Chemisorptive Exoemission Induced by Vibrationally Excited N2O

May 3, 2001 - The pronounced enhancement of the initial emission, Iinit, strongly depends on the mean temperature of the N2O beam, T*. The observed ...
1 downloads 0 Views 80KB Size
Download PDF
3348

Langmuir 2001, 17, 3348-3354

Chemisorptive Exoemission Induced by Vibrationally Excited N2O Molecules Artur Bo¨ttcher* Fritz-Haber-Institut, Faradayweg 4-6, 14195 Berlin, Germany Received November 28, 2000. In Final Form: March 5, 2001 The energy distributions and the kinetics of the electron emission as well as the kinetics of N2 abstraction have been measured during the scattering of vibronically excited N2O molecules on Cs films. The chemisorption of cold N2O molecules leads to a low-energy electron emission which proceeds in two characteristic stages. Whereas the short-term initial chemisorption is characterized by moderate electron emission, the late reaction stage exhibits durable and more efficient exoactivity. The chemisorption of hot N2O molecules inverts this intensity ratio. It raises the intensity of the initial exoemission by more than 1 order of magnitude. Within the late reaction stage, the emission is only slightly affected by the vibronic excitation of the chemisorbing molecules. The pronounced enhancement of the initial emission, Iinit, strongly depends on the mean temperature of the N2O beam, T*. The observed relation, Iinit(T*), can be explained when considering two contributions: The increased probability for dissociation of N2O molecules in the excited bending vibronic mode is due to the long-distance harpooning. The second one originates from ground state molecules which dissociate only when the short-distance interaction with Cs atoms leads to the formation of a Cs+aO-b bond. By comparison of the efficiency of the N2 abstraction with the yield for exoemission, a nondissociative channel for electron emission becomes evident. For Cs layers characterized by low work function values around 1.65 eV, molecules in the stretching vibrational mode also participate in the observed emission.

Introduction The chemisorptive emission of low-energy electrons has been utilized as a phenomenon revealing the elementary dynamics of a molecule-surface interaction.1-13 It is generally accepted that the observed low-energy electron emission manifests a nonadiabatic path of the chemisorptive event, that is, it reflects a kind of surface excitation created when molecular affinity states dive deep below the Fermi level as still unoccupied states. The necessary condition for a possible appearance of such a short living hole is a considerable quenching of the resonant electron transfer at the Fermi level. This resonant process drastically reduces the probability that the hole, that is, the diving affinity state, passes deeply enough for inducing a detectable electron emission (E* > Φ, where E* means the energy position of the hole, and Φ denotes the work function). For metal surfaces marked by low work function * E-mail: [email protected]. Phone: 049-30-8413-5608. Fax: 049-30-8413-5603. (1) Nørskov, J. K.; Newns, D. M.; Lundqvist, B. I. Surf. Sci. 1979, 80, 179. (2) Bo¨ttcher, A.; Imbeck, R.; Morgante, A.; Ertl, G. Phys. Rev. Lett. 1991, 65, 2035. (3) Bo¨ttcher, A.; Grobecker, R.; Imbeck, R.; Morgante, A.; Ertl, G. J. Chem. Phys. 1991, 95, 3756. (4) Greber, T. Surf. Sci. Rep. 1997, 28, 1. (5) Grobecker, R.; Shi, H.; Bludau, H.; Hertel, T.; Greber, T.; Bo¨ttcher, A.; Jacobi, K.; Ertl, G. Phys. Rev. Lett. 1994, 72, 578. (6) Bo¨ttcher, A.; Grobecker, R.; Greber, T.; Ertl, G. Chem. Phys. Lett. 1993, 208, 404. (7) Prince, R. H.; Lambert, R. M.; Foord, J. S. Surf. Sci. 1981, 107, 605. (8) Cox, M. P.; Foord, J. S.; Lambert, R. M.; Prince, R. H. Surf. Sci. 1983, 129, 399. (9) Bo¨ttcher, A.; Morgante, A.; Giessel, T.; Greber, T.; Ertl, G. Chem. Phys. Lett. 1994, 231, 119. (10) Hellberg, L.; Stro¨mqvist, J.; Kasemo, B.; Lundtqvist, B. I. Phys. Rev. Lett. 1995, 74, 4742. (11) Brandt, M.; Greber, T.; Kuhlmann, F.; Bo¨wering, N.; Heinzmann, U. Surf. Sci. 1998, 160, 402. (12) Brandt, M.; Greber, T.; Bo¨wering, N.; Heinzmann, U. Phys. Rev. Lett. 1998, 81, 2376. (13) Greber, T. Appl. Phys. A 1998, 67, 701.

values (Φ < 3 eV), this fundamental condition is automatically fulfilled for strongly electronegative species (Cl, NO2, etc.). In this case, the empty affinity level is placed below the Fermi level even at long surface-molecule distances.6-8 Alternatively, the hole killing due to the resonant electron harpooning can be substantially blocked by depleting the density of states in the outermost layer of the surface.2-4 Consequently, the impinging molecules cross the Fermi region nearly unperturbed without becoming charged and survive as holes at an energy of E* > Φ, where the surface-molecule electron transfer ensures an intense electron emission. The long-distance harpooning in the Fermi zone can be reduced by shortening the passageway of the molecule through the outermost surface regions. For fast molecules, when the mean crossing time is comparable with the harpooning rate, a considerable increase of the survival probability of a molecular hole at E* can be expected. This effect has been convincingly demonstrated by applying the seeded beam technique for oxygen molecules chemisorbing on Cs films9 as well as for more electronegative Cl2 molecules.10 In both cases, the effect was attributed to the increased translation energy. For these homeopolar molecules, the presumable role of vibronic excitation could be definitely excluded.9 However, when assuming that the energy position of the molecular affinity state could be lowered by the excited vibronic mode a considerably increased intensity of exoemission might be expected for hot molecules. Indeed, this prediction has been confirmed by molecular beam experiments on the chemisorption of hot N2O molecules.11 A considerably higher intensity of the exoemission has been found for molecules occupying the first excited vibrational state. This effect was attributed to the harpooning conditioned dissociation of the chemisorbing molecules.11-13 The particular choice of the system under study here, N2O T Cs, is motivated by the mentioned recent achievements in the understanding of the dynamics of the

10.1021/la0016468 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/03/2001

Chemisorptive Exoemission

Langmuir, Vol. 17, No. 11, 2001 3349

chemisorptive event11-13 as well as by the important role of N2O molecules in ozone chemistry of the earth atmosphere. A better understanding of the electron transfer processes between nitrous oxides and alkali metals is necessary to improve the yield of modern catalysts which could more efficiently reduce the N2O pollution.14,15 In this work, some details of the harpooning-mediated model of the chemisorptive cascade, mentioned above, have been examined by showing how the vibronic excitation of N2O molecules approaching Cs films influences the kinetics, the energy distribution, and the total yield of the induced exoemission. It will be clearly shown that the intensity of the initial emission does not scale with the total number of vibronically excited molecules striking the surface. The relationship between the initial emission and the mean temperature of N2O molecules, Iinit(TN) can be explained when including also the short-distance interaction of the ground-state molecules with adsorbed Cs atoms which finally leads to the formation of Cs+aO-b bonds. Moreover, the measurements of the N2 abstraction clearly indicate a substantial contribution of the nondissociative channel to the exoemission. Experimental Section The measurements of the exoemission as well as the preparation and characterization of the sample were performed in an ultrahigh vacuum chamber. The chamber was connected to a three-stage molecular beam apparatus which provided a nearly monoenergetic molecular flux by supersonic gas expansion through a nozzle. The typical molecular flux achieved was around 1013 molecule/(cm2 s). The current molecular flux has been determined by measuring the oxygen load reached by exposing the clean Ru surface to the N2O beam and comparing this value to the one obtained by performing the oxygen deposition once more by flooding the chamber with N2O up to a certain pressure in the region of 10-8-10-7 mbar. The population of the excited vibronic states has been varied by heating the nozzle up to 1000 K. A possible decomposition of molecules taking place at the hot walls of the nozzle can be excluded because no significant changes in the mass spectrum of the scattered nitrous oxides could be detected. The N2O beam was scattered under 22.5° at the surface and the flux of escaping electrons was monitored by an energy analyzer movable around the surface normal. The energy spectra of the emitted electrons were accumulated with an energy resolution of about 0.4 eV within a time interval of about 5 s. The kinetics of exoemission has been obtained via integrating the energy distributions taken during exposure of the Cs films to the N2O beam. The N2 flux resulting from the dissociation of chemisorbing N2O molecules was monitored by a mass spectrometer mounted along the surface normal. The Ru crystal has been cleaned by applying sputtering and annealing cycles according to the procedure described by Madey et al.16 The achieved surface quality was controlled by applying ultraviolet photoelectron spectroscopy (UPS) as well as metastable de-excitation spectroscopy (MDS). Cs films have been deposited by evaporation from the SAES source onto the Ru sample kept at 250 K. The resulting Cs load has been determined by taking thermal desorption spectra of the deposited Cs atoms and comparing the integral intensity with the one obtained for a saturated Cs monolayer (1 ML ) 5.3 × 1014 atom/cm2). Cs submonolayers were prepared via thermal desorption of a thick Cs film up to a certain sample temperature Tcov which defines the remaining Cs coverage, ΘCs. The relationship between ΘCs and Tcov as well the electronic and structural properties of the Cs atoms deposited on Ru(0001) have been previously precisely determined by combining the thermal desorption spectrometry, Auger, low(14) Li, Y.; Armor, J. N. Appl. Catal., B 1992, 1, 21. (15) Yuzaki, K.; Yarimizu, T.; Aoyagi, K.; Ito, S.-I.; Sato, T.; Hayashi, S.; Kunimori, K. Catal. Today 1998, 45, 129. (16) Madey, T. E.; Engelhardt, T.; Menzel, D. Surf. Sci. 1975, 48, 304.

Figure 1. Kinetics of the electron emission as taken during the exposure of a Cs monolayer to hot and cold N2O beams, upper and lower beams, respectively. In both cases, a constant sample temperature of 250 K was kept during the exposure. energy electron diffraction, and other techniques.17-20 The cleanliness of the deposited layers was controlled by analyzing the corresponding UP and MD spectra.

Results and Discussion Figure 1 illustrates the crucial finding of this work, namely, the kinetics of exoemission as taken when exposing a Cs monolayer to a beam of about 1.7 × 1013 N2O molecules/(cm2 s) for the two nozzle temperatures of 300 and 1100 K, lower and upper traces, respectively. The emission intensity observed within the initial exposure interval of about 20 s differs by a factor of about 20. In contrast, the intensity increase induced by vibronically excited molecules is much weaker within the late chemisorption stage. The intensity difference, ∆Iinit, completely disappears for very late chemisorption stages. This quite pronounced effect has been found for all Cs films deposited. Figure 2 shows a similar behavior observed when exposing a thick Cs film (Cs load of 6 ML) to the hot and cold N2O beams, upper and lower curves, respectively. The initial exoemission is again by a factor of about 20 more intense when scattering the hot molecules. Within the late reaction stage, however, the emission induced by hot molecules is on average higher by a factor of 3 than the emission induced by ground-state molecules. For the chemisorption of cold N2O molecules, the time evolution of the sticking probability and the kinetics of the exoemission exhibit quite different trends.20 This fact pointed to the necessity to make a distinction between the initial and the late reaction stages. The pronounced enhancement of exoemission found here only within the initial oxidation stage supports this clear distinction. Whereas the initial electron emission roughly follows the time evolution of the sticking (17) Over, H.; Bludau, B.; Skottke-Klein, M.; Ertl, G.; Moritz, W.; Campbell, C. T. Phys. Rev. B 1992, 45, 8638. (18) Bludau, H.; Over, H.; Hertel, T.; Gierer, M.; Ertl, G. Surf. Sci. 1995, 342, 134. (19) Woratschek, B.; Sesselmann, W.; Ku¨ppers, J.; Ertl, G. J. Chem. Phys. 1987, 86, 2411. (20) Bo¨ttcher, A.; Morgante, A.; Grobecker, R.; Greber, T.; Ertl, G. Phys. Rev. B 1994, 49, 10607.

3350

Langmuir, Vol. 17, No. 11, 2001

Figure 2. Kinetics of the electron emission as taken during exposure of a thick Cs film of 6 monolayers to a N2O beam at nozzle temperatures of 1100 and 300 K, upper and lower beams, respectively. The metallic and nonmetallic oxidation stages are marked by A and B, respectively.

probability, the rather intense exoemission in the late stage is accompanied by a very low value of the sticking coefficient, less than 10-3.21 The initial reaction stage is accompanied by oxygen incorporation into the Cs lattice in the form of O2- ions creating conducting suboxide phases (Cs4O, Cs7O, and Cs11O3).22 Therefore, it is often called the metallic oxidation stage. The oxygen dissolution process becomes complete when the Cs monoxide phase is formed, Cs2O.22-24 Within the nonmetallic oxidation stage, peroxide and superoxide phases replace the monoxides in the topmost layers of the surface.19,24,26 The point where the metallic surface becomes replaced by an insulating layer terminating the Cs film is signalized by a dramatic reduction of the sticking probability as well as by the entire quenching of the N2 emission.21,25 For thick Cs films, the appearance of this transient stage strongly depends on the oxidation conditions, the sample temperature, and the partial pressure of the oxidizing species3. The metal-insulator transition is completed when the upward flux of Cs atoms emerging on top of the surface becomes negligibly low because of the increasing thickness of the created oxide layers. Because of the oxide-like termination of the surface, the long-distance harpooning can obviously be excluded as the step initializing the exoemission.21 Summarizing the findings presented above, it has to be noted that in fact the observed enhancement of the exoemission as induced by raising the nozzle temperature might be due to the increased vibronic and/or kinetic energy of the impinging molecules. To check the possible role of the kinetic energy, a series of seeded-beam experiments has been performed (1% N2O/He mixture at 700 K, Ekin ) 1.4 eV). For the fastest N2O molecules (21) Bo¨ttcher, A.; Niehus, H. Phys. Rev. B, accepted. (22) Su, C. Y.; Lindau, I.; Chye, P. W.; Oh, S.-J.; Spicer, W. E. J. Electron Spectrosc. Relat. Phenom. 1983, 31, 221. (23) Tsai, K.-R.; Harris, P. M.; Lassettre, E. N. J. Phys. Chem. 1955, 60, 338. (24) Su, C. Y.; Lindau, I.; S.-J.; Spicer, W. E. Chem. Phys. Lett. 1982, 87, 523. (25) Bo¨ttcher A. Langmuir 2000, 16, 8858. (26) Bo¨ttcher, A.; Giessel, T. Surf. Sci. 1998, 408, 212.

Bo¨ ttcher

Figure 3. The upper panel shows two representative electron energy distributions taken within the metallic (A) and nonmetallic oxidation stages (B) when scattering cold N2O molecules (TN ) 300 K) on thick Cs film (6 ML). The lower panel shows energy distributions obtained when scattering hot molecules (TN ) 1100 K) on the same Cs film.

scattered on a Cs monolayer, only a rather small increase of the initial exoemission in the range of 40% could be observed. The same increase has been observed when keeping the nozzle at 700 K without seeding the molecular beam. The kinetic energy reached is higher by a factor of 15 than in the case of the nonseeded experiment at a nozzle temperature of 1100 K. Thus, the raised kinetic energy as a possible explanation for the observed enhanced exoemission can clearly be excluded. This finding can be rationalized when considering the accelerating action of the image surface potential on N2O- ions created at long distances to the surface. The mean velocity gained this way may reach values higher than the raised primary velocity. The minor role of the input kinetic energy stresses the fact that the exoemission is not directly generated by the long-distance surface harpooning but mainly by the second electron transfer associated with the dissociation of the chemisorbing molecule. Figure 3 shows the energy distributions of electrons emitted within the initial (A) and late (B) reaction stages, as taken when scattering the cold and hot N2O beams, upper and lower panels, respectively. The relative energy shift of the two distributions in the upper panel (lowenergy edge) reflects the pronounced lowering of the surface work function of about 1.1 eV, as accompanying the transformation of the metallic Cs film into the suband monoxide layers because of the incorporation of O2ions.22,23,26 For the cold N2O beam, the energy profiles taken within stage A are rather weak and about 0.6 eV broad. In contrast, the energy profiles taken within stage B are much more intense and about 0.3 eV wider than the ones found within region A. This pronounced broadening results from two competing electronic processes taking place when the Cs thick film becomes terminated by an insulating layer exhibiting some oxygen-vacancy derived states V* near the Fermi energy, EV < EF.21 An exoactive internal channel becomes active when ballistic electrons from the metallic film pass through the insulating topmost layers and occupy the oxygen-vacancy state. These transitions

Chemisorptive Exoemission

Langmuir, Vol. 17, No. 11, 2001 3351

Figure 4. The initial emission yield, Yinit, as a function of the nozzle temperature, TN, has been measured for a thick Cs film kept at a constant temperature of 250 K during exposure to a N2O beam (left panel). The right panel shows a comparison between the experimental curve Yinit(TN) (full circles) and the probability for the gas-phase dissociation induced by scattering low-energy electrons on thermal N2O molecules, Y(O-) (open circles). The latter curve has been derived from the dependence of O- emission on gas temperature as published by Chantry (ref 27).

are followed by an Auger cascade which carries the electrons over the vacuum level. The external channel is activated by N2O molecules when their affinity state dives deeply below the Fermi level and becomes occupied by electrons from the oxygen-vacancy state.21 Because the energy position where the impinging hole becomes occupied, E*, must not necessarily coincide with the final state of the internal channel Eb, the broadening of the energy distribution in stage B becomes intelligible. Obviously, the proposed scheme is only tentative in nature, because we try to interpret the steady-state measurements in terms of some elementary processes taking place on the 10-12 s scale. The energy distributions A and B taken during the scattering of hot N2O molecules (lower panel in Figure 3) exhibit quite different characteristics. According to the kinetics shown in Figure 2, the corresponding energy profile A is much more intense than the one taken at the late oxidation stages. Whereas the width of profile B is not significantly affected by the increasing number of vibronically excited molecules, profile A exhibits considerable modifications. It is slightly wider and shows a maximum kinetic energy which is larger by at least 0.3 eV. The latter fact indicates that the deepest energy position achieved by the diving hole, E*, carried by vibronically excited molecules, is higher than in the case where the ground-state molecules are chemisorbed. It reaches a value around 3.6 eV. This effect can be explained within the framework of the long-distance harpooning model,10,11 that is, when assuming that the molecules in the excited bending mode, hot N2O molecules, become negatively charged at larger distances to the surface than the ground-state molecules do. Consequently, the ionic intermediates, N2O- ions as well as their dissociation fragments, O-, undergo a pronounced acceleration toward the surface because of the attractive image potential. The high velocity gained this way in the vicinity of the image plane allows higher values of E* to be achieved.

Within the metallic oxidation stage, the model of surface harpooning as the reaction step responsible for the observed exoemission is supported by two experimental findings. The first is the nearly linear relationship between the intensity of the initial exoemission and the sticking probability, Iinit T Sinit.21 Second, the probability for an electron-attachment-induced dissociation of gas-phase N2O molecules has been found to be governed by the increasing occupation degree of the excited bending vibrational mode.27 For low-energy electrons (Ekin f 0), an increase of the yield for the emission of O- ions, Y(O-), by more than 3 orders of magnitude was observed when raising the gas temperature from 295 to 1040 K. However, an inspection of Figures 1 and 2 reveals that within nearly the same temperature interval the intensity of the initial exoemission is only raised by slightly more than a decade. Consequently, the model attributing the observed intensity increase to the gas-phase reaction scheme needs some improvements. Figure 4 shows how the total charge emitted within the initial chemisorption stage, the initial yield Yinit, depends on the nozzle temperature (left panel). The data were collected for a thick Cs film of 6 ML. For sample temperatures up to about 700 K, only a slight enhancement of the exoactivity is observed. For higher temperatures, the exoemission yield increases gradually with the nozzle temperature. This function rather resembles a thresholdlike behavior and not an exponential dependency as expected when relating the exoemission yield to the occupation of highly excited vibronic states which is governed by the Boltzmann distribution. This curve can be fitted to a two-exponential function which mirrors certain hypothetical processes which might be assigned to two quite different activation barriers of 0.02 and 0.65 eV. The right panel allows these two processes to be indicated more precisely. It compares the experimental curve Yinit with the yield for O- abstraction, Y(O-), used (27) Chantry, P. J. J. Chem. Phys. 1969, 51, 3369.

3352

Langmuir, Vol. 17, No. 11, 2001

here as a measure of the dissociation induced by the scattering of low-energy electrons (Ekin < 2 eV) on hot N2O molecules in the gas phase27,28 (open circles). The ascending slope of this line roughly coincides with the Yinit curve only within the high-temperature region of this dependency, HT. In this region, the population of molecules in the bending vibronic mode becomes substantial. In comparison to molecules in the excited bending vibronic mode, the ground-state molecule exhibited a much higher vertical electron affinity (by nearly 1 eV30,31). Consequently, hot molecules may efficiently capture electrons harpooned from the Cs film even at molecule-surface distances long enough to guarantee an essential acceleration of the created ionic intermediates and consequently induce an intense exoemission. According to Chantry,27 an attachment of a low-energy electron (Ekin < 2.5 eV) leads to the dissociation of the created molecular ion with a rather high cross section of 10-18 cm2. This step is accompanied by an efficient abstraction of neutral N2 molecules as recently demonstrated.25 The second dissociation fragment, the released O- ion, is fielded in the image potential of the metallic surface and undergoes an acceleration toward the image plane. The latter process is responsible for the high survival probability of the empty affinity level of the O- ion in the energy region around E* where the second electron harpooning may lead to electron emission. The translational velocity gained before arriving in this energy region may attain a value 3 times higher than the primary thermal velocity of the impinging molecules (1.4 × 1013A/s26). Thus, the electron emission within the HT region can be considered as resulting from a four-step cascade: metal-molecule harpooning, dissociation of the created N2O-, acceleration of O-, and finally a second electron transfer creating O2- initiates the Auger process which is responsible for the electron emission. The time scale of this cascade is limited primarily by the molecular path across the valence region. The flow path takes place within an interval of 10-12 s. Because the monitored emission in fact represents a complicated convolution of all the steps, it is not surprising that only the main trend of the Yinit(TN) curve is roughly reflected by the probability for electron-attachment-mediated dissociation Y(O-). The clearly non-Arrhenius behavior, the nearly two exponential dependence Yinit(TN), resembles the temperature dependence found for the gas-phase Li + N2O reaction.36,37 In this case, the activation energies found could be reproduced within a model of a close-range electron-transfer process.40 The comparison between Yinit and Y(O-) also makes evident that the emission observed in the low-temperature region, LT, cannot be explained only by the increasing population of molecules in the excited vibrational state, that is, to the temperature dependent harpooning-mediated dissociation of N2O molecules at the surface-vacuum interface. When in our experiment with the cold molecular beam (TN ) 300 K) the observed emission would originate exclusively from the small fraction of molecules in the first excited bending mode, then a clearly exponential dependency on the nozzle temperature should be expected, that is, within the LT region the emission intensity should increase at least by one decade. This is evidently not the case. Thus, the reason for the observed dependency Yinit(TN) within the low-temperature range should be a (28) Krishnakumar, E.; Srivastava, S. K. Phys. Rev. B 1990, 41, 2445. (29) Hopper, D. G.; Wahl, A. C.; Wu, R. L. C.; Tiernan, T. O. J. Chem. Phys. 1976, 65, 5474. (30) Bardsley, J. N. J. Chem. Phys. 1969, 51, 3384. (31) Caledonia, G. E. Chem. Rev. 1975, 75, 333.

Bo¨ ttcher

Figure 5. Kinetics of the N2 emission as observed when exposing a Cs monolayer to a hot and a cold N2O beam, upper and lower curves, respectively.

process where the ground-state chemisorbing molecules are involved. This finding seems to be somewhat surprising because recent time-resolved measurements revealed an absence of immediate exoemission when the scattered beam contained ground-state N2O molecules exclusively.11 Instead, a significant delayed emission appeared within an interval of about 20 ms after a beam pulse had been detected. The proposed explanation was based on the assumption that the high vertical electron affinity level of ground-state molecules of about -2.2 eV29 eliminates the possible harpooning as an efficient process leading to the dissociation of impinging molecules. Thus, the puzzle can be solved when assuming that the emission observed in the LT region reflects a delayed process, which occurs when a metastable N2O species in the precursor state decays because of a short-distance interaction with Cs atoms. Such an intermediate step of the chemisorptive cascade has been identified for oxygen chemisorption on Cs films.5 Similar to the gas-phase path, the precursorstate-mediated process should also proceed via the dissociation of N2O molecules followed by a N2 abstraction. It is very likely that this emissive event is associated with the formation of a chemical bond between the O- and the individual Cs atoms which obviously requires a pronounced rearrangement of the electron density distribution in the vicinity of the adsorbate. This concept seems to be nicely supported by recent investigations of the N2 abstraction observed during the chemisorption of thermal N2O molecules on Cs films.23-25 The following experiment elucidates the relation between the gas-phase and the precursor-state-mediated chemisorptive path. The flux of abstracted N2 molecules has been measured during the scattering of hot and cold N2O molecules on a Cs monolayer, upper and lower traces in Figure 5, respectively. In both cases, the N2 emission coincides with the appearance region of the corresponding exoemission (metallic oxidation stage). The kinetic traces reach a maximum immediately after opening the beam; afterward the N2 intensity gradually decays with the progressing oxidation to a background level. The duration time of this emission is inversely proportional to the flux of impinging molecules. The background of this N2 signal, Ih* and Ic*, is mainly determined by the cracking of molecules which after being scattered back on the Cs film approach the hot filament of the mass spectrometer. Thus, only the initial fragment of this emission appearing within the first interval of less than 20 s can be attributed to the

Chemisorptive Exoemission

Cs induced dissociation. When comparing the N2 emission induced by hot and cold molecules, two differences can be stated. For hot molecules, the initial emission, Ih, is 2.5-3 times higher than the corresponding intensity in the cold state, Ic, but the lifetime of this emission is considerably longer for the cold impinging molecules. It means that hot molecules exhibit a much higher mean sticking coefficient which is directly responsible for a more efficient oxidation process. As shown in Figure 1, the corresponding increase of the initial exoemission reaches a factor of 20. So, it is 7 times larger than the corresponding increase of the sticking probability. This kind of discrepancy can be explained when it is assumed that the gas-phase harpooning-mediated dissociation, described above for vibronically excited molecules, is not the only process inducing the exoemission. Thus, the hot N2O molecules should be involved in the postulated emission mechanism but without contributing to the oxidation of the Cs film, without incorporating the oxygen atoms into the surface. There are several possible candidates in the literature of gas-phase reactions which fulfill this requirement, for instance, the associative electron detachment taking place when a vibronically excited N2 molecule collides with an O- ion at hyperthermal energies.30-32 This process proceeds according to the scheme N2 + O- f N2O + e + 0.21 eV31 and therewith prohibits the incorporation of O- ions. Thus, a possible scenario in front of a Cs surface might be that N2 and O- as hot dissociation fragments created immediately after the harpooning will be caught in the precursor state where the emissive association can occur. A second reasonable alternative, well established in the literature, would be the autodetachment of excited molecular ions, N2O-* + M f N2O + e + M, which usually competes with the nonradiative collisional stabilization of the molecular ion, N2O-* + M f N2O- + M.31,32 The last possibility is the cascade proposed by Parkes33 which is released by electron attachment to NO2 and proceeds via the formation of an NO- ion, that is, N2O + e f N2 + Ofollowed by O- + N2O f NO- + NO, and finally NO- + N2O f N2O + e + NO. Obviously, such a cascade can efficiently contribute to the exoemission only when the steady-state concentration of the reagents becomes significantly higher than in the gas phase. This might be the case in the precursor state which acts as a catchment for molecular fragments approaching the surface. Obviously, further work is needed to make a selection and to indicate the most efficient reaction channel which might be responsible for the relation between the intriguing temperature dependence of the N2 abstraction and the exoemission. Nevertheless, this relation indicates the necessity to consider the precursor state as an important stage opening new channels for an exoemission. It also makes clear the role of the nondissociative mechanism in the exoactivity generated by hot molecules. The aim of the next experiment was to prove how the variation of the work function of different Cs films affects the yield of the initial exoemission, Yinit. A common way to realize the concept is to take advantage of the fact that the work function of Cs layers varies dramatically with the Cs load deposited.34-36 It decreases from 2.2 eV at thick Cs layers to about 1.6 eV for a Cs coverage around 0.65 ML. It increases again synchronously with the (32) Christophorou, L. G.; McCorkle, D. L.; Anderson, V. E. J. Phys. B 1971, 4, 1163. (33) Parkes, D. A. J. Chem. Soc., Faraday Trans. 1972, 11, 2121. (34) Maus-Fridrichs, W.; Dickhoff, S.; Wehrhahn, M.; Pu¨lm, S.; Kempter, V. Surf. Sci. 1992, 271, 113. (35) Albano, E. V. Surf. Sci. 1984, 141, 191. (36) Kiskinova, M.; Rangelow, G.; Surnev, L. Surf. Sci. 1986, 172, 57.

Langmuir, Vol. 17, No. 11, 2001 3353

Figure 6. Integral yield of the initial exoemission Yinit versus Cs coverage ΘCs, as obtained when scattering the cold and hot N2O beams, 300 and 1100 K, lower and upper curves, respectively.

decreasing Cs coverage reaching a value of 5.3 eV for a clean, Cs-free ruthenium surface. These dramatic variations reflect the changes of the electronic state of individual adsorbed Cs atoms.20,34 The pronounced work-function minimum appears in the coverage range where the lateral dipole-dipole interaction of adjacent Cs atoms compensates the ionic bonds with the substrate and consequently leads to a lateral delocalization of Cs 6s electrons. Figure 6 shows the yield of initial exoemission measured for cold and hot N2O beams scattered on Cs films differing in coverage (upper and lower curves, respectively). The coverage onset where the emission becomes detectable is common for cold and hot molecules; it lies at 0.45 ML which denotes a threshold work-function value of 2.3 eV. This implies the lowest possible hole energy of 4.6 eV. The striking property of Yinit(ΘCs) for the emission induced by hot molecules is a very strong maximum appearing in a rather narrow coverage region of the minimum work function around ΘCs ) 0.65 ML. Usually, this kind of dependence has been used as an additional support for the long-distance harpooning model which predicts a continuous dependence of the yield on the work function and consequently a clear coincidence between the work-function minimum and the maximum yield for exoemission.1,4,26 However, the profile of our experimental Yinit(ΘCs) curve rather resembles a resonance located in the region around the work-function minimum, 1.6-1.7 eV. The corresponding function Yinit(ΘCs) obtained when scattering cold N2O molecules exhibits a similar dependence, but the observed maximum is much less pronounced and it does not exhibit any resonance-like features. Thus, the most striking behavior, the big difference between the hot and cold exoactivity seen around the work-function minimum, has to be attributed to a vibronic excitation of the chemisorbing molecules which was not contributing to the exoemission observed when scattering the beam on a monolayer (Φ ) 2 eV). Evidently, the small difference in the work-function value of about 0.4 eV triggers a new channel for exoemission. Thus, the additional process might be attributed to the excitation of stretching vibrations which have been

3354

Langmuir, Vol. 17, No. 11, 2001

proposed to play an important role in the electronattachment-mediated bond formation between N2O and Li or Na atoms.37,38 A simple reason for this effect might be the significantly lowered dissociation energy for N2O molecules in a highly excited stretching mode, Ediss < 1.67 eV, which makes likely a non-harpooning-mediated thermal dissociation of impinging hot molecules. For Cs coverages around 0.65 ML, an individual adsorbed atom resembles the one in the gas phase, that is, the ionic bond is fairly compensated by lateral dipole-dipole repulsion but the metallization of the valence electrons is still negligibly low.35,36 The Cs 6s electron remains localized around a Cs core. Thus, the interaction of such a single adatom with the N2O molecule in an excited stretching mode may proceed similarly to the nonactivated reaction path known for the gas-phase oxide formation, Cs + N2O f CsO + N2,39 that is, without any involvement of the nearly free conduction electrons which are responsible for the long-distance harpooning process. Summary The relationship between the initial exoemission and the N2 abstraction, both as functions of the primary (37) Plane, J. M. C.; Rajasekhar, B. J. Phys. Chem. 1989, 93, 313540. (38) Plane, J. M. C.; Nien, C.-Fu. J. Chem. Phys. 1990, 94, 525561. (39) Futerko, P. M.; Fontijn, A. 1991, 95, 8065. (40) Wren, D. J.; Menzinger, M. J. Chem. Phys. 1975, 63, 4557.

Bo¨ ttcher

vibronic excitation of impinging N2O molecules, allows three different channels contributing to the observed exoemission to be distinguished: 1. The coincidence between the Yinit and the gas-phase electron-attachment-mediated dissociation probability, Y(O-) as found for nozzle temperatures higher than 600 K, indicates the dominating role of molecules in the excited bending vibrational state. 2. In the low-temperature region (TN < 600 K), the exoemission has to be assigned to a nondissociative channel of the scattering event. Presumably, a three-body reaction of excited dissociation fragments, N2, NO, or O-, in the precursor state is responsible for the exoemission. 3. The resonant enhancement of the exoemission around the work-function minimum (0.6 < ΘCs < 0.7, Φmin < 1.65 eV) signalizes that an additional reaction channel is involved. It is postulated that this behavior can be attributed to hot N2O molecules in the excited stretching vibrational mode. Acknowledgment. It is a pleasure to acknowledge fruitful discussions with A. Morgante and T. Greber during the experimental work as well as within the stage of processing the experimental data. LA0016468