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Electron stimulated desorption and conversion of N2 adsorbed on a tungsten (110) plane
Surface Science 231 (1990) 344-355 North-Holland
344
ELECTRON ADSORBED
STIMULATED DESORPTION AND CONVERSION ON A TUNGSTEN (110) PLANE
Q.-J. ZHANG Department
OF N,
*, J.C. LIN, N. SHAMIR * * and R. GOMER
of Chemistry and The James Franck Institute,
The University of Chicago, Chicago, 60637 IL, USA
Received 11 October 1989; accepted for publication 20 December 1989
The response of y-N, on W(110) to electron impact has been investigated. The desorption products are principally neutral N, and small amounts of neutral N, which may come from decomposition of N, in the mass analyzer. N: was not detected. N+ is seen for electron energies above 60 eV. Approximately 32% of the initial saturated y-N, layer (in terms of N atoms) is converted by electron impact to chemisorbed atomic N at E_= 150 eV, corresponding to N/W = 0.50. It was possible, by thermal desorption after electron impact for different times to follow both the appearance of N and the desorption of N,. It turns out that the cross section for conversion to adsorbed atomic N is 2.5 x 10-r’ cm2, virtually independent of N, coverage remaining. The ESD signal for N, is not proportional to the amount of N, remaining on the surface. The data indicate that along with desorption there is conversion to a electron impact desorption inactive state (not atomic N) which however is either N, or desorbs thermally like N,. UPS measurements show that such a new molecular state, with 5olr shifted to higher binding energy by - 1 eV is formed by electron impact. The desorption cross section is 1 X lo-i7 cm2, the conversion to inactive N,, 8.5 X lo-l8 cm2. The threshold for N2 desorption is E_= 7-8 eV, suggesting creation of a 50-l state as the desorption mechanism near threshold. Conversion to adsorbed N occurred at E_ 2 12.5 eV within our detection limit (XPS of the N 1s level of atomic N) suggesting a low energy intramolecular N, excitation. N+ is not formed from chemisorbed N but by Coulomb explosion of N,, probably after creation of a 30~’ 2 hole state. The weak binding of N, and the resultant large N,-W separation are postulated to account for the absence of N: : at the large N2-W distance in the ground state vertical transitions lead to the attractive part of any ionic curves reached, so that NC is propelled toward the surface and neutralized before it can desorb as an ion. Similarities and differences with the ESD behavior of CO are rationalized in terms of the differences in 50 and 2~ orbital distributions in the two cases.
1. Introduction The electron and photon stimulated desorption of CO from tungsten and ruthenium has been the subject of numerous studies [l]. Very much less work seems to have been done for N,, particularly from tungsten. It has generally been known that nitrogen is weakly adsorbed as N, on W(110) at 90 K (y-N2) and desorbed with only a relatively small amount of dissociative conversion to atomic N on heating to > 150 K [2] but that dissociative adsorption of y-N, is more efficient via electron impact [3]. It seemed interesting to investigate the
* Permanent address: Modem Physics Institute, Fudan University Shanghai, People’s Rep. of China. ** Permanent address: Nuclear Research Center-Negev, Beersheva, Israel. 0039-6028/90/$03.50
effect of electrons in more detail and to look for similarities and differences with the isoelectronic (and isobaric) CO molecule.
2. Experimental The apparatus and procedures were identical to those described previously in a study of the ESD behavior of CO from W(110) [l]. The main points are the following: The electron source consisted of a 0.003 inch diameter W filament located 3 mm in front of the crystal. The latter consisted of a rectangular slab of tungsten, 2 mm wide, 1.4 cm long and 0.15 mm thick, mounted as described previously [4]. In most experiments, particularly cross section determinations, where current uniformity over the crystal is important the filament
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
Q. -J. Zhang et al. / Desorption and conversion of N2 a&orbed on a W(ll0)
consisted of a central straight section running the length of the crystal, and coiled end sections, - 2 cm in length each, shielded from the crystal by Pt foil. While this arrangement insures current tmiformity over the straight section to within *lo% [5], the Ri drop over this section was estimated to be - 2 V. Although crystal-filament voltages refer to the midpoint of the filament this Ri drop introduces some uncertainty into threshold values. The latter were therefore determined both with this arrangement and with a simple straight filament. For the latter, current is highly concentrated near the center of the filament [5] so that the potential of the midpoint is probably a better measure of the crystal-filament voltage. The Ri drop over the hot section of this filament was < 1 V. In fact the differences in threshold values by the two arrangements were quite small, as will be shown in section 3. The crystal could be uniformly covered with N, by admitting the latter into the main chamber or the central section of the front face only could be covered with N, by means of a cryoshielded effusion source [4]. The cryoshield consisted of a liquid Hz cooled cylindrical annulus with a Cu front surface containing a rectangular hole matching the central section of the crystal; the latter could be positioned - 1 mm in front of this mask. Detection of neutral desorption products was greatly facilitated by using the cryoshield, which prevented N, from reaching the main chamber, since any gas reflected from the crystal condensed on the cold Cu face plate of the cryoshield. Background was further suppressed by 100% square wave modulation of the filament-crystal voltage and phase sensitive detection of the mass analyzer output via an HR-8 Lock-in amplifier. Modulation frequencies of - 20 Hz were used throughout. Desorption products entered the mass analyzer directly so that the latter acted as a flux detector. Initial coverages were varied by varying the exposure time, using the cryoshielded effusion source. It was shown in separate experiments [6] by temperature programmed desorption that the amount of N, adsorbed at 100 K is proportional to the impinged dose i.e. that the sticking coefficient is nearly constant to very high coverage, in agreement with ref. [2]. Thus coverages were
plane
345
estimated from exposure times relative to that for maximum coverage.
3. Results and discussion 3.1. Desorption products The absence of adsorbed atomic N, before electron bombardment at 90 K as well as its production by electron impact could be demonstrated by XPS. Fig. 1 shows a spectrum of the N 1s peak for adsorption at 90 K. Only the two peaks, characteristic of N, at E, = 406.6 and 400.6 eV appear [7]. After electron impact they disappear and a new peak at E, = 397 eV, typical of atomic N on this plane appears [7]. At an electron energy of 150 eV the maximum amount of N formed by electron impact is - 0.32 of the total nitrogen initially present if electron yields entering the CMA per incident photon are the same for atomic and molecular N. It will be shown in a comparison paper [6] that maximum initial N, coverage on W(110) probably corresponds to N/W = - 0.70 so that after conversion by electron impact the
11II
830
111,,1,,,,,,,,,11,,,,,,,,
840 Electron
850 Energy
860
eV
Fig. 1. XPS spectra of Nls peaks for N, adsorbed on W(110) at 90 K (virgin), after massive electron impact, V, =150 V, and after heating a virgin layer to 200 K. The number of scans was 4 for the virgin and electron’ bombarded layers, 5 for the heated one. Note the absence of the molecular peaks at 841.7 and 847.7 eV kinetic energy after heating and the absence of the peak attributed to atomic N at 851.3 eV electron kinetic energy before heating or electron impact. To convert to binding energy, relative to EF, 1248.3 eV must be substracted from the kinetic energies.
346
Q. J. Zhang et al. / Desorption and conversion of N2 adsorbed on a W(ll0)
maximum N coverage corresponds to N/W = 0.50. This value is in good agreement with that deduced previously by Shamir, Lin and Gomer [8] from a comparison of various Auger data. The desorption products of electron impact were neutral N,, small amounts of neutral N (which may correspond to dissociative ionization of N, in the mass analyzer) but only traces of NC. N+ was observed but only for electron energies of > 60 eV. Since chemisorbed N is a product of electron impact on y-N, attempts were also made to look for N or N+ from chemisorbed N, produced either by electron impact or thermally. No measurable N or N+ could be observed, indicating extremely small cross sections for ESD from chemisorbed N. 3.2. Cross sections Total disappearance cross sections were determined in the usual way from the decay of the mass spectrometer signals of various species, using - d8/dt = (3,ja, ePiutt,
=
dl&/dt = e#j,
200
I
I
400 Time,
/
600
I
I
,
800
Seconds
Fig. 2. Plots of log of the mass spectrometer signals M for the indicated species versus time t and the indicated relative coverages. Potential difference between crystal and electron source V, = 150 eV. Current density on crystal 93.3 PA/cm’ for the N and N, cmves and 187 PA/cm* for the Nf curve. These values refer to current during “on” periods. Times shown have been corrected for 50% duty cycle, i.e. are half the actual times. Curves are arbitrarily displaced from each other and only their slopes can be directly compared.
(I)
where ut is the sum of disappearance cross sections, j the electron current density and 8 the coverage of the ad-species contributing to desorption and/or conversion with 0, initial coverage and the assumption that @fk
I
0
plane
(2)
where Mk is the mass spectrometer signal for desorbed species k and ck an appropriate proportionality constant involving the rate of desorption of k, and its detection efficiency in the mass spectrometer. Fig. 2 shows plots of log M versus time for neutral N,, N, and N+ decay. The N, and N curves show two distinct regimes, despite the uniformity of the electron current distribution over the crystal. This is in contrast to CO desorption [l] where only one regime is seen when the current distribution is uniform. For N+ two regimes also occur when the initial N, coverage is d 0.5. The faster initial and slower final decay regimes are seen at all coverages, except 8 = 0.15 where the signal for N, and N is too small to identify the slow decay regime. The initial and final regime
disappearance cross sections are summarized in table 1. The slow regime cross sections deduced from N, decay are somewhat higher than those found from N decay. It will be shown shortly that only 50% of initially adsorbed N, is desorbed as N2 and that the slow regime thus corresponds to desorption of only a very small fraction of desorbable N,. It is also subject to considerable error because the signal is quite small and background must be subtracted from it. (Despite modulation there is some background left.) Thus, while there seems to be a slow regime the actual value of the cross section in it is difficult to determine quantitatively from ESD signal decay. The cross sections deduced from N+ decay are smaller by almost an order of magnitude than those from N, or N. This means either that N+ comes only from a state decaying more slowly than those yielding N, and N or that the decay rate of N+ is not a good measure of total population decay even if N+ comes from that population. This could come about if the reneutralization of N+ decreased with decreasing N, coverage, so that the slope of the
Q. -J. Zhang et al. / Desorption and conversion of N2 a&orbed on a W(1 IO) plane Table 1 Summary of disappearance cross sections from ESD signal decay Relative coverage
Of
:rn2)
From N2 decay 2.3 x lo-” 1.0 0.78 2.8x10-” 0.44 2.9x10-” 2.9x10-” 0.15
7.0x10-‘* 5.4x10-” 6.2x10-” -
From N decay 1.0 0.78 0.48
4.6~10-‘~ 3.0x10-‘* 4.0x lo-‘*
1.8x10-” 1.5 x lo-” 2.0x10-”
From N + decay 1.2xlo-ls 1.0 1.6~10-‘~ 0.78 3.1 x lo-‘* 0.48 4.0x10-‘* 0.15
I
(cm’ ) 116 87 53 22.5
86.5 58 37.5
1.3xlo-‘s 2.1 x 10-1s
Cross sections were determined as described in the text. The subscript i stands for initial, the subscript f for final regime. Z stands for mass spectrometer signal at t = 0. The Z values for different species do not scale.
log N+ versus t curve would be smaller than that of the log B versus t curve. Some support for this possibility comes from the behavior of CO+ from CO/Cu,/W(llO) or CO/Cu,/W(llO), where the CO+ signal actually goes through a maximum as ESD proceeds, i.e. is definitely not proportional to CO population [l]. The disappearance cross sections deducible from the log MN versus t curves are very similar to those from the log MN, curves, as indicated in table 1. It is not clear whether N is in fact a primary desorption product or an artifact. What is observed of course is an N+ signal in the mass spectrometer and this could come from ionization of desorbed N or from dissociative ionization of N, in the mass analyzer. For a static gas pressure of lop9 Torr the ratio of N+/N: caused by ionization in the mass spectrometer is 0.05. Experiments with thermally desorbing N, indicate a ratio 0.044 for 150 K desorption and 0.038 for desorption (as N2) of the atomically bound, i.e. converted N at 1240 K. Thus the ratio is in fact affected by the velocity of N, and its rotational and possibly vibrational excitations. At an electron energy of 150 eV we found N/N, = 0.02 in
347
ESD, i.e. even less than in low temperature thermal desorption or in static N, gas. The mass spectrometer ionizer efficiency for N is 0.7 that of N, at 70-100 eV, based on the efficiency versus mass curve supplied by UTI. The transmission coefficient for N is 1.05 times that for N2, and the detection efficiency in the multiplier for N is 0.77 of that for N,. Thus the N signal must be multiplied by 1.76 before comparison with N,. This still leaves the N signal very much smaller than the N, signal. This indicates that genuine electron induced neutral N desorption accounts for a very small fraction of total desorption, if any, and also that the velocity of desorbed N, is even greater than that of thermal desorption of 1250 K. Near threshold, i.e. for filament-crystal voltages I’, = 3-4 V, the ratio N/N, = 0.1. It will be shown in the next section that this is almost certainly not the result of a genuine increase in the relative cross sections but more likely a velocity effect for desorbing N,. Thus the relatively small differences in disappearance cross sections deduced from N, and N decay probably have no particular significance as far as adsorption states are concerned. For the states responsible for N, desorption both initial and final total cross sections seem virtually independent of coverage for 6 > 0.5. This is again in contrast with CO where cross sections decreased somewhat as initial coverage was decreased, although only a single disappearance cross section was observed [l]. In order to go beyond what can be learned from ESD signal decay alone we also interrupted electron bombardment at various times and ascertained the amount of N, left on the surface as well as the amount of atomic N formed by carrying out temperature programmed thermal desorption. The N2 and N coverages relative to those without electron bombardment could then be obtained by comparing the areas under the corresponding TPD peaks. It is shown in the companion paper to this one, describing thermal desorption [6] that 10% of the initial y-N, is converted thermally and this was used for calibrating the amount of adsorbed N produced by ESD. It was also assumed that 10% of N, left after any stage in the electron bombardment would also be converted to adsorbed N during subsequent thermal desorption;
348
Q. J. Zhang et al. / Desorplion and conversion ofNJ adwrbed
the amounts of N, left and N formed are corrected in this way. Unfortunately XPS is too insensitive to check this assumption, which could therefore be a source of error. If there were no thermal conversion at all (after appreciable creation of atomic N by electron impact) the amount of N, left on the surface, assumed to be 1.1 times the N, TPD peak area, would in fact be less by a factor of 1.1. Similarly the assumption of 10% thermal conversion of N, would, if wrong, give too low a value of atomic N produced by electron impact by an amount of 0.1 times the N, TPD peak area. For small electron induced conversion the assumption of 10% thermal conversion on heating is probably quite good. For large amounts of conversion the amount of N2 left is small and so the error in the amount of electron induced dissociation cannot be very large. However it is possible that the total amount of N, desorbed is underestimated by our assumption. It is also clear that the amount of Nz desorbed (in coverage units relative to initial N, coverage) is given by f& = 1 - &, - e;*,
(3)
where the superscripts stand for desorbed and converted y-N, and BNZ is the amount of y-N, remaining after t seconds of electron bombard-
00°
on a W(I i0) plane
100
200 300 400 t (seconds 1 Fig. 4. Amount of N, desorbed, 8&, in units of saturation coverage at 100 K versus ESD time. Conditions as in fig. 3.
ment. In these runs no attempt to detect the ESD signal was made so the electron current was not modulated. Comparison with ESD was made by correcting the time scale of a normal ESD run with modulated current for the 50% duty cycle. Fig. 3 shows an ESD signal MN2 and the amount of y-N, remaining (in units of dNz) for electron impact on a saturated layer prepared at 100 K. The most obvious feature is that there is still 25% of initial y-N, left by the time the ESD N, signal has decayed to 4% of its initial value. Thus the ESD signal is clearly not proportional to BNz.We will return to this point shortly. Figs. 4 and 5 show the amount of N, desorbed, 0& and converted to atomic N, Bh, versus time. All coverages
0.3-’
'
'
'
'
'
'
'
'
'
'
'
'
'
'
'
T'I
Fig. 3. EST) decay signal for N,, MN2, and molecular N, coverage, BN2 versus electron impact time. Both curves are for a saturated layer at 100 K and are normalized to values at t = 0. Current densities on crystal are 93.3 PA/cm’. For the ESD decay signal square wave modulation was used and the real times have been halved on the graph as in fig. 2. V, = 150 V.
4 0
’ ’ ’
’
100
( i ’ 200 ’ ’ ’ ’ 300’ ’ t (seconds
I
I
’
400
,j
900
1
Fig. 5. Amount of N, converted to adsorbed atomic N, tI$ in units of initial N2 saturation coverage at 100 K versus tmx. Conditions as in fig. 3.
349
Q.-J. Zhang et al. / Desorption and conversion of N2 adsorbed on a W(I IO) plane
0.6 z" ? 0.3 z" H 0.2
0
0.1
200
400
600
800
1000
t (seconds)
Fig. 7. Log 19~~versus t, for the data of fig. 3.
. 9 a, apparent l
e N,. Within experimental
eN2
h,,(o)
Fig. 6. Apparent desorption and conversion cross sections, ed and crcas determined from eqs. (4) and (5) and the derivatives of the curves in figs. 4 and 5 versus coverage of N, remaining, BN2. Also shown is MN&.+ which should parallel the o, curve. The apparent q, values vary strongly with BN2 as discussed in the text, while 0, is effectively constant.
are expressed in units of total C?,* at t = 0. It is possible to differentiate these curves graphically and to use the relations.
error, which is fairly large for the graphically obtained dt9$Jdt values the agreement between the curves is good. Finally fig. 7 shows log 8,2 versus t. This curve decays rapidly at first and eventually with constant slope corresponding to a cross section of 2.5 x lOPi cm*, identical to that of conversion deduced from the data of fig. 5 and eq. (5). If it is assumed that only a fraction of adsorbed N,, 8*, contributes to N, desorption, while the total population contributes to conversion the data of figs. 6, 7 and 8 can be explained quantitatively. With the above assumption and ut = a, + cd -d&Jdt
=jU,(f&
- e*)
+ju#*,
(6)
where t?* stands for the desorption active fraction
and
(5) which would be valid if desorption and conversion occurred from the total adsorbed N, population. Fig. 6 shows plots of the apparent desorption and conversion cross sections so obtained versus Q. The conversion cross section is independent of coverage within experimental error, at a, = 2.5 x lo-l8 cm*, while the desorption cross section is evidently not, in agreement with the earlier observation that the amount of N, remaining on the surface is not proportional to the ESD signal, MN2. The latter is proportional to de$Jdt and thus provides, at short times at least, a more accurate measure of the relation of desorption rate to coverage. Fig. 6 also shows MN,/&, versus
t (seconds) Fig. 8. Log MN2 versus t for a layer saturated at 100 K, then heated for 38 s at 132 K to remove the thermally labile fraction of adsorbate, before ESD at 100 K. Current density 93.3 PA/cm’, V, =150 V. oi = 1.7 x 10-r’ cd, er = 5.5 x 10-t* cm’.
Q. J. Zhang et al. / Desorption and conversion of N2 adsorbed on a W(I 10) plane
350
of N, (in units of ~~~(0)) and is given by a* =: @$ e-J=%t_
(7)
where @a*is the value of B * at t = 0. Thus - d&/dt
=je,
( t&,
- 02 e-‘“l’) t-j@?,* eeJ”I’
=~cT@,~+juJ?,* e-@lr,
(8)
with solution BN2/f&(0) = [l - 6,*(1 - e-Judf)] ebJ4’ = (1 _ e$) e-&i + e; e-jotf.
(91
Since a, = 2.2 x fO_“cm2, an order of magnitude larger than a,, the long time behavior of 61N2is just given by (1 - e,*) e-j’@ and this is observed since the slope of the long time portion of the curve in fig. 8 corresponds to a cross section of 2.5 x lo-‘* cm2, i.e. a,. Further, the intercept at t = 0 of this portion yields 1 - 02 = 0.5, so that fl,* = 0.5. Thus half of the initial N2 population is desorption active. With @a*= 0.5 and 0; = 2.2 X 10-“r7 cm2 it is now possible to calculate flN2/8NZ(O)versus t from eq. (9) and the results of this are shown in fig. 7 as the solid line. The agreement with the experimental points is very good. We can therefore conclude that u, = 2.2 x lo-r7 - 0.25 x lo-l7 = 1.95 x 10-‘7cm2 for this model. The value of @a*= 0.5 also agrees with the data of fig. 4 showing B& versus 1 since this curve levels off near f3$z/t?,z(0) = 0.5. It is totally unclear why only 50% of the initial N, coverage should be desorbable by electron impact. For lower initial coverages the disappearance cross section is unchanged and the initial values of MN1 scale with coverage, as indicated in table I. Thus 50% of any initial coverage would also have to be electron desorption active. It is shown in the companion paper to this one [6] that two regimes are seen in thermal desorption. 25% of N,, regardless of initial coverage, desorbs very rapidly, the remainder more slowly with E = 10.3 kcal, I = 1015s-i. This suggests two binding states, probably corresponding to different sites or types of adsorption, with desorption from an immobile layer. It would be tempting to associate the states seen in thermal desorption with the desorption active and inactive components. Nowever, the
amounts do not match and there is still a large and rapidly decaying ESD signal from a layer prepared by saturation coverage at 100 K, and then heated for 38 s to 132 K, which led to desorption of the labile thermal fraction (fig. 8). Thus this correlation cannot be made. Although the existence of a distinct electron stimulated desorption active fraction of y-N, cannot be ruled out it seems more likely that electron impact itself creates a new state (6) which is electron desorption inactive but can still lead to conversion to atomic N. If we designate the coverage of the ESD active state by 8* as before we have -d&*/dt=j(u,
+ ud + a,)@*,
@* = @* e-J(%+os+W
9
0
(101 01)
where us is the conversion cross section to the desorption inactive state 6. The latter is assumed however to be either molecular or at least to desorb thermally at the normal y-N, desorption temperature as N, and thus to form part of BN, as determined in TPD. Then
‘d ($f__)(l _ e-j(u~+a~)f) 4v*/4.J2(o> = [ 1 - a, x
e-iv,
(13)
where a, =u, f us + a,. This form is mathematically identical to eq. (9). If ~~*/~~~(O)= 1, i.e. if inirially the entire y-N, population is desorption active, the role of f?,,*= 0.5 in the previous analysis is taken over by ud/et. Analogously the amount of N, desorbed, 19& is given by a$, = (cd/i&)(1 - e-‘“‘z),
(14)
which will also approach 0.5 asymptotically as in fig. 4. Since ~~~/~~*(O~ of eq. (13) is of the same form as eq. (9), the two models yield identical results and differ only in their interpretation. If the electron induced conversion to (6) is correct, 50% of the total disappearance cross sec-
Q.-J. Zhang et al. / Desorption and cqnversion of Nz a&orbed on a W(lI0) plane
tion is a, = 1.1 X lo-l7 cm2, that for inactivation us = 8.5 x lo-‘* cm2 and a, = 2.5 x lo-‘* cm2. Electron induced conversion is also appealing because it accounts for the invariance of a, with coverage and with predesorption of the thermally labile fraction of y-N, in a natural way. While little can be said about the physical nature of the 6 state it could correspond to N, adsorbed lying down in a partially dissociated state i.e. with a N-N double rather than triple bond and a correspondingly abnormally long N-N bond length, but not truly dissociated. It is physically appealing that electron induced dissociation into adsorbed N, but not electron induced desorption as N, from lying down N, should be observed. N, TPD peaks after ESD did not seem to differ from those without electron impact, suggesting no very gross differences between y-N, and 6-N, in desorption. This could come about if heating first restores F-N, to the y-form (which then desorbs normally) rather than causing dissociation of 6-N,. In order to test the hypothesis of the creation of a new molecular state by electron bombardment UPS experiments were carried out. Fig. 9 shows He1 spectra of y-N, before and after electron bombardment at 150 V with the same current density used in fig. 3 for 200, and 400 s, as well as
a
I
,
I
I
,
I
,
351
with 4 times this current for an additional 600 s. The latter spectrum is that of atomic N. The spectra after 200 and 400 s of electron impact clearly show that the strong y-N, peak at E, - 7.5 eV has broadened and shifted to E, - 8.5 eV. The low binding side of the new peak may still have some 5a character, but whatever this state is, it is not atomic N or y-N,. Attempts were also made to look for S-N, via XPS and work function changes. The N 1s peaks of y-N, weaken as the atomic N 1s peak grows in with electron dose but it was impossible to detect a new peak, either between the y-N, and atomic N peaks, or to either side of these. Fig. 10 shows work function changes as function of electron bombardment time at the same current density as that in fig. 3. For y-N, adsorption to the coverage of the A+ minimum + increases monotonically with time. For saturation coverage where A+ - 100 meV, as indicated by fig. 12 of the companion paper [6], + first decreases slightly because of y-N, desorption and then increases as atomic N and 6-N, are formed. 6-N, does not show up by an unusually large negative or positive dipole moment. “Lying down” N, has been reported on Fe(ll1) by Grunze et al. [9] and on Cr(ll0) and Cr,/W(llO) by Shinn and Tsang [lo]. In both of
I
500L
Countsf
11
5
Binding
Energy
(eV)
II 6
Binding
It 7
11 0
11 9
I
IO
Energy (eV)
Fig. 9. (a) He I spectra of y-N, after electron bombardment at 150 V: (1) no electron bombardment, (2) 200 s at 5.9 x lOI electrons cm-* SC’, (3) 400 s at this current density, (4) additional exposure for 600 s at 23.6 X 1014 electrons cm-* s-‘. (b) Enlargement of the region where changes in the molecular spectra are most pronounced: (1) corresponds to -f-N,, (2) to a mixture of y-N, and the new state, which is present almost exclusively in (3) while (4) corresponds to atomic N. Energies are relative to E,.
352
Q.-J. Zhang et al. / Desorption and conversion of N, adsorbed on a W(I 10) plane
N,/W(IIO)
tEsD(set) Fig. 10. Ag versus electron bombardment time at 90 K. Current density on crystal 93.3 PA/cm’. V, = 150 V. (0) Saturated y-N, layer. (X) N, dose corresponding to work function minimum.
these cases the lying down species is a precursor to dissociation (on heating) and shows a Nls peak distinct from that of standing y-N, or atomic N, between the former and the latter. Shinn and Tsang also find in UPS a peak at E, - 8.4 eV which is very close to our 6-N, value. Thus 6-N, seems to be similar but not identical to the lying down N, species seen on Fe(ll1) on Cr(ll0) and on Cr,/W(llO). The main differences are first that 6-N, on W(110) is formed only be electron bombardment and second that is not a precursor to thermal dissociation into N but rather desorbs as N, on heating. Most likely these differences have to do with the geometry and electronic properties of the substrates. 6-N, on W(110) also does not show a distinct N 1s peak in XPS, but this could simply be the result of a broader 6-N, peak on W(110) than on Fe(ll1) or Cr(ll0). We consider finally the slow decay regime in ESD. Fig. 11 shows, in addition to log MN1 also log(d$$/dt) versus t. These curves should necessarily be superimposable since they must be proportional to each other, barring changes in angular or velocity distribution of desorbing N, as time and hence coverage changes. Although the df3&/dt values obtained graphically are not very accurate the agreement is reasonable at short time. For t > 100 s both curves show slower decay. The
effect is much more pronounced for MN2. Thus there probably is a slow decay regime, but as already pointed out this affects only a small fraction of desorbable N,. It is noteworthy that in this and (d0$2/dt)/B,z still decrease regime %&& with coverage, as indicated in fig. 6. Thus a mechanism similar to that at high N, coverage must still be operative in this regime. Possibly this could result from the decreased probability of forming 6-N, when the surface contains a large fraction of atomic N, or from a genuine 8 dependence of a,. It is interesting to compare the magnitudes of the various cross sections for N, on W(110) with corresponding ones for CO. The desorption cross section for N, is roughly 10 times bigger than that for CO/W(llO) and 1.5 times that for CO/C&’ W(110) [l]. The conversion cross section for N2 to cross secatomic N is - 2 times the dissociation tion for CO on W(110) [l] (CO does not dissociate on Cu,/W(llO) [l]). The larger cross sections for N, must reflect the smaller probability of making recapturing transitions to the ground state. This is undoubtedly caused by the smaller matrix elements for such processes, arising from the fact that both the filled 5a and empty 277 orbitals are
0
200
400
t (seconds
eSC
1
Fig. 11. Log MN2, (0), and ded/dt, (x), versus t for the data of fig. 3. Both curves are normalized to their values at t = 0.
Q. J. Zhang et al. / Desorption and conversion of Nz o&orbed on a W(I 10) plane
less concentrated on a N atom (including that binding to the surface) than they are on C in CO. This also accounts for the much weaker binding of N, to the substrate. Judging by comparison with CO on Cu,/W(llO), where UPS suggests no rrbackbonding [I], the differences in 5a orbitals seem more in important than those in 2s both for adsorption energy and cross sections. The latter have already been discussed. Binding energies of CO on Cu,/W(llO) are 0.87 eV at low and 0.65 eV at high coverage [ll]. On W(110) the CO binding energy can only be estimated from the range of desorption temperatures (300-400 K) [lo] at l-l.5 eV. Even the lowest of these values exceeds the binding energy of N, on W(llO), 0.45 eV (less for part of the adsorbate).
I1
40
I
60
I
I
80
f
I
353
I
too
I
120
t
I
I
140
v, Votts Fig. 13. N’ signal versus Vt, near threshold, obtained with a straight filament. The yield is not corrected for the fraction of current hitting the crystal. Its decrease above V, =140 V is probably the result of less current hitting the front surface of the crystal. (See ref. [l].) Also shown is background curve.
3.3. Threshold measurements Fig. 12 shows typical curves of signal versus crystal-filament voltage, V, near threshold for N, and N; Fig. 13 shows values for N+. As discussed previously [l] the actual electron energy E_ must be increased by the emitter work function (p, = 4.5 eV if the incident electron is able to utilize all its energy to the Fermi level of the substrate:
E_= V,,+&.
(15)
If the incident electron winds up at the vacuum
I
’
I
*
I
3
I
I
I
’
I
I
I
N2/W(IIO)
---. I
2
,
I
I
4
6
v,
volts
8
I
IO
Fig. 12. Yields versus crystal-filament voltage Va for a straight filament for N, and N near threshold. Also shown are background curves obtained with a clean crystal. These measurements were taken without voltage modulation and are not corrected for the fraction of cnrrent hitting the crystal.
level of the substrate the available electron energy would be
where (p, is the work function of the substrate. Intermediate cases are of course possible. The uncertainties in the determination of threshold energies arising from Ri effects have already been noted. For the straight filament the threshold for N, was V,, = 3-4 eV, without use of square wave modulation of V,. For the filament with coiled end sections, and square wave modulation the threshold was V,, = 2-3 eV. If eq. (15) is applicable E_ is thus 6.5-8.5 eV within the uncertainties of the measurements. The N,/W(llO) UPS spectrum shown in fig. 11 is very similar to that of CO/W(llO), and it therefore seems reasonable that the same peak assignments can be made. The peak at 7.5 eV below E, would then correspond to 5a, and this suggests that desorption of neutral N, at least near threshold arises from creation of a (screened) 5a-’ single hole state, as postulated for CO desorption [l]. The apparent threshold for neutral N is V, = 3-4 volts, by both straight and coiled end filaments, corresponding to E_= 7.5-8.5 eV. It is dubious whether the N signal is in fact real. First, it is extremely small and second this energy is insufficient to break the N-N bond, 9.8 eV (plus
354
Q.-J. Zhang et al. / Desorption and conversion of Nz adsorbed on a W(II0) plane
0.45eV N, binding energy to the substrate) unless some of the energy gained in chemisorption of a remaining N atom can be utilized. If this were so however, there should still be a great deal of converted N observable on the surface, near the N threshold, particulary since the N/N, ratio observed in the mass analyzer is then 0.1 as already pointed out. However the conversion threshold lies near V, = 8 volts, as will be discussed presently. Thus the apparent N threshold may simply correspond to the smallest detectable signal of Nf produced in the mass spectrometer from N,. The threshold for conversion to chemisorbed N was probed by looking for the atomic Nls XPS peak, which is quite distinct from that of molecular nitrogen, as already pointed out. This threshold seemed to be V,, = 8 V, within the detection limit suggesting E _= 12.5 eV if all energy is utilized or 12.5 minus some fraction of the substrate work function, (5.2 eV for the surface saturated with N2) if it is not. This suggests that a fairly low intramolecular excitation of N, may be responsible for formation of chemisorbed N. The threshold for N+ formation was V, = 6065 eV. This clearly indicates that N+ is formed only by an intramolecular excitation involving multiple hole states. For N+ from N,/Ru(lOO) Feulner, Treichler, and Menzel see a similar threshold which they attribute to 3a-* [12]. At first glance the almost total absence of NC seems surprising. In the case of CO/W(llO) CO+ is easily observed although its desorption cross section is estimated to be 10-22-10-21 cm2, depending on coverage [l]. The threshold for CO+ formation seems to correspond either to creation of 5a-= or possibly to 4a-’ [l]. The comparable energies for N: would be quite similar, namely V, = 12.5 V, but virtually no NC could be seen up to the highest energy used, V,, = 150 eV. This suggests that the various ionic curves are still attractive at the relatively large N,-W distance corresponding to the minimum of the y-N, potential curve, which is only weakly binding, as already noted, so that NC ions formed by vertical transitions to the 5ad2 or 4a-’ curves from the ground state move toward the surface and are neutralized before they can escape as ions. In this respect the N,/W(llO) system seems to be quite
similar to inert gases on tungsten, where only neutral atoms are desorbed, although the ESD thresholds correspond to ion formation [13].
4. Conclusion The ESD behavior of N2 adsorbed on W(110) shows some similarities to and some differences from the behavior of the isoelectronic, isobaric CO molecule. The thresholds and mechanisms for desorption of the neutral molecules seem to be nearly identical because of the similar electronic structures and positions in energy of the 5u levels. Cross sections for disappearance are much higher for N, than for CO because N, is more weakly bound. The absence of NC contrasts with the abundant presence of CO+ and can probably also be explained by the weak binding and hence large N,-W surface separation. The electron induced creation of an (electron) desorption inactive state is a new and surprising result of the present work and does not seem to occur for CO.
Acknowledgements This work was supported in part by NSF Grant DMR 861-7186. We have also benefitted from the Materials Research Laboratory of the National Science Foundation at the University of Chicago. The XPS and work function measurements as function of electron bombardment time were carried out by Dr. Y.-B. Zhao, whom we wish to thank.
References [l] J.C. Lin and R. Gamer. Surf. Sci. 218 (1989) 407, and references therein. [2] J.T. Yates, Jr., R. Klein and T.E. Madey, Surf. Sci. 58 (1976) 469. [3] A. Polak and G. Ehrlich, J. Vat. Sci. Technol. 14 (1977) 407. [4] Ch. Steinbriichel and R. Gomer, Surf. Sci. 67 (1977) 21. [5] J.C. Lin and R. Gomer, Surf. Sci. 172 (1986) 183. [6] J.C. Lin, N. Shamir, Y.B. Zhao and R. Gomer, Surf. Sci. 231 (1990) 333.
Q.-J. Zhang et al. / Desorption and conversion of N2 acisorbed on a W(Il0) plane [7] J.C. Fuggle and D. Menzel, Surf. Sci. 79 (1979) 1. [S] N. Shamir, J.C. Lin and R. Gomer, Surf. Sci. 214 (1989) 74. [9] M. Grunze, M. Golze, W. Hirschwald, H.-J. Freund, H. Pulm, U. Seip, G. Ertl and J. Ktippers, Phys. Rev. Lett. 53 (1984) 850.
355
[lo] N. Shinn and K.-L. Tsang, J. Vat. Sci. Technol., in press. [ll] M. Chelvayohan and R. Gomer, Surf. Sci. 186 (1987) 412. [12] P. Feulner, R. Treichler, and D. Menzel, Phys. Rev. B 24 (1981) 7427, and private communication. [13] Q.-J. Zhang, R. Gomer and D.R. Bowman, Surf. Sci. 129 (1983) 535.
344
ELECTRON ADSORBED
STIMULATED DESORPTION AND CONVERSION ON A TUNGSTEN (110) PLANE
Q.-J. ZHANG Department
OF N,
*, J.C. LIN, N. SHAMIR * * and R. GOMER
of Chemistry and The James Franck Institute,
The University of Chicago, Chicago, 60637 IL, USA
Received 11 October 1989; accepted for publication 20 December 1989
The response of y-N, on W(110) to electron impact has been investigated. The desorption products are principally neutral N, and small amounts of neutral N, which may come from decomposition of N, in the mass analyzer. N: was not detected. N+ is seen for electron energies above 60 eV. Approximately 32% of the initial saturated y-N, layer (in terms of N atoms) is converted by electron impact to chemisorbed atomic N at E_= 150 eV, corresponding to N/W = 0.50. It was possible, by thermal desorption after electron impact for different times to follow both the appearance of N and the desorption of N,. It turns out that the cross section for conversion to adsorbed atomic N is 2.5 x 10-r’ cm2, virtually independent of N, coverage remaining. The ESD signal for N, is not proportional to the amount of N, remaining on the surface. The data indicate that along with desorption there is conversion to a electron impact desorption inactive state (not atomic N) which however is either N, or desorbs thermally like N,. UPS measurements show that such a new molecular state, with 5olr shifted to higher binding energy by - 1 eV is formed by electron impact. The desorption cross section is 1 X lo-i7 cm2, the conversion to inactive N,, 8.5 X lo-l8 cm2. The threshold for N2 desorption is E_= 7-8 eV, suggesting creation of a 50-l state as the desorption mechanism near threshold. Conversion to adsorbed N occurred at E_ 2 12.5 eV within our detection limit (XPS of the N 1s level of atomic N) suggesting a low energy intramolecular N, excitation. N+ is not formed from chemisorbed N but by Coulomb explosion of N,, probably after creation of a 30~’ 2 hole state. The weak binding of N, and the resultant large N,-W separation are postulated to account for the absence of N: : at the large N2-W distance in the ground state vertical transitions lead to the attractive part of any ionic curves reached, so that NC is propelled toward the surface and neutralized before it can desorb as an ion. Similarities and differences with the ESD behavior of CO are rationalized in terms of the differences in 50 and 2~ orbital distributions in the two cases.
1. Introduction The electron and photon stimulated desorption of CO from tungsten and ruthenium has been the subject of numerous studies [l]. Very much less work seems to have been done for N,, particularly from tungsten. It has generally been known that nitrogen is weakly adsorbed as N, on W(110) at 90 K (y-N2) and desorbed with only a relatively small amount of dissociative conversion to atomic N on heating to > 150 K [2] but that dissociative adsorption of y-N, is more efficient via electron impact [3]. It seemed interesting to investigate the
* Permanent address: Modem Physics Institute, Fudan University Shanghai, People’s Rep. of China. ** Permanent address: Nuclear Research Center-Negev, Beersheva, Israel. 0039-6028/90/$03.50
effect of electrons in more detail and to look for similarities and differences with the isoelectronic (and isobaric) CO molecule.
2. Experimental The apparatus and procedures were identical to those described previously in a study of the ESD behavior of CO from W(110) [l]. The main points are the following: The electron source consisted of a 0.003 inch diameter W filament located 3 mm in front of the crystal. The latter consisted of a rectangular slab of tungsten, 2 mm wide, 1.4 cm long and 0.15 mm thick, mounted as described previously [4]. In most experiments, particularly cross section determinations, where current uniformity over the crystal is important the filament
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
Q. -J. Zhang et al. / Desorption and conversion of N2 a&orbed on a W(ll0)
consisted of a central straight section running the length of the crystal, and coiled end sections, - 2 cm in length each, shielded from the crystal by Pt foil. While this arrangement insures current tmiformity over the straight section to within *lo% [5], the Ri drop over this section was estimated to be - 2 V. Although crystal-filament voltages refer to the midpoint of the filament this Ri drop introduces some uncertainty into threshold values. The latter were therefore determined both with this arrangement and with a simple straight filament. For the latter, current is highly concentrated near the center of the filament [5] so that the potential of the midpoint is probably a better measure of the crystal-filament voltage. The Ri drop over the hot section of this filament was < 1 V. In fact the differences in threshold values by the two arrangements were quite small, as will be shown in section 3. The crystal could be uniformly covered with N, by admitting the latter into the main chamber or the central section of the front face only could be covered with N, by means of a cryoshielded effusion source [4]. The cryoshield consisted of a liquid Hz cooled cylindrical annulus with a Cu front surface containing a rectangular hole matching the central section of the crystal; the latter could be positioned - 1 mm in front of this mask. Detection of neutral desorption products was greatly facilitated by using the cryoshield, which prevented N, from reaching the main chamber, since any gas reflected from the crystal condensed on the cold Cu face plate of the cryoshield. Background was further suppressed by 100% square wave modulation of the filament-crystal voltage and phase sensitive detection of the mass analyzer output via an HR-8 Lock-in amplifier. Modulation frequencies of - 20 Hz were used throughout. Desorption products entered the mass analyzer directly so that the latter acted as a flux detector. Initial coverages were varied by varying the exposure time, using the cryoshielded effusion source. It was shown in separate experiments [6] by temperature programmed desorption that the amount of N, adsorbed at 100 K is proportional to the impinged dose i.e. that the sticking coefficient is nearly constant to very high coverage, in agreement with ref. [2]. Thus coverages were
plane
345
estimated from exposure times relative to that for maximum coverage.
3. Results and discussion 3.1. Desorption products The absence of adsorbed atomic N, before electron bombardment at 90 K as well as its production by electron impact could be demonstrated by XPS. Fig. 1 shows a spectrum of the N 1s peak for adsorption at 90 K. Only the two peaks, characteristic of N, at E, = 406.6 and 400.6 eV appear [7]. After electron impact they disappear and a new peak at E, = 397 eV, typical of atomic N on this plane appears [7]. At an electron energy of 150 eV the maximum amount of N formed by electron impact is - 0.32 of the total nitrogen initially present if electron yields entering the CMA per incident photon are the same for atomic and molecular N. It will be shown in a comparison paper [6] that maximum initial N, coverage on W(110) probably corresponds to N/W = - 0.70 so that after conversion by electron impact the
11II
830
111,,1,,,,,,,,,11,,,,,,,,
840 Electron
850 Energy
860
eV
Fig. 1. XPS spectra of Nls peaks for N, adsorbed on W(110) at 90 K (virgin), after massive electron impact, V, =150 V, and after heating a virgin layer to 200 K. The number of scans was 4 for the virgin and electron’ bombarded layers, 5 for the heated one. Note the absence of the molecular peaks at 841.7 and 847.7 eV kinetic energy after heating and the absence of the peak attributed to atomic N at 851.3 eV electron kinetic energy before heating or electron impact. To convert to binding energy, relative to EF, 1248.3 eV must be substracted from the kinetic energies.
346
Q. J. Zhang et al. / Desorption and conversion of N2 adsorbed on a W(ll0)
maximum N coverage corresponds to N/W = 0.50. This value is in good agreement with that deduced previously by Shamir, Lin and Gomer [8] from a comparison of various Auger data. The desorption products of electron impact were neutral N,, small amounts of neutral N (which may correspond to dissociative ionization of N, in the mass analyzer) but only traces of NC. N+ was observed but only for electron energies of > 60 eV. Since chemisorbed N is a product of electron impact on y-N, attempts were also made to look for N or N+ from chemisorbed N, produced either by electron impact or thermally. No measurable N or N+ could be observed, indicating extremely small cross sections for ESD from chemisorbed N. 3.2. Cross sections Total disappearance cross sections were determined in the usual way from the decay of the mass spectrometer signals of various species, using - d8/dt = (3,ja, ePiutt,
=
dl&/dt = e#j,
200
I
I
400 Time,
/
600
I
I
,
800
Seconds
Fig. 2. Plots of log of the mass spectrometer signals M for the indicated species versus time t and the indicated relative coverages. Potential difference between crystal and electron source V, = 150 eV. Current density on crystal 93.3 PA/cm’ for the N and N, cmves and 187 PA/cm* for the Nf curve. These values refer to current during “on” periods. Times shown have been corrected for 50% duty cycle, i.e. are half the actual times. Curves are arbitrarily displaced from each other and only their slopes can be directly compared.
(I)
where ut is the sum of disappearance cross sections, j the electron current density and 8 the coverage of the ad-species contributing to desorption and/or conversion with 0, initial coverage and the assumption that @fk
I
0
plane
(2)
where Mk is the mass spectrometer signal for desorbed species k and ck an appropriate proportionality constant involving the rate of desorption of k, and its detection efficiency in the mass spectrometer. Fig. 2 shows plots of log M versus time for neutral N,, N, and N+ decay. The N, and N curves show two distinct regimes, despite the uniformity of the electron current distribution over the crystal. This is in contrast to CO desorption [l] where only one regime is seen when the current distribution is uniform. For N+ two regimes also occur when the initial N, coverage is d 0.5. The faster initial and slower final decay regimes are seen at all coverages, except 8 = 0.15 where the signal for N, and N is too small to identify the slow decay regime. The initial and final regime
disappearance cross sections are summarized in table 1. The slow regime cross sections deduced from N, decay are somewhat higher than those found from N decay. It will be shown shortly that only 50% of initially adsorbed N, is desorbed as N2 and that the slow regime thus corresponds to desorption of only a very small fraction of desorbable N,. It is also subject to considerable error because the signal is quite small and background must be subtracted from it. (Despite modulation there is some background left.) Thus, while there seems to be a slow regime the actual value of the cross section in it is difficult to determine quantitatively from ESD signal decay. The cross sections deduced from N+ decay are smaller by almost an order of magnitude than those from N, or N. This means either that N+ comes only from a state decaying more slowly than those yielding N, and N or that the decay rate of N+ is not a good measure of total population decay even if N+ comes from that population. This could come about if the reneutralization of N+ decreased with decreasing N, coverage, so that the slope of the
Q. -J. Zhang et al. / Desorption and conversion of N2 a&orbed on a W(1 IO) plane Table 1 Summary of disappearance cross sections from ESD signal decay Relative coverage
Of
:rn2)
From N2 decay 2.3 x lo-” 1.0 0.78 2.8x10-” 0.44 2.9x10-” 2.9x10-” 0.15
7.0x10-‘* 5.4x10-” 6.2x10-” -
From N decay 1.0 0.78 0.48
4.6~10-‘~ 3.0x10-‘* 4.0x lo-‘*
1.8x10-” 1.5 x lo-” 2.0x10-”
From N + decay 1.2xlo-ls 1.0 1.6~10-‘~ 0.78 3.1 x lo-‘* 0.48 4.0x10-‘* 0.15
I
(cm’ ) 116 87 53 22.5
86.5 58 37.5
1.3xlo-‘s 2.1 x 10-1s
Cross sections were determined as described in the text. The subscript i stands for initial, the subscript f for final regime. Z stands for mass spectrometer signal at t = 0. The Z values for different species do not scale.
log N+ versus t curve would be smaller than that of the log B versus t curve. Some support for this possibility comes from the behavior of CO+ from CO/Cu,/W(llO) or CO/Cu,/W(llO), where the CO+ signal actually goes through a maximum as ESD proceeds, i.e. is definitely not proportional to CO population [l]. The disappearance cross sections deducible from the log MN versus t curves are very similar to those from the log MN, curves, as indicated in table 1. It is not clear whether N is in fact a primary desorption product or an artifact. What is observed of course is an N+ signal in the mass spectrometer and this could come from ionization of desorbed N or from dissociative ionization of N, in the mass analyzer. For a static gas pressure of lop9 Torr the ratio of N+/N: caused by ionization in the mass spectrometer is 0.05. Experiments with thermally desorbing N, indicate a ratio 0.044 for 150 K desorption and 0.038 for desorption (as N2) of the atomically bound, i.e. converted N at 1240 K. Thus the ratio is in fact affected by the velocity of N, and its rotational and possibly vibrational excitations. At an electron energy of 150 eV we found N/N, = 0.02 in
347
ESD, i.e. even less than in low temperature thermal desorption or in static N, gas. The mass spectrometer ionizer efficiency for N is 0.7 that of N, at 70-100 eV, based on the efficiency versus mass curve supplied by UTI. The transmission coefficient for N is 1.05 times that for N2, and the detection efficiency in the multiplier for N is 0.77 of that for N,. Thus the N signal must be multiplied by 1.76 before comparison with N,. This still leaves the N signal very much smaller than the N, signal. This indicates that genuine electron induced neutral N desorption accounts for a very small fraction of total desorption, if any, and also that the velocity of desorbed N, is even greater than that of thermal desorption of 1250 K. Near threshold, i.e. for filament-crystal voltages I’, = 3-4 V, the ratio N/N, = 0.1. It will be shown in the next section that this is almost certainly not the result of a genuine increase in the relative cross sections but more likely a velocity effect for desorbing N,. Thus the relatively small differences in disappearance cross sections deduced from N, and N decay probably have no particular significance as far as adsorption states are concerned. For the states responsible for N, desorption both initial and final total cross sections seem virtually independent of coverage for 6 > 0.5. This is again in contrast with CO where cross sections decreased somewhat as initial coverage was decreased, although only a single disappearance cross section was observed [l]. In order to go beyond what can be learned from ESD signal decay alone we also interrupted electron bombardment at various times and ascertained the amount of N, left on the surface as well as the amount of atomic N formed by carrying out temperature programmed thermal desorption. The N2 and N coverages relative to those without electron bombardment could then be obtained by comparing the areas under the corresponding TPD peaks. It is shown in the companion paper to this one, describing thermal desorption [6] that 10% of the initial y-N, is converted thermally and this was used for calibrating the amount of adsorbed N produced by ESD. It was also assumed that 10% of N, left after any stage in the electron bombardment would also be converted to adsorbed N during subsequent thermal desorption;
348
Q. J. Zhang et al. / Desorplion and conversion ofNJ adwrbed
the amounts of N, left and N formed are corrected in this way. Unfortunately XPS is too insensitive to check this assumption, which could therefore be a source of error. If there were no thermal conversion at all (after appreciable creation of atomic N by electron impact) the amount of N, left on the surface, assumed to be 1.1 times the N, TPD peak area, would in fact be less by a factor of 1.1. Similarly the assumption of 10% thermal conversion of N, would, if wrong, give too low a value of atomic N produced by electron impact by an amount of 0.1 times the N, TPD peak area. For small electron induced conversion the assumption of 10% thermal conversion on heating is probably quite good. For large amounts of conversion the amount of N2 left is small and so the error in the amount of electron induced dissociation cannot be very large. However it is possible that the total amount of N, desorbed is underestimated by our assumption. It is also clear that the amount of Nz desorbed (in coverage units relative to initial N, coverage) is given by f& = 1 - &, - e;*,
(3)
where the superscripts stand for desorbed and converted y-N, and BNZ is the amount of y-N, remaining after t seconds of electron bombard-
00°
on a W(I i0) plane
100
200 300 400 t (seconds 1 Fig. 4. Amount of N, desorbed, 8&, in units of saturation coverage at 100 K versus ESD time. Conditions as in fig. 3.
ment. In these runs no attempt to detect the ESD signal was made so the electron current was not modulated. Comparison with ESD was made by correcting the time scale of a normal ESD run with modulated current for the 50% duty cycle. Fig. 3 shows an ESD signal MN2 and the amount of y-N, remaining (in units of dNz) for electron impact on a saturated layer prepared at 100 K. The most obvious feature is that there is still 25% of initial y-N, left by the time the ESD N, signal has decayed to 4% of its initial value. Thus the ESD signal is clearly not proportional to BNz.We will return to this point shortly. Figs. 4 and 5 show the amount of N, desorbed, 0& and converted to atomic N, Bh, versus time. All coverages
0.3-’
'
'
'
'
'
'
'
'
'
'
'
'
'
'
'
T'I
Fig. 3. EST) decay signal for N,, MN2, and molecular N, coverage, BN2 versus electron impact time. Both curves are for a saturated layer at 100 K and are normalized to values at t = 0. Current densities on crystal are 93.3 PA/cm’. For the ESD decay signal square wave modulation was used and the real times have been halved on the graph as in fig. 2. V, = 150 V.
4 0
’ ’ ’
’
100
( i ’ 200 ’ ’ ’ ’ 300’ ’ t (seconds
I
I
’
400
,j
900
1
Fig. 5. Amount of N, converted to adsorbed atomic N, tI$ in units of initial N2 saturation coverage at 100 K versus tmx. Conditions as in fig. 3.
349
Q.-J. Zhang et al. / Desorption and conversion of N2 adsorbed on a W(I IO) plane
0.6 z" ? 0.3 z" H 0.2
0
0.1
200
400
600
800
1000
t (seconds)
Fig. 7. Log 19~~versus t, for the data of fig. 3.
. 9 a, apparent l
e N,. Within experimental
eN2
h,,(o)
Fig. 6. Apparent desorption and conversion cross sections, ed and crcas determined from eqs. (4) and (5) and the derivatives of the curves in figs. 4 and 5 versus coverage of N, remaining, BN2. Also shown is MN&.+ which should parallel the o, curve. The apparent q, values vary strongly with BN2 as discussed in the text, while 0, is effectively constant.
are expressed in units of total C?,* at t = 0. It is possible to differentiate these curves graphically and to use the relations.
error, which is fairly large for the graphically obtained dt9$Jdt values the agreement between the curves is good. Finally fig. 7 shows log 8,2 versus t. This curve decays rapidly at first and eventually with constant slope corresponding to a cross section of 2.5 x lOPi cm*, identical to that of conversion deduced from the data of fig. 5 and eq. (5). If it is assumed that only a fraction of adsorbed N,, 8*, contributes to N, desorption, while the total population contributes to conversion the data of figs. 6, 7 and 8 can be explained quantitatively. With the above assumption and ut = a, + cd -d&Jdt
=jU,(f&
- e*)
+ju#*,
(6)
where t?* stands for the desorption active fraction
and
(5) which would be valid if desorption and conversion occurred from the total adsorbed N, population. Fig. 6 shows plots of the apparent desorption and conversion cross sections so obtained versus Q. The conversion cross section is independent of coverage within experimental error, at a, = 2.5 x lo-l8 cm*, while the desorption cross section is evidently not, in agreement with the earlier observation that the amount of N, remaining on the surface is not proportional to the ESD signal, MN2. The latter is proportional to de$Jdt and thus provides, at short times at least, a more accurate measure of the relation of desorption rate to coverage. Fig. 6 also shows MN,/&, versus
t (seconds) Fig. 8. Log MN2 versus t for a layer saturated at 100 K, then heated for 38 s at 132 K to remove the thermally labile fraction of adsorbate, before ESD at 100 K. Current density 93.3 PA/cm’, V, =150 V. oi = 1.7 x 10-r’ cd, er = 5.5 x 10-t* cm’.
Q. J. Zhang et al. / Desorption and conversion of N2 adsorbed on a W(I 10) plane
350
of N, (in units of ~~~(0)) and is given by a* =: @$ e-J=%t_
(7)
where @a*is the value of B * at t = 0. Thus - d&/dt
=je,
( t&,
- 02 e-‘“l’) t-j@?,* eeJ”I’
=~cT@,~+juJ?,* e-@lr,
(8)
with solution BN2/f&(0) = [l - 6,*(1 - e-Judf)] ebJ4’ = (1 _ e$) e-&i + e; e-jotf.
(91
Since a, = 2.2 x fO_“cm2, an order of magnitude larger than a,, the long time behavior of 61N2is just given by (1 - e,*) e-j’@ and this is observed since the slope of the long time portion of the curve in fig. 8 corresponds to a cross section of 2.5 x lo-‘* cm2, i.e. a,. Further, the intercept at t = 0 of this portion yields 1 - 02 = 0.5, so that fl,* = 0.5. Thus half of the initial N2 population is desorption active. With @a*= 0.5 and 0; = 2.2 X 10-“r7 cm2 it is now possible to calculate flN2/8NZ(O)versus t from eq. (9) and the results of this are shown in fig. 7 as the solid line. The agreement with the experimental points is very good. We can therefore conclude that u, = 2.2 x lo-r7 - 0.25 x lo-l7 = 1.95 x 10-‘7cm2 for this model. The value of @a*= 0.5 also agrees with the data of fig. 4 showing B& versus 1 since this curve levels off near f3$z/t?,z(0) = 0.5. It is totally unclear why only 50% of the initial N, coverage should be desorbable by electron impact. For lower initial coverages the disappearance cross section is unchanged and the initial values of MN1 scale with coverage, as indicated in table I. Thus 50% of any initial coverage would also have to be electron desorption active. It is shown in the companion paper to this one [6] that two regimes are seen in thermal desorption. 25% of N,, regardless of initial coverage, desorbs very rapidly, the remainder more slowly with E = 10.3 kcal, I = 1015s-i. This suggests two binding states, probably corresponding to different sites or types of adsorption, with desorption from an immobile layer. It would be tempting to associate the states seen in thermal desorption with the desorption active and inactive components. Nowever, the
amounts do not match and there is still a large and rapidly decaying ESD signal from a layer prepared by saturation coverage at 100 K, and then heated for 38 s to 132 K, which led to desorption of the labile thermal fraction (fig. 8). Thus this correlation cannot be made. Although the existence of a distinct electron stimulated desorption active fraction of y-N, cannot be ruled out it seems more likely that electron impact itself creates a new state (6) which is electron desorption inactive but can still lead to conversion to atomic N. If we designate the coverage of the ESD active state by 8* as before we have -d&*/dt=j(u,
+ ud + a,)@*,
@* = @* e-J(%+os+W
9
0
(101 01)
where us is the conversion cross section to the desorption inactive state 6. The latter is assumed however to be either molecular or at least to desorb thermally at the normal y-N, desorption temperature as N, and thus to form part of BN, as determined in TPD. Then
‘d ($f__)(l _ e-j(u~+a~)f) 4v*/4.J2(o> = [ 1 - a, x
e-iv,
(13)
where a, =u, f us + a,. This form is mathematically identical to eq. (9). If ~~*/~~~(O)= 1, i.e. if inirially the entire y-N, population is desorption active, the role of f?,,*= 0.5 in the previous analysis is taken over by ud/et. Analogously the amount of N, desorbed, 19& is given by a$, = (cd/i&)(1 - e-‘“‘z),
(14)
which will also approach 0.5 asymptotically as in fig. 4. Since ~~~/~~*(O~ of eq. (13) is of the same form as eq. (9), the two models yield identical results and differ only in their interpretation. If the electron induced conversion to (6) is correct, 50% of the total disappearance cross sec-
Q.-J. Zhang et al. / Desorption and cqnversion of Nz a&orbed on a W(lI0) plane
tion is a, = 1.1 X lo-l7 cm2, that for inactivation us = 8.5 x lo-‘* cm2 and a, = 2.5 x lo-‘* cm2. Electron induced conversion is also appealing because it accounts for the invariance of a, with coverage and with predesorption of the thermally labile fraction of y-N, in a natural way. While little can be said about the physical nature of the 6 state it could correspond to N, adsorbed lying down in a partially dissociated state i.e. with a N-N double rather than triple bond and a correspondingly abnormally long N-N bond length, but not truly dissociated. It is physically appealing that electron induced dissociation into adsorbed N, but not electron induced desorption as N, from lying down N, should be observed. N, TPD peaks after ESD did not seem to differ from those without electron impact, suggesting no very gross differences between y-N, and 6-N, in desorption. This could come about if heating first restores F-N, to the y-form (which then desorbs normally) rather than causing dissociation of 6-N,. In order to test the hypothesis of the creation of a new molecular state by electron bombardment UPS experiments were carried out. Fig. 9 shows He1 spectra of y-N, before and after electron bombardment at 150 V with the same current density used in fig. 3 for 200, and 400 s, as well as
a
I
,
I
I
,
I
,
351
with 4 times this current for an additional 600 s. The latter spectrum is that of atomic N. The spectra after 200 and 400 s of electron impact clearly show that the strong y-N, peak at E, - 7.5 eV has broadened and shifted to E, - 8.5 eV. The low binding side of the new peak may still have some 5a character, but whatever this state is, it is not atomic N or y-N,. Attempts were also made to look for S-N, via XPS and work function changes. The N 1s peaks of y-N, weaken as the atomic N 1s peak grows in with electron dose but it was impossible to detect a new peak, either between the y-N, and atomic N peaks, or to either side of these. Fig. 10 shows work function changes as function of electron bombardment time at the same current density as that in fig. 3. For y-N, adsorption to the coverage of the A+ minimum + increases monotonically with time. For saturation coverage where A+ - 100 meV, as indicated by fig. 12 of the companion paper [6], + first decreases slightly because of y-N, desorption and then increases as atomic N and 6-N, are formed. 6-N, does not show up by an unusually large negative or positive dipole moment. “Lying down” N, has been reported on Fe(ll1) by Grunze et al. [9] and on Cr(ll0) and Cr,/W(llO) by Shinn and Tsang [lo]. In both of
I
500L
Countsf
11
5
Binding
Energy
(eV)
II 6
Binding
It 7
11 0
11 9
I
IO
Energy (eV)
Fig. 9. (a) He I spectra of y-N, after electron bombardment at 150 V: (1) no electron bombardment, (2) 200 s at 5.9 x lOI electrons cm-* SC’, (3) 400 s at this current density, (4) additional exposure for 600 s at 23.6 X 1014 electrons cm-* s-‘. (b) Enlargement of the region where changes in the molecular spectra are most pronounced: (1) corresponds to -f-N,, (2) to a mixture of y-N, and the new state, which is present almost exclusively in (3) while (4) corresponds to atomic N. Energies are relative to E,.
352
Q.-J. Zhang et al. / Desorption and conversion of N, adsorbed on a W(I 10) plane
N,/W(IIO)
tEsD(set) Fig. 10. Ag versus electron bombardment time at 90 K. Current density on crystal 93.3 PA/cm’. V, = 150 V. (0) Saturated y-N, layer. (X) N, dose corresponding to work function minimum.
these cases the lying down species is a precursor to dissociation (on heating) and shows a Nls peak distinct from that of standing y-N, or atomic N, between the former and the latter. Shinn and Tsang also find in UPS a peak at E, - 8.4 eV which is very close to our 6-N, value. Thus 6-N, seems to be similar but not identical to the lying down N, species seen on Fe(ll1) on Cr(ll0) and on Cr,/W(llO). The main differences are first that 6-N, on W(110) is formed only be electron bombardment and second that is not a precursor to thermal dissociation into N but rather desorbs as N, on heating. Most likely these differences have to do with the geometry and electronic properties of the substrates. 6-N, on W(110) also does not show a distinct N 1s peak in XPS, but this could simply be the result of a broader 6-N, peak on W(110) than on Fe(ll1) or Cr(ll0). We consider finally the slow decay regime in ESD. Fig. 11 shows, in addition to log MN1 also log(d$$/dt) versus t. These curves should necessarily be superimposable since they must be proportional to each other, barring changes in angular or velocity distribution of desorbing N, as time and hence coverage changes. Although the df3&/dt values obtained graphically are not very accurate the agreement is reasonable at short time. For t > 100 s both curves show slower decay. The
effect is much more pronounced for MN2. Thus there probably is a slow decay regime, but as already pointed out this affects only a small fraction of desorbable N,. It is noteworthy that in this and (d0$2/dt)/B,z still decrease regime %&& with coverage, as indicated in fig. 6. Thus a mechanism similar to that at high N, coverage must still be operative in this regime. Possibly this could result from the decreased probability of forming 6-N, when the surface contains a large fraction of atomic N, or from a genuine 8 dependence of a,. It is interesting to compare the magnitudes of the various cross sections for N, on W(110) with corresponding ones for CO. The desorption cross section for N, is roughly 10 times bigger than that for CO/W(llO) and 1.5 times that for CO/C&’ W(110) [l]. The conversion cross section for N2 to cross secatomic N is - 2 times the dissociation tion for CO on W(110) [l] (CO does not dissociate on Cu,/W(llO) [l]). The larger cross sections for N, must reflect the smaller probability of making recapturing transitions to the ground state. This is undoubtedly caused by the smaller matrix elements for such processes, arising from the fact that both the filled 5a and empty 277 orbitals are
0
200
400
t (seconds
eSC
1
Fig. 11. Log MN2, (0), and ded/dt, (x), versus t for the data of fig. 3. Both curves are normalized to their values at t = 0.
Q. J. Zhang et al. / Desorption and conversion of Nz o&orbed on a W(I 10) plane
less concentrated on a N atom (including that binding to the surface) than they are on C in CO. This also accounts for the much weaker binding of N, to the substrate. Judging by comparison with CO on Cu,/W(llO), where UPS suggests no rrbackbonding [I], the differences in 5a orbitals seem more in important than those in 2s both for adsorption energy and cross sections. The latter have already been discussed. Binding energies of CO on Cu,/W(llO) are 0.87 eV at low and 0.65 eV at high coverage [ll]. On W(110) the CO binding energy can only be estimated from the range of desorption temperatures (300-400 K) [lo] at l-l.5 eV. Even the lowest of these values exceeds the binding energy of N, on W(llO), 0.45 eV (less for part of the adsorbate).
I1
40
I
60
I
I
80
f
I
353
I
too
I
120
t
I
I
140
v, Votts Fig. 13. N’ signal versus Vt, near threshold, obtained with a straight filament. The yield is not corrected for the fraction of current hitting the crystal. Its decrease above V, =140 V is probably the result of less current hitting the front surface of the crystal. (See ref. [l].) Also shown is background curve.
3.3. Threshold measurements Fig. 12 shows typical curves of signal versus crystal-filament voltage, V, near threshold for N, and N; Fig. 13 shows values for N+. As discussed previously [l] the actual electron energy E_ must be increased by the emitter work function (p, = 4.5 eV if the incident electron is able to utilize all its energy to the Fermi level of the substrate:
E_= V,,+&.
(15)
If the incident electron winds up at the vacuum
I
’
I
*
I
3
I
I
I
’
I
I
I
N2/W(IIO)
---. I
2
,
I
I
4
6
v,
volts
8
I
IO
Fig. 12. Yields versus crystal-filament voltage Va for a straight filament for N, and N near threshold. Also shown are background curves obtained with a clean crystal. These measurements were taken without voltage modulation and are not corrected for the fraction of cnrrent hitting the crystal.
level of the substrate the available electron energy would be
where (p, is the work function of the substrate. Intermediate cases are of course possible. The uncertainties in the determination of threshold energies arising from Ri effects have already been noted. For the straight filament the threshold for N, was V,, = 3-4 eV, without use of square wave modulation of V,. For the filament with coiled end sections, and square wave modulation the threshold was V,, = 2-3 eV. If eq. (15) is applicable E_ is thus 6.5-8.5 eV within the uncertainties of the measurements. The N,/W(llO) UPS spectrum shown in fig. 11 is very similar to that of CO/W(llO), and it therefore seems reasonable that the same peak assignments can be made. The peak at 7.5 eV below E, would then correspond to 5a, and this suggests that desorption of neutral N, at least near threshold arises from creation of a (screened) 5a-’ single hole state, as postulated for CO desorption [l]. The apparent threshold for neutral N is V, = 3-4 volts, by both straight and coiled end filaments, corresponding to E_= 7.5-8.5 eV. It is dubious whether the N signal is in fact real. First, it is extremely small and second this energy is insufficient to break the N-N bond, 9.8 eV (plus
354
Q.-J. Zhang et al. / Desorption and conversion of Nz adsorbed on a W(II0) plane
0.45eV N, binding energy to the substrate) unless some of the energy gained in chemisorption of a remaining N atom can be utilized. If this were so however, there should still be a great deal of converted N observable on the surface, near the N threshold, particulary since the N/N, ratio observed in the mass analyzer is then 0.1 as already pointed out. However the conversion threshold lies near V, = 8 volts, as will be discussed presently. Thus the apparent N threshold may simply correspond to the smallest detectable signal of Nf produced in the mass spectrometer from N,. The threshold for conversion to chemisorbed N was probed by looking for the atomic Nls XPS peak, which is quite distinct from that of molecular nitrogen, as already pointed out. This threshold seemed to be V,, = 8 V, within the detection limit suggesting E _= 12.5 eV if all energy is utilized or 12.5 minus some fraction of the substrate work function, (5.2 eV for the surface saturated with N2) if it is not. This suggests that a fairly low intramolecular excitation of N, may be responsible for formation of chemisorbed N. The threshold for N+ formation was V, = 6065 eV. This clearly indicates that N+ is formed only by an intramolecular excitation involving multiple hole states. For N+ from N,/Ru(lOO) Feulner, Treichler, and Menzel see a similar threshold which they attribute to 3a-* [12]. At first glance the almost total absence of NC seems surprising. In the case of CO/W(llO) CO+ is easily observed although its desorption cross section is estimated to be 10-22-10-21 cm2, depending on coverage [l]. The threshold for CO+ formation seems to correspond either to creation of 5a-= or possibly to 4a-’ [l]. The comparable energies for N: would be quite similar, namely V, = 12.5 V, but virtually no NC could be seen up to the highest energy used, V,, = 150 eV. This suggests that the various ionic curves are still attractive at the relatively large N,-W distance corresponding to the minimum of the y-N, potential curve, which is only weakly binding, as already noted, so that NC ions formed by vertical transitions to the 5ad2 or 4a-’ curves from the ground state move toward the surface and are neutralized before they can escape as ions. In this respect the N,/W(llO) system seems to be quite
similar to inert gases on tungsten, where only neutral atoms are desorbed, although the ESD thresholds correspond to ion formation [13].
4. Conclusion The ESD behavior of N2 adsorbed on W(110) shows some similarities to and some differences from the behavior of the isoelectronic, isobaric CO molecule. The thresholds and mechanisms for desorption of the neutral molecules seem to be nearly identical because of the similar electronic structures and positions in energy of the 5u levels. Cross sections for disappearance are much higher for N, than for CO because N, is more weakly bound. The absence of NC contrasts with the abundant presence of CO+ and can probably also be explained by the weak binding and hence large N,-W surface separation. The electron induced creation of an (electron) desorption inactive state is a new and surprising result of the present work and does not seem to occur for CO.
Acknowledgements This work was supported in part by NSF Grant DMR 861-7186. We have also benefitted from the Materials Research Laboratory of the National Science Foundation at the University of Chicago. The XPS and work function measurements as function of electron bombardment time were carried out by Dr. Y.-B. Zhao, whom we wish to thank.
References [l] J.C. Lin and R. Gamer. Surf. Sci. 218 (1989) 407, and references therein. [2] J.T. Yates, Jr., R. Klein and T.E. Madey, Surf. Sci. 58 (1976) 469. [3] A. Polak and G. Ehrlich, J. Vat. Sci. Technol. 14 (1977) 407. [4] Ch. Steinbriichel and R. Gomer, Surf. Sci. 67 (1977) 21. [5] J.C. Lin and R. Gomer, Surf. Sci. 172 (1986) 183. [6] J.C. Lin, N. Shamir, Y.B. Zhao and R. Gomer, Surf. Sci. 231 (1990) 333.
Q.-J. Zhang et al. / Desorption and conversion of N2 acisorbed on a W(Il0) plane [7] J.C. Fuggle and D. Menzel, Surf. Sci. 79 (1979) 1. [S] N. Shamir, J.C. Lin and R. Gomer, Surf. Sci. 214 (1989) 74. [9] M. Grunze, M. Golze, W. Hirschwald, H.-J. Freund, H. Pulm, U. Seip, G. Ertl and J. Ktippers, Phys. Rev. Lett. 53 (1984) 850.
355
[lo] N. Shinn and K.-L. Tsang, J. Vat. Sci. Technol., in press. [ll] M. Chelvayohan and R. Gomer, Surf. Sci. 186 (1987) 412. [12] P. Feulner, R. Treichler, and D. Menzel, Phys. Rev. B 24 (1981) 7427, and private communication. [13] Q.-J. Zhang, R. Gomer and D.R. Bowman, Surf. Sci. 129 (1983) 535.