- Email: [email protected]
Spectroscopic measurement of plume emission from femtosecond laser ablation
Thin Solid Films 453 – 454 (2004) 340–344
Spectroscopic measurement of plume emission from femtosecond laser ablation ` d, Denis Moreaua, O. Alberta, R. Benzergab, C. Boulmer-Leborgneb, E. Millonc, J. Perriere J. Etcheparea,* a
´ Ecole Polytechnique, 91761 Palaiseau Cedex, France LOA, CNRS UMR 7639, ENS Techniques Avancees, b ´ ´ GREMI, Universite´ d’Orleans, 14 rue d’Issoudun BP6744, 45067 Orleans Cedex 2, France c LSMCL, Universite´ de Metz, 57078 Metz Cedex 3, France d ´ Paris VI et VII, Tour 23, 2 Place Jussieu, 75251 Paris Cedex 05, France GPS, CNRS UMR 7588, Universites
Abstract We present a time and space resolved emission spectroscopy technique that enables a full characterisation in three dimensions density of emitting species generated by femtosecond pulsed laser ablation. Applied to Al neutral atoms, it reveals a population that, at the lowest fluences used, can be roughly described by a half range Maxwell–Boltzmann function. This simple configuration evolves towards a more complex population as the energy density increases. Specificities connected to the measured emitted spectral rays are evidenced and taken into account. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 79.20.D Keywords: Femtosecond laser ablation; Time resolved emission spectroscopy
1. Introduction A good understanding of the plume expansion related to femtosecond ablation has an important role in the elaboration of thin films by pulsed laser deposition. We present a spectroscopic measurement of the emitting species that enables a full description of the kinetics of the various species, which escape from the target before their deposition upon a substrate. This paper will focus on neutral’s behavior for two essential reasons: their population is highly preponderant; their study is less extensively developed than the ones coming from time of flight mass spectrometer measurements. Main emphasis will be put on Al atoms, but comparison to other targets will be frequently used. The ultimate goal one can expect from any spectroscopic technique concerns the species density description at any point in the volume that covers the plume. As an emission spectroscopy measurement is usually performed orthogonally to the direction of propagation, it results in signal integration *Corresponding author. Tel.: q33-169-319-787; fax: q33-1-69319996. E-mail address: [email protected] (J. Etchepare).
over a more or less well defined volume. We have therefore developed a specific procedure that enabled us to reach a full three-dimensional spatial resolution compatible with a quantitative estimation of velocity distribution. 2. Experimental results 2.1. Experimental set-up and procedure A schematic view of the experimental set-up is given in Fig. 1. It has been already described elsewhere w1x. We recall the characteristics of the laser beam: ls620 nm; Dts100 fs; rep. rate: 10 Hz; spot size diameter 150 mm; energy density from 1012 to 1014 Wycm2. The residual pressure in the interaction chamber is F10y5 mbar. Special attention has been paid on the geometrical characteristics and constraints of the optical elements. The plasma is imaged on the entrance slit plane of a HR460 Jobin Yvon spectrometer with a 152 mm focal length objective. It is used in a ‘4f’ configuration giving a y1 magnification and a numerical aperture of 2.8 to allow an optimal collection of the plasma-emitted light.
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:.1016/j.tsf.2003.11.098
D. Moreau et al. / Thin Solid Films 453 – 454 (2004) 340–344
341
Fig. 1. Experimental apparatus.
The vertical and horizontal entrance slit widths of the spectrometer are set to approximately 300 mm. This corresponds to a compromise between a good enough spectral resolution of 2 nm, a measurable signal on the photomultiplier even for weak spectral lines, and a depth of field of a few millimeters to image the plasma plume. This depth of field is important to allow the measurement of a slice of the plasma at a given distance from the target with the vertical entrance slit of the spectrometer and a precise right-angled parallelepiped volume in the double slit case. The Hamamatsu H6780-2 photomultiplier placed behind the output slit of the spectrometer has a rise time of 0.78 ns. It allows a time of flight measurement of the fastest species ejected from the plasma 500 mm away from the target surface or more. The use of an aperture (double slit case) enabled obtaining a two-dimensional resolution (as the signal is integrated over x axis). A lot of (10–20) measurements are performed by tuning this aperture along y-axis by identical elementary steps. Due to axial symmetry of the plume, the experimental curves can thus be treated by Abel inversion w2x in order to reach a three-dimensional resolution. Such a calculation leads to an ensemble of S(z,r,t) curves (Fig. 2a), with rsyx2qy2. If one uses xs0.0, r values correspond to y ones from the preceding experimental measures. Fig. 2b gives an overview of the Abel calculation accuracy and opportunity: curve 1 corresponds to experimental findings at ys0.0 mm; curve 2 is the summation from curves obtained by Abel inversion (part a). Their comparison is a good test of the validity of Abel inversion. Differences in shapes between first curve of part a and curves of part b evidence the necessity of Abel inversion.
If the velocity distribution of the species can be written under the form of a maxwellian function, one can describe the emitted signal as w3x: 2 2 2z w 1 m Žzyut. qy qx |, SŽz,r,t.A 3 expxy t 2kT t2 y ~
(1)
where m is the mass of the particle, u the flow velocity, and T the translational temperature. 2.2. Results and discussions As otherwise noticed, the experimental parameters are as follows. The results concern 396 nm emission line from Al I. This line is the most intense we have detected in the visible spectral range and can be easily isolated from other emission lines. An posteriori justification for such a choice will also be given hereafter. Energy density is of 2.5=1013 Wycm2, roughly 10 times above the ablation threshold. The most probable speed vz (which corresponds to the maximum of the time resolved signal for a given z value) is measured to be 7=103 mys. A fairly good fit (Fig. 3) of the three-dimensional curves to Eq. (1) can be obtained by using a same set of parameters for all of them: Ts(5.1"0.1)=104 K (or 4.4 eV) and u'0. The essential point is that a half-range Maxwell–Boltzmann distribution has been found to be satisfactory for describing the whole volume species. This collision less-like behaviour for neutral atoms is in high contrast with well-documented TOF results, that dealt as a matter of fact with ions species w4,5x. As the fluence increases, the curves evolve toward a more complicated form that
D. Moreau et al. / Thin Solid Films 453 – 454 (2004) 340–344
342
can be accounted for by the appearance of a full range maxwellian population. Its contribution takes place at earlier delays and is necessary to a good fit of the temporal curves, even if the two contributions are not directly time-resolved on the experimental curves (Fig. 4). This second population, which proportion increases with fluence, is at the origin of the increase of the most probable speed as a function of fluence, a result that recalls published works on ions species w11x. Another characterization of the plasma expansion that relates to its angular distribution has been readily reported in the literature to be described in a parametric form. The most general expression w6,7x, using polar coordinates, corresponds to a sum over i sinusoidal functions, of power ni and weight ai: SŽq.s8aicosnq
Fig. 3. Emitted signal from successive lateral elementary volumes and their fit using (Eq. (1)) with zs1.46 mm, yi2qxi2sri2 and Ts 5.1=104 K. The most probable velocity vzs7=103 mys.
(2)
i
Fig. 2. Part (a): calculated signals after Abel inversion from successive lateral elementary volumes (steps of 0.1 mm); part (b): curve 1 corresponds to the measured emitted signal, curve 2 to the sum of the curves from the previous figure.
D. Moreau et al. / Thin Solid Films 453 – 454 (2004) 340–344
343
Fig. 4. Angular emission distribution deduced from a time integration of curves from Fig. 2a.
Fig. 5. Emitted signal from an elementary volume on axis (zs1.46 mm) at ls396 nm under 5=1013 Wycm2 fluence. The overall fit is made of two populations: a full range maxwellian population (T1s 3.9=104 K) and a half range maxwellian population (T2s1.9=104 K, u2s6000 mys).
Fig. 7. Grotrian diagram of Al I, showing the nearest levels connected to the analyzed emission lines.
Fig. 6. Emitted signals at ls396 and 669 nm from neutral Al species. The two curves have been normalized.
The angular distribution of the species (Fig. 5) has been obtained by an integration of the experimental curves. Fitted to Eq. (2), and using only one population (is1) the angular distribution of neutrals is particularly wide (with ns3"1), at the lowest used fluence. This corroborates the results obtained from the fit of time and space resolved curves at the same fluence. The fact that n value is higher than 1, as expected when us0, is in concordance with the fact that even at this fluence, a
better fit than the one depicted in Fig. 3 can be obtained by using a two population model. Emitting species kinetics is measured as a function of delay from the impact between the laser pulse and the target. The kinetics generally evidenced do exist over a temporal range that can reach several ms, much longer than the known radiative lifetimes w8x. An analysis from various emitting wavelengths is therefore of unique interest. Fig. 6 presents kinetics that correspond to 396 and 669 nm emission lines from Al I, lines which attributions w9,10x are reported in Fig. 7. (In this last figure, arrows are used to show the documented allowed transitions.) The temporal behavior of 396 and 669 nm lines is drastically different, the second emission coming
D. Moreau et al. / Thin Solid Films 453 – 454 (2004) 340–344
344
significantly later than the first one. A lot of processes can be invoked to try explaining this discrepancy. All of them w11x have as a basis, even different routes for forming excited state species (by post ablation processes), or non-equivalent delayed emissions. Radiation trapping occurs when the emission from excited atoms is absorbed and reemitted several times before being detected. As the efficiency of the process is highest for the regions of highest density, one can expect revealing this effect by comparing temporal signals from elementary volumes more or less close to z propagation axis. None of them has been found not to obey Eq. (1). Electron impact excitation necessitates ey and atoms to be proximate and cannot explain by itself such a time lag. Electron ion recombination leads to the formation of highly excited Rydberg states; these can only radiate by cascade mechanisms. We have in fact measured a continuous decrease in the integrated intensities for these two lines as a function of distance z to the target from 0.5 to 5.0 mm. We therefore deduced that 3s2 (1S)5p´3s2(1S)4s emission is not the essential way for populating 4s state. Moreover, as the 396 nm line appears first, its population cannot be a main consequence of de-excitation from the upper 5p state which gives rise to the 669 nm line. In fact, we suppose that 3s2(1S)5p and 3s2(1S)4s populations come by cascade mechanisms and by independent routes from upper Rydberg excited states. As the routinely invoked processes are not convincing at all, we propose interpreting the time lag accompanying the 669 nm emission line by its longer radiative lifetime (6=10y7 s) than the one of the 396 nm emission line (2=10y8 s). A convolution between the intrinsic atomic kinetics (Eq. (1)) and the radiative lifetime tr: S9Žz,r,t.sSŽz,r,t.Øexpy
t tr
(3)
enabled us to succeed in describing the two lines behavior by a same set of parameters. In the temporal range of interest, the convolution product affects only the 669 nm line. Emission kinetics from the 396 nm line can be therefore directly used as a basis for Al I parameters estimation. 3. Conclusions We have focused in this paper on the behavior of Al neutral species. At the opposite to findings from ns
pulse ablation and from ion species produced when using fs pulses, we have clearly evidenced the contribution to the ablation plume from a half range Maxwellian population, which we suppose to be the only one at fluences close to ablation threshold. Its contribution still exists at markedly higher energy density values. It diminishes when the energy density that impinges the target increases and a population that possesses a stream velocity, as having undergone a lot of collisions, then takes place. In fact, as compared to ions behavior where two discrete populations have been described w12x, we are obviously in the presence of a mixing from populations, which characteristics and proportions evolve continuously as a function of energy density. The spectral analysis brings several interesting aspects about the species excitation trajectory. At the opposite of OES results published on graphite w13x, Al plume is primarily made of excited atom species. Their existence has been detected at the lowest distance that we could reach with our apparatus (f0.5 mm) and the corresponding signal is still measurable at several mm from the target. References w1x E. Millon, O. Albert, J.C. Loulergue, J. Etchepare, D. Hulin, W. Seiler, J. Perriere, J. Appl. Phys. 88 (2000) 6937. w2x K. Bockasten, J. Opt. Soc. Am. 51 (1961) 943. w3x J.C.S. Kools, T.S.B. Baller, S.T. de Zwart, J. Dieleman, J. Appl. Phys. 71 (9) (1992) 4547. w4x R. Stoian, D. Ashkenasi, A. Rosenfeld, E.E.B. Campbell, Phys. Rev. B 62 (2000) 13 167. w5x M.E. Ye, C.P. Grigoropoulos, J. Appl. Phys. 89 (2001) 5183. w6x D.B. Chrisey, G.H. Hubler, Pulsed Laser Deposition of Thin Films, John Wiley and Sons, 1994. w7x F. Antoni, C. Fuchs, E. Fogarassy, Appl. Surf. Sci. 96–98 (1996) 50. w8x F. Claeyssens, R.J. Lade, K.N. Rosser, M.N.R. Ashfold, J. Appl. Phys. 89 (2001) 697. w9x W.C. Martin, J. Sugar, A. Musgrove, G.R. Dalton, W.L. Wiese, J.R. Fuhr, D.E. Kelleher, National Institute of Standards and Technology Atomic Spectra Database, NIST, GaithersburgMD, 1995. w10x A.R. Striganov, N.S. Sventitskii, Tables of Spectral Lines of Neutral and Ionized Atoms, IFIyPlenum, New York–Washington, 1968. w11x K.L. Saenger, J. Appl. Phys. 66 (1989) 4435. w12x F. Qian, V. Cracium, R.K. Singh, S.D. Dutta, P.P. Pronko, J. Appl. Phys. 86 (1999) 2281. w13x F. Claeyssens, M.N.R. Ashfold, E. Sofoulakis, C.G. Ristoscu, D. Anglos, C. Fotakis, J. Appl. Phys. 91 (2002) 6162.
Spectroscopic measurement of plume emission from femtosecond laser ablation ` d, Denis Moreaua, O. Alberta, R. Benzergab, C. Boulmer-Leborgneb, E. Millonc, J. Perriere J. Etcheparea,* a
´ Ecole Polytechnique, 91761 Palaiseau Cedex, France LOA, CNRS UMR 7639, ENS Techniques Avancees, b ´ ´ GREMI, Universite´ d’Orleans, 14 rue d’Issoudun BP6744, 45067 Orleans Cedex 2, France c LSMCL, Universite´ de Metz, 57078 Metz Cedex 3, France d ´ Paris VI et VII, Tour 23, 2 Place Jussieu, 75251 Paris Cedex 05, France GPS, CNRS UMR 7588, Universites
Abstract We present a time and space resolved emission spectroscopy technique that enables a full characterisation in three dimensions density of emitting species generated by femtosecond pulsed laser ablation. Applied to Al neutral atoms, it reveals a population that, at the lowest fluences used, can be roughly described by a half range Maxwell–Boltzmann function. This simple configuration evolves towards a more complex population as the energy density increases. Specificities connected to the measured emitted spectral rays are evidenced and taken into account. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 79.20.D Keywords: Femtosecond laser ablation; Time resolved emission spectroscopy
1. Introduction A good understanding of the plume expansion related to femtosecond ablation has an important role in the elaboration of thin films by pulsed laser deposition. We present a spectroscopic measurement of the emitting species that enables a full description of the kinetics of the various species, which escape from the target before their deposition upon a substrate. This paper will focus on neutral’s behavior for two essential reasons: their population is highly preponderant; their study is less extensively developed than the ones coming from time of flight mass spectrometer measurements. Main emphasis will be put on Al atoms, but comparison to other targets will be frequently used. The ultimate goal one can expect from any spectroscopic technique concerns the species density description at any point in the volume that covers the plume. As an emission spectroscopy measurement is usually performed orthogonally to the direction of propagation, it results in signal integration *Corresponding author. Tel.: q33-169-319-787; fax: q33-1-69319996. E-mail address: [email protected] (J. Etchepare).
over a more or less well defined volume. We have therefore developed a specific procedure that enabled us to reach a full three-dimensional spatial resolution compatible with a quantitative estimation of velocity distribution. 2. Experimental results 2.1. Experimental set-up and procedure A schematic view of the experimental set-up is given in Fig. 1. It has been already described elsewhere w1x. We recall the characteristics of the laser beam: ls620 nm; Dts100 fs; rep. rate: 10 Hz; spot size diameter 150 mm; energy density from 1012 to 1014 Wycm2. The residual pressure in the interaction chamber is F10y5 mbar. Special attention has been paid on the geometrical characteristics and constraints of the optical elements. The plasma is imaged on the entrance slit plane of a HR460 Jobin Yvon spectrometer with a 152 mm focal length objective. It is used in a ‘4f’ configuration giving a y1 magnification and a numerical aperture of 2.8 to allow an optimal collection of the plasma-emitted light.
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:.1016/j.tsf.2003.11.098
D. Moreau et al. / Thin Solid Films 453 – 454 (2004) 340–344
341
Fig. 1. Experimental apparatus.
The vertical and horizontal entrance slit widths of the spectrometer are set to approximately 300 mm. This corresponds to a compromise between a good enough spectral resolution of 2 nm, a measurable signal on the photomultiplier even for weak spectral lines, and a depth of field of a few millimeters to image the plasma plume. This depth of field is important to allow the measurement of a slice of the plasma at a given distance from the target with the vertical entrance slit of the spectrometer and a precise right-angled parallelepiped volume in the double slit case. The Hamamatsu H6780-2 photomultiplier placed behind the output slit of the spectrometer has a rise time of 0.78 ns. It allows a time of flight measurement of the fastest species ejected from the plasma 500 mm away from the target surface or more. The use of an aperture (double slit case) enabled obtaining a two-dimensional resolution (as the signal is integrated over x axis). A lot of (10–20) measurements are performed by tuning this aperture along y-axis by identical elementary steps. Due to axial symmetry of the plume, the experimental curves can thus be treated by Abel inversion w2x in order to reach a three-dimensional resolution. Such a calculation leads to an ensemble of S(z,r,t) curves (Fig. 2a), with rsyx2qy2. If one uses xs0.0, r values correspond to y ones from the preceding experimental measures. Fig. 2b gives an overview of the Abel calculation accuracy and opportunity: curve 1 corresponds to experimental findings at ys0.0 mm; curve 2 is the summation from curves obtained by Abel inversion (part a). Their comparison is a good test of the validity of Abel inversion. Differences in shapes between first curve of part a and curves of part b evidence the necessity of Abel inversion.
If the velocity distribution of the species can be written under the form of a maxwellian function, one can describe the emitted signal as w3x: 2 2 2z w 1 m Žzyut. qy qx |, SŽz,r,t.A 3 expxy t 2kT t2 y ~
(1)
where m is the mass of the particle, u the flow velocity, and T the translational temperature. 2.2. Results and discussions As otherwise noticed, the experimental parameters are as follows. The results concern 396 nm emission line from Al I. This line is the most intense we have detected in the visible spectral range and can be easily isolated from other emission lines. An posteriori justification for such a choice will also be given hereafter. Energy density is of 2.5=1013 Wycm2, roughly 10 times above the ablation threshold. The most probable speed vz (which corresponds to the maximum of the time resolved signal for a given z value) is measured to be 7=103 mys. A fairly good fit (Fig. 3) of the three-dimensional curves to Eq. (1) can be obtained by using a same set of parameters for all of them: Ts(5.1"0.1)=104 K (or 4.4 eV) and u'0. The essential point is that a half-range Maxwell–Boltzmann distribution has been found to be satisfactory for describing the whole volume species. This collision less-like behaviour for neutral atoms is in high contrast with well-documented TOF results, that dealt as a matter of fact with ions species w4,5x. As the fluence increases, the curves evolve toward a more complicated form that
D. Moreau et al. / Thin Solid Films 453 – 454 (2004) 340–344
342
can be accounted for by the appearance of a full range maxwellian population. Its contribution takes place at earlier delays and is necessary to a good fit of the temporal curves, even if the two contributions are not directly time-resolved on the experimental curves (Fig. 4). This second population, which proportion increases with fluence, is at the origin of the increase of the most probable speed as a function of fluence, a result that recalls published works on ions species w11x. Another characterization of the plasma expansion that relates to its angular distribution has been readily reported in the literature to be described in a parametric form. The most general expression w6,7x, using polar coordinates, corresponds to a sum over i sinusoidal functions, of power ni and weight ai: SŽq.s8aicosnq
Fig. 3. Emitted signal from successive lateral elementary volumes and their fit using (Eq. (1)) with zs1.46 mm, yi2qxi2sri2 and Ts 5.1=104 K. The most probable velocity vzs7=103 mys.
(2)
i
Fig. 2. Part (a): calculated signals after Abel inversion from successive lateral elementary volumes (steps of 0.1 mm); part (b): curve 1 corresponds to the measured emitted signal, curve 2 to the sum of the curves from the previous figure.
D. Moreau et al. / Thin Solid Films 453 – 454 (2004) 340–344
343
Fig. 4. Angular emission distribution deduced from a time integration of curves from Fig. 2a.
Fig. 5. Emitted signal from an elementary volume on axis (zs1.46 mm) at ls396 nm under 5=1013 Wycm2 fluence. The overall fit is made of two populations: a full range maxwellian population (T1s 3.9=104 K) and a half range maxwellian population (T2s1.9=104 K, u2s6000 mys).
Fig. 7. Grotrian diagram of Al I, showing the nearest levels connected to the analyzed emission lines.
Fig. 6. Emitted signals at ls396 and 669 nm from neutral Al species. The two curves have been normalized.
The angular distribution of the species (Fig. 5) has been obtained by an integration of the experimental curves. Fitted to Eq. (2), and using only one population (is1) the angular distribution of neutrals is particularly wide (with ns3"1), at the lowest used fluence. This corroborates the results obtained from the fit of time and space resolved curves at the same fluence. The fact that n value is higher than 1, as expected when us0, is in concordance with the fact that even at this fluence, a
better fit than the one depicted in Fig. 3 can be obtained by using a two population model. Emitting species kinetics is measured as a function of delay from the impact between the laser pulse and the target. The kinetics generally evidenced do exist over a temporal range that can reach several ms, much longer than the known radiative lifetimes w8x. An analysis from various emitting wavelengths is therefore of unique interest. Fig. 6 presents kinetics that correspond to 396 and 669 nm emission lines from Al I, lines which attributions w9,10x are reported in Fig. 7. (In this last figure, arrows are used to show the documented allowed transitions.) The temporal behavior of 396 and 669 nm lines is drastically different, the second emission coming
D. Moreau et al. / Thin Solid Films 453 – 454 (2004) 340–344
344
significantly later than the first one. A lot of processes can be invoked to try explaining this discrepancy. All of them w11x have as a basis, even different routes for forming excited state species (by post ablation processes), or non-equivalent delayed emissions. Radiation trapping occurs when the emission from excited atoms is absorbed and reemitted several times before being detected. As the efficiency of the process is highest for the regions of highest density, one can expect revealing this effect by comparing temporal signals from elementary volumes more or less close to z propagation axis. None of them has been found not to obey Eq. (1). Electron impact excitation necessitates ey and atoms to be proximate and cannot explain by itself such a time lag. Electron ion recombination leads to the formation of highly excited Rydberg states; these can only radiate by cascade mechanisms. We have in fact measured a continuous decrease in the integrated intensities for these two lines as a function of distance z to the target from 0.5 to 5.0 mm. We therefore deduced that 3s2 (1S)5p´3s2(1S)4s emission is not the essential way for populating 4s state. Moreover, as the 396 nm line appears first, its population cannot be a main consequence of de-excitation from the upper 5p state which gives rise to the 669 nm line. In fact, we suppose that 3s2(1S)5p and 3s2(1S)4s populations come by cascade mechanisms and by independent routes from upper Rydberg excited states. As the routinely invoked processes are not convincing at all, we propose interpreting the time lag accompanying the 669 nm emission line by its longer radiative lifetime (6=10y7 s) than the one of the 396 nm emission line (2=10y8 s). A convolution between the intrinsic atomic kinetics (Eq. (1)) and the radiative lifetime tr: S9Žz,r,t.sSŽz,r,t.Øexpy
t tr
(3)
enabled us to succeed in describing the two lines behavior by a same set of parameters. In the temporal range of interest, the convolution product affects only the 669 nm line. Emission kinetics from the 396 nm line can be therefore directly used as a basis for Al I parameters estimation. 3. Conclusions We have focused in this paper on the behavior of Al neutral species. At the opposite to findings from ns
pulse ablation and from ion species produced when using fs pulses, we have clearly evidenced the contribution to the ablation plume from a half range Maxwellian population, which we suppose to be the only one at fluences close to ablation threshold. Its contribution still exists at markedly higher energy density values. It diminishes when the energy density that impinges the target increases and a population that possesses a stream velocity, as having undergone a lot of collisions, then takes place. In fact, as compared to ions behavior where two discrete populations have been described w12x, we are obviously in the presence of a mixing from populations, which characteristics and proportions evolve continuously as a function of energy density. The spectral analysis brings several interesting aspects about the species excitation trajectory. At the opposite of OES results published on graphite w13x, Al plume is primarily made of excited atom species. Their existence has been detected at the lowest distance that we could reach with our apparatus (f0.5 mm) and the corresponding signal is still measurable at several mm from the target. References w1x E. Millon, O. Albert, J.C. Loulergue, J. Etchepare, D. Hulin, W. Seiler, J. Perriere, J. Appl. Phys. 88 (2000) 6937. w2x K. Bockasten, J. Opt. Soc. Am. 51 (1961) 943. w3x J.C.S. Kools, T.S.B. Baller, S.T. de Zwart, J. Dieleman, J. Appl. Phys. 71 (9) (1992) 4547. w4x R. Stoian, D. Ashkenasi, A. Rosenfeld, E.E.B. Campbell, Phys. Rev. B 62 (2000) 13 167. w5x M.E. Ye, C.P. Grigoropoulos, J. Appl. Phys. 89 (2001) 5183. w6x D.B. Chrisey, G.H. Hubler, Pulsed Laser Deposition of Thin Films, John Wiley and Sons, 1994. w7x F. Antoni, C. Fuchs, E. Fogarassy, Appl. Surf. Sci. 96–98 (1996) 50. w8x F. Claeyssens, R.J. Lade, K.N. Rosser, M.N.R. Ashfold, J. Appl. Phys. 89 (2001) 697. w9x W.C. Martin, J. Sugar, A. Musgrove, G.R. Dalton, W.L. Wiese, J.R. Fuhr, D.E. Kelleher, National Institute of Standards and Technology Atomic Spectra Database, NIST, GaithersburgMD, 1995. w10x A.R. Striganov, N.S. Sventitskii, Tables of Spectral Lines of Neutral and Ionized Atoms, IFIyPlenum, New York–Washington, 1968. w11x K.L. Saenger, J. Appl. Phys. 66 (1989) 4435. w12x F. Qian, V. Cracium, R.K. Singh, S.D. Dutta, P.P. Pronko, J. Appl. Phys. 86 (1999) 2281. w13x F. Claeyssens, M.N.R. Ashfold, E. Sofoulakis, C.G. Ristoscu, D. Anglos, C. Fotakis, J. Appl. Phys. 91 (2002) 6162.