Sampling modulation technique in radio-frequency helium glow discharge emission source by use of pulsed laser ablation

An emission excitation source comprising a high-frequency diode-pumped Q-switched Nd:YAG laser and a radio-frequency powered glow discharge lamp is proposed. In this system sample atoms ablated by the laser irradiation are introduced into the lamp chamber and subsequently excited by the helium glow discharge plasma. The pulsed operation of the laser can produce a cyclic variation in the emission intensities of the sample atoms whereas the plasma gas species emit the radiation continuously. The salient feature of the proposed technique is the selective detection of the laser modulation signal from the rest of the continuous background emissions, which can be achieved with the phase sensitive detection of the lock-in amplifier. The arrangement may be used to estimate the emission intensity of the laser ablated atom, free from the interference of other species present in the plasma. The experiments were conducted with a 13.56 MHz radio-frequency (rf) generator operated at 80 W power to produce plasma and the laser at a wavelength of 1064 nm (pulse duration:34 ns, repetition rate:7 kHz and average pulse energy of about 0.36 mJ) was employed for sample ablation. The measurements resulted in almost complete removal of nitrogen molecular bands (N₂⁺ 391.44 nm). Considerable reduction (about 75%) in the emission intensity of a carbon atomic line (C I 193.03 nm) was also observed.     

Effect of discharge parameters on emission yields in a radio-frequency glow-discharge atomic-emission source

A study is performed on a radio-frequency glow-discharge atomic-emission (rf-GD-AES) source to determine the factors effecting the emission yields for both metallic and nonconductive sample types. Specifically, these studies focus on determining how the operating parameters (power and pressure) influence emission yields. The results follow predicted patterns as determined by Langmuir probe diagnostic studies of a similar source. In particular, discharge gas pressure is the key operating parameter as slight changes in pressure may significantly affect the emission yield of the analyte species. RF power is less important and is shown to produce only relatively small changes in the emission yield over the ranges typically used in rf-GD analyses. These studies indicate that the quantitative analysis of layered materials, depth-profiling, may be adversely affected if the data collection scheme, i.e. the quantitative algorithm, requires changing the pressure during an analysis to keep the operating current and voltage constant. A direct relationship is shown to exist between the Ar (discharge gas) emission intensity and that of sputtered species for nonconductors. This observance is used to compensate for differences in emission intensities observed in the analysis of various thickness nonconductive samples. The sputtered element emission signals are corrected based on the emission intensity of an Ar (1) transition, implying that quantitative analysis of nonconductive samples is not severely limited by the availability of matrix matched standards. © 1997 Elsevier Science B.V.     

A modified glow-discharge source for emission spectroscopy

A glow-discharge emission source of the type described by Grimm for spectrochemical analysis has been modified to incorporate a secondary discharge of     

Monitoring nano-flow rate of water by atomic emission detection using helium radio-frequency plasma

Recently, high-performance nano-scale flow pumping systems have been developed for micro and miniaturized analysis systems. A novel device capable of measuring and monitoring nanoliter scale flow rates has been required for the further development of the pumping system. In this study, an atomic emission detector using helium radio-frequency plasma (RFP-AED) was used for the measurement of the nanoliter scale flow rate of water by quantitatively detecting the emission from hydrogen in the water molecules. Monitoring nano-flow rates of water in the range up to 1.0 microl min(-1), and the change in the flow rate by the indication of the ratio of the emissions of H (656.3 nm) and He (667.8 nm) were successful. At present, the lowest flow rate that could be determined reproducibly was 4 nl min(-1) calculated as five times the standard deviation of the background noise. Additionally, similar evaluations for the deviation of each flow rate by using the RFP-AED and a flow-injection system were produced.     

Optical emission spectroscopy of nitrogen species and plasma plume induced by laser ablation combined with pulse modulated radio-frequency discharge

Combined laser ablation and pulse modulated radio-frequency (RF) discharge for deposition of CNalpha films was studied by the use of optical emission spectroscopy. Chemically active nitrogen was produced by RF discharge, concentrated between two small electrodes. Influence of RF power, nitrogen pressure, modulation frequency and pulse rate on nitrogen species production was researched. For the same system plasma expansion rate, kinetic energy and concentration of carbon ions emitted by laser from graphite target were determined by Langmuir probes measurement.     

Spatial and intensity modulation of nanowire emission induced by mobile charges

Single-molecule optical experiments carried out in conjunction with externally applied electric fields show deliberate spatial and intensity control over CdSe nanowire (NW) emission. In particular, by applying external fields to electrically isolated (single) NWs, their emission can be localized in areas of the wire closest to the positive electrode. In a few cases, the resulting emission intensity increases over the corresponding zero-field value by nearly an order of magnitude. More often than not, factors of 2-3 are seen. Reversing the field polarity causes the emission to localize in opposite regions of the wire. Emission from individual NWs can therefore be modulated. Complementary ac electric field measurements show that the effect persists up to 500 kHz. To explain the phenomenon, the effective passivation of surface trap states by mobile carriers is speculated. This, in turn, causes local changes in the NW emission quantum yield (QY). To verify the presence of such mobile charges, both ensemble and single NW bundle electrophoresis experiments are conducted. By investigating subsequent NW rotational and translational dynamics, an estimate for the number of mobile carriers is determined. A lower limit (best case) linear charge density of approximately 0.45-1.2 mobile electrons per micrometer of the wire is obtained. Apart from self-consistently explaining the field-induced NW emission modulation, the resulting data and subsequent analysis also suggests that the same mobile carriers may be the root cause of NW emission intermittency. Furthermore, given the ubiquity of stray charges, the resulting hypothesis may have additional applicability toward explaining blinking in other systems, a problem of current interest especially within the context of colloidal QDs.     

Expansion phenomena of aerosols generated by laser ablation under helium and argon atmosphere

Specific expansion phenomena of aerosols generated by near infrared femtosecond laser ablation (NIR-fs-LA) of brass under helium and argon atmosphere were studied. For this purpose, particles were visualized by light scattering using a pulsed laser source. Aerosols were found to be captured in symmetric vortices when striking a solid boundary during their kinetic stage of expansion. Furthermore, high-repetitive LA resulted in the formation of a complex, macroscopic flow pattern driven by a pressure gradient locally built up. Our data indicate that aerosols released under those conditions experience only minor losses of around 1% if they get in contact with the inner walls of ablation cells operated at atmospheric pressures.     

Control of d.c. bias current in radio-frequency glow discharge source and its emission characteristics

A novel technique to control r.f.-powered Grimm-style glow discharge plasmas is described. A new channel of d.c. current driven by the self-bias potential is opened by using a low-pass filter circuit and a load resistor; as the result, a large number of electrons can flow along the d.c. circuit channel including the plasma body from the grounded electrode to the sample electrode. This phenomenon is effective for improvement of the detection sensitivity in the optical emission spectrometry. Atomic emission lines having lower excitation energies are predominantly enhanced by a factor of 10–20. The conduction of the d.c. bias current could promote these excitations.     

Bulk analysis by IR laser ablation inductively coupled plasma atomic emission spectrometry

Two laser ablation systems dedicated to bulk analysis were evaluated for steel and PVC samples, using inductively coupled plasma atomic emission spectrometry detection. These systems were characterized by the use of a Nd:YAG laser operating at 1064 nm, the absence of observation device and a large laser spot size. The 1064 nm wavelength was selected to avoid the use of frequency-multiplying optics, and to be less critical to the sampling position. Calibration graphs and limits of detection are given for both types of materials. LODs were in the range 3-120 microg/g for steel, and in the range 0.07-15 microg/g for PVC. In the case of steel samples, similar calibration graph slopes were obtained between polished and unpolished samples.     

Bulk analysis by IR laser ablation inductively coupled plasma atomic emission spectrometry

Two laser ablation systems dedicated to bulk analysis were evaluated for steel and PVC samples, using inductively coupled plasma atomic emission spectrometry detection. These systems were characterized by the use of a Nd:YAG laser operating at 1064 nm, the absence of observation device and a large laser spot size. The 1064 nm wavelength was selected to avoid the use of frequency-multiplying optics, and to be less critical to the sampling position. Calibration graphs and limits of detection are given for both types of materials. LODs were in the range 3–120 μg/g for steel, and in the range 0.07–15 μg/g for PVC. In the case of steel samples, similar calibration graph slopes were obtained between polished and unpolished samples.     

Stability of micron-sized spheres formed by pulsed laser ablation of metals in superfluid helium and water

It has been shown that micrometer-sized balls, resulting from the condensation of products of laser ablation of fusible metals in both superfluid helium and water, are in the state of strong internal tension counterbalanced by external compression. By radiation-induced or chemical damage to the integrity of their surface, they break up, ejecting a plurality of nanoparticles. The empty shells of the microspheres, which nonetheless remain intact, are identical to the “hollow spheres” of unclear origin that have been observed previously under laser ablation in usual liquids. The metastability of the microparticles produced by ablation in a liquid should be taken into consideration in their use in engineering and medicine.     

Time-resolved Fourier transform infrared emission spectroscopy of laser ablation products

Time-resolved Fourier transform infrared spectroscopy was applied for observations of emission spectra from ablation products induced by a Nd:YLF laser with a 2.5 kHz repletion rate. The infrared emission spectra from Fe, Cu, Zn, and Al atoms were observed in the 2.5–5 μm region. The observed emission spectrum from iron ablation in the 2500 cm⁻¹ region agrees very well with solar absorption spectrum, where new lines have been detected in the present experiment in addition to the lines observed from a hollow cathode discharge. When O₂ was added to the carbon ablation, emissions from vibrationally excited CO were observed with non-equilibrium vibrational distribution.     

Emission characteristics of Cu in Dc voltage modulation applied glow discharge optical emission spectrometry.

In order to obtain the depth profile of a thin film, we investigated the emission characteristics of a voltage modulation glow discharge to optimize the modulation parameters (modulation voltage, offset voltage, and modulation frequency). In this study, a phase-sensitive detection method with a lock-in amplifier to the modulation technique led to a higher sensitivity and a larger signal-to-noise ratio in the emission analysis compared to the normal dc amplification method. Upon increasing the maximum voltage, the emission intensity of the Cu atomic line (CuI 239.34 nm) increased linearly at a modulation voltage of 400 V and an offset voltage of 300 V. On the other hand, the emission intensity was gradually reduced when a modulation frequency increased. It is advantageous for surface analysis that the voltage modulation technique gives a lower sputtering rate rather than the conventional dc discharge.     

Optical emission spectroscopy and modeling of plasma produced by laser ablation of titanium oxides

In the present study, the time evolution of electron number density, of electron, atom and ion temperatures, of plasma produced by KrF excimer laser ablation of titanium dioxide and monoxide targets, are investigated by temporally and spatially resolved optical emission spectroscopy over a wide range of laser fluence from 1.7 to 6 J cm⁻², oxygen pressures of 10⁻²–10⁻¹ torr and in a vacuum. A state-to-state collisional radiative model is proposed for the first time to interpret the experimental results at a distance of 0.6 mm from the target surface, in vacuum and for a time delay from 100 to 300 ns from the beginning of the laser pulse. In particular, we concentrate our attention on problems concerning the existence of the local thermodynamic conditions in the laser-induced plasma and deviation from them, as observed in our experiment. The numerical model proposed for calculating the electron number density and the population densities of atoms and ions in excited states give good quantitative agreement with the experimental results of the optical emission spectroscopy measurements.     

Calibration graphs for steels by IR laser ablation inductively coupled plasma atomic emission spectrometry

Infrared laser ablation (IRLA) was studied as a sample-introduction technique for the analysis of steels by inductively coupled plasma atomic emission spectrometry (ICP–AES). A comparison of two IRLA–ICP–AES systems based on Q-switched nanosecond Nd?:?YAG lasers was performed. The beam of the Lina-Spark atomizer (LSA Sarl, Cully, Switzerland) based on the Surelite I-20 laser (Continuum, USA) was moved along a circle. A Perkin–Elmer Optima 3000 DV ICP system was used both with lateral and axial viewing modes. A laboratory-made ablation system based on the Brilliant laser (Quantel) was coupled to a Jobin-Yvon 170 Ultrace ICP (lateral viewing, polychromator part employed). A sample was rotated along a circle during ablation. Linearity of calibration plots was verified at least up to 19% Cr and 12% Ni without internal standardization for both LA–ICP–AES systems. Other elements examined were Mo up to 3%, Mn up 1.5%, Si up to 1.7%, and Cu up to 0.15%. The reproducibility was in the range 5–1 %RSD for a mass percentage 0.5–20% of steel constituents. The relative uncertainty of the centroids of the calibration lines was in the range from ± 4% to ± 12% for Cr, Ni, Mn, Mo, and Si, and from ± 8% to ± 19% for Cu. The lowest determinable quantities were calculated for calibration dependencies. Performances of both the IR-LA–ICP–AES were comparable.     

Calibration graphs for steels by IR laser ablation inductively coupled plasma atomic emission spectrometry

Infrared laser ablation (IRLA) was studied as a sample-introduction technique for the analysis of steels by inductively coupled plasma atomic emission spectrometry (ICP-AES). A comparison of two IRLA-ICP-AES systems based on Q-switched nanosecond Nd: YAG lasers was performed. The beam of the LINA-Spark atomizer (LSA Sarl, Cully, Switzerland) based on the Surelite 1-20 laser (Continuum, USA) was moved along a circle. A Perkin-Elmer Optima 3000 DV ICP system was used both with lateral and axial viewing modes. A laboratory-made ablation system based on the Brilliant laser (Quantel) was coupled to a Jobin-Yvon 170 Ultrace ICP (lateral viewing, polychromator part employed). A sample was rotated along a circle during ablation. Linearity of calibration plots was verified at least up to 19% Cr and 12% Ni without internal standardization for both LA-ICP-AES systems. Other elements examined were Mo up to 3%, Mn up 1.5%, Si up to 1.7%, and Cu up to 0.15%. The reproducibility was in the range 5-1 %RSD for a mass percentage 0.5-20% of steel constituents. The relative uncertainty of the centroids of the calibration lines was in the range from +/- 4% to +/- 12% for Cr, Ni, Mn, Mo, and Si, and from +/- 8% to +/- 19% for Cu. The lowest determinable quantities were calculated for calibration dependencies. Performances of both the IR-LA-ICP-AES were comparable.     

Characterization of acoustic signals produced by ultraviolet laser ablation inductively coupled plasma atomic emission spectrometry

A simple device was designed to measure the acoustic signal accompanying laser ablation. The potential use of this signal for laser ablation-inductively coupled plasma atomic emission was examined. A frequency quadrupled pulsed Nd:YAG laser radiation was used for the ablation of glass, steel and ceramic samples. The relation between the acoustic signal, the laser energy, the analyte signal and the amount of ablated material was studied and evidence of the use of the acoustic signal for the exact focusing of the laser beam onto the sample surface was given. A more intense acoustic signal was observed for the exact focusing with a formation of larger ablation craters in glass and ceramics. Received: 25 June 1998 / Revised: 25 September 1998 / Accepted: 30 September 1998     

Characterization of acoustic signals produced by ultraviolet laser ablation inductively coupled plasma atomic emission spectrometry

A simple device was designed to measure the acoustic signal accompanying laser ablation. The potential use of this signal for laser ablation-inductively coupled plasma atomic emission was examined. A frequency quadrupled pulsed Nd:YAG laser radiation was used for the ablation of glass, steel and ceramic samples. The relation between the acoustic signal, the laser energy, the analyte signal and the amount of ablated material was studied and evidence of the use of the acoustic signal for the exact focusing of the laser beam onto the sample surface was given. A more intense acoustic signal was observed for the exact focusing with a formation of larger ablation craters in glass and ceramics.     

Characterization of laser ablation and ionization in helium and argon: A comparative study by time-of-flight mass spectrometry

Laser ablation and ionization in ambient helium and argon gases were studied by multiple-stage time-of-flight mass spectrometry. Measurements made at different gas pressures indicated that there exists an optimal pressure for adequately cooling energetic ions and reducing multiply charged ions that are higher for He than for Ar. The temporal distributions of ions were compared at various laser fluences and gas pressures, and the broad distributions for He could be ascribed to elastic scattering and thermodynamic processes. The diffusion of ions in He resulted in a longer delay before the instrument registered its maximal signal. Ions with different masses were observed to have the same kinetic energies in He, which was confirmed using the SIMION software, while ion movement was hydrodynamically controlled in Ar. The velocities of singly and doubly charged ions were also studied, and doubly charged ions showed much higher kinetic energy because of their frontal location in the plasma expansion.     

Analysis of glass by UV laser ablation inductively coupled plasma atomic emission spectrometry. Part 2. Analytical figures of merit

Two lasers working in the UV part of the spectrum have been used for the direct analysis of glass samples by laser ablation ICP-AES. An XeCl excimer laser (308 nm) and a Nd:YAG laser operating at the third harmonic (355 nm) and the fourth harmonic (266 nm) have been selected. The energy was 70 mJ and 5 mJ for the excimer laser and the Nd:YAG laser, respectively, with a 10 Hz repetition rate. Figures of merit such as repeatability, reproducibility, accuracy and limits of detection have been studied. Si was used as an internal standard to improve the repeatability, the reproducibility and the accuracy. Use of internal standardardization led to an RSD of less than 1% for most elements and to a linear calibration graph irrespective of the colour of the glass samples. Limits of detection in the solid were of the same magnitude as those obtained using sample dissolution and pneumatic nebulization. Results confirmed that the XeCl laser provided the best results of detection whereas the Nd:YAG laser, particularly at 266 nm, was less sensitive to glass colour.