EXPANSION Nguyen Thanh Tung1,2*, Ha Cong Nguyen3 Abstract: The speed of gas-phase small molecules is an important parameter for directly and indirectly measuring physical and chemical properties of materials at sub-nano scale like magnetic moment, absorption cross-section, and reactivity rate. In this report, Pb atoms are produced in gas phase using pulsed laser ablation technique and accelerated into time-of-flight mass spectrometer under a supersonic expansion. Charged particles are recorded at the end of the spectrometer by a micro-channel-plate detector. A high-resolution vacuum-compatible chopper is employed to chop atomic beam pulses into a very thin slice. The speed distribution of the atomic beam is determined by analyzing the time synchronization between the laser ablation, the chopper, and the detector. Under our typical investigation, most of the atoms travel at a speed of 1650 m/s. The velocity-selected atoms can be controlled at an uncertainty of 1/1000, depending on conditions of atom formation. Keywords: Atomic velocity, Gas-phased atom, Atomic beam. 1. INTRODUCTION In chemical physics, a cluster refers to a countable number of atoms that are bonded together naturally or under certain conditions. The properties of clusters have been of a great interest owing to the large fraction of their surface atoms, their quantum size effects, and the possibilities to have unlimited mixtures of elements that do not exist in bulk forms [1]. Before embedding these clusters into a matrix of materials for promising applications, studying isolated clusters in gas phase, where the influences of substrate or unexpected ambient interactions can be ignored, would be important to understand their fundamental properties [2]. Since 1980s, clusters of simple metals, transition metal, and semiconductor atoms have been investigated extensively in the gas phase, showing a strongly size-dependent behavior, which results in intriguing chemical and physical applicability [3-6]. The pulsed laser ablation technique in combination with the molecular beam spectroscopy has been widely utilized to produce gas-phased clusters that are often introduced into measuring and/or interacting chambers through a nozzle, resulting in a supersonic beam with narrow distribution of relatively high velocities [7-11]. The velocity of tiny clusters/atoms is a must-know parameter in many experiments, for examples, magnetic/electric deflection of gas-phase clusters, dynamical interactions of gases, or photo-absorption/dissociation of molecules [1016]. However, very little information about this parameter since precisely measuring the cluster velocity needs to overcome a variety of practical difficulties, from vacuum incompatibility, magnetic distortion by iron motor, positioning the cluster beam, and high-resolution time synchronization (microsecond scale). Early investigations used to qualitatively extract cluster speeds by estimating the flying time of the whole molecular beam, which often costs a considerable uncertainty [15]. In this paper, we present a recently-built state-of-art experimental configuration, in which the small gas-phase atoms of lead are produced, and their speed is controlled at a high-precision level. Importantly, some insight into the Tạp chí Nghiên cứu KH&CN quân sự, Số 45, 10 - 2016 125 Vật lý influence of different formation parameters on the atom velocity is extensively discussed for versatile applications and purposes. 2. EXPERIMENTAL CONFIGURATION The cluster source is similar to what described in Refs. [9,10], that is implemented in KU Leuven. In particular, the pure lead clusters, Pbn, are produced in a pulsed laser vaporization source coupled to a dual-reflectron high-resolution time-of-flight mass spectrometer. The schematic drawing of our experimental setup is shown in Fig. 1. In the source chamber, a lead target is ablated by a 10 Hz Nd:YAG laser at 532 nm laser light. After vaporization, high-purified helium gas is introduced to initiate cluster formation. The clusters stay in the source chamber for several hundreds of microseconds before expanding out into the vacuum chamber through the nozzle, forming a supersonic beam with narrow distribution of relatively high velocities. However, before the cluster beam enters the extraction chamber, it will be chopped by a vacuum compatible chopper installed right at the central of the beam path. The chopper system includes a photodiode, a chopper head, a chopper blade, and a control unit, which are all non-magnetic materials. The speed of the chopper can approach a maximal value of 7600 rpm, fed by a 12 V source via electrical feedthroughs. The chopping frequency is measured via the photo-diode with a resolution of 0.01 Hz. After passing through the chopper, the clusters will enjoy a free-flying path about 0.8 m before ionized by a pulsed excimer laser in the extraction chamber. Ionized clusters (cations) will be accerlerated into the time-offligth mass spectrometer and finally detected by a multi-channel-plate detector. The chopper is synchronized with the ablation laser, the extraction optics, and the detector by a multi-channel Stanford clock (nanosecond delay generator). The cluster velocity is then determined by analyzing the flying time and distance between the chopper and the extraction optics. Detailed description about the experimental configuration can be found in Ref. [17]. In this study, only Pb atoms are produced in the atomic beam to demonstrate the operating principle. Larger clusters of different size and composition can be realized based on similar mechanism. Figure 1. Illustration of the velocity-selected gas-phase cluster setup. The apparatus consists of the laser ablation, the chopper, the extraction, and the time-of-flight spectrometer. 126 N. T. Tung, H. C. Nguyên, “Speed determination of atomic gas… supersonic expansion.” Nghiên cứu khoa học công nghệ 3. RESULTS AND DISCUSSIONS To be more specific, the chopper uses a blade with two small rectangular apertures (2 mm x 8 mm) of which the atomic beam can pass through. Smaller or greater apertures can be considered depending on the typical beam shape, the signal intensity recorded by the detector, and the level of measuring precision. In general, at the same chopping frequency, greater apertures allow more intense signal reaching the final detector but the passed pulse length, consequently, will be longer. This eventually reduces the precision of the measured speed. In contrast, smaller apertures give shorter pulse length and therefore, the atom speed will be determined more accurately at a price of signal intensity. 2,0 Signal intensity (V) (a) 10 Hz 20 Hz 50 Hz 100 Hz 200 Hz 1,5 1,0 0,5 0,0 -1,2 -0,8 -0,4 0,0 0,4 0,8 1,2 1,6 Opening time (ms) 2,0 Calculated opening time Measured opening time 1,6 Time (ms) 1,2 0,8 0,4 (b) 0,0 0 50 100 150 200 Frequency (Hz) Figure 2. (a) Measured intensity and living time of the signal passing through the aperture according to various chopping frequencies. (b) Calculated and measured opening time of the chopping aperture 2 mm x 8 mm. Tạp chí Nghiên cứu KH&CN quân sự, Số 45, 10 - 2016 127 Vật lý The measured intensity and living time of the signal passing through the aperture are presented in Fig. 2(a). The opening time is defined via blocking and unblocking time of the photo-diode. As the chopping aperture is right in front of the photo-diode, the light flux going to the detector is proportional to the exposed area of the photo-sensor. Since the detectable area is about 10% larger than the aperture area, the maximum signal is 1.8 V, which is slight smaller than the upper limit of 2 V set by the photo-diode. When the aperture starts emerging in front of the photo-diode area, the signal gradually increases. The increasing speed is understandably proportional to the chopping frequency. Once the aperture entirely falls into the photodiode area, the recorded signal is unchanged (~1.8 V). Increasing the chopping frequency obviously leads to narrower living time of the signal probed by the photo-diode. In other words, the opening time for the molecular beam is smaller. The calculated and measured opening times of the 2 mm x 8 mm aperture are given in Fig. 2(b). The measured opening time is the FWHM of the recorded signal with an uncertainty of 10%. It can be seen that the opening time decreases as increasing the chopping frequency, in line with the calculated results. The recorded opening time at 10, 20, 50, 100 and 200 Hz is about 1.594, 0.792, 0.364, 0.184 and 0.084 ms, respectively. For more precise speed measurements, the chopping frequency is selected to be 200 Hz. Our test results show that higher chopping frequencies can be achieved up to 300 Hz with an opening time of 0.05 ms. However, the atom signal passing through the aperture and reaching the final detector is considerable small, which might increase the data-acquisition time. Atom intensity (counts) 40 Set 1 Set 2 Set 3 Set 4 30 20 10 0 500 1000 1500 2000 2500 3000 3500 4000 Atomic velocity (m/s) Figure 3. Typical velocity distribution of Pb atoms measured at 200 Hz chopping frequency. Figure 3 shows the typical velocity distribution of Pb atoms. The chopping frequency is 200 Hz and the cryogenic gas pressure is about 5 bar. It should be noted that the distribution profile can be slightly changed upon the atom/cluster formation conditions, i. e., gas pressure, timing the laser ablation, shape of formation chamber, size and mass of atoms/clusters. To eliminate the influence of laser instability and 128 N. T. Tung, H. C. Nguyên, “Speed determination of atomic gas… supersonic expansion.” Nghiên cứu khoa học công nghệ systematical uncertainty, we performed four independent sets of experiments. The acquisition time for each experiment is 180 seconds corresponding to 36,000 consecutive measurements (recorded at 200 Hz chopping frequency). The molecular beam has a velocity (from the chopper to the extraction) varied between 1300 and 3000 m/s but most of them prefer to travel around 1650 m/s. A cut-off velocity at 1300 m/s suggests that clusters with lower speeds might not be formed. Further examination show that the cut-off velocity increases with increasing the cryogenic gas pressure but the population of the atomic beam shifts away from the optimal point. Due to the long flying path from the source until the final detector and short opening time at the chopper, the atom signal drastically reduces, down to several tens of counts for each single experiment. The distribution of atom/clusters could shift to the higher side if the size of atoms/clusters decreases, meaning that lighter atoms/clusters travel at greater speeds. The distribution could be also narrower for specific atoms and cluster sizes. The size selection can be carried by a wire-type mass gate installed in mass spectrometer [18] for photofragmentation or magneticdeflection experiments. The velocity selection can be done by timing the extraction optics, i. e., choosing the time where the clusters with selected velocity enter the extraction while blocking others using a voltage barrier. This value can be down to 100 ns for the current setting, which results a 0.15 mm (corresponding to a fractional value of 1/107) separation for an atom travel at 1500 m/s. Since the opening time of the 200-Hz chopper is about 84 µs and the total free-flying is about 0.8 m, ít gives a measured velocity uncertainty that decreases with decreasing the atomic speed. 4. CONCLUSIONS An experimental approach using the gas-phase molecular mass spectroscopy coupled to a vacumm-compatible chopper to determine the speed of sub-nano neutral particles like individual atoms and atomic clusters has been presented. The typical velocity distribution of Pb atoms has been measured, showing that they prefer a velocity of 1650 m/s with a separation of about 1/107. The influences of other experimental conditions on the atom/cluster velocity are discussed. It is shown that the velocity of atoms/clusters can be precisely measured and selected, paving the way for further practical implemenations to characterize the physical and chemical properties of nano molecules. Acknowledgements: This work was supported by the Institute of Materials Science, Vietnam Academy of Science and Technology under grant number HTCBT07.16. REFERENCES [1]. M. L. Cohen and W. D. Knight, Phys. Today 43, 42 (1990). [2]. W. A. de Heer, Rev. Mod. Phys. 65, 611 (1993). [3]. W. D. Knight, K. Clemenger, W. A. de Heer, W. A. Saunders, M. Y. Chou, and M. L. Cohen, Phys. Rev. Lett. 52, 2141 (1984). [4]. K. Clemenger, Phys. Rev. B 32, 1359 (1985). [5]. M. D. Morse, Chem. Rev. 86, 1049 (1986). [6]. J. A. Alonso, Chem. Rev. 100, 637 (2000). Tạp chí Nghiên cứu KH&CN quân sự, Số 45, 10 - 2016 129 Vật lý [7]. S. Neukermans, E. Janssens, H. Tanaka, R. E. Silverans, and P. Lievens, Phys. Rev. Lett. 90, 033401 (2003). [8]. E. Janssens, S. Neukermans, H.M.T. Nguyen, M.T. Nguyen, P. Lievens, Phys. Rev. Lett. 94, 113401 (2005). [9]. N. T. Tung, N. M. Tam, M. T. Nguyen, P. Lievens, and E. Janssens, J. Chem. Phys. 141, 044311 (2014). [10]. N. T. Tung, E. Janssens, P. Lievens, Appl. Phys. B 114, 497 (2014). [11]. S. Bhattacharyya, N. T. Tung, J. D. Haeck, K. Hansen, P. Lievens, and E. Janssens, Phys. Rev. B 87, 054103 (2014). [12]. W. A. de Heer, P. Milani, and A. Chatelain, Phys. Rev. Lett. 65, 488 (1990). [13]. X. Xu, S. Yin, R. Moro, and W. A. de Heer, Phys. Rev. B 78, 054430 (2008). [14]. I. M. L. Billas, A. Chatelain, and W. A. de Heer, Science 265, 1682 (1994). [15]. M. B. Knickelbein, Phys. Rev. Lett. 86, 5255 (2001). [16]. F. W. Payne, W. Jiang, and L. A. Bloomfield, Phys. Rev. Lett. 97, 193401 (2006). [17]. N. T. Tung, “Stablity of bimetallic clusters and development of a magnetic deflection setup”, PhD thesis (KU Leuven, Leuven, 2014). [18]. P. R. Vlasak, D. J. Beussman, M. R. Davenport, and C. G. Enke, Rev. Sci. Instrum. 67, 68 (1996). TÓM TẮT XÁC ĐỊNH TỐC ĐỘ CỦA CHÙM NGUYÊN TỬ TRONG PHA KHÍ DƯỚI ĐIỀU KIỆN GIÃN NỞ SIÊU ÂM Tốc độ của chùm nguyên tử hoặc phân tử trong pha khí là một thông số quan trọng trong các phép đo trực tiếp hoặc gián tiếp các tính chất vật lý và hóa học của vật liệu ở thang dưới nano mét, ví dụ như mô-men từ, tiết diện hấp thụ, và tốc độ phản ứng. Trong bài báo này, nguyên tử chì được tạo ra bằng các bốc bay laser xung trong pha khí và được gia tốc vào khối phổ kế trong điều kiện giãn nở siêu âm. Các hạt mang điện sẽ được ghi lại ở cuối khối phổ kể bằng một đầu thu vi kênh. Một bộ lọc độ phân giải cao phù hợp với môi trường chân không được sử dụng để cắt chùm nguyên tử thành một lát mỏng với tốc độ đồng nhất. Phân bố tốc độ của chùm nguyên tử được xác định bằng cách phân tích thời gian đồng bộ giữa thời điểm bốc bay, lọc, và thu tín hiệu. Trong điều kiện thí nghiệm, hầu hết các nguyên tử bay với vận tốc 1650 m/s. Các nguyên tử có thể được lọc với vận tốc nhất định, có độ chính xác lên tới 1/107, tùy thuộc vào điều kiện hình thành các nguyên tử hoặc phân tử. Từ khóa: Chùm nguyên tử, Chùm phân tử, Tốc độ trong pha khí. Nhận bài ngày 07 tháng 9 năm 2016 Hoàn thiện ngày 30 tháng 9 năm 2016 Chấp nhận đăng ngày 26 tháng 10 năm 2016 Địa chỉ: 1 Institute of Materials Science, Vietnam Academy of Science and Technology, Vietnam; Laboratory of Solid-state Physics and Magnetism, KU Leuven, Belgium; 3 Academy of Military Science and Technology. * Email: tungnt@ims.vast.ac.vn 2 130 N. T. Tung, H. C. Nguyên, “Speed determination of atomic gas… supersonic expansion.”