摘要:
Laser-induced breakdown spectroscopy at metal–water interfaces Honoh Suzuki, Hisanobu Nishikawa and I-Yin Sandy Lee 1 Introduction Laser-induced breakdown spectroscopy (LIBS) is a promising tool for the analysis of trace impurities in metals1 and aerosols.2,3 In the technique, a laser is either focused in the gas phase or directed onto the solid surface to cause optical breakdown. The resulting laser-induced plasma decomposes and excites all species within the plasma volume, and is characterized by a continuum spectrum containing discrete atomic emission lines. The breakdown is a combination of diverse phenomena including laser ablation, plasma generation and electron emission; the physical basis of the phenomena is in active research.4–6 The process of LIB is known to be highly inhomogeneous in spatial and temporal profiles.7 The spatially resolved1 and imaging8 studies on LIBS at the metal–air interface revealed that the elemental-line emission extends over a few millimeters above the metal surface.In view of this, LIBS at the metal– liquid interface is expected to be substantially different from that at the metal–air interface, especially for processes occurring in the gas or liquid phase. By comparing LIB spectra in air and in water, one may be able to distinguish various lightemitting processes involved in the LIB event. Understanding and modelling of the strong continuum emission is also helpful for its use as a convenient pulsed light source for spectroscopy. In this letter, we report LIB spectra at metal–water interfaces and describe a simple model based on thermal diffusion and blackbody radiation.Three common metals, Al, Ti and Pt, are chosen because of their largely different optical and thermal properties.Fig. 1 A schematic of the experimental setup. Department of Chemistry, Toyama University, 3190 Gofuku, Toyama, Japan 930-8555. E-mail: honoh@sci.toyama-u.ac.jp Received 15th March 2002, Accepted 7th May 2002 Published on the Web 17th May 2002 Visible light emission from metal–water interfaces at laser-induced breakdown (LIB) has been observed for aluminium, titanium and platinum. The spectra are found to consist of broadband continuum without any discrete atomic lines, which may be useful as a pulsed light source for spectroscopy.A simple thermal diffusion model combined with blackbody radiation is described, the numerical result of which agrees fairly well with the observed spectra. 2 Experimental A schematic of the experimental setup is shown in Fig. 1. The excitation light source is a Q-switched Nd: yttrium–aluminum– garnet (YAG) laser (l ~ 1064 nm) with a pulse width of 5 ns and pulse energy of 450 mJ (Continuum, Surelite I-10). The pulse duration is monitored using a fast InGaAs PIN photodiode (Hamamatsu, G3476-01) and a digitizing oscilloscope with 2.5 GHz sampling speed (Tektronix, TDS3032). The laser pulse energy is separately measured with a high-energy power meter (Ophir, 10A-P). The collimated laser beam, which has a spot diameter of about 5 mm and TEM00 near-Gaussian pro- file, transmits into a quartz cell and hits the sample surface at an incident angle of ca.45u. Each laser pulse initiates breakdown at the metal surface. The sample, either in the air or immersed in water, is attached to a motorized 2D translator and displaced between shots so that a fresh surface is always used. The incident intensity is well below the breakdown threshold of bulk water. The emitted light is collected into an optical fiber connected to a CCD spectrometer (Ocean Optics, CHEM2000), which is triggered with a delay generator (SRS, DG535) to record time-integrated emission spectra. The trigger signal provides pretriggering with respect to the laser pulse and enables the system to open the CCD gate before the plasma starts emitting.Only the central area (ca. 0.8 mm in diameter) of the breakdown spot is imaged into the fiber so that the spatial variation of the emission is minimized. A rather short distance (30 mm) between the sample and the collecting lens has limited the availability of incident and detection angles; within the limits, the dependence of the emission spectra on these angles is found to follow the ordinary cosine law, so that the angles are chosen to optimize the emission signal under the geometrical constraints. The spectrometer’s sensitivity over 360–850 nm is calibrated with a NIST-traceable standard light source (Ocean Optics, LS-1-CAL). Doubly distilled water is used. Sample metal plates (Nilaco Corporation, Al, 99.5%; Ti, 99.5%; Pt, 99.98%) are used without further treatment.3 Results and discussion Typical LIB spectra for Al, Ti, and Pt in air and in water are presented in Fig. 2. The spectra in air consist of sharp lines and continuum. These lines are identified with the element’s atomic(I) and ionic(II) emission.9 Time-resolved LIBS studies3,7 showed that the duration of the continuum emission is short (v1 ms) compared with those of the elemental lines 88 PhysChemComm, 2002, 5(13), 88–90 This journal is # The Royal Society of Chemistry 2002 Paper DOI: 10.1039/b202597cFig. 2 Laser-induced breakdown spectra at metal–air and metal–water interfaces. The spectra in air are displaced for clarity. The dashed lines indicate the theoretical fluence Fd based on the heat diffusion model.(w10 ms), which was roughly confirmed in our experiment by altering the pretriggering delay time between the CCD activation and the breakdown. In contrast, the spectra in water show broadband continuum only; the atomic and ionic lines are completely quenched at the metal–water interface. The spectral profile of the continuum in water is similar to the continuum in air (i.e., background to the elemental lines), indicating that the physical process behind it is primarily the same in both cases. The continuum is more intense at shorter wavelengths for all the metals examined. It is also three to four times more intense for Ti than those for Al and Pt. Such continuum emission without discrete elemental lines may be useful as a convenient pulsed light source for timeresolved UV-visible spectroscopy. The present result shows that two distinct processes can be identified, which react differently to the environment at the metal surface.The first process, which causes the continuum emission, may primarily reside in the metal’s skin depth where water has little influence on it, whereas the second process ablates metal atoms and ions out of the surface to generate the elemental lines, which are effectively quenched in the presence of water. The strong continuum emission, commonly observed at the early stage of LIB, is in general a complicated phenomenon including Doppler and Stark broadening, bremsstrahlung and blackbody emission.10 In the case of dense plasmas, however, photons emitted in the interior are absorbed and reemitted several times before they escape, so that the smoothed spectrum approaches that of a blackbody. To examine the possibility to explain the observed spectra at the metal–water interface in terms of the blackbody emission, we performed a simple thermal diffusion analysis.Because of the large thermal conductivity of metals, the conductive heat flux through the metal dominates; conduction and convection into water is negligible in the nanosecond time scale. Due to the small thermal diffusion length and the small radiation penetration depth, the heat conduction problem can be treated as one-dimensional with the surface heat flux boundary condition, 2KhT/hx|x ~ 0 ~ AI(t), where K is the thermal conductivity, A the surface absorptivity at the laser wavelength, and I(t) the incident laser pulse intensity as a function of time.In this model, local thermal equilibrium is assumed, i.e., T(electron) c T(ion) c T(lattice), which may be plausible for laser heating of solid surface in the nanosecond region.11 It is also assumed that melting, vaporization and ablation occur in the small skin-depth layer (v25 nm) and do not alter the heat diffusion process significantly; the latent heat for melting and vaporization is small compared with the incident laser power density (c 0.6 GW cm22). For constant thermal properties, the transient surface temperature is analytically obtained:12 LI(t) (1) T(t){T t{t eq~ 2 t p 0 � Lt dt, ......... ......(2) K where Teq is the equilibrium temperature and a ~ K/(rCp) is the thermal diffusivity. At a very high temperature (¢46104 K), radiative heat loss from the surface to water becomes significant. In such cases, the boundary condition is modified to incorporate the total radiative flux from the surface:13 R(t)~n2s(T4(t){Teq 4 ), gate a pp p A e’ F Qp 0 where n is the refractive index of water and s (~5.670 6 1028 W m22 K24) is the Stefan–Boltzmann constant. The radiation loss has a T4 dependence and serves as a limiting mechanism for the surface temperature not to exceed ca. 5 6 104 K for our laser power density. The time-integrated spectral radiation fluence at the detector is given byd(l,h,w)~C �t l(T(t),l,h,w)Ebl(T(t),l)dt, (3) where l is the wavelength, h and w the polar and azimuthal angles, tgate the CCD gating time, e0l(T(t), l, h, w) the directional spectral emissivity and Ebl(T(t), l) the blackbody (Planck) emissive power.13,14 The constant, C ~ Sem Vfab cosh/Sfib, is a view factor that depends on the imaged surface area Sem, the fiber cross section Sfib, the solid angle V and the aberration correction factor fab of the detecting optics.The value of fab has been estimated with a ray-tracing program for optical design.15 By knowing the thermal and optical constants9 and neglecting relatively small temperature dependence of the emissivity,13 the surface temperature T and the thermal radiation fluence Fd expected for our experimental condition can be estimated.T and thus Fd strongly depend on the absorptivity A, which is largely different for each metal.9 Thus, a naive use of the material absorptivities results in Fd values differing by five orders of magnitude between Al (A~0.05) and Ti (A~0.45), and is incompatible with the observed spectra. Above the breakdown threshold, the metal surface is covered by a dense plasma layer which is highly absorbing. Accordingly, we assume high absorptivity and emissivity regardless of the metal, i.e., Aye0l y1. This may represent a model surface covered by a plasma sheath, which acts as a highly absorbing and emitting coating; nonetheless, it is so thin that the surface transient temperature is still governed and characterized by the material thermal diffusivity.Based on this model, the transient temperature T(t) has been numerically calculated (Fig. 3). T(t) quickly decays in less than 30 ns and most thermal photons emit in the nanosecond time scale, which is consistent with the observed short lifetime of the 89 PhysChemComm, 2002, 5(13), 88–90Fig. 3 Transient surface temperature profiles based on the heat diffusion model. The dashed line indicates the incident intensity I(t) (arbitrary scale). Fcontinuum emission. The spectral fluence Fd is also compared with the experimental spectra in Fig. 2. In spite of the oversimplified nature and limitation of our model, the agreement of d with the experimental spectra is fairly good; it is noteworthy that no adjustable parameter is used in our model calculation.In particular, the theory correctly reproduces the more intense emission for Ti than those for Al and Pt, which results from the higher T(t) (Fig. 3) because of the smaller thermal conductivity of titanium. The theory is less satisfactory for Al, where a better estimate for the thermal conductivity under laser-induced breakdown might improve the theory. In summary, the LIB spectra at the metal–water interface have been observed to provide insight into the continuumemitting process. A simple model combining thermal diffusion and blackbody emission has successfully reproduced the continuum emission spectra. The model can be used for 90 PhysChemComm, 2002, 5(13), 88–90 understanding and predicting the emitting intensity, spectrum and temporal profile based on the experimental conditions such as the metal’s material properties, incident laser power and pulse duration, and thus helps its utilization as a spectroscopic light source.Future work will better quantify the spatial and temporal profiles of the LIB process at solid–liquid interfaces, and lead to more precise modelling that incorporates important aspects neglected here, such as the accurate absorptivity, temperature dependence of the thermal diffusivity, and the ablation loss. This work has been financially supported by Grants-in-Aid for Scientific Research No. 10559016 and 12878073 from the Ministry of Education, Science and Culture of Japan.References 1 D. E. Kim, K. J. Yoo, H. K. Park, K. J. Oh and D. W. Kim, Appl. Spectrosc., 1997, 51, 22–29. 2 D. W. Hahn, Appl. Phys. Lett., 1998, 72, 2960–2962. 3 L. J. Radziemski, T. R. Loree, D. A. Cremers and N. M. Hoffman, 4 R. C. Issac, P. Gopinath, G. K. V. V. P. N. Nampoori and 5 S. Amoruso, M. Armenante, R. Bruzzese, N. Spinelli, R. Velotta 6 S. S. Mao, X. Mao, R. Greif and R. E. Russo, Appl. Phys. Lett., 7 B. C. Castle, K. Visser, B. W. Smith and J. D. Winefordner, Appl. 8 B. C. Castle, K. Visser, B. W. Smith and J. D. Winefordner, Anal. Chem., 1983, 55, 1246–1252. C. P. G. Vallabhan, Appl. Phys. Lett., 1998, 73, 163–165. and X. Wang, Appl. Phys. Lett., 1999, 75, 7–9. 2000, 76, 31–33. Spectrosc., 1997, 51, 1017–1024. Spectrochim. Acta, 1997, B52, 1995–2009. 9 CRC Handbook of Chemistry and Physics, ed. D. R. Lide, CRC Press, Boca Raton, FL, 76th edn., 1995. 10 H. R. Griem, Principles of Plasma Spectroscopy, Cambridge University Press, Cambridge, 1997. 11 P. P. Pronko, P. A. VanRompay, R. K. Singh, F. Qian, D. Du and X. Liu, Mater. Res. Soc. Symp. Proc., 1996, 397, 45–51. 12 S. Chen and C. P. Grigoropoulos, Appl. Phys. Lett., 1997, 71, 3191–3193. 13 M. F. Modest, Radiative Heat Transfer, McGraw-Hill, New York, 1993. 14 R. Siegel and J. R. Howell, Thermal Radiation Heat Transfer, Taylor & Francis, New York, 4th edn., 2002. 15 OSLO-Lt. Optics Software for Layout and Optimization, Lambda Research Corporation, Littleton, MA, 2
ISSN:1460-2733
DOI:10.1039/b202597c
出版商:RSC
年代:2002
数据来源: RSC