J. Phys. Chem. 1981, 85,3840-3844
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coadsorption are identical. With other hydrocarbons of higher desorption temperatures and, therefore, stronger bonding to the metal surface as the unsaturated hydrocarbons, the D2 TDS curves peak a t temperatures much lower than those corresponding to the same amount of D2 adsorbed on the clean Pt surface. The study of the adsorption of olefins on Pt(ll1) and its interaction with hydrogen will be reported in a subsequent publication.
Conclusion 1. At temperatures of 110 K, alkanes C4HZn+2with n 2 4 adsorbed readily on a Pt(ll1) surface. The main desorption peaks occurring at 166 and 195 K for C4H10 and C a l 2 , respectively. Desorption is a first-order process w t h the following kinetic parameters: v = 1011.0*'.7s-l and E = 8.2 f 1.2 kcal mol-' for C4H10; v = 1011.6&1.3s-l and E = 10.2 f 1.0 kcal mol-' for C6H12. Multilayers of hydrocarbons are formed at 110 K for high exposures.
2. The presence of patches of subsurface oxygen gives rise to new adsorption sites of lower binding energy. Saturation of the clean Pt regions and the subsequent formation of multilayers require lower hydrocarbon exposures in this circumstance. 3. Preadsorbed deuterium interacts repulsively with the adsorbed hydrocarbons, lowering its desorption temperature and resulting in the formation of multilayers a t exposures lower than those required on the clean Pt surface. At high deuterium coverages, a more rapid decrease in the binding energy of the hydrocarbon is observed which may be due to its adsorption on D-covered regions.
Acknowledgment. This work was partialy supported by the Spanish-American Cooperation Program, which we gratefully acknowledge, and by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S.Department of Energy, under Contract W-7405-ENG-48.
Secondary Ion Mass Spectrometry of Metal Halides. 1. Stability of Alkali Iodide Clusters T. M. Barlak, J. E. Campana," R. J. Colton, J. J. DeCorpo, and J. R. Wyatt Naval Research Laboratoty, Chemistry Division, Washington, D.C. 20375 (Received: May 20, 1981; In Flnal Form: August 6, 1981)
Secondary ion mass spectrometry (SIMS) by ion bombardment of alkali iodides (MI) results in cluster ions of the form [M(MI),]+. The secondary ion intensity distributions are reported to n = 22 for NaI and KI, n = 19 for CsI, n = 16 for RbI, and n = 2 for LiI by using a high-performanceSIMS instrument. The ion intensity distributions are discussed in terms of ion abundance measurements and cluster ion formation and emission. Anomalies observed in the ion intensity distributions appear to be dependent on cluster ion stability. Hypothetical cluster ion structures are proposed to correlate structural stability with the enhanced cluster ion intensities at a few n values. Two unimoleculm decompositionsof metastable secondary cluster ions me studied and reported for the first time in SIMS.
Introduction Clusters are microscopically small atomic aggregates with physical properties so unlike other forms of matter that they have been called the "fifth state of matter".' Clusters play a central role in the vapor-to-condensed phase transition of all matter. Such transitions were initially investigated in the atmospheric sciences where the bulk of early cluster research was related to water nucleation. Renewed cluster research received impetus from the importance of clusters in a wide variety of industrial processes. For example, metallic clusters are important in the fabrication of solid-state devices and are useful as heterogeneous catalysts. Cluster nucleation effects are of fundamental importance in the operation of engines, turbines, boilers, etc. In addition, the exhaust gases from these energy converters are rich in cluster species. Although the importance of these clusters in our environment continues to be assessed, little is known about the structure, the stability and the thermodynamic properties of the fifth state of matter. During the development of a high-performance secondary ion mass spectrometry (SIMS) instrument, alkali halide cluster ions of the form [M(MX),]+ were observed. It was first observed with our instrument that the overall (1) Stein, G. D. Phys. Teach. 1979, 17, 503-12.
cluster ion intensity distribution falls off with increasing n values and that at certain n values the ion intensity is enhanced or reducedS2 Previous studies of these alkali halide ion clusters to intermediate values of n (e.g., to n = 13 for NaF) show a monotonic decrease in ion intensity vs. n and indicate that the cluster ions are table.^-^ The anomalous behavior of the ion intensity distributions obtained with the high-mass analysis capability of our SIMS instrument prompted further investigation of the alkali iodides. The purpose of this paper is to report the alkali iodide cluster ion intensities of the extended-mass species obtained by SIMS and to offer interpretation of these data.
Experimental Section Alkali iodide samples were prepared by grinding reagent-grade salt crystals into fine powders and pressing pellets from this powder. Hydraulic pressures of 1.1X loe N/m2 (1.5 X lo4lb/in.2) were used to form 1.3-cm diameter pellets of 1-2-mm thickness. The pellets were mounted (2) Colton, R. J.; Campana, J. E.; Barlak, T. M.; DeCorpo, J. J.; Wyatt, J. R. Reu. Sci. Instrum. 1980, 51, 1685-9. (3) Taylor, J. A.; Rabalais, J. W. Surf. Sci. 1978, 74, 229-36. (4) Honda, F.; Fukuda, Y.; Rabalais, J. W. J. Chem. Phys. 1979, 70, 4834-6. (5) Honda, F.; Lancaster, G. M.; Fukuda, Y.; Rabalais, J. W. J. Chern. Phys. 1978,69, 4931-7.
This article not subject to U S . Copyright. Published 1981 by the American Chemical Society
Secondary Ion Mass Spectrometry of Metal Halides
on Ag foil surfaces by wetting the Ag-pellet interface with water and drying the composite, or the pellets were mechanically attached to the sample carousel. Other samples were prepared by evaporation of aqueous salt solution directly on the Ag foil. The cluster ion intensity ratios were within experimental error regardless of the sample preparation procedure. Target samples were mounted on a twelve-sided turret which was placed in an ultrahigh-vacuum (UHV) chamber. The chamber was evacuated to lo4 torr before beginning ion bombardment. The primary ion beam source (Kratos/3M Model 432-1 minibeam ion gun, Minneapolis, MN) was maintained at 5 kV, and a 1-kV potential was applied to the sample carousel (secondary ion acceleration voltage) so that the resultant primary ion beam energy was 4 keV. The angle of primary ion beam incidence was 60” to the sample surface normal. The Ar+ ion beam fluxes used in these experiments were 10-6-10-4 A/cm2. Assuming a sputter coefficient of 1,these beam fluxes correspond to monolayer lifetimes of 0.1-10 s, i.e., the condition for “dynamic” SIMS. We found no major difference between the cluster ion ratios of NaI in the dynamic or “static” modes of operation. The SIMS instrument, which was constructed in this laboratory, is based on a double-focusing mass analyzer and does not have the energy and mass dependent ion transmission effects which are characteristic of quadrupole SIMS instrumentation.2 The energy bandwidth of this instrument is 16 eV for 1000-eV ions; thus, all ions within the energy bandwidth are focused at the collector. The ion transmission (secondary ions detected/secondary ions emitted) of this instrument was measured as 1 X lo4. The ion detector is a continuous dynode particle multiplier (Channeltron, Galileo-Electro Optics Corp.) coupled with a pulse-counting system (Ortec 9000 series). The pulsecounting system could measure ion signals over 6 orders of magnitude. The energy of the secondary ions impacting the dynode of the particle multiplier was 4.3 keV (1-keV secondary ions and a -3.3-kV multiplier high voltage for positive ion detection). Measurement of ion signals (e.g., M+, [M(MI),]+, ...) differing in intensity by greater than lo6was accomplished by defocusing the ion beam and measuring the series of extremely intense cluster ions. As the cluster ion size increased, the ion signals reached acceptable levels at which time the ion optics were “retuned” to fully transmit the ion beam. The previous few ion intensities in the series were then remeasured to establish a scaling factor between the “focused” and “unfocused” measurements. Using such an approach, we recorded CsI cluster ion intensities ranging over 9 orders of magnitude. Typical count rates from the RbI experiment ranged from 8 X lo6 counts per second (cps) for Rb+ to -1 cps for [Rb(RbI)16]+.The total “noise” level measured between [Rb(RbI),]+ peaks at m/z -3500 was estimated a t -0.3 cps. The data presented here are an average of the peak heights of several mass spectra.
The Journal of Physical Chemistry, Vol. 85, No. 25, 1981 3841
I
-
Results The largest observed cluster ion from CsI corresponded to the n = 19 species at m / z 5069; this mass limit was imposed by the maximum magnetic field (1.1 T) and a 1-kV secondary ion acceleration voltage. The largest observed cluster ion for the other alkali iodides was determined by the minimum detectable signal, which is dependent on secondary ion yield, cluster ion stability, and instrumental artifacts. The largest observed n value for NaI and KI was n = 22, while for RbI this values was n
~
-6’o0
2
4
6
I
869
8
_LLu
10 12 14 16 18 20 22 n-
1,699 mlz-
, ,-
2,529
3,359
Figure 1. SIMS spectrum of K I cluster Ions, [K(KI),]+, to n = 22.
1
2
6
10
,
,
20 30
LOG n-
Flgure 2. Log-log plot of the normalized ion intensity I(n) for KI cluster ions vs. the cluster number n. The line represents the linear leastsquares fit.
TABLE I: Values of A and B from Least-Squares Fit of D a t a t o log I , = A t B log n for the Alkali Iodides A B LiI +o.oo - 5.78 Na I -0.07 -4.58 KI +0.26 -4.54 RbI t 0.49 -4.50 CSI t 0.43 -5.70
= 16. Cluster ions from LiI were observed only for n = 1, 2. Figure 1 summarizes the KI mass spectrum (mass
range, ca. m / z 3700). Log-log plots of ion intensity vs. n (log 1,/11 vs. log n) decrease linearly and are in agreement with previously published results.6 The linearity of the logarithmic data is empirical in that no quantitative theory exists. However, the slope of the linear least-squares fit (Le., B in log I , = A + B log n) is a relative measure of the extent and distribution of the alkali iodide cluster ions. Values of A and B for the alkali iodides are presented in Table I. Atomic ions (i.e., M+) were excluded from the log-log plots. The broad energy spectrum of atomic ions (e.g., ~ result 9.0-eV full width at half-maximum for C S + )may in some energy discrimination even with the 16-eV energy bandwidth of our mass spectrometer. In contrast the [M(MI),]+ ions for n = 1 are sharply peaked at low energy (- 1 e V 5 so that energy discrimination is reduced. Differences in the ion energy spectrum of the atomic and molecular species are indicative of the ionization and emission processes.‘j (6)Wittmaack, K. Phys. Lett. A 1979, 69, 322-5.
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Barlak et al.
T A B L E 11: Log Relative Intensities (log I,/I,,) for the Alkali I o d i d e s
to (mass)-1/2,the velocities of all cluster ions except [M(MI),]+ are below the velocity threshold for kinetic emission (5.5 X lo4m / ~ ) This . ~ threshold has been shown n LiI NaI KI RbI CsI to apply to metal cluster ions.’l 1 0.00 0.00 0.00 0.00 0.00 Another ion-to-electron conversion mechanism is po2 1.14 1.26 0.98 0.52 1.34 tential emission. This Auger process requires that the 3 2.42 1.95 1.42 2.18 ionization potential of the ion exceed twice the work 4 2.83 2.61 1.94 2.86 5 3.53 3.03 2.11 3.51 function of the converter surface. Potential secondary 6 3.66 3.00 2.18 3.58 electron emission by alkali metals does not occur because 7 4.06 3.52 3.44 4.23 of the low ionization potentials of alkali metals.12 Unless 8 4.26 3.69 3.61 4.38 the work function of the continuous dynode multiplier is 9 4.60 3.84 3.12 4.40
