Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact

We study the desorption of metastable atoms induced by electronic transitions in solid Ne by low energy electron impact. Time-of-flight spectra and the angular distribution of metastable atoms desorbed from the surface of an annealed sample show increased kinetic energy, higher signal intensity, a...

Ausführliche Beschreibung

Gespeichert in:

Bibliographische Detailangaben
Veröffentlicht in:	Физика низких температур
Datum:	2012
Hauptverfasser:	Kato, H., Tachibana, T., Hirayama, T.
Format:	Artikel
Sprache:	English
Veröffentlicht:	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2012
Schlagworte:	Low Temperature Spectroscopy and Radiation Effects
Online Zugang:	https://nasplib.isofts.kiev.ua/handle/123456789/117430
Tags:	Tag hinzufügen Keine Tags, Fügen Sie den ersten Tag hinzu!
Назва журналу:	Digital Library of Periodicals of National Academy of Sciences of Ukraine
Zitieren:	Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact / H. Kato, T. Tachibana, T. Hirayama // Физика низких температур. — 2012. — Т. 38, № 8. — С. 949-952. — Бібліогр.: 9 назв. — англ.

Institution

Digital Library of Periodicals of National Academy of Sciences of Ukraine

id	nasplib_isofts_kiev_ua-123456789-117430
record_format	dspace
spelling	Kato, H. Tachibana, T. Hirayama, T. 2017-05-23T14:58:59Z 2017-05-23T14:58:59Z 2012 Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact / H. Kato, T. Tachibana, T. Hirayama // Физика низких температур. — 2012. — Т. 38, № 8. — С. 949-952. — Бібліогр.: 9 назв. — англ. 0132-6414 PACS: 79.20.La, 71.35.Gg https://nasplib.isofts.kiev.ua/handle/123456789/117430 We study the desorption of metastable atoms induced by electronic transitions in solid Ne by low energy electron impact. Time-of-flight spectra and the angular distribution of metastable atoms desorbed from the surface of an annealed sample show increased kinetic energy, higher signal intensity, and a narrower angular distribution compared with those measured using an unannealed sample. These results are explained by considering the sample’s surface conditions in the framework of the cavity ejection mechanism. Our results for annealed solid Ne show that when the sample temperature increases from 5 to 7 K, the width of the angular distribution increases by about 10%. A simple trajectory calculation qualitatively reproduces our experimental results. One of us (T.H.) is grateful to Dr. Elena V. Savchenko for her valuable discussions and comments as well as continuous encouragement since we first met in 1994. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by the Rikkyo University Special Fund for Research (Rikkyo SFR). en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Low Temperature Spectroscopy and Radiation Effects Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact Article published earlier
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
title	Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact
spellingShingle	Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact Kato, H. Tachibana, T. Hirayama, T. Low Temperature Spectroscopy and Radiation Effects
title_short	Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact
title_full	Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact
title_fullStr	Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact
title_full_unstemmed	Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact
title_sort	temperature effect of metastable atom desorption from solid ne by low-energy electron impact
author	Kato, H. Tachibana, T. Hirayama, T.
author_facet	Kato, H. Tachibana, T. Hirayama, T.
topic	Low Temperature Spectroscopy and Radiation Effects
topic_facet	Low Temperature Spectroscopy and Radiation Effects
publishDate	2012
language	English
container_title	Физика низких температур
publisher	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
format	Article
description	We study the desorption of metastable atoms induced by electronic transitions in solid Ne by low energy electron impact. Time-of-flight spectra and the angular distribution of metastable atoms desorbed from the surface of an annealed sample show increased kinetic energy, higher signal intensity, and a narrower angular distribution compared with those measured using an unannealed sample. These results are explained by considering the sample’s surface conditions in the framework of the cavity ejection mechanism. Our results for annealed solid Ne show that when the sample temperature increases from 5 to 7 K, the width of the angular distribution increases by about 10%. A simple trajectory calculation qualitatively reproduces our experimental results.
issn	0132-6414
url	https://nasplib.isofts.kiev.ua/handle/123456789/117430
citation_txt	Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact / H. Kato, T. Tachibana, T. Hirayama // Физика низких температур. — 2012. — Т. 38, № 8. — С. 949-952. — Бібліогр.: 9 назв. — англ.
work_keys_str_mv	AT katoh temperatureeffectofmetastableatomdesorptionfromsolidnebylowenergyelectronimpact AT tachibanat temperatureeffectofmetastableatomdesorptionfromsolidnebylowenergyelectronimpact AT hirayamat temperatureeffectofmetastableatomdesorptionfromsolidnebylowenergyelectronimpact
first_indexed	2025-11-27T04:18:14Z
last_indexed	2025-11-27T04:18:14Z
_version_	1850799629869776896
fulltext	© Haruaki Kato, Takayuki Tachibana, and Takato Hirayama, 2012 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 8, pp. 949–952 Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact Haruaki Kato1, Takayuki Tachibana2, and Takato Hirayama1 1 Department of Physics, Rikkyo University, Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan E-mail: hirayama@rikkyo.ac.jp 2 Department of Physics, Tokyo University of Science, Shinjuku-ku, Tokyo 162-8601, Japan Received May 16, 2012 We study the desorption of metastable atoms induced by electronic transitions in solid Ne by low energy elec- tron impact. Time-of-flight spectra and the angular distribution of metastable atoms desorbed from the surface of an annealed sample show increased kinetic energy, higher signal intensity, and a narrower angular distribution compared with those measured using an unannealed sample. These results are explained by considering the sam- ple’s surface conditions in the framework of the cavity ejection mechanism. Our results for annealed solid Ne show that when the sample temperature increases from 5 to 7 K, the width of the angular distribution increases by about 10%. A simple trajectory calculation qualitatively reproduces our experimental results. PACS: 79.20.La Photon- and electron-stimulated desorption; 71.35.Gg Exciton-mediated interactions. Keywords: metastable atom, desorption, electronic transition, time-of-flight spectra. 1. Introduction Desorption induced by electronic transitions (DIET) in rare gas solids by elementary excitation has been investi- gated over the past 20 years [1]. Cavity ejection (CE) is one of the mechanisms that leads to excited atom desorp- tion such that an excited atom created on the surface of the solid by incident electrons or photons is repelled by sur- rounding atoms in the ground state and desorbs from the surface. When this mechanism is applicable, desorbed ex- cited atoms are expected to show a narrow angular distri- bution that is concentrated near the surface normal direc- tion. In real samples, this narrow angular distribution may become broader because of either imperfections in the crystal or lattice vibration, or both. Sakurai et al. [2] measured the angular distribution of metastable atoms desorbed from the surface of solid Ne by low energy photon irradiation. They succeeded in qualita- tively reproducing their experimental results using a simple trajectory calculation. To observe the effects caused by conditions of the sam- ple surface as well as those owing to lattice vibration, we measured time-of-flight spectra and the angular distribu- tion of atoms desorbed from the surfaces of unannealed and annealed samples at various temperatures. We will discuss the results within the framework of the CE mecha- nism, and we also performed a simple trajectory calcula- tion of desorbed excited Ne atoms. 2. Experimental setup Figure 1 shows a side view of the experimental setup, which was similar to the one we used in our previous work [3–5], except for the addition of a low-energy electron gun and a channel electron multiplier (CEM) for the detection of desorbed metastable atoms. The main chamber was evacuated by a series of turbo molecular pumps and a Ti-get- ter pump, resulting in a pressure of approximately 3·10–8 Pa. The sample film was prepared on a polycrystalline Cu disk of 10 mm diameter, which was fixed to a mechanical cryostat and cooled down to 5.0 K. The cryostat was sur- rounded by a heat shield that was kept at about 40 K. The cryostat can be rotated without breaking the vacuum, which enables us to measure the angular distribution of de- sorbed particles. The sample film was condensed on the Cu disk by filling the chamber with gaseous Ne to a pressure of 10–6–10–4 Pa. The film thickness was estimated from the exposure assuming a condensation coefficient to be unity. Present address: Department of Physics, Rikkyo University, Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan Haruaki Kato, Takayuki Tachibana, and Takato Hirayama 950 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 8 The temperature of the sample was controlled in the range between 5 and 100 K by a heater mounted in the cryostat. The temperature stability during the measure- ments was 0.1± K. The sample temperature was measured using a Si-diode (LakeShore, DT-670A-SD) attached to the Cu substrate. Solid Ne was formed at a substrate tem- perature of 5.0 K, and was annealed at 7.0 K. An electron gun, which produces an electron beam with an energy between 17 and 100 eV, was fixed to the heat shield, so that the incident angle of the electron beam to the sample surface was fixed during the measurements of the angular distribution. For our measurements of the time- of-flight spectra, the electron beam was pulsed by applying a pulsed voltage to the first grid of the electron gun. The width and frequency of the pulsed beam were approxi- mately 10 μs and 500 Hz, respectively. Desorbed metastable atoms were directly detected by a CEM that was fixed to the chamber wall. The distance between the sample and the CEM was 287 mm. The life- times of the Ne metastable states (2p53s 3P0.2) are known to be much longer than the flight time in the present experi- ments (typically less than 1 ms). Desorbed ions and emit- ted electrons were rejected by applying a positive voltage to the grid mesh in front of the CEM and a negative volt- age to the entrance funnel of CEM, respectively. 3. Experimental results 3.1. Time-of-flight spectra Figure 2 shows the time-of-flight spectra of metastable atoms desorbed from the surface of solid Ne measured at the sample normal direction (θ = 0°). The incident electron energy was 30 eV and the sample thickness was 300 atom- ic layers. The strongest peak at flight time tf = 0 is caused by the emitted VUV light from the sample. The higher kinetic energy peak (tf ~ 80 μs, Ek = 1.4 ± 0.1 eV) is owing to metastable Ne atoms that are desorbed through the excimer dissociation (ED) process. The peak at tf ~ 210 μs (Ek = 0.18 ± 0.02 eV) is caused the cavity ejection (CE) mechanism. Figure 2 shows that the CE peak of the annealed sample (solid line) exhibits a higher intensity. A slight increase of the kinetic energy (~ 0.6%) was also observed in the spec- trum of the annealed sample. The kinetic energy did not change in the spectra of the annealed sample measured at temperatures between 5.0 and 7.0 K within the experi- mental statistical uncertainties. 3.2. Angular distribution The time-of-flight spectra measured at θ = 0, 10, and 20° from the sample normal direction are shown in Fig. 3. The intensity of the CE peak significantly decreases as the angle increases. The CE peak area as a function of obser- vation angle (angular distribution) is plotted in Fig. 4. This clearly shows that the sample that annealed at 7.0 K (open circles) exhibits a narrower angular distribution than the unannealed sample (solid circles). The angular distribution of the CE peak area of the an- nealed sample was measured as a function of temperature. The temperature dependence of the full width at half max- imum (FWHM) of the distribution is shown in Fig. 5. The FWHM increases by approximately 10% when the sample temperature increases from 5.0 to 7.0 K. Fig. 1. (Color online) Schematic of the experimental setup (side view). Si Diode Solid Ne/Cu CEM Mechanical Cryostat Heat Shield 30 mm Electron Gun Fig. 2. (Color online) Time-of-flight spectra of metastable atoms desorbed from the surface of solid Ne measured in the sample normal direction (θ = 0°). The incident electron energy was 30 eV and the thickness of the sample was 300 atomic layers. The solid line and dots represent the spectra from annealed and unannealed Ne solid, respectively. In te ns ity , a rb . u ni ts 0 100 200 300 400 500 Without annealing With annealing at 7.0 K Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 8 951 4. Annealing effect Our results for the time-of-flight spectra and the angular distribution indicate that – the signal intensity detected in the sample normal di- rection increases in the annealed sample, – the angular distribution of the unannealed sample is broader than that of the annealed sample, and – the kinetic energy of the desorbed metastable atoms increases in the annealed sample. The motive force associated with desorption through the CE mechanism is the repulsive force from ground state atoms surrounding the excited atom, which should be axi- ally symmetric around the sample normal direction. The desorbed excited atoms should, therefore, exhibit a narrow angular distribution towards the sample normal direction, as observed in the present study. If the surface is rough, as shown in Fig. 6,a, which is the case if the sample is formed at low temperatures, atoms desorb in the direction normal to the local surface. This results in a broadening of the angular distribution, as seen in Fig. 4. This also explains why the signal intensity detected in the sample normal di- rection increases when the sample is annealed. Cui et al. pointed out, based on molecular dynamics calculations [6], that the kinetic energy of excited atoms desorbed from the surface of solid Ar increases as the number of surrounding atoms increases. It can be reasona- bly assumed that the number of surrounding atoms in the annealed sample — see Fig. 6,b — is larger than that in the unannealed sample: see Fig. 6,a. Therefore we attribute the increase in the kinetic energy in the annealed sample to the flatness of the sample surface, i.e., the average number of atoms surrounding the excited (desorbing) atom. Fig. 3. (Color online) Time-of-flight spectra of metastable atoms desorbed from the surface of annealed solid Ne measured at θ = 0, 10 and 20°. The incident electron energy was 30 eV and the thickness of the sample was 300 atomic layers. θ = 0° θ = 10° θ = 20° 0 100 200 300 400 500 In te ns ity , a rb . u ni ts Flight time, sμ Fig. 4. (Color online) Angular distribution of desorbed metastable Ne atoms through the CE mechanism. Solid and open circles are the results measured with an unannealed sample and a sample annealed at 7.0 K, respectively. Solid lines are the best-fitting curves using a Gaussian fitting function. In te ns ity , a rb . u ni ts Angle, deg. –40 –20 0 20 40 Without annealing With annealing at 7.0 K FW H M , d eg 34 33 32 31 30 29 28 Measurements Simulation 18.4 18.2 18.0 17.8 18.6 W id th , d eg 5.0 5.5 6.0 6.5 7.0 Sample temperature, K Fig. 5. (Color online) FWHM of the angular distribution of de- sorbed metastable Ne atoms through the CE mechanism as a func- tion of the sample temperature (solid circles). The solid line is the result of our simple trajectory calculation. See text for details. Fig. 6. (Color online) Schematic of the surface of the unannealed (a) and the annealed (b) sample. Solid circles represent Ne atoms and the arrows point in the direction of the desorption through the CE mechanism. See text for details. a b Unannealed sample Annealed sample Haruaki Kato, Takayuki Tachibana, and Takato Hirayama 952 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 8 5. Temperature effect The present results show that the width of the angular distribution becomes broader as the sample temperature increases. Sakurai et al. [2] pointed out, based on their simple trajectory calculation, that the angular spread can- not be explained by the zero-point energy of the atom at the lattice point alone, but it is essentially caused by vibra- tional displacement of Ne atoms around the lattice point. Therefore we attribute the broadening of the angular distri- bution at higher sample temperatures to an increase in the vibrational amplitude. To confirm this idea, we performed a simple trajectory calculation, which will be discussed below. 6. Trajectory calculation The trajectory of desorbing excited atoms that are cre- ated at the (111) surface of solid Ne is calculated as de- scribed in [2]. Briefly, an excited Ne atom in the 2p53s (1P1) state is surrounded by atoms in the ground state (1S0). The potential surface around the excited atom is calculated by superposition of the pair potential between the excited atom and the ground state atoms. The potential between ground state atoms is calculated using the Len- nard-Jones 6–12 potential, and that between Ne* (1P1) and Ne (1S0) is calculated based on the theoretical results of Kunsch and Coletti [7]. Ground state atoms within a dis- tance equal to twice the lattice constant (36 atoms) are in- cluded in the potential calculation. It is assumed that the ground state atoms do not move during desorption of the excited atom. The potential thus calculated is inserted into a SIMION potential array file [8]. The trajectory of a desorbing excit- ed Ne atom is calculated by placing a positive ion with a kinetic energy E0 at a distance R(T) from the lattice point. R(T) is estimated as 3 0 22( ) ( ) E kT R T K + = and K is a constant calculated using 2 0 0 1 2 Kx E= , where x0 = 0.032 nm and E0 = 0.01 eV are the mean square amplitude and the energy of the zero-point vibration in the Ne crystal, respectively [9]. Ions are distributed along a circle of radius R(T) at intervals of 15° and the direction of the initial motion is towards and outwards from the cen- ter of the circle, i.e., the lattice point. The calculated angular spread as a function of tempera- ture is shown as the solid line in Fig. 5. Although the cal- culated angular spread is about 60% of the measured FWHM, our calculation qualitatively reproduces the exper- imental temperature dependence. 7. Summary We have measured the time-of-flight spectra and the angular distribution of metastable Ne atoms desorbed from the surface of annealed and unannealed solid Ne at temper- atures between 5.0 and 7.0 K by low energy electron im- pact. The dependence on the sample surface conditions and temperature found in the present work are well explained in the framework of the CE mechanism. A simple trajecto- ry calculation qualitatively reproduced the results obtained here. Acknowledgments One of us (T.H.) is grateful to Dr. Elena V. Savchenko for her valuable discussions and comments as well as con- tinuous encouragement since we first met in 1994. This work was partially supported by a Grant-in-Aid for Scien- tific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by the Rikkyo University Special Fund for Research (Rikkyo SFR). 1. For recent review, see T. Hirayama and I. Arakawa, J. Phys. Cond. Matter 18, S1563 (2006) and references therein. 2. J. Sakurai, T. Nagai, M. Abo, T. Hirayama, and I. Arakawa, J. Vac. Soc. Jpn. 38, 414 (1995). 3. S. Fujita, K. Fukai, T. Tachibana, T. Koizumi, and T. Hira- yama, J. Phys. Conf. Ser. 163, 012083 (2009). 4. K. Fukai, S. Fujita, T. Tachibana, T. Koizumi, and T. Hira- yama, J. Phys. Cond. Matter 22, 084007 (2010). 5. T. Tachibana, K. Fukai, T. Koizumi, and T. Hirayama, J. Phys. Cond. Matter 22, 475002 (2010). 6. S. Cui, R.E. Johnson, and P. Cummings, Phys. Rev. B39, 9580 (1989). 7. P.L. Kunsch and F. Coletti, J. Chem. Phys. 70, 726 (1979). 8. http://simion.com/ 9. H.R. Lyde, in: Rare Gas Solid, M.L. Klein, and J.A. Ven- ables (eds.), Academic Press, London (1976), Vol. 1, p. 382.

Temperature effect of metastable atom desorption from solid Ne by low-energy electron impact

Institution

Ähnliche Einträge