Surface Science 453 ( 2000 ) L328–L331 www.elsevier.nl/ locate/susc Surface Science Letters Comment on ‘‘The ‘infinite velocity method’: a means of concentration calibration in secondary ion mass spectrometry?’’ [Surf. Sci. 429 (1999) 84] P.A.W. van der Heide * Materials Research science and Engineering Center (MRSEC), University of Houston, Houston, TX 77204-5641, USA Received 30 August 1999; accepted for publication 8 February 2000 Keywords: Quantum effects; Secondary ion mass spectroscopy As known to numerous practitioners, quantifi- dependence. The IV method was developed as a result of trends noted in experimental data. These cation of the secondary ion emissions in secondary ion mass spectrometry (SIMS) can be a highly trends are also consistent with presently accepted theory describing secondary ion emission. complex issue mainly because of the severe depen- dence of secondary ion intensities on sample com- In a recently published paper [5] the validity of IV method was questioned by suggesting that all position, i.e. the matrix effect [1]. The aim of the infinite velocity (IV ) method of the above mentioned assumptions are erroneous and that quantitative analysis via such a method for SIMS [2], first mentioned in Ref. [3] not Ref. [4 ] as stated in Ref. [5], was to try circumvent is not possible. These suggestions have far reaching ion yield matrix effects by extrapolating the trend implications as not only do they challenge the IV passing through secondary ion intensities collected method, but they also question the validity of over several different emission energy windows, to many previous 1/v distribution studies (Refs. [6,7], infinite velocity (or more precisely to the inverse to name a few). velocity, 1/v=0, intercept). The theory being that In defense of the IV method, there now exists matrix effects are removed at the 1/v=0 limit. To experimental evidence supporting the prediction do this, required the use of several somewhat that ion yield matrix effects are indeed removed broad assumptions. These are (a) that the sputtered [6–8]. This is further substantiated by the fact that population is isotropic, (b) that this can be IV data from Cs+ impact scales with the concen- described by the commonly used Sigmund– tration irrespective of the matrix the ions departed Thompson distribution, and (c) that the sputtered from [2,8–13]. Such a dependence would not exist population does not exhibit a significant mass if matrix effects were present. This observation however, is only noted if (a) the secondary ion yields exhibit an exponential dependence on emis- * Fax: +1-713-743-2787. sion velocity (the assumption used in numerous E-mail address: pvdheide@bayou.uh.edu ( P.A.W. van der Heide) theoretical studies including the electron tunneling 0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0039-6028(00)00364-2