790
Anal. Chem. 1983, 55,790-792
characterization, spectroscopy can provide valuable information aboutthe processes to aid in enhancing selectivity of chromatographic separations. - -
LITERATURE CITED Snyder, L. R. Anal. Chem. 1974, 46, 1384. Jaroniec, M.; Rozylo, J. K.; Golklewlcz. W. J . C h r m t o g r . 1979, 178,
(12) Petruska, J. A. I n “Systematlcs of Electronic Spectra of Conjugated Molecules”; Platt, J. R., Ed.; Wlley: New York, 1964. (13) ~ ~ a c ~R.~ N.~ l“The l , ~~~~l~~ Transform and itS Appllcatlons”; McGraw-HIII: New York. 1978. (14) Ogan, K.; Katz, E.; Siavin, W.-Anal. Chem. 1979, 51, 1315. (15) Jorgensen, W. L. J . Am. Chem. SOC.1980, 102, 543. (16) Maglni, M.; Paschlna, G.; Piccaluga, G. J . Chem. Phys. 1982, 77, 2051.
Mary J. Wirth* David A. Hahn Ronald A. Holland
27.
Boehm, R. E.; Martire, D. E. J . fhys. Chem. 1980, 84, 3620. Lochmuller, C. H.; Marshall, D. 6.; Wlider, D. R. Anal. Chlm. Acfa 1980, 130, 31. Lochmuller, C. H.; Marshall, D. 6.; Harrls, J. M. Anal. Chlm. Acfa 1981, 731, 263. Colin, H.: Guiochon. G. J . Chromatoor. 1977. 141. 289. Sleight, R. B. J . Chromatogr. 1973,-83, 31. Wise, S. A.; Bonnett, W . J.; Guenther, F. R.; May, W. E. J . Chromatogr. Scl. 1981, 19, 457. Schweizer, K. S.; Chandler, D. J . Chem. Phys. 1982, 76, 2296. Oxtoby, D. W. J . Chem. Phys. 1981, 74, 5371. Qeorge, S. M.; Auweter, H.; Harrls, C. B. J . Chem. Phys. 1980, 73,
5573.
Department of Chemistry University of Wisconsin-Madison Madison, Wisconsin 53706
RECEIVED for review September 3,1982. Accepted December 22, 1982* We gratefully the research Of the National Science Foundation.
Investigation of Adsorption of Benzene on Nickel(001) by Secondary Ion Mass Spectrometry Sir: It is now possible, using secondary ion mass spectrometry (SIMS) or fast atom bombardment (FAB), to form gas-phase molecular ions directly from organic and inorganic solids (1-3). This ionization scheme, where the sample is bombarded by a particle with 1-5 keV of kinetic energy, has proven particularly useful for the analysis of thermally labile organic solids such as amino acids (I,4 ) and for the characterization of biomolecules with molecular weights io the 1000-10000 dalton range (5). The mechanism of ejection of these large molecular species has been the subject of numerous speculations since it appears improbable at first thought that a large organic molecule held together by tens of electronvolts of energy could withstand impact by a 1000-eV primary particle. Although no rigorous approach has been developed to fully explain the observations, a number of conceptual advances in understanding the ejection mechanism have been made utilizing classical dynamics calculations to model the dissipation of energy of the primary particle (6). For benzene adsorbed on Ni(001) for example, molecular ejection is favored since (i) the primary ion energy is rapidly dissipated in the solid such that atomic collisions with the benzene molecule are often only a few electronvolts, (ii) the organic molecule possesses many internal vibrational modes which can absorb excess energy from a violent collision, and (iii) the metal substrate atom is larger than a carbon atom which allows it to strike two or three carbon atoms simultaneously, pushing the entire molecule in one direction (6). It remains to be shown how many of these concepts are extendable to molecules of much higher molecular weight. In this paper, we present preliminary SIMS experimental resulta for the adsorption of benzene on Ni(001) with the goal of comparing our observation to the previous benzene/Ni(001) classical dynamics calculations (6, 7). The results show that, in agreement with studies by other techniques, that the adsorption occurs molecularly at room temperature reaching monolayer coverage after approximately 1langmuir benzene exposure. In this environment, the only molecular ion that is observed with significant intensity is NiCsHe+,in contrast to the very complex spectra found from Ni surfaces exposed to much higher quantities of benzene. We also present evidence for the mechanism of cationization of benzene by Ni, which is predicted to occur via recombination of benzene with 0003-2700/83/0355-0790$01 S O / O
Ni+ ions over the surface of the solid. In general, this model system is utilized to better understand the SIMS spectra of organic solids and to evaluate the importance of various experimental parameters on the quality of the mass spectra.
EXPERIMENTAL SECTION Experiments were performed in an ion-pumped stainless steel torr after vacuum chamber with base pressure of 3.0 X bakeout. The primary ion was generated by a Physical Electronics sputter gun that had been modified to be differentially pumped by a 6-in. trapped diffusion pump. The gun could produce an ion beam with energies ranging from 75 to 1500 eV. With this arrangement we could perform SIMS measurements at an optorr with the ion gun normally erating pressure of 1.0 X operated at 3 X lo4 torr of Ar. An Extranuclear 162-8quadrupole mass spectrometer was employed to perform the secondary ion detection. The spectrometer was fitted with a Bessel box type energy analyzer with energy resolution typically between 1and 2 eV. In addition, the filter could be electrically scanned for use in observing secondary ion energy distributions. The primary ion was incident on the sample at 45O and the analyzer was positioned to detect ejected ions at 45” with a half-angle acceptance of 16”. The signal was amplified by a Channeltron electron multiplier coupled to a PAR 1120 amplifier-discriminator. The nickel(001) crystal was cut and finally polished with 0.05-pm alumina to i0.5” orientation. Before the crystal was mounted, it was etched in a 31:1:5 mixture of concentrated HN03, H2S04,H3P04,and CH,COOH and rinsed in deionized water and ethanol. The crystal was then mounted on a sample holder that could be resistively heated to 1300 K as monitored by a chromel-alumel thermocouple. The sample gas consisted of spectra grade benzene (Fischer) which had been degassed through a series of freeze-pump-thaw cycles. The benzene was admitted through a variable leak valve, with the pressure measured by a nude Bayard-Alpert ionization gauge calibrated for air. Crystal cleaning was restricted to cycles of heating in oxygen (5 X lo-‘ torr, 1200 K,10 min) and hydrogen (1 X lo4 torr, 1200 K, 10 min) followed by sputtering and annealing at 1200 K. The surface was considered clean when no Of and NiO+ peaks could be observed in the SIMS spectra and when the Niz+/Ni+ratio approached 0.25 (8). RESULTS AND DISCUSSION The SIMS spectrum of the clean Ni surface is given in Figure la. Note that the only peaks other than Ni+ and Niz+ are the ubiquitous Na+ and K+ contamination peaks. This 0 I983 Amerlcan Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4,APRIL 1983
(a1
Nit
791
'f---;h 1 e
J
?l
0
lb) Ni*
r a LL IK
!i *
I
W
3
Figure 1. SIMS spectra for various coverages of benzene: (a) clean Nl(001);(b) 3 langmuir benzene exposure: (c) 4500 langmulr benzene exposure; (d) frozen benzene from ref 12. All spectra except (d) were obtained at 298 K using 3 nAlcm2of 1 keV Ar+ Ions. surface was then exposed to a dose equal to 3 L of benzene at room temperature resulting in the SIMS spectrum shown in Figure lb. As a result of the chemisorption of benzene, the Ni+ and Ni2+signals are greatly enhanced with respect to the clean surface. In addition, peaks beginning a t mass 136 are also observed and are assigned to the cationized species NiC6H6+. No other significant ions in the mass spectrum are found below mass 200. These spectra persist in a qualitatively similar way for all exposures between zero and several hundred Langmuir exposure. As illustrated in Figure 2, the Ni+ and NiC6H6+ion yields both exhibit a rapid initial rise followed by a saturation near 1.0 langmuir. A saturation point at 3 langmuir exposure has been observed for benzene on Ni(001) by electron energy loss spectroscopy (EELS) and corresponds to the 4 4 X 4) ordered overlayer structure observed by LEED (9, 10). The EELS measurements as well as chemical measurements which show (11) that molecular benzene is displaced from Ni(001) by P(CH3)3at room temperature suggest that the adsorption occurs associatively. Thermal desorption studies associated with our SIMS experiments show loss of benzene from the surface at -500 K, further suggesting that the same benzene overlayer is being characterized by all three methods. Although there are no major changes in the types of observable cluster ions ejected from the benzene surface at monolayer coverage or below, the situation alters dramatically as the coverage is increased beyond the monolayer point. After exposures of Ni(001) to 4500 langmuir benzene, for example, as shown in Figure IC,the NiC6H6+peak is strongly attenuated, while fragment peaks at mass 15 (CH3+),27 (C2H3+),29
.IO
.36
90
1.25 EXPOSURE (L)
Figure 2. Normalized intensity plot for (-) benzene exposure.
------ __
1.80 2100
4500
NI+ and (---) NiC,H,+ vs.
(C2H5+),39 (K+,C3H3+),41 (K+,C3H5+),and 43 (C3H7+)become quite prominent. The presence of C3H3+and C3H5+ fragments at mass 39 and 41, respectively, is suggested by the change in the isotopic ratio normally expected for K+ alone. Note that the spectrum obtained at 2100 langmuir (7) is nearly identical with the one shown in Figure ICexcept that the weak NiC6H6+ion signal not observed previously has now been extracted from the noise. The exact nature of the surface composition is not clear at this stage. Apparently, several layers of benzene condense on the Ni surface. This condensation does not proceed indefinitely, however, since Ni+ and Ni2+ion signals are still observed. The increased complexity of the mass spectrum is perpetuated as the amount of benzene is increased further as illustrated by Lancaster and co-workers (12)who have reported the spectrum of frozen benzene, reproduced in Figure Id. This spectrum, obtained at 77 K, exhibits intense fragment peaks, the parent ion C6H5+,and numerous clusters containing greater than six carbon atoms. Many of the low mass fragmentation peaks are of different relative intensity than those in Figure IC.The series of spectra shown in Figure lb-d, then, clearly reveal that there is a dramatic, matrix-dependent alteration in the fundamental character of the benzene SIMS spectrum. It is possible to gain a certain level of understanding of the molecular ejection mechanism by focusing on the 4 4 X 4) structure and by comparing our results to results of recent classical dynamics calculations for the same system (7). At the initial level of comparison, the classical dynamics model appears to be quite helpful. It predicts that the major species which ejects, other than Ni, is the molecular benzene itself with very small contributions from the C,H, (with n < 6) fragments. In addition, a sizable component of the spectrum is predicted to be NiC6HG. This organometallic species is found to form via a recombination of ejected Ni atoms and benzene molecules above the surface while still interacting with the solid. In addition, the model predicts that most of the benzene molecules (-75%) do not have sufficient internal energy to decay unimolecularly during the several microsecond passage to the detector. As shown in Figure lb, our experimental spectrum agrees in many respects with the predicted spectrum. The major difference is that the molecular ions C6H6+or C6H6-are not observed. A similar observation was made for CO adsorbed on Ni(001) where no CO+ (or CO-) could be detected but where a large NiCO+ signal was observed (13). This discrepency is due in part to the relatively large difference in ionization potential between Ni and C & + This difference implies that the number of C6H6+ions (IcBH6+) relative to the number of Ni+ ions IN^+) will be very small as indicated by an expression of the form (14) 1C6He+/1Ni+
= exp[(IPNi - I P C , H , ) / d
where I P N i and IPc,, are the respective ionization potentials of 7.6 and 9.3 eV, and t o is a parameter generally between 0.25 eV and 0.5 eV (15). For this case, assuming that the yields
792
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
provides additional evidence that the cationization reaction occurs via a recombination process. Finally, it is important to note that the NiC6H6+ion maximum intensity corresponds to the 4 4 X 4) structure. Presumably at lower coverages, the Ni+ ion yield has not attained its maximum value and hence limits the intensity of the NiC6H6+species. At greater than monolayer coverages, the Ni yield is diminished since Ni atoms cannot penetrate through the overlayer to eject. This result is important to consider when utilizing cationization to enhance the yield of molecular ions of solid compounds by preparing thin films of the sample on a metal foil.
ENERGY e V
Figure 3. Normalized secondary Ion energy dlstrlbutions for: (---) NI+ clean surface: (0)NI+ 3 langmuir benzene: ( 0 )NIC6H6+ 3 langmulr
benzene.
of Ni and C6H6are of the same order, Ic&+/IN~+ g so that virtually all the C6H6molecules should be ejected as neutrals. A similar agreement applies to the ejection of negative ions. By following the predictions of the classical dynamics calculations and by considering experimental evidence from other systems (16),the cationization product is almost certainly formed between an ionized Ni atom and a neutral benzene molecule as Also, in agreement with the calculations, there is very little fragmentation observed in the experimental spectrum. With this insight, it is possible to speculate about the reasons for the strong matrix effects observed when the benzene concentration is increased beyond the monolayer point. In this case, the benzene matrix itself, rather than the metal lattice, must dissipate all of the kinetic energy of the incident particle. It is reasonable to assume that these energetic collisions will lead to numerous fragments which further scramble by recombination reactions above the surface. For metallic systems these collisions occur mainly between metal atoms and only a few actually involve the benzene molecule itself. The argument that the classical dynamics calculations provide a reasonable description of the nuclear motion that leads to benzene ejection is also enhanced by examining the kinetic energy distributions of the ejected species. To make a comparison between the calculated distributions of the neutrals and the measured distributions of the *ions, it is necessary to consider the influence of the image charge created in the metal by the departing ion. For the clean surface, the Ni+ ion distribution, as shown in Figure 3, is much broader than that calculated for the neutrals as presented in ref 7 . The broadening has been shown to arise, in part, from the increased attraction of -3.6 eV between the ion and the metal due to the image force (17). When the Ni surface is exposed to benzene, the calculated neutral energy distributions for both Ni and NiC6H6are very similar to the measured distributions (see Figure 6b of ref 7 ) for Ni+ and NiC6H6+assuming an image energy of 1.8 eV. The decrease in this value presumably arises due to screening of the image charge by the benzene overlayer. This effect also provides an explanation for the sharpness of the energy distributions of the benzene covered surface relative to the clean surface. Further, the fact that the experimental energy distributions for Ni+ and Nice&+ are very similar to the corrected calculated distributions
CONCLUSIONS From this SIMS investigation of the adsorption of benzene on Ni(001) at room temperature we have found (i) the chemisorption of benzene on Ni(001) appears to form a stable superstructure on the surface as indicated by the saturation of the NiC6H6+signal. From the presence of this adsorbed structure we observe a SIMS spectrum with very different character to that reported in a previous study for frozen benzene. (ii) A condensed phase of benzene can be initiated at room temperature on the surface by increasing the dose. From this we see a spectrum approaching the spectrum of frozen benzene. (iii) Preliminary evidence from the comparison of experimental energy distributions of NiC6H6+and Ni+ with the classical dynamics model of this system implies that the formation of NiC6H6+proceeds via Ni+ and C6H6 recombination above the surface to produce the cationized species NiC6H6+. Registry No. Benzene, 71-43-2;nickel, 7440-02-0. LITERATURE CITED Bennlnghoven, A.; Jaspers, D.; Slchtermann, W. K. Appl. Phys, 1976, I. f. ,. 35. Pierce, J.; Busch, K. L.; Walton, R. A.; Cooks, R. G. J . Am. Chem. Sac. 1081, 103, 2583. Barber, M.; Bordoll, R. S.;Sedgewick, R. D.; Tyler, A. N. J . Chem. SOC..Chem. Commun. 1061. 325. Grade, H.; Wlnograd, N.; Cooks, R. G. J . Am. Chem. SOC.1077, 99, 7725. Wllllams, D. H.; Bradley, C.; Bojeson, G.; Santlkarn, S.;Taylor, L. C. E. J . Am. Chem. SOC. 1981, 103, 5700. Garrlson, B. J. J . Am. Chem. SOC.1980, 102, 6553. Garrlson, B. J. J . Am. Chem. SOC. 1982, 104, 6211. Flelsch, T.; Wlnograd, N.; Delgass, W. N. Surf. Sci. 1978, 78, 141. Bertollnl, J. C.; Rousseau, J. Surf. Sci. 1978, 89,467. Bertollnl. J. C.; Dalmal-Imellk, G.; Rousseau, J. Surf. Sci. 1977, 6 7 , 476. Frlend, C. M.; Muetterties, E. L. J . Am. Chem. Sac. 1981, 103, 773. Lancaster, G. M.; Honda, F.; Fukuda, Y.; Rabalais, J. W. J . Am. Chem. SOC. 1070, 101, 1951. Flelsh, T.; Ott, G. L.; Wlnograd, N.; Delgass, W. N. Surf. Scl. 1978, 78, 141. N~rrskov,J. K.; Lundqulst, B. I.Phys. Rev. B : Condens. Matter 1970, 19, 5661. Yu, M. L. Phys. Rev. Lett. 1981, 4 7 , 1325. Wlnograd, N.; Garrlson, B. J.; Flelsch, T.; Delgass, W. N.; Harrison, D. E. Jr., J . Vac. Scl. Techno/. 1079, 76, 629. Glbbs, R. A.; Holland, S. P.; Foley, K. E.; Garrlson, B. J.; Wlnograd, N. Phys. Rev. B : Condens. Matter 1081, 2 4 , 6178.
Nicholas Winograd* E. J. Karwacki The Pennsylvania State University 152 Davey Laboratory Department of Chemistry University Park, Pennsylvania 16802 RECEIVED for review October 25,1982. Accepted January 11, 1983. The financial support of the National Science Foundation, the Office of Naval Research, the Air Force Office of Scientific Research, and the donors of the Petroleum Research Fund, administered by the American Chemical Society, is greatly acknowledged.
