- Email: [email protected]
Diagnostic of an expanding laser-produced lithium plasma using ICCD frame photography and shadowgraphy
Applied Surface Science 127–129 Ž1998. 1035–1040
Diagnostic of an expanding laser-produced lithium plasma using ICCD frame photography and shadowgraphy William Whitty, Jean-Paul Mosnier
)
School of Physical Sciences, Dublin City UniÕersity, GlasneÕin, Dublin 9, Ireland
Abstract The expansion of a laser-produced lithium plasma is characterized using two different high-speed imaging techniques. Firstly, a sequence of frames of the luminous plume is recorded using an interference filterrgated ICCD camera combination. Expansion velocities are estimated from these images. The conditions, in which the radial distributions of emitters could be recovered using Abel inversion, are discussed. Secondly, shadowgraphs obtained with a synchronized tunable dye laser light source are recorded at different probe wavelengths in the vicinity of the Li 0 670.7-nm resonance. The fringe patterns observed in these images are interpreted in terms of strong refractive index gradients within the plasma. The effect of anomalous dispersion is observed and strongly modifies the appearance of the shadowgraphs. q 1998 Elsevier Science B.V. PACS: 52.70 La; 32.70 Jz Keywords: Laser plasma; High-speed imaging; Anomalous dispersion; Shadowgraphy; Lithium
1. Introduction Laser-produced plasmas are characterized by steep and rapidly varying electron densityrrefractive index and temperature gradients. Their spatio-temporal evolution is driven by collisional processes between electrons, ions, neutrals and photons. Laser plasmas are intense sources of line and continuum electromagnetic radiation due to the relative strength of the radiative processes. Thus, the motion of the excited plasma-emitting light by de-excitation or recombination in ions or neutrals can be determined using high-speed imaging. It provides information on the ‘local’ structure and dynamics of the constituent particles provided that a radiation model linking ) Corresponding author. Tel.: q44-353-1-704-5303; fax: q44353-1-704-5384; [email protected].
observed light intensities to particle distributions exist. At least one severe complication in this recovery process arises from the fact that the recorded 2D-intensities are necessarily integrated along the observed plasma depth, hence, requiring an appropriate transformation of the data. In the present work, two different high-speed imaging techniques, namely, framing camera imaging and shadowgraphy, are used. We now present their essential features. High-speed photographic recording of expanding laser plasmas using framing image-converter cameras is a well-established technique w1,2x. More recently, the technique has been refined by the introduction of gated ICCD ŽIntensified Charge Coupled Device. cameras which allow direct digital recording, thus, greatly facilitating computer processing of the images as well as providing improved sensitivity. Such imaging devices have been widely used in the
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 7 . 0 0 6 0 3 - X
1036
W. Whitty, J.-P. Mosnierr Applied Surface Science 127–129 (1998) 1035–1040
area of pulsed laser deposition ŽPLD. of materials Žsee, e.g., Ref. w3x.. Despite the technique’s ease of implementation, the retrieval of quantitative information characterizing the plume expansion from the images is usually complex. By narrowing down the spectral range of the imaged light to that of a known emission line, it becomes possible to track the evolution of the corresponding excited state. This can be carried out by placing a tuned interference filter in front of the camera, in which case, the measured intensities are integrated over the line profile. Spectrally-resolved images can also be obtained if an imaging spectrometer is used w4x. The shadowgraph technique consists of passing a pencil of light through the test section and letting it fall directly, or via an imaging lens, onto a recording device such as a photographic plate or a CCD matrix. It has long been used in the study of compressible gas flow w5x and plasmas, including laser-produced plasmas Žsee, e.g., Ref. w2x and references therein.. One can show that the intensity modulations in the shadowgraph depend upon the second derivative of the refractive index of the traversed medium w5x. The technique is thus suitable for the study of expanding laser plasmas where the refractive index exhibits very rapid changes. Intense pulsed laser light sources proved particularly useful to record the shadowgraphs of bright laser plasmas and the technique has commonly been applied over the years. Numerous references to such early work in the 60s and 70s can be found in Ref. w2x. Michaelis and Willi w6x showed that when probing a laser plasma of steep refractive index gradient with a pencil of laser light, the rays traversing the denser regions suffer greater refraction and may interfere with the rays that did not interact with the plasma, thus producing a pattern of bright and dark fringes called refractive fringes. The authors showed, using an optical path ray analysis, that such refractive fringes could be used to determine the electron density distributions of laserproduced plasmas Žsee also Refs. w7,8x for improved analysis.. More recently, the use of tunable dye laser light as the probe beam brought a further refinement to the technique and was frequently applied as a diagnostic tool in the area of Pulsed Laser Deposition ŽPLD. of materials w9–11x. One should point out here that shadowgraphs obtained with laser light may be regarded as single beam holograms of laser plas-
mas. Holograms of laser plasmas were first recorded in the 60s Žsee, e.g., Ref. w2x.. The strength of the interaction between electromagnetic radiation of angular frequency v and matter, is embodied in the value of the index of refraction nv . The value of nv largely determines the intensity distribution of light in the images obtained with the methods just described, and is related to the particle densities of the probed medium, thereby explaining the principle of the diagnostic. It is wellknown that the index of refraction of a plasma at optical frequencies is largely due to the free electron contribution w12x. However, in the vicinity of an atomic resonance, significant additions must be made to the refractive index. Since we are concerned with such a situation Žsee Section 2., we write the complex index of refraction n˜ s n y i k of a plasma in the vicinity of an atomic resonance of angular frequency v 0 , weighed oscillator strength f and width G in the form w12,13x: n˜ 2 y 1 s
1
ž /
4p Nfe 2
1
me
v 02 y v 2 q i Gv
4p´ 0
q n 2e y 1
Ž 1.
where N is the number of atoms per unit volume and m e the electron mass. The free electron contribution is equal to: n 2e s
(
1y
v p2
Ž 2.
v2
where v p is the plasma frequency for an electron density Ne such that:
v p2 s
1
4p Ne e 2
4p´ 0
me
ž /
.
Ž 3.
From Eqs. Ž1. and Ž2., we see that the electron gas contribution to the phase index of refraction is negative and slightly less than unity. On the other hand, the complex term gives rise to the effect of anomalous dispersion Žsee Section 3 for details.. In Sections 2 and 3, we give a brief description of the experimental set-up used to record frames and shadowgraphs of a laser-produced lithium plasma expanding either in vacuum or in a low-pressure argon atmosphere. We then present typical images
W. Whitty, J.-P. Mosnierr Applied Surface Science 127–129 (1998) 1035–1040
1037
obtained in both configurations and provide some elements of their physical interpretation.
2. Experimental and results The basic configuration used for both types of experiments is shown in Fig. 1a. The arrangement for shadowgraphy is illustrated in Fig. 1b. The lithium plasma is created by a Nd:YAG laser Ž0.4 J, 15 ns. using a slightly defocused 120-mm uncorrected plano-convex lens ŽFL., providing a target irradiance of ; 3 = 10 10 Wrcm2 Ž; 450 Jrcm2 .. The dye laser beam used to probe the plasma is initially expanded ŽBE. to cover the entire sensitive area of the CCD. The optical FWHM ŽFull-Width Half Maximum. is 8 ns, thus, introducing temporal resolution. The experiment is synchronized using a master pulse from a PC ŽPC486. to a digital delay generator ŽDG., thus controlling the inter-laser triggering delay Ž DT . to an accuracy of less than 3 ns. Background light emission from the plasma is eliminated by the introduction of neutral density filters andror a tuned interference filter ŽND q IF.. Blocking the dye laser beam and substituting the CCD detector for a gated image intensifierrCCD combination coupled to a zoom lens Žwhile retaining Fig. 2. Temporal and spatial evolution of Li 0 Ž670.7 nm.. Ža,b,c. Expanding into vacuum. Žd,e,f. Expanding into 200 mTorr of argon. Ža,d. DT s10 ns. Žb,e. DT s100 ns. Žc,f. DT s 250 ns. All units are in millimeter.
Fig. 1. Ža. Basic experimental configuration: ŽCCD. Charge Coupled Device, ŽPC486. 486 Personal Computer, ŽDG. Delay Generator. Žb. Detailed view of the apparatus used for shadowgraphy: ŽBE. Beam Expander, ŽFL. Focusing Lens, ŽNDqIF. Neutral Density FiltersqTuned Interference Filter.
the neutral density filters and tuned interference filter. provides the arrangement for frame photography. An uncorrected plano convex lens ŽFL. of 190-mm focal length was used to create the plasma in this case. The spot size was ; 5 mm in diameter, resulting in an irradiance of ; 2.7 = 10 8 Wrcm2 Ž; 4 Jrcm2 .. Synchronization is achieved as before. Successive frames of the lithium plasma are obtained by varying the triggering delay between the laser ŽNd:YAG 0.7 J; 15 ns. used to create the plasma and the gating pulse Ž8 ns FWHM. to the image intensifier. The temporal and spatial evolution of a lithium plume expanding into vacuum as well as into a low
1038
W. Whitty, J.-P. Mosnierr Applied Surface Science 127–129 (1998) 1035–1040
teristic is the strong spatial anisotropy of the early expansion w1x: the plume remains mostly confined around the axis constituting the target normal with a velocity component Žlongitudinal. along this axis larger than the velocity component Žradial. perpendicular to the target normal. The density contour shows a certain asymmetry in the very early stage probably due to inhomogeneities in the energy distribution of the laser beam. In order to estimate the longitudinal velocity of neutral lithium, we follow the procedure first described in Ref. w14x. At a fixed distance from the target on the axis of expansion Ž y s 0 mm., we record the integrated intensity variations of the Li 0 670.7-nm emission line as a function of time. This procedure is then repeated for different positions d along the axis ŽFig. 4.. A plot of d as a function of the time corresponding to the peak of the distribution curve yields the desired quantity. A value
Fig. 3. Shadowgraphs of Li 0 Ž670.7 nm. at DT s 70 ns. Ža. l probe s669.7 nm. Žb. l probe s670.7 nm. Žc. l probe s672.5 nm.
pressure gas Ž200 mTorr of argon. is shown in Fig. 2 for different time delays. The camera is tuned to the 2s–2p Ž670.7 nm. Li 0 transition using an interference filter centered at 670.7 nm with a 9.3 nm ŽFWHM. spectral window. Shadowgraphs at different probe wavelengths in the vicinity of the same transition are presented in Fig. 3.
3. Analysis and discussion First, we consider the sequence of frames presented in Fig. 2a,b,c depicting the expansion in vacuo of the laser-produced lithium plasma. Charac-
Fig. 4. The integrated intensity variations of Li 0 Ž670.7 nm. at fixed distances d Žmm. as a function of time Žns..
W. Whitty, J.-P. Mosnierr Applied Surface Science 127–129 (1998) 1035–1040
of ; 2.4 = 10 6 cm sy1 is obtained. It should be compared with the advance velocity of the plasma front which is recovered by measuring the position of the leading luminous edge of the plume as a function of time, corresponding to frames obtained without the interference filter Žnot shown here.. The subsequent velocity shows the linearity typical of a free expansion with a measured slope of ; 3.5 = 10 6 cm sy1 . An identical procedure was applied to the leading edge of the plume in the direction normal to the expansion axis, i.e., parallel to the target surface. A value of ; 5 = 10 5 cm sy1 was obtained. In the case of Fig. 2d,e,f, Žexpansion in 200 mTorr of argon. the physical picture is as follows: the leading edge of the luminous plume advances linearly as a function of time Ž Õ f 3.3 = 10 6 cm sy1 . until about 200 ns after the creation of the plasma. Then, the isointensity contour lines appear to pile up at the front edge in a manner reminiscent of the spherical wavefronts of a shock wave. In this regime, we fitted the time evolution of the plasma leading edge to the power law x s kt n and obtained n s 0.42 Žsee, e.g., Ref. w15x.. We also note in Fig. 2a,b,c from the isointensity contour lines that the shape of the intensity level distribution along the expansion axis remains fairly constant with time. This suggests a self-similar expansion for the plume, i.e., the form of the density profile does not change with time. However, this would only apply if a linear relationship between light intensity levels and atomic densities could be established. We now quantify the relationship between the measured 2D images and the 3D distribution of emitters within the plume. In our experimental conditions, the light intensity recorded by a pixel Ž x, y . is given by Žsee Figs. 1 and 2 for the orientation of the axes.:
1039
of the integral Eq. Ž4. provided that: Ža. the observed light rays are virtually paraxial, Žb. the plasma is optically thin, in which case the emission coefficient ´n Ž r . is linearly proportional to N Ž r ., and Žc. there is radial symmetry in the Ž y, z . plane. It is necessary that these three conditions be met simultaneously for the Abel inversion procedure to be applicable. From the images presented in Fig. 2, the condition Žc. of radial symmetry seems readily satisfied. As regards Ža. and Žb., further experimental work would be required to establish their validity. Furthermore, even if the necessary conditions were satisfied, the Abel transform procedure would still only provide the distribution of the corresponding excited atomic densities in the plume. An appropriate radiation model for the plasma would then be required to obtain ground state population densities and possibly electronic densities. We now provide a qualitative interpretation of the shadowgraphs of Fig. 3a,b,c in terms of a model based on the refraction of light rays through the plasma. The value of the complex phase index of refraction is given by Eq. Ž1., from which we extract n and k ŽFig. 5. for typical values of the atomic and electronic densities corresponding to the early phase of the plume expansion. At 669.7 nm ŽFig. 3a., the electronic and atomic contributions add up to less than unity and the observed pattern is identical to
qzr2
IŽ x , y. s
Hyzr2 Hline´ Ž r . d zd n n
Ž 4.
in which ´n Ž r . is the emission coefficient at the photon frequency n , and z the total length of the emitting plasma chord. The radial distribution of emitters N Ž r . could be obtained by Abel inversion
Fig. 5. The real Ž n. and imaginary Ž k . parts of the plasma refractive index as a function of wavelength Žnm.. Ž G f 0.4 nm, N s1=10 18 cmy3 , Ne s1=10 19 cmy3 , f s 0.75, l0 s670.7 nm..
1040
W. Whitty, J.-P. Mosnierr Applied Surface Science 127–129 (1998) 1035–1040
those reported previously in Refs. w6,7x. Steep density gradients refract the incident beam toward regions of higher index and the bright and dark fringes Žrefractive fringes. on the outer part of the shadowgraph are due to optical interferences between the undisturbed part of the incident beam and the refracted rays. The situation changes dramatically at 670.7 nm Žon resonance. ŽFig. 3b. and 672.5 nm ŽFig. 3c.. The atomic contribution to the real part of the index dominates and becomes greater than unity. The steep density gradients steer the beam in the direction opposite to that of Fig. 3a since the regions of higher index now correspond to the regions of higher density. The effect is particularly pronounced in Fig. 3c, as can be seen from the position of the target surface. In order to obtain a rough estimate of the atomic density, the method used by El-Astal and Morrow w16x was followed. From Fig. 3c and other data Žnot shown here., the maximum deflection angle was estimated at ; 7 mrad. For a plume optical length of 4 mm, this corresponds to a uniform refractive index gradient of 1.75 my1 . This value along with an oscillator strength of 0.748 w17x corresponds to a Li atom density gradient of ; 6 = 10 25 my4 . Complete modelling of the present experiment is currently under construction, from which improved atomic density estimates are expected. Nonetheless, the series of shadowgraphs of Fig. 3 provide direct evidence of the preponderant role played by refraction when scanning an absorption resonance in a laser plasma plume with an external tunable source. Strong distortion of the intrinsic line shape is likely to result Žsee, e.g., Ref. w16x..
Acknowledgements The authors wish to thank Forbairt for financial assistance, M. Hopkins for the loan of the dye laser, and E.T. Kennedy for reading the manuscript.
References w1x J.F. Ready, Effects of High Power Laser Radiation, Academic Press, Orlando, 1971. w2x T.P. Hughes, Plasmas and Laser Light, Adam Hilger, London, 1975. w3x D.B. Geohegan, Appl. Phys. Lett. 62 Ž1993. 1463. w4x V. Bulatov, L. Xu, I. Schechter, Anal. Chem. 68 Ž1996. 2966. w5x J.W. Beams, in: R.W. Ladenburg, B. Lewis, R.N. Pease, H.S. Taylor ŽEds.., Physical Measurements in Gas Dynamics and Combustion, Oxford Univ. Press, London, 1955, p. 26. w6x M.M. Michaelis, O. Willi, Opt. Commun. 36 Ž1981. 153. w7x P.F. Cunningham, R.N. Campbell, M.M. Michaelis, J. Phys. E: Sci. Instr. 19 Ž1986. 957. w8x R. Benattar, C. Popovics, J. Appl. Phys. 54 Ž1983. 603. w9x R.M. Gilgenbach, P.L. Ventzek, Appl. Phys. Lett. 58 Ž1991. 1597. w10x P.E. Dyer, P.H. Key, D. Sands, H.V. Snelling, F.X. Wagner, Appl. Surf. Sci. 86 Ž1995. 18. w11x R. Srinivasan, J. Appl. Phys. 73 Ž1993. 2743. w12x R.A. Alpher, D.R. White, Phys. Fluids 2 Ž1959. 162. w13x R.W. Ditchburn, Light, 3rd ed., Academic Press, New York, 1976. w14x B.C. Boland, F.E. Irons, R.W.P. McWhirter, J. Phys. B ŽProc. R. Soc. 1 Ž2. Ž1968. 1180. w15x B. Angleraux, C. Girault, C. Champeaux, F. Garrelie, C. Germain, A. Catherinot, Appl. Surf. Sci. 96–98 Ž1996. 117. w16x A.H. El-Astal, T. Morrow, J. Appl. Phys. 80 Ž1996. 1156. w17x D.A. Verner, E.M. Verner, G.J. Ferland, At. Data Nucl. Data Tables 64 Ž1996. 1.
Diagnostic of an expanding laser-produced lithium plasma using ICCD frame photography and shadowgraphy William Whitty, Jean-Paul Mosnier
)
School of Physical Sciences, Dublin City UniÕersity, GlasneÕin, Dublin 9, Ireland
Abstract The expansion of a laser-produced lithium plasma is characterized using two different high-speed imaging techniques. Firstly, a sequence of frames of the luminous plume is recorded using an interference filterrgated ICCD camera combination. Expansion velocities are estimated from these images. The conditions, in which the radial distributions of emitters could be recovered using Abel inversion, are discussed. Secondly, shadowgraphs obtained with a synchronized tunable dye laser light source are recorded at different probe wavelengths in the vicinity of the Li 0 670.7-nm resonance. The fringe patterns observed in these images are interpreted in terms of strong refractive index gradients within the plasma. The effect of anomalous dispersion is observed and strongly modifies the appearance of the shadowgraphs. q 1998 Elsevier Science B.V. PACS: 52.70 La; 32.70 Jz Keywords: Laser plasma; High-speed imaging; Anomalous dispersion; Shadowgraphy; Lithium
1. Introduction Laser-produced plasmas are characterized by steep and rapidly varying electron densityrrefractive index and temperature gradients. Their spatio-temporal evolution is driven by collisional processes between electrons, ions, neutrals and photons. Laser plasmas are intense sources of line and continuum electromagnetic radiation due to the relative strength of the radiative processes. Thus, the motion of the excited plasma-emitting light by de-excitation or recombination in ions or neutrals can be determined using high-speed imaging. It provides information on the ‘local’ structure and dynamics of the constituent particles provided that a radiation model linking ) Corresponding author. Tel.: q44-353-1-704-5303; fax: q44353-1-704-5384; [email protected].
observed light intensities to particle distributions exist. At least one severe complication in this recovery process arises from the fact that the recorded 2D-intensities are necessarily integrated along the observed plasma depth, hence, requiring an appropriate transformation of the data. In the present work, two different high-speed imaging techniques, namely, framing camera imaging and shadowgraphy, are used. We now present their essential features. High-speed photographic recording of expanding laser plasmas using framing image-converter cameras is a well-established technique w1,2x. More recently, the technique has been refined by the introduction of gated ICCD ŽIntensified Charge Coupled Device. cameras which allow direct digital recording, thus, greatly facilitating computer processing of the images as well as providing improved sensitivity. Such imaging devices have been widely used in the
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 7 . 0 0 6 0 3 - X
1036
W. Whitty, J.-P. Mosnierr Applied Surface Science 127–129 (1998) 1035–1040
area of pulsed laser deposition ŽPLD. of materials Žsee, e.g., Ref. w3x.. Despite the technique’s ease of implementation, the retrieval of quantitative information characterizing the plume expansion from the images is usually complex. By narrowing down the spectral range of the imaged light to that of a known emission line, it becomes possible to track the evolution of the corresponding excited state. This can be carried out by placing a tuned interference filter in front of the camera, in which case, the measured intensities are integrated over the line profile. Spectrally-resolved images can also be obtained if an imaging spectrometer is used w4x. The shadowgraph technique consists of passing a pencil of light through the test section and letting it fall directly, or via an imaging lens, onto a recording device such as a photographic plate or a CCD matrix. It has long been used in the study of compressible gas flow w5x and plasmas, including laser-produced plasmas Žsee, e.g., Ref. w2x and references therein.. One can show that the intensity modulations in the shadowgraph depend upon the second derivative of the refractive index of the traversed medium w5x. The technique is thus suitable for the study of expanding laser plasmas where the refractive index exhibits very rapid changes. Intense pulsed laser light sources proved particularly useful to record the shadowgraphs of bright laser plasmas and the technique has commonly been applied over the years. Numerous references to such early work in the 60s and 70s can be found in Ref. w2x. Michaelis and Willi w6x showed that when probing a laser plasma of steep refractive index gradient with a pencil of laser light, the rays traversing the denser regions suffer greater refraction and may interfere with the rays that did not interact with the plasma, thus producing a pattern of bright and dark fringes called refractive fringes. The authors showed, using an optical path ray analysis, that such refractive fringes could be used to determine the electron density distributions of laserproduced plasmas Žsee also Refs. w7,8x for improved analysis.. More recently, the use of tunable dye laser light as the probe beam brought a further refinement to the technique and was frequently applied as a diagnostic tool in the area of Pulsed Laser Deposition ŽPLD. of materials w9–11x. One should point out here that shadowgraphs obtained with laser light may be regarded as single beam holograms of laser plas-
mas. Holograms of laser plasmas were first recorded in the 60s Žsee, e.g., Ref. w2x.. The strength of the interaction between electromagnetic radiation of angular frequency v and matter, is embodied in the value of the index of refraction nv . The value of nv largely determines the intensity distribution of light in the images obtained with the methods just described, and is related to the particle densities of the probed medium, thereby explaining the principle of the diagnostic. It is wellknown that the index of refraction of a plasma at optical frequencies is largely due to the free electron contribution w12x. However, in the vicinity of an atomic resonance, significant additions must be made to the refractive index. Since we are concerned with such a situation Žsee Section 2., we write the complex index of refraction n˜ s n y i k of a plasma in the vicinity of an atomic resonance of angular frequency v 0 , weighed oscillator strength f and width G in the form w12,13x: n˜ 2 y 1 s
1
ž /
4p Nfe 2
1
me
v 02 y v 2 q i Gv
4p´ 0
q n 2e y 1
Ž 1.
where N is the number of atoms per unit volume and m e the electron mass. The free electron contribution is equal to: n 2e s
(
1y
v p2
Ž 2.
v2
where v p is the plasma frequency for an electron density Ne such that:
v p2 s
1
4p Ne e 2
4p´ 0
me
ž /
.
Ž 3.
From Eqs. Ž1. and Ž2., we see that the electron gas contribution to the phase index of refraction is negative and slightly less than unity. On the other hand, the complex term gives rise to the effect of anomalous dispersion Žsee Section 3 for details.. In Sections 2 and 3, we give a brief description of the experimental set-up used to record frames and shadowgraphs of a laser-produced lithium plasma expanding either in vacuum or in a low-pressure argon atmosphere. We then present typical images
W. Whitty, J.-P. Mosnierr Applied Surface Science 127–129 (1998) 1035–1040
1037
obtained in both configurations and provide some elements of their physical interpretation.
2. Experimental and results The basic configuration used for both types of experiments is shown in Fig. 1a. The arrangement for shadowgraphy is illustrated in Fig. 1b. The lithium plasma is created by a Nd:YAG laser Ž0.4 J, 15 ns. using a slightly defocused 120-mm uncorrected plano-convex lens ŽFL., providing a target irradiance of ; 3 = 10 10 Wrcm2 Ž; 450 Jrcm2 .. The dye laser beam used to probe the plasma is initially expanded ŽBE. to cover the entire sensitive area of the CCD. The optical FWHM ŽFull-Width Half Maximum. is 8 ns, thus, introducing temporal resolution. The experiment is synchronized using a master pulse from a PC ŽPC486. to a digital delay generator ŽDG., thus controlling the inter-laser triggering delay Ž DT . to an accuracy of less than 3 ns. Background light emission from the plasma is eliminated by the introduction of neutral density filters andror a tuned interference filter ŽND q IF.. Blocking the dye laser beam and substituting the CCD detector for a gated image intensifierrCCD combination coupled to a zoom lens Žwhile retaining Fig. 2. Temporal and spatial evolution of Li 0 Ž670.7 nm.. Ža,b,c. Expanding into vacuum. Žd,e,f. Expanding into 200 mTorr of argon. Ža,d. DT s10 ns. Žb,e. DT s100 ns. Žc,f. DT s 250 ns. All units are in millimeter.
Fig. 1. Ža. Basic experimental configuration: ŽCCD. Charge Coupled Device, ŽPC486. 486 Personal Computer, ŽDG. Delay Generator. Žb. Detailed view of the apparatus used for shadowgraphy: ŽBE. Beam Expander, ŽFL. Focusing Lens, ŽNDqIF. Neutral Density FiltersqTuned Interference Filter.
the neutral density filters and tuned interference filter. provides the arrangement for frame photography. An uncorrected plano convex lens ŽFL. of 190-mm focal length was used to create the plasma in this case. The spot size was ; 5 mm in diameter, resulting in an irradiance of ; 2.7 = 10 8 Wrcm2 Ž; 4 Jrcm2 .. Synchronization is achieved as before. Successive frames of the lithium plasma are obtained by varying the triggering delay between the laser ŽNd:YAG 0.7 J; 15 ns. used to create the plasma and the gating pulse Ž8 ns FWHM. to the image intensifier. The temporal and spatial evolution of a lithium plume expanding into vacuum as well as into a low
1038
W. Whitty, J.-P. Mosnierr Applied Surface Science 127–129 (1998) 1035–1040
teristic is the strong spatial anisotropy of the early expansion w1x: the plume remains mostly confined around the axis constituting the target normal with a velocity component Žlongitudinal. along this axis larger than the velocity component Žradial. perpendicular to the target normal. The density contour shows a certain asymmetry in the very early stage probably due to inhomogeneities in the energy distribution of the laser beam. In order to estimate the longitudinal velocity of neutral lithium, we follow the procedure first described in Ref. w14x. At a fixed distance from the target on the axis of expansion Ž y s 0 mm., we record the integrated intensity variations of the Li 0 670.7-nm emission line as a function of time. This procedure is then repeated for different positions d along the axis ŽFig. 4.. A plot of d as a function of the time corresponding to the peak of the distribution curve yields the desired quantity. A value
Fig. 3. Shadowgraphs of Li 0 Ž670.7 nm. at DT s 70 ns. Ža. l probe s669.7 nm. Žb. l probe s670.7 nm. Žc. l probe s672.5 nm.
pressure gas Ž200 mTorr of argon. is shown in Fig. 2 for different time delays. The camera is tuned to the 2s–2p Ž670.7 nm. Li 0 transition using an interference filter centered at 670.7 nm with a 9.3 nm ŽFWHM. spectral window. Shadowgraphs at different probe wavelengths in the vicinity of the same transition are presented in Fig. 3.
3. Analysis and discussion First, we consider the sequence of frames presented in Fig. 2a,b,c depicting the expansion in vacuo of the laser-produced lithium plasma. Charac-
Fig. 4. The integrated intensity variations of Li 0 Ž670.7 nm. at fixed distances d Žmm. as a function of time Žns..
W. Whitty, J.-P. Mosnierr Applied Surface Science 127–129 (1998) 1035–1040
of ; 2.4 = 10 6 cm sy1 is obtained. It should be compared with the advance velocity of the plasma front which is recovered by measuring the position of the leading luminous edge of the plume as a function of time, corresponding to frames obtained without the interference filter Žnot shown here.. The subsequent velocity shows the linearity typical of a free expansion with a measured slope of ; 3.5 = 10 6 cm sy1 . An identical procedure was applied to the leading edge of the plume in the direction normal to the expansion axis, i.e., parallel to the target surface. A value of ; 5 = 10 5 cm sy1 was obtained. In the case of Fig. 2d,e,f, Žexpansion in 200 mTorr of argon. the physical picture is as follows: the leading edge of the luminous plume advances linearly as a function of time Ž Õ f 3.3 = 10 6 cm sy1 . until about 200 ns after the creation of the plasma. Then, the isointensity contour lines appear to pile up at the front edge in a manner reminiscent of the spherical wavefronts of a shock wave. In this regime, we fitted the time evolution of the plasma leading edge to the power law x s kt n and obtained n s 0.42 Žsee, e.g., Ref. w15x.. We also note in Fig. 2a,b,c from the isointensity contour lines that the shape of the intensity level distribution along the expansion axis remains fairly constant with time. This suggests a self-similar expansion for the plume, i.e., the form of the density profile does not change with time. However, this would only apply if a linear relationship between light intensity levels and atomic densities could be established. We now quantify the relationship between the measured 2D images and the 3D distribution of emitters within the plume. In our experimental conditions, the light intensity recorded by a pixel Ž x, y . is given by Žsee Figs. 1 and 2 for the orientation of the axes.:
1039
of the integral Eq. Ž4. provided that: Ža. the observed light rays are virtually paraxial, Žb. the plasma is optically thin, in which case the emission coefficient ´n Ž r . is linearly proportional to N Ž r ., and Žc. there is radial symmetry in the Ž y, z . plane. It is necessary that these three conditions be met simultaneously for the Abel inversion procedure to be applicable. From the images presented in Fig. 2, the condition Žc. of radial symmetry seems readily satisfied. As regards Ža. and Žb., further experimental work would be required to establish their validity. Furthermore, even if the necessary conditions were satisfied, the Abel transform procedure would still only provide the distribution of the corresponding excited atomic densities in the plume. An appropriate radiation model for the plasma would then be required to obtain ground state population densities and possibly electronic densities. We now provide a qualitative interpretation of the shadowgraphs of Fig. 3a,b,c in terms of a model based on the refraction of light rays through the plasma. The value of the complex phase index of refraction is given by Eq. Ž1., from which we extract n and k ŽFig. 5. for typical values of the atomic and electronic densities corresponding to the early phase of the plume expansion. At 669.7 nm ŽFig. 3a., the electronic and atomic contributions add up to less than unity and the observed pattern is identical to
qzr2
IŽ x , y. s
Hyzr2 Hline´ Ž r . d zd n n
Ž 4.
in which ´n Ž r . is the emission coefficient at the photon frequency n , and z the total length of the emitting plasma chord. The radial distribution of emitters N Ž r . could be obtained by Abel inversion
Fig. 5. The real Ž n. and imaginary Ž k . parts of the plasma refractive index as a function of wavelength Žnm.. Ž G f 0.4 nm, N s1=10 18 cmy3 , Ne s1=10 19 cmy3 , f s 0.75, l0 s670.7 nm..
1040
W. Whitty, J.-P. Mosnierr Applied Surface Science 127–129 (1998) 1035–1040
those reported previously in Refs. w6,7x. Steep density gradients refract the incident beam toward regions of higher index and the bright and dark fringes Žrefractive fringes. on the outer part of the shadowgraph are due to optical interferences between the undisturbed part of the incident beam and the refracted rays. The situation changes dramatically at 670.7 nm Žon resonance. ŽFig. 3b. and 672.5 nm ŽFig. 3c.. The atomic contribution to the real part of the index dominates and becomes greater than unity. The steep density gradients steer the beam in the direction opposite to that of Fig. 3a since the regions of higher index now correspond to the regions of higher density. The effect is particularly pronounced in Fig. 3c, as can be seen from the position of the target surface. In order to obtain a rough estimate of the atomic density, the method used by El-Astal and Morrow w16x was followed. From Fig. 3c and other data Žnot shown here., the maximum deflection angle was estimated at ; 7 mrad. For a plume optical length of 4 mm, this corresponds to a uniform refractive index gradient of 1.75 my1 . This value along with an oscillator strength of 0.748 w17x corresponds to a Li atom density gradient of ; 6 = 10 25 my4 . Complete modelling of the present experiment is currently under construction, from which improved atomic density estimates are expected. Nonetheless, the series of shadowgraphs of Fig. 3 provide direct evidence of the preponderant role played by refraction when scanning an absorption resonance in a laser plasma plume with an external tunable source. Strong distortion of the intrinsic line shape is likely to result Žsee, e.g., Ref. w16x..
Acknowledgements The authors wish to thank Forbairt for financial assistance, M. Hopkins for the loan of the dye laser, and E.T. Kennedy for reading the manuscript.
References w1x J.F. Ready, Effects of High Power Laser Radiation, Academic Press, Orlando, 1971. w2x T.P. Hughes, Plasmas and Laser Light, Adam Hilger, London, 1975. w3x D.B. Geohegan, Appl. Phys. Lett. 62 Ž1993. 1463. w4x V. Bulatov, L. Xu, I. Schechter, Anal. Chem. 68 Ž1996. 2966. w5x J.W. Beams, in: R.W. Ladenburg, B. Lewis, R.N. Pease, H.S. Taylor ŽEds.., Physical Measurements in Gas Dynamics and Combustion, Oxford Univ. Press, London, 1955, p. 26. w6x M.M. Michaelis, O. Willi, Opt. Commun. 36 Ž1981. 153. w7x P.F. Cunningham, R.N. Campbell, M.M. Michaelis, J. Phys. E: Sci. Instr. 19 Ž1986. 957. w8x R. Benattar, C. Popovics, J. Appl. Phys. 54 Ž1983. 603. w9x R.M. Gilgenbach, P.L. Ventzek, Appl. Phys. Lett. 58 Ž1991. 1597. w10x P.E. Dyer, P.H. Key, D. Sands, H.V. Snelling, F.X. Wagner, Appl. Surf. Sci. 86 Ž1995. 18. w11x R. Srinivasan, J. Appl. Phys. 73 Ž1993. 2743. w12x R.A. Alpher, D.R. White, Phys. Fluids 2 Ž1959. 162. w13x R.W. Ditchburn, Light, 3rd ed., Academic Press, New York, 1976. w14x B.C. Boland, F.E. Irons, R.W.P. McWhirter, J. Phys. B ŽProc. R. Soc. 1 Ž2. Ž1968. 1180. w15x B. Angleraux, C. Girault, C. Champeaux, F. Garrelie, C. Germain, A. Catherinot, Appl. Surf. Sci. 96–98 Ž1996. 117. w16x A.H. El-Astal, T. Morrow, J. Appl. Phys. 80 Ž1996. 1156. w17x D.A. Verner, E.M. Verner, G.J. Ferland, At. Data Nucl. Data Tables 64 Ž1996. 1.