Ion Kinetic Energy Distributions and Mechanisms of Pulsed Laser

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J. Phys. Chem. C 2008, 112, 16556–16560

Ion Kinetic Energy Distributions and Mechanisms of Pulsed Laser Ablation on Al† Jon I. Apin˜a´niz, Borja Sierra, Roberto Martı´nez, Asier Longarte, Carolina Redondo, and Fernando Castan˜o* Departamento de Quı´mica Fı´sica, Facultad de Ciencia y Tecnologı´a, UniVersidad del Paı´s Vasco, Apdo. 644. 48940 Leioa, and Departamento de Quı´mica Fı´sica, Facultad de Farmacia, UniVersidad del Paı´s Vasco, Apdo. 450. 01006 Vitoria-Gasteiz, Spain ReceiVed: June 25, 2008; ReVised Manuscript ReceiVed: July 23, 2008

A detailed investigation on the ion kinetic energy distributions of ions ejected in the nanosecond pulsed laser ablation of aluminum is reported. For laser fluences just over threshold, the emerging ions fit shifted neat Maxwell-Boltzmann-Coulomb (MBC) distributions. For fluences higher than ∼1.3 J/cm2, the Al+ distributions split into two MBC contributions peaked at different energies. It is demonstrated that the observed Al+ ion distribution has two components, one fast, correlated with the direct multiphoton laser ionization, and the other slow, associated with electron-Al0 collisions in the solid. A similar behavior is observed at higher fluences for all Al ion distributions indicating that the electron-impact ionization of Al rate constants is faster than that of recombination and other possible collision channels. In addition, the linear relationship between the Coulomb velocities and the ion charges and the behavior of Coulomb energy of the ions versus the laser fluence suggest the appearance of an electric field within the metal/laser interaction volume that impels the ions up to the high velocities measured. A discussion of the application of this type of mechanisms to other metals is advanced. 1. Introduction The models to explain the properties of metals and also the linear interactions of their surface with electromagnetic radiation were great and hallmark successes of early 20th century physics.1 Linear interactions of electromagnetic radiation with metals were of much concern at the beginning of the 20th century. When a photon with an energy (hν) higher than a metal “work function” (W) impinges on its surface, the energy excess (hν - W) is quantitatively allocated as “averaged” kinetic energy on the emerging electrons.2 It is worth noting that the metal work function itself is not a universal constant but has a distribution with first and second moments.3 The advent of highpower lasers permits one to study nonlinear and multiphoton absorption effects and has enhanced the fundamental interest in the radiation/metal interactions, adding a number of new applications in technological and scientific fields. In spite of the large number of applications of metal ablation,4 practitioners in the field lack a quantitative theory to design their experiments, and it is often substituted by rules of thumb that permit a sensible approach to practical problems. In this work, a model grounded on basic physical principles is applied to understand the multifaceted nonlinear high energy radiation/ metal surface interactions.5 Aluminum has been chosen in this study to explore the intricacies and resources of its ablation. To pave the way, singular attention is given to the species that by and large draw out most of the energy of the process, the ions, and their kinetic energy distributions. The analysis of these distributions has led to consideration of as yet unexplored processes in ablation, such as electron-ion collisions leading to ionization5 (in Al and/or recombination in other metals) and the buildup of a positively charged electrostatic field in the * Corresponding author. Fernando Castan˜o, Departamento de Quı´mica Fı´sica, Universidad del Paı´s Vasco, Barrio Sariena s/n. 48940 Leioa, Spain. Phone: +34 94 601 2533, Fax: +34 94 601 3500, e-mail: [email protected]. † Dedicated to the late Dr. David Husain (U. Cambridge, UK).

narrow layer penetrated by the radiation, which ultimately causes the propulsion of ions at very high velocities. The positively charged field is left behind by the photoelectrons ejected by the laser multiphoton absorption. This interpretation leads to a fresh elucidation of the Al ablation process, which also applies to other metals.4 2. Experimental Section The experimental setup has been presented elsewhere,5 and only a few details relevant to the present study are summarized here. Laser ablation was conducted with a 10 Hz Nd:YAG laser (Quantel model Brilliant 450 mJ/pulse, 5 ns/pulse at 532 nm), impinging on an Al target (Goodfellow 99.999%), housed in a vacuum stainless steel chamber (background pressure 5.10-7 mbar). Laser fluences between 0.8 and 6 J/cm2 were generated with the aid of an attenuator (Newport M-935-10) and the laser beam focused with a 50 cm quartz lens onto a ∼0.7 mm diameter spot (as measured with a photosensitive paper). A portion of the ejected ions normal to the sample were selected through a 1 mm diameter diaphragm, routed to a second vacuum chamber (2 × 10-7 mbar) and finally focused onto the entrance of an electrostatic ion energy analyzer (EEA) (Comstock AC-901) located some 20 cm from the target. The selected ion energy/charge (E/Z) ratio depends on the voltage difference between the analyzer plates (∆V) and their dimensions (2.54 × ∆V eV in our experiment). It is worth noting that the energy and temporal resolution only depend on geometry parameters (target-EEA distance and solid angle coverage by the analyzer entrance).6 To improve mass resolution, a 73 cm long drift tube was added to the exit of the EEA. The resulting ions impinge on a multichannel plate (MCP) detector whose electric signal is routed to a digital scope (Tektronix TDS360) and finally to a computer for analysis and storage. The detection is triggered by the laser Q-switch. Figure 1a depicts an example of mass spectrum taken at E/Z) 360 eV transmission energy and 5.3

10.1021/jp805610h CCC: $40.75  2008 American Chemical Society Published on Web 10/01/2008

Pulsed Laser Ablation on Al

J. Phys. Chem. C, Vol. 112, No. 42, 2008 16557 distribution. It is worth noting that all Al ion KEDs produced with fluences below 1.2 J/cm2 fit single MBC distributions. The fluence threshold for production of Al+ was extrapolated from the distribution maxima versus laser fluence to be 0.8 J/cm2 (other thresholds will be discussed below). Higher laser fluences create a large unbalanced positive electrostatic field within the radiation/surface plasma, favoring a high number of electron-ion, ion-ion, and ion-neutral collisions, leading to recombination, ionization, disproportionation, comproportionation, and charge transfer,4,11 that smear out the single MBC distributions. These dynamic effects are expected to influence the shape of the distributions,6,12 but have not been properly identified so far and, consequently, not reported. As collisions lead to ions observed with a different charge than the precursor or source species, they are expected to be accelerated (for a time that depends on the rate of the formation process) proportionally to their own charge and hence observed as an independent MBC distribution. For example, if ionization is the dominant process (Al0 + e- f Al+ + 2e-), Al0 and Al+ species will contribute to the observed Al+ MBC observed distribution. Consequently, the overall distribution is a sum of MBC distributions with their characteristic Coulomb velocities, i.e.

Figure 1. (a) Example of mass spectrum obtained by nanosecond pulsed laser ablation of Al at a fluence of 5.3 J/cm2 and 360 eV ion kinetic energy to charge ratio. (b) Single Maxwell-Boltzmann-Coulomb kinetic energy distribution of Al+ ions at laser fluence just over threshold (1.1 J/cm2).

J/cm2 fluence. Al+, Al2+, and Al3+ peaks appear at 360, 720, and 1080 eV, respectively. Kinetic energy distributions (KEDs) of the ablated ions are worked out from the spectra scanned at a series of EEA voltages.7 Figure 1b shows an example of the KED of Al+ ion at laser fluence just over threshold (see below). Steady results are hard to obtain as it requires ablation on fresh material and an safe, educated method to collect and average the results. 3. Results and Discussion One-dimensional expansion of a thermalized gas of atoms with an overlapped gravitational or any other field fits the Maxwell-Boltzmann distribution law.8 If, in addition, the sample would contain thermalized ions emerging from an electrostatic field, their total energy would be that of the added energies. The ablation of a metal with a laser fluence over the characteristic threshold yields electrons, ions, and a broadband radiation (at times shorter than 1 µs). Just over the threshold, the ion kinetic energy distribution has to be of MaxwellBoltzmann-Coulomb (MBC) type,9 namely

(

F(Vx) ) A

m 2πkT

3⁄2

)

(

V3x exp -

m(Vx - (Vk + Vc))2 2kT

)

(1)

where Vx, Vk, and Vc are the Maxwellian, the center-of-mass, and the Coulomb velocities, and A is a constant proportional to the contribution of the ion considered. In nanosecond pulsed ablation, the center-of-mass velocity Vk is smaller than the Coulomb one5 and is computed10 as the sonic velocity according to

Vk )

( γkT m )

1⁄2

(2)

where γ ) 5/3 for metal monatomic species. Figure 1b shows the experimental kinetic energy distribution of Al+ at a laser fluence of 1.1 J/cm2 and its best fitting to a single MBC

F(Vx) )

∑ i

m Ai 2πkT

(

3⁄2

)

V3x

(

)

m(Vx - (Vk + Vc,i))2 exp (3) 2kT

Figure 2 shows a few examples of experimental KEDs of Al ions created at pulsed laser fluence in the 1.5-6.0 J/cm2 range. The distributions were taken for ions driven out normal to the target surface and derived by integration of the ion bands (Figure 1a) at a variety of kinetic energies chosen with the EEA. For fluences over 1.2 J/cm2, a significant change in the Al+ distribution shape is readily observed, indicative of the contribution of other source species, in particular, that of a charge one unit lower than the observed. Similar behavior is observed for higher-charged Al ions. Table 1 collects the ions observed for a number of fluences, the source of the ions seen in the distributions, and the numerical parameters of the MBC distributions plotted in Figure 2. It is worth noting that, for experiments carried out at the same fluence and, hence, identical metal ablation conditions, the Coulomb velocities, Vc, of the ions created from the same source match each other within the experimental error (Table 1). The same argument applies to the total or shift velocity, Vk + Vc, determined directly from the experiments and henceforth measured to a better accuracy. The center-of-mass velocity Vk is computed from eq 2 and thus is very close for ions of the same species. Henceforth, it is interpreted as the occurrence of ionization processes within the surface/radiation plasma in times shorter than the laser pulse (5 ns) (see below). In turn, as ion distributions are well-characterized and in good and systematic agreement with the observed ones, they do have to fulfill the thermalized conditions imposed by the derivation of MBC distributions from basic principles and thereby the average magnitudes considered have the standard physical meaning. As stressed above, the comparison of the features of the ion distributions in experiments at the same fluence has particular relevance. The interest is even higher for high fluences because the number of ions created is higher. Indeed, the relationship between the Coulomb velocity and the charge for fluences of 4.5 and 5.3 J/cm2 shown in Figure 3 has a nearly linear dependence. The constant slope in the plot suggests that the ejected ions are exposed to an identical (positive) electrostatic field, and hence, the average velocity is proportional to the

16558 J. Phys. Chem. C, Vol. 112, No. 42, 2008 charge. This conclusion is of paramount interest in understanding the ablation process. The contrast between the straightforward mechanism of the photoelectric effect and the involved interaction of high intensity laser radiation with metals (Al in this case) is worth examining. Ejection of electrons, although it requires absorption of at least two photons, is completed in the short time characteristic of the induced absorption (few to hundredths femtoseconds) and leaves behind an electrostatic positive field in the metal skin where the laser light impacted. This field will spread out toward the metal bulk with a characteristic lifetime that depends on the ion-electron rate collisions. The electrostatic field accelerates the local ions to attain experimental velocities as high as (3-6) × 104 ms-1 (traveling ∼150-300 µm in 5 ns). The constant slope in Figure 3 suggests that the total pulse may be divided into a number of subpulses, that might well be on the order of tens of picosecondssfor the sake of argumentsto provide traveled distances of nanometers and so a sensible averaged electrostatic field. The system has to fulfill the conservation laws, even making an allowance for averaged electrostatic fields. The energy conservation law was used by Einstein in the photoelectric effect because the metal electron “work function” was an empirical accessible energy, but it cannot be extended to the ejection of electrons/ions for two reasons: the metal ion “work function” has its own distribution, and the number of absorbed photons by the sample to access the channel of concern is not a constant number and also has its own distribution. The momentum of the ejected ion would fulfill the equation, mVc ) Ft, where m is the ion mass, Vc the Coulomb velocity, F the impulsive force, and t the time. The impulse, F, is related to the electrostatic field, E, so that:

Apin˜a´niz et al.

mVc ) ZeEt

(4)

Therefore, the velocity, and hence the momentum, is proportional to the ion charge, as has been established in Figure 3. The linear dependence between velocity and charge is indeed a key assumption in deriving the MBC distributions.8 The split of the interacting nanosecond laser pulse into short pulses is a possible argument in a dynamic system that absorbs light and releases electrons, creating the appropriate conditions to drive out ions and expel them over and over again. An alternative interpretation to the process consists of the accumulation of photoelectrons to build up a large electrostatic positive field in the plasma radiation/surface bursting at the end of the laser pulse and ejecting a bunch of ions. Observations aimed at distinguishing between the two mechanisms are not simple to implement, and experiments with picosecond lasers are being planned. The comparison among the momenta of ions with different charges affected by the electrostatic field sheds light onto the relative residence time of each. The ion velocity ratio is

V1 Z1t1 ) V2 Z2t2

(5)

For equal residence time, the velocity ratio is equal to the ion charge ratio, i.e., 2 for the Al2+/Al+, 3 for Al3+/Al+, and so forth. However, the experimental ratios are 1.3 and 1.6 at 5.3 J cm-2. Hence, the higher the ion charge, the shorter the residence time or, in other words, the later the ion is formed. In the case that the Al+ stays for as long as the laser pulse, the average time for the Al2+ will be (5.0/1.3) ) 3.8 ns and for the Al3+ 3.1 ns. In the case of a shorter interaction time, the average residence time will follow the same ratio.

Figure 2. Experimental kinetic energy distributions of the Al ions produced by laser ablation at (a) 1.5 J/cm2, (b) 2.0 J/cm2, (c) 3.3 J/cm2, and (d) 5.3 J/cm2 fluences. Note the systematic contribution of the Al(Z-1)+ ion onto the experimental distribution of AlZ+ ion for fluences higher than 1.3 J/cm2.

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TABLE 1: Summary of Parameters Obtained by Fitting the Experimental Results to Full-Range Maxwell-Boltzmann-Coulomb Distributionsa

a Vk and Vc are the center-of-mass and the Coulomb velocities, kT the ion distribution temperature, and A the normalization constant (eq 3). Note the closeness of the total velocities of the same source ion observed in the Al(n)+ and Al(n+1)+ distributions at all studied fluences (indicated with a square bracket).

Figure 3. Relationship between shift velocity, Vc, and ion charge in the laser ablation of Al at fluences of 4.5 and 5.3 J/cm2.

Assuming that the detector is insensitive to the ion kinetic energy, the ratio under the areas of the precursor and the source ions in the MBC distribution is the “branching ratio”. Straightforward calculations for the ionization process of Al+ lead to a branching ratio close to 3.5 for a fluence of 2.0 mJ/cm2 and only 1.5 for 5.3 mJ/cm2. For the neutral Al0 associated and later observed as Al+ ion, its velocity is much higher than that expected for neutrals (where the Coulomb velocity is expected to be zero). Although the original source is a neutral species, the electron-ion collisions produce Al+, and it is observed in the same ion punch as Al+

and thereby part of its lifetime is elapsed as a charged ion. For slow ion-electron rate constants, the contribution of the neutral to the ion distribution is small, and most of it is created at the end of the pulse and with Coulomb velocities close to zero. For high ion-electron rate constants, the ions are created rapidly and are accelerated for a long time. Therefore, the expected Coulomb velocity of a neutral source appearing as an ion is larger than when it is observed as neutral. The average acceleration time interval may also be modeled from the resulting Al0 and Al+ velocity ratio. The velocities depicted in Table 1 allow the computation of the residence times as source and ionized species. So, for Al0 observed in the Al+ group at 4.5 J/cm2 the time ratio is 1.3 (i.e., in our experiments, close to 2.7 ns). The argument of the neutral dragging may be further strengthened, noting that the Al+ ion partner energy of the neutral increases with the fluence (cf. also Figure 2) and hence the energy of the neutral must increase with the charge of the partner. Figure 4 shows an almost linear dependence between the apparent Coulomb energy of the neutral and the fluence, with an energy intercept close to zero. Figure 4 also shows the Coulomb energies of the ejected Al+ and Al2+ ions as a function of the laser fluence deposited on the Al surface. The shapes are related to the appearance of ions of higher charges and allow a sensible extrapolation of the fluence threshold for Al+ and Al2+ ions (0.8 and 1.4) to be obtained. The appearance thresholds are related to the energy needed to eject the ion from the surface of the metal soaked in

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Apin˜a´niz et al. rate constant for both processes. Some other metals studied with the same experimental setup, namely, Mn, Ga, and so forth, also have distribution sidebands.

Figure 4. Plot of the squared velocity (and energy (E ) 1/2mV2)) of the ejected Al+ and Al2+ ions and the Al0 neutral as a function of the laser fluence. The “ion work function” for Al+ and Al2+ ions are given by the extrapolated onset, i.e., 0.8 and 1.4 J/cm2, respectively.

an electrostatic field that we name, following the photoelectric effect nomenclature, the “ion work function”. However, we do not know if the ions do not depart from the surface because either the electrostatic field is not large enough to accelerate the ion or it has no internal energy to be allocated as translational energy (the latter may include electronic and phonon energies, assumed to be smaller than the former and therefore negligible). In this model, there is an electron density threshold below which the produced electrostatic field is not large enough to enable ejection of ions. Therefore, the ion work function threshold should be also regarded as a key feature of the metal. For pulsed laser fluences high enough to yield ions, the laser energy should balance the “electron work function” of the metal, the internal (kinetic) energy of the ejected electron (both very fast processes in the femtosecond regime), the metal “ion work function”, the kinetic energy of the ion ejected, and other energies relaxed to the solid. The first two processes take place on a shorter time scale and can be considered separately, extending Einstein′s balance for the linear photoelectric effect to that of multiphoton absorption. Left behind are a space charge and other energies included in the empirical work function. For pulsed lasers, the interaction process is followed by further absorption of radiation, further ejection, and electron and phonon relaxation (commonly known as photothermal effect).13 The picture in this short time is repeated over and over again, building up an electrostatic field in the metal skin that relaxes with a typical characteristic time. For long laser pulses, the two processes mix together. Nanosecond pulsed lasers, even in the region of a few nanoseconds, do not have the appropriate scales to avoid smearing of the effects, but they can be analyzed, although qualitatively. Experiments on the femtosecond scale (50 fs, 400 nm) carried out on Al (Al+, Al2+, Al3+) at a number of fluences and also on Fe, Ni, FeNi, etc.4 show neat MBC distributions with neither recombination nor ionization contributions. In these short times, the ion thresholds are characteristic of each metal (for the same laser type, polarization, and wavelength) and may be used for identification purposes. In the nanosecond pulse regime, we have also observed sidebands on the precursor distributions in other metals. So, Fe has a recombination band but no traces of ionization (ionization rate constants are known to be much smaller than recombination ones).14,15 The ionization process is known to have an inefficient electron-Fe rate constant. In contrast, Ni depicts both recombination and ionization contributions and hence a comparable

4. Conclusions Ablation of Al metal with nanosecond pulsed lasers yields overlapped Al ions MBC distributions for fluences above a threshold. The contributions to the ion kinetic energy distribution come mainly from the unaffected or precursor ion itself and other species producing the ion by collision with electrons (i.e., the distribution for observed Al+ ion, comes from precursor Al+ and electron collisions with neutral Al, Al0 + e- f Al+ + 2e-). The ionization rate constant in the Al metal plasma (metal/ laser radiation) is several orders of magnitude higher than that of recombination, and thus ablation kinetic energy distributions can be used to find out the (relative) rate constants in plasmas. The electrical field left behind by the prompt electron ejection lasts longer than the laser pulse, and during that time, it accelerates the ions proportional to their charge. In the case of source Al0 observed as Al+, the acceleration is proportional to the residence time of Al0 in the plasma as Al+, providing a time scale for the process. Energy distributions following ablation yield information on the electron-atom/ion ionization and recombination rate constants that may be readily used to identify pure metals and alloys, its geometrical structure, and electron surface density. Ionization thresholds also may be used for identification purposes. Indeed, the method may compete, at least for metals and alloys, with well-established laser-induced breakdown spectroscopy LIBS, whose radiation emitting species are final products (>1 µs) of the relaxation energy excess of the metal/laser plasma.16 Acknowledgment. The authors are grateful to MEC (grantin-aid ZQU2004-7188 and Consolider Grant CSD2007-00013) for partial support of this work; to GV (Vitoria) and to UPV/ EHU for the award of a Consolidated Research Group Grant (2001-2008). B.S. and J.I.A. thank MEC (Madrid) and UPV/ EHU for graduate fellowships associated to Projects. References and Notes (1) Ashcroft, N. W. and Mermin N. D. In Solid State Physics; Thomson Learning: Andover, U.K., 1976. (2) Einstein, A. Ann. Physik 1905, 17, 132. (3) Hu¨fner, S. In Photoelectron Spectroscopy; 3rd ed.; Springer: New York, 2003. (4) (a) Rubahn, H.-G. In Laser Applications in Surface Science and Technology; J. Wiley & Sons: West Sussex, 1999. (b) Phipps, C. R. In Laser Ablation and its Applications; Springer: New York, 2007. (5) Ecija, P.; Sa´nchez Rayo, M. N.; Martı´nez, R.; Sierra, B.; Redondo, C.; Basterretxea, F. J.; Castan˜o, F. Phys. ReV. A 2008, 77, 032904. (6) Imhof, R. E.; Adams, A.; King, G. C. J. Phys. E 1976, 9, 138. (7) Arago´n, C.; Aguilera, J. A.; Blanco, F.; Campos, J. Vacuum 1994, 45, 923. (8) Castellan, G. W. In Physical Chemistry, 3rd ed.; Addisson-Wesley: Reading, 1983. (9) Torrisi, L.; Gammino, S.; Ando`, L.; La`ska, L. J. Appl. Phys. 2002, 91, 4685. (10) Kelly, R. J. Chem. Phys. 1992, 92, 5047. (11) Atkins, P. W. In Concepts in Physical Chemistry; Oxford University Press: Oxford, 1995. (12) Demtro¨der, W.; Jantz, W. Plasma Phys. 1970, 12, 691. (13) Dudley, W. W. In UV lasers: effects and applications in material science; Cambridge University Press: Cambridge, 1996. (14) Nahar, S. N.; Bautista, M. A.; Pradham, A. K. Astrophys. J. 1997, 479, 497. (15) Woodcock, K. R. S.; Vondrak, T.; Meech, S. R.; Plane, J. M. C. Phys. Chem. Chem. Phys. 2006, 8, 1812. (16) Gonza´lez, A.; Ortiz, M.; Campos, J. Appl. Spectrosc. 1995, 42, 1632.

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