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Investigation of the effect of laser parameters on the target, plume and plasma behavior during and after laser-solid interaction
Accepted Manuscript Title: Investigation of the effect of laser parameters on the target, plume and plasma behavior during and after laser-solid interaction Authors: A. Stancalie, S. Ciobanu, D. Sporea PII: DOI: Reference:
S0169-4332(17)30599-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.226 APSUSC 35335
To appear in:
APSUSC
Received date: Revised date: Accepted date:
29-11-2016 9-2-2017 26-2-2017
Please cite this article as: A.Stancalie, S.Ciobanu, D.Sporea, Investigation of the effect of laser parameters on the target, plume and plasma behavior during and after laser-solid interaction, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.226 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of the effect of laser parameters on the target, plume and plasma behavior during and after laser-solid interaction
A. Stancalie1, S. Ciobanu2, D. Sporea1
1
National Institute for Laser, Plasma and Radiation Physics, Atomistilor 409, P.O.Box MG-36, Magurele-Ilfov, 077125 Romania 2 University Politehnica of Bucharest, Faculty of Applied Sciences, 313 Splaiul Independentei, 060042 Bucharest, Romania
[email protected]; [email protected]; [email protected]
Corresponding author : Dr Andrei Stancalie, National Institute for Laser, Plasma and Radiation Physics, Centre for Advanced Laser Technologies, Photonic Investigation Laboratory, Atomistilor 409, P.O.Box MG-36, Magurele-Ilfov, 077125 Romania Tel: +4021 457 45 50 ext. 2500/ Fax : + 4021 457 42 43 http://www.inflpr.ro
Highlights Detailed theoretical and experimental analysis is performed for a wide range of laser operating conditions, typical for laser induced breakdown spectroscopy (LIBS) and laser ablation (LA) experiments on copper metallic target.
New results on recent both experimental and theoretical characterization of the plume expansion using spectroscopic methods are presented.
Recent both experimental and theoretical characterization of the plume expansion using spectroscopic methods are given in this work. To check the departure of the copper ions from the local thermodynamic equilibrium conditions, the results from synthetic spectra generated with FLYCHK program packages have been compared with measurements.
The Atomic data Analysis System (ADAS) programs package has been used for modeling the levels population density distribution and the corresponding line intensity ratios in the electron density and temperature range as output from experiments
Abstract. A detailed theoretical and experimental analysis is performed for a wide range of laser operating conditions, typical for laser induced breakdown spectroscopy (LIBS) and laser ablation (LA) experiments on copper metallic target. The plasma parameters were experimentally estimated from the line intensities ratio which reflects the relative population of neutral excited species in the plasma. In the case of LA experiments the highest temperature observed was 8210 ± 370 K. In case of LIBS measurements, a maximum temperature of 8123 K has been determined. The experimental results are in good agreement with a stationary, hydrodynamic model. We have theoretically investigated the plasma emission based on the generalized collisional-radiative model as implemented in the ADAS interconnected set of computer codes and data collections. The ionic population density distribution over the ground and excited states
into the cooper plasma is graphically displayed as output from the code. The theoretical line intensity ratios are in good agreement with experimental values for the electron density and temperature range measured in our experiments.
Keywords: laser induced plasma, laser-ablation, and laser induced breakdown spectroscopy.
1. Introduction The growing interest on laser-induced plasma has generated a variety of theoretical models and experimental works in order to improve the basic knowledge and determine the best experimental conditions. High energy from a pulse laser over a small space generates ablation plasma. Plasma formation by intense laser irradiation of solids was the subject of many experimental investigations. In parallel with the experiments, computer modelling is needed to improve the understanding of the results as well as to give reliable predictions in the design of new experiments. The laser ablation (LA) of metallic target and the subsequent expansion of the evaporated material plume [1-9], as well as the laser-induced breakdown spectroscopy (LIBS) techniques [10-12] stand in the use of short laser pulses as the energy source to vaporize samples and excite the emission of electromagnetic radiation from its elements and/or molecular fragments. Although LIBS and LA are based on different mechanisms of detection, the processes occurring during and after laser-solid interaction are quite similar. In the case of metallic target the laser causes heating of the solid target, followed by melting and evaporation of some of the target material. The LIBS based experiments, intended specifically to study the effect of laser parameters on the evaporation process, plume and plasma characterization [13,14], have
indicated the signal intensity dependence on laser irradiance [15] and the effect of the laser-pulse duration on laser-induced plasma emission [16,17]. A commonly accepted idea on laser-induced plasma expansion is that photons strike a solid surface and that particles are therefore emitted during the laser pulse. It has been shown [18, and references herein] that at low laser intensities the radiation is absorbed by inverse bremsstrahlung in a low density expansion fan, such that plasma density is much less than the critical density at which the plasma frequency equals the laser frequency. As the laser intensity is increased the temperature and the density at which the absorption occurs also increase. A limit is reached when the density equals the critical density since propagation of the laser radiation is not possible at higher density. At this critical value strong non-linear absorption occurs. The main difficulty in modelling LA is that both dynamics of plume and plasma chemical reactions occurring in it should be investigated [19]. LIBS and LA make use of similar laser operating conditions with pulse durations in the order of several nanoseconds (ns) down to fs-pulse duration, and the laser irradiance varying from 108 till above 1010 W/cm2. The plume expansion takes place in vacuum or in background gases. In the present study we report recent both experimental and theoretical characterization of the plume expansion using spectroscopic methods to investigate the time averaged plasma evolution. At least two different areas of physics are involved in the analysis of laser-produced plasma experiments. They correspond to successive and generally independent steps in the calculation. The first area is the description of laser light absorption, heat transport, and plasma hydrodynamics in geometry suitable to the experimental configuration of target irradiation. For a given set of laser irradiation, parameters, and target specifications these codes calculate the time evolution of quantities such as electron density and temperature throughout the plasma. This implies the use of one - dimensional or sometimes two-dimensional hydrodynamics codes. The 2D-codes, when available, are used in a first approach only to determine whether two-
dimensional effects play an important role in the plasma evolution. These simulations are by far more costly in terms of computer-time. Hence some particular parts of the modeling, such as atomic physics discussed below, cannot be treated by two-dimensional simulations in such detailed way as they are by one-dimensional simulation. The second area is the detailed calculation of the plasma ionization kinetics in a situation where neither the local thermal equilibrium (LTE, fulfilled for high density plasma) nor the coronal equilibrium (CE, fulfilled for low density plasmas) is valid, and where the system may be strongly non-stationary. Thus an extensive set of atomic data is required to calculate the distribution among the ionization charge states of the target element, as well as the excited level populations, and line emission intensities. Part of the accuracy of these calculations thus relies on that of the atomic data. The detailed calculation of ionization kinetics is usually coupled to the above hydrodynamics area through the plasma energy balance equation. The paper is structured as follows. Section 2 gives details on the experiments and results. In order to obtain a better insight in the exact influence of these laser parameters on laser-ablation and laser-plasma formation we present comparatively, recent results obtained from LIBS to previously reported [20] characteristics from LA spectroscopic investigation of a plasma in air produced by the first harmonics of a Q-switched pulsed Nd: YAG laser. The plasma experimental parameters were estimated [21] from spectroscopic data. In the LIBS experiments, the Applied Photonics LIBS-6 instrument allowed modifying the laser irradiance at the sample surface by changing the pulse energy or the laser focusing distance. The Boltzmann plots corresponding to above mentioned experiments give the excitation temperature. Section 3 gives our concluding remarks. 2.
Experimental details and results
Full details on the LA experimental set-up and measurements have already been reported [20, 21]. We only recall here the principal ideas and results. For completeness, Table 1 reproduces from Ref. 21 the laser characteristics for LA and LIBS experiments to which present investigation is applied. The plasma on copper target has been produced by the second harmonic ( = 0.532 m, 180 mJ energy) of a Q-switched pulsed Nd: YAG laser (λ=1.064 µm, 360mJ energy in the first harmonic), of 4.5 ns pulse duration, and 0.1 ÷ 10Hz frequency used for excitation.
The plasma emission spectra were recorded while the target was rotated to have a clean area under irradiation. For the typical conditions used in LA experiments, the observed emission lines correspond to neutral and single ionized copper ions. Taking into account the finite dimension of the focal spot we developed a hydrodynamic model describing the plasma production as a steady state process. A simple theoretical model with two homogeneous rectangular regions corresponding to the hot emissive central area (the heating zone) and to the cold absorbing surrounding area, respectively, indicated radiative transfer for the Cu I resonance lines at 324.75 and 327.40nm for laser-produced copper plasma in air. This model allows a distinction of thermal and kinetic energy in the plasma. The plasma production was considered as a steady state process, as suggested by our earlier experimental investigation, showing a heating of the target layer by layer. This steady state is reached if the time which an electron ion pair spends within the hot central zone is less than the duration of the laser pulse. In the case of nanosecond pulses this assumption is generally fulfilled [22]. Temperature measurements were performed over a wide range of light intensities on copper plasma. In addition, our model assumes one-dimensional geometry. In this case the plasma interacts with the
incoming beam during the whole pulse duration. In reality the time of interaction is reduced due to more rapid decrease of the plasma density in 3D geometry, as well as due to the decreasing light intensity with increasing distance from the focal plane of the lens system. Another significant approximation of the model is the application of the absorption coefficient derived by combining Maxwell equations with equation describing the plasma properties [23]. Accordingly, the main absorption in the plasma takes place in the heating zone. The validity of this approximation is limited to the density region where the electron density is less than the electron density at which the corresponding plasma frequency equals the frequency of the incident light (ne < nep). Considering an energy balance between the incoming light flux and the thermal- and kinetic energy transported by the plasma flow, we were able to determine the electron temperature at the local thermodynamic equilibrium for neutral atoms [24]. The temperature calculation was made over a wide range of light intensities. We have used the Cu I line intensities at 510.55, 515.32, 521.82 and 522.01 nm and the Boltzmann plot method for the plasma in local thermodynamic equilibrium (LTE). Assuming the LTE for neutral cooper atoms, the line intensity, for optical thin plasmas is: Eu , k
gu , k kT N hc I k Cu e Aul , k 4 ZCu T k
(1)
where N Cu represents the total number of the Cu species in the plume, g u ,k and Eu ,k are the statistical weight and the energy of the upper level for the line k with wavelength k , the index l is for the lower level, Aul ,k is the emission rate for the spontaneous emission, h is the Planck
constant, c the vacuum light speed, and ZCu T is the partition function. In the present analysis the electron temperature was obtained from the slope -1/kT of the best fit of Boltzmann plot to a straight line:
ln
Ik k hcN Cu 1 ln Eu ,k g u ,k Aul ,k 4Z Cu kT
(2)
in the plane with the energy Eu ,k of the upper level on the horizontal axis and ln
Ik k on gu , k Aul , k
the vertical axis. The atomic parameters used in the evaluation of electron temperature, as level degeneracy, oscillator strengths and radiative transition probabilities, were taken from the spectral database of National Institute of Standards and Technology (NIST) [25] and are shown in Table 2. The mean electron temperature obtained from the Cu I line intensities ratio in the copper ablation experiment was of 8210 ± 370 K. The estimated LA plasma electron density was of ne = 3.15·1019cm-3.
Combined with other methods of analysis and other power sources, LIBS enhances detection capabilities. In the present work we have performed LIBS experiments where the laser –induced copper plasmas is generated using an Nd: YAG laser (wavelength 1064 nm, pulse width 4.5ns, repetition rate 20Hz) from sample placed in air at atmospheric pressure. Applied Photonics LIBS-6 allows modifying the laser irradiance at the sample surface by changing the pulse energy or the laser focusing distance. The experimental set-up has been presented in details in Ref. [21]. All the experimental operations, including sample movement, settings of the laser and of the spectral acquisition parameters were controlled by the personal computer. The samples were 1mm thick copper of 99.999% purity. The laser beam was focused at right angles onto the sample surface by a lens of 128mm focal length at 1064nm. The LIBS plasma spectrum has been recorded for series of laser pulse energy: 50mJ, 80mJ, 100mJ, 110mJ, and 150mJ keeping the frequency (10Hz) and the pulses accumulation rate constant. Calculations with the plasma
ablation model show that the initial plasma, both temperature and electron density are found to be significantly higher for the higher irradiance. The relative line intensities of atomic copper have been used to determine the electron temperature. The emission spectrum corresponding to the wavelengths domain of 200-900nm is displayed in Figure 1a, with a detail in Figure 1b for wavelengths between 508 and 526 nm. The pulse energy was 150mJ, 10 Hz repetition rate and 20 pulses accumulation rate. The integration time was 1ms and the integration time delay of 1.27 s. Figure 2 displays the Boltzmann plot based on the line intensities at 510.5nm, 515.3nm, 521.8nm and 522.0nm, excluding the background continuum emission. The corresponding determined electron temperature for this shot was T= 9350±133 K.
The estimated electron density is of 7.4 1015 cm-3. In order to examine the relevance of the plasma thermodynamic regime on the spectrally resolved intensities we have performed a temporal analysis of the plasma parameters based on FLYCHK program packages [26]. The solution for the population density for the case where the states are in LTE is obtained in the framework of statistical mechanics. The equations for the ratio of the ground states to the successive ionization stages is provided by the Saha-Boltzmann equation, while the ratio of the excited states to the ground states within each ionization stage is given by the Boltzmann equation. To solve for the total absolute number densities, the code uses the charge neutrality constraint that requires that the total ion density be related to the given electron density by the mean charge of the ions. Figure 3 gives the mean-ion charge distribution, and the corresponding total radiative loss, versus electron temperature into the plasma for three electron density values. The range of plasma parameters corresponds to measured values in our LIBS experiments.
It should be noted that the FLYCHK code is a „zero- dimension code”, in that there are no information on conditions of the plasma other than the local conditions specified. This indicates that a correct tratement of radiative transfer or gradients are outside of the scope of this suite of codes. The Atomic Data Analysis System (ADAS) [27] interconnected set of computer codes and data collections has been used for modeling the levels population density distribution N (q, i) of the i-th excited level (i =1 refers to the ground state) and the corresponding line intensity ratios in the electron density and temperature range as output from experiments. For a number of ten electron temperature values and eight electron densities, ADAS has been used to model the population density distribution over the excited states into the plasma, and the corresponding transitions line intensity. At each time the variation of this population is expressed by a rate equation involving the collisional excitation and deexcitation rates, spontaneous decay probabilities, radiative and 3-body recombination rates and the collisional ionisation rate. These processes connect the excited levels of an ionization stage to the ground state of the next higher one. We have addopted the following suitable approximations: (a) only the ground state of each ionization stage was significantly populated, and (b) ionization and recombination from these ground states were the dominat processes in the population transfer between successive ionisation stages. Figure 4 displays, as an example, the plot of the population density of the ground and first excited states of Cu I as output from the code.
In this figure the state population density is given as ratio to the ground population density times the electron density, for a range of electron density values. Application refers to 3d104s(2S1/2) ground and 3d104p ( 2 P3o/ 2 ), 3d105s ( 2 S1 / 2 ) and 3d104d(2D5/2) metastable states population. The electron temperature included into the calculation is 0.7eV.
Conclusions This work is part of general investigation which started a few years ago to comparatively study the effect of laser parameters on laser produced copper plasma. In order to get presented results, two experiments have been compared, namely the laser ablation spectroscopy and the laser induced breakdown spectroscopy on copper plasma. The experiments have been carried out using the second and the first harmonics of an Nd: YAG laser, respectively. The theoretical methods for investigating the plasma plume behavior and its main parameters calculation use spectral atomic copper main lines in the visible range. The fitting of the Saha-Boltzmann plot to a straight line provides an apparent averaged ionization temperature, of 8210±370 K in the LA experiment, and of 8123K in the LIBS experiment. A simple hydrodynamic model for the heating of plasma generated at the interaction of the laser beam with copper target has been used to define the range of LTE during the plasma expansion. Following this model, for nanosecond pulse duration and a finite focal spot diameter, the plasma production may be considered as a steady state problem. In our treatment the plasma is considered as a continuous particle flow originating at the target. In LIBS experiments different value of laser fluence has been used, keeping the laser frequency and the accumulation rate constant, in order to determine the electron temperature and density. In both cases, the heating of the plasma is due to energy absorption of the electrons from the radiation field by inverse bremsstrahlung. The corresponding absorption coefficient has been determined neglecting the factor (1-ne/nep)-1/2 which in turn limits the dependence of the absorption coefficient on the electron temperature in the density region ne < nep. We theoretically investigated the corresponding mean ion charge, the radiation loss, the population density distribution on the excited states and the line intensity ratios using FLYCHK
code. Following presented results we conclude that laser beam absorption and the plasma shielding is especially important near the target, where the Cu vapor, ion and electron densities reach their maximum (i.e. in laser-ablation experiment). Other phenomena such as the formation of early-stage plasma can start to take place at ps-laser pulses, which has not been considered here. The general tendency of the relative importance of the different absorption mechanisms, as well as the overall amount of laser absorption, seems to be more or less correctly predicted here, and will be further investigated. Comparing the laser fluence (which is defined as the laser irradiance integrated over the entire pulse) effect on target and plume characteristics it was observed that it increases with pulse duration. In parallel with the experiments, we have theoretically investigated the line intensity ratios for a set of plasma parameters and results are in good agreement to the experimental values for the transition considered in this work. We used the ADAS routines which solve the coupled rate equation for wide range of plasma parameters. Large amount of data (not presented here) has been obtained for cross sections and rates of the elementary processes into the plasma.
ACKNOWLEDGEMENTS
A. Stancalie and D. Sporea acknowledge financial support of The National Authority for Scientific Research and Innovation, Program NUCLEU PN 1647, LAPLAS IV. Some of the equipment used in this research were purchased in the frame of the project “Center for Advanced Lasers Technologies (CETAL)”, contract 8PM/2010, financed by UEFISCDI (RO).
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Figure captions Figure 1. (a). LIBS on copper plasma. The laser pulse energy was 150mJ at a repetition rate of 10Hz. The accumulation rate was 20, the integration time was 1ms and the time delay was 1.27 s. The emission spectrum corresponds to the wavelengths domain of 200-900nm, and (b) 508526nm, respectively.
Figure 2. The Boltzmann plot based on the line intensities at 510.5nm, 515.3nm, 521.8nm and 522.0nm, excluding the background continuum emission. The corresponding determined electron temperature for this shot was T= 9350±133 K.
Figure 3. (Left) The mean-ion charge distribution versus electron temperature for three values of electron density (lower) 1010cm-3, (middle) 1015cm-3, (higher) 1019cm-3 ; (Right) The total radiative losses versus electron temperature for electron density (from lowest to highest) of 1015cm-3, 1016cm-3, 1017cm-3 and 1019cm-3. A FLYCHK simulation.
Figure 4. Population density (in cm3) of the ground and first excited states of Cu I relative to ground state population times electron density, as function of the electron density (in cm-3 units). Te = 0.7 eV. There are three curves in this graph, labeled by 1, 2, 3. They correspond to 3d104p ( 2 P3o/ 2 ), 3d104d( 2 D5 / 2 ) and 3d105s(2S1/2) level population ratios to the ground 3d104s(2S1/2) population, respectively.
A
b Fig 1
Fig 2
Left
Right Fig 3
Fig 4
Table1. Laser –induced plasma formation conditions. Target Background gas Laser Irradiance Laser pulse duration Laser wavelengths
Table 2.
Copper Free atmosphere 108 -1010 W/cm2 0.3 – 10 ns 532nm, 1064 nm
Atomic parameters for the lines of Cu I used in the electron temperature evaluation:
wavelength, , radiative transition probability, Aul , upper level energy, Eu, and lower level energy, El , and the spectroscopic designation of corresponding transition. (nm)
Aul(s-1)
Eu(cm-1)
El(cm-1)
transition (upper - lower)
510.5541 515.3235 521.8202 522.0070
2.00E+06 6.00E+07 7.50E+07 3/20E+07
30783.697 49935.195 49942.051 49935.195
11202.618 30535.324 30783.697 30783.697
3d104p 2P°3/2 - 3d94s2 2D5/2 3d104d 2D3/2 - 3d104p 2P°1/2 3d104d 2D5/2 - 3d104p 2P°3/2 3d104d 2D3/2 - 3d104p 2P°3/2
S0169-4332(17)30599-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.226 APSUSC 35335
To appear in:
APSUSC
Received date: Revised date: Accepted date:
29-11-2016 9-2-2017 26-2-2017
Please cite this article as: A.Stancalie, S.Ciobanu, D.Sporea, Investigation of the effect of laser parameters on the target, plume and plasma behavior during and after laser-solid interaction, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.226 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of the effect of laser parameters on the target, plume and plasma behavior during and after laser-solid interaction
A. Stancalie1, S. Ciobanu2, D. Sporea1
1
National Institute for Laser, Plasma and Radiation Physics, Atomistilor 409, P.O.Box MG-36, Magurele-Ilfov, 077125 Romania 2 University Politehnica of Bucharest, Faculty of Applied Sciences, 313 Splaiul Independentei, 060042 Bucharest, Romania
[email protected]; [email protected]; [email protected]
Corresponding author : Dr Andrei Stancalie, National Institute for Laser, Plasma and Radiation Physics, Centre for Advanced Laser Technologies, Photonic Investigation Laboratory, Atomistilor 409, P.O.Box MG-36, Magurele-Ilfov, 077125 Romania Tel: +4021 457 45 50 ext. 2500/ Fax : + 4021 457 42 43 http://www.inflpr.ro
Highlights Detailed theoretical and experimental analysis is performed for a wide range of laser operating conditions, typical for laser induced breakdown spectroscopy (LIBS) and laser ablation (LA) experiments on copper metallic target.
New results on recent both experimental and theoretical characterization of the plume expansion using spectroscopic methods are presented.
Recent both experimental and theoretical characterization of the plume expansion using spectroscopic methods are given in this work. To check the departure of the copper ions from the local thermodynamic equilibrium conditions, the results from synthetic spectra generated with FLYCHK program packages have been compared with measurements.
The Atomic data Analysis System (ADAS) programs package has been used for modeling the levels population density distribution and the corresponding line intensity ratios in the electron density and temperature range as output from experiments
Abstract. A detailed theoretical and experimental analysis is performed for a wide range of laser operating conditions, typical for laser induced breakdown spectroscopy (LIBS) and laser ablation (LA) experiments on copper metallic target. The plasma parameters were experimentally estimated from the line intensities ratio which reflects the relative population of neutral excited species in the plasma. In the case of LA experiments the highest temperature observed was 8210 ± 370 K. In case of LIBS measurements, a maximum temperature of 8123 K has been determined. The experimental results are in good agreement with a stationary, hydrodynamic model. We have theoretically investigated the plasma emission based on the generalized collisional-radiative model as implemented in the ADAS interconnected set of computer codes and data collections. The ionic population density distribution over the ground and excited states
into the cooper plasma is graphically displayed as output from the code. The theoretical line intensity ratios are in good agreement with experimental values for the electron density and temperature range measured in our experiments.
Keywords: laser induced plasma, laser-ablation, and laser induced breakdown spectroscopy.
1. Introduction The growing interest on laser-induced plasma has generated a variety of theoretical models and experimental works in order to improve the basic knowledge and determine the best experimental conditions. High energy from a pulse laser over a small space generates ablation plasma. Plasma formation by intense laser irradiation of solids was the subject of many experimental investigations. In parallel with the experiments, computer modelling is needed to improve the understanding of the results as well as to give reliable predictions in the design of new experiments. The laser ablation (LA) of metallic target and the subsequent expansion of the evaporated material plume [1-9], as well as the laser-induced breakdown spectroscopy (LIBS) techniques [10-12] stand in the use of short laser pulses as the energy source to vaporize samples and excite the emission of electromagnetic radiation from its elements and/or molecular fragments. Although LIBS and LA are based on different mechanisms of detection, the processes occurring during and after laser-solid interaction are quite similar. In the case of metallic target the laser causes heating of the solid target, followed by melting and evaporation of some of the target material. The LIBS based experiments, intended specifically to study the effect of laser parameters on the evaporation process, plume and plasma characterization [13,14], have
indicated the signal intensity dependence on laser irradiance [15] and the effect of the laser-pulse duration on laser-induced plasma emission [16,17]. A commonly accepted idea on laser-induced plasma expansion is that photons strike a solid surface and that particles are therefore emitted during the laser pulse. It has been shown [18, and references herein] that at low laser intensities the radiation is absorbed by inverse bremsstrahlung in a low density expansion fan, such that plasma density is much less than the critical density at which the plasma frequency equals the laser frequency. As the laser intensity is increased the temperature and the density at which the absorption occurs also increase. A limit is reached when the density equals the critical density since propagation of the laser radiation is not possible at higher density. At this critical value strong non-linear absorption occurs. The main difficulty in modelling LA is that both dynamics of plume and plasma chemical reactions occurring in it should be investigated [19]. LIBS and LA make use of similar laser operating conditions with pulse durations in the order of several nanoseconds (ns) down to fs-pulse duration, and the laser irradiance varying from 108 till above 1010 W/cm2. The plume expansion takes place in vacuum or in background gases. In the present study we report recent both experimental and theoretical characterization of the plume expansion using spectroscopic methods to investigate the time averaged plasma evolution. At least two different areas of physics are involved in the analysis of laser-produced plasma experiments. They correspond to successive and generally independent steps in the calculation. The first area is the description of laser light absorption, heat transport, and plasma hydrodynamics in geometry suitable to the experimental configuration of target irradiation. For a given set of laser irradiation, parameters, and target specifications these codes calculate the time evolution of quantities such as electron density and temperature throughout the plasma. This implies the use of one - dimensional or sometimes two-dimensional hydrodynamics codes. The 2D-codes, when available, are used in a first approach only to determine whether two-
dimensional effects play an important role in the plasma evolution. These simulations are by far more costly in terms of computer-time. Hence some particular parts of the modeling, such as atomic physics discussed below, cannot be treated by two-dimensional simulations in such detailed way as they are by one-dimensional simulation. The second area is the detailed calculation of the plasma ionization kinetics in a situation where neither the local thermal equilibrium (LTE, fulfilled for high density plasma) nor the coronal equilibrium (CE, fulfilled for low density plasmas) is valid, and where the system may be strongly non-stationary. Thus an extensive set of atomic data is required to calculate the distribution among the ionization charge states of the target element, as well as the excited level populations, and line emission intensities. Part of the accuracy of these calculations thus relies on that of the atomic data. The detailed calculation of ionization kinetics is usually coupled to the above hydrodynamics area through the plasma energy balance equation. The paper is structured as follows. Section 2 gives details on the experiments and results. In order to obtain a better insight in the exact influence of these laser parameters on laser-ablation and laser-plasma formation we present comparatively, recent results obtained from LIBS to previously reported [20] characteristics from LA spectroscopic investigation of a plasma in air produced by the first harmonics of a Q-switched pulsed Nd: YAG laser. The plasma experimental parameters were estimated [21] from spectroscopic data. In the LIBS experiments, the Applied Photonics LIBS-6 instrument allowed modifying the laser irradiance at the sample surface by changing the pulse energy or the laser focusing distance. The Boltzmann plots corresponding to above mentioned experiments give the excitation temperature. Section 3 gives our concluding remarks. 2.
Experimental details and results
Full details on the LA experimental set-up and measurements have already been reported [20, 21]. We only recall here the principal ideas and results. For completeness, Table 1 reproduces from Ref. 21 the laser characteristics for LA and LIBS experiments to which present investigation is applied. The plasma on copper target has been produced by the second harmonic ( = 0.532 m, 180 mJ energy) of a Q-switched pulsed Nd: YAG laser (λ=1.064 µm, 360mJ energy in the first harmonic), of 4.5 ns pulse duration, and 0.1 ÷ 10Hz frequency used for excitation.
The plasma emission spectra were recorded while the target was rotated to have a clean area under irradiation. For the typical conditions used in LA experiments, the observed emission lines correspond to neutral and single ionized copper ions. Taking into account the finite dimension of the focal spot we developed a hydrodynamic model describing the plasma production as a steady state process. A simple theoretical model with two homogeneous rectangular regions corresponding to the hot emissive central area (the heating zone) and to the cold absorbing surrounding area, respectively, indicated radiative transfer for the Cu I resonance lines at 324.75 and 327.40nm for laser-produced copper plasma in air. This model allows a distinction of thermal and kinetic energy in the plasma. The plasma production was considered as a steady state process, as suggested by our earlier experimental investigation, showing a heating of the target layer by layer. This steady state is reached if the time which an electron ion pair spends within the hot central zone is less than the duration of the laser pulse. In the case of nanosecond pulses this assumption is generally fulfilled [22]. Temperature measurements were performed over a wide range of light intensities on copper plasma. In addition, our model assumes one-dimensional geometry. In this case the plasma interacts with the
incoming beam during the whole pulse duration. In reality the time of interaction is reduced due to more rapid decrease of the plasma density in 3D geometry, as well as due to the decreasing light intensity with increasing distance from the focal plane of the lens system. Another significant approximation of the model is the application of the absorption coefficient derived by combining Maxwell equations with equation describing the plasma properties [23]. Accordingly, the main absorption in the plasma takes place in the heating zone. The validity of this approximation is limited to the density region where the electron density is less than the electron density at which the corresponding plasma frequency equals the frequency of the incident light (ne < nep). Considering an energy balance between the incoming light flux and the thermal- and kinetic energy transported by the plasma flow, we were able to determine the electron temperature at the local thermodynamic equilibrium for neutral atoms [24]. The temperature calculation was made over a wide range of light intensities. We have used the Cu I line intensities at 510.55, 515.32, 521.82 and 522.01 nm and the Boltzmann plot method for the plasma in local thermodynamic equilibrium (LTE). Assuming the LTE for neutral cooper atoms, the line intensity, for optical thin plasmas is: Eu , k
gu , k kT N hc I k Cu e Aul , k 4 ZCu T k
(1)
where N Cu represents the total number of the Cu species in the plume, g u ,k and Eu ,k are the statistical weight and the energy of the upper level for the line k with wavelength k , the index l is for the lower level, Aul ,k is the emission rate for the spontaneous emission, h is the Planck
constant, c the vacuum light speed, and ZCu T is the partition function. In the present analysis the electron temperature was obtained from the slope -1/kT of the best fit of Boltzmann plot to a straight line:
ln
Ik k hcN Cu 1 ln Eu ,k g u ,k Aul ,k 4Z Cu kT
(2)
in the plane with the energy Eu ,k of the upper level on the horizontal axis and ln
Ik k on gu , k Aul , k
the vertical axis. The atomic parameters used in the evaluation of electron temperature, as level degeneracy, oscillator strengths and radiative transition probabilities, were taken from the spectral database of National Institute of Standards and Technology (NIST) [25] and are shown in Table 2. The mean electron temperature obtained from the Cu I line intensities ratio in the copper ablation experiment was of 8210 ± 370 K. The estimated LA plasma electron density was of ne = 3.15·1019cm-3.
Combined with other methods of analysis and other power sources, LIBS enhances detection capabilities. In the present work we have performed LIBS experiments where the laser –induced copper plasmas is generated using an Nd: YAG laser (wavelength 1064 nm, pulse width 4.5ns, repetition rate 20Hz) from sample placed in air at atmospheric pressure. Applied Photonics LIBS-6 allows modifying the laser irradiance at the sample surface by changing the pulse energy or the laser focusing distance. The experimental set-up has been presented in details in Ref. [21]. All the experimental operations, including sample movement, settings of the laser and of the spectral acquisition parameters were controlled by the personal computer. The samples were 1mm thick copper of 99.999% purity. The laser beam was focused at right angles onto the sample surface by a lens of 128mm focal length at 1064nm. The LIBS plasma spectrum has been recorded for series of laser pulse energy: 50mJ, 80mJ, 100mJ, 110mJ, and 150mJ keeping the frequency (10Hz) and the pulses accumulation rate constant. Calculations with the plasma
ablation model show that the initial plasma, both temperature and electron density are found to be significantly higher for the higher irradiance. The relative line intensities of atomic copper have been used to determine the electron temperature. The emission spectrum corresponding to the wavelengths domain of 200-900nm is displayed in Figure 1a, with a detail in Figure 1b for wavelengths between 508 and 526 nm. The pulse energy was 150mJ, 10 Hz repetition rate and 20 pulses accumulation rate. The integration time was 1ms and the integration time delay of 1.27 s. Figure 2 displays the Boltzmann plot based on the line intensities at 510.5nm, 515.3nm, 521.8nm and 522.0nm, excluding the background continuum emission. The corresponding determined electron temperature for this shot was T= 9350±133 K.
The estimated electron density is of 7.4 1015 cm-3. In order to examine the relevance of the plasma thermodynamic regime on the spectrally resolved intensities we have performed a temporal analysis of the plasma parameters based on FLYCHK program packages [26]. The solution for the population density for the case where the states are in LTE is obtained in the framework of statistical mechanics. The equations for the ratio of the ground states to the successive ionization stages is provided by the Saha-Boltzmann equation, while the ratio of the excited states to the ground states within each ionization stage is given by the Boltzmann equation. To solve for the total absolute number densities, the code uses the charge neutrality constraint that requires that the total ion density be related to the given electron density by the mean charge of the ions. Figure 3 gives the mean-ion charge distribution, and the corresponding total radiative loss, versus electron temperature into the plasma for three electron density values. The range of plasma parameters corresponds to measured values in our LIBS experiments.
It should be noted that the FLYCHK code is a „zero- dimension code”, in that there are no information on conditions of the plasma other than the local conditions specified. This indicates that a correct tratement of radiative transfer or gradients are outside of the scope of this suite of codes. The Atomic Data Analysis System (ADAS) [27] interconnected set of computer codes and data collections has been used for modeling the levels population density distribution N (q, i) of the i-th excited level (i =1 refers to the ground state) and the corresponding line intensity ratios in the electron density and temperature range as output from experiments. For a number of ten electron temperature values and eight electron densities, ADAS has been used to model the population density distribution over the excited states into the plasma, and the corresponding transitions line intensity. At each time the variation of this population is expressed by a rate equation involving the collisional excitation and deexcitation rates, spontaneous decay probabilities, radiative and 3-body recombination rates and the collisional ionisation rate. These processes connect the excited levels of an ionization stage to the ground state of the next higher one. We have addopted the following suitable approximations: (a) only the ground state of each ionization stage was significantly populated, and (b) ionization and recombination from these ground states were the dominat processes in the population transfer between successive ionisation stages. Figure 4 displays, as an example, the plot of the population density of the ground and first excited states of Cu I as output from the code.
In this figure the state population density is given as ratio to the ground population density times the electron density, for a range of electron density values. Application refers to 3d104s(2S1/2) ground and 3d104p ( 2 P3o/ 2 ), 3d105s ( 2 S1 / 2 ) and 3d104d(2D5/2) metastable states population. The electron temperature included into the calculation is 0.7eV.
Conclusions This work is part of general investigation which started a few years ago to comparatively study the effect of laser parameters on laser produced copper plasma. In order to get presented results, two experiments have been compared, namely the laser ablation spectroscopy and the laser induced breakdown spectroscopy on copper plasma. The experiments have been carried out using the second and the first harmonics of an Nd: YAG laser, respectively. The theoretical methods for investigating the plasma plume behavior and its main parameters calculation use spectral atomic copper main lines in the visible range. The fitting of the Saha-Boltzmann plot to a straight line provides an apparent averaged ionization temperature, of 8210±370 K in the LA experiment, and of 8123K in the LIBS experiment. A simple hydrodynamic model for the heating of plasma generated at the interaction of the laser beam with copper target has been used to define the range of LTE during the plasma expansion. Following this model, for nanosecond pulse duration and a finite focal spot diameter, the plasma production may be considered as a steady state problem. In our treatment the plasma is considered as a continuous particle flow originating at the target. In LIBS experiments different value of laser fluence has been used, keeping the laser frequency and the accumulation rate constant, in order to determine the electron temperature and density. In both cases, the heating of the plasma is due to energy absorption of the electrons from the radiation field by inverse bremsstrahlung. The corresponding absorption coefficient has been determined neglecting the factor (1-ne/nep)-1/2 which in turn limits the dependence of the absorption coefficient on the electron temperature in the density region ne < nep. We theoretically investigated the corresponding mean ion charge, the radiation loss, the population density distribution on the excited states and the line intensity ratios using FLYCHK
code. Following presented results we conclude that laser beam absorption and the plasma shielding is especially important near the target, where the Cu vapor, ion and electron densities reach their maximum (i.e. in laser-ablation experiment). Other phenomena such as the formation of early-stage plasma can start to take place at ps-laser pulses, which has not been considered here. The general tendency of the relative importance of the different absorption mechanisms, as well as the overall amount of laser absorption, seems to be more or less correctly predicted here, and will be further investigated. Comparing the laser fluence (which is defined as the laser irradiance integrated over the entire pulse) effect on target and plume characteristics it was observed that it increases with pulse duration. In parallel with the experiments, we have theoretically investigated the line intensity ratios for a set of plasma parameters and results are in good agreement to the experimental values for the transition considered in this work. We used the ADAS routines which solve the coupled rate equation for wide range of plasma parameters. Large amount of data (not presented here) has been obtained for cross sections and rates of the elementary processes into the plasma.
ACKNOWLEDGEMENTS
A. Stancalie and D. Sporea acknowledge financial support of The National Authority for Scientific Research and Innovation, Program NUCLEU PN 1647, LAPLAS IV. Some of the equipment used in this research were purchased in the frame of the project “Center for Advanced Lasers Technologies (CETAL)”, contract 8PM/2010, financed by UEFISCDI (RO).
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Figure captions Figure 1. (a). LIBS on copper plasma. The laser pulse energy was 150mJ at a repetition rate of 10Hz. The accumulation rate was 20, the integration time was 1ms and the time delay was 1.27 s. The emission spectrum corresponds to the wavelengths domain of 200-900nm, and (b) 508526nm, respectively.
Figure 2. The Boltzmann plot based on the line intensities at 510.5nm, 515.3nm, 521.8nm and 522.0nm, excluding the background continuum emission. The corresponding determined electron temperature for this shot was T= 9350±133 K.
Figure 3. (Left) The mean-ion charge distribution versus electron temperature for three values of electron density (lower) 1010cm-3, (middle) 1015cm-3, (higher) 1019cm-3 ; (Right) The total radiative losses versus electron temperature for electron density (from lowest to highest) of 1015cm-3, 1016cm-3, 1017cm-3 and 1019cm-3. A FLYCHK simulation.
Figure 4. Population density (in cm3) of the ground and first excited states of Cu I relative to ground state population times electron density, as function of the electron density (in cm-3 units). Te = 0.7 eV. There are three curves in this graph, labeled by 1, 2, 3. They correspond to 3d104p ( 2 P3o/ 2 ), 3d104d( 2 D5 / 2 ) and 3d105s(2S1/2) level population ratios to the ground 3d104s(2S1/2) population, respectively.
A
b Fig 1
Fig 2
Left
Right Fig 3
Fig 4
Table1. Laser –induced plasma formation conditions. Target Background gas Laser Irradiance Laser pulse duration Laser wavelengths
Table 2.
Copper Free atmosphere 108 -1010 W/cm2 0.3 – 10 ns 532nm, 1064 nm
Atomic parameters for the lines of Cu I used in the electron temperature evaluation:
wavelength, , radiative transition probability, Aul , upper level energy, Eu, and lower level energy, El , and the spectroscopic designation of corresponding transition. (nm)
Aul(s-1)
Eu(cm-1)
El(cm-1)
transition (upper - lower)
510.5541 515.3235 521.8202 522.0070
2.00E+06 6.00E+07 7.50E+07 3/20E+07
30783.697 49935.195 49942.051 49935.195
11202.618 30535.324 30783.697 30783.697
3d104p 2P°3/2 - 3d94s2 2D5/2 3d104d 2D3/2 - 3d104p 2P°1/2 3d104d 2D5/2 - 3d104p 2P°3/2 3d104d 2D3/2 - 3d104p 2P°3/2