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Formation of large water clusters by IR laser resonant desorption of ice
Applied Surface Science 248 (2005) 238–242 www.elsevier.com/locate/apsusc
Formation of large water clusters by IR laser resonant desorption of ice C. Mihesan, M. Ziskind *, B. Chazallon, C. Focsa, J.L. Destombes Laboratoire de Physique des Lasers, Atomes et Mole´cules (UMR 8523), Centre d’Etudes et de Recherches Lasers et Applications, Universite´ des Sciences et Technologies de Lille, F59655 Villeneuve d’Ascq Cedex, France Available online 23 March 2005
Abstract Large H3O+(H2O)n clusters, up to n 100, were produced by IR laser resonant desorption of an ice matrix using an optical parametric oscillator (OPO) at 3.1 mm. The velocity distribution has been analyzed to characterize the laser–sample interaction and the plume dynamics desorption. The velocity distribution curves of the clusters show two distinct components corresponding to a phase explosion regime followed by vaporization. We discuss the shift of these curves with mass and the formation mechanism. A distinction in the desorption process is made between small and large clusters. # 2005 Elsevier B.V. All rights reserved. PACS: 36.40.Qv; 36.40.Mr; 68.43.Tj Keywords: Water; Clusters; Desorption induced by photon stimulation
1. Introduction In the last decade, the production of hydrated clusters has become a subject of interest for various fields of chemistry, biology and medicine, i.e. water in tissues, production of biomolecules, etc. Laser ablation techniques like matrix assisted laser deso* Corresponding author. Present address: Universite´ de Lille 1, UFR de Physique – Laboratoire PhLAM, Baˆt. P5, bur. 037, F59655 Villeneuve d’Ascq Cedex, France. Tel.: +33 3 20336330; fax: +33 3 20336463. E-mail address: [email protected] (M. Ziskind).
rption and ionization (MALDI) and laser resonant desorption (LRD) are suitable tools for such an undertaking, avoiding the use of heavy pulsed crossing beam apparatus involved in many experiments in the past [1,2]. The ejection of hydrated clusters during the interaction between laser radiation and water-based samples is an indication of the complexity of the desorption mechanism. The characterization of the ejected products is needed to understand these processes and optimally exploit the capabilities of the laser ablation techniques. In a previous article [3], we have demonstrated the efficiency of a pulsed mid-infrared LiNbO3 optical
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.026
C. Mihesan et al. / Applied Surface Science 248 (2005) 238–242
parametric oscillator (OPO) for the IR desorption of ice in the 3 mm range. The resonant character of the process has been established and a phase explosion model has been proposed for the laser–ice interaction. This study was also extended to doped-ice samples with Na/K salt [4], formaldehyde [5], methanol, ethanol and tryptophan [6]. All these studies showed a common trend. Above the desorption threshold (0.5 mJ/pulse), massive ejection of H3O+(H2O)n clusters occurs. In this paper, we give a better insight on the desorption process and a possible formation mechanism using a velocity analysis of the desorption products.
2. Experimental setup The experimental setup has been described in detail elsewhere [3,4]. Briefly, the desorption of the ice sample is performed by an OPO operating at 10 Hz repetition rate with a laser pulse width of 10 ns. The laser wavelength is set to 3.1 mm, in coincidence with the optical absorption peak of the ice matrix. The laser beam is focused on the sample at normal incidence with a typical energy of 3 mJ/pulse corresponding to a power density of 450 MW/cm2. Multi-photon ionization of the desorbed neutral particles present in the plume is achieved by the UV pulse of a frequency quadrupled Nd:YAG laser (l = 266 nm, 60 mJ/pulse, 80 GW/cm2). Both lasers are synchronized by a digital four channel delay/pulse generator. The produced ions
239
are mass analyzed using a 1-m long Wiley–McLaren time-of-flight mass spectrometer equipped with a reflectron (residual pressure: 109 Torr). Ice samples are generated by freezing 3 ml of pure water on a copper sample holder in liquid nitrogen. The holder is introduced in a vacuum chamber on a finger pre-cooled at T 100 K by a liquid nitrogen flow. The gate valve which separates the chamber from the mass spectrometer is opened once the pressure in the chamber is low enough, i.e. of the order of 107 Torr. The sample is then translated into the desorption zone of the mass spectrometer. This technique allows us to produce samples a few millimeters thick, while those obtained by deposition of pure water vapor [3] are only a few micrometers thick. Therefore, one can greatly increase the number of laser impacts on the sample and then increase the signal-to-noise ratio of the recorded spectra.
3. Results and discussion 3.1. Cluster production and size distribution Fig. 1 shows a typical mass spectrum evidencing the presence of large H3O+(H2O)n clusters (up to n 100). In comparison, water clusters with n 20 have been reported by the IR-FEL ablation (l = 5.9 mm) of frozen aqueous solutions of a protein by Baltz-Knorr et al. [7] and by the 266 nm laser desorption of frozen CeCl3/D2O solutions which
Fig. 1. A typical mass spectrum obtained by laser resonant desorption of a frozen water sample at 90 K.
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produces D3O+(D2O)n with n = 0–5 [8]. Such a discrepancy could be partly due to the limitations of the ionization techniques employed or to the nonresonant character of the desorption process. We have also observed that the formation of the clusters is greatly facilitated if cracks are present in the sample. Thus, no cluster with n higher than 30 has ever been detected during the desorption of samples obtained by deposition and supposed to be very homogeneous (see Fig. 4 in ref. [3]) while one can easily observe large clusters during the desorption of frozen water sample even after few impacts. This is consistent with the work on mixed water–organic solute clusters developed by Kosevich and co-workers [9–11]. Model for low-temperature fast-atom bombardment (LT-FAB) mass spectra shows that the formation of mixed clusters is a consequence of the structural inhomogeneity of the samples. Our observation could translate this assumption to the case of pure samples. The study of the size distribution of the clusters offers valuable insight on the stability and structural properties of these complexes. Thus, one can see in Fig. 1 that the abundance of clusters is characterized by successive local minima and maxima. The maxima indicate a higher stability for the corresponding cluster. The size distribution could be fitted by the sum of five lognormal curves characterizing the agglomeration process [4,12]. According to Wang et al. [13], each curve corresponds to a distinct structural configuration and the transition to a new lognormal curve coincides with a new geometry of the molecular aggregate. 3.2. Velocity distribution and formation mechanism The velocity distribution of the products present in the plume has been recorded by varying the time delay, Dt, between the desorption and ionization pulses, to characterize the laser–sample interaction and the desorption plume dynamics. Fig. 2 shows one distribution for three small water clusters, H3O+(H2O)n with n = 1, 3 and 6, and two larger ones, H3O+(H2O)35 and H3O+(H2O)40. According to previous works [5,14,15], the velocity distribution curves of the first clusters demonstrate the presence of two main distinct components. Around Dt = 25 ms, the first component, corresponding to fast particles has been shown previously to result from a phase
Fig. 2. Velocity distributions of three small water clusters (at the top) and two large clusters (at the bottom). The solid curve was obtained from Eq. (1) with M = 1.9 and uz = 1100 m/s.
explosion regime, i.e. the sudden release of the laser– sample interaction volume into the gas phase. It is due to a very fast heating of the sample surface up to the water thermodynamic critical temperature, Tc, where an important homogeneous nucleation rate occurs [16,17]. The fast component can be described by a shifted Maxwell-like distribution: " 2 # 3 vz g 2 vz f ðvz Þ ¼ exp M 1 (1) 2 uz uz with translational velocity vz ¼ Dz=Dt, where Dz = 35 mm, is the distance between the desorption and ionization spots, superimposed on the stream velocity uz, M is the Mach number and g the adiabatic coefficient Cp/Cv (g = 1.33). The solid line in Fig. 2 is the theoretical velocity distribution curve corresponding to the first cluster obtained using this model with uz = 1100 m/s and M = 1.9. These values indicate a moderate supersonic
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close to Tc. The corresponding peak can be fitted by the classical Maxwell–Boltzmann equation [20]: h m i f ðvz Þ ¼ v3z exp ¯ v2z (2) 2kT
Fig. 3. Velocity distributions of H2O, H3O+(H2O)2 and H3O+(H2O)4. The clusters have been artificially amplified by a factor 30 and 50, respectively, and translated to allow a direct comparison between the curves. The energy of the ionization laser was reduced to a few mJ/pulse to avoid any saturation of the water signal.
expansion similar to our previous studies on frozen aqueous solutions of organic [5,6] and non-organic [4] species. Moreover, the observed cluster velocities (and H2O; see Fig. 3) mass independence suggests the thermalization of desorbed species following a formalism developed by Kelly and Dreyfus [18,19]. According to this model, water and its clusters acquire a common center of mass velocity due to collisions via the formation of a Knudsen layer and same values for M and uz during the adiabatic expansion. Finally, the surface temperature can be deduced from calculations (see Ref. [4] for details). The value obtained is 590 K, close to the water critical temperature Tc 647 K and consistent with a phase explosion regime. The efficiency of the mechanism decreases with the mass of the cluster and is possibly coupled to a fragmentation process becoming negligible for n = 25–30 (see below). At larger delays, the second component corresponds to the normal vaporization regime, which results from complex non-radiative effects and thermal diffusion into the ice sample [20]. In contrast to the phase explosion, the normal vaporization regime occurs for any clusters and over a broad range of temperatures, i.e. there is no threshold temperature
where T designates the average temperature at which the process occurs, m is the mass of the cluster and k the Boltzmann constant. Fig. 3 displays both monomer and cluster velocity distributions. One can see an important broad tail of the second component of the water compared with the clusters. Such an observation has already been reported for C60 and C70 [21] and has been mainly attributed to a different vapor pressure for the two species. This argument cannot be applied to the present experiment. In particular, the exact coincidence of the cluster curves implies a similar behaviour for all clusters during vaporization, despite a different vapor pressure [22].
Fig. 4. Evolution of the first clusters peaks with Dz. Dt has been changed to obtain the same velocity class (corresponding to the slow component). This required the modification of the distance between the desorption laser and the target. This also alters the power density at the surface sample and does not allow absolute comparison. Small peaks between the clusters are the consequence of the fragmentation of the ions in the time-of-flight mass spectrometer.
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As suggested in a previous article [3], the slow tail of the water velocity distribution could be rather a consequence of by-product formation. Slower clusters (Dt > 70 ms) spend a long time in the desorption plume and have a potentially higher probability to fragment into H2O accounting for the broadening of its slow component. This hypothesis is corroborated by a set of velocity distributions in which Dz has been changed by a few millimeters. In Fig. 4, the contribution of the clusters decreases with Dz, indicating a fragmentation process. Finally, this assumption is supported by recent molecular dynamics simulations based on the ‘‘breathing sphere’’ model [23,24]. These simulations also emphasize that individual molecules and small clusters predominantly originate closer to the ice surface, and then have a larger stream velocity, while heavier clusters originate deeper in the solid and then have a smaller velocity [24]. Fig. 2 suggests such a segregation between the small clusters which tend to come from a phase explosion and vaporization regime and the larger ones which result from vaporization only.
4. Conclusion The present results show the efficiency of the IR laser resonant desorption of an ice matrix to generate large H3O+(H2O)n clusters. By recording the velocity distribution of the desorbed products, i.e. monomer, small and large clusters, it is possible to give a good insight on the laser–ice sample interaction. In particular, we point out a clear distinction between small and larger clusters, which tend to originate from different formation regimes, i.e. phase explosion and normal vaporization. This is consistent with the observed fragmentation process of the slower clusters and supported by simulations reported in [24].
Acknowledgements The Centre d’Etudes et de Recherches Lasers et Applications is supported by the Ministe`re charge´ de la Recherche, the Re´ gion Nord-Pas de Calais and the
Fonds Europe´ en de De´ veloppement Economique des Re´ gions. This research is partially supported by the Groupement de Recherche ‘‘Reactivite´ a` la Surface de la Glace’’.
References [1] I.V. Hertel, C. Hu¨ glin, C. Nitsch, C.P. Schults, Phys. Rev. Lett. 67 (1991) 1767. [2] U. Buck, C.S. Steinbach, J. Phys. Chem. A 102 (1998) 1124. [3] C. Focsa, B. Chazallon, J.L. Destombes, Surf. Sci. 528 (2003) 189. [4] C. Focsa, J.L. Destombes, Chem. Phys. Lett. 347 (2001) 390. [5] C. Mihesan, N. Lebrun, M. Ziskind, B. Chazallon, C. Focsa, J.L. Destombes, Surf. Sci. 566 (2004) 650. [6] M. Ziskind, C. Mihesan, N. Lebrun, B. Chazallon, C. Focsa, J.L. Destombes, Appl. Phys. A 79 (2004) 991. [7] M.L. Baltz-Knorr, K.E. Scriver, R.F. Haglund, Appl. Surf. Sci. 197–198 (2002) 11. [8] C.C. Han, Y.-L. Han, Y.C. Chen, Int. J. Mass Spectrom. 189 (1999) 157. [9] O.A. Boryak, I.O. Stepanov, M.V. Kosevich, V.S. Shelkovsky, V.V. Orlov, Yu.P. Blagoy, Eur. Mass Spectrom. 2 (1996) 329. [10] O.A. Boryak, M.V. Kosevich, V.S. Shelkovsky, Yu.P. Blagoy, Rapid Commun. Mass Spectrom. 10 (1996) 197. [11] M.V. Kosevich, G. Czira, O.A. Boryak, V.S. Shelkovsky, K. Ve´ key, J. Mass Spectrom. 33 (1998) 843. [12] I. Pocsik, Z. Phys. D 20 (1991) 395. [13] C.R. Wang, R.B. Huang, Z.Y. Liu, L.S. Zheng, Chem. Phys. Lett. 227 (1994) 103. [14] W.C. Natzle, D. Padowitz, S.J. Sibener, J. Chem. Phys. 88 (1988) 7975. [15] L.M. Cousins, R.J. Levis, S.R. Leone, J. Chem. Phys. 91 (1989) 5731. [16] J. Sunner, M.G. Ikonomou, P. Kebarle, Int. J. Mass Spectrom. Ion Proc. 82 (1988) 221. [17] A. Miotello, R. Kelly, Appl. Phys. Lett. 67 (1995) 3535. [18] R. Kelly, Phys. Rev. A 46 (1992) 860. [19] R. Kelly, R.W. Dreyfus, Nucl. Instrum. Methods B 32 (1988) 341. [20] R. Kelly, A. Miotello, Nucl. Instrum. Methods B 122 (1997) 374. [21] P. Wurz, K.R. Lykke, M.J. Pellin, D.M. Gruen, J. Appl. Phys. 70 (1991) 6647. [22] B.-J. Mhin, S.-J. Lee, K.S. Kim, Phys. Rev. A 48 (1993) 3764. [23] L.V. Zhigilei, P.B.S. Kodali, B.J. Garrison, J. Phys. Chem. B 102 (1998) 2845. [24] L.V. Zhigilei, Appl. Phys. A 76 (2003) 339.
Formation of large water clusters by IR laser resonant desorption of ice C. Mihesan, M. Ziskind *, B. Chazallon, C. Focsa, J.L. Destombes Laboratoire de Physique des Lasers, Atomes et Mole´cules (UMR 8523), Centre d’Etudes et de Recherches Lasers et Applications, Universite´ des Sciences et Technologies de Lille, F59655 Villeneuve d’Ascq Cedex, France Available online 23 March 2005
Abstract Large H3O+(H2O)n clusters, up to n 100, were produced by IR laser resonant desorption of an ice matrix using an optical parametric oscillator (OPO) at 3.1 mm. The velocity distribution has been analyzed to characterize the laser–sample interaction and the plume dynamics desorption. The velocity distribution curves of the clusters show two distinct components corresponding to a phase explosion regime followed by vaporization. We discuss the shift of these curves with mass and the formation mechanism. A distinction in the desorption process is made between small and large clusters. # 2005 Elsevier B.V. All rights reserved. PACS: 36.40.Qv; 36.40.Mr; 68.43.Tj Keywords: Water; Clusters; Desorption induced by photon stimulation
1. Introduction In the last decade, the production of hydrated clusters has become a subject of interest for various fields of chemistry, biology and medicine, i.e. water in tissues, production of biomolecules, etc. Laser ablation techniques like matrix assisted laser deso* Corresponding author. Present address: Universite´ de Lille 1, UFR de Physique – Laboratoire PhLAM, Baˆt. P5, bur. 037, F59655 Villeneuve d’Ascq Cedex, France. Tel.: +33 3 20336330; fax: +33 3 20336463. E-mail address: [email protected] (M. Ziskind).
rption and ionization (MALDI) and laser resonant desorption (LRD) are suitable tools for such an undertaking, avoiding the use of heavy pulsed crossing beam apparatus involved in many experiments in the past [1,2]. The ejection of hydrated clusters during the interaction between laser radiation and water-based samples is an indication of the complexity of the desorption mechanism. The characterization of the ejected products is needed to understand these processes and optimally exploit the capabilities of the laser ablation techniques. In a previous article [3], we have demonstrated the efficiency of a pulsed mid-infrared LiNbO3 optical
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.026
C. Mihesan et al. / Applied Surface Science 248 (2005) 238–242
parametric oscillator (OPO) for the IR desorption of ice in the 3 mm range. The resonant character of the process has been established and a phase explosion model has been proposed for the laser–ice interaction. This study was also extended to doped-ice samples with Na/K salt [4], formaldehyde [5], methanol, ethanol and tryptophan [6]. All these studies showed a common trend. Above the desorption threshold (0.5 mJ/pulse), massive ejection of H3O+(H2O)n clusters occurs. In this paper, we give a better insight on the desorption process and a possible formation mechanism using a velocity analysis of the desorption products.
2. Experimental setup The experimental setup has been described in detail elsewhere [3,4]. Briefly, the desorption of the ice sample is performed by an OPO operating at 10 Hz repetition rate with a laser pulse width of 10 ns. The laser wavelength is set to 3.1 mm, in coincidence with the optical absorption peak of the ice matrix. The laser beam is focused on the sample at normal incidence with a typical energy of 3 mJ/pulse corresponding to a power density of 450 MW/cm2. Multi-photon ionization of the desorbed neutral particles present in the plume is achieved by the UV pulse of a frequency quadrupled Nd:YAG laser (l = 266 nm, 60 mJ/pulse, 80 GW/cm2). Both lasers are synchronized by a digital four channel delay/pulse generator. The produced ions
239
are mass analyzed using a 1-m long Wiley–McLaren time-of-flight mass spectrometer equipped with a reflectron (residual pressure: 109 Torr). Ice samples are generated by freezing 3 ml of pure water on a copper sample holder in liquid nitrogen. The holder is introduced in a vacuum chamber on a finger pre-cooled at T 100 K by a liquid nitrogen flow. The gate valve which separates the chamber from the mass spectrometer is opened once the pressure in the chamber is low enough, i.e. of the order of 107 Torr. The sample is then translated into the desorption zone of the mass spectrometer. This technique allows us to produce samples a few millimeters thick, while those obtained by deposition of pure water vapor [3] are only a few micrometers thick. Therefore, one can greatly increase the number of laser impacts on the sample and then increase the signal-to-noise ratio of the recorded spectra.
3. Results and discussion 3.1. Cluster production and size distribution Fig. 1 shows a typical mass spectrum evidencing the presence of large H3O+(H2O)n clusters (up to n 100). In comparison, water clusters with n 20 have been reported by the IR-FEL ablation (l = 5.9 mm) of frozen aqueous solutions of a protein by Baltz-Knorr et al. [7] and by the 266 nm laser desorption of frozen CeCl3/D2O solutions which
Fig. 1. A typical mass spectrum obtained by laser resonant desorption of a frozen water sample at 90 K.
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produces D3O+(D2O)n with n = 0–5 [8]. Such a discrepancy could be partly due to the limitations of the ionization techniques employed or to the nonresonant character of the desorption process. We have also observed that the formation of the clusters is greatly facilitated if cracks are present in the sample. Thus, no cluster with n higher than 30 has ever been detected during the desorption of samples obtained by deposition and supposed to be very homogeneous (see Fig. 4 in ref. [3]) while one can easily observe large clusters during the desorption of frozen water sample even after few impacts. This is consistent with the work on mixed water–organic solute clusters developed by Kosevich and co-workers [9–11]. Model for low-temperature fast-atom bombardment (LT-FAB) mass spectra shows that the formation of mixed clusters is a consequence of the structural inhomogeneity of the samples. Our observation could translate this assumption to the case of pure samples. The study of the size distribution of the clusters offers valuable insight on the stability and structural properties of these complexes. Thus, one can see in Fig. 1 that the abundance of clusters is characterized by successive local minima and maxima. The maxima indicate a higher stability for the corresponding cluster. The size distribution could be fitted by the sum of five lognormal curves characterizing the agglomeration process [4,12]. According to Wang et al. [13], each curve corresponds to a distinct structural configuration and the transition to a new lognormal curve coincides with a new geometry of the molecular aggregate. 3.2. Velocity distribution and formation mechanism The velocity distribution of the products present in the plume has been recorded by varying the time delay, Dt, between the desorption and ionization pulses, to characterize the laser–sample interaction and the desorption plume dynamics. Fig. 2 shows one distribution for three small water clusters, H3O+(H2O)n with n = 1, 3 and 6, and two larger ones, H3O+(H2O)35 and H3O+(H2O)40. According to previous works [5,14,15], the velocity distribution curves of the first clusters demonstrate the presence of two main distinct components. Around Dt = 25 ms, the first component, corresponding to fast particles has been shown previously to result from a phase
Fig. 2. Velocity distributions of three small water clusters (at the top) and two large clusters (at the bottom). The solid curve was obtained from Eq. (1) with M = 1.9 and uz = 1100 m/s.
explosion regime, i.e. the sudden release of the laser– sample interaction volume into the gas phase. It is due to a very fast heating of the sample surface up to the water thermodynamic critical temperature, Tc, where an important homogeneous nucleation rate occurs [16,17]. The fast component can be described by a shifted Maxwell-like distribution: " 2 # 3 vz g 2 vz f ðvz Þ ¼ exp M 1 (1) 2 uz uz with translational velocity vz ¼ Dz=Dt, where Dz = 35 mm, is the distance between the desorption and ionization spots, superimposed on the stream velocity uz, M is the Mach number and g the adiabatic coefficient Cp/Cv (g = 1.33). The solid line in Fig. 2 is the theoretical velocity distribution curve corresponding to the first cluster obtained using this model with uz = 1100 m/s and M = 1.9. These values indicate a moderate supersonic
C. Mihesan et al. / Applied Surface Science 248 (2005) 238–242
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close to Tc. The corresponding peak can be fitted by the classical Maxwell–Boltzmann equation [20]: h m i f ðvz Þ ¼ v3z exp ¯ v2z (2) 2kT
Fig. 3. Velocity distributions of H2O, H3O+(H2O)2 and H3O+(H2O)4. The clusters have been artificially amplified by a factor 30 and 50, respectively, and translated to allow a direct comparison between the curves. The energy of the ionization laser was reduced to a few mJ/pulse to avoid any saturation of the water signal.
expansion similar to our previous studies on frozen aqueous solutions of organic [5,6] and non-organic [4] species. Moreover, the observed cluster velocities (and H2O; see Fig. 3) mass independence suggests the thermalization of desorbed species following a formalism developed by Kelly and Dreyfus [18,19]. According to this model, water and its clusters acquire a common center of mass velocity due to collisions via the formation of a Knudsen layer and same values for M and uz during the adiabatic expansion. Finally, the surface temperature can be deduced from calculations (see Ref. [4] for details). The value obtained is 590 K, close to the water critical temperature Tc 647 K and consistent with a phase explosion regime. The efficiency of the mechanism decreases with the mass of the cluster and is possibly coupled to a fragmentation process becoming negligible for n = 25–30 (see below). At larger delays, the second component corresponds to the normal vaporization regime, which results from complex non-radiative effects and thermal diffusion into the ice sample [20]. In contrast to the phase explosion, the normal vaporization regime occurs for any clusters and over a broad range of temperatures, i.e. there is no threshold temperature
where T designates the average temperature at which the process occurs, m is the mass of the cluster and k the Boltzmann constant. Fig. 3 displays both monomer and cluster velocity distributions. One can see an important broad tail of the second component of the water compared with the clusters. Such an observation has already been reported for C60 and C70 [21] and has been mainly attributed to a different vapor pressure for the two species. This argument cannot be applied to the present experiment. In particular, the exact coincidence of the cluster curves implies a similar behaviour for all clusters during vaporization, despite a different vapor pressure [22].
Fig. 4. Evolution of the first clusters peaks with Dz. Dt has been changed to obtain the same velocity class (corresponding to the slow component). This required the modification of the distance between the desorption laser and the target. This also alters the power density at the surface sample and does not allow absolute comparison. Small peaks between the clusters are the consequence of the fragmentation of the ions in the time-of-flight mass spectrometer.
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As suggested in a previous article [3], the slow tail of the water velocity distribution could be rather a consequence of by-product formation. Slower clusters (Dt > 70 ms) spend a long time in the desorption plume and have a potentially higher probability to fragment into H2O accounting for the broadening of its slow component. This hypothesis is corroborated by a set of velocity distributions in which Dz has been changed by a few millimeters. In Fig. 4, the contribution of the clusters decreases with Dz, indicating a fragmentation process. Finally, this assumption is supported by recent molecular dynamics simulations based on the ‘‘breathing sphere’’ model [23,24]. These simulations also emphasize that individual molecules and small clusters predominantly originate closer to the ice surface, and then have a larger stream velocity, while heavier clusters originate deeper in the solid and then have a smaller velocity [24]. Fig. 2 suggests such a segregation between the small clusters which tend to come from a phase explosion and vaporization regime and the larger ones which result from vaporization only.
4. Conclusion The present results show the efficiency of the IR laser resonant desorption of an ice matrix to generate large H3O+(H2O)n clusters. By recording the velocity distribution of the desorbed products, i.e. monomer, small and large clusters, it is possible to give a good insight on the laser–ice sample interaction. In particular, we point out a clear distinction between small and larger clusters, which tend to originate from different formation regimes, i.e. phase explosion and normal vaporization. This is consistent with the observed fragmentation process of the slower clusters and supported by simulations reported in [24].
Acknowledgements The Centre d’Etudes et de Recherches Lasers et Applications is supported by the Ministe`re charge´ de la Recherche, the Re´ gion Nord-Pas de Calais and the
Fonds Europe´ en de De´ veloppement Economique des Re´ gions. This research is partially supported by the Groupement de Recherche ‘‘Reactivite´ a` la Surface de la Glace’’.
References [1] I.V. Hertel, C. Hu¨ glin, C. Nitsch, C.P. Schults, Phys. Rev. Lett. 67 (1991) 1767. [2] U. Buck, C.S. Steinbach, J. Phys. Chem. A 102 (1998) 1124. [3] C. Focsa, B. Chazallon, J.L. Destombes, Surf. Sci. 528 (2003) 189. [4] C. Focsa, J.L. Destombes, Chem. Phys. Lett. 347 (2001) 390. [5] C. Mihesan, N. Lebrun, M. Ziskind, B. Chazallon, C. Focsa, J.L. Destombes, Surf. Sci. 566 (2004) 650. [6] M. Ziskind, C. Mihesan, N. Lebrun, B. Chazallon, C. Focsa, J.L. Destombes, Appl. Phys. A 79 (2004) 991. [7] M.L. Baltz-Knorr, K.E. Scriver, R.F. Haglund, Appl. Surf. Sci. 197–198 (2002) 11. [8] C.C. Han, Y.-L. Han, Y.C. Chen, Int. J. Mass Spectrom. 189 (1999) 157. [9] O.A. Boryak, I.O. Stepanov, M.V. Kosevich, V.S. Shelkovsky, V.V. Orlov, Yu.P. Blagoy, Eur. Mass Spectrom. 2 (1996) 329. [10] O.A. Boryak, M.V. Kosevich, V.S. Shelkovsky, Yu.P. Blagoy, Rapid Commun. Mass Spectrom. 10 (1996) 197. [11] M.V. Kosevich, G. Czira, O.A. Boryak, V.S. Shelkovsky, K. Ve´ key, J. Mass Spectrom. 33 (1998) 843. [12] I. Pocsik, Z. Phys. D 20 (1991) 395. [13] C.R. Wang, R.B. Huang, Z.Y. Liu, L.S. Zheng, Chem. Phys. Lett. 227 (1994) 103. [14] W.C. Natzle, D. Padowitz, S.J. Sibener, J. Chem. Phys. 88 (1988) 7975. [15] L.M. Cousins, R.J. Levis, S.R. Leone, J. Chem. Phys. 91 (1989) 5731. [16] J. Sunner, M.G. Ikonomou, P. Kebarle, Int. J. Mass Spectrom. Ion Proc. 82 (1988) 221. [17] A. Miotello, R. Kelly, Appl. Phys. Lett. 67 (1995) 3535. [18] R. Kelly, Phys. Rev. A 46 (1992) 860. [19] R. Kelly, R.W. Dreyfus, Nucl. Instrum. Methods B 32 (1988) 341. [20] R. Kelly, A. Miotello, Nucl. Instrum. Methods B 122 (1997) 374. [21] P. Wurz, K.R. Lykke, M.J. Pellin, D.M. Gruen, J. Appl. Phys. 70 (1991) 6647. [22] B.-J. Mhin, S.-J. Lee, K.S. Kim, Phys. Rev. A 48 (1993) 3764. [23] L.V. Zhigilei, P.B.S. Kodali, B.J. Garrison, J. Phys. Chem. B 102 (1998) 2845. [24] L.V. Zhigilei, Appl. Phys. A 76 (2003) 339.