Journal of Quantitative Spectroscopy & Radiative Transfer, 2003
We present observations of high-n transitions in the spectra of neon-like Cu19+ and Zn20+ ions ob... more We present observations of high-n transitions in the spectra of neon-like Cu19+ and Zn20+ ions observed in a variety of laser-produced plasmas (LPPs). The spectra are recorded with spectral resolution λ/Δλ=3000–8000 from three different laser sources: a -pulse length Nd:glass, a ps-pulse length Nd:glass, a -pulse length XeCl laser. These spectral observations are used to classify the transitions in the 2p–nd and 2s–np Rydberg series of the Ne-like copper and zinc ions, and to derive their first and second ionization energies. The plasma X-ray emission is simulated with a steady-state collisional-radiative model that includes the effect of self-absorption on line intensities, and the effect of hot-electron populations on the ionization balance. These spectra, along with those in our other recent work, provide a comprehensive set of data that can be used to test the accuracy of atomic-structure calculations, and to demonstrate the importance of opacity and hot electron effects on high-n transitions.
Laser produced expanding plasmas exhibit appreciable spatial charge separations as measured by a ... more Laser produced expanding plasmas exhibit appreciable spatial charge separations as measured by a low impedance Faraday cup. This can result from energetic electrons drifting with the positive ions. A Faraday cup collector with a magnetic filter in front is shown to measure the correct ion component as compared to other charge collector configurations.
Tin, lithium, and xenon laser-produced plasmas are attractive candidates as light sources for ext... more Tin, lithium, and xenon laser-produced plasmas are attractive candidates as light sources for extreme ultraviolet lithography (EUVL). Simulation of the dynamics and spectral properties of plasmas created in EUVL experiments plays a crucial role in analyzing and interpreting experimental measurements, and in optimizing the 13.5 nm radiation from the plasma source. Developing a good understanding of the physical processes in
The application of x-ray spectroscopy methods enabled a space-resolved observation of the high-en... more The application of x-ray spectroscopy methods enabled a space-resolved observation of the high-energy Rydberg satellite series 1s2lnl'-->1s22l+hnu and the Lybeta satellites 2l3l'-->1s2l+hnu in dense laser-produced plasmas. Ab initio atomic structure calculations including relativistic and QED effects show excellent agreement with the precise wavelengths measurements. Satellite transitions with n>3 split into three distinct groups. For n>5, the transitions are found to merge with the Lybeta satellite transitions 2l3l'-->1s2l+hnu and the series limits approach the Lybeta line. This leads to emission on the red and even on the blue wing of the Lybeta line. Spatially and spectrally resolved intensity distribution discovered strong spatial restrictions of the Rydberg satellite intensities near the target surface. On the contrary, all resonance lines show strong emissions up to 10-mm distance from the target and large optical thickness. This enabled us to develop essentially opacity-free and spatially localized temperature diagnostics based on Rydberg satellite emission. Total x-ray emission spectra are established with the help of the MARIA suite of kinetic codes. The derived properties of the spectral distribution make the high-energy Rydberg satellites of general interest for the wide class of dense plasma experiments.
Journal of Electron Spectroscopy and Related Phenomena, 1996
A soft X-ray microscope was constructed using grazing incidence mirrors and a laser-produced plas... more A soft X-ray microscope was constructed using grazing incidence mirrors and a laser-produced plasma source. A Q-switched Nd-YAG laser (1.064μm, 530mJ, 6ns pulse) and a Wolter type-I mirror objective were used. The spatial resolution estimated by the edge profile (25% – 75%) was about 60nm using 5 – 7nm soft X-rays from an Al target. Hydrated biological specimens, such as human blood cells, could be observed using an environmental chamber with Si3N4 windows (thickness: 0.1μm), a back-illuminated CCD camera, and 3–5nm soft X-rays from a Ta target.
Journal of Quantitative Spectroscopy & Radiative Transfer, 2006
Precise spectroscopic observations of K-shell emission from highly stripped Al ions immersed in d... more Precise spectroscopic observations of K-shell emission from highly stripped Al ions immersed in dense, constrained-flow laser-produced plasma is reported. By using a vertical dispersion Johann spectrometer, the time-integrated spectra of the Al Heα group were measured with a high spectral and spatial resolution. The complex spectral profiles modified by the satellite formation, line broadening and frequency shifts were decomposed into individual pseudo-Voigt components by using a code GASPED based on a problem-dependent genetic algorithm. The method uses eight operators tailored to the problem of spectral decomposition and variable-size genomes to fit the data with a varying number of spectral lines. The spectra fitting was based on anticipatory theoretical knowledge of the satellite structure simulated by the multilevel collisional-radiative code MARIA and on an assumption of the aggregate plasma-induced shift of the parent lines and their satellites. The analysis of the spectral profiles revealed systematic red shifts of the resonance and the intercombination lines. Their magnitude is commensurate with predictions of the atomic data and spectral line shape codes combined with the 1D hydrodynamic modeling of the plasma conditions and independent electron density measurements. The results obtained corroborate the feasibility of an accurate decomposition of the spectral profiles encompassing optically thick and thin lines overlapped by a strong satellite emission.
The wavelength dependence of laser-produced breakdown in air, CO and CO2 has been studied using t... more The wavelength dependence of laser-produced breakdown in air, CO and CO2 has been studied using the four Nd:YAG harmonics (266 nm, 355 nm, 532 nm and 1064 nm) and the ArF-excimer laser (193 nm). Breakdown thresholds at these wavelengths are reported for air, CO and CO2. A significant reduction in the breakdown thresholds for both CO and CO2 is apparent when comparing 193 nm with the four Nd:YAG harmonics. This reduction is attributed to the resonance-enhanced two-photon ionization of metastable carbon atoms generated in the laser focus at the ArF-laser wavelength. In addition to reporting breakdown thresholds, the laser-produced plasmas in CO and CO2 are characterized in terms of plasma temperatures and electron densities which are measured by time-resolved emission spectroscopy. Electron densities range from 9 × 1017 cm−3 to 1 × 1017 cm−3. Excitation temperatures range from 22 000 K at 0.2 µs to 11 000 K at 2 µs. Ionization temperatures range from 22 000 K at 0.1 µs to 16 000 K at 2 µs. Evidence is presented to indicate that, like ArF-laser-produced plasmas, Nd:YAG-laser-produced plasmas formed in CO and CO2 are in or near a state of Local Thermodynamic Equilibrium (LTE) soon after their formation.
An investigation is made of the blow-off characteristics of a two component laser produced plasma... more An investigation is made of the blow-off characteristics of a two component laser produced plasma. Approximate similarity solutions describing the plasma expansion are given and it is shown that the inherent inhomogeneity of the problem limits the total growth of the associated two-stream instability.
An analytical formula for maximizing radiation efficiency from a laser-produced plasma is derived... more An analytical formula for maximizing radiation efficiency from a laser-produced plasma is derived. The maximum efficiency is achieved when the plasma expansion distance during laser heating is equal to the laser absorption length. The dependence of the radiation efficiency on the plasma density is confirmed by experiments using a particle-cluster target. By creating a relatively uniform density plasma with a 300 microm diameter by dispersing SnO(2) particles coated on a Si wafer, the conversion efficiency at 14 nm, as high as 4 times as that for a Sn plate target, is achieved.
We present an analysis of dielectronic satellite spectra of Ne-like ion resonance lines for eleme... more We present an analysis of dielectronic satellite spectra of Ne-like ion resonance lines for elements from Cr to Ni. For these low-Z elements, we use spectra from strongly underionized CO2-laser produced plasma to minimize the emission from open L-shell ions. This simplifies the spectra and allows the identification of satellite lines caused by radiative transitions from autoionizing states of sodium
Journal of Quantitative Spectroscopy & Radiative Transfer, 1997
In the present work, we report the frequency spectra of the continuum generated in a laser produc... more In the present work, we report the frequency spectra of the continuum generated in a laser produced plasma of Xe, in the region 3000–5800 Å. From these studies it can be concluded that we have identified the photorecombination thresholds (radiative recombination, RR) to the excited levels of Xe I and Xe II and some autoionizing resonances nl−El′ transitions where with nl we label bound levels and with El′ auto-ionizing levels are indicated.
Detailed radiative-transfer simulations are presented for plasma that is prepared in a predominan... more Detailed radiative-transfer simulations are presented for plasma that is prepared in a predominantly heliumlike ground state and then photopumped. The plasma conditions in the simulation are based on those determined in a line-coincidence experiment described in the preceding paper [C. A. Back, C. Chenais-Popovics, and R. W. Lee, Phys. Rev. A 44, 6730 (1991)]. The measured emitted flux of the resonance line of the heliumlike aluminum ion, at 7.757 Å, is used as input for the photon pump. Predictions of the fluorescence are directly compared with experimental results. The synthetic spectra are calculated, and the effect of the photopumping on the level populations and ionization balance is discussed. Also a description is included of the equivalent two-level-atom radiative-transfer scheme that is used to calculate the self-consistent solution of the level populations and the radiation field.
Preliminary experiments using an X-ray streak filter spectrometer employing a Au/CsI bifurcated p... more Preliminary experiments using an X-ray streak filter spectrometer employing a Au/CsI bifurcated photocathode to investigate the X-ray history of UV (350 nm) laser produced plasmas are reported. Novel use of the streak spectrometer permits a direct and precise fiducial record of the laser pulse to be made from specularly scattered laser light, in true streak time relative to the X-ray record. Initial data indicates a constant 700 eV temperature during the laser pulse; time-resolved particulars are compared to simulations made by the design code LILAC.
With the construction of novel XUV sources, such as VUV FEL’s, XUV laser-matter interaction will ... more With the construction of novel XUV sources, such as VUV FEL’s, XUV laser-matter interaction will become available at ultra-high intensities. But even tabletop facilities such as XUV lasers or High Harmonic Generation, are starting to reach intensities high enough to produce dense plasmas. XUV laser-matter interaction is studied by a 1D hydrodynamic Lagrangian code with radiative transfer for a range of interesting XUV sources. Heating is found to be very different for Z = 12-14 elements having L-edges around the XUV laser wavelength. Possible absorption mechanisms were investigated in order to explain this behaviour, and interaction with cold dense matter proved to be dominant. Plasma sensitivity to XUV laser parameters such as energy, pulse duration, and wavelength was also studied, covering ranges of existing XUV lasers. We found that XUV laser-produced plasmas could be studied using tabletop lasers, paving the way for future VUV-FEL high intensity experiments.
Journal of Quantitative Spectroscopy & Radiative Transfer, 2006
The structures observed in profiles of the LγLγ line of Al XIII emitted from the laser-produced p... more The structures observed in profiles of the LγLγ line of Al XIII emitted from the laser-produced plasma are identified as the Langmuir-wave-caused dips (L-dips). Hitherto, the L-dips were found in hydrogen and deuterium spectral lines only; we report the first experiment dedicated to their identification in higher-Z plasmas. The L-dip phenomenon has a fundamental importance as a nonlinear dynamic-resonance effect of essentially multifrequency nature. Besides, the L-dips can be practically used for measuring the amplitude of the Langmuir waves. The latter diagnostic application might be very important in future experiments with laser-produced plasmas, as no other method was suggested in the literature for this purpose yet.
Presented are the results of experiments in which jets are created through the collision of two l... more Presented are the results of experiments in which jets are created through the collision of two laser-produced plasmas. These experiments use a simple ''v-foil'' target design: two thin foils are placed at an angle of 140 to each other and irradiated with a high-energy laser. The plasmas from the rear face of these foils collide and drive plasma jets moving with a velocity of $300 km s À1 . By carefully choosing the foil thickness and material to suit the laser conditions available, it has proven possible to create plasma jets for which the relevant scaling parameters show significant overlap with those of outflows associated with young stellar objects (YSOs). Preliminary results are also shown from experiments to study the effect of an ambient gas on jet propagation. Nominally identical experiments are conducted either in vacuum or in an ambient medium of 5 mbar of nitrogen gas. The gas is seen to increase the jet collimation and to introduce shock structures at the head of the outflow.
A stigmatic spectrograph composed of a toroidal mirror and a concave grating both working at graz... more A stigmatic spectrograph composed of a toroidal mirror and a concave grating both working at grazing incidence has been built and applied to the observation of laser-produced plasmas. Spatial resolution of 20-30 pm over a region of 1-mm extension and spectral resolution of -1500 have been obtained with a considerable increase in speed of the spectrograph. By varying the relative positions of the optical elements, it is possible to change the wavelength of stigmatic condition in the range 10-300 A. Examples of observations of a beryllium plasma are shown.
Journal of Quantitative Spectroscopy & Radiative Transfer, 2003
We present observations of high-n transitions in the spectra of neon-like Cu19+ and Zn20+ ions ob... more We present observations of high-n transitions in the spectra of neon-like Cu19+ and Zn20+ ions observed in a variety of laser-produced plasmas (LPPs). The spectra are recorded with spectral resolution λ/Δλ=3000–8000 from three different laser sources: a -pulse length Nd:glass, a ps-pulse length Nd:glass, a -pulse length XeCl laser. These spectral observations are used to classify the transitions in the 2p–nd and 2s–np Rydberg series of the Ne-like copper and zinc ions, and to derive their first and second ionization energies. The plasma X-ray emission is simulated with a steady-state collisional-radiative model that includes the effect of self-absorption on line intensities, and the effect of hot-electron populations on the ionization balance. These spectra, along with those in our other recent work, provide a comprehensive set of data that can be used to test the accuracy of atomic-structure calculations, and to demonstrate the importance of opacity and hot electron effects on high-n transitions.
Laser produced expanding plasmas exhibit appreciable spatial charge separations as measured by a ... more Laser produced expanding plasmas exhibit appreciable spatial charge separations as measured by a low impedance Faraday cup. This can result from energetic electrons drifting with the positive ions. A Faraday cup collector with a magnetic filter in front is shown to measure the correct ion component as compared to other charge collector configurations.
Tin, lithium, and xenon laser-produced plasmas are attractive candidates as light sources for ext... more Tin, lithium, and xenon laser-produced plasmas are attractive candidates as light sources for extreme ultraviolet lithography (EUVL). Simulation of the dynamics and spectral properties of plasmas created in EUVL experiments plays a crucial role in analyzing and interpreting experimental measurements, and in optimizing the 13.5 nm radiation from the plasma source. Developing a good understanding of the physical processes in
The application of x-ray spectroscopy methods enabled a space-resolved observation of the high-en... more The application of x-ray spectroscopy methods enabled a space-resolved observation of the high-energy Rydberg satellite series 1s2lnl'-->1s22l+hnu and the Lybeta satellites 2l3l'-->1s2l+hnu in dense laser-produced plasmas. Ab initio atomic structure calculations including relativistic and QED effects show excellent agreement with the precise wavelengths measurements. Satellite transitions with n>3 split into three distinct groups. For n>5, the transitions are found to merge with the Lybeta satellite transitions 2l3l'-->1s2l+hnu and the series limits approach the Lybeta line. This leads to emission on the red and even on the blue wing of the Lybeta line. Spatially and spectrally resolved intensity distribution discovered strong spatial restrictions of the Rydberg satellite intensities near the target surface. On the contrary, all resonance lines show strong emissions up to 10-mm distance from the target and large optical thickness. This enabled us to develop essentially opacity-free and spatially localized temperature diagnostics based on Rydberg satellite emission. Total x-ray emission spectra are established with the help of the MARIA suite of kinetic codes. The derived properties of the spectral distribution make the high-energy Rydberg satellites of general interest for the wide class of dense plasma experiments.
Journal of Electron Spectroscopy and Related Phenomena, 1996
A soft X-ray microscope was constructed using grazing incidence mirrors and a laser-produced plas... more A soft X-ray microscope was constructed using grazing incidence mirrors and a laser-produced plasma source. A Q-switched Nd-YAG laser (1.064μm, 530mJ, 6ns pulse) and a Wolter type-I mirror objective were used. The spatial resolution estimated by the edge profile (25% – 75%) was about 60nm using 5 – 7nm soft X-rays from an Al target. Hydrated biological specimens, such as human blood cells, could be observed using an environmental chamber with Si3N4 windows (thickness: 0.1μm), a back-illuminated CCD camera, and 3–5nm soft X-rays from a Ta target.
Journal of Quantitative Spectroscopy & Radiative Transfer, 2006
Precise spectroscopic observations of K-shell emission from highly stripped Al ions immersed in d... more Precise spectroscopic observations of K-shell emission from highly stripped Al ions immersed in dense, constrained-flow laser-produced plasma is reported. By using a vertical dispersion Johann spectrometer, the time-integrated spectra of the Al Heα group were measured with a high spectral and spatial resolution. The complex spectral profiles modified by the satellite formation, line broadening and frequency shifts were decomposed into individual pseudo-Voigt components by using a code GASPED based on a problem-dependent genetic algorithm. The method uses eight operators tailored to the problem of spectral decomposition and variable-size genomes to fit the data with a varying number of spectral lines. The spectra fitting was based on anticipatory theoretical knowledge of the satellite structure simulated by the multilevel collisional-radiative code MARIA and on an assumption of the aggregate plasma-induced shift of the parent lines and their satellites. The analysis of the spectral profiles revealed systematic red shifts of the resonance and the intercombination lines. Their magnitude is commensurate with predictions of the atomic data and spectral line shape codes combined with the 1D hydrodynamic modeling of the plasma conditions and independent electron density measurements. The results obtained corroborate the feasibility of an accurate decomposition of the spectral profiles encompassing optically thick and thin lines overlapped by a strong satellite emission.
The wavelength dependence of laser-produced breakdown in air, CO and CO2 has been studied using t... more The wavelength dependence of laser-produced breakdown in air, CO and CO2 has been studied using the four Nd:YAG harmonics (266 nm, 355 nm, 532 nm and 1064 nm) and the ArF-excimer laser (193 nm). Breakdown thresholds at these wavelengths are reported for air, CO and CO2. A significant reduction in the breakdown thresholds for both CO and CO2 is apparent when comparing 193 nm with the four Nd:YAG harmonics. This reduction is attributed to the resonance-enhanced two-photon ionization of metastable carbon atoms generated in the laser focus at the ArF-laser wavelength. In addition to reporting breakdown thresholds, the laser-produced plasmas in CO and CO2 are characterized in terms of plasma temperatures and electron densities which are measured by time-resolved emission spectroscopy. Electron densities range from 9 × 1017 cm−3 to 1 × 1017 cm−3. Excitation temperatures range from 22 000 K at 0.2 µs to 11 000 K at 2 µs. Ionization temperatures range from 22 000 K at 0.1 µs to 16 000 K at 2 µs. Evidence is presented to indicate that, like ArF-laser-produced plasmas, Nd:YAG-laser-produced plasmas formed in CO and CO2 are in or near a state of Local Thermodynamic Equilibrium (LTE) soon after their formation.
An investigation is made of the blow-off characteristics of a two component laser produced plasma... more An investigation is made of the blow-off characteristics of a two component laser produced plasma. Approximate similarity solutions describing the plasma expansion are given and it is shown that the inherent inhomogeneity of the problem limits the total growth of the associated two-stream instability.
An analytical formula for maximizing radiation efficiency from a laser-produced plasma is derived... more An analytical formula for maximizing radiation efficiency from a laser-produced plasma is derived. The maximum efficiency is achieved when the plasma expansion distance during laser heating is equal to the laser absorption length. The dependence of the radiation efficiency on the plasma density is confirmed by experiments using a particle-cluster target. By creating a relatively uniform density plasma with a 300 microm diameter by dispersing SnO(2) particles coated on a Si wafer, the conversion efficiency at 14 nm, as high as 4 times as that for a Sn plate target, is achieved.
We present an analysis of dielectronic satellite spectra of Ne-like ion resonance lines for eleme... more We present an analysis of dielectronic satellite spectra of Ne-like ion resonance lines for elements from Cr to Ni. For these low-Z elements, we use spectra from strongly underionized CO2-laser produced plasma to minimize the emission from open L-shell ions. This simplifies the spectra and allows the identification of satellite lines caused by radiative transitions from autoionizing states of sodium
Journal of Quantitative Spectroscopy & Radiative Transfer, 1997
In the present work, we report the frequency spectra of the continuum generated in a laser produc... more In the present work, we report the frequency spectra of the continuum generated in a laser produced plasma of Xe, in the region 3000–5800 Å. From these studies it can be concluded that we have identified the photorecombination thresholds (radiative recombination, RR) to the excited levels of Xe I and Xe II and some autoionizing resonances nl−El′ transitions where with nl we label bound levels and with El′ auto-ionizing levels are indicated.
Detailed radiative-transfer simulations are presented for plasma that is prepared in a predominan... more Detailed radiative-transfer simulations are presented for plasma that is prepared in a predominantly heliumlike ground state and then photopumped. The plasma conditions in the simulation are based on those determined in a line-coincidence experiment described in the preceding paper [C. A. Back, C. Chenais-Popovics, and R. W. Lee, Phys. Rev. A 44, 6730 (1991)]. The measured emitted flux of the resonance line of the heliumlike aluminum ion, at 7.757 Å, is used as input for the photon pump. Predictions of the fluorescence are directly compared with experimental results. The synthetic spectra are calculated, and the effect of the photopumping on the level populations and ionization balance is discussed. Also a description is included of the equivalent two-level-atom radiative-transfer scheme that is used to calculate the self-consistent solution of the level populations and the radiation field.
Preliminary experiments using an X-ray streak filter spectrometer employing a Au/CsI bifurcated p... more Preliminary experiments using an X-ray streak filter spectrometer employing a Au/CsI bifurcated photocathode to investigate the X-ray history of UV (350 nm) laser produced plasmas are reported. Novel use of the streak spectrometer permits a direct and precise fiducial record of the laser pulse to be made from specularly scattered laser light, in true streak time relative to the X-ray record. Initial data indicates a constant 700 eV temperature during the laser pulse; time-resolved particulars are compared to simulations made by the design code LILAC.
With the construction of novel XUV sources, such as VUV FEL’s, XUV laser-matter interaction will ... more With the construction of novel XUV sources, such as VUV FEL’s, XUV laser-matter interaction will become available at ultra-high intensities. But even tabletop facilities such as XUV lasers or High Harmonic Generation, are starting to reach intensities high enough to produce dense plasmas. XUV laser-matter interaction is studied by a 1D hydrodynamic Lagrangian code with radiative transfer for a range of interesting XUV sources. Heating is found to be very different for Z = 12-14 elements having L-edges around the XUV laser wavelength. Possible absorption mechanisms were investigated in order to explain this behaviour, and interaction with cold dense matter proved to be dominant. Plasma sensitivity to XUV laser parameters such as energy, pulse duration, and wavelength was also studied, covering ranges of existing XUV lasers. We found that XUV laser-produced plasmas could be studied using tabletop lasers, paving the way for future VUV-FEL high intensity experiments.
Journal of Quantitative Spectroscopy & Radiative Transfer, 2006
The structures observed in profiles of the LγLγ line of Al XIII emitted from the laser-produced p... more The structures observed in profiles of the LγLγ line of Al XIII emitted from the laser-produced plasma are identified as the Langmuir-wave-caused dips (L-dips). Hitherto, the L-dips were found in hydrogen and deuterium spectral lines only; we report the first experiment dedicated to their identification in higher-Z plasmas. The L-dip phenomenon has a fundamental importance as a nonlinear dynamic-resonance effect of essentially multifrequency nature. Besides, the L-dips can be practically used for measuring the amplitude of the Langmuir waves. The latter diagnostic application might be very important in future experiments with laser-produced plasmas, as no other method was suggested in the literature for this purpose yet.
Presented are the results of experiments in which jets are created through the collision of two l... more Presented are the results of experiments in which jets are created through the collision of two laser-produced plasmas. These experiments use a simple ''v-foil'' target design: two thin foils are placed at an angle of 140 to each other and irradiated with a high-energy laser. The plasmas from the rear face of these foils collide and drive plasma jets moving with a velocity of $300 km s À1 . By carefully choosing the foil thickness and material to suit the laser conditions available, it has proven possible to create plasma jets for which the relevant scaling parameters show significant overlap with those of outflows associated with young stellar objects (YSOs). Preliminary results are also shown from experiments to study the effect of an ambient gas on jet propagation. Nominally identical experiments are conducted either in vacuum or in an ambient medium of 5 mbar of nitrogen gas. The gas is seen to increase the jet collimation and to introduce shock structures at the head of the outflow.
A stigmatic spectrograph composed of a toroidal mirror and a concave grating both working at graz... more A stigmatic spectrograph composed of a toroidal mirror and a concave grating both working at grazing incidence has been built and applied to the observation of laser-produced plasmas. Spatial resolution of 20-30 pm over a region of 1-mm extension and spectral resolution of -1500 have been obtained with a considerable increase in speed of the spectrograph. By varying the relative positions of the optical elements, it is possible to change the wavelength of stigmatic condition in the range 10-300 A. Examples of observations of a beryllium plasma are shown.
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