The chemisorption of nitrogen at activated sites on a polycrystalline tungsten surface

SURFACE

SCIENCE 24 (1971) 587-611 0 North-Holland

THE CHEMISORPTION

OF NITROGEN

ON A POLYCRYSTALLINE

HAROLD IBM Research

F. WINTERS Laboratory,

Publishing Co.

AT ACTIVATED

TUNGSTEN

and DONALD San Jose, California

SITES

SURFACE

E. HORNE 95114, U.S.A.

Received 23 July 1970 The exposure of a polycrystalline tungsten surface to atomic nitrogen or bombardment by Nz+ is shown to enhance chemisorption. Ion bombardment causes the saturation coverage to be increased by about 50% over that found after exposure to the molecular gas. A somewhat greater saturation coverage is obtained by exposure to neutral atomic nitrogen. The additional gas found in these experiments is desorbed at lower temperatures than nitrogen chemisorbed in p sites. These results are interpreted on the basis of chemisorption at endothermic adsorption sites. 1. Introduction

The presence of energetic electrons (> 10 eV) in a nitrogen atmosphere causes adsorption. This adsorption is in addition to the chemisorption that one normally expects. In two previous papers, it was shown that this adsorption results from the presence of ionic and atomic nitrogenlsg). Teloy and Jackel have independently come to the same conclusion in similar experimentss). It was previously proposed that the adsorption of N: resulted from its dissociation upon impact with the surface2). The results indicated that the dissociation energy was derived primarily from the kinetic energy of the ion although it was suggested that some energy was obtained from the neutralization reaction which left a vibrationally-excited molecule. These experiments were carried out on surfaces which were not atomically clean. Furthermore, the properties of the adsorbed gas were not investigated. In this paper we present results of an investigation concerning activated adsorption on a clean, polycrystalline tungsten surface. The chemisorption of nitrogen on tungsten has been studied in great detail using flash filament techniquesd-11). However, in these experiments the surface was invariably populated by exposure to the molecular gas. It will be shown that this technique allows only a fraction of the available adsorption sites to be filled. It will also be shown that additional sites can be populated by exposure to the atomic gas and by ion or electron bombard587

588

HAROLD

F. WINTERS

AND DONALD

E.HORNE

ment of the surface. These results will be interpreted ence of endothermic adsorption sites.

on the basis of the pres-

2. Equipment and procedures 2.1. APPARATUS The experimental tube shown in fig. 1, is a modified version of tube D which has been previously described a,12). The tube was modified by mounting three filaments (F,, F,, F3) symmetrically with respect to the electron beam as indicated in fig. 1. F, and F, are the tungsten surfaces used in most of these experiments. F, was made from either iridium or rhodium. The filaments were constructed from 0.005 inch wire. Each filament was 22 cm long. Each filament has a guard ring which electrostatically prevents ions from bombarding the cold surface near the filament ends. F, was used to evaporate nickel which subsequently gettered the CO present in the predominately nitrogen atmosphere. This cleansing operation made it possible to work at high partial pressures of N, (3.0 x 10d4 torr) while maintaining the concentration of the impurity gases at a relatively low level. The partial pressure of these gases depended strongly on the amount of excitation and ion bombardment which had recently occurred in the experimental tube. Nevertheless, the amount of impurity gases desorbed from the tungsten surface after a lengthy experiment was in the worst case ~5% of the amount of nitrogen desorbed. 2.2. PROCEDURE Electrons are accelerated from F, into region V and collected at electrode E. The adsorption of species created by the electron beam can be studied by two different methods. First, adsorption at the tungsten surface can be investigated using flash filament techniques. Secondly, the amount of nitrogen sorbed anywhere in the tube can be studied by measuring the pressure drop in a closed volume using procedures which have been previously described13 2>. These two experiments are complimentary as demonstrated by the following illustration. N: bombardment of F, causes the ion to dissociate and the resulting atoms are adsorbed either at the surface of F, or are reflected from this surface and adsorbed on other parts of the tube. The ratio of the number remaining at F, to the number reflected can be found by relating the information obtained from these two experiments. Electrode F was typically held 5 V positive and electrode G and H, 5 V negative with respect to C through E. This arrangement provides a small drawing out field for ions without drastically altering their energy distribution. Retarding potential measurements indicate that the energy spread of

THE CHEMISORPTION

the ions arriving was smaller than over 100 PA. The emission current emission current.

OF NITROGEN

AT ACI’IVATED

SITES

589

at S was less than 1.5 eV when the electron beam current 15 uA and increased to about 10 eV with beam currents data on total adsorption probability was taken at the lower while the flash desorption experiments used the higher Consequently, the energy of the ions bombarding F, and

w

0

Fig. 1.

Schematic

in.

diagram of the experimental tube. FI, Fz, and FB are the surfaces to be studied.

590

HAROLD

F. WINTERS

AND

DONALD

E. HORNE

F, was only known to + 5 eV. The potential difference between electrode A and F, was electronically controlled so as to maintain a constant electron current to E. Electrode B was held 3 V positive with respect to C so that ions trapped on the electron beam would not bombard the LaB, emission filament. The electron beam was focussed by a 100 to 200 gauss coaxial magnetic field. The electron energy was controlled by the potential difference between F, and C, while the energy of the ions arriving at F,, F,, F, and S was controlled by biasing the whole electrode structure with respect to ground. It is necessary for interpretation of the data that the incident ion flux be uniformly distributed over the tungsten surface. This uniformity was demonstrated to be the case by the following experiment. A chemisorbed layer of nitrogen was sputtered with At-’ under vacuum conditions where the partial pressure of N, was estimated to be less than 1.0 x 10-i’ torr. Since physical sputtering results from momentum transfer between the impinging ion and the surface atoms, one expects the nitrogen sputtering rate to be proportional to the surface concentration and the ion current density. One also expects that the nitrogen will leave the surface in the atomic form. Therefore, with uniform ion bombardment the amount of nitrogen on the surface is described by the following equation: dN=-s where s is the sputtering No the saturated surface Hence,

N n, dt, No

(1)

ratio measured at full coverage, ni the ion flux, coverage, and N the surface coverage at time t.

lnN=-s

ni t+lnN,. No

(2)

A semi-log plot of N versus t should yield a straight line if eq. (2) is valid and if there is no nitrogen adsorption during the run. Fig. 2 shows these data for 300 eV Ar+ ions. Similar curves have been obtained for energies down to 50 eV. We interpret these results as indicating that the ion flux is distributed in an approximately uniform manner over the surface. On the other hand, when the spiral filament was replaced by a straight wire, uniform bombardment was not obtained as judged by this type of experiment. 2.3. THERMAL

DESORPTION SPECTRA

The thermal desorption experiments reported in this paper were conducted in the following manner. The filament was flashed to 2400°K at -2.0 x 10e4 torr of 30N2. The temperature was then held at -900°K for -300 sec.

THE CHEMISORFTION

OF NITROGEN

AT ACTIVATED

SITES

591

Next, the filament was allowed to sit for 200 set at room temperature. This treatment yielded a saturated chemisorbed layer which contained a reproducible number of atoms. The electron beam was then used to create atoms and ions in region V some of which subsequently bombarded F, and F,.

In14

10'3 0

8

\2

Number of Incident

Fig.2.

Sputtering of chemisorbed

16 Art

20

24

28

Ions X lOI

nitrogen by Ar+. The argon ion energy was 300 eV.

Often one of the tungsten wires was exposed to neutral atoms and bombarded with ions while the other was only exposed to atoms. Thus, the difference in the coverage of the two filaments was traceable to ion bombardment. The nitrogen was then pumped away and the filaments flashed. The number of desorbed molecules was obtained from the pressure increase. The thermal desorption spectra were obtained by linearly increasing the temperature of the tungsten wire at a rate of 40”K/sec while a quadrupole residual gas analyzer was sitting at the mass 30 peak. The RGA output was electronically differentiated, thus yielding the desorption spectra. 3. Chemisorption 3.1. MECHANISMS

LEADING

at activated sites (experimental)

TO ADSORPTION

AT ACTIVATED

SITES

Electron impact-activated chemisorption must result from the creation of an excited species whose lifetime is of sufficient length so that the particle

592

HAROLD

F. WINTERS

AND

DONALD

E. HORNE

can reach the surface and thus be adsorbed. For electron impact with a gas phase molecule the only states meeting this criteria are ions, atoms, and metastable species. We have shown in previous papers that the presence of both atoms and ions can lead to activated adsorption on various surfaces19 2). Moreover, there have been suggestions by several investigatorsis) that the presence of metastable molecules or atoms can cause enhanced adsorption. However, we feel that the evidence is definitely against this hypothesis. At least for gas phase excitation it can be shown that the presence of metastable nitrogen does not cause adsorption on the type of surfaces normally found in vacuum system. The cross sections for metastable creationis), ionizationla), and adsorptioni) taken under similar conditions in previous experiments in this laboratory indicate differences in the threshold energies and the shape of the cross section curves which conclusively show that the presence of metastable molecular nitrogen does not lead to enhanced adsorption on surfaces such as nickel, tungsten, nichrome, and glass which were present in the experimental tube. We will also assume on the basis of this evidence, that metastable nitrogen does not produce adsorption on the well-defined tungsten surface studied in this paper. Ermrich and Van 0ostrom14y15 ) and, subsequently, Yates and Madey16) and Plummerl7) have shown that electron bombardment of nitrogen adsorbed in the y state (T= - 19O’C) also produces activated adsorption. It should be noted that y nitrogen is at least in part a weakly-bound molecular species. Ermrich and Van Oostrom have suggested that their data might be interpreted on the basis of an adsorption stabilized negative nitrogen molecular ion’s), a neutral excited stateld), or perhaps by molecular dissociationi4). We believe the correct interpretation to be that the electron dissociates the sorbed molecule and the resulting atoms are adsorbed at activated sites. This interpretation is verified by the fact that the thermal desorption spectra found in our work is similar to that found by Yates and Madey. Fig. 3 shows thermal desorption spectra for a tungsten surface exposed to molecular nitrogen (A) and molecular and atomic nitrogen (E). In the three middle spectra (B, C, D) the surface was exposed to molecular nitrogen and bombarded with ions of 20 eV, 100 eV, and 300 eV, respectively. They were also exposed to a small amount of atomic nitrogen, however, the adsorption due to ion bombardment was much greater than that due to atom exposure. These data show that the same states are populated exposure to neutral atomic nitrogen as are populated by bombardment with ions. Furthermore, these spectra are similar to those found by Madey and Yates Is) for electron bombardment of y nitrogen. Hence, we conclude that electron bombardment of adsorbed molecular nitrogen, exposure to atoms, and ion bombardment fills the same sites. There may also be other types of activated sites such as

THE CHEMISORPTION

OF NITROGEN

AT ACTIVATED

SITES

593

the x state observed by Ermrich, which are not evident from the desorption spectra. The fact that these activated sites are populated by exposure to neutral atoms suggests that the adsorbed species are atoms rather than molecules.

I

I

6

8

I

IO

I

12

I

14

I

16

T(“K/lOe

Fig. 3. Thermal desorption spectra from a polycrystalline tungsten wire illustrating desorption from activated sites. The tungsten was exposed to > 10-l torr-see of molecular nitrogen and then given the following treatments: (A) None. (B) Bombarded by 2.0 x 1Ol5Nz+ (20 eV). (C) Bombarded by 2.0 x 1015Nz+ (100 eV). (D) Bombarded by 2.0 x 1Ol5N2+ (300 eV). (E) Exposed to neutral atomic nitrogen.

This interpretation is consistent with the threshold energies (-9 eV) needed to populate these sites by electron and ion bombardment. Furthermore, based on this model, one would expect the decomposition of molecules such as NH,, NO, etc. to cause enhanced adsorption. This has been reported for NO by Madey and Yatesls) and for NH, by Matsushita and Hansenle). 3.2. ADSORPTION RESULTING FROM ION BOMBARDMENT Ion impact with the surface may cause sorption by two methods. The ion can become physically trapped in the lattice or it can dissociate with the subsequent adsorption of the atoms on the surface2). Experiments with

594

HAROLD

F. WINTERS

AND

DONALD

E. HORNE

noble gases indicate that gas trapping via lattice penetration is quite improbable at energies below 100 eVa0). By analogy we expect most of the adsorption resulting from low energy N: bombardment to be caused by dissociation of the molecule. This analysis agrees with earlier work and also with the data presented in this paper. It should be noted that ionic adsorption cannot be interpreted as resulting from surface damage followed by damage-induced chemisorption. This possibility was eliminated by the following experiment. A partial pressure of 2.0 x 10m5 torr of N, was maintained in 2.0 x 10m4 torr of argon. The tungsten surface was then bombarded with ions which were about 90% Ar’ and 10% N:. The surface damage during bombardment was primarily caused by argon ions. The damage did not, however, cause enhanced adsorption of nitrogen from the molecular gas. The small amount of adsorption which was observed could be quantitatively accounted for by Nl bombardment. The probability of adsorption is defined as the ratio the number of molecules which disappear from the gas phase (i.e., the number sorbed on the bombarded surface or surrounding surfaces) to the number of ions which hit the surface. The sticking probability is defined as the ratio of the number of molecules adsorbed on the bombarded surface to the number of molecular ions hitting that surface. Fig. 4 shows the probability of adsorption and the

_ _____Nickel foil -Tungsten foil - -.- -Tungsten fhment --------Mdylodenum foil

0

I

50

100

150

200

250

300

N; Energy (64)

Fig. 4.

Probability of ionic adsorption on various surfaces. (Analysis of probable errors indicates that most of the data should be accurate to 25 %.)

sticking probability for Nl as a function of ion energy. The data were obtained using the technique described in the previous section. The curves for MO and Ni have previously been publisheds) while the data on the two tungsten surfaces were taken during this set of experiments. Similar results have been obtained on platinum surfaces by Teloys). All probability of

THE CHEMISORFTION

OF NITROGEN

AT ACllVATED

SITES

595

adsorption curves have a threshold around 9 eV and approach unity for energies less than 100 eV. Since asdorption is assumed to result from dissociation of the ion, the data of fig. 4 represent the probability that a molecular ion will be dissociated upon impact with the surface. The fact that similar probability functions have now been obtained for five different metals, suggests that these results can be generalized to many types of surfaces. The Nl which is dissociated does not necessarily adsorb on the surface at which the initial collision occurs. The ionic sticking probability obtained from flash filament techniques is substantially less than the probability of ionic adsorption resulting from collision with the tungsten surface. For example, at an ion energy of 100 eV, the sticking probability is -0.2 while the probability of adsorption is close to unity (see fig. 4). This result indicates that 80% of the ions are reflected as atoms and adsorbed at activated sites on the surrounding surfaces. It should be noted that experiments conducted in this laboratory suggest that most surfaces have activated sites. It is also interesting to note that the sticking probability is almost independent of ion energy between 20 eV and 150 eV while the probability of adsorption changes from 0.3 to 1.0. The energy of an ion approaching a metal surface is composed of three parts, i.e., the excitation energy which is represented by the ionization potential, the interaction energy between the surface and the ion, and kinetic energy of the ion. The ion is neutralized into the ground electronic state of the molecule or atom via a two-electron process while it is still several angstroms from the surface. This happens before the ion begins to interact strongly with the surfacesi*ss). Th e energy released in the neutralization process is often quantitatively related to the energy of the emitted secondary electrons and hence is not in general expected to promote an activated chemical reaction. However, in the case of nitrogen and tungsten, there is some evidence that whereas the ion is neutralized into the ground electronic state it does not go into the ground vibrational levelsi). It has been suggested in a previous papers) that this vibrational energy might contribute to the activation energy for the sorption reaction. Nevertheless, this vibrational energy is expected to be small because the minima in the potential energy curves for the ground state of the molecular ion and the ground state of the molecule occur at approximately the same internuclear distancess). Hence, application of the Franck-Condon principle to the neutralization reaction suggests that the molecule will be in a rather low vibrational level. The attractive interaction energy between the molecule and a tungsten surface whose j3 sites are saturated is also small since adsorption does not occur at room temperature. Therefore, the kinetic energy of the ion should be the primary source for the activation energy of adsorption.

HAROLD F. WINTERS AND DONALD E. HORNE

596

As previously stated, all of the data on the N+ adsorption can be interpreted on the basis of a two-step process where the ion is dissociated upon impact with the surface as the result of its kinetic energy and then the resulting atoms are adsorbed at activated sites either on the impacted surface or on the surrounding surfaces. In particular, the threshold energy for adsorption of ions at about 9 eV (see fig. 4) suggests this interpretation. 3.3. ACTIVATED SITES ON TUNGSTEN

(EXPERIMENTAL)

3.3.1. Isotopic mixing Tungsten has planes which will chemisorb nitrogen (100) as the result of exposure to the molecular gas and those which will not (110). The following experiments were conducted to determine the location of activated sites with regard to these planes. A clean tungsten surface was exposed to 28N2 until it was saturated. The 28N2 was pumped away and 30N2 leaked into the system. The surface was then bombarded with 50 eV 3oN+ or exposed to neutral ’ 5N. This process should cause the activated sites on planes which do not chemisorb nitrogen to be filled with 15N while the p sites would be filled with i4N. If i5N is sorbed on planes which do not contain 14N, then it should, with a high degree of probability, be desorbed as 30N224). On the other hand, i5N adsorbed on planes which contain i4N would probably be desorbed as a mixture of 30N2 and 29N2. Therefore, complete isotopic mixing upon desorption would indicate that the ’ ‘N was adsorbed on planes containing 14N. A small amount of mixing would indicate that 15N was result adsorbed on planes which do not contain 14N while an intermediate suggests that both of the above cases occur. Table

1 compares

the observed

TABLE 1 Mixing

experiment

(p sites filled with 28N~, then activated chemisorption a) Bombardment with 6.25 x 1014 N2+ (50 eV)

occurs with 30Nz)

2sNz

29N2

30N2

Observed

11.2

22.1

12.1

Total mix - calculated

10.9

22.6

11.8

b) Exposure

Observed Total mix - calculated

to neutral atomic nitrogen 28Nz

29N2

2ON2

14.8 14.3

25 26

12.3 11.8

THE CHEMISORFTION

OF NITROGEN

AT ACTIVATED

SITES

597

mass spectrometer peak heights with those calculated assuming total mixing. There was always complete mixing within experimental error implying that the first case was true, i.e., activated sites on a tungsten surface occur on the same planes as the nonactivated sites. It should be noted that Yates and Madeyls) also found isotopic mixing when “N was adsorbed in their h state and 14N in the p states. These results do not prove that activated sites do not occur on other planes. It may be that these planes were absent on the surface of our filament or that the activation energy for surface diffusion was so small that desorption occurred at room temperature. It should be mentioned in this regard that adsorption at activated sites does occur on an iridium surface (F, - see fig. 1) which does not normally chemisorb Nz on any plane. 3.3.2. The effect of oxygen on the adsorption of nitrogen The experiments presented so far indicate that nitrogen is adsorbed as atoms at activated sites which occur on the same crystal planes as nonactivated sites. Furthermore, only about half the sites are filled by exposure

“‘1’4”

6

8

IO

12

16

18

T(°Kl/lOz

Fig. 5. Thermal desorption spectra from tungsten illustrating the effect of oxygen. The tungsten was exposed to > 10-l torr-set of nitrogen then given the following treatments: (Top) none. (Middle) exposed to neutral atomic nitrogen. (Bottom) exposed to molecular oxygen at N 3 x lo-5 torr, then exposed to molecular and atomic nitrogen under conditions similar to those under which the middle spectra was obtained.

598

HAROLD

F. WINTERS

AND

DONALD

E. HORNE

to molecular nitrogen whereas exposure to molecular oxygen is known to fill most of the available sites. Therefore, an experiment was conducted where the tungsten surface was exposed to oxygen prior to exposure to molecular and atomic nitrogen. Fig. 5 shows the results. In the top spectra the surface was exposed only to molecular nitrogen. It should be noted that the maximum desorption rate occurs at a lower temperature than the spectra shown elsewhere in this paper. This decrease resulted from prior exposure to oxygen and is identical with results previously reported by Yates and Madeyls). The explanation of the effect is not known. The normal spectra does, however, return after elimination of the oxygen, outgassing the tube, and repeatedly flashing the filament. In the middle spectra the tungsten wire was exposed to both atomic and molecular nitrogen. The amount of activated adsorption is indicated by the difference between the areas under the top two curves. It is interesting to note that whereas oxygen exposure has reduced the desorption temperature for the l3 state, the temper-

I

I

I

I

7

9

II

13

I

15

I

17

T(“K~lO*

Fig. 6. Thermal desorption spectra from tungsten illustrating the effect of bombardment by Arf. The tungsten was exposed to > 10-r torr-see of nitrogen, then given the following treatments: (A) None. (B) Bombarded with 1.8 x 1Ol5Arf [75 eV]. (C) Bombarded with 9.1 x 1014Nz+ [75 eV]. (D) Bombarded with 9.1 x 1Or4N-Z+and then with 1.8 x 10ls Ar+ [75 eV].

THE CHEMISORPTION

OF NITROGEN

AT ACTIVATED

SITFS

599

ature has remained approximately the same for the h state. The treatment of the tungsten was the same for the bottom and middle spectra except that the tungsten wire was exposed to 0, at -3 x lo-’ torr for 300 seconds in the bottom one prior to exposure to molecular and atomic nitrogen. This data shows that oxygen adsorption prevents both activated and nonactivated adsorption of nitrogen. 3.3.3. Sputtering experiment If indeed the h sites and /3 sites are intimately mixed, then it is of interest to examine the desorption spectra when both sites are only partially full. Results of this type of experiment are shown in fig. 6. The top curve is the spectra obtained after exposure to molecular nitrogen. The next is similar except that after exposure to nitrogen the wire was bombarded with 1.8 x 1Or5 Ar+ under conditions where the partial pressure of N2 was in the 10-i’ torr region. The ratio of the areas beneath the curves (-4) is an indication of the amount of nitrogen sputtered by the argon. Note that the damage caused by the bombardment did not change the spectra. Only the b1 and bz peaks occur after bombardment. Results similar to these have also been obtained after 300 eV bombardment. In the third spectra there was exposure to molecular nitrogen and then bombardment with 9 x lOi N: ions which always produces the h, peak. The bottom spectra is identical except that after bombardment with Nl the tungsten wire was again bombarded with 1.8 x 1015 Ar+ ions. There are two things to note about this spectra. The activated peak has completely disappeared and the amount of gas desorbed from the wire is greater than that found in curve B. This suggests that some of the gas originally adsorbed at activated sites is being desorbed from the nonactivated b sites. In all probability the gas originally adsorbed at the activated h sites relaxes into the empty l3 sites as the result of ion bombardment or possibly during the temperature sweep. We have never been able to obtain desorption spectra which contained a h peak when the surface coverage was substantially less than 6 x 1014 atoms/cm’. 3.3.4. Activation by thermal methods There has been no report in the literature that activated sites could be populated by thermal means. However, it was considered worthwhile to look for this kind of activated adsorption using high exposures and large sensitivities. The results are shown in fig. 7 for an exposure of 10-l torr-sec. The top curve shows typical desorption spectra obtained with about 5 x 1OL3 atoms at activated sites. It is estimated that 1.0 x 1Ol3 atoms would be detectable. The bottom four spectra were obtained after holding the wire at 300°K 600 “K, 760”K, and 900°K during exposure to nitrogen. There is

600

HAROLD

F. WINTERS

AND

DONALD

E. HORNE

no evidence for thermal activation. This indicates surface (from the fi sites) nor molecules from the sites as the result of the high temperature of the other indications that the activation energy for

that neither atoms on the gas phase enter activated tungsten. There are also adsorption is quite high.

I

I

I

I

I

I

6

8

IO

12

14

16

Tc”K)/lO’

Fig. 7. Thermal desorption spectra from tungsten illustrating the lack of thermal activation. The small peak at N 800°K in the top spectra is from a small piece of iridium welded to the tungsten.

The threshold energy for populating these sites by ion bombardment is -9 eV (see fig. 4). Furthermore Ermrichls) has reported a threshold -9 eV (5 eV+ Work Function) for electron bombardment nitrogen. The subsequent interpretation will be based on this evidence for high activation energy and would have to be modified somewhat if this were not the case. 4. Proposed model At this time we would

like to propose

a model which accounts

for the

THE CHEhfISORPTlON

experimental

OF NITROGEN

AT ACTIVATED

results in a simple and straightforward

SITES

manner.

601

The adsorption

of CO, NO, and O2 causes a saturation density of about 1.O x 10’ 5 molecules/ and 1.0 to 1.4 x 1Or5 atoms/cm2, respectcm’, 1.4 x 10’ 5 molecules/cm2, ively2s~r8~se). This suggests that there are -1.4x 1Ol5 sites on a tungsten surface which can be filled by any one of several gases. However, exposure to nitrogen only fills about half this number of sites. On our particular surface, for example, exposure to molecular nitrogen resulted in the adsorption of -6.0 x 1Ol4 atoms/cm’. We suggest that the activated sites discussed in this paper are the ones which are still empty after exposure to molecular nitrogen. We also suggest that the binding energy for atoms at activated sites is substantially less than that for atoms at nonactivated sites (possibly because of lateral interaction between the sorbed atoms). The isotope mixing experiments

Fig. 8.

Schematic

potential energy diagram for nitrogen chemisorbed on tungsten. binding energy for nitrogen atoms in the h state. Ed is the activation energy for surface diffusion. The dashed and solid curves indicate energies in a direction perpendicular to the surface while the dotted curves show the potential energy in the plane of the surface. The dotted curves illustrate schematically the formation of a molecule on the surface by two atoms which had been sorbed at h sites, E, is the unknown

602

HAROLD

F. WINTERS

AND DONALD

E. HORNE

(see table 1 and ref. 16) and the relaxation of nitrogen at activated sites into empty p sites (see fig. 6) is reasonable according to this model because of the proximity of the two sets of sites. This model is also consistent with the number of sorbed atoms. Ion bombardment, for example, increases the nitrogen coverage to N 1.0 x IO” atoms/cm’ (see fig. 11). Moreover, exposure to neutral atomic nitrogen can produce a coverage of greater than 1.2 x 1015 atoms/cm’. (Compare the areas under the curves A and E of fig. 3.) Furthermore, the absence of adsorption on an oxygenated surface (fig. 5) may simply indicate according to this model that the sites are already filled with oxygen atoms. The energetics of processes which occur when a clean tungsten surface is exposed to nitrogen are represented schematically by the potential energy diagram shown in fig. 8. In this diagram, the potential energy of a nitrogen atom as part of a diatomic molecule in the gas phase is assumed to be zero. The nitrogen is probably bound as atoms in the p state and as molecules in the a and y states. Moreover, there are two types of dissociative chemisorption which though formally equivalent are physically distinct, i.e., endothermic adsorption where the energy change in going from the molecular gas to the sorbed atoms is positive (the h states) and exothermic adsorption where the energy change is negative (the p state). The general characteristics of these types of adsorption have been discussed from a theoretical point of view by Ehrlichz4). Many of the interpretations presented in this paper are based on ideas suggested in his work. We believe that activated adsorption occurs into a site which is described schematically by the dashed h curve in fig. 8. This binding state for a sorbed atom is endothermic in the sense that the energy of the sorbed atom is greater than its energy would be as part of a gas phase molecule. This explains why exposure to the molecular gas does not populate these sites. Whereas the site is endothermic, nevertheless, the atom is still strongly chemisorbed since it would take -3 eV to desorb it as an atom into the gas phase. The assignment

of the energy range for this state is based on the following

arguments. The atoms at endothermic sites would minimize their energy by desorbing as a molecule. Hence, the rate limiting step for desorption from these sites is expected to be the activation energy for surface diffusion. One estimates this to be -2.5 eV from the desorption spectra27). Hence, states like 1’ would cause desorption of atoms rather than molecules. Most of the gas was desorbed from the tungsten surface in our experiment as molecules. Evidence for this statement will be presented in the next section. This indicates that the activation energy for surface diffusion is less than the activation energy for the desorption of atoms.

THE CHEMISORF’TION

OF NITROGEN

AT ACTIVATED

SITES

603

An activated exothermic site (illustrated schematically by the 6 state in fig. 8) has an activation energy for adsorption which is less than the activation energy for desorption. However, the experimental evidence previously discussed suggests a high activation energy for adsorption ( > 9 eV/molecule), therefore, the adsorption of nitrogen atoms at activated exothermic sites has been rejected as an explanation of our data. The elimination of energies less than 0 and greater than -2.5 eV suggests the binding energy is between 0 and 2.5. eV. It should be noted that a high activation energy for adsorption is consistent with atoms at endothermic sites. It seems quite likely, for example, that during ion bombardment one atom may stick to the surface while the other is reflected into the gas phase. The threshold energy for this process at a h site would be -7 eV. On the other hand, if two h sites were filled as the result of electron impact with a weakly-bound molecule, the threshold energy could be -9 eV. This assumes that both atoms move from the original site occupied by the molecule and hence the activation energy would be 2 (EA + Ed) where Ed is the activation energy for surface diffusion and EA is the energy of an atom at a h site. The interpretation presented in this paper assumes that the activation energy for desorption is also the activation energy for surface diffusion. The experimental value (-45 to 60 kcal/mole) is higher than that obtained for tungsten surfaces which are not saturated with nitrogenas). However, if surface diffusion proceeds via a hopping mechanism from site to site, then it is not unreasonable to assume that the activation energy should increase when all the low energy sites are filled.

5. Miscellaneous

experimental results

The desorption of an atom from an exothermic site would require over 4.9 eV. Therefore, if nitrogen is predominately desorbed as atoms, it is almost certain that they were originally at endothermic sitesz4). However, the absence of atomic desorption does not indicate the absence of endothermic sites. In our experiments, desorbed atoms would either be readsorbed on the surrounding surfaces causing no net pressure increase, or they would recombine with atoms which had been previously sorbed and then desorb as a molecule. The large pressure increase observed during flash desorption indicates that we are not desorbing atoms which are then readsorbed. However, it remains to be proved that atoms are not being desorbed from the tungsten surface and subsequently recombining with other atoms at the walls thus causing a net pressure increase during the flash. This process was ,shown not to exist by the following experiment. 30N2 was

604

HAROLD

F. WINTERS

AND DONALD

E. HORNE

adsorbed at activated and nonactivated sites on the walls while 28N2 was adsorbed in both l3 and h sites on the tungsten. A process like that described above would produce a large 29N 2 peak while desorption of molecules from the tungsten would produce a large 28N2 peak. It was found that most of the desorbed gas was 28N2 thus eliminating the possibility that a substantial number of atoms were being desorbed from the tungsten. Whereas there is no evidence for atom desorption from tungsten, it appears that atoms are desorbed from some of the surrounding surfaces. The tungsten and the surrounding surfaces were exposed to atoms created by electrons in region V. The electron beam was then turned off and the tungsten wire flashed to 2300°K while the nitrogen pressure was -3 x 10m4 tort-. It was then allowed to adsorb nitrogen for -400 sec. Fig. 9 (bottom) shows the desorp-

I

I

I

I

I

I

6

8

IO

12

14

16

1

16

T P-K $0’

Fig. 9. Thermal desorption spectra from tungsten illustrating the adsorption of atoms which were desorbed from the surrounding surfaces. The top spectra illustrates a surface exposed to molecular nitrogen.

tion spectra. The presence of a h-, peak indicates that atoms were impinging on the surface. Since the electron beam was off these atoms must have originated from the walls. This indicates that some of the surrounding surfaces have endothermic sites. It should be noted that a small h, peak was always observed in our desorption spectra unless precautions were taken to eliminate

THE CHEhfISORPTlON

OF NITROGEN

AT ACTIVATED

SITES

605

the nitrogen sorbed on the walls of the experimental tube. Nitrogen atoms desorbed from the walls were also found to adsorb on the iridium wire which does not normally chemisorb any nitrogen. Rigbyss) has examined the simultaneous adsorption of CO and N,. His desorption spectra showed a small peak at approximately the same temperature as our h, peak. Consequently, we examined the characteristics of our desorption spectra as a function of CO pressure. Fig. 10 shows the results.

I

I

I

I

I

I

6

8

IO

12

14

16

T(“K)/lO’

Fig. 10. Thermal desorption spectra from tungsten illustrating the affect of CO. (A) Exposed only to molecular nitrogen; (B) exposed to molecular nitrogen in the usual manner then exposed to CO (PCO = 2 x 1O-5 torr) and NZ (PN2 = 2 X 1O-5 torr) for 500 seconds; (C) exposed to a mixture of CO (PCO = 5 x 1O-s torr) and N2 (PN2 = 3 X 1O-4 torr) in the manner described in section 2.

Curve A was exposed to N, in the usual manner. Curve B was first exposed to nitrogen in the usual manner and then exposed to both CO(Pc, =2 x IO-’ torr) and N, (P,,=2 x IO-’ torr) for 500 sec. The CO exposure did not significantly change the desorption spectra. The tungsten filament was then flashed in a mixture of CO (Pc, = 5 x lO-‘j torr) and nitrogen (p& = 3 x 10m4 torr) in the manner described in section 2. The results are shown in curve C. The CO has reduced the amount of sorbed nitrogen but there is no h peak. Hence, we conclude that the CO does not produce the low temperature h peaks observed in our experiments. There is some evidence that the h, peak (see fig. 3) may be structure

606

HAROLD

F. WINTERS

AND

DONALD

E. HORNE

sensitive. A tungsten wire was observed to give spectra like those shown in fig. 3 for several months. After being open to the atmosphere for some time, the h, peak was found to be much smaller and in some cases, it disappeared. The h, peak did not seem to be effected by this procedure.

6. Sticking probability, recombination

and saturation

Fig. 11 shows the coverage as a function of time for bombardment with N:. All p sites and a few h sites were full at t = 0. The increased coverage results from further adsorption at h sites. It is interesting to note that the saturation coverage is about the same for all ion energies to 100 eV. An analytical expression can be derived for the coverage as a function of time based on the following assumptions: 1) It is assumed that the amount of gas adsorbed at exothermic sites is constant during the experiment. (Presumably an atom that is desorbed from an exothermic site via sputtering or recombination will almost immediately be replaced by adsorption from the molecular gas.) 2) Recombination via surface diffusion does not occur at room temperature. 3) The sticking probability for ions and atoms is assumed proportional to the number of empty sites. Similar results are obtained if they are assumed constant. 4) The probability of a gaseous atom or ion recombining with a surface

5 ?z

1.0 -

ss i 0

V, = 20

A=l.63.10”

ATOMS/Set

$-=8.5d04

V, = 50

A-3.54.10”

ATOMS/Set

&21

V,= 100

A=4,02~IO”

ATOMS/Set

&=2.3

1000 Time

2000 Bed

.10-s Id3

3000

Fig. 11. Coverage versus time for a polycrystalline tungsten wire. Vs is the energy of the bombarding ions. The points on the various curves were determined experimentally while the curves themselves were obtained from eq. (5). B/No and A are constants which yield the best fit between eq. (5) and the experimental points, Vs = 0; ni = 0; VS = 20, nrr6.8 x 10r1ions/cm2-set; V, = 50, nr~ 1.19 x 1Ora; V, = 100, nrz 1.62 X 10r2.

THE CHEMISORPTION

atom

at an endothermic

OF NITROGEN

site is assumed

AT ACTIVATED

607

SITES

to be proportional

to the number

of filled sites. The first assumption is in our opinion the most questionable. For example, an atom from an endothermic site may fill an empty exothermic site. On the other hand, an exothermic site which is emptied by some mechanism may stay empty thus causing a net decrease in the surface concentration. Despite these uncertainties, it is felt that the first assumption is the most reasonable one to make with the present information. The second assumption is based on the experimental result that the number of atoms at endothermic sites does not decrease with time when exposed to vacuum. Based on the four assumptions, the following equation can be written :

where N, is the density of endothermic sites, N is the number of filled sites, CQthe ion sticking probability, a2 the atom sticking probability, fil the ion recombination probability, /I2 the atom recombination probability, ni the incident ion flux, n, the incident atom flux, and S is the sputtering probability for atoms at endothermic sites. The solution for N yields:

where A = alni

+ tLZne,

B = mini + a2ne + /YIni + P2ne + Sni ;

N, is the number of atoms sorbed in endothermic sites at t = 0. This quantity is measured and is usually quite small. It results from the fact that during an experiment atoms are sorbed on most surfaces in the tube. Some of these atoms are desorbed as atoms and subsequently adsorbed on the tungsten surface. Thus, even in the absence of energetic electrons or ions, there can be some adsorption into endothermic sites. This accounts for the small increase in concentration observed in fig. 1 (Vs=O). When the tube has been thoroughly outgassed, this behavior is not observed. The total coverage (NJ is the number of atoms at exothermic sites (N’) plus the number Nat endothermic sites, i.e., N,

=

!; [

1 _

,-

WW]

+ ;

B

0

e-WW~

+

N’

.

608

HAROLD

F. WINTERS

AND DONALD

E. HORNE

There are two undetermined parameters in eq. (5), i.e., AN,,/B and B/N,. Fig. 6 compares the theoretical curve from eq. (5) with the experimental data for three different ion energies (VJ. The two constants are adjusted for the best fit. The agreement is very satisfactory considering the simplicity of the model. Our data indicates that eq. (5) accurately describes the nitrogentungsten systems for ion energies less than 150 eV. At higher energies trapping in the lattice becomes important and the sorption characteristics change. The saturation coverage is given by

Nsa,=

ANo B

GLlni + CC*n,

+ cq,

+

(6)

Plni + j32ne+ Sflj

This equation is quite sensitive to the ratio of n,/ni and to the values of the various constants. Under the condition of most of our experiments Nsacr 1.5 N’. However, under conditions where the filaments are biased against ions, the saturation coverage was substantially higher. The experiments were long and somewhat unreliable under these conditions. Nevertheless, the results indicated that N,,,>2N’. This result is to be compared with that of Yates and Madey who have also found that electron bombardment of molecular nitrogen in the y state increases the surface concentration by at least a factor of two. Let A, be the value of A which is found during an experiment where the filament is biased to repel ions, i.e., A, = cc,n,. In the same manner, let A, be the value of A found in an identical except that ion bombardment is occurring. Then,

(7) experiment

A, = ct,ni + c(~TI~. Subtracting

(7) from (8) and dividing

(8)

by Hi yields

CI~= (A, - A,)/ni. The ion sticking probabilities obtained from this type of analysis are shown in fig. 4. They are quite independent of ion energy. This should be contrasted with the probability of ionic adsorption shown in the same figure. Our experiments are complicated by the fact that the ratio of n,/ni is a function of the ion energy. However, consider the ideal case where n, = 0. Eq. (5) then becomes

N, = c( -+ ‘krs I

No { 1 - exp [ - (x1 + CI~+ S) nit/No]}

+ N'

THE CHEMISORPTION

OF NITROGEN

AT ACTIVATED

609

SITES

It should be noted that if n, =0, then N, =O. In our experiments, n, is substantially smaller than n, and, therefore, eq. (8) is approximately true. Since ~1~and the saturation coverage (see fig. 6) are both almost independent of ion energy, the quantity /I1 + S must be a constant. Furthermore, nit is just the total number of ions which have bombarded the surface (NJ. Therefore, N, = C, (1 - e-C2Ni) + N’ , (9) where C, and C, are constants independent of ion energy. It is not clear, however, how they depend on the angle of incidence. All of the data shown in fig. 11 (V. =20, 50, 100) is described quite accurately by this equation. The small deviations are adequately explained by the fact that n, is not zero. 7. Trapping in the lattice The adsorption of noble gas ions becomes significant at energies between 50 eV and 100 eV depending upon the type of surface so~ss*sr).For example, the sticking probability of neon at 75 eV is about low3 and is increasing rather sharplyso). These atoms are believed to be sorbed as the result of being physically trapped in the lattice. By analogy with neon and considering a 150 eV N: as equivalent to two 75 eV atoms, one would expect nitrogen trapping in the tungsten lattice to become significant at about 150 eV. A definite change in the saturation characteristics of the surface begins to occur at about this energy. Fig. 12 shows the surface concentration as a

28r

-Ar+(300eV) I

0

2

4

I

I

,

I

I

I

I

I

I,

1

16 N”nlbe8r

Fig. 12.

,

6 of Ir(iLl+

:&S

x ‘To15

Coverage versus number of incident ions.

,

18

,

(

20

610

HAROLD

F. WINTERS

AND

DONALD

E. HORNE

function of the number of bombarding ions for 300 eV Nl. The saturation occurring at a coverage of about 9 x lOi atoms/cm2 (see fig. 11) is no longer evident. Other experiments indicate the saturation coverage of at least 8.0 x lOi atoms/cm2 can be obtained under these conditions. The differences in the saturation coverages between low and high energy bombardment is believed to be caused by lattice penetration and the resultant trapping of the impinging atoms. The saturation coverage for the noble gases is about 80 times less than the equivalent nitrogen coverage. This is believed to result from different release mechanisms. Ion-induced re-emission of previously adsorbed noble gases is very efficient. The movement of surface atoms as the result of bombardment probably releases the trapped gas. On the other hand, nitrogen reacts chemically with tungsten and is not expected to be released by the motion of these atoms. 8. Conclusion There are a large number of sites on a tungsten surface which are not populated by exposure to the molecular gas. These sites can be populated, however, by many mechanisms including N: bombardment, exposure to atomic nitrogen, and electron bombardment of physically-adsorbed molecular nitrogen. Atoms adsorbed at these endothermic sites have energies which are somewhere between 0 and 4.8 eV (see fig. 1). These atoms are believed to be desorbed as molecules via recombination with atoms from i3 or other h sites. The rate limiting step for desorption is thought to be that of surface diffusion. This generally has a low activation energy thus explaining the desorption at rather low temperatures. The general characteristics of various gases at endothermic sites on a wide variety of surfaces are expected to be similar to those found for tungsten. Acknowledgments The author gratefully acknowledges valuable discussions with Bernard Wood of Stanford Research Institute and John Coburn of IBM. We would also like to thank Curtis Erickson and Joe Schlaegel for designing and constructing much of the experimental apparatus. References 1) H. F. Winters, D. E. Home and E. E. Donaldson, J. Chem. Phys. 41 (1964) 2766. 2) H. F. Winters, J. Chem. Phys. 44 (1966) 1472. 3) E. Teloy, in: Trans. Third Intern. Vacuum Congress, Ed. H. Adam (Pergamon, New York, 1967) Vol. 2, Pt. 3. p. 613 (1965).

THE CHEMISORPTION

4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25) 26) 27) 28) 29) 30) 31)

OF NITROGEN

AT ACTIVATED

SITES

611

J. A. Becker and C. D. Hartman, J. Phys. Chem. 57 (1953) 157. G. Ehrlich, J. Phys. Chem. 60 (1956) 1388. G. Ehrlich, J. Chem. Phys. 34 (1961) 29. P. A. Redhead, in: Proc. Symp. Electron Vacuum Physics, Hungary 1962, p. 89. L. J. Rigby, Can. J. Phys. 43 (1965) 532. T. Oguri, J. Phys. Sot. Japan 18 (1963) 1280. T. E. Madey and J. T. Yates, Jr., J. Chem. Phys. 44 (1966) 1675. P. Kisliuk, J. Chem. Phys. 30 (1959) 174. H. F. Winters, J. Chem. Phys. 43 (1965) 926. For a discussion of this subject along with the appropriate references, G. Carter see: and J. S. Colligon, Ion Bombardment of Solids (American Elsevier, New York, 1968) p. 370. W. Ermrich and A. Van Oostrom, Solid State Commun. 5 (1967) 471. W. Ermrich, Nuovo Cimento Suppl. [l] 5 (1967) 582. J. T. Yates and T. E. Madey, The Structure and Chemistry of Solid Surfaces, Ed. G. A. Somorjai (Wiley, New York, 1969) p. 59-l. W. Plummer, J. W. Gadzuk, and R. D. Young, Solid State Commun. 1 (1969) 487. John T. Yates and T. E. Madey, J. Chem. Phys. 45 (1966) 1623. Kun-Ichi Matsushita and R. S. Hansen, J. Chem. Phys. 52 (1970) 4877. E. V. Komelsen, Can. J. Phys. 42 (1964) 364. F. M. Propst and E. Ltischer, Phys. Rev. 132 (1963) 1037. H. D. Hagstrum, Phys. Rev. 96 (1954) 336. F. R. Gilmore, Rand Corporation Rept. RM-4034-PR. For a discussion of the subject see: G. Ehrlich, J. Chem. Phys. 31 (1959) 1111. J. Anderson and P. J. Estrup, J. Chem. Phys. 46 (1967) 563. J. H. Singleton, J. Chem. Phys. 47 (1967) 73. Data contained in ref. 19 suggest the activation energy may be somewhat lower than this value. See for example, Gert Ehrlich, Brit. J. Appl. Phys. 15 (1964) 349. L. J. Rigby, Can. J. Phys. 42 (1964) 1256. F. Brown and J. A. Davies, Can. J. Phys. 41 (1963) 844. E. Kay and H. F. Winters, in: Trans. Third Intern. Vacuum Congress, Ed. H. Adam (Pergamon, New York, 1967) Vol. 2, Pt. 2, p. 351 (1965).