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Laser induced charge transfer from Ni to CO adsorbed on (111) and (110) surfaces
Surface Science 0 North-Holland
93 (1980) 107-116 Publishing Company
LASER INDUCED CHARGE TRANSFER
FROM Ni TO CO ADSORBED ON
(111) AND (110) SURFACES *
H.W. RUDOLF and W. STEINMANN Sckth
Received
Physik dcr UniversitdtMtincl~en,Sehellin~strasse 4, D-8000 Miinchen 40, Gcrruany
21 August
1979; accepted
for publication
5 November
1979
Ni(ll0) and (111) surfaces covered with up to one monolayer of CO were irradiated with the light of a dye laser in the photon energy range 2.0 to 3.4 eV. Two-photon photocmission was obscrvcd when the laser light was focussed. Upon defocussing a signal was measured which did not depend on the potential of the sample and showed a linear intensity dependence. It is caused by electrons transferred from the Ni substrate into adsorbate states. The signal vanishes for photon energies below 2 eV. This shows that the adsorbate state lies at most 2 cV above the fermi level. The lifetime 7 of the electrons in the adsorbate states is cstirnated to be lo-” < 7 < 10-7 s. No tluoresccnce in the photon energy range above 1 eV could be detected.
1. Introduction
The bonding of simple molecules on transition metal surfaces has been investigated for considerable time using various methods. Nickel has been of particular interest in this context because of its wide application as a catalyst. In order to understand the processes governing catalysis, the adsorption has to be known in detail. Photoemission experiments have been very important in clarifying chemisorption. In particular the occupied levels of adsorbed CO on Ni surfaces and other transition metal surfaces have been determined by UPS. Recently it has been possible to identify the orbitals of the CO-Ni system in terms of the free CO molecule by measuring the angular distribution of the photoelectrons and its dependence on the polarisation of the incident light [ 11. The occupied levels of CO on Ni are thus well known. A point which is not yet well understood concerns the binding energy of CO on d-band metal surfaces. It varies considerably, e.g. by a factor of 2 between Cu and Ni. This cannot be accounted for by the position of the occupied levels in both the adsorbate and the metal [2]. Blyholder [3] and Doyen and Ertl [4] have emphasized that the binding energy may be influenced by antibonding molecular orbitals close to the fermi energy. A review of the experimental results concerning CO and other adsorbates on Ni has recently been given by Yu et al. [2]. These authors * Work supported
by Deutschc
Forschungsgerneinschaft,
107
Sonderforschungsbereich
128
point
out that the binding
levels
1 77.5 cr. 4
themselves range
0.
in the variation
of the d-bands.
vicinity
energy
of the
The levels
not only by the position
in tlic elcctrcmic
of the energy
Fermi energy.
electroncgativity.
is detctminctt
hut also by changes
distribution
to these
Like the chemical
hand
of mc~leculcs
of the CO
which
manifest
of 1l1e ptlotoclectrons
contributing
the chemisorption
structure effects
between
on metal
in tl~c
art‘ located atoms
surfaces
in ttlc
of different seems
to 1~2
brought about by ttre formation of pairs of honding a11d antihonding orbitals originating from the hyhridi/.ation of the Inotecular and the metal orbitals. In order to compare
theoretical
results
ing and antihonding
and experimental
orbitals
data,
have to be known.
the positions
of both the hond-
In CPS-experiments,
this requires
both levels to be occupied. Even if this necessary condition is fulfilled the antibonding orbital can be identified only under specially fortunate circumstances. An example is O2 on C’u where the structureless range between tltc top of the d-band and the fermi level is modulated by the antibonding orbital of the adsorbate which can he observed by variation of the cross-section (If the photoc\;citatioll 121. It transition metals with partially filled d-bands. like Ni. are used as ;I substrate the antibonding orbital lies either in the trange of tt~e d-bands and can thus hardly tic detected. or above the Fermi Icvel. In tlris case photoemission can not 1~ used and instead methods like surface reflection and electron energy loss spectroscopy 15 1 have to he applied. These experiments Ilowcvcr yield only the energy between levels hut not the absolute position of the lcvct itself. A suitable
method
for determining
is tu use a level below initial
state
for the
excitation
Absorption
measurements
that
states
initial
show
levels have to be used as initial of partial
states
and insulators
can be detected excited electron.
in photoemission method
into
tile lcvets under
region
and near large dispersion.
for this purpc)se.
yield have been successful
in semiconductors
An alternative
3 rather
levels above the t;ermi energy
the pcjGtic)n of which is well known,
of electrons
in the visible
normally
using the method
the unoccupied
the Formi energy.
diffcrcncc
lJV
suffer from tllf f‘;lct Sufficiently deep core
Pl~otoemission in finding
cxperimcnts
unoccupied
[6]. t f tbc levels lie below
the vacuum
only via Auger transitions
or energy
of determining
electronic
states
as an
consideration.
between
levels
level they
losses of the
the Fermi level
and vacuum level is two-photon photoemission. In this process one photon is used to excite an electron into the intermediate lcvet which is empty if the system is in its ground state. The second photon excites the electron from the intermediate state into a level above the vacuum energy so that it can be detected as a photoelectron and, by measuring the kinetic cnelgy, the p~~sition of the intermediate state with respect to the vacuum level can be determined *. This cxperhent requires ;I * In addition
to the wsonant t\vo-photon c*citatiun via real interlncdiatc states, tllere ;ITCnonvia virtual intermediate states. The cross scL3ion of the resonant cxcita-
resonant transitions
larper and it can bc shown
tion is much to two-photon
hroatlcninp
~~lrotoemission
or the levels in sdids
[ 8) that the contribution
is small compared lo the rcson:mt
is taken into account.
uf nonresonant excitations
transitions
cvcn il_ lifctimc
rather high intensity since the lifetitnc of the intermediate states is considerabiy shorter than in molecules or ions [IO]. A sufficietirly high intensity is only available from lasers. Furthermore, the photon energy has to bc high enough, i.e. of the order of the work function or the photoelectric threshold for one photon photoemission. Clean Ni surfaces have work functions around 5 eV, which increase up to about 6 eV if the surface is covered with certain adsorbates. e.g.. CO. We have studied two photon I)hotuennissiott front metal surfaces. The first experiments were carried out with thin films of Al [7]. Recently, we have applied this method to Ni( 110) and (1 11) surfaces. As a light source we have used a dye laser which was putnpcd by a Nz laser. This establishes a continuously tunable light source of sufficiently high intensity in the photon energy range from 2 to 3.4 eV. With clean Ni surfaces we found two-photon photoemission for all photon energies exceeding half the work function, i.e. PKL> 3.5 eV. The details and results of this experiment will be reported elsewhere [X]. When the sut-face was covered with a monolayer of CO. the photoelectric yield decreased drastically. This is at least partly due to the litnited photon energy: with Xc; < 0.8 eV the maximum twophoton energy exceeded the work function by less than 1 eV. As regards VLII- aim to determine electronic states between the Fermi level and the vacuum level, we are limited by the maximum photon energy to states close to the center of the energy range in question. In order to extend the energy range to be investigated by twophoton photoemission, the photon energy has to he increased by using, e.g., excimer lasers. These lasers are not continuously tunable. but the intermediate states or resonances should show up in the energy distribution of the photoelectrons. Although the maximum photon energy of our light source was not high enough to find the two-photon photoemission via an intermediate adsorbate state. we have found experimental evidence for such states in CO Ni( 1 1 1) and CO- Ni( 1 IO) systems which we report in this papet.
2. Experimental The experimental arrangement is schematically shown in fig. 1. The dye laser emits light pulses with a width of 3 ns and about 50 kW peak power at a repetition rate between 5 and 50 cps. The light of the dye laser irradiates the surface of a Ni single crystal which is mounted in a UHV system. In the two-photon photoetnission experiments the light is focussed to irradiate a spot of about 0.1 mm diameter with an intensity of about lo8 W/ctn’. Focussing is essential since the two-photon effect depends quadratically on the intensity. The phenomenon reported in this paper is a one-photon effect. Due to the linear dependence on the light intensity the signal does not change when the light is defocussed since the product of intensity and irradiated area remains constant. Therefore it was not necessary to focus the beam in these experiments.
r-l
N, -laser
charge sensltlve
The crystal was cleaned by argon ion bombardment and subsequent annealing. The structure and cleanlyncss of the surface was controlled in situ by a LEED Auger system. The pressure was in the low to-” Torr range. Adsorption layers were produced by a bakeable gas doser system. The electric signals were measured and processed by standard pulse electronics as shown in fig. 1. The sensitivity of the system was about IO-’ 7 A s.
3. Results and discussion The maximum kinetic energy of photoelectrons produced by two-photon absorption is cUo = 2Rw - $J where Q is the work function of the emitter. If the potential of the emitter is raised above a cutoff, i.e. U> Uo, the photoelectric current vanishes. We have observed this effect. well known from one-photon photoemission, with clean as well as with oxygen-covered surfaces as emitters in twophoton photoemission. When the surface was covered with CO, the signal did not
H. W. Rudolf;
W. Strirlmam
/ Laser induced
charge trarfsfer
from
.Vi to CO
111
slgnal amphtude rel units
Fig. 2. The signal amplitude is plotted as a function of the retarding voltage for photoelectrons applied to the sample. (a) The dashed curve gives the photocurrent from a clean Ni surface: the current vanishes for positive sample potential higher than l/u = 2nw - @ where @ is the work function. (b) The solid curve gives the signal amplitude after adsorption of CO on the surface There remains a signal amplitude for I/ > Uo. (c) The dash-dotted curve shows the signal amplitude from the CO covered surface after defocussing of the beam. No signal was obtained from the clean surface in this case.
vanish even if a potential U 9 U, was applied (see fig. 2). The remaining signal was obviously not due to photoelectrons leaving the emitter, since the signal amplitude did not change noticeably when the applied voltage was varied up to 1000 V. The following explanations cannot account for this effect: (a) Positive ions leaving the emitter would lead to a signal with opposite polarity. (b) Photoelectrons produced by scattered light in parts of the system which are at ground potential can also be ruled out with the polarity argument. (c)Positive ions flying towards the emitter would produce a current of the observed polarity by they would be prevented from doing so by the high positive potential. (d) Photodesorption of CO- or any other negative ions cannot occur at high positive sample potentials. Consequently the signal cannot be caused by charge transport in the vacuum. (e) The signal could arise from photodesorption of neutral CO molecules which would change the work function. This process can be ruled out because it would lead to a signal of opposite polarity, since a desorption would result in a decrease of
the work function which is equivalent to a decrease of ncgativc cl~a~gc in flout of the xurfacc. Furtlierrnorc, n desorption of either neutral 01. charged adsorbate ~nckcult5 should result in ;I decrease of the signal under irradiation. This was not observed. Thcrnml effects can be cxcludcd since the signal did not chnnge when the light Iw~ ~3s defocussed (SW fig. 2).
lj.set
k I
H
gru dt
c ‘t -=T
I_/
Lip. 3. The response of the charge sensitive presmplit‘icr is compared with various input s+als: (a) Shape of tire incoming light pulse; the time SLY& for (a) (d) is given in this fip~rr. (b) ‘I‘hc time dependence trf the xsulting two-photon phutoernission signal is proportional tu I*. (c) The tilne depcndcncc of the input sigmd caused by charging of thr adsorbatc: for lifetimes of the electrons in the adsorbate states, 7, short compared to the pulacwidth, 7, the char~c transfer into the adsorbatc states occurs only during the rise time of the light pulse: this positive charging signal is followed by a negative pulse due to the tlowback of the cbar~c stored in the adsorbatc during the decrease of the light intensity; the time integral over the whole signal must vanish since no net charge transtcr between sample and anodc has occurred. (d) I,or 7 P, T, the charge stored in the adsorbate is just the integral over the charge excited by the light into the adsorbate states; the negative part of the signal ia govcrncd by the decaytime of the charge. (c) The response of the amplifier tu a single positive pulse with width T 15 1 MS:the time scale to (e) and (1) is given here. (1) The rcsponsc of the amplifier to a double pulse like (c); the output pulse is narrower than the rcsponsc (c) to a single pulse (h); the narrowing is observed for decay times 7 < 1 O-’ s: for longer decay times the response is sinlila to (e).
There remains only one possible explanation for the signal at high positive potential of the emitter: An increase of negative charge in the adsorbate layer. The absorbed photons excite electrons in the Ni substrate which consequently occupy intermediate levels in the adsorbate. This would lead to a charge transport over the distance of the order of lo-” m which therefore produces an accordingly small signal per electron. The large number of photons however guarantees that sufficiently many elementary charge transfer processes take place within the width of the light pulse to produce an observable signal. Two consequences come to mind immediately: (a) The phenomenon should be a one-photon effect; therefore the signal should depend linearly on the intensity. (b) The lifetime of the electrons in the intermediate state should be finite. This should produce a signal which is qualitatively different from a pulse produced in photoemission as shown in fig. 3. The different intensity dependence can qualitatively be seen in fig. 2. The quadratic effect vanishes upon defocussing since it is proportional to I* A, where Z gives the intensity and A the illuminated area. I* A is proportional to l/A if the radiation flux Z.4 remains constant. The linear effect, which does not depend on the emitter potential U, does not change upon defocussing since the signal amplitude is proportional to IA. Fig. 4 shows a quantitative proof of the intensity dependence. The
Fig. 4. The intensity dependcnces of the two-photon photoemission signal and the charging signal for CO covered surface are compared. Solid line: linear response of the charging signal. Dash-dotted line: quadratic response of the two-photon photoemission signal.
difference in the pulse shape, shown in figs. 3b and 3~2,could not be demonstrated directly since the time constant of the preamplifier was not small enough. We have. however, simulated the two input pulses with a pulse generator and observed that the shapes of the output pulses were different (see figs. 3e and 30. The output pulse of the linear signal observed with a defocussed light beam had a shape as shown in fig. 3f. which is compatible with lifetimes shorter than 1O-7 s. On the other hand, the signal should rapidly decrease if the lifetime became shorter than the pulse width of the exciting light. Ilence WC can estimate the lifetime T to he in the range 10m7 > 7 > 1O-‘O s. llic lower end of this range requires a quantitative estimate of the charge transfer and its effect on the signal. The latter can readily he obtained if the geometry of the emitter 4lector arrangement is idealized by a parallel plate condenser. In this case a charge qe which is transported over the distance d at the emitter induces a charge qC = y&/D at the collector where D is the distance between emitter and collector. With d = lo-” m. D = 10-l1x1, 3 X 1Ol4 photons per pulse and a yield of lo-’ electrons transferred to the adsorbate layer per photon absorbed, one obtains an input signal of 5 X IO-l6 A s. which is more than one order of magnitude above the noise level. The height of the signal measured was typically of the order of IO-l4 A s. If one calculates the effect for a geometry which is closer to reality than a parallel plate condenser, one finds that qJq, > d/D.Due to the rather complicated geometry of our arrangement. it is difficult to arrive at 3 good estimate for the ratio qc/qe. However, assuming cl = 1O-" 111. (lc/qe - 1O-7 is a plausible figure. Thus the measured signal can bc accounted for by the proposed process. The uncertainty introduced by the geometry and the yield leads to an equivalent uncertainty for the lower limit of the estimated lifetime of electrons in the intermediate udsorhute states. The signal depends rather strongly on the photon energy in the range over which the laser could be tuned. This dependence is shown in fig. 5. The increase of the signal with increasing photon energy can he explained if one assumes that the adsorbate state can he populated not only by electrons excited directly into it, hut also by electrons excited to higher energies and subsequently scattered into the adsorbate level by electron electron interaction. The data shown in fig. 5 prove that the adsorbate level involved lies at most 3 eV above the Fermi level. Alternatively. the data could he accounted for if one assumes a hand of adsorbate states, extending from 3 eV above the Fermi level upwards. The threshold may in fact lie below 2 eV, since the sensitivity of our arrangement may not have been high enough to find the effect at lower photon energies. We have attempted to detect a possible fluorescence arising from the radiative decay of the adsorbate state. The photocathode of the multiplier limited the lower end of the photon energy range to 1 eV. Using the N2 laser with a photon cncrgy of -3.7 eV, we did not find any increase of the signal over the dark current when the laser light hit the sample. The sensitivity was such that no more than one fluorescence photon in the energy range 1 < ?zti G 3 eV per 10’ laser photons absorbed
electronsperobsphoton rel lnts
NI (111I
0
co
photon energy L
I
2
2.5
Fig. 5. The amplitude of the charging signal as a function corrected for rctlectivity of the sample.
3 av of photon
energy.
The signal has been
have been emitted. Therefore we conclude that either the adsorbate state decays by a radiationless process or the photon energy of the fluorescence is smaller than 1 eV. In the latter case, the initial distance between the adsorbate state and the Fermi level does not necessarily have to be smaller than 1 eV, since an appreciable relaxation of the system, leading to a decrease of the energy of the intermediate adsorbate state, could occur during the lifetime. The energy difference between the relaxed adsorbate state and the Fermi level may be rather small. This would explain the surprisingly long lifetime of lo-” s or more we have to assume in order to account for the effect. The CO-Ni system has recently been investigated theoretically by Yu [9] who considered a cluster of five Ni atoms and one CO molecule in a geometry corresponding to a (100) surface. He finds a cluster state originating from the interaction of the 5a CO orbital with s-d hybridized Ni states which lies 0.7 eV above the Fermi energy. The electron density pattern of this state extends well into the metal, but has also considerable constributions at the CO molecule. It is an antibonding orbital which would lead to a decrease in the binding energy upon population. This in turn would cause an increase of the dipole moment, enhancing the effect which leads to the signal we have observed. It would also result in a relaxation which would bring the adsorbate level considerably closer to the Fermi level. In conclusion, this adsorbate state calculated by Yu could be responsible for our results. It lies within the energy range of which we could so far only determine the upper limit. Experiments with higher laser photon energy, leading to photoionization of the adsorbate state, are necessary to measure the position of the adsorbate level after relaxation. could
Acknowledgements
The authors wish to thank D. Rieger, V. Saile and T. Wegehaupt for their substantial contribution to the fluorescence measurement, and J. Hermanson for valuable comments.
References [ I] [Z] [3] [4] [5] [6] (71 [8] [9]
[lo]
C.L. Allpn. T. Gustafson and I<.W. Plummcr. Chcm. Phys. Letters 47 (1977) 127. K.Y. Yu.W.C. Spicer, I. Lindau, P. Pianetta and S.1‘. Lin, Surface Sci. 57 (1976) 157 G. Blyholder. J. Phys. Chem. 68 (1964) 2772. G. Doyen and k. lktl, Surface Sci. 43 (1974) 197. G.W. Rubloffand J.L. I:rccouf, Phys. Rev. 1~17 (1978) 4680. D.E. 1:astman and J.L. I:reeouf, Phys. Rev. Letters 33 (1974) 1601. H.W. Rudolf and W. Steinmann, Phys. Letters 61A (1977) 471. H.W. Rudolf, D. Riegcr and W. Stcinmann, to be published. H.L. Yu. J. Chem. Phys. 69 (1978) 1755. VI. Bondybcy and T.A. Miller, J. Chem. Phys. 69 (1978) 3597.
93 (1980) 107-116 Publishing Company
LASER INDUCED CHARGE TRANSFER
FROM Ni TO CO ADSORBED ON
(111) AND (110) SURFACES *
H.W. RUDOLF and W. STEINMANN Sckth
Received
Physik dcr UniversitdtMtincl~en,Sehellin~strasse 4, D-8000 Miinchen 40, Gcrruany
21 August
1979; accepted
for publication
5 November
1979
Ni(ll0) and (111) surfaces covered with up to one monolayer of CO were irradiated with the light of a dye laser in the photon energy range 2.0 to 3.4 eV. Two-photon photocmission was obscrvcd when the laser light was focussed. Upon defocussing a signal was measured which did not depend on the potential of the sample and showed a linear intensity dependence. It is caused by electrons transferred from the Ni substrate into adsorbate states. The signal vanishes for photon energies below 2 eV. This shows that the adsorbate state lies at most 2 cV above the fermi level. The lifetime 7 of the electrons in the adsorbate states is cstirnated to be lo-” < 7 < 10-7 s. No tluoresccnce in the photon energy range above 1 eV could be detected.
1. Introduction
The bonding of simple molecules on transition metal surfaces has been investigated for considerable time using various methods. Nickel has been of particular interest in this context because of its wide application as a catalyst. In order to understand the processes governing catalysis, the adsorption has to be known in detail. Photoemission experiments have been very important in clarifying chemisorption. In particular the occupied levels of adsorbed CO on Ni surfaces and other transition metal surfaces have been determined by UPS. Recently it has been possible to identify the orbitals of the CO-Ni system in terms of the free CO molecule by measuring the angular distribution of the photoelectrons and its dependence on the polarisation of the incident light [ 11. The occupied levels of CO on Ni are thus well known. A point which is not yet well understood concerns the binding energy of CO on d-band metal surfaces. It varies considerably, e.g. by a factor of 2 between Cu and Ni. This cannot be accounted for by the position of the occupied levels in both the adsorbate and the metal [2]. Blyholder [3] and Doyen and Ertl [4] have emphasized that the binding energy may be influenced by antibonding molecular orbitals close to the fermi energy. A review of the experimental results concerning CO and other adsorbates on Ni has recently been given by Yu et al. [2]. These authors * Work supported
by Deutschc
Forschungsgerneinschaft,
107
Sonderforschungsbereich
128
point
out that the binding
levels
1 77.5 cr. 4
themselves range
0.
in the variation
of the d-bands.
vicinity
energy
of the
The levels
not only by the position
in tlic elcctrcmic
of the energy
Fermi energy.
electroncgativity.
is detctminctt
hut also by changes
distribution
to these
Like the chemical
hand
of mc~leculcs
of the CO
which
manifest
of 1l1e ptlotoclectrons
contributing
the chemisorption
structure effects
between
on metal
in tl~c
art‘ located atoms
surfaces
in ttlc
of different seems
to 1~2
brought about by ttre formation of pairs of honding a11d antihonding orbitals originating from the hyhridi/.ation of the Inotecular and the metal orbitals. In order to compare
theoretical
results
ing and antihonding
and experimental
orbitals
data,
have to be known.
the positions
of both the hond-
In CPS-experiments,
this requires
both levels to be occupied. Even if this necessary condition is fulfilled the antibonding orbital can be identified only under specially fortunate circumstances. An example is O2 on C’u where the structureless range between tltc top of the d-band and the fermi level is modulated by the antibonding orbital of the adsorbate which can he observed by variation of the cross-section (If the photoc\;citatioll 121. It transition metals with partially filled d-bands. like Ni. are used as ;I substrate the antibonding orbital lies either in the trange of tt~e d-bands and can thus hardly tic detected. or above the Fermi Icvel. In tlris case photoemission can not 1~ used and instead methods like surface reflection and electron energy loss spectroscopy 15 1 have to he applied. These experiments Ilowcvcr yield only the energy between levels hut not the absolute position of the lcvct itself. A suitable
method
for determining
is tu use a level below initial
state
for the
excitation
Absorption
measurements
that
states
initial
show
levels have to be used as initial of partial
states
and insulators
can be detected excited electron.
in photoemission method
into
tile lcvets under
region
and near large dispersion.
for this purpc)se.
yield have been successful
in semiconductors
An alternative
3 rather
levels above the t;ermi energy
the pcjGtic)n of which is well known,
of electrons
in the visible
normally
using the method
the unoccupied
the Formi energy.
diffcrcncc
lJV
suffer from tllf f‘;lct Sufficiently deep core
Pl~otoemission in finding
cxperimcnts
unoccupied
[6]. t f tbc levels lie below
the vacuum
only via Auger transitions
or energy
of determining
electronic
states
as an
consideration.
between
levels
level they
losses of the
the Fermi level
and vacuum level is two-photon photoemission. In this process one photon is used to excite an electron into the intermediate lcvet which is empty if the system is in its ground state. The second photon excites the electron from the intermediate state into a level above the vacuum energy so that it can be detected as a photoelectron and, by measuring the kinetic cnelgy, the p~~sition of the intermediate state with respect to the vacuum level can be determined *. This cxperhent requires ;I * In addition
to the wsonant t\vo-photon c*citatiun via real interlncdiatc states, tllere ;ITCnonvia virtual intermediate states. The cross scL3ion of the resonant cxcita-
resonant transitions
larper and it can bc shown
tion is much to two-photon
hroatlcninp
~~lrotoemission
or the levels in sdids
[ 8) that the contribution
is small compared lo the rcson:mt
is taken into account.
uf nonresonant excitations
transitions
cvcn il_ lifctimc
rather high intensity since the lifetitnc of the intermediate states is considerabiy shorter than in molecules or ions [IO]. A sufficietirly high intensity is only available from lasers. Furthermore, the photon energy has to bc high enough, i.e. of the order of the work function or the photoelectric threshold for one photon photoemission. Clean Ni surfaces have work functions around 5 eV, which increase up to about 6 eV if the surface is covered with certain adsorbates. e.g.. CO. We have studied two photon I)hotuennissiott front metal surfaces. The first experiments were carried out with thin films of Al [7]. Recently, we have applied this method to Ni( 110) and (1 11) surfaces. As a light source we have used a dye laser which was putnpcd by a Nz laser. This establishes a continuously tunable light source of sufficiently high intensity in the photon energy range from 2 to 3.4 eV. With clean Ni surfaces we found two-photon photoemission for all photon energies exceeding half the work function, i.e. PKL> 3.5 eV. The details and results of this experiment will be reported elsewhere [X]. When the sut-face was covered with a monolayer of CO. the photoelectric yield decreased drastically. This is at least partly due to the litnited photon energy: with Xc; < 0.8 eV the maximum twophoton energy exceeded the work function by less than 1 eV. As regards VLII- aim to determine electronic states between the Fermi level and the vacuum level, we are limited by the maximum photon energy to states close to the center of the energy range in question. In order to extend the energy range to be investigated by twophoton photoemission, the photon energy has to he increased by using, e.g., excimer lasers. These lasers are not continuously tunable. but the intermediate states or resonances should show up in the energy distribution of the photoelectrons. Although the maximum photon energy of our light source was not high enough to find the two-photon photoemission via an intermediate adsorbate state. we have found experimental evidence for such states in CO Ni( 1 1 1) and CO- Ni( 1 IO) systems which we report in this papet.
2. Experimental The experimental arrangement is schematically shown in fig. 1. The dye laser emits light pulses with a width of 3 ns and about 50 kW peak power at a repetition rate between 5 and 50 cps. The light of the dye laser irradiates the surface of a Ni single crystal which is mounted in a UHV system. In the two-photon photoetnission experiments the light is focussed to irradiate a spot of about 0.1 mm diameter with an intensity of about lo8 W/ctn’. Focussing is essential since the two-photon effect depends quadratically on the intensity. The phenomenon reported in this paper is a one-photon effect. Due to the linear dependence on the light intensity the signal does not change when the light is defocussed since the product of intensity and irradiated area remains constant. Therefore it was not necessary to focus the beam in these experiments.
r-l
N, -laser
charge sensltlve
The crystal was cleaned by argon ion bombardment and subsequent annealing. The structure and cleanlyncss of the surface was controlled in situ by a LEED Auger system. The pressure was in the low to-” Torr range. Adsorption layers were produced by a bakeable gas doser system. The electric signals were measured and processed by standard pulse electronics as shown in fig. 1. The sensitivity of the system was about IO-’ 7 A s.
3. Results and discussion The maximum kinetic energy of photoelectrons produced by two-photon absorption is cUo = 2Rw - $J where Q is the work function of the emitter. If the potential of the emitter is raised above a cutoff, i.e. U> Uo, the photoelectric current vanishes. We have observed this effect. well known from one-photon photoemission, with clean as well as with oxygen-covered surfaces as emitters in twophoton photoemission. When the surface was covered with CO, the signal did not
H. W. Rudolf;
W. Strirlmam
/ Laser induced
charge trarfsfer
from
.Vi to CO
111
slgnal amphtude rel units
Fig. 2. The signal amplitude is plotted as a function of the retarding voltage for photoelectrons applied to the sample. (a) The dashed curve gives the photocurrent from a clean Ni surface: the current vanishes for positive sample potential higher than l/u = 2nw - @ where @ is the work function. (b) The solid curve gives the signal amplitude after adsorption of CO on the surface There remains a signal amplitude for I/ > Uo. (c) The dash-dotted curve shows the signal amplitude from the CO covered surface after defocussing of the beam. No signal was obtained from the clean surface in this case.
vanish even if a potential U 9 U, was applied (see fig. 2). The remaining signal was obviously not due to photoelectrons leaving the emitter, since the signal amplitude did not change noticeably when the applied voltage was varied up to 1000 V. The following explanations cannot account for this effect: (a) Positive ions leaving the emitter would lead to a signal with opposite polarity. (b) Photoelectrons produced by scattered light in parts of the system which are at ground potential can also be ruled out with the polarity argument. (c)Positive ions flying towards the emitter would produce a current of the observed polarity by they would be prevented from doing so by the high positive potential. (d) Photodesorption of CO- or any other negative ions cannot occur at high positive sample potentials. Consequently the signal cannot be caused by charge transport in the vacuum. (e) The signal could arise from photodesorption of neutral CO molecules which would change the work function. This process can be ruled out because it would lead to a signal of opposite polarity, since a desorption would result in a decrease of
the work function which is equivalent to a decrease of ncgativc cl~a~gc in flout of the xurfacc. Furtlierrnorc, n desorption of either neutral 01. charged adsorbate ~nckcult5 should result in ;I decrease of the signal under irradiation. This was not observed. Thcrnml effects can be cxcludcd since the signal did not chnnge when the light Iw~ ~3s defocussed (SW fig. 2).
lj.set
k I
H
gru dt
c ‘t -=T
I_/
Lip. 3. The response of the charge sensitive presmplit‘icr is compared with various input s+als: (a) Shape of tire incoming light pulse; the time SLY& for (a) (d) is given in this fip~rr. (b) ‘I‘hc time dependence trf the xsulting two-photon phutoernission signal is proportional tu I*. (c) The tilne depcndcncc of the input sigmd caused by charging of thr adsorbatc: for lifetimes of the electrons in the adsorbate states, 7, short compared to the pulacwidth, 7, the char~c transfer into the adsorbatc states occurs only during the rise time of the light pulse: this positive charging signal is followed by a negative pulse due to the tlowback of the cbar~c stored in the adsorbatc during the decrease of the light intensity; the time integral over the whole signal must vanish since no net charge transtcr between sample and anodc has occurred. (d) I,or 7 P, T, the charge stored in the adsorbate is just the integral over the charge excited by the light into the adsorbate states; the negative part of the signal ia govcrncd by the decaytime of the charge. (c) The response of the amplifier tu a single positive pulse with width T 15 1 MS:the time scale to (e) and (1) is given here. (1) The rcsponsc of the amplifier to a double pulse like (c); the output pulse is narrower than the rcsponsc (c) to a single pulse (h); the narrowing is observed for decay times 7 < 1 O-’ s: for longer decay times the response is sinlila to (e).
There remains only one possible explanation for the signal at high positive potential of the emitter: An increase of negative charge in the adsorbate layer. The absorbed photons excite electrons in the Ni substrate which consequently occupy intermediate levels in the adsorbate. This would lead to a charge transport over the distance of the order of lo-” m which therefore produces an accordingly small signal per electron. The large number of photons however guarantees that sufficiently many elementary charge transfer processes take place within the width of the light pulse to produce an observable signal. Two consequences come to mind immediately: (a) The phenomenon should be a one-photon effect; therefore the signal should depend linearly on the intensity. (b) The lifetime of the electrons in the intermediate state should be finite. This should produce a signal which is qualitatively different from a pulse produced in photoemission as shown in fig. 3. The different intensity dependence can qualitatively be seen in fig. 2. The quadratic effect vanishes upon defocussing since it is proportional to I* A, where Z gives the intensity and A the illuminated area. I* A is proportional to l/A if the radiation flux Z.4 remains constant. The linear effect, which does not depend on the emitter potential U, does not change upon defocussing since the signal amplitude is proportional to IA. Fig. 4 shows a quantitative proof of the intensity dependence. The
Fig. 4. The intensity dependcnces of the two-photon photoemission signal and the charging signal for CO covered surface are compared. Solid line: linear response of the charging signal. Dash-dotted line: quadratic response of the two-photon photoemission signal.
difference in the pulse shape, shown in figs. 3b and 3~2,could not be demonstrated directly since the time constant of the preamplifier was not small enough. We have. however, simulated the two input pulses with a pulse generator and observed that the shapes of the output pulses were different (see figs. 3e and 30. The output pulse of the linear signal observed with a defocussed light beam had a shape as shown in fig. 3f. which is compatible with lifetimes shorter than 1O-7 s. On the other hand, the signal should rapidly decrease if the lifetime became shorter than the pulse width of the exciting light. Ilence WC can estimate the lifetime T to he in the range 10m7 > 7 > 1O-‘O s. llic lower end of this range requires a quantitative estimate of the charge transfer and its effect on the signal. The latter can readily he obtained if the geometry of the emitter 4lector arrangement is idealized by a parallel plate condenser. In this case a charge qe which is transported over the distance d at the emitter induces a charge qC = y&/D at the collector where D is the distance between emitter and collector. With d = lo-” m. D = 10-l1x1, 3 X 1Ol4 photons per pulse and a yield of lo-’ electrons transferred to the adsorbate layer per photon absorbed, one obtains an input signal of 5 X IO-l6 A s. which is more than one order of magnitude above the noise level. The height of the signal measured was typically of the order of IO-l4 A s. If one calculates the effect for a geometry which is closer to reality than a parallel plate condenser, one finds that qJq, > d/D.Due to the rather complicated geometry of our arrangement. it is difficult to arrive at 3 good estimate for the ratio qc/qe. However, assuming cl = 1O-" 111. (lc/qe - 1O-7 is a plausible figure. Thus the measured signal can bc accounted for by the proposed process. The uncertainty introduced by the geometry and the yield leads to an equivalent uncertainty for the lower limit of the estimated lifetime of electrons in the intermediate udsorhute states. The signal depends rather strongly on the photon energy in the range over which the laser could be tuned. This dependence is shown in fig. 5. The increase of the signal with increasing photon energy can he explained if one assumes that the adsorbate state can he populated not only by electrons excited directly into it, hut also by electrons excited to higher energies and subsequently scattered into the adsorbate level by electron electron interaction. The data shown in fig. 5 prove that the adsorbate level involved lies at most 3 eV above the Fermi level. Alternatively. the data could he accounted for if one assumes a hand of adsorbate states, extending from 3 eV above the Fermi level upwards. The threshold may in fact lie below 2 eV, since the sensitivity of our arrangement may not have been high enough to find the effect at lower photon energies. We have attempted to detect a possible fluorescence arising from the radiative decay of the adsorbate state. The photocathode of the multiplier limited the lower end of the photon energy range to 1 eV. Using the N2 laser with a photon cncrgy of -3.7 eV, we did not find any increase of the signal over the dark current when the laser light hit the sample. The sensitivity was such that no more than one fluorescence photon in the energy range 1 < ?zti G 3 eV per 10’ laser photons absorbed
electronsperobsphoton rel lnts
NI (111I
0
co
photon energy L
I
2
2.5
Fig. 5. The amplitude of the charging signal as a function corrected for rctlectivity of the sample.
3 av of photon
energy.
The signal has been
have been emitted. Therefore we conclude that either the adsorbate state decays by a radiationless process or the photon energy of the fluorescence is smaller than 1 eV. In the latter case, the initial distance between the adsorbate state and the Fermi level does not necessarily have to be smaller than 1 eV, since an appreciable relaxation of the system, leading to a decrease of the energy of the intermediate adsorbate state, could occur during the lifetime. The energy difference between the relaxed adsorbate state and the Fermi level may be rather small. This would explain the surprisingly long lifetime of lo-” s or more we have to assume in order to account for the effect. The CO-Ni system has recently been investigated theoretically by Yu [9] who considered a cluster of five Ni atoms and one CO molecule in a geometry corresponding to a (100) surface. He finds a cluster state originating from the interaction of the 5a CO orbital with s-d hybridized Ni states which lies 0.7 eV above the Fermi energy. The electron density pattern of this state extends well into the metal, but has also considerable constributions at the CO molecule. It is an antibonding orbital which would lead to a decrease in the binding energy upon population. This in turn would cause an increase of the dipole moment, enhancing the effect which leads to the signal we have observed. It would also result in a relaxation which would bring the adsorbate level considerably closer to the Fermi level. In conclusion, this adsorbate state calculated by Yu could be responsible for our results. It lies within the energy range of which we could so far only determine the upper limit. Experiments with higher laser photon energy, leading to photoionization of the adsorbate state, are necessary to measure the position of the adsorbate level after relaxation. could
Acknowledgements
The authors wish to thank D. Rieger, V. Saile and T. Wegehaupt for their substantial contribution to the fluorescence measurement, and J. Hermanson for valuable comments.
References [ I] [Z] [3] [4] [5] [6] (71 [8] [9]
[lo]
C.L. Allpn. T. Gustafson and I<.W. Plummcr. Chcm. Phys. Letters 47 (1977) 127. K.Y. Yu.W.C. Spicer, I. Lindau, P. Pianetta and S.1‘. Lin, Surface Sci. 57 (1976) 157 G. Blyholder. J. Phys. Chem. 68 (1964) 2772. G. Doyen and k. lktl, Surface Sci. 43 (1974) 197. G.W. Rubloffand J.L. I:rccouf, Phys. Rev. 1~17 (1978) 4680. D.E. 1:astman and J.L. I:reeouf, Phys. Rev. Letters 33 (1974) 1601. H.W. Rudolf and W. Steinmann, Phys. Letters 61A (1977) 471. H.W. Rudolf, D. Riegcr and W. Stcinmann, to be published. H.L. Yu. J. Chem. Phys. 69 (1978) 1755. VI. Bondybcy and T.A. Miller, J. Chem. Phys. 69 (1978) 3597.