Measurements of Ionization Potentials with Laser-induced Plasma Spectroscopy International Science Press ISSN:

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1 I R A M P 7(1), June 2016, pp Measurements of Ionization Potentials wi Laser-induced Plasma Spectroscopy International Science Press ISSN: Measurements of Ionization Potentials wi Laser-induced Plasma Spectroscopy ASHRAF M. EL SHERBINI 1, MOHAMED M. HAGRASS 2 AND CHRISTIAN G. PARIGGER 3* 1 Laboratory of Laser and New Materials, Faculty of Science, Cairo University, Giza, Egypt 2 Faculty of Engineering, Electrical Engineering Department, Alexandria University, Egypt 3 The University of Tennessee/University of Tennessee Space Institute, Center for Laser Applications, 411 B.H. Goeert Parkway, Tullahoma, TN , USA *Corresponding auor. cparigge@tennessee.edu ABSTRACT: The first ionization potential of neutral atoms is determined at e resholds of -induced optical breakdown. The induced plasmas at e surface of aluminum, silver, lead, indium and copper are created in laboratory air wi focused, 5-ns pulsed Nd:YAG, 1064 nm IR radiation. At fixed spot size of 2 ± 0.1 mm, e fluence is varied from 16 to 3 J/cm 2. For several selected elements at show strong atomic emission lines, e first ionization potentials of Al I 396.2, Ag I 520.9, Pb I and 406.2, In I and Cu I nm are measured to amount to 5.9 ± 0.2, 7.6 ± 0.3, 7.4 ± 0.2, 5.8 ± 0.1 and 7.7 ± 0.2 ev, respectively. The measured ionization energies for e different targets nicely agree wi tabulated values, consequently, is agreement can be interpreted as confirmation for e -induced plasma reshold model. PACS Codes: Mf, m, r, Jm, Fi, d Keywords: Laser ablation, plasma diagnostics, atomic spectroscopy, plasma ionization, spectroscopy. 1. INTRODUCTION The generation of -induced plasma requires sufficient irradiance to initiate e avalanche type optical breakdown process [1-9]. Plasma is described as e four state of matter and is mainly composed of four different species: Atoms, electrons, ions and radiation field at are distributed according to well-known equilibrium distribution functions. The minimum energy flux required for plasma formation is called reshold fluence,, (J/cm 2 ). The fluence resholds [1-9] depend on parameters of e material at include density, latent heat of vaporization, coefficient of conductivity and specific heat as well as on excitation waveleng and ionization energy [1, 2, 10-12]. In is work, bulk metallic targets are investigated to infer e ionization potentials [13] using selected lines of aluminum, silver, lead, indium and copper. The measured first ionization potentials are expected to agree wi published values, ereby confirming e model for e breakdown resholds of bulk material [1, 2]. 2. LASER-INDUCED THRESHOLD At e irradiance reshold, e amount of energy just needed to vaporize e target material in e focal volume is given by e term [5],, which is constant for each material and can be calculated from classical material-dependent properties. However, in order to ionize is vapor, an additional -dependent term [1, 2], International Review of Atomic and Molecular Physics, 7 (1), January-June

2 Ashraf M. EL Sherbini Mohamed M. Hagrass and Christian G. Parigger = C lt, needs to be considered [1, 2]. Here, l T is e conduction or diffusion leng (m), is e waveleng and i is e first ionization potential energy. The reshold term for e fluence,, originates from e timeaveraged ponderomotive energy for linear polarization [14, 15]. The proportional relation of e reshold fluence wi e inverse square waveleng and linear conduction leng was already studied for nano-materials of different sizes [2, 3, 4]. The constant C is composed of a combination of electromagnetic constants [1], including m e, 0, c and e denoting mass of e electron, vacuum permittivity, speed of light and elementary charge, respectively, i 2 = / 2 = e o. (11) C m c e m (2) Recent work discussed measurements of resholds,, and confirmed at e complimentary -dependent contribution,, at different irradiaton wavelengs needs to be be added to describe fluence resholds of -induced plasma at e surface of bulk material [1, 2], 15 i = l. 2 T l aser (3) The contribution,, amounts to and e conduction or diffusion leng [1, 2, 5], l T, can be expressed as l = L l, (4) 32 International Review of Atomic and Molecular Physics, 7 (1), January-June 2016 v T = / C, (5) T T p where T is e conductivity coefficient of e material (W/m K), is e density (kg/m 3 ), is e pulse duration (s), C p is e specific heat at constant pressure (J/kg K) and is e latent heat of vaporization in (J/kg). For e evaluation of e ionization energies, Eq. (3) is suggested and applied in is work, and it can be expressed in frequently encountered units for fluence, waveleng and conduction leng, 2 2 i e V = 27.9 (J/cm ) m. (6) l In is formula, should be larger an e classical vaporization term,. Therefore, at a fixed irradiation waveleng and wi knowledge of classical properties, one can measure e ionization potential, i, by decreasing e energy to find e fluence reshold,, for plasma generation. 3. EXPERIMENTAL DETAILS The experimental configuration for e ionization energy measurements includes a Q-switched Nd:YAG device, focusing and attenuation optics, and optical fiber coupled to a spectrometer equipped wi an intensified detector [1, 2]. The Nd:YAG pulsed is operated at e fundamental waveleng, = 1064 nm, wi pulse duration, = 5 ns, delivering an output energy of 470 mj per pulse. The radiation is focused wi a convex lens of 1 m focal leng. The focal spots show radii of 2 ± 0.1 mm, measured using heat sensitive paper (Kentek or Quantel heat sensitive paper). The fluences are varied in e range of 16 to 3 J/cm 2 wi a set of calibrated glass attenuators. The variation of e radiation is monitored using a 4% reflective beam splitter wi an absolutely calibrated power T

3 Measurements of Ionization Potentials wi Laser-induced Plasma Spectroscopy meter (Ophir model 1z02165). The emitted radiation from e plasma is collected wi an optical fiber of 25 m diameter, connected to e detection system comprised of a spectrograph (SE 200 Echelle Spectrograph) and an intensified charge coupled device (ICCD Andor-iStar DH734-18F). The camera is used to record e time-resolved spectra in e range of 200 to 1000 nm. The fiber tip is positioned at a distance of 18 ± 2 mm from e plasma expansion axis. Absolute calibration of e detector system is accomplished wi a deuterium-halogen light source (Ocean Optics DH-2000-CAL) [16]. In order to obtain clear optical signals from different plasmas, e light emissions were collected over ree different shots. The strongest spectral line intensity for each material was selected, e.g., e In I at nm and Al I at nm lines. The isolated lines are selected to record e plasma emission at different irradiation levels, and e signal to background ratio, S/B, is monitored. 4. RESULTS AND DISCUSSION The target materials used in our work are selected to cover a relatively wide range of expected reshold values from 0.4 for Pb to 3.5 J/cm 2 for Cu and diverse properties. Table 1 summarizes e and physical parameters. This table shows at ere is an apparent increase of isobaric specific heat of materials except for aluminum which has relatively large heat capacity. There is an apparent increase in e conductivity ( T ) and latent heat of vaporization (L v ) except for copper and aluminum. Neier one of e physical quantities is responsible of e reshold of plasma ignition alone but only a combination of ese quantities may be responsible for e generation of -induced plasma. Table 1 Thermal and physical parameters of e elements used in is work in SI units for density,, latent heat of vaporization, L v, conductivity coefficient,, specific heat (isobaric), C (J/kg K) and recommended waveleng, (nm). For indium and T p aluminum, lines emerging from resonance transitions are indicated. The listed data are taken from e NIST data base [17]. r L v T C p (kg/m 3 ) (10 6 J/kg) (W/m K) (J/kg K) (nm) Pb In Al Ag Cu Regarding e lead Pb I line, e midpoint nm of e two prominent lines at wavelengs of and nm is considered because ey appear actually as a single line even at lower irradiance levels. Stark broadening and choice of resolution instrumental bandwid (0.12 nm) resulted in overlap of e two lines separated by 0.5 nm. Table 2 shows e variation of e combination of conductivity, specific heat, density and pulse duration, labeled conduction leng, l T, and e heat enalpy per unit volume, L v (J/m 3 ). The table also shows e term,, see Equation (4). The investigated elements are arranged in Table 1 in ascending order of e predicted reshold fluences according to Eq. (6) at e waveleng of 1064 nm. As can be noted from e data in Table 2, ere is an increase from Pb to Cu in e conduction leng and e density heat enalpy product of e materials except for silver because of e its relatively large conductivity. The contribution, = L v l T, shows an increase but is consistently smaller an e eoretically predicted contribution from e fluence,, indicated in Eq. (3). Figure 1 illustrates typical recorded copper emission spectra. The integrated spectral intensity is obtained by evaluating e area of e lines after subtracting e continuum or background. International Review of Atomic and Molecular Physics, 7 (1), January-June

4 Ashraf M. EL Sherbini Mohamed M. Hagrass and Christian G. Parigger Table 2 Element parameters: Density heat enalpy product, L v, conduction leng, l T, term,, and predicted resholds from Equation (6),. L v l T (kj/cm 3 ) (nm) (J/cm 2 ) (J/cm 2 ) Pb In Al Ag Cu Figure 1: Measured spectra of e nm Cu I line versus waveleng and fluence. Cu Figure 2: Signal to background ratio, S/B, for e nm Cu I line and reshold, = 3.4 J/cm 2, at S/B = International Review of Atomic and Molecular Physics, 7 (1), January-June 2016

5 Measurements of Ionization Potentials wi Laser-induced Plasma Spectroscopy The plasma-reshold fluence is determined via backward extrapolation at e recommended signal-to-background, S/B, value of ree as indicated in Figure 2. The log-log plot in Figure 2 shows a linear decrease in e S/B ratio for smaller fluences in e range of 10 to 3 J/cm 2. For larger signal levels, e recorded spectral intensities of e lines tend to saturate. The saturation may be attributed to self-absorption effects at e relatively large fluences. Table 3 exp Measured resholds,, ionization energies,, and comparison wi tabulated ionization energies,, and work functions, i tab W tab, for bulk metallic targets. exp i tab W tab (J/cm 2 ) (ev) (ev) (ev) Pb 0.42 ± ± In 0.80 ± ± Al 2.10 ± ± Ag 2.55 ± ± Cu 3.40 ± ± Table 3 shows e experimentally measured reshold fluences corresponding to each element, e measured first ionization energy utilizing e plasma reshold fluence, e tabulated standard values for e ionization energies and for e work functions. The work function is defined as e minimum energy required generating free electrons from e surface of a solid. The work functions of e different elements show nearly constant values in e range of 4.1 to 4.7 ev. However, e discussed model for -induced breakdown resholds in laboratory air at standard ambient temperature and pressure, indicates at e ionization energy is e significant term for fluence resholds in Eq. (6) raer an work function. Excellent agreement of measured and tabulated first ionization potentials can be noticed in Table 3 for e different elements. This agreement can be viewed as a furer confirmation of e validity of e term as suggested previously in generation of plasma in laboratory air at or near e surface of different target materials [1, 2]. 5. CONCLUSION The induced plasma reshold dependence on waveleng and ionization energy was utilized to provide means to measure e first ionization potential of elements. The measured ionization energies for e different targets nicely agree wi tabulated values. In turn, is agreement can be interpreted as confirmation for e -induced plasma reshold model. As an extension to is work, studies wi different wavelengs will be reported. ACKNOWLEDGMENT The auors acknowledge e continued interest and comments from Professor Th. M. EL Sherbini and ank for support in part by e Center for Laser Applications at e University of Tennessee Space Institute and in part by e Laboratory of Laser and New Materials at Cairo University. REFERENCES [1] A.M. EL Sherbini, C.G. Parigger, Spectrochim. Acta Part B 116 (2016) 8. [2] A.M. EL Sherbini, C.G. Parigger, Spectrochim. Acta Part B 124 (2016) 79. [3] E.G. Gamaly, A.V. Rode, B. Luer-Davies, J. Appl. Phys. 85 (1999) International Review of Atomic and Molecular Physics, 7 (1), January-June

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