Xenon ion laser-induced fluorescence using a visible tunable diode laser near 680 nm

Laser-induced fluorescence (LIF) measurements have been performed for the first time in a low temperature (Te approximately 0.6 eV) Xe plasma using a tunable diode laser in the visible range of wavelengths. The transition in Xe II involved the (3P1)5d[3]7/2 metastable state and the excitation wavelength was found to be 680.570+/-0.001 nm (air). LIF measurements of I 2 in a room temperature iodine gas cell were used to monitor the wavelength of the laser during the measurements.     

An efficient magneto-optical trap of metastable krypton atoms

We report a magneto-optical trap of metastable krypton atoms with a trap loading rate of 3×10(11) atoms/s and a trap capture efficiency of 3×10(-5). The system starts with an atomic beam of metastable krypton produced in a liquid-nitrogen cooled, radio-frequency driven discharge. The metastable beam flux emerging from the discharge is 1.5×10(14) atoms/s/sr. The flux in the forward direction is enhanced by a factor of 156 with transverse laser cooling. The atoms are then slowed inside a Zeeman slower before captured by a magneto-optic trap. The trap efficiency can be further improved, possibly to the 10(-2) level, by gas recirculation. Such an atom trap is useful in trace analysis applications where available sample size is limited.     

High-flux beam source of fast neutral helium

A high-flux beam source of fast neutral helium has been constructed by extending the designs of previous authors. The source is a dc or pulsed electric discharge in an expanding gas nozzle. The beam produced has a flux on the order of 10(15) atoms/s sr and a mean velocity on the order of 10(7) cm/s. The composition of the beam has been determined by the use of particle detectors and by the observation of the excitation of certain target gases. An upper bound of 3.7 x 10(-5) has been estimated for the He(2(3)S(1))/He((1)S(0))beam density ratio and a value of 0.2 found for the He(+)/He(1(1)S(0)) beam density ratio.     

Measurement of axial neutral density profiles in a microwave discharge ion thruster by laser absorption spectroscopy with optical fiber probes

In order to reveal the physical processes taking place within the "μ10" microwave discharge ion thruster, internal plasma diagnosis is indispensable. However, the ability of metallic probes to access microwave plasmas biased at a high voltage is limited from the standpoints of the disturbance created in the electric field and electrical isolation. In this study, the axial density profiles of excited neutral xenon were successfully measured under ion beam acceleration by using a novel laser absorption spectroscopy system. The target of the measurement was metastable Xe I 5p(5)((2)P(0) (3/2))6s[3/2](0) (2) which absorbed a wavelength of 823.16 nm. Signals from laser absorption spectroscopy that swept a single-mode optical fiber probe along the line of sight were differentiated and converted into axial number densities of the metastable neutral particles in the plasma source. These measurements revealed a 10(18) m(-3) order of metastable neutral particles situated in the waveguide, which caused two different modes during the operation of the μ10 thruster. This paper reports a novel spectroscopic measurement system with axial resolution for microwave plasma sources utilizing optical fiber probes.     

Three-dimensional analysis of microwave generated plasmas with extended planar laser-induced fluorescence

We present the development and application of a diagnostic system for the analysis of microwave generated low-pressure plasmas, which might also be used for the investigation of the edge regions in magnetically confined fusion plasmas. Our method uses planar laser-induced fluorescence, which is produced by excitation of neutral metastable atoms through a short, intense, pulsed laser. The beam expansion optics consist of an uncommon setup of four lenses. By controlled shifting of an element of the optics sideways, the location of the laser sheet in the plasma is scanned perpendicular to the excitation plane. Together with a spectrometer observing different observation volumes along the beam path, we are able to map absolute three-dimensional (3D) population density distributions of the metastable ((2)P(12) (o)) 3s[12](0) (o) state of Ne I in an electron cyclotron resonance heating (ECRH) plasma. This optical tomography system was used to study the influence of the microwave power and mode on the spatial structure of the plasma. The results show that the population density of the neutral neon in this metastable state is found to be in the range of 10(16) m(-3), and that its spatial distribution is associated with the 3D structure of the magnetic field. We also report that the spatial distribution strongly varies with the mode structure, which depends on the microwave power.     

Electron beam-induced thickening of the protective oxide layer around Fe nanoparticles

There are many circumstances in science where the process of measuring the properties of a system alters the system. An imaging process can exert an inadvertent effect on the object being observed. Consequently, what we observe does not necessarily represent what had been present before the observation. Normally, this effect can be ignored if the consequence of such a change is believed not to be significant. The expansion of nanostructured materials has made high-resolution transmission electron microscopy one of the indispensable tools for probing the characteristics of nanomaterials. Modification of nanoparticles by the electron beam during their imaging has been widely noticed and this is generally believed to be due to electron beam-induced heating effect, defect formation in the particles, charging of the particle, or excitation of surrounding gases. However, an explicit experimental identification of which process dominates is often very hard to establish. We report the thickening of native oxide layer on iron nanoparticle under electron beam irradiation. Based on atomic level imaging, electron diffraction, and computer simulation, we have direct evidence that the protecting oxide layer formed on Fe nanoparticle at room temperature in air or oxygen continues to grow during an electron beam bombardment in the vacuum system typical of most TEM systems. Typically, the oxide layer increases from approximately 3 to approximately 6 nm following approximately 1h electron beam exposure typically with an electron flux of 7 x 10(5)nm(-2)s(-1) and an vacuum of approximately 3 x 10(-5)Pa. Partial illumination of a nanoparticle and observation of the shell thickening conclusively demonstrates that many of the mechanisms postulated to explain such processes are not occurring to a significant extent. The observed growth is not related to the electron beam-induced heating of the nanoparticle, or residual oxygen ionization, or establishment of an electrical field, rather it is related to electron beam-facilitated mass transport across the oxide layer (a defect-related process). The growth follows a parabolic growth law.     

Frequency stabilization of an external-cavity diode laser to metastable argon atoms in a discharge

A laser stabilization scheme using magnetic dichroism in a RF plasma discharge is presented. This method has been used to provide a frequency stable external-cavity diode laser that is locked to the 4s[3/2](2) → 4p[5/2](3) argon laser cooling transition at 811.53 nm. Using saturated absorption spectroscopy, we lock the laser to a Doppler free peak which gave a locking range of 20 MHz when the slope of the error signal was maximized. The stability of the laser was characterized by determining the square root Allan variance of laser frequency fluctuations when the laser was locked. A stability of 129 kHz was measured at 1 s averaging time for data acquired over 6000 s.     

Fast ion beam-plasma interaction system

A device has been constructed for the study of the interaction between a fast ion beam and a target plasma of separately controllable parameters. The beam of either hydrogen or helium ions has an energy of 1-4 keV and a total current of 0.5-2 A. The beam energy and beam current can be varied separately. The ion source plasma is created by a pulsed (0.2-10-ms pulse length) discharge in neutral gas at up to 3 x 10(-3) Torr. The neutrals are pulsed into the source chamber, allowing the neutral pressure in the target region to remain less than 5 x 10(-5) Torr at a 2-Hz repetition rate. The creation of the source plasma can be described by a simple set of equations which predict optimum source design parameters. The target plasma is also produced by a pulsed discharge. Between the target and source chambers the beam is neutralized by electrons drawn from a set of hot filaments. Currently under study is an unstable wave in a field-free plasma excited when the beam velocity is nearly equal to the target electron thermal velocity (v(beam) approximately 3.5 x 10(7) cm/s, Te = 0.5 eV).     

A modification of the laser detonation-type hyperthermal oxygen atom beam source for a long-term operation

It has been an impedimental problem, for the laser detonation-type atom beam generator, that a poppet in the pulsed supersonic valve is rapidly eroded by the irradiation of powerful laser light and high temperature plasma. In order to operate the atom beam source for a long duration, a modification was made to hide the poppet from direct irradiation of laser and plasma. The alteration of device configuration resulted in great improvement in endurance of poppet more than 300,000 repetitions. Morphology of a polyimide film exposed to approximately 200,000 pulses of hyperthermal oxygen atom beam showed a shaglike carpet structure, which is a characteristic to that exposed to energetic oxygen atoms. A flux of the oxygen atom beam was estimated to be 5x10(14) atoms/cm2/pulse at a location of 30 cm away from the nozzle throat.     

Variable rowland radius laboratory vacuum surface-sensitive x-ray absorption fine structure spectrometer

A new laboratory x-ray spectrometer for surface-sensitive extended x-ray absorption fine structure [(S)EXAFS] and surface-sensitive x-ray absorption near-edge structure [(S)XANES] measurements is described. The spectrometer employs a 12 kV mA rotating anode generator. It has a monochromator equipped with a set of exchangeable curved crystals of Johann or Johansson type with different cell parameters, orientations, and Rowland radii. The computer controlled movement system based on nine stepping motors allows all the main elements of the spectrometer to be positioned freely relative to the x-ray source and gives an opportunity to use sophisticated scanning modes (for example, a mode with a focus spot position on a sample surface instead of an exit slit). The whole x-ray beam line is completely enclosed in a vacuum chamber that is directly connected to the x-ray generator, thereby preventing the absorption of x rays in the air. This layout allows a wide x-ray photon energy range from a few keV up to dozens of keV. A registration of x rays transmitted through the sample with proportional counter- and photoelectrons emitted from the sample with channeltron is used to carry out bulk- and surface-sensitive measurements, respectively. Using a 25 x 200 kV mA power regime of a rotating anode x-ray generator, a photon flux of 2.5 x 10(5) counts/s was registered at the Cu K edge, where the energy resolution was about 5 eV. High near surface sensitivity is demonstrated by the EXAFS spectra of Cu K and Hf LIII edges measured from 3 nm Cu and Hf oxide films.     

Excitation of high-intensity laser radiation of semiconductor targets by a subnanosecond electron beam

The results of the excitation of СdS semiconductor targets by a subnanosecond electron beam (EB) with an electron energy of 60–230 keV are presented. The maximum intensity of laser radiation from targets for a 1-mm EB diameter exceeded 10⁷ W/cm² at an efficiency of ~10%. Lasing was initiated at the leading edge of the EB current; laser radiation then reproduced the shape of the excitation pulse. At low excitation levels, a single-mode lasing regime with the wavelength λ = 522 nm was observed. The maximum power of laser radiation (10 MW) was achieved on a multielement CdS semiconductor target. The duration of laser pulses changed in the range of 100–500 ps.     

全固态复合内腔和频570nm连续波黄光激光器

研制了全固态连续波570nm黄光激光器,黄激光分别由两片Nd:YAG的1444nm和946nm谱线非线性和频产生,两条谱线分别对应各自的晶体能级跃迁4F3/2-4I15/2和4F3/2-4I9/2。实验采用复合腔结构,利用KTP晶体II类临界相位进行内腔和频。测量了570nm黄激光输出功率随泵浦功率的变化,结果表明,当注入到两片Nd:YAG晶体的泵浦功率分别为24W和15W时,获得了560mW的连续波570nm黄激光输出,其4h功率稳定度优于±2.8%。在输出功率为560mW时,采用光束质量分析仪测量了激光输出光斑质量,结果显示,在570nm最大和频激光输出时的光束质量因子M2为2.3。所提出的复合内腔和频技术可为新波长激光器的发展提供参考。     

Pulsed blowdown-electron gun facility for low-temperature and high-pressure supersonic flow electronic transition lasers

The design of a small compact pulsed blowdown apparatus coupled to a cold cathode electron gun is described. This system allows the excitation of a gas mixture in supersonic flow at temperatures of 80 and 120 K and density of 1 amagat with a maximum electron beam current density of 20 A cm(-2) at 300 kV and pulse length of 0.5 mus. With this facility it has been possible, using aerodynamic cooling, to achieve high-power, long-pulse ultraviolet laser operation in a high-pressure active medium on XeF and Ne ii ion transitions. A strong increase in fluorescence intensity was also obtained for numerous atomic lines and molecular bands of interest for the developement of high-power, high-efficiency, visible and ultraviolet lasers.     

A source of gas, vapor, metal, and carbon ions based on a low-pressure hollow-cathode discharge

A compact source of gas, vapor, metal, and carbon ions based on a cold-hollow-cathode reflective discharge has been developed, in which a 6-mm-diameter flat target (Cu, Mo, W, C) is installed on the bottom of the cold cathode insulated from it. The density of the ion flow from cathode plasma reaches 100 mA/cm² at an accelerating voltage of up to 10 kV and a discharge current of 0.2-0.5 A. Vapors produced during ion sputtering of the target are ionized in the cathode and anode cavities. A beam containing ions of the plasma-producing gas and vapor is extracted throug h the channel in the reflector cathode. A fraction of the vapor of the sputtered target, the flow of which is sufficient for growing layers at a rate of ∼0.03 nm/s at a distance of 10 cm from the emission channel under the action of an ion beam, is extracted together with ions. The fraction of metal ions in the extracted beam is 0.05-0.10. The total current of the ion beam is 20-30 mA.     

A novel vacuum ultra violet lamp for metastable rare gas experiments

We report on a new design of a vacuum ultra violet (VUV) lamp for direct optical excitation of high laying atomic states, e.g., for excitation of metastable rare gas atoms. The lamp can be directly mounted to ultra-high vacuum vessels (p ≤ 10(-10)mbar). It is driven by a 2.45 GHz microwave source. For optimum operation, it requires powers of ~20 W. The VUV light is transmitted through a magnesium fluoride window, which is known to have a decreasing transmittance for VUV photons with time. In our special setup, after a run-time of the VUV lamp of 550 h the detected signal continuously decreased to 25% of its initial value. This corresponds to a lifetime increase of two orders of magnitude compared to previous setups or commercial lamps.     

Development of a time-resolved attenuated total reflectance spectrometer in far-ultraviolet region

A far-ultraviolet transient absorption spectrometer based on time-resolved attenuated total reflectance (ATR) has been developed and tested for aqueous solutions of phenol and tryptophan in the region 170-185 nm. In this region, a stable tunable laser was not available, and therefore, white light from a laser-driven Xe lamp source was used. The time resolution, which was determined by the time response of a continuous light detector, was 40 ns. A new ATR cell where a sample liquid is exchanged continuously by a flow system was designed to reduce efficiently the stray light from the excitation light. We have tested the performance of the instrument by using aqueous solutions of phenol and tryptophan, whose photochemistry is already well known. Phenol and tryptophan have very strong absorptions due to a π-π? transition near 180 nm. Even for dilute solutions (10(-3) mol dm(-3)), we could observe decreases in their concentrations due to photochemistry that occurred upon their irradiation with a fourth harmonic generation laser pulse produced by an Nd:YAG laser. The sensitivity of the spectrometer was about 10(-4) abs, which corresponded to a concentration variation of 10(-3) mol dm(-3) for phenol and tryptophan.     

Online monitoring of gold nanoparticles and induced aggregation by photothermal lens microscopy

In this paper, we have developed inverted photothermal lens microscopy (PTLM) with a flow through optical cell for online detection of gold nanoparticles (AuNPs) to monitor the AuNP production process and aggregation. The indirect detection of the plasmonic signals of AuNPs was performed using the thermal lens phenomena. The measurements were made using continuous-wave excitation (532?nm) and probe laser beams (652?nm). The changes of the transmitted probe beam’s intensity were detected with a photodiode. The results show the possibility of online determination of AuNPs in micro and nanovolumes in the range of 1.7–13.7 particles for the focal volume of the excitation beam. The obtained limit of detection and relative standard deviation for 10 repeated measurements of AuNPs were 0.65?nM and 1.43%, respectively. Therefore, this system was successfully applied for screening of aggregation in process applications.     

Transmissive x-ray beam position monitors with submicron position- and submillisecond time resolution

We present the development of fast transmissive center-of-mass x-ray beam position monitors with a large active area, based on a thinned position sensitive detector in both a duo- and a tetra-lateral variant. The detectors were tested at BESSY beamlines BL14.1, KMC-1, and KMC-2 and yielded signal currents of up to 3 microA/100 mA ring current at 10 keV photon energy using the monochromatic focused beam of BL14.1. The active area sizes were 1 x 1 and 3 x 3 mm(2) for the duo-lateral and 5 x 5 mm(2) for the tetra-lateral devices, with the duo-lateral detectors currently being available in sizes from 1 x 1 to 10 x 10 mm(2) and thicknesses between 5 and 10 microm. The presented detectors' thicknesses were measured to be 5 and 8 microm with a corresponding transmission of up to 93% at 10 keV and 15% at 2.5 keV. Up to a detection bandwidth of 10 kHz, the monitors provide submicron position resolution. For lower detection bandwidths, the signal-to-noise reaches values of up to 6 x 10(4) at 10 Hz, corresponding to a position resolution of better than 50 nm for both detector sizes. As it stands, this monitor design approach promises to be a generic solution for automation of state-of-the-art crystal monochromator beamlines.     

Qualitative and quantitative flow visualization technique using ozone

We have developed a new flow-visualization technique based on the absorption of ultraviolet light by ozone. Ozone is an excellent tracer, because as a gas it has the same effective physical properties as air. Ozone strongly absorbs the principal line (253.7 nm) of a mercury lamp, varepsilon=310 (atm cm)(-1), where I/Io=exp(-varepsiloncl) such that when an ozone-traced flow passes between a mercury lamp and a fluorescent screen, a sharp, shadow-like image of the ozone tracer is cast on the screen. Quantitative photometry can be carried out by replacing the screen with ultraviolet detectors that yield the path-integrated column density of ozone in the flow. High-speed quantitative point monitoring (10 Hz at 10 ppb O3) is possible with capillary probes and chemiluminescent analysis.     

An optically pumped XeF(C-A) laser with repetitive rate of 10 Hz

A novel XeF(C-A) laser which can be operated in repetition mode has been developed based on surface discharge optical pumping technique. Its maximum repetitive rate is up to 10 Hz. The influence of repetitive rate and gas flow rate on the stability of output energy is studied and the main factor which influences the stability of output energy is analyzed. The experimental results show that increasing the gas flow rate into laser chamber can improve the stability of the output energy. The ideal output energy results of 20 laser pulses under different repetitive rates and their optimal experimental conditions are presented. Output energies of more than 4 J and better stability can be obtained when the laser device operates at 1, 2, and 5 Hz, respectively. When the gas feed rate is larger than 53 l/s, the stability of output energy is improved obviously at the repetitive rate of 10 Hz, and the average energy of 20 laser pulses is up to 3.2 J.