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Stimulated Raman Spectroscopy of a K-Promoted Catalyst Surface: Spectroscopic Evidence of K* Rydberg State Formation Leif Holmlid* Reaction Dynamics Group, Department of Chemistry, Go¨ teborg University, SE-412 96 Go¨ teborg, Sweden Received July 5, 2000. In Final Form: November 14, 2000 The direct spectroscopic observation of K* Rydberg states with principal quantum number n ) 5 and 6 by anti-Stokes stimulated Raman spectroscopy at a K-promoted iron oxide surface (commercial catalyst for styrene production) proves that such states are formed thermally at surfaces of alkali-promoted heterogeneous catalysts. The K* states can be detected at 1 bar air pressure downward and at normal catalyst operating temperature in a vacuum. They exist in the boundary layer at the surface. Previous reports of the detection of K* Rydberg states from such catalysts using field ionization and laser ionization in a vacuum are thus confirmed. The implications for the reactivity of alkali-promoted catalysts are discussed.
1. Introduction It is known that the alkali metal promoter atom at a heterogeneous catalyst surface can form long-lived Rydberg states at the surface by thermal excitation.1-7 Such states8,9 carry an excitation energy of several eV (up to 4.34 eV for K and 3.89 eV for Cs), that is, several hundred kJ/mol (up to 419 kJ/mol for K and 376 kJ/mol for Cs). It is thus very likely that they take part in the reaction steps at the catalyst surface. The most long-lived Rydberg states are circular states with large angular momentum quantum numbers, which have radiative lifetimes of 0.18 s for circular states with principal quantum number n ) 40.14 Their lifetimes increase with increasing excitation energy. Their desorption from typical alkali-promoted catalyst surfaces was proved by accepted methods for detection of Rydberg states several years ago,3,4 and a number of studies have been published also on the formation of promoter metal clusters in Rydberg states at catalytically active surfaces.4,5,11 From our group, an early description was also published on the possible mechanism of the Rydberg state reactions,12 which is likely to be an important part of the alkali promoter action. A review will soon be published which deals with the Rydberg desorption and reaction processes of the alkali promoter.13 Despite these advances, Rydberg states of the alkali promoter atoms have not been widely discussed. The main problem seems to be that a straightforward spectroscopic * Phone: +46-31 7722832. Fax: +46-31 7723107. E-mail:
[email protected]. (1) Lundin, J.; Engvall, K.; Holmlid, L.; Menon, P. G. Catal. Lett. 1990, 6, 85. (2) Engvall, K.; Holmlid, L.; Menon, P. G. Appl. Catal. 1991, 77, 235. (3) Engvall, K.; Holmlid, L. Appl. Surf. Sci. 1992, 55, 303. (4) Engvall, K.; Kotarba, A.; Holmlid, L. Catal. Lett. 1994, 26, 101. (5) Holmlid, L. Z. Phys. D 1995, 34, 199. (6) Engvall, K.; Kotarba, A.; Holmlid, L. J. Catal. 1999, 181, 256. (7) Kotarba, A.; Baranski, A.; Hodorowicz, S.; Sokolowski, J.; Szytula, A.; Holmlid, L. Catal. Lett. 2000, 67, 129. (8) Rydberg States of Atoms and Molecules; Stebbings, R. F., Dunning, F. B., Eds.; Cambridge University Press: Cambridge, 1983. (9) Gallagher, T. F. Rydberg Atoms; Cambridge University Press: Cambridge, 1994. (10) Beigman, I. L.; Lebedev, V. S. Phys. Rep. 1995, 250, 95. (11) Kotarba, A.; Engvall, K.; Pettersson, J. B. C.; Svanberg, M.; Holmlid, L. Surf. Sci. 1995, 342, 327. (12) Pettersson, J. B. C.; Mo¨ller, K.; Holmlid, L. Appl. Surf. Sci. 1989, 40, 151. (13) Holmlid, L.; Menon, P. G. Appl. Catal., A, in print.
proof is expected. Unfortunately, Rydberg states are difficult to detect by spectroscopic means, because of several special features, the first of which is the long distance between the ion core and the Rydberg electron which gives small transition rates and thus long radiative lifetimes.8,14 This is true especially for the most long-lived so-called circular states, in which the electron orbit is characterized by an angular momentum quantum number l slightly less than n. The fact that the radiative lifetimes of the circular states are so large means that the lowintensity spontaneous light emission from them is difficult to measure. It also means that diffusional motion of the Rydberg atoms or molecules will rapidly remove them from the area of detection. Quenching by collisions will also be important during the long radiative lifetime. The best method to detect Rydberg states is still the field ionization method,8,9 which gives ion formation from Rydberg states at very weak field strengths due to the small energy remaining to the ionization limit for the Rydberg electron. For example, a field strength of 100 V/cm ionizes all Rydberg states with principal quantum numbers n higher than 40. This method has been widely employed. Recently, spectroscopic methods to observe even very long-lived Rydberg states have been developed, like stimulated Raman scattering using pulsed lasers.15 Here, we report the first direct spectroscopic observation of Rydberg states at a catalyst surface, using anti-Stokes Raman scattering with an inexpensive continuous-wave He-Ne laser. 2. Experimental Section The experiments are carried out in an apparatus with a pressure of 1 bar (air) - 1 × 10-5 mbar. The commercial iron oxide catalyst sample is placed inside a folded Ta foil with a flat surface cut in the exposed part. It is held in the vertical direction by two arms, which are connected to an ac power supply. A heating current of up to 40 A can be fed through the Ta foil to heat the sample. The catalyst initially contains 8 wt % K and is used in large-scale production of styrene from ethyl benzene in the chemical industry. The laser is a CW (continuous-wave) 10 mW He-Ne laser (Melles-Griot) with plane polarized light at 632.8 nm that is brought in to the sample through a semitransparent (14) Alber, G.; Zoller, P. Phys. Rep. 1991, 199, 231. (15) Svensson, R.; Holmlid, L. Phys. Rev. Lett. 1999, 83, 1739.
10.1021/la000951q CCC: $20.00 © 2001 American Chemical Society Published on Web 12/28/2000
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A typical anti-Stokes Raman spectrum from the catalyst sample is shown in Figure 1. The sharpest peaks are identified as deexcitation peaks for transitions between high-l states 6 f 4 and 5 f 4 in K atoms, as shown in Table 1. In the table, a comparison is also made with the corresponding transitions 12 f 8 and 10 f 8 in K+, because such transitions were identified in the laser Raman study of Cs* Rydberg states in ref 15. The agreement with the transitions in K is considerably better than for the ones in K+, and it is concluded that the transitions take place in K atoms (K* Rydberg states). If the transitions took place in K+, a transition 11 f 8 should also exist at approximately 525.4 nm. No such transition is observed. In the present study, the conditions are much milder than in ref 15, with lower densities of Rydberg states, and thus less condensation to clusters. Further, a much weaker laser is used here which will not introduce any collective deexcitation processes in such clusters.
The transitions observed are anti-Stokes Raman, and no corresponding Stokes Raman lines are identified. Thus, the system is inverted because of thermal excitation to Rydberg states. This excitation is due to desorption from covalently bonded states on the surface, as discussed in ref 6 for another catalyst surface and proved for the somewhat less complex system of K atoms at a graphite surface in ref 18. The main reason for the thermal desorption to Rydberg states is that the state of K which correlates with desorbed ground-state K atoms is not stable on the surface. Thus, very little flux can desorb from the surface to the K ground state outside the surface, but almost all K atoms desorb to K* Rydberg states. The signal is taken out in the direction opposite to the laser beam. The size of the Raman signal and the different angular widths around the laser beam for different lines show that the process taking place is stimulated electronic Raman scattering.19 The observations of these atomic transitions in K can be made at room temperature in air and in a vacuum and at typical process temperature (900 K) in a vacuum. Their intensity does not change with the pressure but decreases slightly after prolonged heating in a vacuum above 1000 K. It could be thought that the excitation to K* Rydberg states is due to the laser which excites K atoms in the gas phase to Rydberg states. However, the energy in the laser quanta (1.95 eV relative to an excitation energy of 3.96 eV for the 6H state and 3.80 eV for the 5G state) and the low fluence (10 mW power in CW) prohibit any such effects. If K ground-state atoms existed in the gas phase, Stokes lines would also have been observed, which is not the case. Thus, the formation of the K* Rydberg states is not due to the laser. The 4F state is the lowest unoccupied maximum-l Rydberg state in the K atom, as it also is in the Cs atom as pointed out in ref 15. This means that it is a very likely lower level in transitions from higher high-l Rydberg states in the K atom. For transitions from low-l states such as 7P and similar states, 4F will probably not be engaged, even if it is allowed according to the selection rules for Raman scattering. The observed width of the transitions (1.2-1.8 nm) shows that they do not take place in free K atoms but in atoms coupled to the surface in the surface boundary layer6,11 or possibly to gas molecules. Thus, no selection rules apply. At temperatures of 1300 K and with air admission at 5 × 10-4 mbar, the atomic transitions disappear temporarily (on the order of hours before they return at a lower temperature). This indicates that the observed K atoms exist in the surface boundary layer, which has to be replenished by diffusion from the catalyst bulk. The fact that the intensity of these lines does not change strongly with air pressure supports the conclusion that the K* Rydberg atoms exist in the surface boundary layer. The intensity relation between the two atomic transitions cannot be used to determine any excitation temperature, because the relative amounts of K atoms going into any of the Rydberg states is determined by the desorption kinetics and not by energetics. The light in the two transitions is also emitted into cones of different sizes due to different phase-matching conditions,16,17 which is easily observed experimentally. The spectroscopic confirmation of the thermal formation of K Rydberg states at the catalyst surface supports the
(16) Demtro¨der, W. Laser Spectroscopy, Basic Concepts and Instrumentation, 2nd ed.; Springer: Berlin, 1996. (17) Svanberg, S. Atomic and Molecular Spectroscopy, 2nd ed.; Springer: Berlin, 1992.
(18) Holmlid, L. J. Phys. Chem. A 1998, 102, 10636. (19) White, J. C. In Tunable lasers, 2nd ed.; Mollenauer, L. F., White, J. C., Pollock, C. R., Eds.; Topics in Applied Physics Vol. 59; Springer: Berlin, 1992.
Figure 1. Stimulated Raman spectrum containing the two atomic lines from K. The two broader bands have a molecular origin. The He-Ne laser emits at 632.8 nm. Table 1. The Two Atomic Transitions Observed, Compared to Transitions in K and K+a observed wavelength (nm) anti-Stokes wavenumber upper level in K (cm-1) lower level in K (cm-1) difference (cm-1) upper level in K+ (cm-1) lower level in K+ (cm-1) difference (cm-1) a
(cm-1)
509.3
546.6
3832 -3046 (6H) -6882 (4F) 3836 -2926 (12R) -6737 (8J) 3811
2492 -4393 (5G) -6882 (4F) 2489 -4268 (10R) -6737 (8J) 2469
The energy levels are given relative to the ionization limit.
glass mirror and a glass window. The light from the sample is taken out in the opposite direction relative to the laser beam, through the semitransparent mirror to a spectrometer. A microscope objective (×40) can be used to widen the laser beam and increase the viewing angle inside the chamber, giving similar results. The light from the sample is focused by gold mirrors onto the entrance slit of a 0.5 m spectrometer (Spex 500M, ISA Inc.) with a grating with 1800 lines/mm and a photo multiplier detector (Hamamatsu R928 in current mode). The spectrometer is run under computer control (SpectraMax 32, ISA Inc.). The different angular cones for different Raman lines due to phasematching conditions16,17 are studied by readjustments of the mirrors and also by changing the entrance slit width.
3. Results and Discussion
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idea that the promoter function of alkali is coupled to the formation of Rydberg states.12 The Rydberg states that exist in the boundary layer of the surface may diffuse thermally out from the surface to a distance up to 1 µm without actually desorbing,6 because of the very long range dispersion interaction between the surface and the Rydberg electron. At the same time as they make such large jumps out from the surface, they will also move rapidly along the surface.6 Because of the extremely large cross sections expected for many collisional processes involving Rydberg states, the Rydberg atoms may act as very efficient “scavengers” of the space close to the actual solid catalyst surface, bringing back attached and chemically activated molecules to the catalyst surface. The formation of excited clusters in collisions of an external flux entering the boundary layer proves that such processes exist.20 For example, the proposed Rydberg action for the adsorption of N2 onto the ammonia catalyst involves the formation of a Rydberg complex K*-N-N or (K-N-N)* which is still bound to the surface.4 In this way, the N2 molecule may reach the surface in an excited and chemically (20) Wang, J.; Engvall, K.; Holmlid, L. J. Chem. Phys. 1999, 110, 1212.
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activated state and at a much higher rate than would be possible by molecular diffusion to active sites on the surface. Thus, the formation of K Rydberg states may have a profound influence on the reaction rate and should provide many new opportunities for improved design and functionality of heterogeneous catalysts. 4. Conclusions Transitions between high-l Rydberg states of K atoms can be observed by stimulated Raman scattering at the surface of a commercial K-promoted catalyst (styrene production). The Rydberg states cannot be formed by the weak laser used for the spectroscopy, and the fact that the only transitions observed are of the anti-Stokes type shows that the Rydberg states are formed thermally. They are observed at room temperature in air as well as at typical process temperatures (900 K) in a vacuum. The K* Rydberg states reside in the boundary layer at the catalyst surface. Acknowledgment. The author extends sincere thanks to P. Govind Menon from whom the catalyst samples were obtained. LA000951Q
