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Comparison of Pd plasmas produced at 532 nm and 1064 nm by a Nd:YAG laser ablation
Nuclear Instruments and Methods in Physics Research B 268 (2010) 2285–2291
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Comparison of Pd plasmas produced at 532 nm and 1064 nm by a Nd:YAG laser ablation L. Torrisi a,b, F. Caridi c,*, L. Giuffrida a,b a
Dipartimento di Fisica, Università di Messina, Viale F. Stagno dÕAlcontres, 31, 98166 Messina, Italy INFN-Laboratori Nazionali del Sud, Via S. Sofia 44, Catania, Italy c Facoltà di Scienze MM. FF. NN., Università di Messina, Viale F. Stagno dÕAlcontres, 31, 98166 Messina, Italy b
a r t i c l e
i n f o
Article history: Received 8 January 2010 Received in revised form 16 February 2010 Available online 1 April 2010 Keywords: Palladium Pulsed laser deposition Time-of-flight Laser ablation
a b s t r a c t A comparison between the laser ablation of a palladium target in vacuum, by using 1064 nm and 532 nm Nd:YAG laser wavelengths, with an intensity of about 109 W/cm2, is reported. Nanosecond pulsed ablation produces high non-isotropic emission of neutrals and ions. For both wavelengths, mass quadrupole spectrometry and time-of-flight measurements allow estimation of the atomic and ionic species emitted from the plasma and of their energy distributions. Ions show Coulomb–Boltzmann-shifted distributions depending on their charge state. Surface profiles of the ablated craters permitted to study the ablation threshold and yields of palladium in vacuum vs. the laser fluence. The plasma temperature and density was evaluated by the experimental data. A special regard is given to the ion acceleration process occurring inside the plasma, due to the high electrical field generated inside the plasma. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The pulsed laser ablation (PLA) of metal targets at different wavelengths represents an innovative technique employed in the last years for several purposes: generation of non-equilibrium plasma with peculiar properties (high ion density, high charge state, high plasma temperature,. . .); generation of high ion energy and high directional emission; deposition of thin films assisted by ion implantation; laser ion source (LIS) and ion injection in high energy accelerators [1]; etc. PLA and LIS represent only two of many possible applications, whereas the energy distributions of neutral and ion species emitted from the laser-generated plasma become critical parameters in order to control and to optimize the application. Energetic ions, for example, permit to induce ion implantation effects in the exposed surfaces, improving the film–substrate adhesion, or to induce activation and growth of nanostructure thin films [2]. Nanosecond lasers produce PLA by means of high pulse intensity that generates hot plasma at the target surface; the plasma plume expands in vacuum at supersonic velocity along the normal to the target surface. At high laser intensity, the fast evaporative process interacts with the same laser pulse and, due to the inverse Bremsstrahlung effect, generates high charge states, high fractional ionization and plasma super-heating effects [3].
* Corresponding author. Tel.: +39 0906765120; fax: +39 0906765372. E-mail address: [email protected] (F. Caridi). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.03.029
In this work the Nd:YAG laser irradiation of a Pd target placed in vacuum, by using 1064 nm (fundamental) and 532 nm (second harmonics) wavelengths, both with an intensity of about 109 W/cm2, is reported. The obtained plasmas are investigated especially in terms of neutral and ion emissions and of the anisotropic self-generated electric field, due to the fast charge separation effects, responsible of the high directive ion acceleration. Palladium has the unusual property to absorb very high hydrogen concentrations, up to concentrations higher with respect to its atomic density. The diffusivity of the absorbed hydrogen can be controlled by the temperature. Thus the hydrogen content and reserve may be employed for many applications. For example palladium can be employed as a good catalyst and used for hydrogenation and dehydrogenation reactions or, in PLD case, the palladium target can be used as a source of intense proton beam generation. 2. Materials and methods The employed laser is a Q-switched Nd:YAG operating at 1064 nm and 532 nm wavelengths (first and second harmonics, respectively), 3 ns pulse duration, single shot or 1–10 Hz repetition rate mode. At 1064 nm the laser pulse energy can be varied in the 1–300 mJ range while at 532 nm it can be selected between 1 mJ and 150 mJ energy values. The laser beam was focused through a convergent lens on a target placed inside a vacuum chamber at 106 mbar. The optimum focalization distance (50 cm) was
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determined minimizing the spot dimension observed on the target. The spot was about 1 mm in diameter at the laser incidence angle, uinc, of 0°. Thus at the laser pulse energy of 100 mJ the corresponding laser intensity is 4 109 W/cm2. The incidence angle could be changed by the operator moving a vacuum feedthrough at which the target holder is fixed, in order to optimize the ion signals recorded by various detectors placed at different detection angles, udet. The employed target consists of a Pd sheet, with a polished 2 cm2 surface and 1 mm thickness. The target could be moved vertically with the vacuum feedthrough, so that each laser shot could hit a fresh flat surface. An ion collector (IC), consisting of a collimated Faraday cup placed along the normal to the target surface, was used for
time-of-flight (TOF) measurements at a laser incidence angle of 45° and at a flight distance L = 1.2 m. The experimental IC spectra are recorded with a fast storage oscilloscope and represented in terms of ion time distributions, which follows the following ‘‘Coulomb–Boltzmann-shifted” relationship [4]:
2 ! rffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffi!2 3 m L m 3=2 L4 ckT 2zeV 0 5 exp 4 FðtÞ ¼ A 2pkT 2kT t m m t5
ð1Þ where m is the ion mass; kT, the equivalent temperature (in eV); L, the target–detector distance; c, the adiabatic coefficient; ze, the ion
Fig. 1. Photo of the experimental set-up (a) and scheme of the IC, MQS and CCD assembly (b).
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charge and V0, the equivalent acceleration voltage developed in the non-equilibrium plasma. A mass quadrupole spectrometer (MQS, Balzer-Prisma 300), operating in the mass range 1–300 amu, with 1 amu resolution, about 1 ppm sensitivity with the use of a SEM detector, permits to analyze the partial pressure of the gas produced by the laser ablation in the vacuum chamber at a background pressure of 106 mb. At a laser incidence angle of 45°, the MQS is placed behind the target for a residual partial pressure gas analysis. A fast CCD camera (Pixefly), triggered with the laser pulse, was employed in order to photograph the plasma plume and to measure the plasma volume at 5 ls exposition time from the laser shot. At a laser incidence angle of 45°, the CCD detection angle was of 90° with respect to the expansion direction. Fig. 1 shows a photo of the experimental set-up (a) and a scheme of the IC, MQS and CCD assembly (b). Finally, a surface profiler (Tencor P-10) was employed in order to measure the crater depth profiles and shapes (with 10 nm depth resolution) and to calculate the ablation yield in terms of removed mass obtained for a single laser shot. Fig. 2. The ablation yield, given in terms of amount of removed atoms per laser pulse, as a function of the laser fluence.
Fig. 3. The plasma volume, calculated from the fast CCD camera images at 5 ls exposition time from the laser shot, as a function of the laser energy (a) and the exponential decrease of the plasma atomic density with the laser energy at 5 ls expansion time (b).
Fig. 4. MQS spectra comparison obtained for a laser ablation at 100 mJ laser energy, 1 Hz repetition rate, 1064 nm (a) and 532 nm (b) wavelengths for the mass range 1– 20 amu, 100–115 amu and 210–215 amu.
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3. Results The palladium ablation yield was calculated through the volume of the ablated craters measured with their depth profiles. The inset of Fig. 2 (top) shows a typical depth profile of a Pd crater produced by 532 nm, 50 mJ, 30 laser shot irradiation, at 0° laser incidence angle.
The depth profiles, performed along two orthogonal directions along the crater diameter, permit to calculate accurately the crater volume. Consequently, the ablation yield, given in terms of amount of removed atoms per laser pulse, is calculable through the Pd density (12 g/cm3), as reported in Fig. 2 vs. the laser fluence. The ablation yield at 1064 nm is higher with respect to 532 nm, in agreement with the higher absorption coefficient of free electrons
Fig. 5. IC time-of-flight spectra comparison recorded at 1064 nm, 50 mJ (a) and 100 mJ (b) and at 532 nm, 50 mJ (c) and 100 mJ (d).
Fig. 6. Fit of the IC TOF spectra recorded at 1064 nm, 50 mJ (a) and 100 mJ (b) and at 532 nm, 50 mJ (c) and 100 mJ (d) reporting the ion equivalent temperature evaluation.
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in the IR region. Moreover, these results show the existence of an ablation threshold, below which no ablation occurs and above which the ablation yield increases linearly with the laser fluence. The experimental ablation thresholds, obtained by the crater profiles, are 0.75 J/cm2 at 532 nm and 0.9 J/cm2 at 1064 nm, in good agreement with the classic ablative model proposed by Torrisi et al. [5], that gives a theoretical threshold of 1.1 J/cm2 for both wavelengths. Fig. 3a shows the plasma volume, calculated from the fast CCD camera images at 5 ls exposition time from the laser shot, as a function of the laser energy. The ellipsoidal volume increases linearly for both wavelengths and, at a fixed laser energy, it is higher at 1064 nm, as can be evaluated from the CCD camera images reported as an inset. The number of atoms removed from the crater for laser shot and the visible CCD plasma volume permits to evaluate the plasma atomic density. This density is very high just after the laser pulse, higher than 1017/cm3 at the used experimental conditions [6], and decreases exponentially with the time due to the supersonic plasma expansion process. In the first instants of the plasma formation, the plasma density increases with the laser pulse energy. However for long time, of the order of some microseconds, the high laser energy increases the plasma temperature and consequently the plasma expansion velocity and, as a result,
a decrement of the atomic plasma density may occurs with the laser energy. Fig. 3b shows the exponential decrease of the plasma atomic density with the laser energy at 5 ls exposition time from the laser shot. This density was evaluated from the ablated mass reported in Fig. 2 and from the plasma volume calculated from the fast CCD camera images and reported in Fig. 3a. The faster decreasing of the atomic plasma density with the laser pulse energy at 532 nm with respect to 1064 nm is due to the higher equivalent temperature induced at this wavelength, as will be demonstrated in the following. Fig. 4 show a comparison between the MQS spectra obtained for a laser ablation at 100 mJ laser energy, 1 Hz repetition rate, at 1064 nm (a) and 532 nm (b) wavelengths. For the MQS position in the vacuum chamber, the MQS signal comes only from neutral species. It is possible to observe that during the laser ablation process, subtracting the background signal, only hydrogen (2 amu) and oxygen (16 amu) peaks appeared in the mass range 1–20 amu. The Pd neutral was detected as monoatomic (106 amu), as compounds with hydrogen (PdH5 at 111 amu and PdH6 at 112 amu) and biatomic molecules (Pd2 at 212 amu), both at 1064 nm and 532 nm laser wavelength irradiations. It is possible to note that
Fig. 7. Normalized Pd TOF ion yield vs. ion charge state obtained at 50 mJ (a) and 100 mJ (b) for both wavelengths and comparison with the Pd ionization crosssections calculated according to Lotz theory.
Fig. 8. Comparison of the ion peak energy vs. charge state, for the two wavelengths, as a function of the ion charge state, at 50 mJ (a) and at 100 mJ (b).
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this last Pd detection (inset II) is much lower (8 pA) than the other ones (inset I). The inset III reports a typical shape of the Pd detection (106 amu) as a function of the time for a single laser shot. It is possible to observe a fast increase of the peak at the laser shot and a slow decreasing due to the pump system velocity. Moreover, the spectra comparison at the two wavelengths shows that the Pd neutral detection is about an order of magnitude higher than at 1064 nm, in good agreement with the ablation yield results reported in Fig. 2. In order to evaluate the plasma ion temperature, ion TOF measurements were developed as described in the following. Fig. 5 reports the IC time-of-flight spectra recorded at 50 mJ (a, c) and 100 mJ (b, d) and at 1064 nm and 532 nm, respectively. The TOF start time at t = 0 is given by the laser photopeak. These experimental spectra were fitted though Eq. (1) giving as parameter the amplitude A, the equivalent temperature kT and the V0 acceleration voltage developed inside the expanding plasma. The spectrum shown in Fig. 6a, as an example, represents the convolution of all ions arriving at the IC detector at 1064 nm and 50 mJ, which contribution can be evaluated by a deconvolution process based on the Coulomb–Boltzmann-shifted time function. The deconvolution was obtained with the following charge states: m+ (with m ranging beH+; Onþ 2 (with n ranging between 1 and 2); O tween 1 and 3); Pdp+ (with p ranging between 1 and 4), in agreement with the ionization potentials of the elements below about 60 eV. This last energy value was obtained through a comparison between the mean hydrogen ions energy, measured by TOF, and the ionization potentials of the detected ion species, as reported in the inset. The measured mean hydrogen ion energy, EH, is 57 eV. The fit process obtained with the Coulomb–Boltzmannshifted time function (whose analytical expression contains the fit parameter m/2pkT) gives an equivalent ion plasma temperature, kT, of 15 eV and a regular ion energy shift towards higher energy indicating a mean value of the accelerating voltage, V0, developed inside the plasma, of 22 V/charge state [6]. In this case the mean ion charge state, hzi, is 1.39. Fig. 6b shows, the results obtained ablating Pd at 1064 nm and 100 mJ. This fit, performed with H, O and Pd ions, gives an equivalent ion plasma temperature and accelerating voltage of 17 eV and 38 V, respectively. In this case the mean ion charge state is 1.49 and the mean H+ ion energy is 78 eV, as reported in the inset. Thus a significant increment of the temperature is obtained by the laser pulse increase. The procedure of ion time distribution fit of experimental TOF spectra was repeated for the other employed experimental conditions, obtained by varying the laser wavelength and pulse energy. The TOF fit of palladium ablated at 532 nm and 50 mJ is reported in Fig. 6c. It gives an equivalent ion plasma temperature and accelerating voltage of 16.5 eV and 25 V, respectively, a mean ion charge state of 1.55 and a mean H+ ion energy of 63 eV, as reported in the inset. The TOF fit of palladium ablated at 532 nm and 100 mJ is reported in Fig. 6d. It gives an equivalent ion plasma temperature and accelerating voltage of 20 eV and 43 V, respectively, a mean ion charge state of 1.57 and a mean H+ ion energy of 90 eV. Thus data elaboration confirms that the equivalent plasma temperature is higher at 532 nm with respect to 1064 nm. Moreover obtained data indicate that the visible radiation increases the equivalent acceleration voltage, the mean charge state, the mean proton energy and the total ion yield emission with respect to the infrared radiation. Only the neutral yield emission appears higher at 1064 nm with respect to the second harmonics. The TOF data fits have been performed taking in consideration that the used ion charge state distributions should be in agreement with the trend of the electron ionization cross-section of palladium. To this reason the comparisons between the normalized
ion charge state distributions and the ionization cross-sections based on Lotz theory [7] have been performed in Fig. 7. The cross-sections were calculated using the experimental mean electron energy, Ee, given by the best temperature fit (Ee = 3kT/2) assuming the plasma to be in near local thermal equilibrium (NTLE) (Te Ti). Fig. 7 shows the comparison of the Pd TOF ion data elaboration at 50 mJ (a) and 100 mJ (b) for both wavelengths. The experimental distribution of the ion yield for the 1064 nm (squares), 532 nm (circles) and the calculated electron ionization cross-section (Lotz theory) show a good agreement. The ion yield is higher at 532 nm, in good agreement with the mean ion charge state reported in Fig. 6 that is higher for this wavelength. Finally, Fig. 8 reports that the ion peak energy, for the two wavelengths, increases linearly with the ion charge state, at 50 mJ (a) and at 100 mJ (b). This trend is measured by the regular energy shift between the ion charge states evaluated though the equivalent voltage V0. At 532 nm the peak energy is higher with respect to that at 1064 nm, however the two peak energies tend to be comparable increasing the laser pulse energy.
4. Discussion and conclusions Literature does not report many data about the comparison between the Pd laser ablation at 109 W/cm2 intensity, in vacuum, by using 1064 nm and 532 nm Nd:YAG laser wavelengths. This comparison is possible by evaluating the ablation yield, the mass quadrupole spectrometer analysis and the IC time-of-flight spectrometry. Many differences occur between the Pd plasma produced at 1064 nm and 532 nm. The first difference can be observed in Fig. 2, where the different ablation yield vs. laser fluence at the two wavelengths is showed. By using 5 J/cm2 laser fluence the ablation yield at 1064 nm is about 4.2 1015 Pd atoms/pulse, while it decreases of about a factor seventeen at 532 nm, demonstrating that the IR radiation is more efficient for the ablation process with respect to the visible one. This first result can be explained on the basis of the high absorption coefficient of IR in metals. Another observation concerns the differences at the two wavelengths. By using IR radiation the ablation threshold is 0.9 J/cm2, about a factor 1.2 higher than the visible one, as a consequence of the higher penetration depth. Fig. 3 shows the plasma volume, measured with 5 ls exposition time and 100 mJ laser pulse energy vs. the laser energy at the two wavelengths. The plasma volume measured at 1064 nm in such conditions is about 1 cm3, while that one measured at 532 nm is about a factor five lower. Such significant difference can be due to a different angular distribution of the ejected particles from the target surface. In agreement with the literature [8], the IR pulse radiation produces an angular distribution larger than the visible one. Moreover the higher ablation yield induced by IR radiation may produce higher particles interactions and scattering, that influence the total CCD image of the plasma volume. The measurements of the plasma density, obtained calculating the number of removed particles with respect to the plasma volume and measured at 5 ls exposition time vs. laser energy, indicate that the density is very similar for the two wavelengths at low laser energy (50 mJ). At higher values (100 mJ) the plasma density at 1064 nm is about a factor 1.5 higher than the 532 nm one. Fig. 4 compares the MQS spectra of Pd ablation obtained at 1064 nm and 532 nm. The comparison shows that the IR radiation produces H2, O, Pd, Pd2 and PdHx compounds emission higher than the visible one, in agreement with the ablation yield measurements. Fig. 6 reports the ion yield vs. the time-of-flight, showing the possible deconvolution process of the acquired spectra, obtained at the two wavelengths with different laser pulse energies. The IR
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radiation at 100 mJ produces a Pd plasma equivalent temperature, acceleration voltage and average charge state lower than those ones produced with the visible radiation. It means that higher kinetic energy is transferred to the particles (neutrals and ions) due to the second harmonic radiation. This result is in agreement with the ablation yield values. In fact the 1064 nm radiation transfers a low kinetic energy to the free plasma particles. The same figure shows that also the TOF proton peak is characterized by a higher velocity using the visible radiation with respect to the IR one. This last result is better underlined in the comparisons of Fig. 8, reporting the ion peak energy of the TOF spectra vs. the ion charge states, for Pd plasma generated at 50 mJ and 100 mJ for both wavelengths. The results obtained in this work are of special interest concerning some aspects of the laser-generated Pd plasma characterization. The 532 nm seems to be more effective of the fundamental in terms of plasma equivalent temperature, equivalent acceleration voltage and mean charge state. The 1064 nm radiation, instead, seems to be more effective of the second harmonics in terms of total ion removing (ablation yield) and atomic plasma density at times of the order of some microseconds. The high ion velocity along the normal to the target surface indicates that ions are submitted to a Coulomb acceleration due to a high electrical field self-generated in the non-equilibrium plasma [9]. This electric field is higher at 532 nm with respect to 1064 nm, as demonstrated by the measurements of proton energy delivered by the Pd target. The calculation of the electric field is not simple because many parameters influence it, such as the laser irradiation conditions, the used electron temperature and density and the NLTE conditions. Moreover, the temperature and the density depend strongly on
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the time and on the space and, consequently, the Debye length too [10]. The most confirmed model supposes that the electric field is due to a space–charge separation between the fast electrons and the slow ion clouds, respectively, ejected from the target along the normal direction [11]. Further investigations are in progress to evaluate in detail this possible kinetic mechanism of the electric field generation. Acknowledgements Authors thank the INFN-Commission V for the useful support given to this research. A special thank to Prof. L. Catani and Prof. A. Valentini, Referees of the PLEIADI INFN-Project in the ambit of which this work is developed. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
S. Gammino, L. Torrisi, G. Ciavola, et al., J. Appl. Phys. 96 (5) (2004) 2961. M. Nesladek, Semic. Sci. Tech. 20 (2005) R19–R27. D. Giulietti, L.A. Gizzi, Rivista. Nuovo. Cimento. 21 (10) (1998) 1–93. F. Caridi, L. Torrisi, L. Giuffrida, Nucl. Instr. and Meth. in Phys. Res. B 268 (2010) 499–505. L. Torrisi, A. Borrielli, D. Margarone, Nucl. Instr. and Meth. B 255 (2007) 373–379. D.H.H. Hofmann, R. Bock, A.Y. Faenov, et al., Nucl. Instr. Meth. B 9 (18) (2000) 161–163. G.D. Shirkov, G. Zschornack, Electron Impact Ion Sources for Charged Heavy Ions, Vieweg, 1996. L. Torrisi, F. Caridi, A. Picciotto, A. Borrielli, Nucl. Instr. Meth. B 252 (2006) 183– 189. V. Nassisi, A. Pedone, Rev. Sci. Instr. 7 (1) (2003) 68. L.C. Chen, in: D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. F. Caridi, L. Torrisi, D. Margarone, A. Borrielli, Laser Part. Beams 26 (2008) 265– 271.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Comparison of Pd plasmas produced at 532 nm and 1064 nm by a Nd:YAG laser ablation L. Torrisi a,b, F. Caridi c,*, L. Giuffrida a,b a
Dipartimento di Fisica, Università di Messina, Viale F. Stagno dÕAlcontres, 31, 98166 Messina, Italy INFN-Laboratori Nazionali del Sud, Via S. Sofia 44, Catania, Italy c Facoltà di Scienze MM. FF. NN., Università di Messina, Viale F. Stagno dÕAlcontres, 31, 98166 Messina, Italy b
a r t i c l e
i n f o
Article history: Received 8 January 2010 Received in revised form 16 February 2010 Available online 1 April 2010 Keywords: Palladium Pulsed laser deposition Time-of-flight Laser ablation
a b s t r a c t A comparison between the laser ablation of a palladium target in vacuum, by using 1064 nm and 532 nm Nd:YAG laser wavelengths, with an intensity of about 109 W/cm2, is reported. Nanosecond pulsed ablation produces high non-isotropic emission of neutrals and ions. For both wavelengths, mass quadrupole spectrometry and time-of-flight measurements allow estimation of the atomic and ionic species emitted from the plasma and of their energy distributions. Ions show Coulomb–Boltzmann-shifted distributions depending on their charge state. Surface profiles of the ablated craters permitted to study the ablation threshold and yields of palladium in vacuum vs. the laser fluence. The plasma temperature and density was evaluated by the experimental data. A special regard is given to the ion acceleration process occurring inside the plasma, due to the high electrical field generated inside the plasma. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The pulsed laser ablation (PLA) of metal targets at different wavelengths represents an innovative technique employed in the last years for several purposes: generation of non-equilibrium plasma with peculiar properties (high ion density, high charge state, high plasma temperature,. . .); generation of high ion energy and high directional emission; deposition of thin films assisted by ion implantation; laser ion source (LIS) and ion injection in high energy accelerators [1]; etc. PLA and LIS represent only two of many possible applications, whereas the energy distributions of neutral and ion species emitted from the laser-generated plasma become critical parameters in order to control and to optimize the application. Energetic ions, for example, permit to induce ion implantation effects in the exposed surfaces, improving the film–substrate adhesion, or to induce activation and growth of nanostructure thin films [2]. Nanosecond lasers produce PLA by means of high pulse intensity that generates hot plasma at the target surface; the plasma plume expands in vacuum at supersonic velocity along the normal to the target surface. At high laser intensity, the fast evaporative process interacts with the same laser pulse and, due to the inverse Bremsstrahlung effect, generates high charge states, high fractional ionization and plasma super-heating effects [3].
* Corresponding author. Tel.: +39 0906765120; fax: +39 0906765372. E-mail address: [email protected] (F. Caridi). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.03.029
In this work the Nd:YAG laser irradiation of a Pd target placed in vacuum, by using 1064 nm (fundamental) and 532 nm (second harmonics) wavelengths, both with an intensity of about 109 W/cm2, is reported. The obtained plasmas are investigated especially in terms of neutral and ion emissions and of the anisotropic self-generated electric field, due to the fast charge separation effects, responsible of the high directive ion acceleration. Palladium has the unusual property to absorb very high hydrogen concentrations, up to concentrations higher with respect to its atomic density. The diffusivity of the absorbed hydrogen can be controlled by the temperature. Thus the hydrogen content and reserve may be employed for many applications. For example palladium can be employed as a good catalyst and used for hydrogenation and dehydrogenation reactions or, in PLD case, the palladium target can be used as a source of intense proton beam generation. 2. Materials and methods The employed laser is a Q-switched Nd:YAG operating at 1064 nm and 532 nm wavelengths (first and second harmonics, respectively), 3 ns pulse duration, single shot or 1–10 Hz repetition rate mode. At 1064 nm the laser pulse energy can be varied in the 1–300 mJ range while at 532 nm it can be selected between 1 mJ and 150 mJ energy values. The laser beam was focused through a convergent lens on a target placed inside a vacuum chamber at 106 mbar. The optimum focalization distance (50 cm) was
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determined minimizing the spot dimension observed on the target. The spot was about 1 mm in diameter at the laser incidence angle, uinc, of 0°. Thus at the laser pulse energy of 100 mJ the corresponding laser intensity is 4 109 W/cm2. The incidence angle could be changed by the operator moving a vacuum feedthrough at which the target holder is fixed, in order to optimize the ion signals recorded by various detectors placed at different detection angles, udet. The employed target consists of a Pd sheet, with a polished 2 cm2 surface and 1 mm thickness. The target could be moved vertically with the vacuum feedthrough, so that each laser shot could hit a fresh flat surface. An ion collector (IC), consisting of a collimated Faraday cup placed along the normal to the target surface, was used for
time-of-flight (TOF) measurements at a laser incidence angle of 45° and at a flight distance L = 1.2 m. The experimental IC spectra are recorded with a fast storage oscilloscope and represented in terms of ion time distributions, which follows the following ‘‘Coulomb–Boltzmann-shifted” relationship [4]:
2 ! rffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffi!2 3 m L m 3=2 L4 ckT 2zeV 0 5 exp 4 FðtÞ ¼ A 2pkT 2kT t m m t5
ð1Þ where m is the ion mass; kT, the equivalent temperature (in eV); L, the target–detector distance; c, the adiabatic coefficient; ze, the ion
Fig. 1. Photo of the experimental set-up (a) and scheme of the IC, MQS and CCD assembly (b).
L. Torrisi et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2285–2291
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charge and V0, the equivalent acceleration voltage developed in the non-equilibrium plasma. A mass quadrupole spectrometer (MQS, Balzer-Prisma 300), operating in the mass range 1–300 amu, with 1 amu resolution, about 1 ppm sensitivity with the use of a SEM detector, permits to analyze the partial pressure of the gas produced by the laser ablation in the vacuum chamber at a background pressure of 106 mb. At a laser incidence angle of 45°, the MQS is placed behind the target for a residual partial pressure gas analysis. A fast CCD camera (Pixefly), triggered with the laser pulse, was employed in order to photograph the plasma plume and to measure the plasma volume at 5 ls exposition time from the laser shot. At a laser incidence angle of 45°, the CCD detection angle was of 90° with respect to the expansion direction. Fig. 1 shows a photo of the experimental set-up (a) and a scheme of the IC, MQS and CCD assembly (b). Finally, a surface profiler (Tencor P-10) was employed in order to measure the crater depth profiles and shapes (with 10 nm depth resolution) and to calculate the ablation yield in terms of removed mass obtained for a single laser shot. Fig. 2. The ablation yield, given in terms of amount of removed atoms per laser pulse, as a function of the laser fluence.
Fig. 3. The plasma volume, calculated from the fast CCD camera images at 5 ls exposition time from the laser shot, as a function of the laser energy (a) and the exponential decrease of the plasma atomic density with the laser energy at 5 ls expansion time (b).
Fig. 4. MQS spectra comparison obtained for a laser ablation at 100 mJ laser energy, 1 Hz repetition rate, 1064 nm (a) and 532 nm (b) wavelengths for the mass range 1– 20 amu, 100–115 amu and 210–215 amu.
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3. Results The palladium ablation yield was calculated through the volume of the ablated craters measured with their depth profiles. The inset of Fig. 2 (top) shows a typical depth profile of a Pd crater produced by 532 nm, 50 mJ, 30 laser shot irradiation, at 0° laser incidence angle.
The depth profiles, performed along two orthogonal directions along the crater diameter, permit to calculate accurately the crater volume. Consequently, the ablation yield, given in terms of amount of removed atoms per laser pulse, is calculable through the Pd density (12 g/cm3), as reported in Fig. 2 vs. the laser fluence. The ablation yield at 1064 nm is higher with respect to 532 nm, in agreement with the higher absorption coefficient of free electrons
Fig. 5. IC time-of-flight spectra comparison recorded at 1064 nm, 50 mJ (a) and 100 mJ (b) and at 532 nm, 50 mJ (c) and 100 mJ (d).
Fig. 6. Fit of the IC TOF spectra recorded at 1064 nm, 50 mJ (a) and 100 mJ (b) and at 532 nm, 50 mJ (c) and 100 mJ (d) reporting the ion equivalent temperature evaluation.
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in the IR region. Moreover, these results show the existence of an ablation threshold, below which no ablation occurs and above which the ablation yield increases linearly with the laser fluence. The experimental ablation thresholds, obtained by the crater profiles, are 0.75 J/cm2 at 532 nm and 0.9 J/cm2 at 1064 nm, in good agreement with the classic ablative model proposed by Torrisi et al. [5], that gives a theoretical threshold of 1.1 J/cm2 for both wavelengths. Fig. 3a shows the plasma volume, calculated from the fast CCD camera images at 5 ls exposition time from the laser shot, as a function of the laser energy. The ellipsoidal volume increases linearly for both wavelengths and, at a fixed laser energy, it is higher at 1064 nm, as can be evaluated from the CCD camera images reported as an inset. The number of atoms removed from the crater for laser shot and the visible CCD plasma volume permits to evaluate the plasma atomic density. This density is very high just after the laser pulse, higher than 1017/cm3 at the used experimental conditions [6], and decreases exponentially with the time due to the supersonic plasma expansion process. In the first instants of the plasma formation, the plasma density increases with the laser pulse energy. However for long time, of the order of some microseconds, the high laser energy increases the plasma temperature and consequently the plasma expansion velocity and, as a result,
a decrement of the atomic plasma density may occurs with the laser energy. Fig. 3b shows the exponential decrease of the plasma atomic density with the laser energy at 5 ls exposition time from the laser shot. This density was evaluated from the ablated mass reported in Fig. 2 and from the plasma volume calculated from the fast CCD camera images and reported in Fig. 3a. The faster decreasing of the atomic plasma density with the laser pulse energy at 532 nm with respect to 1064 nm is due to the higher equivalent temperature induced at this wavelength, as will be demonstrated in the following. Fig. 4 show a comparison between the MQS spectra obtained for a laser ablation at 100 mJ laser energy, 1 Hz repetition rate, at 1064 nm (a) and 532 nm (b) wavelengths. For the MQS position in the vacuum chamber, the MQS signal comes only from neutral species. It is possible to observe that during the laser ablation process, subtracting the background signal, only hydrogen (2 amu) and oxygen (16 amu) peaks appeared in the mass range 1–20 amu. The Pd neutral was detected as monoatomic (106 amu), as compounds with hydrogen (PdH5 at 111 amu and PdH6 at 112 amu) and biatomic molecules (Pd2 at 212 amu), both at 1064 nm and 532 nm laser wavelength irradiations. It is possible to note that
Fig. 7. Normalized Pd TOF ion yield vs. ion charge state obtained at 50 mJ (a) and 100 mJ (b) for both wavelengths and comparison with the Pd ionization crosssections calculated according to Lotz theory.
Fig. 8. Comparison of the ion peak energy vs. charge state, for the two wavelengths, as a function of the ion charge state, at 50 mJ (a) and at 100 mJ (b).
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this last Pd detection (inset II) is much lower (8 pA) than the other ones (inset I). The inset III reports a typical shape of the Pd detection (106 amu) as a function of the time for a single laser shot. It is possible to observe a fast increase of the peak at the laser shot and a slow decreasing due to the pump system velocity. Moreover, the spectra comparison at the two wavelengths shows that the Pd neutral detection is about an order of magnitude higher than at 1064 nm, in good agreement with the ablation yield results reported in Fig. 2. In order to evaluate the plasma ion temperature, ion TOF measurements were developed as described in the following. Fig. 5 reports the IC time-of-flight spectra recorded at 50 mJ (a, c) and 100 mJ (b, d) and at 1064 nm and 532 nm, respectively. The TOF start time at t = 0 is given by the laser photopeak. These experimental spectra were fitted though Eq. (1) giving as parameter the amplitude A, the equivalent temperature kT and the V0 acceleration voltage developed inside the expanding plasma. The spectrum shown in Fig. 6a, as an example, represents the convolution of all ions arriving at the IC detector at 1064 nm and 50 mJ, which contribution can be evaluated by a deconvolution process based on the Coulomb–Boltzmann-shifted time function. The deconvolution was obtained with the following charge states: m+ (with m ranging beH+; Onþ 2 (with n ranging between 1 and 2); O tween 1 and 3); Pdp+ (with p ranging between 1 and 4), in agreement with the ionization potentials of the elements below about 60 eV. This last energy value was obtained through a comparison between the mean hydrogen ions energy, measured by TOF, and the ionization potentials of the detected ion species, as reported in the inset. The measured mean hydrogen ion energy, EH, is 57 eV. The fit process obtained with the Coulomb–Boltzmannshifted time function (whose analytical expression contains the fit parameter m/2pkT) gives an equivalent ion plasma temperature, kT, of 15 eV and a regular ion energy shift towards higher energy indicating a mean value of the accelerating voltage, V0, developed inside the plasma, of 22 V/charge state [6]. In this case the mean ion charge state, hzi, is 1.39. Fig. 6b shows, the results obtained ablating Pd at 1064 nm and 100 mJ. This fit, performed with H, O and Pd ions, gives an equivalent ion plasma temperature and accelerating voltage of 17 eV and 38 V, respectively. In this case the mean ion charge state is 1.49 and the mean H+ ion energy is 78 eV, as reported in the inset. Thus a significant increment of the temperature is obtained by the laser pulse increase. The procedure of ion time distribution fit of experimental TOF spectra was repeated for the other employed experimental conditions, obtained by varying the laser wavelength and pulse energy. The TOF fit of palladium ablated at 532 nm and 50 mJ is reported in Fig. 6c. It gives an equivalent ion plasma temperature and accelerating voltage of 16.5 eV and 25 V, respectively, a mean ion charge state of 1.55 and a mean H+ ion energy of 63 eV, as reported in the inset. The TOF fit of palladium ablated at 532 nm and 100 mJ is reported in Fig. 6d. It gives an equivalent ion plasma temperature and accelerating voltage of 20 eV and 43 V, respectively, a mean ion charge state of 1.57 and a mean H+ ion energy of 90 eV. Thus data elaboration confirms that the equivalent plasma temperature is higher at 532 nm with respect to 1064 nm. Moreover obtained data indicate that the visible radiation increases the equivalent acceleration voltage, the mean charge state, the mean proton energy and the total ion yield emission with respect to the infrared radiation. Only the neutral yield emission appears higher at 1064 nm with respect to the second harmonics. The TOF data fits have been performed taking in consideration that the used ion charge state distributions should be in agreement with the trend of the electron ionization cross-section of palladium. To this reason the comparisons between the normalized
ion charge state distributions and the ionization cross-sections based on Lotz theory [7] have been performed in Fig. 7. The cross-sections were calculated using the experimental mean electron energy, Ee, given by the best temperature fit (Ee = 3kT/2) assuming the plasma to be in near local thermal equilibrium (NTLE) (Te Ti). Fig. 7 shows the comparison of the Pd TOF ion data elaboration at 50 mJ (a) and 100 mJ (b) for both wavelengths. The experimental distribution of the ion yield for the 1064 nm (squares), 532 nm (circles) and the calculated electron ionization cross-section (Lotz theory) show a good agreement. The ion yield is higher at 532 nm, in good agreement with the mean ion charge state reported in Fig. 6 that is higher for this wavelength. Finally, Fig. 8 reports that the ion peak energy, for the two wavelengths, increases linearly with the ion charge state, at 50 mJ (a) and at 100 mJ (b). This trend is measured by the regular energy shift between the ion charge states evaluated though the equivalent voltage V0. At 532 nm the peak energy is higher with respect to that at 1064 nm, however the two peak energies tend to be comparable increasing the laser pulse energy.
4. Discussion and conclusions Literature does not report many data about the comparison between the Pd laser ablation at 109 W/cm2 intensity, in vacuum, by using 1064 nm and 532 nm Nd:YAG laser wavelengths. This comparison is possible by evaluating the ablation yield, the mass quadrupole spectrometer analysis and the IC time-of-flight spectrometry. Many differences occur between the Pd plasma produced at 1064 nm and 532 nm. The first difference can be observed in Fig. 2, where the different ablation yield vs. laser fluence at the two wavelengths is showed. By using 5 J/cm2 laser fluence the ablation yield at 1064 nm is about 4.2 1015 Pd atoms/pulse, while it decreases of about a factor seventeen at 532 nm, demonstrating that the IR radiation is more efficient for the ablation process with respect to the visible one. This first result can be explained on the basis of the high absorption coefficient of IR in metals. Another observation concerns the differences at the two wavelengths. By using IR radiation the ablation threshold is 0.9 J/cm2, about a factor 1.2 higher than the visible one, as a consequence of the higher penetration depth. Fig. 3 shows the plasma volume, measured with 5 ls exposition time and 100 mJ laser pulse energy vs. the laser energy at the two wavelengths. The plasma volume measured at 1064 nm in such conditions is about 1 cm3, while that one measured at 532 nm is about a factor five lower. Such significant difference can be due to a different angular distribution of the ejected particles from the target surface. In agreement with the literature [8], the IR pulse radiation produces an angular distribution larger than the visible one. Moreover the higher ablation yield induced by IR radiation may produce higher particles interactions and scattering, that influence the total CCD image of the plasma volume. The measurements of the plasma density, obtained calculating the number of removed particles with respect to the plasma volume and measured at 5 ls exposition time vs. laser energy, indicate that the density is very similar for the two wavelengths at low laser energy (50 mJ). At higher values (100 mJ) the plasma density at 1064 nm is about a factor 1.5 higher than the 532 nm one. Fig. 4 compares the MQS spectra of Pd ablation obtained at 1064 nm and 532 nm. The comparison shows that the IR radiation produces H2, O, Pd, Pd2 and PdHx compounds emission higher than the visible one, in agreement with the ablation yield measurements. Fig. 6 reports the ion yield vs. the time-of-flight, showing the possible deconvolution process of the acquired spectra, obtained at the two wavelengths with different laser pulse energies. The IR
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radiation at 100 mJ produces a Pd plasma equivalent temperature, acceleration voltage and average charge state lower than those ones produced with the visible radiation. It means that higher kinetic energy is transferred to the particles (neutrals and ions) due to the second harmonic radiation. This result is in agreement with the ablation yield values. In fact the 1064 nm radiation transfers a low kinetic energy to the free plasma particles. The same figure shows that also the TOF proton peak is characterized by a higher velocity using the visible radiation with respect to the IR one. This last result is better underlined in the comparisons of Fig. 8, reporting the ion peak energy of the TOF spectra vs. the ion charge states, for Pd plasma generated at 50 mJ and 100 mJ for both wavelengths. The results obtained in this work are of special interest concerning some aspects of the laser-generated Pd plasma characterization. The 532 nm seems to be more effective of the fundamental in terms of plasma equivalent temperature, equivalent acceleration voltage and mean charge state. The 1064 nm radiation, instead, seems to be more effective of the second harmonics in terms of total ion removing (ablation yield) and atomic plasma density at times of the order of some microseconds. The high ion velocity along the normal to the target surface indicates that ions are submitted to a Coulomb acceleration due to a high electrical field self-generated in the non-equilibrium plasma [9]. This electric field is higher at 532 nm with respect to 1064 nm, as demonstrated by the measurements of proton energy delivered by the Pd target. The calculation of the electric field is not simple because many parameters influence it, such as the laser irradiation conditions, the used electron temperature and density and the NLTE conditions. Moreover, the temperature and the density depend strongly on
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the time and on the space and, consequently, the Debye length too [10]. The most confirmed model supposes that the electric field is due to a space–charge separation between the fast electrons and the slow ion clouds, respectively, ejected from the target along the normal direction [11]. Further investigations are in progress to evaluate in detail this possible kinetic mechanism of the electric field generation. Acknowledgements Authors thank the INFN-Commission V for the useful support given to this research. A special thank to Prof. L. Catani and Prof. A. Valentini, Referees of the PLEIADI INFN-Project in the ambit of which this work is developed. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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