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Characterization of a UV–VUV light source based on a gas-target ns-laser-produced plasma
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 254 (2007) 193–199 www.elsevier.com/locate/nimb
Characterization of a UV–VUV light source based on a gas-target ns-laser-produced plasma Tonia M. Di Palma *, Antonio Borghese Istituto Motori – CNR, Via Marconi 8, 80125 Napoli, Italy Received 22 September 2006; received in revised form 3 November 2006 Available online 22 December 2006
Abstract We report measurements of the temporal and spatial evolution of plasmas, produced on gaseous targets by focused ns-Nd:YAG laser. Characterization of the UV–VUV light source includes time-resolved visualization of the spatial growth and the spectroscopic signatures of plasmas produced on pulsed, supersonic jets of helium, argon, nitrogen and xenon gases into a vacuum chamber. Photon fluxes of up to 1012 photons cm2 nm1/pulse have been measured in the wavelength region 100–260 nm within the first 30 ns following the laser pulse. Also discussed for comparison are plasma signatures in helium, argon and nitrogen gases at standard temperature and pressure. The results indicate availability of photon fluxes, at typical laser repetition rates, that are at least one order of magnitude higher than those achieved from commercial c.w. lamp light sources. 2006 Elsevier B.V. All rights reserved. PACS: 42.72.Bj; 07.60.Rd; 82.50.Hp Keywords: UV–VUV light sources; Laser-produced plasmas; Vacuum-UV spectroscopy
1. Introduction Intense and tunable UV/vacuum-UV light sources are required in many areas of science, ranging from chemistry and biology to atomic and solid state physics. Also, in the industrial processing of inorganic and polymeric materials, the interactions of matter with ultraviolet photons are of relevance because the induced surface modifications (morphology and composition) can greatly improve adhesions and coating depositions and other relevant surface properties [1–3]. Usually, the ultraviolet radiation for laboratory applications is obtained from glow discharge lamps or from lasers operated on higher harmonics. The former have drawbacks in terms of achievable irradiances and photon fluxes, whereas the latter suffer from the lack of continuous and wide tunability. In the last years, laser-pro*
Corresponding author. Fax: +39 081 2396097. E-mail address: [email protected] (T.M. Di Palma).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.11.059
duced plasmas (LPPs) have been increasingly considered as pulsed, bright and broadband light sources, due to their intense radiative emission, the short duration and the small size. Most efforts involve microlithography applications in the extreme-UV and X regions [4]. Also, LPPs have proved to be versatile and powerful diagnostic tools in many experiments and applications [5–7]. However, very few studies have so far considered the production of radiation in the vacuum-UV wavelength range 100–200 nm, corresponding to photon energies of 6–12 eV, wherein notably the ionization potentials (IP) of most molecules fall. Here, the perspective is to use an LPP as a continuously tunable single-photon ionization (SPI) source in compact (‘tabletop’) laboratory configurations, for time-of-flight mass spectrometry experiments, which usually are carried out at large scale synchrotron facilities [8]; they include the near/above threshold one-photon ionization spectroscopy [9] and the trace species detection with capability of isomer discrimination through IP spectral analysis.
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T.M. Di Palma, A. Borghese / Nucl. Instr. and Meth. in Phys. Res. B 254 (2007) 193–199
LPP’s characterizations are often carried out in terms of the target’s state (either solid, liquid or gaseous), of the laser pulse duration, basically from the ns- down to the fs-scale, and of the plasma parameters, mainly electron temperatures and densities versus space/time. LPPs from solid targets [10,11] provide high radiative fluxes, yet they are afflicted by debris emission which limits practical applications. More recently, despite their lower conversion efficiency, LPPs from supersonic gas jet targets have been reported as a feasible alternative, since they are intrinsically debris-free [12,13] and also because the sharp vacuum–gas boundary strongly limits the spatial extension of the expanding plasma [4]. The laser pulse duration, whether on the ns- or on the fsscale, determines the physical processes involved in the laser–target interaction. Although the plasma produced by high-intensity short-pulse lasers show unequalled and promising features, such us average electron temperature up to 1 keV and the emission of ultrafast X-ray pulses [14], they are still in the exploration stage and yet unfeasible for widespread uses as light sources. In this paper we focus rather on ns- (‘slow’) LPPs from gaseous targets, which are better suited for practical applications [15], with the aim of providing additional information through a parametric description of their properties. We report the spatial, temporal and spectral descriptions of LPPs, produced on supersonic jets of different gases (He, N2, Ar, Xe) in the 100–260 nm emission band. Also reported are the corresponding features of LPPs from 105 Pa quiescent gases, as a reference condition. The study has been carried out in the embedded-in-the-chamber working configuration, specifically developed for the debris-free LPP-based light source, in order to provide high photon fluxes to the VUV–UV light–matter interaction region.
2. Experimental set-up Fig. 1 shows the optical layout of the overall light source and the experimental set-up for the spectral analysis and the LPP imaging. The vacuum chamber is equipped with a pulsed valve, a diffraction grating and a laser focusing lens. The pulsed beam from a Nd-YAG Q-switched laser (k = 1064 nm, 200 mJ, 7 ns duration at FWHM) is enlarged to 20 mm and focused by a 100 mm f.l. plano-convex lens on the axis of a solenoidal pulsed valve (Parker mod. 99, 0.8 mm nozzle throat, 45 half-angle conical nozzle). The beam enlargement and the f/5 lens aperture help reaching an irradiance estimated as nearly 5 · 1013 W cm2 in the beam waist. The plasma is produced on a supersonic pulsed jet (250 ls duration, 1 MPa back-pressure) at 2.5 mm above the exit nozzle. He, Ar, N2 and Xe gases are used as targets and the chamber is kept at a pressure below 102 Pa; otherwise, the chamber is filled with He, Ar or N2 at 100 kPa when plasma is produced in standard temperature and pressure (STP) conditions; heretofore, we will refer to them as ‘‘jet’’ and ‘‘STP’’ conditions, respectively. The spark images are monochromatized at 532 nm with an interference filter and acquired with a 1:2 magnification with an intensified CCD (Andor-Tech Ltd.) along an optical axis orthogonal to the laser beam. A flat-field concave diffraction grating (Jobin-Yvon, 70 mm o.d., 210 mm focal length, 600 grooves/mm, 8 nm/mm dispersion, blazed at 220 nm) collects the plasma radiation and disperses it on a 25 mm line-shaped volume on a glass coated with sodium salicylate scintillator. The quantum efficiency gscint ffi 60% and the fluorescence decay time sscint ffi 7 ns of the scintillator are retrieved by coupled measurements of the direct UV emission of the spark at 210 nm and the corresponding fluorescence from the scintillator. The grating is positioned along
Fig. 1. Top-view of the experimental set-up showing the 200 mm i.d. vacuum chamber equipped with pulsed valve, laser focusing lens and diffraction grating. The optical layout for the plasma production and the spectral dispersion is fully assembled on the left arm. The bottom arm is used for LPP visualization and the right arm is used for acquisition of spectra.
T.M. Di Palma, A. Borghese / Nucl. Instr. and Meth. in Phys. Res. B 254 (2007) 193–199
an optical axis at an angle of about 30 with respect to the incident laser (backward collection), chosen as a trade-off between the requirement of collecting the widest emission solid angle (DX = 0.087 sr in our case) at the smallest non-obscuring angle of the laser focusing lens. This arrangement maximizes the fraction of the overall photons delivered to the interaction volume, expected to be higher in the backward direction due to the emission anisotropy of the plasma [16]. The grating is aligned so that an embedded-in-the-chamber spectrograph results, which has the LPP coincident with its nominal ‘‘input pin-hole’’ and the grating exit focal plane crossing the center of the chamber. The resulting 100–260 nm dispersed spectrum falls partially outside the flat field of the grating (200–400 nm) and may lead to defocusing of the VUV wing and to reduction of the nominal resolution of the spectrograph at Dk/k 2% BW at 160 nm when the spectra are acquired from sparks on jet. The scintillator fluorescence produced by the VUV/UV dispersed radiation of the LPP is imaged with a 1:1 magnification on the 6 · 25 mm (256 · 1024 pixel) MCP/CCD detector along the right arm in Fig. 1. The temporal analysis of both the spark images and the spectra is carried out with a 2 ns gate pulse to the MCP. 3. Results and discussion 3.1. Sizes and shapes of the LPP Fig. 2 shows a series of images of LPPs in Ar at increasing time delays from laser onset, produced in jet (left-side)
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and STP (right-side) conditions. The jet and the laser beam axis are shown as a vertical and a horizontal dashed line, respectively. In the top-right image of Fig. 2 also shown is the contour of the valve body, drawn by a solid line. The monochromatic emission intensities (z-scale) are reported in false-colors and in units of counts/pixel. The plasma develops through quite different spatial shapes and sizes in the two conditions. On one hand, in the STP gas, the plasma elongates axially and progressively towards the laser beam, incoming from left, due to the build up of a laser-sustained plasma wavefront, which in turn shields the plasma and leads it to extinction downstream. The effect is well known and is typical of uniform gas conditions [17]. From Fig. 2 we estimate an average speed of the plasma wavefront, moving towards the focusing lens in the first 8 ns from the plasma onset, of about 2.5 · 107 cm/s, according to previous data on argon LPPs produced in similar condition [18]. In addition, localized bright spots appear in the images, resulting from interference effects of the non-uniform laser electric field upstream the beam waist, due to the spherical aberration of the focusing lens [19]. On the other hand, the plasma in jet conditions appears instead to be locked to the position, determined by the intersection of the laser beam and the conical gas jet. The effect is responsible of the high space/time reproducibility as well as of the small size, which are keyfeatures of the light source in applications under vacuum conditions. The transverse sizes of the plasma in Fig. 2 reach values of several hundred microns within the first 30 ns in both
Fig. 2. Images of sparks produced in jet (left-side) in STP conditions (left-side), acquired at different timing after the laser onset. The CCD gate aperture was 2 ns.
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gas conditions, which determines the spectral resolution of our spectrograph (see Fig. 1). In STP conditions, the backward shift of the plasma leads to further defocusing of the spectra on the output focal plane. The resulting low spectral resolution of our slit-less configuration does not allow us to apply ordinary spectroscopic techniques to plasma emission analysis. Z-scales in Fig. 2 show that peak emission intensities from jet targets are much lower than those in STP gas; the effect is primarily due to the correspondingly lower gas density n(r) in the jet, which decreases downstream as n(r) = 0.15 Æ n0 Æ (d/r)2 [20], where n0 = 2.5 Æ 1020 cm3 is the number gas density at the backpressure of 1 MPa, d = 0.8 mm is the nominal nozzle throat and r the distance from the nozzle. At the position of the laser beam waist, we evaluate n(2.5 mm) ffi 4 Æ 1018 cm3. It is worth noting that the actual gas density might be even lower, since the short stroke of the valve poppet determines an effective throat smaller than the nominal value [21]. The quite different laser absorption mechanisms and space/time evolutions of the plasma in the jet and STP conditions limit the extent to which further comparative analysis can be carried out. 3.2. Spectral analysis Fig. 3 reports the emission spectra, dispersed in the VUV–UV wavelength range 100–260 nm and time integrated in the first 30 ns, of LPPs in He, Ar, N2 and Xe in jets (solid lines) and in He, Ar and N2 at STP (open circles), respectively. The absolute calibration of intensities has been performed by accounting for the geometrical factors arising
from the light collection optics, the quantum efficiency of scintillator and CCD photocathode, and the gain of the MCP intensifier. The vertical scales report the photon fluxes collected from plasmas in gas jets onto the scintillator plane, expressed in units of photons cm2 nm1 pulse1, whereas those acquired in STP gas are firstly reduced by the indicated factors and then reported on the same scale, for sake of quantitative comparison. Data are not corrected for the grating efficiency, which affects strongly the emission spectra below 120 nm, due to the vanishing reflectivity of the MgF2 coating of the aluminum grating [22]. The time-integrated emission spectra result from contributions of both excitation and relaxation processes. The former involves the primary effect of the laser radiation: the laser-sustained plasma is very hot and dense and the free–free radiation (f–f or bremsstrahlung) and free–bound (f–b) electron recombination are the main emission mechanisms, which contribute as a spectral continuum. The latter comprises the plasma expansion and relaxation following the laser pulse, when bound–bound (b–b) electronic transitions of ions and neutrals occur and emission lines emerge from the vanishing continuum [23]. The emission spectrum produced in He gas jet is dominated by the strong Balmer-a line at k = 164 nm, arising from singly ionized atoms [24], whereas a weak continuum also appears in the STP condition. The case of the He plasma highlights the major differences between the LPPs produced in jet and STP gas conditions, in terms of intensity and resolution. As the emission line arises from ions, there is no re-absorption from neutral He, whether the plasma pocket is surrounded or not by the gas in the chamber; therefore, the strong decrease of the emission intensity
Fig. 3. UV–VUV spectra of the sparks of argon, nitrogen, helium and xenon in jet (solid lines) and in uniform gas (open circles), integrated in the first 30 ns from the plasma onset.
T.M. Di Palma, A. Borghese / Nucl. Instr. and Meth. in Phys. Res. B 254 (2007) 193–199
of He plasma in the gas jet case, compared to that at STP conditions, is mainly due to the lower local gas density at the chosen distance downstream the nozzle exit, which is expected to be lowered at least by two orders of magnitude with respect to that at back-pressure, and to the smaller dimension of the plasma. Moreover, a broader and blueshifted line is observed from He plasma produced in STP condition, which results from different effects: (a) the larger size of the plasma, since the fraction of laser energy absorbed by the gas is much larger then in the jet case; (b) the defocusing due to the backward spatial shift of the plasma, similar to that of Ar in Fig. 2, which also accounts for the geometrical effect of the apparent ‘‘blueshift’’ of the He line in Fig. 3 and (c) possibly a Starkbroadening, which however cannot be estimated with the available spectral resolution. The spectra of Ar and N2 in STP conditions have similar shapes and both show a strong continuum, involving emission dominated by f–f and f–b transitions. Furthermore, the emission from Ar is nearly twice more intense than that from N2, over the whole examined wavelength range, mainly due to the higher Ar atomic number. As noted above, the pronounced decrease of emission below the 120 nm is essentially due to the reduced efficiency of the grating; however, some features due to absorption of the surrounding gas survive and are observable in both spectra. In particular, the sharper decrease of the nitrogen spectrum below 140 nm is due to the additional absorption of molecular nitrogen [25], while the expected Ar absorption lines at 105 and 106 nm [24] appear in the Ar spectrum, though blue-shifted at around 100 nm due to the plasma spatial drift described above. As compared to the spectra from plasmas in STP conditions, those of the jet cases have much lower intensities and show more resolved structures superimposed to a continuum. The atomic nitrogen has a very complex spectrum in 90–260 nm [24,25], therefore no assignment has been attempted. In the argon spectrum the line at 105 nm of Ar I is clearly observable, even if at a low intensity. Other unassigned structures may well be emission lines of Ar III. The spectrum of the plasma in Xe jet is by far more intense than those of the other gases and does not show spectral structures. The spectral shape and intensity result from a number of causes: (a) the higher laser energy absorption, allowed by the lower ionization potential of Xe [24]; (b) the higher electron density, expected from the higher atomic number; (c) the extremely congested spectrum of Xe in this wavelength range, comprising very close lines from single or multiple ions [24]; (d) enhanced emission from either clusters formed in the supersonic expansion or even excimers formed in the plasma [26]. The photon fluxes of up to 1012 photons cm2 nm1 pulse1 at about 180 nm from Xe jets in Fig. 3 correspond to 10 lW cm2 nm1 at 10 Hz laser repetition rate and are at least one order of magnitude larger than those achieved by commercial D2 glow discharge lamps (e.g. Hamamatsu L2D2).
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Quantitative analysis of the electron density ne and temperature Te as functions of space and time, requires the application of dedicated diagnostics, e.g. laser interferometry and high resolution emission/absorption spectroscopy. Published data from laser interferometry on nitrogen LPPs in uniform gas and jet [27], have provided values of the electron density in the range ne ffi 1 to 3 Æ 1018 cm3, with a weak dependence on neutral gas density (n ffi 1 to 4 Æ 1018 cm3) for plasmas produced in uniform gas. Also, previous emission/absorption spectroscopy experiments on air LPPs at STP conditions [6], have given values of Te as high as 105 K within the very first nanoseconds, decreasing to nearly 4.104 K at the end of the laser pulse and down to 2.104 K after 30 ns; Te values measured for argon LPPs in STP conditions are slightly larger (3 · 104 K for time >30 ns) [28]. The images in Fig. 2 allow us to estimate Te from gas dynamics arguments, by considering that the pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi blast wave expands with a radial speed vP cZk B T e =m [29], where c = 5/3 for Ar, and Z ffi ne/n, kB and m are the ionization level, the Boltzmann constant and the atomic mass, respectively. The evaluation of vP from the images of Fig. 2 at times beyond 30 ns gives values of the order of 4 Æ 103 m/s in jet and 5 Æ 103 m/s in STP conditions, which correspond, respectively, to values of Z Te 3 Æ 104 K and Z Te 4 Æ 104 K. From the electron and neutral densities reported for plasma produced in uniform gas (Z 1) [27,28], we estimate temperature values which are consistent with those cited above. 3.3. Temporal analysis The temporal analysis of data has been carried out by assuming a flat spectral response of the sodium salicylate and by accounting for the decay time s of its fluorescence [30]; accordingly, all the spectra at each wavelength have been corrected for by means of a numerical de-convolution, using the relationships between the light signal incident onto the scintillator I(t, k) and the positiondependent induced fluorescence IF(t, x), where x / k, given by [31] t t0 Iðt0 ; kÞ exp dt0 ) Iðt; kÞ s 0 dI F ðt; xÞ ¼ I F ðt; xÞ þ s dt
I F ðt; xÞ ¼
1 s
Z
t
The time-resolved VUV–UV plasma brightness, namely the emission intensity integrated over the whole 100–260 nm band, is reported in Fig. 4(a) and (b) for the indicated gases in jet and STP conditions. It includes the contribution of the continuum (f–f + f–b), which prevails in the first nanoseconds, and that of the lines (b–b), which appear later and last longer. The He emission has a peculiar temporal evolution, due the features of its spectrum, which reduces to the strong
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Fig. 4. UV–VUV emission temporal evolution of the plasma produced in the jet (a) and in uniform gas (b) starting from the onset of the plasma formation.
Balmer-a line, superimposed to a weak continuum only in ‘‘STP’’ conditions. Fig. 4(b) shows that the Ar and nitrogen plasma emission in ‘‘STP’’ conditions lasts considerably longer than the laser pulse width (FWHW >50 ns for Ar and >30 ns for nitrogen). This effect is due to the expansion dynamics following the laser interaction, shown in Fig. 2, where the plasma wave clearly appears as a wide source of emission. Notably, the radiation pulse has a duration less than 20 ns (FWHW) when the plasmas are produced in gas jets of Ar and nitrogen; the faster decay of the plasma in jet conditions constitutes a further premium feature of LPPs as light sources for pulsed operation. The time evolution of the Xe emission in the jet condition shows a tail longer than in Ar and N2, due to a slower overall relaxation of the b–b transitions between numberless ionized and excited states. 4. Conclusions The paper reports on measurements of shapes, spectral emissions and brightness evolutions of plasmas produced in supersonic jet of He, Ar, N2 and Xe in vacuum. For a comparison, also reported are the corresponding observations in uniform (STP) gas conditions of He, Ar and N2. Time-resolved imaging of the plasmas has shown that the conical gas jet limits the plasma length along the laser axis to values well below 1 mm all over the plasma life, while the sparks produced in STP conditions elongate
backward far above 1 mm since the first nanoseconds from the plasma onset. The corresponding transverse sizes are of some hundreds microns within the first 30 ns, which determine the overall spectral resolution on the output focal plane of our optical configuration. The spectrograph geometry, due to its low f/number and to the slit-less input light source, allows to produce photon fluxes in the UV–VUV of about 1011–1012 photons cm2 nm1 per laser pulse, depending on the jet target gas. From the quantitative comparison of the brightness of laser plasmas produced in different surrounding gas conditions, the photon fluxes produced in jet targets are one order of magnitude lower than those obtained in STP gas, primarily as an effect of the lower gas density in the jet; nevertheless, the irradiance of the LPP in jet, for typical Nd-YAG laser repetition rates is still much larger than those of conventional c.w. glow discharges lamps. Time-resolved analysis of the emitted radiation reveals decay-times of emission intensities in jet targets shorter than those in uniform gases, which correlate with the lack of the plasma back-streaming expansion, inhibited by the sharp vacuum–gas jet boundary. Finally, the light source based on the LPP from gas jet targets has high stability and reproducibility, in terms of size, position and emission intensity. Our findings suggest that LPPs from gas jets may be usefully exploited as debris-free, point-like, pulsed, tunable and efficient UV–VUV light sources, suitable for many application. Acknowledgement This work was supported by the Italian M.I.U.R.-D.D. 1105/02 – Contract on Project No. 193. References [1] V. Skurat, Nucl. Instr. and Meth. B 208 (2003) 27. [2] M.R. Wertheimer, A.C. Fozza, A. Hollander, Nucl. Instr. and Meth. B 151 (1999) 65. [3] N. Shirahata, K. Oda, S. Asakura, A. Fuwa, Y. Yokogawa, T. Kameyama, A. Hozumi, J. Vac. Sci. Technol. A 22 (4) (2004) 1615. [4] S. Kranzusch, C. Peth, K. Mann, Rev. Sci. Instr. 74 (2) (2003) 969. [5] R. Flesh, M.C. Schurmann, J. Plenge, H. Meiss, M. Hunnekuhl, E. Ruhl, Phys. Rev. A 62 (2000) 52723. [6] A. Borghese, S.S. Merola, Appl. Opt. 37 (18) (1998) 3977. [7] D.A. Cremers, L.J. Radziemski, Handbook of Laser Induced Breakdown Spectroscopy, John Wiley & Sons Ltd., West Sussex, England, 2006. [8] W. Sun, K. Yokoyama, J. Robinson, A. Suits, D.M. Neumark, J. Chem. Phys. 110 (1999) 4363. [9] T.M. Di Palma, A. Latini, M. Satta, M. Varvesi, A. Giardini, Chem. Phys. Lett. 284 (1998) 184. [10] J.M. Bridges, C.L. Corner, T.J. Mcllrath, Appl. Opt. 25 (13) (1986) 2208. [11] W. Shaikh, G. Hirst, R.M. Allott, I.C.E. Turcu, M. Folkard, K. Ledingham, R. Donovan, N. Khan, IEEE J. Sel. Top. Quantum Electron. 5 (6) (1999) 1522. [12] P. Laporte, N. Damany, H. Damany, Opt. Lett. 12 (12) (1987) 987. [13] N. Xu, V. Majidi, Appl. Spectrosc. 47 (8) (1993) 1134. [14] M.M. Murnane, H.C. Kapteyn, M.D. Rosen, R.W. Falcone, Science 251 (1991) 531.
T.M. Di Palma, A. Borghese / Nucl. Instr. and Meth. in Phys. Res. B 254 (2007) 193–199 [15] L.J. Radziemsky, D.A. Cremers, Laser induced Plasma and Applications, Marcel Dekker, Inc., New York, 1989. [16] M. Kanouff, H. Shields, L. Bernardez, D. Chenoweth, G. Kubiak, J. Appl. Phys. 90 (8) (2001) 3726. [17] Y.B. Zeldovich, Y.P. Raizer, Physics of Shock Wave and High Temperature Hydrodynamic Phenomena, Dover Publication, Inc., New York, 2002. [18] C.V. Bindu, S.S. Harilal, M.S. Tillack, F. Najmabadi, Appl. Spectrosc. 58 (6) (2004) 719. [19] Y.L. Ken, J.W.L. Lewis, C. Parigger, J. Quant. Spectrosc. Radiat. Transfer 67 (2000) 91. [20] O.F. Hagena, Surf. Sci. 106 (1981) 101. [21] R. Jung, J. Osterholz, O. Willi, M. Galimberti, L.A. Gizzi, M. Borghesi, S. Kar, C.A. Cecchetti, R. Heathcote, D. Neely, Central Laser Facility Annual Report 2004/2005, p. 23.
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[22] J.A.R. Samson, Techniques of Vacuum Ultraviolet Spectroscopy, Wiley, New York, 1967. [23] A. Miziolek, V. Palleschi, I. Schechter, Laser-Induced Breakdown Spectroscopy, Cambridge University Press, Cambridge, 2006. [24]. [25] A. Lofthus, P.H. Krupenie, J. Phys. Chem. Ref. Data 6 (1) (1977) 113. [26] H. Langhoff, J. Phys. B: At., Mol. Opt. Phys. 27 (1994) L709. [27] J.U. Kim, H.J. Lee, K. Kim, G.H. Kim, H. Suk, J. Appl. Phys. 94 (9) (2003) 5497. [28] S.H. Hariral, Appl. Opt. 43 (19) (2004) 3931. [29] T.R. Clark, H.M. Milchberg, Phys. Rev. Lett. 78 (1997) 2373. [30] G.J. Baker, B. Brocklehurst, I.R. Holton, J. Phys. B: At., Mol. Opt. Phys. 20 (1987) L305. [31] R.S. Sigmond, Br. J. Appl. Phys. 17 (1966) 1307.
Nuclear Instruments and Methods in Physics Research B 254 (2007) 193–199 www.elsevier.com/locate/nimb
Characterization of a UV–VUV light source based on a gas-target ns-laser-produced plasma Tonia M. Di Palma *, Antonio Borghese Istituto Motori – CNR, Via Marconi 8, 80125 Napoli, Italy Received 22 September 2006; received in revised form 3 November 2006 Available online 22 December 2006
Abstract We report measurements of the temporal and spatial evolution of plasmas, produced on gaseous targets by focused ns-Nd:YAG laser. Characterization of the UV–VUV light source includes time-resolved visualization of the spatial growth and the spectroscopic signatures of plasmas produced on pulsed, supersonic jets of helium, argon, nitrogen and xenon gases into a vacuum chamber. Photon fluxes of up to 1012 photons cm2 nm1/pulse have been measured in the wavelength region 100–260 nm within the first 30 ns following the laser pulse. Also discussed for comparison are plasma signatures in helium, argon and nitrogen gases at standard temperature and pressure. The results indicate availability of photon fluxes, at typical laser repetition rates, that are at least one order of magnitude higher than those achieved from commercial c.w. lamp light sources. 2006 Elsevier B.V. All rights reserved. PACS: 42.72.Bj; 07.60.Rd; 82.50.Hp Keywords: UV–VUV light sources; Laser-produced plasmas; Vacuum-UV spectroscopy
1. Introduction Intense and tunable UV/vacuum-UV light sources are required in many areas of science, ranging from chemistry and biology to atomic and solid state physics. Also, in the industrial processing of inorganic and polymeric materials, the interactions of matter with ultraviolet photons are of relevance because the induced surface modifications (morphology and composition) can greatly improve adhesions and coating depositions and other relevant surface properties [1–3]. Usually, the ultraviolet radiation for laboratory applications is obtained from glow discharge lamps or from lasers operated on higher harmonics. The former have drawbacks in terms of achievable irradiances and photon fluxes, whereas the latter suffer from the lack of continuous and wide tunability. In the last years, laser-pro*
Corresponding author. Fax: +39 081 2396097. E-mail address: [email protected] (T.M. Di Palma).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.11.059
duced plasmas (LPPs) have been increasingly considered as pulsed, bright and broadband light sources, due to their intense radiative emission, the short duration and the small size. Most efforts involve microlithography applications in the extreme-UV and X regions [4]. Also, LPPs have proved to be versatile and powerful diagnostic tools in many experiments and applications [5–7]. However, very few studies have so far considered the production of radiation in the vacuum-UV wavelength range 100–200 nm, corresponding to photon energies of 6–12 eV, wherein notably the ionization potentials (IP) of most molecules fall. Here, the perspective is to use an LPP as a continuously tunable single-photon ionization (SPI) source in compact (‘tabletop’) laboratory configurations, for time-of-flight mass spectrometry experiments, which usually are carried out at large scale synchrotron facilities [8]; they include the near/above threshold one-photon ionization spectroscopy [9] and the trace species detection with capability of isomer discrimination through IP spectral analysis.
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LPP’s characterizations are often carried out in terms of the target’s state (either solid, liquid or gaseous), of the laser pulse duration, basically from the ns- down to the fs-scale, and of the plasma parameters, mainly electron temperatures and densities versus space/time. LPPs from solid targets [10,11] provide high radiative fluxes, yet they are afflicted by debris emission which limits practical applications. More recently, despite their lower conversion efficiency, LPPs from supersonic gas jet targets have been reported as a feasible alternative, since they are intrinsically debris-free [12,13] and also because the sharp vacuum–gas boundary strongly limits the spatial extension of the expanding plasma [4]. The laser pulse duration, whether on the ns- or on the fsscale, determines the physical processes involved in the laser–target interaction. Although the plasma produced by high-intensity short-pulse lasers show unequalled and promising features, such us average electron temperature up to 1 keV and the emission of ultrafast X-ray pulses [14], they are still in the exploration stage and yet unfeasible for widespread uses as light sources. In this paper we focus rather on ns- (‘slow’) LPPs from gaseous targets, which are better suited for practical applications [15], with the aim of providing additional information through a parametric description of their properties. We report the spatial, temporal and spectral descriptions of LPPs, produced on supersonic jets of different gases (He, N2, Ar, Xe) in the 100–260 nm emission band. Also reported are the corresponding features of LPPs from 105 Pa quiescent gases, as a reference condition. The study has been carried out in the embedded-in-the-chamber working configuration, specifically developed for the debris-free LPP-based light source, in order to provide high photon fluxes to the VUV–UV light–matter interaction region.
2. Experimental set-up Fig. 1 shows the optical layout of the overall light source and the experimental set-up for the spectral analysis and the LPP imaging. The vacuum chamber is equipped with a pulsed valve, a diffraction grating and a laser focusing lens. The pulsed beam from a Nd-YAG Q-switched laser (k = 1064 nm, 200 mJ, 7 ns duration at FWHM) is enlarged to 20 mm and focused by a 100 mm f.l. plano-convex lens on the axis of a solenoidal pulsed valve (Parker mod. 99, 0.8 mm nozzle throat, 45 half-angle conical nozzle). The beam enlargement and the f/5 lens aperture help reaching an irradiance estimated as nearly 5 · 1013 W cm2 in the beam waist. The plasma is produced on a supersonic pulsed jet (250 ls duration, 1 MPa back-pressure) at 2.5 mm above the exit nozzle. He, Ar, N2 and Xe gases are used as targets and the chamber is kept at a pressure below 102 Pa; otherwise, the chamber is filled with He, Ar or N2 at 100 kPa when plasma is produced in standard temperature and pressure (STP) conditions; heretofore, we will refer to them as ‘‘jet’’ and ‘‘STP’’ conditions, respectively. The spark images are monochromatized at 532 nm with an interference filter and acquired with a 1:2 magnification with an intensified CCD (Andor-Tech Ltd.) along an optical axis orthogonal to the laser beam. A flat-field concave diffraction grating (Jobin-Yvon, 70 mm o.d., 210 mm focal length, 600 grooves/mm, 8 nm/mm dispersion, blazed at 220 nm) collects the plasma radiation and disperses it on a 25 mm line-shaped volume on a glass coated with sodium salicylate scintillator. The quantum efficiency gscint ffi 60% and the fluorescence decay time sscint ffi 7 ns of the scintillator are retrieved by coupled measurements of the direct UV emission of the spark at 210 nm and the corresponding fluorescence from the scintillator. The grating is positioned along
Fig. 1. Top-view of the experimental set-up showing the 200 mm i.d. vacuum chamber equipped with pulsed valve, laser focusing lens and diffraction grating. The optical layout for the plasma production and the spectral dispersion is fully assembled on the left arm. The bottom arm is used for LPP visualization and the right arm is used for acquisition of spectra.
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an optical axis at an angle of about 30 with respect to the incident laser (backward collection), chosen as a trade-off between the requirement of collecting the widest emission solid angle (DX = 0.087 sr in our case) at the smallest non-obscuring angle of the laser focusing lens. This arrangement maximizes the fraction of the overall photons delivered to the interaction volume, expected to be higher in the backward direction due to the emission anisotropy of the plasma [16]. The grating is aligned so that an embedded-in-the-chamber spectrograph results, which has the LPP coincident with its nominal ‘‘input pin-hole’’ and the grating exit focal plane crossing the center of the chamber. The resulting 100–260 nm dispersed spectrum falls partially outside the flat field of the grating (200–400 nm) and may lead to defocusing of the VUV wing and to reduction of the nominal resolution of the spectrograph at Dk/k 2% BW at 160 nm when the spectra are acquired from sparks on jet. The scintillator fluorescence produced by the VUV/UV dispersed radiation of the LPP is imaged with a 1:1 magnification on the 6 · 25 mm (256 · 1024 pixel) MCP/CCD detector along the right arm in Fig. 1. The temporal analysis of both the spark images and the spectra is carried out with a 2 ns gate pulse to the MCP. 3. Results and discussion 3.1. Sizes and shapes of the LPP Fig. 2 shows a series of images of LPPs in Ar at increasing time delays from laser onset, produced in jet (left-side)
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and STP (right-side) conditions. The jet and the laser beam axis are shown as a vertical and a horizontal dashed line, respectively. In the top-right image of Fig. 2 also shown is the contour of the valve body, drawn by a solid line. The monochromatic emission intensities (z-scale) are reported in false-colors and in units of counts/pixel. The plasma develops through quite different spatial shapes and sizes in the two conditions. On one hand, in the STP gas, the plasma elongates axially and progressively towards the laser beam, incoming from left, due to the build up of a laser-sustained plasma wavefront, which in turn shields the plasma and leads it to extinction downstream. The effect is well known and is typical of uniform gas conditions [17]. From Fig. 2 we estimate an average speed of the plasma wavefront, moving towards the focusing lens in the first 8 ns from the plasma onset, of about 2.5 · 107 cm/s, according to previous data on argon LPPs produced in similar condition [18]. In addition, localized bright spots appear in the images, resulting from interference effects of the non-uniform laser electric field upstream the beam waist, due to the spherical aberration of the focusing lens [19]. On the other hand, the plasma in jet conditions appears instead to be locked to the position, determined by the intersection of the laser beam and the conical gas jet. The effect is responsible of the high space/time reproducibility as well as of the small size, which are keyfeatures of the light source in applications under vacuum conditions. The transverse sizes of the plasma in Fig. 2 reach values of several hundred microns within the first 30 ns in both
Fig. 2. Images of sparks produced in jet (left-side) in STP conditions (left-side), acquired at different timing after the laser onset. The CCD gate aperture was 2 ns.
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gas conditions, which determines the spectral resolution of our spectrograph (see Fig. 1). In STP conditions, the backward shift of the plasma leads to further defocusing of the spectra on the output focal plane. The resulting low spectral resolution of our slit-less configuration does not allow us to apply ordinary spectroscopic techniques to plasma emission analysis. Z-scales in Fig. 2 show that peak emission intensities from jet targets are much lower than those in STP gas; the effect is primarily due to the correspondingly lower gas density n(r) in the jet, which decreases downstream as n(r) = 0.15 Æ n0 Æ (d/r)2 [20], where n0 = 2.5 Æ 1020 cm3 is the number gas density at the backpressure of 1 MPa, d = 0.8 mm is the nominal nozzle throat and r the distance from the nozzle. At the position of the laser beam waist, we evaluate n(2.5 mm) ffi 4 Æ 1018 cm3. It is worth noting that the actual gas density might be even lower, since the short stroke of the valve poppet determines an effective throat smaller than the nominal value [21]. The quite different laser absorption mechanisms and space/time evolutions of the plasma in the jet and STP conditions limit the extent to which further comparative analysis can be carried out. 3.2. Spectral analysis Fig. 3 reports the emission spectra, dispersed in the VUV–UV wavelength range 100–260 nm and time integrated in the first 30 ns, of LPPs in He, Ar, N2 and Xe in jets (solid lines) and in He, Ar and N2 at STP (open circles), respectively. The absolute calibration of intensities has been performed by accounting for the geometrical factors arising
from the light collection optics, the quantum efficiency of scintillator and CCD photocathode, and the gain of the MCP intensifier. The vertical scales report the photon fluxes collected from plasmas in gas jets onto the scintillator plane, expressed in units of photons cm2 nm1 pulse1, whereas those acquired in STP gas are firstly reduced by the indicated factors and then reported on the same scale, for sake of quantitative comparison. Data are not corrected for the grating efficiency, which affects strongly the emission spectra below 120 nm, due to the vanishing reflectivity of the MgF2 coating of the aluminum grating [22]. The time-integrated emission spectra result from contributions of both excitation and relaxation processes. The former involves the primary effect of the laser radiation: the laser-sustained plasma is very hot and dense and the free–free radiation (f–f or bremsstrahlung) and free–bound (f–b) electron recombination are the main emission mechanisms, which contribute as a spectral continuum. The latter comprises the plasma expansion and relaxation following the laser pulse, when bound–bound (b–b) electronic transitions of ions and neutrals occur and emission lines emerge from the vanishing continuum [23]. The emission spectrum produced in He gas jet is dominated by the strong Balmer-a line at k = 164 nm, arising from singly ionized atoms [24], whereas a weak continuum also appears in the STP condition. The case of the He plasma highlights the major differences between the LPPs produced in jet and STP gas conditions, in terms of intensity and resolution. As the emission line arises from ions, there is no re-absorption from neutral He, whether the plasma pocket is surrounded or not by the gas in the chamber; therefore, the strong decrease of the emission intensity
Fig. 3. UV–VUV spectra of the sparks of argon, nitrogen, helium and xenon in jet (solid lines) and in uniform gas (open circles), integrated in the first 30 ns from the plasma onset.
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of He plasma in the gas jet case, compared to that at STP conditions, is mainly due to the lower local gas density at the chosen distance downstream the nozzle exit, which is expected to be lowered at least by two orders of magnitude with respect to that at back-pressure, and to the smaller dimension of the plasma. Moreover, a broader and blueshifted line is observed from He plasma produced in STP condition, which results from different effects: (a) the larger size of the plasma, since the fraction of laser energy absorbed by the gas is much larger then in the jet case; (b) the defocusing due to the backward spatial shift of the plasma, similar to that of Ar in Fig. 2, which also accounts for the geometrical effect of the apparent ‘‘blueshift’’ of the He line in Fig. 3 and (c) possibly a Starkbroadening, which however cannot be estimated with the available spectral resolution. The spectra of Ar and N2 in STP conditions have similar shapes and both show a strong continuum, involving emission dominated by f–f and f–b transitions. Furthermore, the emission from Ar is nearly twice more intense than that from N2, over the whole examined wavelength range, mainly due to the higher Ar atomic number. As noted above, the pronounced decrease of emission below the 120 nm is essentially due to the reduced efficiency of the grating; however, some features due to absorption of the surrounding gas survive and are observable in both spectra. In particular, the sharper decrease of the nitrogen spectrum below 140 nm is due to the additional absorption of molecular nitrogen [25], while the expected Ar absorption lines at 105 and 106 nm [24] appear in the Ar spectrum, though blue-shifted at around 100 nm due to the plasma spatial drift described above. As compared to the spectra from plasmas in STP conditions, those of the jet cases have much lower intensities and show more resolved structures superimposed to a continuum. The atomic nitrogen has a very complex spectrum in 90–260 nm [24,25], therefore no assignment has been attempted. In the argon spectrum the line at 105 nm of Ar I is clearly observable, even if at a low intensity. Other unassigned structures may well be emission lines of Ar III. The spectrum of the plasma in Xe jet is by far more intense than those of the other gases and does not show spectral structures. The spectral shape and intensity result from a number of causes: (a) the higher laser energy absorption, allowed by the lower ionization potential of Xe [24]; (b) the higher electron density, expected from the higher atomic number; (c) the extremely congested spectrum of Xe in this wavelength range, comprising very close lines from single or multiple ions [24]; (d) enhanced emission from either clusters formed in the supersonic expansion or even excimers formed in the plasma [26]. The photon fluxes of up to 1012 photons cm2 nm1 pulse1 at about 180 nm from Xe jets in Fig. 3 correspond to 10 lW cm2 nm1 at 10 Hz laser repetition rate and are at least one order of magnitude larger than those achieved by commercial D2 glow discharge lamps (e.g. Hamamatsu L2D2).
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Quantitative analysis of the electron density ne and temperature Te as functions of space and time, requires the application of dedicated diagnostics, e.g. laser interferometry and high resolution emission/absorption spectroscopy. Published data from laser interferometry on nitrogen LPPs in uniform gas and jet [27], have provided values of the electron density in the range ne ffi 1 to 3 Æ 1018 cm3, with a weak dependence on neutral gas density (n ffi 1 to 4 Æ 1018 cm3) for plasmas produced in uniform gas. Also, previous emission/absorption spectroscopy experiments on air LPPs at STP conditions [6], have given values of Te as high as 105 K within the very first nanoseconds, decreasing to nearly 4.104 K at the end of the laser pulse and down to 2.104 K after 30 ns; Te values measured for argon LPPs in STP conditions are slightly larger (3 · 104 K for time >30 ns) [28]. The images in Fig. 2 allow us to estimate Te from gas dynamics arguments, by considering that the pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi blast wave expands with a radial speed vP cZk B T e =m [29], where c = 5/3 for Ar, and Z ffi ne/n, kB and m are the ionization level, the Boltzmann constant and the atomic mass, respectively. The evaluation of vP from the images of Fig. 2 at times beyond 30 ns gives values of the order of 4 Æ 103 m/s in jet and 5 Æ 103 m/s in STP conditions, which correspond, respectively, to values of Z Te 3 Æ 104 K and Z Te 4 Æ 104 K. From the electron and neutral densities reported for plasma produced in uniform gas (Z 1) [27,28], we estimate temperature values which are consistent with those cited above. 3.3. Temporal analysis The temporal analysis of data has been carried out by assuming a flat spectral response of the sodium salicylate and by accounting for the decay time s of its fluorescence [30]; accordingly, all the spectra at each wavelength have been corrected for by means of a numerical de-convolution, using the relationships between the light signal incident onto the scintillator I(t, k) and the positiondependent induced fluorescence IF(t, x), where x / k, given by [31] t t0 Iðt0 ; kÞ exp dt0 ) Iðt; kÞ s 0 dI F ðt; xÞ ¼ I F ðt; xÞ þ s dt
I F ðt; xÞ ¼
1 s
Z
t
The time-resolved VUV–UV plasma brightness, namely the emission intensity integrated over the whole 100–260 nm band, is reported in Fig. 4(a) and (b) for the indicated gases in jet and STP conditions. It includes the contribution of the continuum (f–f + f–b), which prevails in the first nanoseconds, and that of the lines (b–b), which appear later and last longer. The He emission has a peculiar temporal evolution, due the features of its spectrum, which reduces to the strong
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Fig. 4. UV–VUV emission temporal evolution of the plasma produced in the jet (a) and in uniform gas (b) starting from the onset of the plasma formation.
Balmer-a line, superimposed to a weak continuum only in ‘‘STP’’ conditions. Fig. 4(b) shows that the Ar and nitrogen plasma emission in ‘‘STP’’ conditions lasts considerably longer than the laser pulse width (FWHW >50 ns for Ar and >30 ns for nitrogen). This effect is due to the expansion dynamics following the laser interaction, shown in Fig. 2, where the plasma wave clearly appears as a wide source of emission. Notably, the radiation pulse has a duration less than 20 ns (FWHW) when the plasmas are produced in gas jets of Ar and nitrogen; the faster decay of the plasma in jet conditions constitutes a further premium feature of LPPs as light sources for pulsed operation. The time evolution of the Xe emission in the jet condition shows a tail longer than in Ar and N2, due to a slower overall relaxation of the b–b transitions between numberless ionized and excited states. 4. Conclusions The paper reports on measurements of shapes, spectral emissions and brightness evolutions of plasmas produced in supersonic jet of He, Ar, N2 and Xe in vacuum. For a comparison, also reported are the corresponding observations in uniform (STP) gas conditions of He, Ar and N2. Time-resolved imaging of the plasmas has shown that the conical gas jet limits the plasma length along the laser axis to values well below 1 mm all over the plasma life, while the sparks produced in STP conditions elongate
backward far above 1 mm since the first nanoseconds from the plasma onset. The corresponding transverse sizes are of some hundreds microns within the first 30 ns, which determine the overall spectral resolution on the output focal plane of our optical configuration. The spectrograph geometry, due to its low f/number and to the slit-less input light source, allows to produce photon fluxes in the UV–VUV of about 1011–1012 photons cm2 nm1 per laser pulse, depending on the jet target gas. From the quantitative comparison of the brightness of laser plasmas produced in different surrounding gas conditions, the photon fluxes produced in jet targets are one order of magnitude lower than those obtained in STP gas, primarily as an effect of the lower gas density in the jet; nevertheless, the irradiance of the LPP in jet, for typical Nd-YAG laser repetition rates is still much larger than those of conventional c.w. glow discharges lamps. Time-resolved analysis of the emitted radiation reveals decay-times of emission intensities in jet targets shorter than those in uniform gases, which correlate with the lack of the plasma back-streaming expansion, inhibited by the sharp vacuum–gas jet boundary. Finally, the light source based on the LPP from gas jet targets has high stability and reproducibility, in terms of size, position and emission intensity. Our findings suggest that LPPs from gas jets may be usefully exploited as debris-free, point-like, pulsed, tunable and efficient UV–VUV light sources, suitable for many application. Acknowledgement This work was supported by the Italian M.I.U.R.-D.D. 1105/02 – Contract on Project No. 193. References [1] V. Skurat, Nucl. Instr. and Meth. B 208 (2003) 27. [2] M.R. Wertheimer, A.C. Fozza, A. Hollander, Nucl. Instr. and Meth. B 151 (1999) 65. [3] N. Shirahata, K. Oda, S. Asakura, A. Fuwa, Y. Yokogawa, T. Kameyama, A. Hozumi, J. Vac. Sci. Technol. A 22 (4) (2004) 1615. [4] S. Kranzusch, C. Peth, K. Mann, Rev. Sci. Instr. 74 (2) (2003) 969. [5] R. Flesh, M.C. Schurmann, J. Plenge, H. Meiss, M. Hunnekuhl, E. Ruhl, Phys. Rev. A 62 (2000) 52723. [6] A. Borghese, S.S. Merola, Appl. Opt. 37 (18) (1998) 3977. [7] D.A. Cremers, L.J. Radziemski, Handbook of Laser Induced Breakdown Spectroscopy, John Wiley & Sons Ltd., West Sussex, England, 2006. [8] W. Sun, K. Yokoyama, J. Robinson, A. Suits, D.M. Neumark, J. Chem. Phys. 110 (1999) 4363. [9] T.M. Di Palma, A. Latini, M. Satta, M. Varvesi, A. Giardini, Chem. Phys. Lett. 284 (1998) 184. [10] J.M. Bridges, C.L. Corner, T.J. Mcllrath, Appl. Opt. 25 (13) (1986) 2208. [11] W. Shaikh, G. Hirst, R.M. Allott, I.C.E. Turcu, M. Folkard, K. Ledingham, R. Donovan, N. Khan, IEEE J. Sel. Top. Quantum Electron. 5 (6) (1999) 1522. [12] P. Laporte, N. Damany, H. Damany, Opt. Lett. 12 (12) (1987) 987. [13] N. Xu, V. Majidi, Appl. Spectrosc. 47 (8) (1993) 1134. [14] M.M. Murnane, H.C. Kapteyn, M.D. Rosen, R.W. Falcone, Science 251 (1991) 531.
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