The Journal of Physical Chemistry, Vo/. 82, No. 7 1 , 1978 1295
Unusual Behavior of Vaporized Magnesium
(21) H. A. Laitinen, C. A. Vincent, and T. M. Bednarski, J . Nectrochem. SOC.,115, 1024 (1968). (22) K. Hauffe and U. Bode, Discuss. Faraday Soc., 68, 281 (1975). (23) P. Kirkov, Nectrochem. Acta, 17, 519 (1972). (24) V. Ya. Davydov, Trans. Faraday Soc., 61, 2254 (1964). (25) L. Balsenc, H. Berthou, and C. K. Jorgenson, Chimia, 29, 64 (1975). (26) M. P. Seah, Surface Sci., 32, 703 (1972). (27) N. R. Armstrong and R. Shepard, manuscript in preparation.
46, 391 (1973). (16) 1. M. Issa, R. M. Issa, M. Ghonheim, and Y. Temerk, Electrochem. Acta, 18, 265 (1973). (17) N. R. Bannerjee and A. S. Hegi, Electrochem. Acta, 335 (1973). (18) I. M. Chaiken and E. L. Smith, J. Bioi. Chem., 244, 5096 (1969). (19) J. H. Scofieid, J . Nectron Specfrosc., 8 , 129 (1976). (20) A. W. C. Lin, N. R. Armstrong, and T. Kuwana, Anal. Chem., 49, 1228 (1977).
Unusual Behavior of Vaporized Magnesium. 2. Evidence for Gas-Surface Exchange L. B. Knight, Jr.,* K. S. Stewart, and W. T. Beaudry Departmenf of Chemistry, Furman University, Greenville, South Carolina 296 73 (Received June 22, 1977; Revised Manuscript Recelved October 25, 7977) Publication costs assisted by the National Science Foundation
Previous experimental evidence indicated that vaporized magnesium atoms in the presence of trace quantities of certain impurity gases can exhibit a substantially reduced sticking coefficient. Magnesium isotopes have been used to determine if the mechanism responsible for producing the low sticking coefficient could involve exchange between gas phase and surface bound magnesium. Evidence for the occurrence of such exchange is discussed.
Introduction Experimental conditions have been recently reported which can cause vaporized magnesium atoms to exhibit a substantially reduced sticking coefficient.’ The simultaneous impingement of magnesium vapor and trace quantities of certain volatile species containing halogen atoms under high vacuum conditions can condition ambient temperature surfaces so that the sticking coefficient of vaporized magnesium atoms is reduced to such an extremely low value that magnesium behaves as if it were a semipermanent gas. The previous report has presented the detailed experimental evidence used to reach the above conclusion. The evidence included the establishment of the identity of the volatile magnesium species as atomic magnesium, the chemical specificity of the effect, and the determination of an approximate value for the sticking coefficient. The sticking coefficient value of 0.04 previously reported for the isolated glass bulb was calculated incorrectly. The original calculations failed to include the possibility of multiple surface collisions of a single atom. We are grateful to one of the reviewers for suggesting a reexamination of this parameter. The experimental data reported in Figure 6 of ref 1 have been employed to determine a revised sticking coefficient value of 1.3 X 10“. The details of this calculation are presented in Appendix
-
1. Efforts have been made to obtain information about the surface mechanism responsible for causing the reduced sticking coefficient. Isotopically labeled magnesium has been employed to determine whether or not exchange between impinging and surface bound magnesium occurs. The experimental results clearly indicate that significant exchange does in fact occur. Furthermore, the evidence seems to indicate that exchange occurs predominantly on glass surfaces of the apparatus. However, this conclusion is only tentative at this stage.
Experimental Section The surface exchange utilized the double cell vaporization assembly shown in Figure 1. Experimental pa0022-385417812082-1295$01.0010
rameters such as the vaporization temperatures, partial pressures of magnesium, and partial pressures of impurity gases are identical with those described in ref 1. This assembly is identical with the single cell vaporization system used previously except it contains two separately controlled tantalum Knudsen cells mounted on watercooled copper electrodes. The double cell flange simply replaces the single cell assembly shown in Figure 1of ref 1. For the surface exchange experiments described in the next section, the 5-L glass sphere was connected between the vaporization chamber and the TOF mass spectrometer via 15-mm teflon stopcocks. Natural magnesium (NatMg = 79% 24Mg,11% 26Mg,10% 25Mg)was loaded into the forward cell while 99.4% 2sMg obtained from the Oak Ridge National Laboratory was placed in the rear vaporization cell. The rear 2sMg cell was surrounded by a water-cooled copper jacket which helped to prevent 24Mg contamination of the 26Mgsource. Since such contamination would invalidate the isotopic exchange results, extensive efforts were expended to prevent the contamination initially and subsequently to prove, by the deliberate design of various experimental procedures, that contamination had not occurred.
Results The basic plan of the exchange studies involved generating volatile magnesium in the “normal” manner until the surfaces in the system were thoroughly conditioned with NatMgfrom the forward cell in Figure 1. The system was taken to be “conditioned” when the Mg+ mass spectrometric signals reached a steady value near maximum. If the system had been conditioned on a given day and no part of the vacuum system had been opened to atmosphere, the time required for subsequent conditioning was about 10 min. Once a NatMgconditioned system was achieved, the forward vaporization cell was switched off and the rear cell containing 2sMgwas switched on. It takes about 10 s for the NatMgcell to cool to the point where no Mg+ ion signal can be detected. Ordinarily, =90 s is the required time for these types of cells to reheat to a point 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 11, 1978
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L. B. Knight, K. S. Stewart, and W. T. Beaudry
DOUBLE CELL ASSEMBLY
24
MASS S E C T SfGNAL a-
SH I ELD
I!
I
_tL/ FLANGE
m
-
2
i 4
*
6
8
,
1 0 TIMEhn)
,
2
,
, 4
,
6
8
Figure 3. The decay of the 24Mgsignal with time as *'Mg only is vaporized. (See text.)
ELECTRODES Flgure 1. Double cell arrangement employed for the surface exchange studies. (See text.)
1
56
0
30
60
90
t20
150
TIME (min)
Flgure 2. The time dependence of the value of 28Mg/24Mgdetermined with the TOF mass spectrometer during 2BMgonly vaporization following conditioning with NatMg. (See text.)
where the Mg+ ion signal returns. After switching off the NatMgcell and switching on the 26Mgcell, the 26Mg/24Mg ion signals were continuously monitored. If no exchange occurred with surface bound 24Mg,the mass spectrometer should show a constant 26Mgsignal and practically no "Mg signal. In fact, a strong 24Mg signal was seen which gradually weakened as the z6Mg vaporization was continued. Both the 26Mgand 24Mgsignals were strongly dependent on the temperature of the 26Mgcell. A typical plot of 26Mg/24Mgvalues is given in Figure 2A. Note that the time scale is quite extended. Extensive experiments were conducted to ascertain that 24Mg did not contaminate the zeMg source during the conditioning period. The following type of blank was conducted to check for such contamination. NatMgwas vaporized from the forward cell and the conditions for generating volatile magnesium were maintained continuously at the maximum value for 10 h. The rear 26Mgcell did not contain magnesium during this blank experiment but was maintained at 200 "C which is approximately 150 "C below the point where the magnesium vaporization rate first produces a measurable Mgf mass spectrometer signal. Following the 10-h period the NatMgcell was switched off, the Mg' signal decayed to zero in 20 s, and the rear empty z6Mgcell was heated from 200 "C up to normal range for magnesium vaporization. In fact, on some blanks its temperature was slowly increased 100 "C above the normal range. This type of blank was repeated numerous times
with the result that no Mg' signal was detected when the empty rear cell was brought up to temperature. The exact same procedure, in every detail, was followed on the real experiments except that the rear cell contained the z6Mg sample. It should be pointed out that the ion signals measured in the mass spectrometer for the magnesium species are quite intense, usually several orders of magnitude above background. Also the measured signal ratio of 24Mgand 25Mg has the expected value when 26Mg is vaporized. The exchange experiments represented by the data in Figure 2A were repeated dozens of times under different experimental conditions such as length of the conditioning period, pressures, metal vaporization rates, choice of impurity gas, previous surface history, and varying lengths of time during which the bulb was isolated from the vaporization chamber during the conditioning period. In general, the value of 26Mg/24Mgincreased more rapidly upon 26Mgvaporization when the surfaces of the apparatus were not as thoroughly preconditioned with 24Mg. The rate of increase was also considerably greater if the "Mg vaporization was delayed several hours following the "Mg conditioning period. Figure 2B shows data taken from an exchange experiment where there was a 20-h delay in 26Mgvaporization. During the 20 h the impurity gas flow, CC14,was also stopped. No noticeable differences in exchange rates were noted for carbon tetrachloride and ethyl bromide as impurity gases. Data for a bulb isolation experiment are presented in Figure 3 where the 24Mg+signal is monitored vs. time during 26Mgvaporization. If the system was thoroughly conditioned on a given day and was not exposed to atmosphere, a conditioning time of approximately 10 min was required for the partial pressure of atomic magnesium to reach its maximum level. Figure 3A shows the 24Mg signal resulting from 26Mgvaporization after conditioning the entire apparatus, metal vaporization chamber and 5-L glass bulb, for 10 min. Note the high initial level and the marked decay of the 24Mgsignal with time as 26Mg is vaporized. Next, a similar 10 min conditioning period with 24Mgwas conducted but with the 5-L bulb isolated from the metal vaporization chamber, although the bulb was being continuously evacuated through the mass spectrometer. Immediately following the 10-min conditioning, the 24Mgcell was switched off, the 26Mgcell switched on in the usual manner, and the teflon stopcock isolating the 5-L bulb was opened, The resulting 24Mgsignal from this experiment, where the metal system was freshly conditioned but not the glass bulb, is shown in Figure 3B. In Figure 3B the 24Mgsignal has been amplified to a greater degree as is evident by the decreased signal-to-noise ratio. This was done so that the initial signal intensity would be similar to that in Figure 3A thus allowing for a more critical determination of the time rate of decay for the %Mgsignal.
Unusual Behavior of Vaporized Magnesium Note that the Figure 3B signal does not change significantly with time relative to Figure 3A. In fact the measured signal level of Figure 3B is approximately the same as that obtained.by extrapolating Figure 3A. (This fact would be more obvious from the figure if the signal amplification had not been increased in Figure 3B.) Since Figure 3B shows a much weaker 24Mgsignal during “Mg vaporization than Figure 3A and since 3B is approximately constant with time, the magnesium exchange appears to be dominated by exchange reactions in the glass bulb as opposed to the metallic part of the system. The increased surface area of the system when the bulb was included in the %Mgconditioning stage, Figure 3A, cannot account for the dramatic differences in the two curves. A series of experiments was conducted to determine if the rate of exchange could be influenced by increasing the temperature of the 5-L bulb. Decay curves such as the one discussed for Figure 3A above were obtained numerous times at T = 23 “C to check for reproducibility. Next the bulb was heated to 103 “C and the experiment repeated in a identical manner. The curves obtained were superimposable on those at 23 “C, hence no temperature effect was observed over this range. This could indicate that the exchange rate is limited by the rate of 26Mgimpingement. It would be interesting to lower the temperature below 23 OC to determine if a temperature dependence could be found which might enable a calculation of an exchange activation energy parameter.
Discussion It is interesting that the initial values of 26Mg/24Mg (henceforth referred to as R) are small. Although the representative data shown in Figure 2 indicate an initial ratio of approximately 4, the initial ratio on certain experiments approached 2. The small ratios are indicative of a large degree of magnesium exchange. The increase in the R value as z6Mg is vaporized indicates gradual depletion of exchangeable surface bound 24Mgand/or increasing amounts of surface bound z6Mg. In extremely well-conditionedsystems the variation of R with time was observed to be approximately linear over the entire time range. (See Figure 2A.) With poorly conditioned systems or with systems where a long delay occurred between the 24Mgconditioning period and the 26Mgvaporization, the variation of R with time was not linear over the entire time range. Under these conditions the slope for R vs. time over the first 30-40 min was a factor of 2 to 3 times greater than the slope eventually obtained after 50 min. However, the slopes obtained after 50 min on these poorly conditioned surfaces are in close agreement with those obtained with the thoroughly conditioned experiments. Hence in the early phases of the R.vs. time measurements with poorly conditioned systems the rapid depletion of potentially exchangeable surface bound 24Mgseems to dominate since a large amount of exchangeable 24Mg should not be present. For the later periods the increasing amounts of surface bound 26Mgcould be the dominant factor. This would qualitatively account for the observation that the slopes obtained for the well-conditioned systems were found to be similar to the slopes observed at later stages for poorly conditioned systems. The large initial slopes observed on those experiments where thorough conditioning with 24Mgwas conducted but where considerable time (-20 h) elapsed before 26Mg vaporization was begun would therefore be accounted for by instability of the surface bound magnesium. Either a chemical change occurs on the surface which strengthens the metal to surface bonding and hence decreases the probability of exchange or the surface active magnesium
The Journal of Physical Chemistry, Vol. 82, No. 11, 1978
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reacts with background gases such as 02 or H’O. The detailed surface mechanism has not been established by this work. However, the volatile metal effect (the reduced metal sticking coefficient) is consistent with the observation of gas-surface exchange. It is interesting that Knudsen in 1916 suggested the possibility of such an occurrence.’ “Let us assume that a small quantity of one of the metals with a low adhesion temperature is enclosed in a glass vessel and that the whole is heated to such a temperature that all the metal is evaporated. We then cool the vessel until its temperature is below the temperature at which the vapor is saturated. If now the probability that an impinging molecule adheres to the wall at the first impact i s nearly zero, then the possibility exists that a perceptible time will elapse before the condensation begins, and, during that time the vapor will be supersaturated. This supersaturation will be almost permanent if single molecules or small aggregates of molecules already condensed could be driven out by other impinging molecules.” The role of the background “impurity” gas which is necessary to cause the volatile effect is not clear. It could act as a simple dispersive agent on the surface to prevent the formation of large magnesium clusters. If this were the case, one would argue that without the gas, the clusters would rapidly grow to the point where surface bound magnesium would not undergo exchange with impinging atoms. This explanation assumes that sur€ace exchange is a necessary condition for maintaining a low effective sticking coefficient. The small initial R values previously discussed certainly indicates that exchange does occur to a degree large enough to significantly influence the effective sticking coefficient. Apparently, there has been no systematic investigation into the effects of “impurity” gases on sticking coefficients of metals. Experiments designed to study the effect of only residual gases on the measured condensation coefficients of silver found little or no effect in the range 4 X 10-5-6 X rnmHgs3The previous magnesium report indicated the chemical specificity regarding the impurity gases for magnesium. Earlier, some investigators attributed “critical phenomena” to the presence of surface films. “It is probable that transition (“critical temperature”) effects in the condensation of metal on metal as observed by Estermann, Cockcroft, and others are conditional by the presence of surface films of gas and vapor and disappear when such films are r e m ~ v e d . ” ~ It is also possible that the gas reacts with the magnesium to produce a surface bound magnesium compound capable of undergoing subsequent magnesium exchange. The experimental evidence does indicate that the surface conditioning phase does require that both gas and metal be simultaneously introduced for a significant time before the metal sticking coefficient is reduced to the point where Mg’ is detected in the mass spectrometer. XPS (X-ray photoelectron spectroscopy) studies are planned so that a more complete understanding of the surface mechanism can be obtained. It has also been suggested that “volatile magnesium” be introduced into an ICR (ion cyclotron resonance) apparatus so that kinetic studies involving the reactions of atomic magnesium with various organic moieties could be studied. The evidence that exchange was observed to occur to a significantly greater extent on the glass portions of the apparatus is not conclusive. It should be pointed out that the initial magnesium vaporization occurs in the metal section and hence those surfaces are exposed to a much greater amount of nonvolatile by-products than the 5-L glass bulb. The surface area of the metal section is ap-
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The Journal of Physical Chemistty, Vol. 82, No. 11, 1978
proximately equal to the glass area. Additional experiments are planned where various initial coatings would be applied to surfaces to determine their influence on the sticking coefficients of various metals vaporized in the presence of trace impurity gases. I t might be possible to produce the “volatile metal effect” for metals other than magnesium with the use of certain preapplied primary coatings.
Acknowledgment. Generous support from Research Corporation through a Cottrell College Science Grant is gratefully acknowledged. Support from the National Science Foundation’s Undergraduate Research Program (EPP-750-4311) and NSF Project Grant (CHE76-18587) is also gratefully acknowledged. Appendix 1. Corrected Sticking Coefficient Calculation
total number atoms lost from gas phase s-“ = (impingement rate cm-2 s-’)(area cm2)(F fraction of all collisions that result in sticking) = zP(t)AF where z is impingement constant ( 2 ~ m k T ) - ’and / ~ , P(t)is the pressure at time t.
Zecchina et al.
& t ) (number of atoms present in gas phase a t time t ) = P(t)VN/RT
where V is the volume of the 5-L glass bulb, and N is Avogadro’s number.
-
--
- ( V N / R T )-= W t ) -zP(t)AF dt P(t) = k l ( t ) , where I ( t ) is the ion signal, Mg+, at time t. dt
Therefore
d In
/ d t = (-R TAz/VN)F
I(t0)
The left-hand side can be obtained from the slope of data in Figure 6, ref 1 (MgI curve). Hence F (the sticking coefficient) can be calculated since a11 other parameters are known.
References and Notes (1) L. B.Knight, Jr., R. D. Brittain, M. Duncan, and C. H. Joyner, J. Phys. Chem., 79, 1183 (1975). (2) M.Knudsen, “Kinetic Theory of Gases”, Methuen, London, 1950, p 20. M. Knudsen, Ann. Phys., 50, 472 (1916). (3) R. A. Ropp, J. P. Hirth, and G. M. Pound, Can. J . Phys., 38, 709 (1960). (4) M. B. Sampson and P. A. Anderson, Phys. Rev., (20) 50,385 (1936).
Infrared Study of Surface Modes on Silica F. BOCCUZZI,S. Coluccla, G. Ghlottl, C. Morterra, and A. Zecchina” Istituto dl Chimica Fisica dell’ UniversiQ di Torino, Corso M. D’Azegllo 48 10 125 Torino, Italy (Received September 7, 1977; Revised Manuscript Received February 18, 1978) Publicatlon costs assisted by Istltuto di Chimica Fisica dell’Unlversitk di Torlno
A large variety of surface IR active modes can be observed on finely divided silica. The surface modes can be classified as impurities and intrinsic modes. Impurities modes are associated with surface OH groups and a fairly complete vibrational assignment is proposed. Intrinsic modes are associated with the clean surface and related to the different strength and geometry of the surface bonds with respect to the bulk.
Introduction The IR spectra of inorganic powders can be different from those of single crystals. If the particles are very small (i.e., the ratio between bulk and surface atoms relatively large) size and shape effects (first-order effects)’ are expected which can lead to new IR active modes. This fact has been shown for both highly i o n i ~ ~and - ~ partially covalent5 solids. However, up to now, these kinds of measurements have been carried out in ambient atmosphere, i.e., in conditions of large surface contamination by H20and COz. As a consequence, the detection of the second-order effect1 (Le., the possible change of the force constants at the clean surface) has been prevented. Ordinarily, a bond relaxation is expected at the clean surface, leading to a softening of the vibrati0ns.l However a stiffening effect is not excluded, due to the change in the chemical bonds and local geometries at the surface.6 It is well known that, in single crystals, IR active localized modes associated to substitutional or interstitial impurities can often be observed. In the same way IR active modes associated with surface impurities or adsorbed species can be observed, and indeed all the applications of IR spectroscopy to the vibrational study of 0022-365417812082-1298$0 1.OOlO
the adsorbed species is based on this principle. In conclusion the IR spectra of very pure finely divided powders are the sum of (1)nearly bulk modes, (2) surface modes due both to first-order (surface truncation) and secondorder (change of the surface bond force constant) effects, and (3) adsorbed impurities. In order to distinguish these three effects careful control of the particle dimensions and the degree of the surface contamination is required. As in surface studies we are mainly interested in the strength of the surface bonds (second-order effect) and the nature of surface species and defects, we must keep the size and shape of the particles fairly constant during a set of experiments. Nonporous silica is a good starting point for this type of investigation since the particle dimensions do not change appreciably even under the severe thermal conditions required for an efficient surface cleaning. Due to the silica preparation method, OH groups are natural surface contaminants. However the term impurity is maintained in agreement with solid-state physics literature.
Experimental Section The silica powder was Aerosil Degussa with a specific surface area of 380 m2/g. The average diameter of the 0 1978 American Chemical Society
