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Angular distributions of C2 and CN formed by laser ablation of graphite in a nitrogen atmosphere
Applied Surface Science 178 (2001) 37±43
Angular distributions of C2 and CN formed by laser ablation of graphite in a nitrogen atmosphere Seung Min Parka,*, Kee Hag Leeb a b
Department of Chemistry, Kyunghee University, Seoul 130-701, South Korea Department of Chemistry, WonKwang University, Iksan 570-749, South Korea Received 8 December 2000; accepted 15 March 2001
Abstract Optical time-of-¯ight measurements have been performed to obtain snapshots of spatial distributions of C2 and CN molecules produced by laser ablation of graphite in a nitrogen atmosphere. The propagation direction of C2 was oriented along the target surface normal regardless of the nitrogen pressure while that of CN was inclined towards the incident laser beam at low nitrogen pressures. This anomalous propagation of CN molecules is attributed to the high activation energy of the reaction C2 N2 ! 2CN, which is facilitated in a hot region generated by laser±plume interaction. # 2001 Elsevier Science B.V. All rights reserved. PACS: 79.20.D; 52.50.J Keywords: Laser ablation; Laser-induced plasma; Plume chemistry
1. Introduction When a high-intensity laser pulse is focused onto a solid target surface, a laser-induced plasma plume is generated and its bright optical emission is observed. In pulsed laser deposition (PLD) of thin ®lms, the target material in the plume is transported to a substrate resulting in the ®lm growth [1]. The axis of the plume is oriented along the target surface normal for the normal angle of laser incidence. In PLD, however, the angle of laser incidence is in general skewed to the target surface since the substrate is mounted facing the target [2]. For such oblique *
Corresponding author. Tel.: 82-2-961-0226; fax: 82-2-966-3701. E-mail address: [email protected] (S.M. Park).
angles of laser incidence, a few cases of plume tilting towards the direction of the incident laser beam have been reported [3±5]. Eryu et al. [3] observed that the spatial distribution of the luminous plume of YBa2Cu3O7 inclined towards the laser beam by means of space±time resolved optical measurements. The plume tilting was attributed to the ejection of slow clusters from the target surface followed by partial decomposition yielding light-emitting particles. Davanloo et al. [4] reported that the direction of the carbon plume produced by laser ablation of graphite target was symmetrical around the bisecting angle between the laser beam and the surface normal. They suspected that the plume tilt was caused by an angled crater on the target surface. Also, vaporization from ``pseudocraters'' consisting of mechanically deformed liquid depressions oriented parallel to the
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 2 4 6 - X
38
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
incident laser beam was expected to cause the skewed plume direction [5]. As mentioned above, only overall tilt of plume has been studied so far by analyzing the luminous plume or by collecting the ablated material on the substrate. In the plume, there exist many different atoms, molecules, ions, and energetic electrons [1]. Will there be any possibility that certain chemical species may show different propagation direction depending on its formation mechanism? Here, we present an experimental result of skewed propagation of a particular chemical species in a reactive laser ablation of graphite. We have examined the propagation of CN and C2 molecules formed by laser ablation of graphite target in a nitrogen atmosphere. Spatially resolved optical timeof-¯ight spectra of the violet band (B 2S ! X 2S) of CN and the swan band (d 3Pg ! a 3Pu) of C2 were recorded to obtain snapshots of molecular emission at different nitrogen pressures. Surprisingly, only CN molecules exhibited a tilted propagation direction. 2. Experimental The experimental setup is illustrated in Fig. 1. Graphite targets (Niraco, 99.99%) with size of
20 mm in diameter were used as purchased without further treatment. The fourth harmonic of Nd:YAG laser (266 nm, 7 ns duration, Quanta±Ray GCR 150-10) was used to ablate the graphite target which was rotated by a standard rotary motion feedthrough. The laser beam was focused onto the graphite target by a S1UV lens with focal length of 30 cm at an incident angle of 528 with respect to the target surface normal. The laser spot size on the target surface was 0.95 mm2. Nitrogen gas (99.999%) was fed to the chamber by a needle valve and the pressure was measured by a full range gauge (Balzers PKR250). The luminescent plasma plume was imaged onto a ®ber optic bundle with a nominal aperture size of 0.55 mm (Spex 700FB) by a lens of 15 cm focal length (f 3:7) and sent to a monochromator (Jobin±Yvon, TMR1000) coupled to a photomultiplier tube (R928). The monochromator was calibrated by using a Hg lamp (Oriel 6335). Spatially resolved measurements of the emission spectra were performed by translating the optical ®ber/lens assembly with increments of 0.5 mm within the image of plasma. The signal from the photomultiplier tube was ampli®ed and fed to a storage oscilloscope (LeCroy 9300) to get optical time-of-¯ight spectra of C2 and CN. A boxcar averager (SR250) was used for signal processing to obtain emission spectra. 3. Results and discussion
Fig. 1. The schematic diagram of the experiment. A 266 nm laser beam is in the x±y plane. The optical ®ber/lens assembly is mounted along the z direction in a 3-dimensional translational stage and scans over the plume in the x±y plane. The spot on the graphite target irradiated by the laser beam is de®ned as (x,y 0,0).
In the spectral range of 340±520 nm, emission bands from C2 (d 3Pg ! a 3Pu, Dv 2, 1, 0) and CN (B 2S ! X 2S, Dv 1, 0, 1) were observed as shown in Fig. 2. Emissions from N, N, C, C, and N2 were not observed in our experimental conditions, presumably hidden in the vibrational progressions of C2 and CN, if any [6]. Three vibrational sequences with band heads at 516.6 nm (Dv 0), 473.7 nm (Dv 1), and 438.2 nm (Dv 2) correspond to the swan bands of C2. The vibrational sequences with band heads at 421.6 nm (Dv 1), 388.3 nm (Dv 0), and 359.0 nm (Dv 1) correspond to the violet bands of CN. To obtain optical time-of-¯ight spectra of C2 and CN molecules, the monochromator was set at 516.3 nm and 388.1 nm, respectively. Typical optical time-of-¯ight spectra of C2 and CN are shown in
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
Fig. 2. Optical emission spectrum produced by laser ablation of graphite in a nitrogen atmosphere measured at a position (x,y 0,4 mm). The laser ¯uence was 1.9 J/cm2 and the nitrogen pressure was 3 Torr.
Fig. 3. The electronic excitation probability in the plume is determined by the density of energetic collisions and thereby the emission intensity of C2 or CN at a given time and position in the plume corresponds to the density of the electronically excited molecules. Although the natural life times of the d 3Pg state of C2 and the B 2S state CN are as short as 102 ns [7] and 64 ns [8], respectively, the optical emission from the excited states persists even 1 ms after arrival of the laser pulse due to collisional excitation of molecules in the plume. By plotting the intensity of the space±time resolved optical emission at a given wavelength, we could get
Fig. 3. Optical time-of-¯ight spectra of C2 and CN measured at positions (x,y 0,2 mm) and (x,y 0,7 mm). The laser ¯uence was 1.9 J/cm2 and the nitrogen pressure was 3 Torr.
39
time-resolved contour graphs which represent the spatial distributions of C2 and CN molecules as shown in Fig. 4. In high vacuum (p 1:0 10 6 Torr), where the effect of collisions between the ejectants and ambient gas molecules in the plume is not signi®cant, C2 molecules are mostly produced by direct laser ablation [9]. The optical emission from C2 molecules in high vacuum reaches at its maximum intensity about 150 ns after irradiation of the graphite target and smears out after 500 ns as shown in Fig. 4(a). C2 molecules are found to propagate perpendicular to the target surface regardless of nitrogen pressure as presented in Fig. 4(a), (c), and (e), which implies that C2 molecules are mostly formed by either direct ablation and or recombination of C atoms. If the photofragmentation of larger carbon clusters had contributed to the enrichment of C2 molecules in the plume signi®cantly, C2 molecules would have been preferentially formed along the laser beam path. Since CN molecules are formed by a bimolecular reaction C2 N2 ! 2CN, no optical emission from CN was detected in high vacuum [10]. As the nitrogen pressure was increased to 100 mTorr, optical emission from CN molecules was observed as shown in Fig. 4(b). It is of note that the propagation direction of CN molecules is inclined towards the incident laser beam at 100 mTorr and 1.0 Torr (Fig. 4(d)) but it is along the target surface normal as the nitrogen pressure is increased to 10 Torr (Fig. 4(f)). Due to the inverse bremsstrahlung absorption of the later part of the 7 ns long laser pulse by the plume already formed through interaction of the early part of the laser pulse with the target surface, the temperature distribution in the plume is not symmetrical around the target surface normal and there may exist an extraordinarily hot region near the target surface along the laser beam. Since the recombination reaction of atoms has no activation barrier, the high temperature region is not favorable at all to the reaction [11]. For the bimolecular reaction C2 N2 ! 2CN which has activation energy as high as 1.8 eV [12], the reaction is expedited in the hot region and thus the reaction by C2 molecules with momentum towards the incident laser beam will be certainly facilitated. In our experiment, formation of angled craters [4] which could result in the inclined propagation of the ejectants is unlikely since such skewed expansion was not observed for C2 molecules. Also, the possibility of
40
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
Fig. 4. Time-resolved images of C2 and CN obtained at different nitrogen pressures. The laser ¯uence was 1.9 J/cm2. (a) C2, 1:0 10 (b) CN, 100 mTorr; (c) C2, 1.0 Torr; (d) CN, 1.0 Torr; (e) C2, 10 Torr; (f) CN, 10 Torr.
6
Torr;
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
Fig. 4. (Continued ).
41
42
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
Fig. 4. (Continued ).
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
photodissociation of slow clusters [3] as a cause of the non-normal expansion of the plume may be ruled out because the plume is inclined to the laser beam at the forefront of the plume. As the pressure increases above few Torr, collisions in the hot region near the target lead to averaging the momentum of the product [13] and the optical emission from CN molecules becomes symmetrical around the target surface normal as depicted in Fig. 4(f). 4. Conclusions We have observed a skewed propagation of CN molecules produced by a reactive laser ablation of graphite in a nitrogen atmosphere. This anomalous propagation of molecules in the plume is considered to originate from the high activation energy of the reaction C2 N2 ! 2CN, which is facilitated in the hot region generated by laser±plume interaction. On the other hand, C2 molecules are formed by recombination reaction of C atoms and their spatial distribution is symmetrical around the target surface normal. Since the angular pro®les of the molecules in the plume may be different according to the formation mechanisms, care must be taken in order to direct the proper chemical species produced by reactive laser ablation to a substrate in PLD, where the typical pressures are in the range from 100 mTorr to 1 Torr [2].
43
Acknowledgements This work was supported by grant no. 2000-112100-001-3 from the Basic Research Program of the Korea Science & Engineering Foundation. References [1] D.H. Lowndes, D.B. Geohegan, A.A. Puretzky, D.P. Norton, C.M. Rouleau, Science 273 (1996) 898. [2] D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [3] O. Eryu, K. Murakami, K. Masuda, A. Kasuya, Y. Nishina, Appl. Phys. Lett. 54 (1989) 2716. [4] F. Davanloo, E.M. Juengerman, D.R. Jander, T.J. Lee, C.B. Collins, E. Matthais, Appl. Phys. A 54 (1992) 369. [5] C. Fuchs, E. Fogarassy, Mater. Res. Soc. Symp. Proc. 169 (1990) 517. [6] E. D'Anna, A. Luches, A. Perrone, S. Acquaviva, R. Alexandrescu, I.N. Mihailescu, J. Zemek, G. Majni, Appl. Surf. Sci. 106 (1996) 126. [7] C. Naulin, M. Costes, G. Dorthe, Chem. Phys. Lett. 143 (1988) 496. [8] K. Kanda, N. Igari, Y. Kikuchi, N. Kishida, J. Igarashi, S. Katsumata, K. Suzuki, J. Phys. Chem. 99 (1995) 5269. [9] D.J. Krajnovich, J. Chem. Phys. 102 (1995) 726. [10] S. Wee, S.M. Park, Opt. Commun. 165 (1999) 199. [11] S.M. Park, H. Chae, S. Wee, I. Lee, J. Chem. Phys. 109 (1998) 928. [12] T. Sommer, T. Kruse, P. Roth, H. Hippler, J. Phys. Chem. A 101 (1997) 3720. [13] S.M. Park, M.I. Khan, G.J. Diebold, Opt. Lett. 15 (1990) 771.
Angular distributions of C2 and CN formed by laser ablation of graphite in a nitrogen atmosphere Seung Min Parka,*, Kee Hag Leeb a b
Department of Chemistry, Kyunghee University, Seoul 130-701, South Korea Department of Chemistry, WonKwang University, Iksan 570-749, South Korea Received 8 December 2000; accepted 15 March 2001
Abstract Optical time-of-¯ight measurements have been performed to obtain snapshots of spatial distributions of C2 and CN molecules produced by laser ablation of graphite in a nitrogen atmosphere. The propagation direction of C2 was oriented along the target surface normal regardless of the nitrogen pressure while that of CN was inclined towards the incident laser beam at low nitrogen pressures. This anomalous propagation of CN molecules is attributed to the high activation energy of the reaction C2 N2 ! 2CN, which is facilitated in a hot region generated by laser±plume interaction. # 2001 Elsevier Science B.V. All rights reserved. PACS: 79.20.D; 52.50.J Keywords: Laser ablation; Laser-induced plasma; Plume chemistry
1. Introduction When a high-intensity laser pulse is focused onto a solid target surface, a laser-induced plasma plume is generated and its bright optical emission is observed. In pulsed laser deposition (PLD) of thin ®lms, the target material in the plume is transported to a substrate resulting in the ®lm growth [1]. The axis of the plume is oriented along the target surface normal for the normal angle of laser incidence. In PLD, however, the angle of laser incidence is in general skewed to the target surface since the substrate is mounted facing the target [2]. For such oblique *
Corresponding author. Tel.: 82-2-961-0226; fax: 82-2-966-3701. E-mail address: [email protected] (S.M. Park).
angles of laser incidence, a few cases of plume tilting towards the direction of the incident laser beam have been reported [3±5]. Eryu et al. [3] observed that the spatial distribution of the luminous plume of YBa2Cu3O7 inclined towards the laser beam by means of space±time resolved optical measurements. The plume tilting was attributed to the ejection of slow clusters from the target surface followed by partial decomposition yielding light-emitting particles. Davanloo et al. [4] reported that the direction of the carbon plume produced by laser ablation of graphite target was symmetrical around the bisecting angle between the laser beam and the surface normal. They suspected that the plume tilt was caused by an angled crater on the target surface. Also, vaporization from ``pseudocraters'' consisting of mechanically deformed liquid depressions oriented parallel to the
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 2 4 6 - X
38
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
incident laser beam was expected to cause the skewed plume direction [5]. As mentioned above, only overall tilt of plume has been studied so far by analyzing the luminous plume or by collecting the ablated material on the substrate. In the plume, there exist many different atoms, molecules, ions, and energetic electrons [1]. Will there be any possibility that certain chemical species may show different propagation direction depending on its formation mechanism? Here, we present an experimental result of skewed propagation of a particular chemical species in a reactive laser ablation of graphite. We have examined the propagation of CN and C2 molecules formed by laser ablation of graphite target in a nitrogen atmosphere. Spatially resolved optical timeof-¯ight spectra of the violet band (B 2S ! X 2S) of CN and the swan band (d 3Pg ! a 3Pu) of C2 were recorded to obtain snapshots of molecular emission at different nitrogen pressures. Surprisingly, only CN molecules exhibited a tilted propagation direction. 2. Experimental The experimental setup is illustrated in Fig. 1. Graphite targets (Niraco, 99.99%) with size of
20 mm in diameter were used as purchased without further treatment. The fourth harmonic of Nd:YAG laser (266 nm, 7 ns duration, Quanta±Ray GCR 150-10) was used to ablate the graphite target which was rotated by a standard rotary motion feedthrough. The laser beam was focused onto the graphite target by a S1UV lens with focal length of 30 cm at an incident angle of 528 with respect to the target surface normal. The laser spot size on the target surface was 0.95 mm2. Nitrogen gas (99.999%) was fed to the chamber by a needle valve and the pressure was measured by a full range gauge (Balzers PKR250). The luminescent plasma plume was imaged onto a ®ber optic bundle with a nominal aperture size of 0.55 mm (Spex 700FB) by a lens of 15 cm focal length (f 3:7) and sent to a monochromator (Jobin±Yvon, TMR1000) coupled to a photomultiplier tube (R928). The monochromator was calibrated by using a Hg lamp (Oriel 6335). Spatially resolved measurements of the emission spectra were performed by translating the optical ®ber/lens assembly with increments of 0.5 mm within the image of plasma. The signal from the photomultiplier tube was ampli®ed and fed to a storage oscilloscope (LeCroy 9300) to get optical time-of-¯ight spectra of C2 and CN. A boxcar averager (SR250) was used for signal processing to obtain emission spectra. 3. Results and discussion
Fig. 1. The schematic diagram of the experiment. A 266 nm laser beam is in the x±y plane. The optical ®ber/lens assembly is mounted along the z direction in a 3-dimensional translational stage and scans over the plume in the x±y plane. The spot on the graphite target irradiated by the laser beam is de®ned as (x,y 0,0).
In the spectral range of 340±520 nm, emission bands from C2 (d 3Pg ! a 3Pu, Dv 2, 1, 0) and CN (B 2S ! X 2S, Dv 1, 0, 1) were observed as shown in Fig. 2. Emissions from N, N, C, C, and N2 were not observed in our experimental conditions, presumably hidden in the vibrational progressions of C2 and CN, if any [6]. Three vibrational sequences with band heads at 516.6 nm (Dv 0), 473.7 nm (Dv 1), and 438.2 nm (Dv 2) correspond to the swan bands of C2. The vibrational sequences with band heads at 421.6 nm (Dv 1), 388.3 nm (Dv 0), and 359.0 nm (Dv 1) correspond to the violet bands of CN. To obtain optical time-of-¯ight spectra of C2 and CN molecules, the monochromator was set at 516.3 nm and 388.1 nm, respectively. Typical optical time-of-¯ight spectra of C2 and CN are shown in
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
Fig. 2. Optical emission spectrum produced by laser ablation of graphite in a nitrogen atmosphere measured at a position (x,y 0,4 mm). The laser ¯uence was 1.9 J/cm2 and the nitrogen pressure was 3 Torr.
Fig. 3. The electronic excitation probability in the plume is determined by the density of energetic collisions and thereby the emission intensity of C2 or CN at a given time and position in the plume corresponds to the density of the electronically excited molecules. Although the natural life times of the d 3Pg state of C2 and the B 2S state CN are as short as 102 ns [7] and 64 ns [8], respectively, the optical emission from the excited states persists even 1 ms after arrival of the laser pulse due to collisional excitation of molecules in the plume. By plotting the intensity of the space±time resolved optical emission at a given wavelength, we could get
Fig. 3. Optical time-of-¯ight spectra of C2 and CN measured at positions (x,y 0,2 mm) and (x,y 0,7 mm). The laser ¯uence was 1.9 J/cm2 and the nitrogen pressure was 3 Torr.
39
time-resolved contour graphs which represent the spatial distributions of C2 and CN molecules as shown in Fig. 4. In high vacuum (p 1:0 10 6 Torr), where the effect of collisions between the ejectants and ambient gas molecules in the plume is not signi®cant, C2 molecules are mostly produced by direct laser ablation [9]. The optical emission from C2 molecules in high vacuum reaches at its maximum intensity about 150 ns after irradiation of the graphite target and smears out after 500 ns as shown in Fig. 4(a). C2 molecules are found to propagate perpendicular to the target surface regardless of nitrogen pressure as presented in Fig. 4(a), (c), and (e), which implies that C2 molecules are mostly formed by either direct ablation and or recombination of C atoms. If the photofragmentation of larger carbon clusters had contributed to the enrichment of C2 molecules in the plume signi®cantly, C2 molecules would have been preferentially formed along the laser beam path. Since CN molecules are formed by a bimolecular reaction C2 N2 ! 2CN, no optical emission from CN was detected in high vacuum [10]. As the nitrogen pressure was increased to 100 mTorr, optical emission from CN molecules was observed as shown in Fig. 4(b). It is of note that the propagation direction of CN molecules is inclined towards the incident laser beam at 100 mTorr and 1.0 Torr (Fig. 4(d)) but it is along the target surface normal as the nitrogen pressure is increased to 10 Torr (Fig. 4(f)). Due to the inverse bremsstrahlung absorption of the later part of the 7 ns long laser pulse by the plume already formed through interaction of the early part of the laser pulse with the target surface, the temperature distribution in the plume is not symmetrical around the target surface normal and there may exist an extraordinarily hot region near the target surface along the laser beam. Since the recombination reaction of atoms has no activation barrier, the high temperature region is not favorable at all to the reaction [11]. For the bimolecular reaction C2 N2 ! 2CN which has activation energy as high as 1.8 eV [12], the reaction is expedited in the hot region and thus the reaction by C2 molecules with momentum towards the incident laser beam will be certainly facilitated. In our experiment, formation of angled craters [4] which could result in the inclined propagation of the ejectants is unlikely since such skewed expansion was not observed for C2 molecules. Also, the possibility of
40
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
Fig. 4. Time-resolved images of C2 and CN obtained at different nitrogen pressures. The laser ¯uence was 1.9 J/cm2. (a) C2, 1:0 10 (b) CN, 100 mTorr; (c) C2, 1.0 Torr; (d) CN, 1.0 Torr; (e) C2, 10 Torr; (f) CN, 10 Torr.
6
Torr;
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
Fig. 4. (Continued ).
41
42
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
Fig. 4. (Continued ).
S.M. Park, K.H. Lee / Applied Surface Science 178 (2001) 37±43
photodissociation of slow clusters [3] as a cause of the non-normal expansion of the plume may be ruled out because the plume is inclined to the laser beam at the forefront of the plume. As the pressure increases above few Torr, collisions in the hot region near the target lead to averaging the momentum of the product [13] and the optical emission from CN molecules becomes symmetrical around the target surface normal as depicted in Fig. 4(f). 4. Conclusions We have observed a skewed propagation of CN molecules produced by a reactive laser ablation of graphite in a nitrogen atmosphere. This anomalous propagation of molecules in the plume is considered to originate from the high activation energy of the reaction C2 N2 ! 2CN, which is facilitated in the hot region generated by laser±plume interaction. On the other hand, C2 molecules are formed by recombination reaction of C atoms and their spatial distribution is symmetrical around the target surface normal. Since the angular pro®les of the molecules in the plume may be different according to the formation mechanisms, care must be taken in order to direct the proper chemical species produced by reactive laser ablation to a substrate in PLD, where the typical pressures are in the range from 100 mTorr to 1 Torr [2].
43
Acknowledgements This work was supported by grant no. 2000-112100-001-3 from the Basic Research Program of the Korea Science & Engineering Foundation. References [1] D.H. Lowndes, D.B. Geohegan, A.A. Puretzky, D.P. Norton, C.M. Rouleau, Science 273 (1996) 898. [2] D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [3] O. Eryu, K. Murakami, K. Masuda, A. Kasuya, Y. Nishina, Appl. Phys. Lett. 54 (1989) 2716. [4] F. Davanloo, E.M. Juengerman, D.R. Jander, T.J. Lee, C.B. Collins, E. Matthais, Appl. Phys. A 54 (1992) 369. [5] C. Fuchs, E. Fogarassy, Mater. Res. Soc. Symp. Proc. 169 (1990) 517. [6] E. D'Anna, A. Luches, A. Perrone, S. Acquaviva, R. Alexandrescu, I.N. Mihailescu, J. Zemek, G. Majni, Appl. Surf. Sci. 106 (1996) 126. [7] C. Naulin, M. Costes, G. Dorthe, Chem. Phys. Lett. 143 (1988) 496. [8] K. Kanda, N. Igari, Y. Kikuchi, N. Kishida, J. Igarashi, S. Katsumata, K. Suzuki, J. Phys. Chem. 99 (1995) 5269. [9] D.J. Krajnovich, J. Chem. Phys. 102 (1995) 726. [10] S. Wee, S.M. Park, Opt. Commun. 165 (1999) 199. [11] S.M. Park, H. Chae, S. Wee, I. Lee, J. Chem. Phys. 109 (1998) 928. [12] T. Sommer, T. Kruse, P. Roth, H. Hippler, J. Phys. Chem. A 101 (1997) 3720. [13] S.M. Park, M.I. Khan, G.J. Diebold, Opt. Lett. 15 (1990) 771.