Sum frequency generation vibrational spectroscopy (SFG-VS) is used to characterize vibrational bonds of molecules at surfaces or interfaces. SFG spectroscopy is particularly attractive because of molecular specificity and intrinsic interfacial sensitivity. Surface sensitivity of the technique arises from the fact that within the electric dipole approximation the nonlinear generation of the sum-frequency (SF) signal from the overlapped visible and infrared beams is forbidden in the media of randomly oriented molecules or in the centrosymmetric media but is allowed at the interface where inversion symmetry is broken. Molecular specificity emerges from the ability to record vibrational spectrum.
In SFG-VS measurements, a pulsed tunable infrared IR (ωIR) laser beam is mixed with a visible VIS (ωVIS) beam to produce an output at the sum frequency (ωSFG = ωIR + ωVIS). SFG signal is generated in visible spectral range, so it can be efficiently measured using sensitive detectors (PMT or CCD).
Heavy Anionic Complex Creates a Unique Water Structure at a Soft Charged Interface
Ion hydration and interfacial water play crucial roles in numerous phenomena ranging from biological to industrial systems. Although biologically relevant (and mostly smaller) ions have been studied extensively in this context, very little experimental data exist about molecular-scale behavior of heavy ions and their complexes at interfaces, especially under technologically significant conditions. It has recently been shown that PtCl6 2− complexes adsorb at positively charged interfaces in a two-step process that cannot fit into well-known empirical trends, such as Hofmeister series. Here, a combined vibrational sum frequency generation and molecular dynamics study reveals that a unique interfacial water structure is connected to this peculiar adsorption behavior. A novel subensemble analysis of molecular dynamics simulation results shows that after adsorption PtCl6 2− complexes partially retain their first and second hydration spheres and that it is possible to identify three different types of water molecules around them on the basis of their orientational structures and hydrogen-bonding strengths. These results have important implications for relating interfacial water structure and hydration enthalpy to the general understanding of specific ion effects.
This in turn influences interpretation of heavy metal ion distribution across, and reactivity within, liquid interfaces.
Probing the Orientation and Conformation of α-Helix and β-Strand Model Peptides on Self-Assembled Monolayers Using Sum Frequency Generation and NEXAFS Spectroscopy
Related products: SFG Spectrometer
The structure and orientation of amphiphilic α-helix and β-strand model peptide films on self-assembled monolayers (SAMs) have been studied with sum frequency generation (SFG) vibrational spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The α-helix peptide is a 14-mer, and the β-strand is a 15-mer of hydrophilic lysine and hydrophobic leucine residues with hydrophobic periodicities of 3.5 and 2, respectively. These periodicities result in the leucine side chains located on one side of the peptides and the lysine side chains on the other side. The SAMs were prepared from the assembly of either carboxylic acid- or methyl-terminated alkyl thiols onto gold surfaces. For SFG studies, the deuterated analog of the methyl SAM was used. SFG vibrational spectra in the C-H region of air-dried peptides films on both SAMs exhibit strong peaks near 2965, 2940, and 2875 cm-1 related to ordered leucine side chains. The orientation of the leucine side chains was determined from the phase of these features relative to the nonresonant gold background. The relative phase for both the R-helix and β-strand peptides showed that the leucine side chains were oriented away from the carboxylic acid SAM surface and oriented toward the methyl SAM surface. Amide I peaks observed near 1656 cm-1 for the R-helix peptide confirm that the secondary structure is preserved on both SAMs. Strong linear dichroism related to the amide π* orbital at 400.8 eV was observed in the nitrogen K-edge NEXAFS spectra for the adsorbed β-strand peptides, suggesting that the peptide backbones are oriented parallel to the SAM surface with the side chains pointing toward or away from the interface. For the α-helix the dichroism of the amide π* is significantly weaker, probably because of the broad distribution of amide bond orientations in the α-helix secondary structure.
Retrieval of complex χ(2) parts for quantitative analysis of sum-frequency generation intensity spectra
Vibrational sum-frequency generation (SFG) spectroscopy has become an established technique for in situ surface analysis. While spectral recording procedures and hardware have been optimized, unique data analysis routines have yet to be established. The SFG intensity is related to probing geometries and properties of the system under investigation such as the absolute square of the second-order susceptibility |χ(2)|2 . A conventional SFG intensity measurement does not grant access to the complex parts of χ(2) unless further assumptions have been made. It is therefore difficult, sometimes impossible, to establish a unique fitting solution for SFG intensity spectra. Recently, interferometric phase-sensitive SFG or heterodyne detection methods have been introduced to measure real and imaginary parts of χ(2) experimentally. Here, we demonstrate that iterative phase-matching between complex spectra retrieved from maximum entropy method analysis and fitting of intensity SFG spectra (iMEMfit) leads to a unique solution for the complex parts of χ(2) and enables quantitative analysis of SFG intensity spectra. A comparison between complex parts retrieved by iMEMfit applied to intensity spectra and phase sensitive experimental data shows excellent agreement between the two methods.
Unified treatment and measurement of the spectral resolution and temporal effects in frequency-resolved sum-frequency generation vibrational spectroscopy (SFG-VS)
The lack of understanding of the temporal effects and the restricted ability to control experimental conditions in order to obtain intrinsic spectral lineshapes in surface sum-frequency generation vibrational spectroscopy (SFG-VS) have limited its applications in surface and interfacial studies. The emergence of high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) with sub-wavenumber resolution [Velarde et al., J. Chem. Phys., 2011, 135, 241102] offers new opportunities for obtaining and understanding the spectral lineshapes and temporal effects in SFG-VS. Particularly, the high accuracy of the HR-BB-SFG-VS experimental lineshape provides detailed information on the complex coherent vibrational dynamics through direct spectral measurements. Here we present a unified formalism for the theoretical and experimental routes for obtaining an accurate lineshape of the SFG response. Then, we present a detailed analysis of a cholesterol monolayer at the air/water interface with higher and lower resolution SFG spectra along with their temporal response. With higher spectral resolution and accurate vibrational spectral lineshapes, it is shown that the parameters of the experimental SFG spectra can be used both to understand and to quantitatively reproduce the temporal effects in lower resolution SFG measurements. This perspective provides not only a unified picture but also a novel experimental approach to measuring and understanding the frequency-domain and time-domain SFG response of a complex molecular interface.
Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy
This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liquid and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liquids. It has been found that molecular interactions at such polymer/liquid interfaces dictate interfacial polymer structures. The molecular structures of silane molecules, which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing molecularlevel understanding of polymer adhesion promotion. The molecular structures of polymer/solid interfaces have been examined using SFG with several different experimental geometries. These results have provided molecularlevel information about polymer friction, adhesion, interfacial chemical reactions, interfacial electronic properties, and the structure of layerbylayer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive analytical techniques.
A structural and temporal study of the surfactants behenyltrimethylammonium methosulfate and behenyltrimethylammonium chloride adsorbed at air/water and air/glass interfaces using sum frequency generation spectroscopy
Molecular scale information about the structure of surfactants at interfaces underlies their application in consumer products. In this study the non-linear optical technique of Sum Frequency Generation (SFG) vibrational spectroscopy has been used to investigate the structure and temporal behaviour of two cationic surfactants used frequently in hair conditioners. SFG spectra of films of behenyltrimethylammonium methosulfate (BTMS) and behenyltrimethylammonium chloride (BTAC) were recorded at the air/water interface and on glass slides following Langmuir Blodgett (LB) deposition. The assignment of the BTMS and BTAC spectral features (resonances) to the C--H stretching modes of the surfactants was consolidated by comparison with the SFG spectrum of deuterated cetyltrimethylammonium bromide (d-CTAB) and by recording spectra on D2O as well as on water. The C--H resonances arise from the methylene and methyl groups of the tail and head-groups of the surfactants. A slow collapse mechanism was observed following film compression of both BTAC and BTMS. The change in molecular structure of the films undergoing this slow collapse was followed by recording sequential SFG spectra in the C--H region, and by monitoring the SFG intensity at specific wavenumbers over time. Additionally, LB deposition onto glass was used to capture the state of the film during the slow collapse, and these SFG spectra showed close similarity to the corresponding spectra on water. Complementary Atomic Force Microscopy (AFM) was used to elucidate the layering of the compressed and relaxed films deposited onto mica by LB deposition.
Structure of the Fundamental Lipopeptide Surfactin at the Air/Water Interface Investigated by Sum Frequency Generation Spectroscopy
The lipopeptide surfactin produced by certain strains of Bacillus subtilis is a powerful bio-surfactant possessing potentially useful antimicrobial properties. In order to better understand its surface behaviour, we have used surface sensitive Sum Frequency Generation (SFG) vibrational spectroscopy in the C-H and C=O stretching regions to determine its structure at the air/water interface. Using surfactin with the leucine groups of the peptide ring perdeuterated we have shown that the majority of the SFG signals arise from the 4 leucine residues. We find that surfactin forms a robust film, and that its structure is not affected by the number density at the interface or by pH variation of the sub-phase. The spectra show that the ring of the molecule lies in the plane of the surface rather than perpendicular to it, with the tail lying above this, also in the plane of the interface.
Quantitative Sum-Frequency Generation Vibrational Spectroscopy of Molecular Surfaces and Interfaces: Lineshape, Polarization, and Orientation
Sum-frequency generation vibrational spectroscopy (SFG-VS) can provide detailed information and understanding of the molecular composition, interactions, and orientational and conformational structure of surfaces and interfaces through quantitative measurement and analysis. In this review, we present the current status of and discuss important recent developments in the measurement of intrinsic SFG spectral lineshapes and formulations for polarization measurements and orientational analysis of SFG-VS spectra. The focus of this review is to present a coherent description of SFG-VS and discuss the main concepts and issues that can help advance this technique as a quantitative analytical research tool for revealing the chemistry and physics of complex molecular surfaces and interfaces.
Second harmonic generation (SHG) is a second order nonlinear optical effect where two photons of frequency ω are converted to one photon of frequency 2ω. SHG is allowed only in media without inversion symmetry.
SHG is a sensitive method used to characterize molecules at surfaces or interfaces because inversion symmetry is broken at the interface.
SHG measurements provide information about: surface coverage, molecular orientation, adsorbtion-desorbtion processes, and reactions at interfaces.
FemtoLux 3 laser for the rapid wide-field second harmonic generation microscopy
Related products: FemtoLux series
FemtoLux 3 laser was used as an illumination source in the wide-field second harmonic generation (SHG) microscope. Relatively high laser pulse energy at a medium pulse repetition frequency allowed for a faster single image acquisition compared to that using laser-scanning. It was also possible to acquire images of relatively large sample areas, which paved the way for the rapid imaging of macroscopic sample areas with microscopic resolution.
In nonlinear optics, z-scan technique is used to measure the non-linear index n2 (Kerr nonlinearity) and the non-linear absorption coefficient Δα via the “closed” and “open” methods to measure both real and imaginary components of the nonlinear refractive index.
For measuring the real part of the nonlinear refractive index, the z-scan setup is used in its closed-aperture form. The sample is typically placed in the focal plane of the lens, and then moved along the z axis, defined by the Rayleigh length. In this form, since the nonlinear material reacts like a weak z-dependent lens, the far-field aperture makes it possible to detect small beam distortions in the original beam. Since the focusing power of this weak nonlinear lens depends on the nonlinear refractive index, it is possible to extract its value by analyzing the z-dependent data acquired by the detector and by interpreting them using an appropriate theory.
For measurements of the imaginary part of the nonlinear refractive index, or the nonlinear absorption coefficient, the z-scan setup is used in its open-aperture form. In open-aperture measurements, the far-field aperture is removed and the whole signal is measured by the detector. By measuring the whole signal, the beam small distortions become insignificant and the z-dependent signal variation is due to the nonlinear absorption entirely.
The main cause of non-linear absorption is two-photon absorption. Due to high pulse intensity and cost effectiveness, picosecond high energy lasers are the most appropriate choice for z-scan measurements.
LIDAR is an acronym for “LIght Detection And Ranging”. LIDAR sends out short laser pulses into the atmosphere, where all along its path, the light is scattered by small particles, aerosols, and molecules of the air and is collected by telescope for analysis. Due to the constant velocity of light, time is related to the scatter’s distance, therefore, the spatial information is retrieved along the beam path. LIDAR uses ultraviolet, visible, or near infrared light to image objects. It can target a wide range of materials, including non-metallic objects, rocks, rain, chemical compounds, aerosols, pollutants, clouds, and even single molecules. LIDAR especially helps in those cases where access with conventional methods is troublesome.
Laser-induced breakdown spectroscopy (LIBS) utilizes a high intensity short laser pulse to convert a very small amount of target material to plasma for optical analysis of the spectra. LIBS can be used on solid, liquid, or gas samples, and, depending on the spectrograph and detector, can detect all elements. LIBS is non-contact, so it can be used in a wide variety of environments, including remote analysis and micro-sampling.
When coupled with appropriate optics and stages, elemental maps of a surface can be created. Multiple LIBS scans can effectively resolve material composition throughout the volume, building a full three dimensional elemental map.
Enhancement of Laser-Induced Breakdown Spectroscopy (LIBS) Detection Limit Using a Low-Pressure and Short-Pulse Laser-Induced Plasma Process
Laser-induced breakdown spectroscopy (LIBS) technology is an appealing technique compared with many other types of elemental analysis because of the fast response, high sensitivity, real-time, and noncontact features. One of the challenging targets of LIBS is the enhancement of the detection limit. In this study, the detection limit of gas-phase LIBS analysis has been improved by controlling the pressure and laser pulse width. In order to verify this method, low-pressure gas plasma was induced using nanosecond and picosecond lasers. The method was applied to the detection of Hg. The emission intensity ratio of the Hg atom to NO (IHg/ INO) was analyzed to evaluate the LIBS detection limit because the NO emission (interference signal) was formed during the plasma generation and cooling process of N2 and O2 in the air. It was demonstrated that the enhancement of IHg/INO arose by decreasing the pressure to a few kilopascals, and the IHg/INO of the picosecond breakdown was always much higher than that of the nanosecond breakdown at low buffer gas pressure. Enhancement of IHg/INO increased more than 10 times at 700 Pa using picosecond laser with 35 ps pulse width. The detection limit was enhanced to 0.03 ppm (parts per million). We also saw that the spectra from the center and edge parts of plasma showed different features. Comparing the central spectra with the edge spectra, IHg/INO of the edge spectra was higher than that of the central spectra using the picosecond laser breakdown process.
Laser Induced Breakdown Spectroscopy and Applications Toward Thin Film Analysis
Laser induced breakdown spectroscopy (LIBS) provides the opportunity to analyze almost any element, from any material, in any environment. Among the many applications of LIBS is the analysis of thin lms and multilayered structures. An automated system was designed and built to conduct LIBS using Nd:YAG and Ti:Sapphire lasers, broadband and high-resolution spectrometers and detectors. This system incorporates the sample manipulation as well as laser and spectrometer control and timing. A series of experiments were conducted to analyze the ability of nanosecond and femtosecond lasers to detect Mg impurities in thin TiO2 lms using LIBS. It was determined that optimal detection occurs early in the plasma ionic/atomic emission with detection capabilities in the parts-per-million range. Another series of experiments were conducted using LIBS to analyze thin transparent organic lms, with specic emphasis on the eect of lm thickness and interplay between lm and substrate. The challenges of ablating and measuring multiple layers have also been explored using various laser wavelengths. The eectiveness of LIBS has been demonstrated for depth proling of CIGS solar cells. Ablation crater and ablation threshold analysis aided in understanding and overcoming some of the obstacles in depth proling. One of the challenges with LIBS is the identication and mitigation of matrix eects. This problem was explored using a Mg tracer element and various compositions of the suspected elements Si, Ca, and Sr which can cause errors in LIBS analysis. The goal of this dissertation is to investigate the ability of LIBS to conduct detailed thin lm analysis for a variety of materials and potential applications. This includes analyzing trace elements from a traditionally noisy background, measuring dicult to ablate thin lms, and the unique challenges associated with multilayered structures.
Ultra-high intensity laser applications span a number of scientific disciplines, such as plasma physics and fusion research, atomic molecular & optical physics, femtosecond chemistry, astrophysics, high energy physics, materials science, biology, and medicine.
Areas where a strong impact is possible include:
- High harmonic generation and attosecond science
- Relativistic effects in interactions with atoms, molecules and electrons
- Ultrafast X-ray science
- High density science
- Fusion energy research
- Particle accelerators
- Thomson scattering
Development and characterization of a laser-plasma soft X-ray source for contact microscopy
In this work, we present a compact laser-produced plasma source of X-rays, developed and characterized for application in soft X-ray contact microscopy (SXCM). The source is based on a double stream gas puff target, irradiated with a commercially available Nd:YAG laser, delivering pulses with energy up to 740 mJ and 4 ns pulse duration at 10 Hz repetition rate. The target is formed by pulsed injection of a stream of high-Z gas (argon) into a cloud of low Z-gas (helium) by using an electromagnetic valve with a double nozzle setup. The source is designed to irradiate specimens, both in vacuum and in helium atmosphere with nanosecond pulses of soft X-rays in the ‘‘water-window” spectral range. The source is capable of delivering a photon fluence of about 1.09 x 103 photon/µm2/pulse at a sample placed in vacuum at a distance of about 20 mm downstream the source. It can also deliver a photon fluence of about 9.31 x 102 - photons/µm2/pulse at a sample placed in a helium atmosphere at the same position. The source design and results of the characterization measurements as well as the optimization of the source are presented and discussed. The source was successfully applied in the preliminary experiments on soft X-ray contact microscopy and images of microstructures and biological specimens with ~80 nm half-pitch spatial resolution, obtained in helium atmosphere, are presented.
EUV spectra from highly charged terbium ions in optically thin and thick plasmas
We have observed extreme ultraviolet (EUV) spectra from terbium (Tb) ions in optically thin and thick plasmas for a comparative study. The experimental spectra are recorded in optically thin, magnetically conned torus plasmas and dense laser-produced plasmas (LPPs). The main feature of the spectra is quasicontinuum emission with a peak around 6.5-6.6 nm, the bandwidth of which is narrower in the torus plasmas than in the LPPs. A comparison between the two types of spectra also suggests strong opacity effects in the LPPs. A comparison with the calculated line strength distributions gives a qualitative interpretation of the observed spectra.
XUV generation from the interaction of pico- and nanosecond laser pulses with nanostructured targets
Laser-produced plasmas are intense sources of XUV radiation that can be suitable for different applications such as extreme ultraviolet lithography, beyond extreme ultraviolet lithography and water window imaging. In particular, much work has focused on the use of tin plasmas for extreme ultraviolet lithography at 13.5 nm. We have investigated the spectral behavior of the laser produced plasmas formed on closely packed polystyrene microspheres and porous alumina targets covered by a thin tin layer in the spectral region from 2.5 to 16 nm. Nd:YAG lasers delivering pulses of 170 ps (Ekspla SL312P )and 7 ns (Continuum Surelite) duration were focused onto the nanostructured targets coated with tin. The intensity dependence of the recorded spectra was studied; the conversion efficiency (CE) of laser energy into the emission in the 13.5 nm spectral region was estimated. We have observed an increase in CE using high intensity 170 ps Nd:YAG laser pulses as compared with a 7 ns pulse.
Conversion efficiency of a laser-plasma source based on a Xe jet in the vicinity of a wavelength of 11 nm
We optimized the parameters of a laser-produced plasma source based on a solid-state Nd: YAG laser (λ = 1.06 nm, pulse duration 4 ns, energy per pulse up to 500 mJ, repetition rate 10 Hz, lens focus distance 45 mm, maximum power density of laser radiation in focus 9 x 1011 W/cm2) and a double-stream Xe/He gas jet to obtain a maximum of radiation intensity around 11 nm wavelength. It was shown that the key factor determining the ionization composition of the plasma is the jet density.With the decreased density, the ionization composition shifts toward a smaller degree of ionization, which leads to an increase in emission peak intensity around 11 nm.We attribute the dominant spectral feature centred near 11 nm originating from an unidentified 4d-4f transition array in Xe+10...+13 ions. The exact position of the peak and the bandwidth of the emission line were determined. We measured the dependence of the conversion efficiency of laser energy into an EUV in-band energy with a peak at 10.82 nm from the xenon pressure and the distance between the nozzle and the laser focus. The maximum conversion efficiency (CE) into the spectral band of 10–12 nm measured at a distance between the nozzle and the laser beam focus of 0.5 mm was CE = 4.25 ± 0.30%. The conversion efficiencies of the source in-bands of 5 and 12 mirror systems at two wavelengths of 10.8 and 11.2 nm have been evaluated; these efficiencies may be interesting for beyond extreme ultraviolet lithography.
53 W average power CEP-stabilized OPCPA system delivering 5.5 TW few cycle pulses at 1 kHz repetition rate
We present a high peak and average power optical parametric chirped pulse amplification system driven by diode-pumped Yb:KGW and Nd:YAG lasers running at 1 kHz repetition rate. The advanced architecture of the system allows us to achieve >53 W average power combined with 5.5 TW peak power, along with sub-220 mrad CEP stability and sub-9 fs pulse duration at a center wavelength around 880 nm. Broadband, background-free, passively CEP stabilized seed pulses are produced in a series of cascaded optical parametric amplifiers pumped by the Yb:KGW laser, while a diode-pumped Nd:YAG laser system provides multi-mJ pump pulses for power amplification stages. Excellent stability of output parameters over 16 hours of continuous operation is demonstrated.
Spectral pulse shaping of a 5 Hz, multi-joule, broadband optical parametric chirped pulse amplification frontend for a 10 PW laser system
Related products: NL940 series
We present a broadband optical parametric chirped pulse amplification (OPCPA) system delivering 4 J pulses at a repetition rate of 5 Hz. It will serve as a frontend for the 1.5 kJ, <150 fs, 10 PW laser beamline currently under development by a consortium of National Energetics and Ekspla. The spectrum of the OPCPA system is precisely controlled by arbitrarily generated waveforms of the pump lasers. To fully exploit the high flexibility of the frontend, we have developed a 1D model of the system and an optimization algorithm that predicts suitable pump waveform settings for a desired output spectrum. The OPCPA system is shown to have high efficiency, a high-quality top-hat beam profile, and an output spectrum demonstrated to be shaped consistently with the theoretical model.
Characterization and calibration of the Thomson scattering diagnostic suite for the C-2W field-reversed configuration experiment
Related products: ANL series
The new C-2W Thomson scattering (TS) diagnostic consists of two individual subsystems for monitoring electron temperature (Te) and density (ne): one system in the central region is currently operational, and the second system is being commissioned to monitor the open field line region. Validating the performance of the TS’s custom designed system components and unique calibration of the detection system and diagnostic as a whole is crucial to obtaining high precision Te and ne profiles of C-2W’s plasma. The major components include a diode-pumped Nd:YAG laser which produces 35 pulses at up to 20 kHz, uniquely designed collection lenses with a fast numerical aperture, and uniquely designed polychromators with filters sets to optimize a Te ranging from 10 eV to 2 keV. This paper describes the design principles and techniques used to characterize the main components of the TS diagnostic on C-2W, as well as the results of Rayleigh scattering calibrations performed for the whole system response.
Thomson scattering systems on C-2W field-reversed configuration plasma experiment
Related products: ANL series
TAE Technologies’ newly constructed C-2W experiment aims to improve the ion and electron temperature in a sustained field-reversed configuration plasma. A suite of Thomson scattering systems has been designed and constructed for electron temperature and density profile measurements. The systems are designed for electron densities of 1×1012 cm-3 to 2×1014 cm-3 and temperature ranges from 10 eV to 2 keV. The central system will provide profile measurements of Te and ne at 16 radial locations from r = -9 cm to r = 64 cm with a temporal resolution of 20 kHz for 4 pulses or 1 kHz for 30 pulses. The jet system will provide profile measurements of Te and ne at 5 radial locations in the open field region from r = -5 cm to r = 15 cm with a temporal resolution of 100 Hz. The central system and its components have been characterized, calibrated, installed and commissioned. A maximum-likelihood algorithm has been applied for data processing and analysis.
Emission properties of ns and ps laser-induced soft x-ray sources using pulsed gas jets
The influcence of the pulse duration on the emission characteristics of nearly debris-free laser-induced plasmas in the soft x-ray region (λ ≈1-5 nm) was investigated, using six different target gases from a pulsed jet. Compared to ns pulses of the same energy, a ps laser generates a smaller, more strongly ionized plasma, being about 10 times brighter than the ns laser plasma. Moreover, the spectra are considerably shifted towards shorter wavelengths. Electron temperatures and densities of the plasma are obtained by comparing the spectra with model calculations using a magneto-hydrodynamic code.