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The emission of atoms and nanoparticles during femtosecond laser ablation of gold
Applied Surface Science 248 (2005) 163–166 www.elsevier.com/locate/apsusc
The emission of atoms and nanoparticles during femtosecond laser ablation of gold M. Vitiello *, S. Amoruso, C. Altucci, C. de Lisio, X. Wang Coherentia-INFM and Dipartimento di Scienze Fisiche, Universita` degli Studi di Napoli Federico II, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy Available online 22 March 2005
Abstract The properties of laser ablated plumes produced in high vacuum by 100 fs Ti:sapphire pulses from a gold solid target were studied. Optical emission spectroscopy showed that the plume was formed by both atomic and non-atomic species in the whole investigated laser intensity range (1012–1013 W/cm2), while atomic force microscopy analysis of deposits carried out at room temperature revealed the nanometric size of the non-atomic component, with a mean particle radius of tens of nanometers. # 2005 Elsevier B.V. All rights reserved. PACS: 52.38.Mf; 52.50.Jm; 79.20.Ds Keywords: Laser ablation; Nanoparticles; Spectroscopy
1. Introduction Recently, materials consisting of metal nanoparticles of about 10 nm radius have attracted great attention due to their optical, electrical, magnetic and mechanical properties, which make them suitable for a large amount of applications in various fields. For example, a great effort is currently in progress to use gold nanoparticles in the development of very sensitive biological sensors [1,2]. The ablation of matter from a solid metallic target with femtosecond (fs) laser pulses is also a topic of great * Corresponding author. Fax: +39 081 676346. E-mail address: [email protected] (M. Vitiello).
present interest. Among the different properties of this laser-solid interaction regime, there is the possibility of creating a highly excited material state followed by a rapid quenching. This process usually results in a material blow off composed of fast ions and atoms, as well as clusters and nanoaggregates of the target material [3,4]. Very recently, fs laser ablation in vacuum became a powerful tool for the production of metal and semiconductor nanoparticles, exhibiting several advantages with respect to previous standard techniques [3,5]. Here we report an experimental analysis of the plasma plume induced at the surface of a gold target by irradiation with intense fs Ti:sapphire laser pulses in high vacuum. In particular, time-gated optical emission spectroscopic measurements allowed us to
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.019
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M. Vitiello et al. / Applied Surface Science 248 (2005) 163–166
observe two different velocity populations in the expanding ablated plume that we correlate to fast atoms and ions, and slower nanoparticles. Atomic force microscopy (AFM) investigation of deposits of less than a monolayer provided the size distribution of the produced nanoparticles. Various theoretical studies have suggested that rapid expansion and cooling of solid-density matter heated by ultrashort laser pulses result in nanoparticles synthesis via several mechanisms. An example is the spinodal decomposition, as well as liquid phase ejection and fragmentation depending on the heating regimes, see Ref. [3–5]. We have analysed the results in terms of a simple phenomenological model of lasersolid irradiation and subsequent adiabatic expansion of the heated material. A good agreement was obtained between the experimental results and theoretical interpretation.
2. Experimental details The setup and techniques of the present experiment are similar to those used in our previous studies [5,6]. Laser pulses of about 100 fs and 1 mJ, produced by a Ti:sapphire (780 nm) source operated at 3 Hz, were focused to a spot size of 2 104 cm2 on the sample surface. The Au target was mounted on a rotating holder and placed in a vacuum chamber evacuated to a residual pressure of 107 mbar. The light emitted by a tiny volume of the plume at a distance d from the target surface was analysed by a 0.25 m monochromator (Jobin Yvon, HR250) coupled to an intensified charged coupled device (ICCD) camera with a minimum temporal gating of 5 ns and an overall spectral resolution of about 0.4 nm. To characterize the nanoparticle phase, a mica substrate was then placed at room temperature in front of the expanding plume, parallel to the target at a distance of 30 mm. The deposited samples were analysed with an AFM equipped with a silicon tip with a radius of less than 10 nm, operating in the tapping mode.
3. Results and discussion A time gated analysis of the plume emission spectrum was carried out in the fluence range 0.3–
Fig. 1. Emission spectra of the gold plume for different time delays t at a distance d = 1 mm for a laser fluence F = 0.6 J/cm2. The pulse duration was about 100 fs.
1.1 J/cm2. We followed the temporal evolution of the plume emission spectrum in the range 200–900 nm, at several distances from the target d = 1, 3, 5, 7, 10 mm, by varying the ICCD acquisition time delay t. The spectra acquired at a distance d = 1 mm and at a laser fluence F = 0.6 J/cm2 are shown in Fig. 1. While for short delays after the laser pulse, up to about 1 ms at d = 1 mm, the plume emission was dominated by atomic species, responsible for a typical line spectrum, at longer delays the emission was characterized by a featureless continuous spectrum, lasting up to about 100 ms. This behaviour, found for various time delays at the investigated distances, demonstrates the presence of two main classes of species, an atomic and a non-atomic one, moving at average velocities of about 3 105 cm/s and 5 103 cm/s, respectively, in the fs laser ablated plume. The emission of similar continuum spectra from hot nanoparticles (NPs) produced by fs laser ablation of different materials has been reported previously [5,7]. To characterize the NPs size we deposited the ablation plume onto mica substrates at different laser fluences. The laser shots were chosen to have almost the same number of incident photons on the gold target. Fig. 2 shows the size distribution of the deposited NPs obtained for a laser fluence of F = 0.5 J/cm2. The size statistics of the nanoparticles at different laser fluences is summarized in Table 1. The size distributions are quite narrow, with FWHM of 10 nm, and all the considered statistical quantities show a general common increasing trend with respect to increasing laser fluence. These results suggest a controllable process with laser intensity in the range
M. Vitiello et al. / Applied Surface Science 248 (2005) 163–166
Fig. 2. Size distribution of Au nanoparticles deposited in high vacuum onto mica substrates at a laser fluence of 0.5 J/cm2. The solid line is a guide to the eye.
3.0 1012–1.1 1013 W/cm2, to produce gold nanoparticles with narrow size distribution. In metals irradiated with ultrashort laser pulses, the laser energy is mainly absorbed by conduction electrons into a surface layer whose thickness is of the order of the inverse of the absorption coefficient a. Then, the electrons transfer energy to the lattice within a characteristic time teq, and the system reaches an equilibrium temperature T0. On this short timescale, the hydrodynamic motion of the heated material can be neglected (isochoric heating), coming into play at times of the order of teq. For 100 fs laser pulses, following Ref. [8] we can calculate this time by: teq ¼
te ti te þ ti
(1)
Table 1 Size statistics of the gold nanoparticles FL (J/cm2)
I (W/cm2)
rm (nm)
s (nm)
rmax (nm)
a
0.3 0.5 0.6 1.1
3.0 1012 5.0 1012 6.0 1012 1.1 1013
8.0 9.8 16.4 14.1
4.7 6.1 10.0 10.9
27 36 48 55
0.59 0.62 0.61 0.77
rm, rmax, and s represent nanoparticles’ mean radius, ‘maximum radius’ (defined as the radius below which 90% of the particles is counted), and the standard deviation of their size distribution, respectively. a is the semi-dispersion parameter (a = s/rm). FL and I are the incident laser fluence and intensity, respectively.
165
where te and ti, are the electrons cooling time and lattice heating time, respectively. We can assume then that before significant expansion starts, a ffisurpffiffiffiffiffiffiffiffiffi face layer of thickness z max(a1, h ¼ Dt eq ) in the target is heated to a temperature T0 = F abs/zCi, where D is the electronic thermal diffusivity, F abs = (1 R) F L is the absorbed fluence (energy per unit surface area), R is the metal reflectivity, and Ci is the lattice specific heat per unit volume. Here h is the characteristic electron thermal diffusion length. When the electrons temperature remains smaller than the Fermi energy, the electron diffusivity can be expressed by D ¼ v2F t=3 avF =3, with an estimated electron collision time t as that necessary to cover the average interatomic distance a at the Fermi velocity vF [9]. For gold we estimate by Eq. (1) a value of teq = 30–40 ps, thus obtaining a thermal penetration depth of h 60–70 nm, which turns out to be greater than the gold optical penetration depth a1 10 nm. Thus, in the investigated fluence range 0.3–1.1 J/ cm2, our calculations give a lattice equilibrium temperature T0 in the range 0.5–1.8 eV, to be compared with the critical temperature of gold Tcr = 0.56 eV [10]. The adiabatic expansion (AE) following the isochoric heating stage can be described approximately by the Gru¨ neisen coefficient G, and the temperature T and density r of the expanding material are related to the initial state T0, r0 through the relation: T = T0(r/r0)G(r) [11]. A good approximation for metals is G = 2(r/r0) for r0/3 < r < r0, while G ! 2/3 for r r0/3 (the perfect gas) [11]. We have constructed the AE pathways by joining the two dependences mentioned assuming a transition to a perfect gas behaviour as soon as the density decreases below r0/3 during the expansion, i.e. G 2/3 for r r0/3. The AE paths of the heated layer following the isochoric heating stage starting at different initial temperatures T0 are shown in Fig. 3, where also a Van der Waals-like (r, T) phase-diagram for gold has been reported [12]. The cross () indicates the critical point of gold (rcr, Tcr). AE paths starting at 0.4 T0 1 eV reach the binodal curve at subcritical temperatures T < Tcr, then crossing the spinodal at supercritical densities r > rcr during the expansion; AE paths starting at 1.0 < T0 < 1.5 eV pass above the critical point
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M. Vitiello et al. / Applied Surface Science 248 (2005) 163–166
4. Conclusions
Fig. 3. AE paths and Van der Waals-like (r T) phase-diagram of gold. The vertical solid line indicates isochoric heating due to laser irradiation. The grey region represents accessible metastable states of the diagram corresponding to undercooled gas (left side) and superheated liquid (right side). The shaded area is the unstable region delimited by the spinodal curve. The ‘’ indicates the critical point. The solid lines represent AE pathways starting at the different initial temperatures: T0 = 0.4 eV (~); T0 = 0.6 eV (&); T0 = 1 eV (^); T0 = 1.2 eV (*); T0 = 1.5 eV ($); T0 = 2 eV (b).
T > Tcr, intersect the spinodal at subcritical densities r < rcr. Paths starting at T0 > 1.5 eV do not cross the spinodal, ending into the metastable undercooled gas region. Thus, the AE paths starting at T0 0.4–1.5 eV, which is quite the same range of lattice temperatures that we estimated above for the investigated fluence range, are the most promising for nanoparticles production, as we observed experimentally. In fact, as AE reaches the spinodal at supercritical or nearcritical densities, phase-decomposition takes place and a relatively large portion of the heated material transforms into nanodroplets [3]. A uniformly heated layer of thickness h was used to estimate the initial temperature T0. The target temperature varies as a function of depth, z, thus cells of materials at different z start their AE paths at different temperatures, being interested by rarefaction at different times. Nevertheless, the simple analysis reported above clearly indicates that laser irradiation of gold with 100 fs laser pulses in the fluence range of 0.3–1.1 J/cm2, can drive the heated material into a state of the phase diagram which leads to the emission of a mixture of phases favouring the formation of nanoparticles, as observed experimentally.
The investigation of the properties of gold ablated plumes, generated by 100 fs Ti:sapphire laser in high vacuum, showed the dynamics of atomic and nonatomic components. The presence of two different velocity populations in the plume was revealed by optical emission spectroscopy. The non-atomic phase is due to nanoparticles which are characterized by a featureless continuum emission spectrum. The sizes of the deposited nanoparticles studied by AFM analysis, showed sizes of 10 nm, with a narrow size distribution. The phenomenological model suggested qualitatively explains the experimental observation. Acknowledgement The authors would like to thank Prof. Luciano Lanotte and co-workers for providing the AFM measurements. References [1] R. Elghanian, J.J. Storhoff, R.C. Mucic, R.L. Letsinger, C.A. Mirkin, Science 277 (1997) 1078. [2] Y. Xiao, F. Patolsky, E. Katz, J.F. Hainfeld, I. Willner, Science 299 (2003) 1877. [3] S. Eliezer, N. Eliaz, E. Grossman, D. Fisher, I. Gouzman, Z. Henis, S. Pecker, Y. Horovitz, M. Fraenkel, S. Maman, Y. Lereah, Phys. Rev. B 69 (2004) 144119. [4] T.E. Glover, J.O.S.A.B. 20 (2003) 125. [5] S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, G. Ausanio, V. Iannotti, L. Lanotte, Appl. Phys. Lett. 84 (2004) 4502. [6] S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, Phys. Rev. B 67 (2003) 224503. [7] O. Albert, S. Roger, Y. Glinec, J.C. Loulergue, J. Etchepare, C. Boulmer-Leborgne, J. Perriere, E. Millon, Appl. Phys. A 76 (2003) 319. [8] S. Nolte, C. Momma, H. Jacobs, A. Tunnermann, J. Opt. Soc. Am. B 14 (1997) 2716. [9] N.W. Ashcroft, N.D. Mermin, Solid State Physics, Holt, Rinehart and Winston, New York, 1976. [10] M.M. Martynyuk, Zh. Fiz. Khimii 57 (1983) 810; M.M. Martynyuk, Sov. J. Phys. Chem. 57 (1983) 494. [11] Ya.B. Zel’dovich, Yu.P. Raizer, Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena, Academic Press, New York, 1966. [12] L.D. Landau, E.M. Lifshitz, Statistical Physics, Part 1, Pergamon Press, Oxford, 1960.
The emission of atoms and nanoparticles during femtosecond laser ablation of gold M. Vitiello *, S. Amoruso, C. Altucci, C. de Lisio, X. Wang Coherentia-INFM and Dipartimento di Scienze Fisiche, Universita` degli Studi di Napoli Federico II, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy Available online 22 March 2005
Abstract The properties of laser ablated plumes produced in high vacuum by 100 fs Ti:sapphire pulses from a gold solid target were studied. Optical emission spectroscopy showed that the plume was formed by both atomic and non-atomic species in the whole investigated laser intensity range (1012–1013 W/cm2), while atomic force microscopy analysis of deposits carried out at room temperature revealed the nanometric size of the non-atomic component, with a mean particle radius of tens of nanometers. # 2005 Elsevier B.V. All rights reserved. PACS: 52.38.Mf; 52.50.Jm; 79.20.Ds Keywords: Laser ablation; Nanoparticles; Spectroscopy
1. Introduction Recently, materials consisting of metal nanoparticles of about 10 nm radius have attracted great attention due to their optical, electrical, magnetic and mechanical properties, which make them suitable for a large amount of applications in various fields. For example, a great effort is currently in progress to use gold nanoparticles in the development of very sensitive biological sensors [1,2]. The ablation of matter from a solid metallic target with femtosecond (fs) laser pulses is also a topic of great * Corresponding author. Fax: +39 081 676346. E-mail address: [email protected] (M. Vitiello).
present interest. Among the different properties of this laser-solid interaction regime, there is the possibility of creating a highly excited material state followed by a rapid quenching. This process usually results in a material blow off composed of fast ions and atoms, as well as clusters and nanoaggregates of the target material [3,4]. Very recently, fs laser ablation in vacuum became a powerful tool for the production of metal and semiconductor nanoparticles, exhibiting several advantages with respect to previous standard techniques [3,5]. Here we report an experimental analysis of the plasma plume induced at the surface of a gold target by irradiation with intense fs Ti:sapphire laser pulses in high vacuum. In particular, time-gated optical emission spectroscopic measurements allowed us to
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.019
164
M. Vitiello et al. / Applied Surface Science 248 (2005) 163–166
observe two different velocity populations in the expanding ablated plume that we correlate to fast atoms and ions, and slower nanoparticles. Atomic force microscopy (AFM) investigation of deposits of less than a monolayer provided the size distribution of the produced nanoparticles. Various theoretical studies have suggested that rapid expansion and cooling of solid-density matter heated by ultrashort laser pulses result in nanoparticles synthesis via several mechanisms. An example is the spinodal decomposition, as well as liquid phase ejection and fragmentation depending on the heating regimes, see Ref. [3–5]. We have analysed the results in terms of a simple phenomenological model of lasersolid irradiation and subsequent adiabatic expansion of the heated material. A good agreement was obtained between the experimental results and theoretical interpretation.
2. Experimental details The setup and techniques of the present experiment are similar to those used in our previous studies [5,6]. Laser pulses of about 100 fs and 1 mJ, produced by a Ti:sapphire (780 nm) source operated at 3 Hz, were focused to a spot size of 2 104 cm2 on the sample surface. The Au target was mounted on a rotating holder and placed in a vacuum chamber evacuated to a residual pressure of 107 mbar. The light emitted by a tiny volume of the plume at a distance d from the target surface was analysed by a 0.25 m monochromator (Jobin Yvon, HR250) coupled to an intensified charged coupled device (ICCD) camera with a minimum temporal gating of 5 ns and an overall spectral resolution of about 0.4 nm. To characterize the nanoparticle phase, a mica substrate was then placed at room temperature in front of the expanding plume, parallel to the target at a distance of 30 mm. The deposited samples were analysed with an AFM equipped with a silicon tip with a radius of less than 10 nm, operating in the tapping mode.
3. Results and discussion A time gated analysis of the plume emission spectrum was carried out in the fluence range 0.3–
Fig. 1. Emission spectra of the gold plume for different time delays t at a distance d = 1 mm for a laser fluence F = 0.6 J/cm2. The pulse duration was about 100 fs.
1.1 J/cm2. We followed the temporal evolution of the plume emission spectrum in the range 200–900 nm, at several distances from the target d = 1, 3, 5, 7, 10 mm, by varying the ICCD acquisition time delay t. The spectra acquired at a distance d = 1 mm and at a laser fluence F = 0.6 J/cm2 are shown in Fig. 1. While for short delays after the laser pulse, up to about 1 ms at d = 1 mm, the plume emission was dominated by atomic species, responsible for a typical line spectrum, at longer delays the emission was characterized by a featureless continuous spectrum, lasting up to about 100 ms. This behaviour, found for various time delays at the investigated distances, demonstrates the presence of two main classes of species, an atomic and a non-atomic one, moving at average velocities of about 3 105 cm/s and 5 103 cm/s, respectively, in the fs laser ablated plume. The emission of similar continuum spectra from hot nanoparticles (NPs) produced by fs laser ablation of different materials has been reported previously [5,7]. To characterize the NPs size we deposited the ablation plume onto mica substrates at different laser fluences. The laser shots were chosen to have almost the same number of incident photons on the gold target. Fig. 2 shows the size distribution of the deposited NPs obtained for a laser fluence of F = 0.5 J/cm2. The size statistics of the nanoparticles at different laser fluences is summarized in Table 1. The size distributions are quite narrow, with FWHM of 10 nm, and all the considered statistical quantities show a general common increasing trend with respect to increasing laser fluence. These results suggest a controllable process with laser intensity in the range
M. Vitiello et al. / Applied Surface Science 248 (2005) 163–166
Fig. 2. Size distribution of Au nanoparticles deposited in high vacuum onto mica substrates at a laser fluence of 0.5 J/cm2. The solid line is a guide to the eye.
3.0 1012–1.1 1013 W/cm2, to produce gold nanoparticles with narrow size distribution. In metals irradiated with ultrashort laser pulses, the laser energy is mainly absorbed by conduction electrons into a surface layer whose thickness is of the order of the inverse of the absorption coefficient a. Then, the electrons transfer energy to the lattice within a characteristic time teq, and the system reaches an equilibrium temperature T0. On this short timescale, the hydrodynamic motion of the heated material can be neglected (isochoric heating), coming into play at times of the order of teq. For 100 fs laser pulses, following Ref. [8] we can calculate this time by: teq ¼
te ti te þ ti
(1)
Table 1 Size statistics of the gold nanoparticles FL (J/cm2)
I (W/cm2)
rm (nm)
s (nm)
rmax (nm)
a
0.3 0.5 0.6 1.1
3.0 1012 5.0 1012 6.0 1012 1.1 1013
8.0 9.8 16.4 14.1
4.7 6.1 10.0 10.9
27 36 48 55
0.59 0.62 0.61 0.77
rm, rmax, and s represent nanoparticles’ mean radius, ‘maximum radius’ (defined as the radius below which 90% of the particles is counted), and the standard deviation of their size distribution, respectively. a is the semi-dispersion parameter (a = s/rm). FL and I are the incident laser fluence and intensity, respectively.
165
where te and ti, are the electrons cooling time and lattice heating time, respectively. We can assume then that before significant expansion starts, a ffisurpffiffiffiffiffiffiffiffiffi face layer of thickness z max(a1, h ¼ Dt eq ) in the target is heated to a temperature T0 = F abs/zCi, where D is the electronic thermal diffusivity, F abs = (1 R) F L is the absorbed fluence (energy per unit surface area), R is the metal reflectivity, and Ci is the lattice specific heat per unit volume. Here h is the characteristic electron thermal diffusion length. When the electrons temperature remains smaller than the Fermi energy, the electron diffusivity can be expressed by D ¼ v2F t=3 avF =3, with an estimated electron collision time t as that necessary to cover the average interatomic distance a at the Fermi velocity vF [9]. For gold we estimate by Eq. (1) a value of teq = 30–40 ps, thus obtaining a thermal penetration depth of h 60–70 nm, which turns out to be greater than the gold optical penetration depth a1 10 nm. Thus, in the investigated fluence range 0.3–1.1 J/ cm2, our calculations give a lattice equilibrium temperature T0 in the range 0.5–1.8 eV, to be compared with the critical temperature of gold Tcr = 0.56 eV [10]. The adiabatic expansion (AE) following the isochoric heating stage can be described approximately by the Gru¨ neisen coefficient G, and the temperature T and density r of the expanding material are related to the initial state T0, r0 through the relation: T = T0(r/r0)G(r) [11]. A good approximation for metals is G = 2(r/r0) for r0/3 < r < r0, while G ! 2/3 for r r0/3 (the perfect gas) [11]. We have constructed the AE pathways by joining the two dependences mentioned assuming a transition to a perfect gas behaviour as soon as the density decreases below r0/3 during the expansion, i.e. G 2/3 for r r0/3. The AE paths of the heated layer following the isochoric heating stage starting at different initial temperatures T0 are shown in Fig. 3, where also a Van der Waals-like (r, T) phase-diagram for gold has been reported [12]. The cross () indicates the critical point of gold (rcr, Tcr). AE paths starting at 0.4 T0 1 eV reach the binodal curve at subcritical temperatures T < Tcr, then crossing the spinodal at supercritical densities r > rcr during the expansion; AE paths starting at 1.0 < T0 < 1.5 eV pass above the critical point
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M. Vitiello et al. / Applied Surface Science 248 (2005) 163–166
4. Conclusions
Fig. 3. AE paths and Van der Waals-like (r T) phase-diagram of gold. The vertical solid line indicates isochoric heating due to laser irradiation. The grey region represents accessible metastable states of the diagram corresponding to undercooled gas (left side) and superheated liquid (right side). The shaded area is the unstable region delimited by the spinodal curve. The ‘’ indicates the critical point. The solid lines represent AE pathways starting at the different initial temperatures: T0 = 0.4 eV (~); T0 = 0.6 eV (&); T0 = 1 eV (^); T0 = 1.2 eV (*); T0 = 1.5 eV ($); T0 = 2 eV (b).
T > Tcr, intersect the spinodal at subcritical densities r < rcr. Paths starting at T0 > 1.5 eV do not cross the spinodal, ending into the metastable undercooled gas region. Thus, the AE paths starting at T0 0.4–1.5 eV, which is quite the same range of lattice temperatures that we estimated above for the investigated fluence range, are the most promising for nanoparticles production, as we observed experimentally. In fact, as AE reaches the spinodal at supercritical or nearcritical densities, phase-decomposition takes place and a relatively large portion of the heated material transforms into nanodroplets [3]. A uniformly heated layer of thickness h was used to estimate the initial temperature T0. The target temperature varies as a function of depth, z, thus cells of materials at different z start their AE paths at different temperatures, being interested by rarefaction at different times. Nevertheless, the simple analysis reported above clearly indicates that laser irradiation of gold with 100 fs laser pulses in the fluence range of 0.3–1.1 J/cm2, can drive the heated material into a state of the phase diagram which leads to the emission of a mixture of phases favouring the formation of nanoparticles, as observed experimentally.
The investigation of the properties of gold ablated plumes, generated by 100 fs Ti:sapphire laser in high vacuum, showed the dynamics of atomic and nonatomic components. The presence of two different velocity populations in the plume was revealed by optical emission spectroscopy. The non-atomic phase is due to nanoparticles which are characterized by a featureless continuum emission spectrum. The sizes of the deposited nanoparticles studied by AFM analysis, showed sizes of 10 nm, with a narrow size distribution. The phenomenological model suggested qualitatively explains the experimental observation. Acknowledgement The authors would like to thank Prof. Luciano Lanotte and co-workers for providing the AFM measurements. References [1] R. Elghanian, J.J. Storhoff, R.C. Mucic, R.L. Letsinger, C.A. Mirkin, Science 277 (1997) 1078. [2] Y. Xiao, F. Patolsky, E. Katz, J.F. Hainfeld, I. Willner, Science 299 (2003) 1877. [3] S. Eliezer, N. Eliaz, E. Grossman, D. Fisher, I. Gouzman, Z. Henis, S. Pecker, Y. Horovitz, M. Fraenkel, S. Maman, Y. Lereah, Phys. Rev. B 69 (2004) 144119. [4] T.E. Glover, J.O.S.A.B. 20 (2003) 125. [5] S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, G. Ausanio, V. Iannotti, L. Lanotte, Appl. Phys. Lett. 84 (2004) 4502. [6] S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, Phys. Rev. B 67 (2003) 224503. [7] O. Albert, S. Roger, Y. Glinec, J.C. Loulergue, J. Etchepare, C. Boulmer-Leborgne, J. Perriere, E. Millon, Appl. Phys. A 76 (2003) 319. [8] S. Nolte, C. Momma, H. Jacobs, A. Tunnermann, J. Opt. Soc. Am. B 14 (1997) 2716. [9] N.W. Ashcroft, N.D. Mermin, Solid State Physics, Holt, Rinehart and Winston, New York, 1976. [10] M.M. Martynyuk, Zh. Fiz. Khimii 57 (1983) 810; M.M. Martynyuk, Sov. J. Phys. Chem. 57 (1983) 494. [11] Ya.B. Zel’dovich, Yu.P. Raizer, Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena, Academic Press, New York, 1966. [12] L.D. Landau, E.M. Lifshitz, Statistical Physics, Part 1, Pergamon Press, Oxford, 1960.