Electron- and Photon-Induced Desorption - Advances in Chemistry

6 Electron- and Photon-Induced Desorption M . J . D R I N K W I N E , Y. SHAPIRA, and D . L I C H T M A N

Radiation Effects on Solid Surfaces Downloaded from pubs.acs.org by CORNELL UNIV on 08/19/16. For personal use only.

Physics Department and Laboratory for Surface Studies, University of Wisconsin—Milwaukee, Milwaukee, Wis. 53201

Electrons and photons can cause the desorption of adsorbed gases as a result of a quantum interaction, as opposed to thermal effects. Electron-induced desorption is fairly inde pendent of the substrate material. Both neutrals and ions can desorb. Maximum cross sections are similar to those for dissociative reactions in electron-bombarded gas phase mole cules (i.e., 10 -10 cm ). Photodesorption is an entirely different process which is very substrate dependent. Photo desorption has been detected primarily from semiconductor substrates where the threshold for the reaction is the band gap energy of the semiconductor, usually 2-5 eV. Photo desorption gives rise only to neutral particle emission with cross sections in the range of 10 -10 cm from appro priate active substrates. -16

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Τ T n t i l the advent of readily available sensitive detection equipment i n ^ the last few decades, evidence of electron- or photon-induced gas desorption as a quantum process was generally masked by thermal effects caused by the need to use relatively high power density beams. Experi ments in recent years have now clearly established both the electronand the photon-induced desorption quantum process. Since electrons and photons of similar energy produce similar effects when interacting with gas phase atoms and molecules, it was anticipated that they would show the same effect with gases adsorbed on solid substrates. It has now been shown that the processes are quite different. Each process w i l l there fore be discussed separately. Electron-Induced Desorption Electron-induced desorption has now been studied i n fair detail for almost 15 yr {1,2). Both ions and neutral particles are desorbed by the electron beam. However, because the detection efficiency of ions is much 171


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greater than that of neutrals, virtually a l l data have been obtained on electron-induced ion desorption. The problem i n obtaining electroninduced neutral desorption is one of signal-to-noise and has as yet not been satisfactorily solved. It is pretty well accepted that electron-induced desorption involves, basically, the interaction of electrons with the adsorbed gas components. Transitions occur to dissociative states giving rise to desorbing fragments. F o r example, i n Figure 1 there are some of the energy levels of the hydrogen molecule. Electron-induced excitation to the various allowed states leads to the reactions as noted. If the molecule is adsorbed on a solid surface, one would expect some modifications of the energy levels. These modifications would not be comparable with the overall energy level values and, therefore, would not change the basic molecular energy level scheme very drastically. Electrons interacting with molecules adsorbed on surfaces should produce the same types of transitions leading to the same resulting components. One would expect the difference to be a relatively small shift i n the energy required for the various transitions compared with that occurring i n the gas phase. If the molecule is raised to an excited or ionized state, there is no reason why it should not remain

NUCLEAR SEPARATION (A)

Figure 1. Some potential energy curves for the hydrogen molecule. ( ) indicate the region of Frank-Condon transitions. Curves III and IV indicate states which lead to fragment production.


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Figure 2. Rehtive surface H* ion current vs. electron probe energy near threshold. Electron current density is lCr A · cm' . Surface temperature is 300K. 7

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bound to the surface as i n the normal neutral state. Should the transition take the molecule to the dissociative state, then some of the fragments w i l l have sufficient excess kinetic energy to be able to leave the surface. This process is generally referred to as electron-induced desorption. It is expected that the cross section for electron interaction with adsorbed gas species is comparable with the value for a similar interaction in the gas phase. Thus, cross sections for production of desorbing ion fragments range from a maximum of 10" c m , and indirectly determined cross sections for neutral desorption range from a maximum of approximately H T c m (2). If one measures the cross section vs. electron energy for the production of ion fragments of adsorbed hydrogen on, for example, a nickel surface, one obtains data as seen i n Figure 2 (3). The desorbing ion fragment is H , and the curve is extremely similar to that obtained i n electron-gas phase interactions. Thus, the threshold and various breaks in the curve undoubtedly relate to the various allowed transitions to dissociative states of the adsorbed hydrogen molecule which contribute to the desorbing H signal. 18

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The interaction of the electrons causing ion desorption appear to be fairly independent of the substrate. This factor is seen i n a collection of recent experiments by various scientists studying carbon monoxide adsorp tion on different metals. Electron bombardment of adsorbed C O generally gives rise to two ion species—0 and CO*. I n this case, a parent molecule ion C O * is seen. The explanation for its appearance is still i n question. The data obtained for C O adsorbed on tungsten, rhenium, and niobium, all i n different laboratories, are shown i n Figure 3 (4,5,6). Except for a slight shift i n the temperature scale caused by slightly different binding energies, one can see clear similarities i n the behavior of the O* and C O * signals from a l l three quite different substrates.

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Figure 3. Relative surface O* and CO* signals caused by electron-induced desorption as the surface temperature is varied. (A) rhenium surface (adapted from Ref. 4), (B) tungsten surface (adapted from Ref. 5), (C) niobium sur face (adapted from Ref. 6). Thus, not only is the general behavior of C O on these different metals quite similar, but the interaction of the electrons causing desorp tion produce almost identical results, even though the substrate metals are significantly different. Additional data obtained by these experi menters indicate no significant difference when using either single crystal or polycrystalline samples. Almost a l l experiments to date have concentrated on using electrons i n the energy range near 100 eV. Some work is done using higher fixed electron energies (500, 1000, 2000 eV, etc.), but these energies are used


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primarily only because of convenience and compatibility of the electron source i n conjunction with other techniques of surface analysis. U n t i l recently there has been very little investigation on what effect increasing the bombarding electron energy above 100 eV has on the desorbing sur face ion signal. Now, however, a system has been developed which ultimately can study desorbing surface ions using electron energies rang ing from 0 to 20 K e V . Some preliminary data using electron energies up to 4000 eV have been obtained. The design of the analyzer-detector is essentially the same as that described by Lichtman and McQuistan ( J ) . A 6-in. radius, 60-degree magnetic sector mass spectrometer combined w i t h an appropriate set of grids at the entrance region (which help focus and direct the incoming desorbing ions) was used on this system. A n extra grid i n the entrance region enables us to determine the kinetic energy distribution of the desorbing ions. A d d e d features include an ion gun, which can sputter clean the target and perform ISS (ion scattering spectroscopy) and SIMS (secondary ion mass spectroscopy) experiments, and a very sensi tive magnetic sector R G A , (residual gas analyzer), w h i c h can monitor the gas phase composition and changes i n partial pressures of gas phase components during E I D (electron induced desorption) experiments. However, the most important feature of the system is the manner i n w h i c h the electron gun has been incorporated into the overall system set-up. Care has been taken i n the design and construction of the system so that electron energy changes are produced by a varying potential on the cathode of the electron source alone. Thus, ion collection efficiencies remain unchanged for a l l electron energies, and secondary electron pro duction i n the electron multiplier detector remains constant since the incoming ion energy is fixed irrespective of the bombarding electron energy. Consequently, the "overall ion optics" of the system remains fixed and any changes in the relative ion signal w i t h changing electron energy reflect a true change between the interaction of the bombarding electrons and the surface adsorbates. W i t h the various features incorporated into this system, many interesting and well controlled'experiments can be easily done, many of which have never been attempted previously. This particularly applies to experiments involving simultaneous combinations of various surface analysis techniques. Preliminary data have been obtained from a degreased 304 stainless steel sample at room temperature using electron bombarding energies varying from 0 to 4000 eV. T h e most common ion signals occur at m/e ratios 1, 16, 19, 35, and 37 indicating the respective ions H , Οι , F i , C W , and C l 7 . Spectra similar to this have been obtained from similarly prepared 304 stainless steel surfaces by other researchers. The H and F* are by far the largest surface signals. The identification of the 19 signal +

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as F is substantiated b y the presence and similar behavior of surface signals at m/e 35 and 37, which are identified as C W and C W b y the 3-to-l signal height characteristic ratio of the natural isotopic composition of chlorine. T h e fluorine contaminant, although generally not detectable b y other means, shows u p quite readily on nearly any stainless steel sample during an E I D experiment A n interesting aspect of these H and F* surface ion signals is that they often appear i n highly sensitive magnetic sector R G A ' s and are referred to as satellite peaks. The H and F surface ions are produced by bombarding stainless steel surfaces inside the ionization chamber of the R G A b y stray electrons from the ionization beam. Since these ions are produced at a surface, they may have more or less energy than the ions produced i n the gas phase, depending on the potential of the surface at w h i c h the surface ions were produced. C o n sequently, the position of the peak, caused b y surface H or F , compared w i t h where it should be i n the spectrum is shifted, giving the term "satellite peaks/' +

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Figures 4 and 5 show the relative ion signal behavior of F* and H * w i t h electron energies of 0-4000 eV. T h e general shape of the curves closely resembles cross section curves for ion production b y electron bombardment of gas phase molecules. Figure 6 compares the curve of Figure 5 with cross section data obtained for gas phase dissociative ionization of H . The three immediate striking features of this comparison are that (1) the general shape of both curves is essentially the same, (2) the surface ion signal reaches a maximum at a higher electron energy, and (3) the surface curve is much broadened with respect to the gas phase curve. The most probable explanation is that the electron-molecule 2

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a function of bombarding electron energy using an electron beam current density, i " = 2 X 10' A/cm . 7

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Figure 6. Comparison of relative EID H* signal from Figure 5 with relative gas phase H* production from H as a function of bombarding electron energy. The H* surface curve has been arbitrarily normalized to match the peak height of the gas phase curve. The general shape of the curves appear simifor, but the surface H* curve is broadened compared with the gas phase H* curve.


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interaction resulting i n ion production for both gas phase ions and surface ions is essentially the same. However, the surface H curve indicates that reflected primary and secondary electrons which are produced by electrons passing the adsorbed hydrogen and striking the substrate may also contribute to the ion signal. These reflected primary and secondary electrons can make a second pass at the adsorbed species producing a twofold result—the ion signal reaches a maximum at an electron energy greater than that shown for the gas phase curve and the ion signal falls off more slowly with increasing electron energy. Both results are logical in view of the fact that the second pass secondary electrons range i n energy from 0 - E w i t h an energy distribution depending to a certain degree on the initial bombarding electron energy. These results are encouraging. They are what one would intuitively expect using the assumption that the adsorbed molecules at the surface behave much the same as they do i n the gas phase when bombarded by electrons. This assumption is substantiated by a considerable amount of data (Figures 2 and 3 ) . Another aspect of these results is that it may be possible to determine absolute cross sections for E I D , something normally very difficult to measure. If one assumes that the second-pass electrons essentially do not contribute to the ion signal for primary electron energies below, for example, 75 eV, then by a point-slope matching method of the surface data to gas phase data for electron energies below 75 eV, the surface ionization curve can be scaled with respect to the gas phase ionization curve. Further experiments should, therefore, hopefully provide quantitative data on both electron-induced ion and neutral particle desorption. +

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Photon-Induced Desorption Considerably greater difficulty has been encountered i n attempting to measure photodesorption ( 7 - 1 2 ) . This difficulty was related to two basic problems. The first is the much greater difficulty i n producing


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controlled beams of photons whose energy is sufficient to cause desorption—that is, the uv and extreme uv range—without simultaneously causing thermal desorption. The second problem is that most early experiments were attempted on clean simple metals. In a l l cases, the data was very difficult to obtain and erratic at best. The increased availability of uv sources and ultra-sensitive detection systems have led to further experiments. Recent results (8) indicate that it is difficult to obtain data from clean simple metals because the cross section for photodesorption of gases adsorbed on these surfaces is very small or perhaps even zero. Significant results were obtained when studying photodesorption from materials such as 304 stainless steel whose surface is a semiconductor, chrome oxide. Some results are shown i n Figure 7. The values obtained for the various metal elements may be real or very possibly caused by scattered uv radiation striking uncovered stainless steel or similar type surfaces. A t any rate, it seems quite clear that i n the photodesorption process the substrate plays a significant role. The importance of investigating semiconductor surfaces i n relation to photodesorption has thus been clearly demonstrated by the results of photodesorption from the chrome oxide surface of stainless steel. T o understand the interaction between photons, adsorbed gases, and semiconductor substrates, an extensively investigated semiconductor, Z n O , (13,14) was chosen for experiments on photodesorption i n our laboratory.

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Figure 7. Relative photodesorption signal obtained from several metal surfaces irradiated with the full spectrum of a low pressure mercury lamp. Flux density = 200 nW/cm at all wavelengths tested. 2

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Figure 8. Transient change in partial pressure of CO ( ) and in total pressure ( ) during a 10-sec uv illumination interval. ZnO sample is at 125°C. t

M a n y years ago, photodesorption of oxygen from Z n O was postulated (15,16) when it was observed that huge persistent changes i n the con ductivity of Z n O could be brought about by band gap radiation i n vacuum while the initial conductivity could be restored almost completely by admitting oxygen into the system. N e w insight into the ZnO/oxygen system has been obtained by recent experiments (17) using careful mass spectrometry of the photodesorption products under controlled irradiation. Short flashes of band gap radiation incident on ZnO surfaces i n a U H V (1 X 10" torr) system produced pressure changes such as the one shown i n Figure 8. The 10-sec flash also caused a very fast and large change i n the C 0 (m/e = 44) partial pressure. N o such change was found i n any of the other masses (between 1 and 50) that could not be accounted for by the cracking pattern of C 0 i n the ionization chamber. To establish the true photodesorptive characteristics of the experi ment, measurements of the photodesorption signal were carried out as a function of illumination intensity. The linear dependence observed indi cates the quantum features of this effect. Furthermore, the photodesorp tion spectral response shown i n Figure 9 indicates clearly the substrate dependence of this effect ( E ( Z n O ) = 3 . 2 e V ) as well as its quantum (and not thermal) characteristics. These results have also been confirmed by measurements on stainless steel (chrome oxide), C d S , and T i 0 and support the accepted model of neutralization of the chemisorbed species, which proves to be C 0 , by photogenerated holes. 9

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A measurement of the photodesorbed C 0 yield as a function of iUumination time is shown i n Figure 10. As the measurement was done by 0.25-sec flashes, Figure 10 actually shows the derivative of the C 0 photodesorption yield w i t h respect to time and indicates the gradual decay of the photodesorption rate. A recent calculation based on these experimental data suggests a value of 10" c m as the cross section for neutralization of the chemisorbed C 0 species by the photogenerated holes. A measurement of the photodesorption signal as a function of temperature yields an activation energy for this process of approximately 0.25 eV. There is a one-to-one correspondence between the C 0 photodesorption rate and the simultaneous rate of the surface conductivity increase. Both rates are fast at the beginning of the illumination period 2

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and decay later to a much slower rate. These data support the accepted model of recombination of chemisorbed species b y photogenerated holes migrating to the surface. O u r measurements show, however, that this chemisorbed species is C 0 , and its photodesorption is directly responsible for the observed surface conductivity changes. T o understand the source of the photodesorbed C 0 species, chemisorption experiments were done with several different gases such as C 0 , C O , and 0 . It was found, however, that only 0 chemisorption could destroy the accumulation layer created during photodesorption. Therefore, these oxygen molecules must oxidize impurity carbon atoms on the surface and, upon electron capture, create a C 0 " ion molecule. Indeed, A E S (Auger electron spectroscopy) measurements showed considerably less carbon impurities on Z n O surfaces which had been subject to prolonged illumination (19). The observations of the impurity carbon signal b y A E S are i n accordance w i t h the fact that after some cycles of photodesorptionchemisorption runs, the photo-activity of the surface diminishes considerably. This activity cannot be rejuvenated by further exposure to oxygen. In other words, cleaned and carbon-depleted Z n O surfaces become inert to photodesorption and chemisorption processes. This was observed by other workers as well (18). Deposition of monolayer amounts of carbon was enough to rejuvenate the surface activity for photodesorption and conductivity to its initial level. 2

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Figure 10. Semi-log plot of the rate of surface conductivity changes and of relative CO photodesorption (O) as a function of net illumination time as obtained from a ZnO powder sample at 300 Κ È

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Thus, the process of photodesorption, i n many cases, seems to proceed according to the following sequence—molecular oxygen is adsorbed on surface impurity carbon atoms; capture of a substrate electron leads to the formation of a chemisorbed C 0 ~ " ion molecule complex; irradiation by band gap light produces electron hole pairs; some of the holes migrate to the surface, combining with the "C0 ~" ion molecule leaving a physisorbed C 0 molecule; and this molecule then desorbs thermally from the substrate. W

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Summary Electron-induced desorption involves primarily the direct interaction of the incoming electron with the adsorbed gas molecule complex. Tran sitions to dissociative states lead to the desorption of molecular fragments as neutrals or ions. Interactions are pretty much independent of the sub strates. Photodesorption seems to be primarily a substrate-dependent semiconductor process. Photons of band gap energy and above produce electron hole pairs. The holes recombine with the chemisorbed complex to produce a physisorbed molecule, generally C 0 , which then thermally desorbs. In photodesorption, only desorbing neutrals are detected. In both E I D and photodesorption, the maximum cross sections are typical of atomic processes, i.e., 10" -10~ cm . 2

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Literature Cited 1. Lichtman, D., McQuistan, R. B., Prog. Nucl. Energy Ser. (1965) IX, 4 (pt. 2) 95. 2. Madey, T. E., Yates, J. T., Jr.,J.Vac. Sci. Technol. (1971) 8, 525. 3. Lichtman, D., Simon, F. N., Kirst, T. R., Surf. Sci. (1968) 9, 325. 4. Ford, R. R., Lichtman, D., Surf. Sci. (1971) 26, 365. 5. Menzel, D., Ber. Bunsenges. Physik. Chem. (1968) 72, 591. 6. Davis, P. R., Donaldson, E. E., Sandstrom, D. R., Surf. Sci. (1973) 34, 177. 7. Peavey, J., Lichtman, D., Surf. Sci. (1971) 27, 649. 8. Fabel, G. W., Cox, S. M., Lichtman, D., Surf. Sci. (1973) 40, 571. 9. Adams, R. O., Donaldson, Ε. E.,J.Chem. Phys. (1965) 42, 770. 10. Lange, W.J.,J.Vac. Sci. Technol. (1965) 2, 74. 11. Genequand, P., Surf. Sci. (1971) 25, 643.

12. Kronauer, P., Menzel D., Adsorption-Desorption Phenom., Proc. Int. Co 2nd (1972) 313.

13. 14. 15. 16. 17. 18. 19.

Heiland, G., Kunstman, P., Surf. Sci. (1969) 13, 72. Many, Α., Crit. Rev. Solid State Sci. (1974) 4, 515. Morrison, S. R., Adv. Catal. (1955) VII, 259. Melnick, D. Α.,J.Chem. Phys. (1957) 26, 1136. Shapira, Y., Cox, S. M., Lichtman, D., Surf. Sci. (1975) 50, 503. Levine, J. D., Willis, Α., Bottoms, W. R., Mark, P., Surf. Sci. (1972) 29, 165. Shapira, Y., Cox, S. M., Lichtman, D., Surf. Sci. (1976) 54, 43. RECEIVED January 5, 1976. Work supported by ERDA grant No. E(11-1)-2425 and by NSF grant No. DMR 74-03947.

Electron- and Photon-Induced Desorption - Advances in Chemistry

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