J. Phys. Chem. 1990, 94, 6299-6305
6299
Kinetic Energy Distributions of Aniline Molecules and Cations following Their UV-Laser- Induced Desorption from a Metal Surface M. Yang and J . P.Reilly* Chemistry Department, Indiana University, Bloomington, Indiana 47405 (Received: July 24, 1989; In Final Form: March 29, 1990)
Aniline molecules and cations are desorbed from a thin metal film by irradiation with UV laser pulses in an internal reflection geometry. Their kinetic energy distributions are measured by two novel methods using a time-of-flight mass spectrometer. To study the neutral distribution, a two-laser experiment is performed in which one pulse desorbs molecules and a second, delayed pulse ionizes the desorbed molecules in a spatially resolved transient grating above the surface. The contribution of ion velocity spread to mass spectral peak broadening is deconvoluted in order to extract the ion speed distribution. With incident UV laser pulses of 0.5 and 1.5 pJ, the kinetic energy distribution of desorbing neutral molecules is found to fit to a Maxwell-Boltzmann curve characterized by a temperature of approximately 700 K. The ion kinetic energy distribution is significantly different, peaking at about 1 eV and having a fwhm of approximately 1 eV, depending on the drawout electric field and the laser intensity.
on a cold s ~ r f a c e . l ~In- adsorption/desorption ~~ experiments on
1. Introduction A number of methods of probing and describing the processes of surface desorption and desorption ionization have been proposed during the past two decades.'+ Desorption studies consist of an energy input by heat, photons, or particle beams, any of which can serve to rupture the adsorbate-surface binding state; this is followed by some type of measurement of the desorbed species. Since the initial kinetic energy distribution and the angular distribution of desorbing species are signatures of the chemical configuration on the surface, their measurement may provide valuable information that is of interest in the field of surface science. Recent investigations of the internal state distribution of desorbing or scattering molecules using resonance-enhanced multiphoton ionization with a tunable laser have revealed particularly detailed information about gas-surface interactions.'-" Products of dissociative or other surface chemical reactions have also been invest iga ted.'*-I5 One of the experimental conditions required to optimize the study of desorption is that desorbing molecules must be distinguishable from ambient gas-phase species. This may be achieved by depositing nonvolatile molecules'618 or by freezing molecules
( I ) Tolk, N. H.; Traum, M.; Tully, J. C.; Madey, T. E., Eds. Desorption Induced by Electronic Transitions (DIET I);Springer-Verlag: Berlin, 1983. (2) Brenig, W.; Menzel, D., Eds. Desorption Induced by Electronic Transitions (DIET II); Springer-Verlag: Berlin, 1986. (3) Barker, J. A.; Auerbach, D. J. Surf. Sci. Rep. 1985, 4, 1. (4) Comsa, G.; David, R. Surf. Sci. Rep. 1985, 5 , 145. (5) Lin, M. C.; Ertl, G. Annu. Rev. Phys. Chem. 1986, 37, 587. (6) Zacharias, H. Appl. Phys. 1988, A47, 37. (7) Haeger, J.; Walther, H. Annu. Reu. Mater. Sci. 1989, 19, 265. (8) Haeger, J.; Shen, Y. R.; Walther, H. Phys. Rev. 1985, A31, 1962. (9) Budde, F.; M d l , A.; Hamza, A. V.; Ferm, P. M.; Ertl, G. Surf. Sci. 1987. 192, 507. (IO) Budde, F.; Hamza, A. V.; Ferm, P. M.; Ertl, G.; Weide, D.; Andresen, P.; Freund, H. J. Phys. Rev. Lett. 1988, 60. 1518. ( I 1) Weide, D.; Andresen, P.; Freund, H. J. Chem. Phys. Lett. 1987, 136, 106. (12) Kutzner, J.: Lindeke, G.; Welge, K. H.; Feldmann, D. J . Chem. Phys. 1989, 90, 548. (13) Harrison, I.; Polanyi, J. C.; Young, P. A. J. Chem. Phys. 1988, 89, 1498. (14) Harrison, I.; Polanyi. J. C.; Young, P. A. J . Chem. Phys. 1988. 89, 1475. (IS) Houle, F. A. J . Chem. Phys. 1987, 87, 1866. (16) Egorov, S. E.;Letokhov, V. S.; Shivanov, A. N. Sou. J . Quantum Electron. (Engl. Transl.) 1984, 14, 940. (17) Hardin, E. D.; Vestal, M. L. Anal. Chem. 1981, 53, 1492. (18) Van der Peyl, G. J. Q.; Isa, K.;Haverkamp, J.; Kistemaker, P. G. Org. Mass Spectrom. 1981. 16, 416.
0022-3654/90/2094-6299$02.50/0
volatile molecules at room temperature, it is impossible to distinguish whether the molecules being ionized are initially adsorbed on the surface or just traveling around nearby in the gas phase, unless the density of desorbing molecules considerably exceeds that in the ambient gas. Largely for this reason, the measurement of the kinetic energy distribution of volatile molecules pulse desorbed from a warm surface has not been previously reported. A laser beam incident upon a metal surface from the gas phase above it is capable of ionizing gas-phase molecules both before and after it strikes the surface. If the light is monochromatic, an interference pattern between the incident and reflected beams develops above the surface. Because destructive interference actually generates a node of the electric field of the light at the surface, we have often found that gas-phase ionization above the surface overwhelms the surface desorption ionization signal even though the number of molecules on the surface exposed to the laser beam is orders of magnitude higher than the number irradiated in the gas phase.21 UV-laser-induced desorption/ionization with prism internal reflection enables us to avoid this problem and monitor only those molecules initially adsorbed on the surface.22 When light strikes the surface from the side of higher refractive index and the incident angle is larger than the critical angle, total internal reflection occurs. In this case there is no energy flow through the interface, an evanescent wave propagates along the surface,23and only molecules on or very close to the surface interact with the laser field. These external and internal methods of UV-laser-induced surface ionization were compared recently, and it was demonstrated that in the latter case the light ionizes molecules initially adsorbed on the surface.u Multiple-peak mass spectra obtained in the external reflection mode all result from gas-phase ionization.22 High light intensities are not required in the internal reflection mode. Normally, a few microjoules is sufficient to induce detectable ionization of molecules adsorbed on an area of IO-) cm2; this is weak enough that deterioration of the surface can be minimized. The fact that adsorbed molecules can be distinguished from ambient gas-phase species permits a detailed examination of adsorption/desorption equilibrium for volatile species at room t e m p e r a t ~ r e . ~ ~ . ~ ~ , ~ ~ The kinetic energy distribution of neutral molecules desorbed by electron i m p a ~ t or ~ ~laser J~ has previously been studied (19) (20) (21) (22) (23) (24) (25)
Wedler, G.; Ruhmann, H. Surf. Sci. 1982, 121, 464. Hussla, 1.; Viswanathan, R. J . Vuc. Sci. Techno/. 1985, 8 3 , 1520. Chai, J. W.; Reilly, J. P. Opt. Commun. 1984, 49, 51. Yang, M.; Reilly, J. P. Opt. Commun. 1989, 71, 193. Born, M.; Wolf, E. Principles of Optics; Pergamon: New York, 1975. Millard, J. R.; Yang, M.; Reilly, J. P. J . Phys. Chem. 1987, 91,4323. Millard, J. R.; Yang, M.; Reilly, J. P. Manuscript in preparation.
0 1990 American Chemical Society
6300 The Journal of Physical Chemistry. Vol. 94, No. 16, 1990 by measuring the time that molecules take to travel from the surface to a detector some distance away. If the desorption process is rapid compared to the flight time, the measured time-dependent signal can be deconvoluted to yield the velocity distribution. The problem with this approach is that to obtain good resolution the detector or ionizing beam must be displaced from the surface by a considerable distance. In this case the signal detected may be very weak. Jf the ionizing beam is focused, it can be much closer to the surface.6 However, there is always a compromise between resolution and sensitivity. In this paper a new method for measuring the velocity distribution of desorbing neutral molecules is demonstrated. Two temporally short ultraviolet laser pulses are utilized. The first pulse desorbs molecules from the surface by internal reflection, while the time-delayed second laser pulse ionizes them in welldefined spatially resolved locations. This enables us to measure the density of the desorbing molecules as a function of their distance from the surface. For a single delay time, an entire density distribution can be extracted. Measurement at different delay times provides corroborating information. One of the principal advantages of the method to be described is that desorbed molecules are probed close to the surface, before their density drops to the level of the ambient gas phase. Adsorbate-surface bond energetics is particularly relevant to the study of desorption processes. Thermal desorption is an activated surface process involving an exponential dependence on input energy. The energy of desorption is supplied by a temperature increase in the solid substrate. The kinetic and internal energy characterizing the desorbates should depend on the temperature of the surface at the instant of desorption which in turn depends on the strength of the surfaceadsorbate bond. In contrast with thermal desorption, it may also be possible with a laser to directly excite an antibonding electronic state that fractures the surfaceadsorbate bond. Less information is available about such processes, although nonthermal velocity distributions would be expected. I n the process of desorption ionization, it is usually not obvious whether ions are formed on the surface and desorbed or desorbed as neutrals and then ionized. In one case we have probed this question by comparing the wavelength dependence of the desorption ionization and the two-step gas-phase ionization of NO.24 The two spectra appear nearly identical except that a higher rotational temperature is observed in the former case. This observation suggests that NO is desorbed as a neutral and then ionized. However, in the case of aniline, the wavelength dependence of the desorption ionization is very different from the gas-phase absorption spectrum so the issue is u n r e ~ o l v e d . ~ ~ When molecules are ionized on or very near to a surface, the primary ion kinetic energy distribution may be modified by image charge attraction and by repulsions among neighboring ions. Some of the primary ions can be drawn back to the surface while others are accelerated away. The velocity of an ion therefore depends (26) Kobrin, P. H.; Schick, G . A.; Baxter, J. P.; Winograd, N. Reu. Sci. Insfrum. 1986, 57, 1354.
(27) Feulner. P.: Menzel, D.; Kreuzer, H. J.; Gortel, Z. W. Phys. Rev. Lerr. 1984, 53, 67 1 .
(28) Burgess. D..Jr.; Cavanagh, R . R.; King, D. S. J . Chem. Phys. 1988, 88. 6556.
(29) Burgess, D. Jr.; Mantell, D. A.; Cavanagh, R. R.; King, D. S. J. Chem. Phys. 1986,85, 3123. (30) Moiseenko, I . F.; Glebovskii, A . A.; Lisachenko, A . A . SOC.Tech. Phys. Leu. (Engl. Transl.) 1983, 9, 599. (31) Chuang, T.J . J . Vac. Sci. Technol. 1985, B3, 1408. (32) Burgess, D., Jr.; Viswanathan, R.; Hussla, 1.; Stair, P. C.: Weitz, E. J . Chem. Phys. 1983, 79, 5200. (33) Bourdon, E. B. D.; Das, P.; Harrison, 1.; Polanyi, J. C.; Segner, J.; Stanners. C. D.: Williams, R . J.: Young, P. A. Faraday Discuss. Chem. Soc. 1986, 82, 343. (34) Bourdon, E. B. D.; Cowin, J. P.; Harrison, 1.; Polanyi, J . C.; Segner, J.; Stanner, C. D.; Young, P. A . J . Phys. Chem. 1984.88, 6100. (35) Natzle, W. C.; Padowitz, D.; Sibener, S. J. J. Chem. Phys. 1988,88, 7975. (36) Egorov, S. E.; Letokhov, V . S.; Shivanov, A. N. Chem. Phys. 1984, 85, 349. (37) Ferm. P. M.; Budde, F.; Hamza, A. V.; Jakubith, S.;Ertl, G.; Weide, D.:Andresen, P.: Freund. H . J. Surf. Sci. 1989. 218, 461.
Yang and Reilly
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Figure 1. T w o different geometries used in laser-induced desorption/ ionization experiments: (a) comparison of internal and external reflection geometries; (b) laser interference pattern generated by external reflection from the metal surface.
critically on where it is formed.3* The observed ion kinetic energy distribution conveys information about whether ions are produced before or after the desorption. 2. Experiments and Results The experimental apparatus has been previously described.24 It consists of a linear time-of-flight (TOF) mass spectrometer with a laser ionization source. A thin metal film coated on a fused silica prism substrate is irradiated from the prism side. The system is mounted in an ultrahigh-vacuum chamber evacuated with a cryopump. The ultimate vacuum in the chamber is in the 1O-IO-Torr region. A frequency-doubled tunable dye laser with a pulse width of 2 ns and pulse energy of 0.5-10 pJ served as a source of light in the 220-300-nm wavelength region. The UV laser beam passed through a pinhole and was focused by a 30-cm focal length lens. A Biomation waveform recorder interfaced to a personal computer digitized the microchannel plate output. The ion signals obtained during 1000 laser pulses were averaged for each measurement. The electric fields in the ionization region were 10-100 V/5.7 mm for ion kinetic energy measurements and 2000 V/6.2 mm for measurements of the neutral speed distribut ion. The right angle prism surface was acid cleaned and then washed with methyl alcohol. The edges of the prism were coated with a thick layer of aluminum, but over a central 2 X 19 mm2 stripe, a very thin gold film was vacuum d e p o ~ i t e d The . ~ ~electrical ~~~~~~ resistance across that gold film was measured to be 1.6 kohm. The transmission of unpolarized He-Ne laser light incident at an angle of 40” (relative to the normal) through this film was measured to be 71%. The film thickness was estimated to be about 1 nm based on its resistivity39and about 6 nm from its optical transparency.m Aniline sample gas was sprayed onto this surface during measurements; the surrounding pressure was about (1 5 2 ) X lo-* and (4-40) X lo-’ Torr during neutral and ion kinetic energy measurements, respectively. In this paper, two experiments will be described. The first is a two-pulse experiment. One laser pulse desorbs neutral molecules, and the second ionizes them. The second experiment involves a single laser pulse that generates desorbed ions; the mass spectral peak profile associated with these ions conveys information about their velocity distribution. Since the desorption and the ion yields are subject to the surface coverage which depends on the time between the laser pulse^,^^^^^ the laser (38) Garrison, B. J . Sur/. Sci. 1986, 167, 1225. (39) Larson, D. C. In Physics of Thin Films; Francomb, M. H . , Hoffman, R . W., Eds.; Academic Press: New York, 1971; Vol. 6, p 81. (40) Vossen, J. L. In Physics of Thin Films; Francomb, M. H., Hoffman, R . W.. Eds.: Academic Press: New York, 1977; Vol. 9. p 1.
Kinetic Energies of Aniline Molecules and Cations
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The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6301
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Channel Number [ I c h = 2 n s l Figure 2. Time-of-flight mass spectra recorded to measure the velocity distribution of desorbed neutral aniline molecules: (a) probe laser alone (external reflection): (b) desorption laser (internal reflection) followed by probe laser: (c) desorption laser alone.
repetition rate has been held constant at 10 Hz in both experiments. ( a ) Measurement of Desorbing Neutral Molecules. When a laser beam is reflected from a metal film, a periodic interference pattern is generated above the film as shown in Figure 1B. The light intensity at the antinodes of this interference pattern is capable of ionizing gas-phase molecules; with a T O F mass spectrometer ions formed at different points within the interference pattern can be distinguished.2' This well-defined distribution of light provides a convenient arrangement for observing the spatial distribution of molecules that have been desorbed from the film by a preceding light pulse. Since a node of the light electric field occurs at the ~ u r f a c e , this ~ ' method of probing does not ionize molecules adsorbed on the surface, but only those present in the gas phase above it.22 I n order to exploit the periodic interference pattern to probe the desorption process, experiments must be performed in two steps. First, a T O F mass spectrum is recorded with the "multiple-peak" probe laser only. Since the density of gas-phase molecules is assumed to be uniform, this provides information about the spatial distribution of the light intensity. An example of such a mass spectrum is displayed in Figure 2A. The intensity profile of the laser beam exhibits a well-defined Gaussian distribution with a diameter of about 100 pm. Since it strikes the surface at a grazing angle of 0.7*,the fluence there is estimated to be 0.1 mJ/cm2 for a pulse energy of 1 pJ. Because of the grazing angle of incidence, the periodicity of the interference pattern, approximately 1 2 pm, is much larger than the light wavelength.2' The second step involves recording a T O F mass spectrum with the probe laser pulse preceded by an internal reflection desorption pulse. Figure 2B shows such a mass spectrum. Since Figure 2A,B is plotted on the same scale, it is clear that the ionization yield is dramatically larger when the probe laser follows a desorption pulse. In fact, with aniline sample pressure of 2 X Torr and laser energy of 1.5 pJ at 288 nm, ion yield increases by up to a factor of 400. In Figure 2C a mass spectrum of the ions generated by the desorption laser only is displayed. As can be seen, this signal is comparatively weak and is temporally separated from the spectrum generated by the probe laser. It therefore does not interfere with our measurement of the desorbed neutral molecule density distribution. The relative density of molecules at various distances from the surface at a particular time delay after the desorption pulse can be estimated by comparing the heights of the peaks in Figure 2A,B. (41) Bernig, P. H . In Physics oJThin Films; Francomb, M. H., Hoffman, R. W., Ed.; Academic Press: New York, 1963; Vol. I , p 69.
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Figure 3. Measured density distribution of aniline molecules desorbed from a gold surface: (a) 70 ns after a 0.5-pJ desorption laser pulse; (b) 70 ns after a 1 . 5 - J laser pulse; (c) 104 ns after a 1.5-pJ laser pulse. Calculated density distributions are plotted for three different angular distributions and three different speed distributions.
The actual distances that each of these multiple peaks correspond to are found by ion time-of-flight calculations. The measured relative densities of desorbed molecules at two different delay times (70 and 104 ns) are displayed as experimental points in parts A, B, and C of Figure 3. The energies of the desorption laser were 0.5, 1.5, and 1.5 pJ, respectively. Since the distances that molecules travel from the surface are proportional to their velocities, the density distributions can be used to determine the speed distribution. The complication is that distance above the surface represents only one component of a molecule's position vector. At a particular delay time, this establishes only one component of the molecule's velocity vector. An angular distribution over which neutral molecules are desorbed must be assumed in order to extract the speed distribution. Conversion of experimental signals to a speed distribution has been discussed previously in the literature.32 Details of our calculations are included in the Appendix, and results only will be presented. The experimental desorbed molecule density distributions are compared with results calculated for three different COS" 6 angular distributions and Maxwell-Boltzmann speed distributions at various temperatures. All are displayed in Figure 3. Satisfactory agreement is obtained in several cases. The data appear to fit best to a cos 6 angular distribution at 700 K. Increasing the exponent of the cosine function leads to a more sharply peaked desorption distribution and a somewhat lower estimated temperature. Because we do not know the exact form of the desorption angular distribution, the error associated with our temperature estimate is f l O O K. For measurements involving different delay times but the same desorption laser energy, one angular and one speed distribution should fit all the data. This is confirmed by comparing parts B and C of Figure 3. The similarity between parts A and B of Figure 3 is rather striking considering that the laser pulse energy differed by a factor of 3 in the two cases. Although the total ion signal was 5 times larger a t the higher pulse energy, the density distribution was measured to be the same within experimental error. ( b ) Kinetic Energy Distribution of Desorbing Ions. In a time-of-flight mass spectrometer, the flight time of an ion is a function of its mass-charge ratio, m / z , ionizing position, X,, and initial kinetic energy, K , along with the drift tube length, L, grid spacings, di,and potentials, Pi.4244 If ionization occurs in the gas phase and the initial kinetic energy of the neutral precursor is small compared to the acceleration energy, then the ion flight time spread is mainly due to the spatial spread of the ionizing position X,. However, when ionization occurs by evanescent wave excitation, as in the internal reflection geometry, the width of the ionizing region is negligible. If the grid spacings and potentials are constant during a measurement, the flight times for various ions of one mass differ only because of their initial kinetic energies. (42) Wiley. W . C.; McLaren, I. H. Reu. Sci. Instrum. 1955, 26, 1150. (43) Opal, R. B.; Owens, K. G.; Reilly, J. P. Anal. Chem. 1985, 57, 1884. (44) Yang, M.; Reilly, J. P. Anal. Insfrum. 1987, 16, 133.
Yang and Reilly
6302 The Journal of Physical Chemistry, Vol. 94, No. Id, 1990
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kinetic energy. 2 K i n e t i c E n e r g y lev1 Figure 6. Kinetic energy distributionsof aniline ions desorbing from an AI surface measured as a function of ion drawout field. The laser pulse energy was 4 FJ at 260 nm. Accelerating voltages were (a) 100, (b) 50, (c) 20, and (d) 10 V. Vertical scales are the same in all cases. 0
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Figure 5. Kinetic energy distributions of aniline ions desorbing from a Au surface as a function of ion drawout electric field. The laser pulse energy was 9 rJ at 260 nm. Accelerating voltages were (a) 100, (b) 50, (c) 20, and (d) I O V. Vertical scales are the same in all cases.
Therefore, a curve of ion signal vs time can be directly inverted to a graph of ion number vs initial kinetic energy
dl
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where tm is the flight time of an ion with zero initial kinetic energy and tk is the flight time shift due to an initial kinetic energy of K. Figure 4 illustrates the conversion of a measured time distribution, 1(zk+tm),into a kinetic energy distribution, I ( K ) . Figures 5 and 6 show the measured ion kinetic energy distributions of aniline molecules desorbing from Au and AI surfaces, respectively, with applied electric fields in the first ion acceleration region of 100, 50, 20, and IO V/5.7 mm. The energy distributions peak at between 0.6 and 1.3 eV, depending on the applied electric field. The fwhm's of the distributions are about 1 eV. They are broader at higher laser intensity or higher sample pressure. Since the thickness of the thin metal film was not carefully controlled and the laser parameters were not always identical, differences between the exact peak locations and shapes observed for the two metal surfaces may not be significant. However, the monotonic shift of the distributions to lower energy as the electric field increases was consistent in every measurement. In Figure 7 the dependence of the mass spectral peak profile on incident laser energy and applied electric field is displayed for the particular case of aniline adsorbed to a gold film. The laser wavelength was 268 nm. For each drawout voltage, the flight time of an ion with zero initial kinetic energy, Tm, is indicated. lons arrive at the detector earlier than Tmbecause of their initial kinetic energy. This flight time shift increases as the laser energy increases from 1.2 to 2.4 pJ. At higher incident laser energies ions do not appear earlier in time. Rather, the mass spectral peak
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Figure 7. Dependence of aniline ion flight time on both the applied drawout field and on the incident laser pulse energy. Vertical scale for each spectrum is different.
profile broadens and more slow ions reach the detector. This interesting dependence is reproduced at three different drawout electric fields as seen in Figure 7.
3. Discussion ( a ) Kinetic Energy of Desorbing Neutral Molecules. From the results summarized in Figure 3 we conclude that in these experiments aniline molecules were desorbed from a gold surface with a roughly 700 K Maxwell-Boltzmann speed distribution. Is 700 K the surface temperature at the moment of desorption? In order to gain some insight into this, the experiment was performed at two different laser pulse energies, 0.5 and 1.5 pJ. The measured kinetic energy distributions were found to be identical within experimental error although about 5 times as many molecules were desorbed at 1.5 pJ as at 0.5 HJ. This result indicates that a certain activation energy is sufficient for the desorption; additional input energy does not increase the kinetic energy of desorbed molecules, but only their probability to desorb. Flash desorption and numerous much slower temperatureprogrammed desorption experiments have previously been done to investigate the temperature dependence of desorption of small molecules from metal surface^.^^-^' Several characteristic tem-
Kinetic Energies of Aniline Molecules and Cations perature peaks are usually reported, suggesting different activation energies for different binding state^.^*,^^ The locations of these peaks appear to depend strongly on the rate of heating, and this is likely to be the case in our experiments. As an example of another complication, two different velocity distributions of NO desorbed from a cold Ag surface by 5 4 s UV laser pulses have been reported?s one of which was not dependent on the laser pulse energy or the photon energy. Two different internal state distributions, thermal and nonthermal, have been observed in the From our measurements we cannot rule out desorption of the possibility that two or more velocity distributions are overlapped. If these were associated with similar temperatures, it would be very difficult to resolve them. However, if the temperatures were quite different, the observed distribution should be non-Maxwellian, and there is no evidence for this in our data. The fact that the velocities of desorbed aniline molecules can be well described by a Maxwell distribution with a reasonable temperature implies that a thermal desorption mechanism adequately explains our data. While it is possible that photon-induced direct excitation of the surface-adsorbate bond could occur, we have no evidence suggesting that it does. An investigation of the dependence of the velocity distribution on the laser wavelength might provide information relevant to this issue. (b) Kinetic Energy of Desorbing Ions. Two observations about the velocity distribution of ions desorbed from the metal film require explanation. First, ions typically receive 1 eV of kinetic energy, and that is much larger than the thermal energy that neutral molecules desorb with. Second, the measured ion velocity distributions exhibit reproducible drawout electric field dependencies. In this section a few possible models for the desorption/ionization process will be considered, particularly with respect to the above observations. While each has its merits, none is capable of explaining the data quantitatively. Thermal Emission Model. The process by which molecules impinging on a hot metal surface in the absence of an applied electric field are ionized has been described by the Saha-Langmuir equation.s0 I n this case the degree of ionization depends on 4, the work function of the metal, and I , the molecular ionization potential: a(E=O) a exp(e[4 - I]/kT} (2) For aniline desorbing from a gold surface at 700 K this exponential is on the order of Comparison of our two experiments a number suggests that our ratio of ions to neutrals is about much larger than that predicted by this equation. This suggests that ionization is not thermally but light induced, a result that is not very surprising. The dependence of ion survival probability on external electric field has also been previously considered in thermal surface ionization studies.50 The degree of ionization in the presence of an accelerating electric field, E, is predicted to increase because the image force attracting the ion to the metal is reduced by an amount e E (normal Schottky effect).s1
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6303
I(u) = Io(u) P,(U) (4) P, has been derived according to Auger neutralization t h e ~ r y ~ ~ , ~ ~ which describes the process whereby an ion near to a surface extracts an electron from it and becomes neutralized. P, is given by P,(u) = exp(-A/au) (5) where u is the ion velocity component normal to the surface. Thus, faster moving ions have a higher escape probability. The constant Ala has been experimentally found to be 2 X lo4 m/s for secondary ion emission of Cu+ from a Cu crystal surface.5s By use of this number and a Maxwell-Boltzmann distribution with a the function I ( u ) can be made temperature of 1000 K for to match one of the experimentally observed kinetic energy distributions. However, according to this theory ion neutralization only occurs at distances very close to the surface (less than 1 A). At larger distances the escape probability is essentially one. Since the evanescent wave of the laser light extends much farther than this, it would be surprising if most of the ions that we observe were not produced much further from the surface. Their velocity distribution should be quite independent of this theory's predictions. Neutral Desorption Followed by Ionization. Suppose that a neutral molecule is desorbed with thermal kinetic energy and then ionized at a distance r from the surface by the evanescent laser light. The dependence of the ion yield on the laser wavelength supports this ionization mechanism.24 If an ion is produced close to the surface, then it immediately induces an image potential
that attracts it to the surface. Here I l k is the Fermi-Thomas screening length.56*57 If an ion is formed at position ro, with kinetic energy less than U(ro),it will be drawn back to the surface. If it is moving away from the surface with a kinetic energy KOthat is larger than U(ro), it can escape whereupon its kinetic energy is then reduced to K = KO- U(ro) (7) For example if ro is 1 nm and I l k is negligible, U(ro) is approximately 0.4 eV. Although this mechanism implies that the kinetic energy of an ion must be smaller than that of its neutral precursor, our experiments have firmly indicated that ion kinetic energies are substantially higher than those of neutrals. In order to explain how ions can acquire their observed kinetic energies, we consider the repulsive force of surface charges. When many ions are produced on or near the surface, some are initially attracted to it by the image potential. We hypothesize that these momentarily build up a positive charge layer. Ions subsequently produced are repelled by this surface charge. The electric field at a distance r above a disk of radius R having a surface charge density QS(in C/m2) iss8
a ( E ) = a(E=O) exp(+e(eE)'12/kT)
(3) However, the electric fields used in this experiment, 10-100 V/5.7 mm, were so small that eq 3 does not predict a significant change in a,further suggesting that the thermal emission model is inappropriate for interpreting our results. Thermal Distribution with Escape Probability. It has recently been suggested that the observed kinetic energy distribution of desorbing ions should be the product of the initial kinetic energy distribution, Io, multiplied by an ion survival probability, PS? (45) Readhead, P. A. Vacuum 1962, 12, 203. (46) Ehrlich, G.J . Appl. Phys. 1961, 32, 4. (47) Dresser, M. J.; Madey, T . E., Jr.; Yates, J. T. Surf. Sci. 1974, 43, 533.
(48) King, D. A. Surf. Sci. 1975, 47, 384. (49) Madey, T.E.; Yates, J . T., Jr. Sur/. Sci. 1977, 63, 203 (50) Kaminsky. M. Atomic and Ionic Impact Phenomena on Metal Surfaces; Springer-Verlag: Berlin, 1965; Chapter 8. (51) Romanow, A. M.; Starodubtsev, S. V. Sou. Pfiys.-Tech. Pfiys. (Engl. Transl.) 1957. 27. 652.
Because r is much smaller than R under our conditions, E, is almost independent of r and R . Although an appropriate value for QS(t) during our experiments is certainly not known, we assume that its time dependence would follow that of the laser pulse profile I(t): (9)
Using various trial numbers for the maximum value of Qs(t), one (52) Tully, T. C. In ref 1 , p 31. (53) Cobas, A.; Lamb, W. E., Jr. Phys. Rev. 1944. 65, 327. (54) Hagstrum, H. D. Phys. Reu. 1954, 96, 336. (55) Vasile, M. J. Surf. Sci. 1982, 115, L141. (56) Dunlap, B. 1.; Gadzuk, J. W. Surf.Sci. 1980, 94, 89. Vukanic, J. Surf. Sci. 1984, 141, 285. (57) Miskovic, Z.; (58) Halliday, D.; Resnick, R. Fundamenrols of Physics: Wiley: New York, 1988: p 557.
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The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
PI:lOv
20
v
50
h
r'
0
,,!
v
81
h
,,-A,? -,,&;a*;
500
Yang and Reilly
1000 0
so0
/' , loo0 0
;
500
,
1,21J 1000
Channel Numbers I1 ch.= 2 n d Figure 8. Calculated maximum ion kinetic energy as a function of surface charge density for a laser ionization pulse width of 2 ns. can calculate the final velocity of an ion accelerated by the surface charge by numerically integrating u = u,, +- e / m S ( E , +- Eo) d t
(10)
Here, fi0 is the initial ion velocity (which is assumed to be thermal) and Eo is the static ion drawout electric field. The distance r in eq 8 is the integral of velocity u over time t . We find that because the dependence of E, on r is so weak, the calculated final ion velocity is virtually independent of initial ion positions ranging from I A to I pm. Figure 8 shows the calculated kinetic energy imparted to ions due to acceleration by surface charges produced by a 2-ns laser pulse. Since the maximum kinetic energies observed in our experiments were approximately 2 eV, this model predicts that the peak value of the surface charge density is about 1 X 1O'O charges/". Dependence on the External Electric Field. As evident in Figures 5 and 6, measured kinetic energy distributions of desorbing ions depend greatly on the applied external electric field. T O F peaks shift from roughly 1 to 0.7 eV when the electric field varies from I O to 100 V/5.7 mm. A possible instrumental artifact that might cause this would be if the ion detection efficiency depended on the desorption angular distribution. Since the detector is small ( 18-mm diameter), a nonnegligible velocity component perpendicular to the acceleration can prevent an ion from being observed. This effect should depend on the magnitude of the ion drawout field. Exact ion trajectory calculations reveal that the energy distributions are shifted by changes in the electric field by only about 8% for a sine distribution and less than 1% for a cosine desorption distribution. These numbers are smaller than those we have experimentally observed, particularly since a cosinelike angular distribution has normally been found in ion emission e~periments.~~f'~I f the application of different electric fields actually changed the desorption angular distribution, this could explain the peak shift. However, it is not clear why this should occur. Several mechanisms to explain the observed ion kinetic energy distributions were considered above. Neutral desorption followed by ionization remains as the most plausible mechanism to explain our results. The kinetic energy distribution of ions is calculated from the repulsive force of surface charges existing during the ionization pulse. Comparing the experimental results in Figure 7 to the calculation in Figure 8, it can be concluded that the surface charge density saturates at higher laser intensity at which point the ion kinetic energy does not further increase. The saturation of the surface charge density, which certainly depends on the ion mobility and lifetime, should depend on the external applied (59) Carter, G.; Colligon, J. S. Ion Bombardmen! of Solids; American Elsevier: New York, 1968. (60) Feulner. P.; Riedle. W.; Menzel, D.Phys. Reu Lett. 1983, 50. 986.
O
0
d
, , ,
,
1
,
I
I
I
Zxlb10
Surface Charge Density [ % n 2 1
Figure 9. Schematic depiction of the regions in which the desorbed molecule density is computed. Ions formed within the circular disks will
reach the detector. electric field. Fewer charges are expected to reach the surface in higher drawout electric fields, resulting in a less repulsive acceleration. Comparison with Other Results. Kinetic energy distributions of desorbing ions have been measured using electron or laser-induced desorption ionization.6447 Observed kinetic energies are unfortunately different in every experiment, ranging from thermal to a few tens of electronvolts. The electric field dependence of the ion kinetic energy distribution has also been previously studied.67 As suggested in this paper, the surface charge density, which will change from one type of experiment to another, can affect the ion velocity distribution. In spite of this, ion kinetic energy information is useful to have since it is relevant to the development of mass spectrometer ion sources.68 4. Conclusion
Two novel methods have been developed to measure the kinetic energies of neutral molecules and ions desorbed from a metal surface by UV laser pulses. The velocities of neutral molecules desorbing from a gold surface under our irradiation conditions were characterized by a Maxwell-Boltzmann distribution with a temperature of approximately 700 K. Higher laser intensity did not change this distribution but only increased the number of desorbing molecules. Aniline ions were desorbed from the gold surface with about 1 f 0.5 eV of kinetic energy. The precise distribution depends on the laser intensity and external drawout electric fields. We suggest that the high kinetic energy of desorbing ions may be a consequence of surface charge repulsion. This implies that ions are produced immediately after the neutral desorption. The shift of the kinetic energy distribution as the electric field is varied is probably due to the inverse dependence of the surface charge density on the applied ion drawout field.
Acknowledgment. This work has been supported by the National Science Foundation and the Research Corporation. James P. Reilly is a Camille and Henry Dreyfus Teacher-Scholar. (61) Readhead, P. A. Nuovo Cimenfo Suppl. 1967, 5, 586. (62) Madey, T.; Yates, J. T., Jr. Surf. Sei. 1968, 1 1 , 327. (63) Nishijima, M.; Propst, F. M. Phy. Rev. 1971, 82, 2368. (64) Higashi, G. S. J . Chem. Phys. 1988,88, 422. ( 6 5 ) Van der Peyl, G. J. Q.;Van der Zande, W.J.; Bederski, K.; Boerboom, A. J. H.; Kistmaker, P. G. Inf. J . Mass Specfrom. Ion Phys. 1983, 47, 7. (66) Tsong, T. T.; Kinkus, T. J. Phys. Reu. 1984, 829, 529. (67) Van der Peyl, G. J. Q.; Van der Zande. W.J.; Kistemaker, P. G. I n f . J . Mass Specrrom. Ion Processes 1984, 62, 5 1. (68) Yang, M.; Reilly, J. P. In!. J . Mass Specfrom. Ion Processes 1987, 75, 209.
6305
J . Phys. Chem. 1990, 94. 6305-6316
Appendix The spatial density distribution of molecules at various time delays after their desorption can be calculated from their initial speed and angular distributions. Let p(r,R,) be the density at a distance r and direction R, then
Thus, the fraction of the total number of desorbed molecules within some region above the surface at a time td is
=
p(r,R,)? d r dR, = R ( r ) I(R,)r2 d r dR,
2(n
+
I)(m/2~kT)'/'
X
td3
represents the number in a unit solid angle dR, at R, and between r a n d r dr. For the case of a pulsed point source on a surface, the density of desorbing molecules is associated with the desorption speed distribution N(v), angular distribution I(R"), and delay time td. Since r = utd, and R, = R, = n, we have
+
R ( r ) I(R,)r2 d r dR, = N(u) i(R,)u2 dv dR, = N(r/td)I(R)r2 d r dR/td3
(11)
Thus, the number of molecules in any region above the surface is obtained by integrating R(r) I(R) over that region. The normalized angular distribution can be described as
i(R) = ( n
+ 1 ) / 2 ~COS" 0
(12)
If the normalized speed distribution is assumed to be MaxwellBoltzmann, then N ( u ) udu ~ = NMB(o) do = 4 n ( m / 2 ~ k T ) ~ /exp(-mv2/2kT) ~u~ dv
(13)
exp(-mr2/2kTfd2) cos" 0 9 d r sin 0 d0 d+ (14) Figure 9 illustrates the regions in which the molecular density has been calculated. Under our experimental conditions the flight time of ions to the detector is approximately 18 ps, and the diameter of the detector face is 18 mm. Therefore, molecules having a perpendicular velocity component larger than 500 m/s are not detected. This implies that at a delay time of 70 ns following the desorption pulse molecules that have traveled further than 35 pm in a direction perpendicular to the ion flight tube axis will not be detected. This implies that for a 70-11s delay the effective ionization regions are cylindrical disks with radii of 35 pm. (For a longer delay the radii are correspondingly larger.) Calculations are performed for disks of 1-pm thickness. For the numerical integration, each disk was divided into 900 rings in which the numbers of molecules at different speeds are calculated and summed up. The calculated number of desorbed molecules in 35-pm-diameter cylindrical disks at various distances from the surface is displayed in Figure 3 for three different temperatures and three different angular distributions.
C-C and C-H Bond Splits of Laser-Excited Aromatic Molecules. 1. Specific and Thermally Averaged Rate Constants U. Brand, H. Hippler, L. Lindemann, and J. Troe* institut f u r Physikalische Chemie der Uniuersitat Gottingen, Tammannstrasse 6, 0-3400 Gottingen, West Germany (Received: August 1 7 , 1989: In Final Form: February 14. 1990)
Toluene, m-, 0-, and p-xylene, mesitylene, ethyl-, isopropyl-, and tert-butylbenzene were irradiated by nanosecond laser flashes at 193 nm. After fast internal conversion to the electronic ground state, the molecules dissociate by C-C or C-H bond splits. The products of the fragmentation were identified by comparison with the UV spectra of products from the photolysis of the corresponding a-bromoalkylbenzenes. Toluene, the xylenes, and mesitylene predominantly dissociate by C-H bond split in the methyl groups, the a-methyl-substituted toluenes dissociate by C-C bond split into methyl + substituted benzyl radicals. Specific rate constants k(E,J) for the dissociations were determined in low-pressure experiments with an extrapolation to zero pressure. Statistical adiabatic channel (SACM) calculationsof k(E,J) are fitted to the experiments and used to calculate the corresponding high-pressure rate constants for thermal dissociation and recombination.
1. Introduction The study of the pyrolysis of aromatic molecules under combustion conditions has presented a number of experimental problems which are difficult to overcome: (i) the primary reaction can be governed by a competition of C-H and C-C bond splits; (ii) secondary reactions can lead to complicated mechanisms; (iii) the falloff curves of the unimolecular bond fission are so broad that extrapolations to the limiting low- and high-pressure rate constants are difficult to perform (see refs 1-3 and work cited therein). Laser excitation experiments offer an elegant way out of this dilemma. The photophysics of aromatic molecules, under irradiation at suitable wavelengths in the UV region, is governed by sufficiently fast internal conversions from excited electronic states to the ground state. In this way energy-selected molecules ( I ) Muller-Markgraf. W. Troe, J. J . Phys. Chem. 1988, 92, 4899. ( 2 ) Brouwer, L. D.; Muller-Markgraf, W.; Troe, J. J . Phys. Chem. 1988, 92, 4905. ( 3 ) Muller-Markgraf, W.; Troe, J. J. Phys. Chem. 1988, 92, 4914.
0022-3654/90/2094-6305$02.50/0
can be prepared whose vibrational excitation is of the same order as in high-temperature thermal excitation situations. By working with isolated molecules one can directly identify the primary dissociation products; branching ratios and specific dissociation rate constants are measured. The isolated molecule data then can be converted to thermal reaction data by using modern unimolecular rate theory. An example of this procedure has been given in ref 2 for the dissociation of toluene. The present work extends this approach to other alkyl-substituted benzenes such as those frequently encountered in today's fuels. It also discusses in more detail the theoretical connections between specific rate constants from laser excitation experiments and thermally averaged rate constants under pyrolysis conditions. The first direct study of a simple bond fission after laser excitation was reported in ref 4 where vibrationally highly excited toluene ( E 623 kJ mol-l = 52080 cm-I) in the electronic ground (4) Hippler, H.; Schubert, V.;Troe, J.; Wendelken, H. J. Chem. Phys. Leu. 1981, 84, 253.
62 1990 American Chemical Society
