Laser Photodetachment Research Articles

Selective detection of 13C by laser photodetachment mass spectrometry

In this paper, we demonstrate how laser photodetachment mass spectrometry (LPMS) can be used to selectively detect 13C − ions in the presence of 12C − ions in a low energy ion beam. An isotopically enriched beam of carbon ions consisting of equal amounts of 13C − and 12C − ions was extracted from an ion source. The ions interacted with a laser beam in a collinear geometry over a distance of 70 cm. Residual atoms produced in the photodetachment process were detected in a neutral particle detector placed downstream of the collinear interaction region. By making use of the Doppler effect we were able to selectively photodetach 13C − ions. The number of detected 13C atoms was 13 times larger than the number of detected 12C atoms. The population of the excited, weakly bound 2D excited state of the C − ion was depleted by the use of a second laser. This significantly reduced the background accompanying the signal arising from the photodetachment of the 4S ground state C − ion. Different applications of the LPMS method will be discussed in the paper.

Isobar suppression in AMS using laser photodetachment

We are investigating the possibility of using laser photodetachment of negative atomic and molecular ions as an additional isobaric selection filter in accelerator mass spectrometry. The aim of this study is to find a possibility to further improve the detection limit for long-lived heavy radionuclides at AMS facilities. We will focus on the astrophysical relevant radionuclide 182Hf, which is one of the isotopes measured with the 3MV tandem AMS facility VERA (Vienna Environmental Research Accelerator) at the University of Vienna. Laser-induced isobar suppression is also of importance for radioactive-beam facilities.The present detection limit for measuring the isotope ratio 182Hf/Hf at VERA is 1×10-11. The limiting factor is the strong background of the stable isobar 182W. Currently this background is suppressed using suitable molecular ions in the injection stage. Selective laser photodetachment of the negative ions at the injector can lead to an additional suppression of the interfering isobar. Test experiments have been carried out at the negative ion laser spectroscopy setup at Göteborg University. In a small ion beam apparatus pulsed tunable laser radiation is used to measure the photodetachment cross-section of different atomic and molecular negative ions. We will present studies of the photodetachment process for various tungsten and hafnium molecules with the aim to find a selective isobaric suppression scheme using laser photodetachment spectroscopy in combination with AMS.

Laser photodetachment diagnostics of electronegative expanding plasmas

The study presents results from measurements of the negative hydrogen ion density by laser photo-detachment in the expanding plasma region of an inductively-driven tandem plasma source. The results obtained show that the negative ion density and the plasma electronegativity increase with increasing the high-frequency power absorbed in the discharge. An optimum gas pressure range [(1-3) Pa] for negative ion production in the expansion chamber is observed.

Investigation of H− production by surface interaction of the plasma generated in “Camembert III” reactor via distributed electron cyclotron resonance at 2.45GHz (abstract)

When considering the state-of-the-art on H− ion sources, ions can be produced either by plasma-surface interaction and/or inside the plasma volume. For the production of negative ions by surface ionization, a low work function material is required. For this purpose, cesium has been used in many cases at LBNL, JAEA, KEK, and in other facilities [M. Bacal, Nucl. FusionNUFUAU0029-5515 46, 250 (2006)]. Despite an enhancement in the negative ion production (by a factor of 2.5 in JAEA source), the use of cesium could lead to many drawbacks in the plasma functioning of ITER, for example. An alternative material to cesium could lead to an important improvement for negative ion source.For this purpose, both theoretical and experimental studies must be undertaken. Surface mechanisms have to be taken into account both for creation and loss mechanisms: (i)By recycling the atomic hydrogen into highly vibrationally excited molecular hydrogen via recombinative desorption on specific surfaces (fresh tantalum on surface increases the negative ion density [M. Bacal, A. A. Ivanov, Jr. et al., Rev. Sci. Instrum.RSINAK0034-674810.1063/1.1699456 75, 1699 (2004)] by more than 60%). It has been shown for a rigid substrate model that both the recombination cross section and the degree of vibrational excitation are highly sensitive to the nature of the surface [B. Jackson and D. Lemoine, J. Chem. Phys.JCPSA60021-960610.1063/1.1328041 114, 474 (2001)].(ii)By surface passivation, which could lead to a substantial decrease in H2 (X,v″) wall losses.In order to understand the fundamental mechanisms of surface production and losses, “Camembert III” experimental setup, recently settled in the LPSC laboratory (Grenoble, France) is used. In this experimental structure, hydrogen multidipolar plasma sustained by microwaves (2.45GHz) presents the potential advantage to operate either in a metallic or a conductive chamber. The inner walls could be then frequently coated, by sputtering or chemical vapor deposition techniques, with no opening of the chamber with various materials. First experiments of H− surface production will be performed on target material. Hence, perfectly knowing the target material (in terms of composition and structural and physical properties), H− ion density production near the target surface will be monitored by laser photodetachment [M. Bacal, Rev. Sci. Instrum.RSINAK0034-674810.1063/1.1310362 71, 3981 (2000)]. Even if this diagnostic gives no direct information on high rovibrationally excited level of the H2 molecule, its implementation and use are far less complicated than vuv LIF.

Infrared photodetachment ofCe−: Threshold spectroscopy and resonance structure

The negative ion of cerium is investigated using tunable laser photodetachment threshold spectroscopy. The relative cross section for photodetachment from ${\mathrm{Ce}}^{\ensuremath{-}}$ is measured over the photon energy range $0.61--0.75\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ using a crossed laser-beam--ion-beam technique. The spectrum of neutral atom production reveals a photodetachment threshold at $0.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which is interpreted as the threshold for the ${\mathrm{Ce}}^{\ensuremath{-}}$ $(4f5{d}^{2}6{s}^{2}\phantom{\rule{0.2em}{0ex}}^{4}H_{7∕2})$ to Ce $(4f5d6{s}^{2}\phantom{\rule{0.2em}{0ex}}^{1}G_{4})$ ground-state to ground-state transition yielding the electron affinity of Ce to be 0.65(3) eV. At least five narrow peaks are observed in the cross section over the range $0.62--0.70\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ due to negative ion resonances, and their energies and widths are measured. The results are compared to other recent experimental and theoretical studies of ${\mathrm{Ce}}^{\ensuremath{-}}$.

Electron Affinity of NO

The electron affinity of NO has been measured to be 0.026 eV by laser photodetachment experiments. This low electron affinity (just 2.5 kJ/mol or 210 cm-1) presents a computational challenge that requires careful attention to several aspects of the computational procedure required to predict the electron affinity of NO from first principles. We have used augmented correlation consistent basis sets with several coupled cluster methods to calculate the molecular energies, bond dissociation energies, bond lengths, vibrational frequencies, and potential energy curves for NO and NO-. The electron affinity of NO, EA0, using the CCSD(T) method and extrapolating to the complete basis set limit, is calculated to be 0.028 eV. The calculated bond dissociation energies, D0, for NO and NO- are 622 and 487 kJ/mol, respectively, compared with experimental values of 626.8 and 487.8 kJ/mol. From the calculated potential energy curves for NO and NO- the vibrational wavefunctions were determined. The calculated vibrational wavefunctions predict Franck-Condon factor ratios in good agreement with the values determined in the photodetachment experiment.

The electron affinity of phosphorus

We have measured the energies of all three fine structure components in the 3PJ ground state of the negative ion of phosphorus using laser photodetachment threshold spectroscopy. The experiment yielded an electron affinity of 746.68(6) meV. The ΔJ = 2–0, 2–1 and 1–0 fine structure splittings were determined to be 32.73(7) meV, 22.48(7) meV and 10.25(3) meV, respectively. In the experiment, a mass selected beam of P− ions was merged with the output from a pulsed infrared optical parametric oscillator. The residual atoms produced in the photodetachment process were detected and used as a monitor of the photon-energy dependence of the relative cross section. The Wigner law was fitted to each of the three observed onsets of production of neutrals in order to extract the threshold energies.

Effect of cross-field transport on H− density profile in magnetized plasmas: Comparison between measurement and simulation

The radial density profile of negative hydrogen ions in magnetized plasma is investigated in the divertor simulator MAP (Material and plasma)-II [S. Kado et al., J. Plasma Fusion Res. 71, 810 (2005)] by measurement and numerical simulation. The laser photodetachment method is used to evaluate the H− density by considering the influence of the magnetic field. The density of H− has a hollow profile and exhibits a peak in the peripheral region, though the electron density and temperature exhibit a peak at the center of the plasma column. The density profile of H− does not agree with the calculation result obtained from the rate equation, in which the local production and extinction rates are balanced, under the present experimental condition. To understand the behavior of negative ions, their trajectories are calculated by numerically solving the equation of motion by considering the effect of collisions. The negative ion density profile calculated from the particle simulation agrees well with the measured negative ion density profile. It is shown that the cross-field H− transport due to the radial electric field with the assistance of the elastic collisions plays an important role in enhancing the negative ion density in the peripheral region.

Measurements of partial cross sections and photoelectron angular distributions for the photodetachment ofFe−andCu−at visible photon wavelengths

Photodetachment cross sections and the angular distributions of photoelectrons produced by the single-photon detachment of the transition metal negative ions ${\mathrm{Fe}}^{\ensuremath{-}}$ and ${\mathrm{Cu}}^{\ensuremath{-}}$ have been measured at four discrete photon wavelengths ranging from 457.9 to $647.1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ $(2.71--1.92\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ using a crossed-beams laser photodetachment electron spectrometry (LPES) apparatus. Photodetachment cross sections were determined by comparing the photoelectron yields from the photodetachment of ${\mathrm{Fe}}^{\ensuremath{-}}$ to those of ${\mathrm{Cu}}^{\ensuremath{-}}$ and ${\mathrm{C}}^{\ensuremath{-}}$, which have known absolute photodetachment cross sections. Using the measured photodetachment cross sections, radiative electron attachment cross sections were calculated using the principle of detailed balance. Angular distributions were determined by measurements of laboratory frame, angle-, and energy-resolved photoelectrons as a function of the angle between the linear laser polarization vector and the momentum vector of the collected photoelectrons. Values of the asymmetry parameter have been determined by nonlinear least-squares fits to these angular distributions. The measured asymmetry parameters are compared to predictions of photodetachment models including Cooper and Zare's dipole approximation theory [J. Cooper and R. N. Zare, J. Chem. Phys. 48, 942 (1968)], and the angular momentum transfer theory developed by Fano and Dill [Phys. Rev. A 6, 185 (1972)].

One-dimensional analysis of the effect of the ambipolar potential on the H− ion density recovery after a laser photodetachment in the high density regime

In order to analyse the effect of the ambipolar potential on the H− ion density recovery in the measurement of the H− ion temperature by the two-laser photodetachment diagnostic technique, the particle-in-cell simulation with a one-dimensional slab model is performed. In particular, a high H− ion density regime with n−0/n+0 > 0.1 (n−0 : H− ion density, n+0 : H+ ion density before the laser shot) has been studied. The recovery time of the H− ion density becomes shorter as the density ratio n−0/n+0 increases. The ambipolar potential formed after the laser shot is almost proportional to the density ratio n−0/n+0. Due to the H− ion acceleration by this ambipolar potential, the H− ion temperature T−d deduced from the H− ion density recovery tends to be larger than the actual temperature T−0. The temperature ratio T−d/T−0 increases with the density ratio n−0/n+0 and the deduced temperature T−d becomes about 1.5 times larger than the actual temperature T−0 for n−0/n+0 = 0.9.

Investigation of H- production in plasma of dc glow discharge

The laser photodetachment technique was applied to investigate the negative ion dynamics in the positive column of a pure hydrogen dc glow discharge at pressures P = 0.1 - 3 Torr. Upon the discharge current modulation, the negative ion H- concentration decayed with the characteristic loss time of hydrogen H(1s2S) atom concentration. The evolution of H atom concentration was investigated by the time-resolved actinometry. The axial electric field was measured by double probe technique. The analysis of the experimental data, i.e. the dependencies of H- and H concentrations as well as reduced electric field on the discharge parameters (discharge current and pressure), allowed us to determine the rate constant of H- production in the discharge, where vibrational excitation and reactions with atoms H greatly influence upon H- production and loss processes. The main processes, which contribute to H- production, are discussed in detail. The vibrational excitation of H2 is shown to increase the rate of dissociative attachment essentially not only via the excitation of the well-known lowthreshold (eth≈3 eV) resonance of H2-(X2∑u+), but also via the excitation of the high-threshold resonances of H2-(2∑g+ (eth≈6 eV), 2Πg, 2Πu (eth≈8-13 eV) etc). Detailed analysis of dissociative attachment via high-threshold resonances allowed us to describe experimental data in the whole range of E/N being investigated, whereas taking any other mechanisms into account (attachment to Rydberg states of H2 etc) failed to do that.

A Concept of Negative Ion Flow Velocity Measurement Using a Laser Photodetachment Velocimetry (LPDV)

AbstractA concept of the thermal and flow velocities measurement for negative ions by means of a laser photodetachment technique, laser photodetachment velocimetry (LPDV), is presented. The difference in the temporal evolution of the photodetached excess electron current between the upstream and the downstream regions corresponds to the flow velocity, while the average of the recovery times corresponds to the thermal velocity, namely the negative ion temperature. Initial results in a divertor/edge plasma simulator MAP(material and plasma)‐II suggest that the parallel flow velocity is small in the periphery of the linear plasma column where the negative ions exist. The negative ion temperature is similar to that of helium ion obtained for helium discharges. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetized Features of Sheath and Collection Region of Photodetached Electrons in Negative Ion Measurement

AbstractThe magnetized features of the sheath and collection region of photodetached electrons (PDE's) are experimentally studied in the linear divertor simulator MAP‐II. The thickness of the electron sheath formed around a cylindrical electrostatic probe is measured by analyzing the variation in the temporal evolution of the laser photodetachment (LPD) signal. The effect of the magnetic field on the electron sheath thickness along the field is revealed by comparing the experimental result to the theoretical thickness derived using the Child‐Langmuir (CL) law. The length of the collection region of PDE's is measured in several magnetized regimes by using the recently developed eclipse‐LPD method. The physical meaning of this collection region is discussed based on the measured electron sheath thickness and the length of the collection region. Finally, the necessary conditions for applying the LPD technique in magnetized plasmas are discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of bias potential upon H− density near a plasma grid of a negative ion source

A molybdenum plasma grid was installed in a 30-cm-long 16-cm-diam 16-pole magnetic multicusp ion source to simulate the effect of a plasma electrode bias of a negative ion source. Effects caused by the bias voltage applied to the electrode upon the plasma parameters, the density of negative hydrogen ions (H−), and the drift velocity of plasma perturbed by the photodetachment of H− were investigated with the direct current laser photodetachment method. The electron density at the distance of 0.5 cm from the plasma grid decreased from 8×1010 to 4×1010cm−3 by increasing the grid bias from 0 to +4V, while the H− density increased from 8×108 to 4.5×109cm−3. The drift velocity of plasma perturbation was changed by a factor of 3 corresponding with the gradient of the plasma potential near the electrode.

Density and velocity of H− in the extraction region of a negative ion source estimated from the change in H− beam current due to a pulse laser injection

A method to estimate H− density and velocity in the extraction region of a negative ion source plasma without a Langmuir probe is developed. It utilizes the laser photodetachment with a Faraday cup (PD-FC). The H− parameters estimated by PD-FC are compared to the results measured by laser-photodetachment with a Langmuir probe (PD-LP). The result shows that H− velocity estimated by PD-FC is in good agreement with the PD-LP result. Meanwhile, H− density estimated by PD-FC is twice larger than the PD-LP result. Though a more detailed research, including more effects that are ignored in this study, is needed for H− density measurement, PD-FC will be a very useful tool for H− measurements especially near an extraction hole in negative ion sources.

D − density distribution and its dependence on plasma parameters in volume negative ion sources

To understand H −/D − negative ion formation mechanism, isotope effects of plasma production and H −/D − production in volume negative ion sources are studied. Axial distributions of H − and D − densities in the source are measured directly by laser photodetachment method. In particular, experimental results concerning dependence of H −/D − density distributions on plasma parameters are presented and discussed. Isotope effect of plasma production in H 2 and D 2 discharge is observed. H − and D − densities have different spatial distributions, respectively, corresponding to those different plasma parameter conditions.

Volume production negative hydrogen ion sources

We review the evolution of volume production negative hydrogen ion sources since the discovery in 1977 of the new phenomenon, designated as volume production and attributed to dissociative electron attachment of low energy electrons to rovibrationally excited molecules. The experimental verification in 2005 of the reality of this mechanism is reported. The magnetically filtered tandem sources, using hot filaments or inductively coupled radio frequency discharges, proposed in order to make use of the volume production mechanism, are used as continuous wave sources for cyclotrons and short pulse sources for synchrotrons. The extraction physics, required to correlate the negative ion and electron densities near the extraction opening with the extracted currents, is discussed taking into account the recently measured H/sup -//D/sup -/ ion temperatures. It is also shown that the extracted negative ion current can be predicted from the directed flow velocity (measured by two laser photodetachment) and the negative ion density measured in the extraction region plasma. Progress in modeling the volume production negative hydrogen ion sources is briefly summarized. Main attention has been paid to some recent topics, such as negative ion temperature and specific for two negative species plasmas transport in a weak transverse magnetic field. A new view on the potential of volume production making use of the vibrationally excited molecules produced on surfaces (plasma electrode, walls) by recombinative desorption is presented.

Development of Laser Photodetachment Technique Using Heated Probe to Eliminate the Effect of Probe Surface Ablation Phenomena

The characteristics of the phenomena caused by laser irradiation to an electrostatic probe in plasmas are studied to avoid the disturbance of the laser photodetachment signals for negative ion density measurement. In helium–hydrogen and hydrogen–methane plasmas, a probe surface ablation phenomenon was observed as an anomalous excess electron current in response to the laser irradiation to the electrostatic probe, while the phenomenon was not observed in pure hydrogen plasmas. Contaminations of the probe surface appear to be the mechanism causing the ablation phenomena. In order to clean the probe surface, a filament-type heated probe, which is the same type of conventional emissive probe, is applied to the laser photodetachment technique. When the surface is cleaned by heating the probe, the ablation phenomenon disappears, and the negative ion density can be evaluated at a sufficiently high laser pulse energy to saturate the photodetachment rate of negative ions. The method developed in this paper is useful for the measurement of negative ion density in plasmas where the probe surface is easily contaminated.

The Singlet–Triplet Separation in CF2: State‐of‐the‐Art Ab Initio Calculations and Franck–Condon Simulations Including Anharmonicity

Geometrical parameters, vibrational frequencies and relative electronic energies of the X2B1 state of CF2- and the X1A1 and ã3B1 states of CF2 have been calculated. Core-electron effects on the computed minimum-energy geometries and relative electronic energies have been investigated, and relativistic contributions to the computed relative electronic energies calculated. Potential energy functions of the X2B1 state of CF2- and the X1A1 and ã3B1 states of CF2 have been determined, and anharmonic vibrational wavefunctions of these states calculated variationally. Franck-Condon factors including anharmonicity and Duschinsky rotation have been computed and used to simulate the ã-X emission spectrum of CF2 determined by S. Koda [Chem. Phys. Lett. 1978, 55, 353] and the 364 nm laser photodetachment spectrum of CF2- obtained by R. L. Schwartz et al. [J. Phys. Chem. A 1999, 103, 8213]. Comparison between theory and experiment shows that the theoretical approach benchmarked in the present study is able to give highly reliable positions for the CF2(X1A1) + e <-- CF2-(X2B1) and CF2(ã3B1) + e <-- CF2-(X2B1) bands in the photoelectron spectrum of CF2- and a reliable singlet-triplet gap for CF2. It is therefore concluded that the same theoretical approach should give reliable simulated CCl2(X1A1) + e <-- CCl2-(X2B1) and CCl2(ã3B1) + e <-- CCl2-(X2B1) bands in the photodetachment spectrum of CCl2- and a reliable singlet-triplet gap for CCl2.

A Combined Ab Initio/Franck–Condon Study of the ÖX̃ Single‐Vibronic‐Level Emission Spectrum of CCl2 and the Photodetachment Spectrum of CCl2−

State-of-the-art ab initio calculations have been carried out on the X1A1, ã3B1 and A1B1 states of CCl2 and the X2B1 state of CCl2-. Franck-Condon factors including anharmonicity have been calculated, between the CCl2 states, and between the CCl2- X2B1 state and the CCl2 states. They are used to simulate the A-X single-vibronic-level (SVL) emission spectra of CCl2 determined by M.-L. Lui et al. [PCCP 2003, 5, 352] and the 364 nm laser photodetachment spectrum of CCl2- obtained by R. L. Schwartz et al. [J. Phys. Chem. A 1999, 103, 8213]. Comparison between simulated and observed spectra confirms the vibrational assignments of the X2B1 SVL emission spectra and the T0 position of the A1B1 state of CCl2. For the photodetachment spectrum of CCl2-, spectral simulation shows that the higher binding energy ã3B1(CCl2) <-- X2B1(CCl2-) band is well separated from the X1A1(CCl2) <-- X2B1(CCl2-) band. It is concluded that the observed second band, which overlaps heavily with the X1A1(CCl2) <-- X2B1(CCl2-) band in the photodetachment spectrum of CCl2- cannot be assigned to the CCl2(ã3B1) + e --> CCl2-(X2B1) detachment process. Further ab initio calculations carried out in the present investigation support the suggestion that the second band in the 364 nm photodetachment spectrum of CCl2- is due to detachment from an excited state of CCl2-, a linear quartet state, to a triplet state of CCl2. These calculations identify the anionic state to be the lowest 4Sigmag- (4Sigma-) state, which photodetaches vertically to the 3Sigmag- (3Sigma-; adiabatically ã3B1) and/or 3Pi(u) (3Pi) states of CCl2 to give the second band observed in the 364 nm photodetachment spectrum of CCl2-.