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[Taku T. Suzuki](https://orcid.org/0000-0001-6041-4297)

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This is the peer reviewed version of the following article: Suzuki, T. (2025), Cryogenic TOF-SIMS Around Sublimation Temperature of Quench-Condensed Noble Gas (Ne, Ar, and Kr) Films. J Mass Spectrom, 60: e5107, which has been published in final form at https://doi.org/10.1002/jms.5107. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Cryogenic TOF‐SIMS Around Sublimation Temperature of Quench‐Condensed Noble Gas (Ne, Ar, and Kr) Films](https://mdr.nims.go.jp/datasets/63ff1789-90e6-4fbc-af9b-af60a009126c)

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Cryogenic TOF-SIMS around sublimation temperature of quenchcondensed-noble gas (Ne, Ar, and Kr) filmsTaku T. Suzuki∗National Institute for Materials Science,1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan(Dated: October 28, 2024)AbstractA possible TOF-SIMS analysis of surface phase transitions has recently been proposed for limitedcases such as polymers and ionic liquids. In the present study, we have extended this analysisto quench condensed noble gas films. The newly developed cryogenic TOF-SIMS allowed bothmeasurements of TOF-SIMS below 4 K and low-energy ion scattering spectroscopy that is usedto prepare a clean surface. It was found that the TOF-SIMS intensity variation by increasing thetemperature at a constant ramp rate (temperature-programmed TOF-SIMS) shows steep changesdue to sublimation. Thus, the possibility of analyzing the surface phase transition at the localregion defined by the incident ion beam of (cryogenic) TOF-SIMS was demonstrated in the presentstudy.∗Corresponding author. E-mail: suzuki.taku@nims.go.jp1I. INTRODUCTIONTOF-SIMS has established itself as a powerful technique for analyzing outermost surfaces.The surface sensitivity of TOF-SIMS is extreme, which is a critical feature for applicationin various scientific and technological fields where the outermost surface plays an importantrole. The application range of TOF-SIMS is expanding with the help of instrumental inno-vations, a deeper understanding of the ion-surface interaction, and accelerated processingfor the measured data.One of the advanced applications of TOF-SIMS may be the analysis of surface phase tran-sitions. Recent studies have suggested that the surface phase transition is detected bymeasuring the TOF-SIMS intensity with increasing sample temperature. For example, Vinxet al. [1] and Poleunis et al. [2] have reported that the temperature dependence of TOF-SIMSsignal intensity shows an inflection point related to the glass transition temperature of poly-mer surfaces. In addition, Souda has indicated that the TOF-SIMS signal intensity measuredby increasing the sample temperature at a constant ramp rate (temperature-programmedTOF-SIMS) shows a steep change at the glass transition, crystallization and melting ofthe ionic liquid surface [3, 4]. Furthermore, Fu et al. have shown that the temperaturedependence of the TOF-SIMS intensity signal shows a step change at the surface glass tran-sition of polystyrene films [5, 6]. Our recent study on quench condensed - hydrogen filmsusing cryogenic TOF-SIMS has also suggested that the solid-to-gas phase transition, that isthe sublimation, is detected by the temperature-programmed TOF-SIMS measurement [7].These studies suggest the possibility of a unique analysis of the structural phase transitionof the local area on the outermost surface defined by the incident ion beam of TOF-SIMS.The phase transition analysis of the surface local region is obviously important not only forfundamental materials science, but also for various practical devices using surface and inter-face properties. However, to the best of our knowledge, the detection mechanism remainsunclear. Therefore, it is not clear that this method of analyzing the surface phase transitionby TOF-SIMS is applicable to materials other than polymers, ionic liquids, and hydrogen.In our previous temperature-programmed TOF-SIMS study on the solidified hydrogen film,a steep change in the intensity was observed at the sublimation temperature [7]. Since the2observed temperature for the intensity variation agrees with the desorption temperatureof a hydrogen molecule from the solid surface, the possibility of the surface melting wasproposed to be ruled out. The surface melting of solid hydrogen has attracted attentionas the approach to realizing the superfluidity of hydrogen. [8, 9] Thus, the existence of thesurface melting on solid hydrogen itself should be revealed, which was the motivation of ourprevious study. The surface melting has often been suggested at solid vapor interfaces thattake place in prior to sublimation. It is crucial for understanding the fundamental process atsurfaces, such as wetting, adsorption, and friction, and therefore, it has been widely studiedboth from experiments and theories [10–15].In the present study, we extended the application of our cryogenic TOF-SIMS to quenchcondensed-noble gas (Ne, Ar, and Kr) films. In the present paper, it will be described thatthe solid-to-gas phase transition, that is sublimation, of the solidified noble gas surfaces isanalyzed by temperature-programmed TOF-SIMS, and therefore, it is useful for the analysisof surface phase transitions.II. EXPERIMENTSExperiments were performed in an ultra-high vacuum (UHV) analysis chamber (7×10−9Pa) configured for home-built TOF-SIMS. The details of the experimental setup have beendescribed elsewhere [7]. Briefly, a 2 keV He+ ion beam is used for the primary ion ofthe TOF-SIMS measurement, which was pulsed by the pulsed electric field with a pulsewidth of 500 ns and a duty ratio of 1.5%. The beam diameter at the sample position wasapproximately 2 mm. The secondary ions were detected by an electrostatic hemisphericalenergy analyzer (VSW, CL50), where the pass energy and the analysis energy of the analyzerwere set to 90 eV and 10 eV, respectively. Thus, the secondary ions measured in TOF-SIMS have a kinetic energy of 10 eV, which was accelerated to 90 eV inside the energyanalyzer. In our TOF-SIMS measurement, the beam fluence was less than 1012 ion·cm−2which is the typical condition for static SIMS. The temperature-programmed TOF-SIMSwas performed by raising the sample temperature at the constant rate of 2 K/min from thequench condensation temperature. The continuous He+ ion beam was used for low-energyHe+ ion scattering spectroscopy (LEIS), which was needed to confirm the cleaning of the3sample surface. Since TOF-SIMS and LEIS share the energy analyzer in our setup, theanalysis point was identical between these techniques.The substrate for quench-condensation of noble gases was tantalum foil of thickness 0.1 mm(purity: 99.95%, Nilaco, Japan). Before the quench condensation, the Ta substrate surfacewas cleaned by several cycles of flash heating to above 1800 K in UHV and 2 keV Ar+ ionsputtering. The successful surface cleaning by the surface cleaning procedure was confirmedby LEIS.We exposed the cleaned Ta substrate surface to noble gases (Ne, Ar, and Kr) below 4 K toprepare the quench-condensed solid film. The purity of the gases was more than 99.999%for Ne, 99.9999% for Ar, and 99.999% for Kr (Suzuki Shokan Co., Ltd). These gases wereintroduced into the UHV chamber via a variable leak valve. The exposure is expressed inthe present paper by Langmuir (L), where 1 L = 1.33×10−4 Pa·s. The partial pressureof the introduced gas during the exposure was controlled to be 1.33×10−7 Pa for 0.1 L orless, 1.33×10−6 Pa for over 0.1 L to 2 L, 1.33×10−5 Pa for over 2 L to less than 25 L, and1.33×10−4 Pa for over 25 L. The TOF-SIMS measurements were performed after recoveryof the post-exposure vacuum below 1×10−6 Pa.We used a GM refrigerator (Iwatani, HE05) together with a homemade sample stage anda radiation shield to construct a UHV-compatible cryostat that allowed sample cooling tobelow 4 K during TOF-SIMS measurement as well as the quench condensation of noblegases. The sample temperature was measured by a silicon diode sensor (Lake Shore, DT-670-SD-1.4H) placed near the sample.III. RESULTS AND DISCUSSIONFigure 1 shows the temperature-programmed TOF-SIMS results on the quenchcondensed- Ne film. The Ne+ intensity is plotted as a function of the increasing sampletemperature for different exposures from 0.3 L to 1000 L. The features of the profile aresummarized as follows. In the low exposure below 3 L, the Ne+ signal vanishes at about22 K (position (i)). The increase of the exposure to 10 L causes the appearance of a new4structure at about 7 K (position (ii)). This new structure changes the shape of the profilewith further increase of exposure: it shows a simple decrease of intensity with the increaseof temperature at position (ii) below 100 L, while above 300 L, it consists of the oppositeincrease of intensity at position (ii) followed by the decrease of intensity at a higher tem-perature. Thus, the intensity variation above 300 L forms a peak starting at position (ii).It is noted that the onset temperatures at which the intensity change starts above 10 L areidentical (position (ii)) between the different exposures.The tendency of the temperature-programmed TOF-SIMS profile summarized above for Neis quite similar to that observed for solidified hydrogen films in our previous study [7]. Inthe case of the hydrogen, the structure at position (i) was attributed to the desorption ofadatoms directly contacting the substrate surface, while the structure at position (ii) wasattributed to the sublimation from the quench-condensed film. The sublimation tempera-ture of Ne reported for the pressure of 10−6 − 10−7 Pa under which the measurement wasperformed in the present study agrees with the temperature of position (ii) [16]; therefore,the structure at position (ii) is attributed to the sublimation. On the other hand, from thesimilarity to the hydrogen case, it is likely that the structure (i) is due to the elimination ofthe Ne adatoms from the Ta surface by desorption.Figure 2 shows temperature-programmed TOF-SIMS profiles on the quench-condensed Arfilm. The essential features of the profile are very similar to those observed for Ne, namelythe signal disappearance at position (i) and the intensity variation starting at position (ii).The intensity variation at position (ii) is dependent on the exposure as similar to Ne: itsimply decreases as the temperature at position (ii) in the initial stage while it oppositelyincreases and forms a peak at larger exposures. The temperature of position (ii) again agreeswith the reported sublimation temperature for Ar [16].Figure 3 shows temperature-programmed TOF-SIMS profiles on the quench-condensed Krfilm. Again, the profile shows a similar trend to that observed for Ne. The temperature ofposition (ii) agrees with the reported sublimation temperature for Kr [16].The full mass spectra are shown in Fig. S1 of supplementary, where the Ne+/Ar+/Kr+5FIG. 1: Temperature-programmed TOF-SIMS profiles of the Ne+ ion measured on the Ne filmquench condensed on the Ta substrate. The Ne film was grown by exposing the Ne gas to the Tasubstrate below 4 K, which was immediately followed by the temperature-programmed TOF-SIMSmeasurement performed by increasing the sample temperature at a constant rate of 2 K/min. Allprofiles are shown in the linear scale in (a) and the selected profiles (0.3 L and 1000 L) are shownin the logarithmic scale in (b).peaks are clearly observed. It has been reported in the paper by Wilson [17] that the SIMSrelative sensitivity factors (RSF) of Ne, Ar and Kr are quite high; hence the ion yields ofthese noble gases are very low. The RSF reported by Wilson are for implanted standardsprepared in Si, GaAs and diamond. Therefore, the reported RSF for Ne, Ar and Kr arefor these noble gases implanted in Si, GaAs or diamond. The situation is different in thepresent study, where the ionization of the noble gases takes place on the solid film of the6FIG. 2: Temperature-programmed TOF-SIMS profiles of the Ar+ ion measured on the Ar filmquench condensed on the Ta substrate. The Ar film was grown by exposing the Ar gas to the Tasubstrate below 4 K, which was immediately followed by the temperature-programmed TOF-SIMSmeasurement performed by increasing the sample temperature at a constant rate of 2 K/min. Allprofiles are shown in the linear scale in (a) and the selected profiles (0.3 L and 1000 L) are shownin the logarithmic scale in (b).noble gases themselves. It is believed that this difference, related to the matrix effect in theionization mechanism, gives a large difference in the ionization yield of noble gases.From the series of measurements shown in Figs. 1 to 3, it is concluded that there are twofeatures in the temperature-programmed TOF-SIMS profiles on a quench-condensed noblegas film: the intensity decrease due to the desorption of noble gas adatoms from the surface7FIG. 3: Temperature-programmed TOF-SIMS profiles of the Kr+ ion measured on the Kr filmquench condensed on the Ta substrate. The Kr film was grown by exposing the Kr gas to the Tasubstrate below 4 K, which was immediately followed by the temperature-programmed TOF-SIMSmeasurement performed by increasing the sample temperature at a constant rate of 2 K/min. Allprofiles are shown in the linear scale in (a) and the selected profiles (0.3 L and 1000 L) are shownin the logarithmic scale in (b).at position (i) and the exposure-dependent intensity variation due to sublimation at position(ii). The temperature of position (i) is higher than that of position (ii). This is probablydue to the smaller interatomic bond strength in the solidified noble gas film compared tothe noble gas adatom-Ta surface bond.Figure 4 shows the TOF-SIMS intensities of Ne+, Ar+, and Kr+ in addition to that of the8Ta-derived monocation as a function of the exposure in measurements separate from thoseof Figs. 1 to 3. The Ta-derived +1 ions are ａ monocation of Ta bound to contaminantmolecules still present on the surface after surface cleaning, such as hydrogen, water andhydrocarbons, in addition to Ta+.The intensity variations are similar between Ne+, Ar+, and Kr+. They show a rapid in-crease in the initial stage of the exposure, which reaches the maximum below 100 L. Furtherincrease of the exposure reduces the intensity of Ar+, while the intensities of Ne+ and Kr+remain almost the same. On the other hand, the intensity of the Ta-derived monocationincreases steeply in the initial stage of the exposure. The exposure amount for the maximumof the Ta-derived monocation agrees with those for the maximum of Ne+, Ar+, and Kr+.Further increase of the exposure drastically decreases the Ta-derived monocation intensity,which vanishes at very large exposures such as 1000 L.It is observed that the Ta-derived monocation intensity is quite small on the clean surface,which increases with the exposure of Ne, Ar, and Kr. This indicates that the secondary ionemission of the Ta-derived monocation is enhanced by the adsorbed Ne, Ar, and Kr on thesurface.It is well-known that the adsorbed molecule by quench condensation forms the low-densitysherbet-like structure because the molecule can not migrate on the surface at low tempera-tures. This sherbet-like film of the noble gas likely does not completely cover the Ta surfaceat the large exposure of 100 L because the Ta-derived monocation signal is detected at thisexposure. The vanish of the Ta-derived monocation signal at larger exposure as 1000 L indi-cates that the Ta surface is fully covered by the solidified noble gas film with this exposure.The TOF-SIMS intensity I is generally dependent on the surface density of the precursorion Cj asI = IpSKjηCj, (1)where Ip the primary ion beam current, S the sputtering yield, Kj the ionization rate, and ηthe instrumental function. Assuming these parameters are constant except for Cj before and9FIG. 4: (Color online) TOF-SIMS intensity of (a) Ne, (b) Ar, and (c) Kr (solid black squares) andTa-derived cations (open red circles) as a function of exposure below 4 K. The data are obtainedfrom those shown in Figs. 1, 2, and 3 at 4 K.after the structural phase transition, the TOF-SIMS intensity changes at a phase transitionthat is normally accompanied by the change of density.The noble gas atoms are stuck at the first position they reach on the surface, which is themechanism of low-density film formation. It is noted that re-adsorption of the sublimatedmolecules smooths the rough surface, hence the density of the film increases. [7] Therefore,10the quench-condensed molecular film is densified at the sublimation temperature while thetotal number of molecules in the film decreases due to desorption. We consider that this isthe mechanism for the appearance of the peak at position (ii). In other word, the intensityvariation observed at position (ii) is considered as a result of two competing effects: theincrease of the film density and the decrease of the surface coverage.As stated in the experimental section, all TOF-SIMS measurements were performed afterrecovery of the post-exposure vacuum below 1×10−6 Pa. Thus, the initial vacuum conditionswere almost the same between the TOF-SIMS measurement with different exposures. How-ever, during the temperature programmed measurements, the partial pressure of the noblegas varied due to the desorption from the sample surface and evacuation by the vacuumpump. Since the amount of the noble gas adsorbing on the surface depends on the exposure,the vacuum behavior during the temperature programmed measurements differed betweendifferent exposures. Therefore, the balance between the desorption and the re-adsorption ofthe noble gas atom which is the mechanism of the TOF-SIMS intensity variation at position(ii) in Figs. 1, 2, and 3 depends on the exposure. This may be related to the exposuredependent profile shape of the temperature programmed TOF-SIMS at position (ii).It is observed in Figs. 1, 2, and 3 that the temperature at which the TOF-SIMS intensitystarts to decrease shifts to the low temperature side with increasing the exposure in the rangeof 0.3-3 L. As mentioned above, the TOF-SIMS intensity above the sublimation temperatureis considered to be determined by two competing effects: the densification of the film andthe decrease of the surface coverage. Thus, the flat profile of the temperature-programmedmeasurement suggests that these two effects are balanced. The decrease of the intensity isattributed to the imbalance between these two effects, indicating that the decrease of thesurface coverage is the primary factor affecting the profile shape. Since the average thicknessof the noble gas film decreases with decreasing the exposure, the temperature at which theimbalance begins shifts to lower temperatures with decreasing the exposure.Surface melting of solidified noble gases has been claimed experimentally by several groups,although we are not aware of recent studies on this topic. [15, 18–21] The surface meltinghas often been analyzed using powder samples to enhance the surface volume ratio, although11the melting effect may be affected by the confinement effect [22]. Since the sublimation tem-perature observed in the present study is consistent with the desorption energy from thesolid surface as reported by Ferreira et al [16], the possibility of the surface melting in priorto sublimation seems less likely.IV. CONCLUSIONWe applied cryogenic TOF-SIMS analysis to quench-condensed noble gas (Ne, Ar, and Kr)films grown on a polycrystalline Ta foil. The temperature-programmed TOF-SIMS profilewas sensitive to the surface structure of the film, which was found to show two prominentfeatures. One is due to the sublimation of the film and the other is due to the eliminationof the noble gas adatom from the surface by the desorption. Therefore, it is suggested that(cryogenic) TOF-SIMS is useful for analyzing the surface solid-to-gas phase transition atthe local point defined by the primary ion beam.AcknowledgementThis work was partly supported by JSPS KAKENHI Grant No. 15K13366 and theInnovative Science and Technology Initiative for Security, ATLA, Japan, Grant NumberJPJ004596.12[1] N. Vinx, P. Leclere, C. Poleunis, A. Delcorte, P. Mathieu, D. Cossement, R. Snyders, D. 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