Secondary ion emission from frozen argon by ²⁵²Cf fission fragments

Abstract

In order to study sputtering of Van-der-Waals solids by MeV ion impact, films of solid argon condensed onto a metallic substrate at a temperature of 10 K were irradiated by "*Cf fission fragments. The secondary ions emitted from the surface were investigated by a time-of-flight technique. The mass spectra are dominated by a cluster series Ar: with n ranging from 1 to about 30. Comparatively high absolute ion yields were determined; the yield of Ar' was 48 ionshmpact. The mass line of Ar + was broadened due to ejection energies of up to 50 eV. The line shape of the cluster ions indicates delayed particle emission. Some of the observed phenomena are not typical for electronic sputtering at MeV energies; they are briefly discussed. These are the first and preliminary results of a systematic exploration of high energy sputtering from solid rare gases.Electronic sputtering of frozen argon has been frequently studied and is one of the best-understood sputtering phenomena (see, for instance, the review article of Johnson and Schou).' In solid argon, swift heavy ions generate, by electronic excitation, mobile excitons, which get self-trapped near the surface and cause sputtering by their decay-either directly or via the formation and subsequent de-excitation of excimers. These processes have been explored mainly by energy distributions of neutral argon atoms combined with luminescence spectroscopy.Little is known about the mechanisms forming ionized sputtered particles. The mean ionization energy of Ar (20.1 eV) is orders of magnitude higher than the cohesive energy (0.08 eV). Therefore, Ar' is produced by relatively highly energetic processes-in accordance with the finding that the ejection energy of argon ions is distinctly higher than that of the neutral atoms. ' The highest electronic sputtering yields (85 Ar atoms per impact) have been observed with 4He beams at an energy loss of 1.3 M e V / m g /~m * .~The aim of the present work was to extend the investigation of sputtering from solid Ar in the high energy loss regime, where additional sputter processes should occur, such as gas flow sputtering"' or ablation due to shock wave propagation. The *' *Cf fission fragments used to irradiate the frozen Ar layers have an energy loss of 36 MeV/mg/cm' (light group of fragments) and 38 MeV/mg/cm' (heavy group). In the following, we present our very first results on ion emission from frozen Ar. (We have also planned systematic experiments with frozen Ne, Kr and Xe including studies of neutral particles.) EXPERIMENTALThe time-of-flight (TOF) mass spectra of Ar ions were measured by means of a plasma desorption mass spectrometry (PDMS) technique developed recently for studies of organic frozen gasesx The liquid-nitrogen trap of this machine has now been replaced by a liquidhelium cryostat, which provides a temperature of 10 K at the sample substrate. The part of the instrument, which holds the cold target, the 12 pC "I' Cf source and the acceleration grid, is sketched in Fig. 1. During the TOF measurements, a steady stream of...

Secondary ion emission from condensed CO bombarded by fission fragments

“…6 and 8, the C n + series observed in the present work exhibits an exponential decrease superposed to a periodic intensity variation where the odd n correspond to local maxima. The negative C n − series has the opposite behavior: the even/odd periodicity presents maxima at even n (C 5 − and C 7 − have the same mass as CO 3 − and (CO) 3 − , respectively). The steep decrease of cluster yields with increasing n is reported only for cluster production in sparks [32]: the mass distributions of carbon clusters produced by laser impact usually show up to n = 11 an increase of intensity [31,33].…”

“…[1][2][3][4][5][6][7][8]. Motivation for such studies comes from material science [1], molecular physics [2][3][4][5] and astrophysics [6][7][8].…”