Resonant Ionization Detection Research Articles

Threshold Photodetachment in a Repulsive Potential

We report on the first experimental observation of a new threshold behavior observed in the 5(2)G partial channel in photodetachment of K(-). It arises from the repulsive polarization interaction between the detached electron and the residual K(5(2)G) atom, which has a large negative dipole polarizability. In order to account for the observation in the K(5(2)G) channel, we have developed a semiclassical model that predicts an exponential energy dependence for the cross section. The measurements were made with collinear laser-ion beams and a resonance ionization detection scheme.

Observation of blue satellite bands and photoassociation at ultracold temperatures

We have observed atomic line self-broadening of Cs near 7P{sub 3/2} and 7P{sub 1/2} atomic lines at ultracold temperatures using a magneto-optical trap and resonant ionization detection. We have observed blue satellite band features at detunings of 560 and 800 MHz, respectively, as well as sharp hyperfine-split photoassociative spectra on the red wings of each line and also on the blue wings. Possible explanations of these features are discussed.

Nuclear charge radii of 8,9Li determined by laser spectroscopy.

The 2s-->3s transition of (6,7,8,9)Li was studied by high-resolution laser spectroscopy using two-photon Doppler-free excitation and resonance-ionization detection. Hyperfine structure splittings and isotope shifts were determined with precision at the 100 kHz level. Combined with recent theoretical work, the changes in the nuclear-charge radii of (8,9)Li were determined. These are now the lightest short-lived isotopes for which the charge radii have been measured. It is found that the charge radii monotonically decrease with increasing neutron number from 6Li to 9Li.

Single photo-electron and photon detection in a mercury resonance ionization photon detector (RID)

Avalanche detection of a laser enhanced ionization (LEI) signal has been studied in a resonance ionization detector (RID) cell containing mercury vapor at room temperature. An avalanche multiplication factor of more than 8000 was achieved. The limit of detection of Hg resonance radiation ( λ = 253.7 nm) was at the level of 0.5 quantum during the lifetime of the excited 6 3P 1 0 state. Detection of radiation from a conventional CW Hg discharge lamp source with a signal to noise ratio of more than 10 4 has been achieved.

Mechanistic study of atomic desorption resulting from the keV-ion bombardment of fcc{001} single-crystal metals.

Energy-resolved angular distributions of Ni and Rh atoms desorbed by keV ${\mathrm{Ar}}^{+}$ ion bombardment have been measured using multiphoton resonance ionization detection. The experimental spectra were simulated using molecular-dynamics calculations which are based on the molecular-dynamics/Monte Carlo corrected effective-medium interaction potential. Important collision events were identified using a recently developed graphical utility which allows easy visualization of atomic motions subsequent to bombardment. Three major microscopic ejection mechanisms were determined, each of which is categorized into three additional interactions. The features which make up the polar angle spectra are assigned to one of these mechanisms. We find that the majority of particles eject due to a collision with an atom from one layer below (${\mathrm{\ensuremath{\Delta}}}_{1}$ mechanism). A mechanism involving a collision due to an atom from the same layer, however, is responsible for a shift in peak position with energy. This investigation strongly reinforces the view that the inherent registry of the atoms in the crystal lattice is the crucial factor in determining the dominant microscopic sequences of events which lead to ejection as well as the macroscopically observable quantities.

High-Spatial-Resolution OH Rotational Temperature Measurements in an Atmospheric-Pressure Flame Using an Indium-Based Resonance Ionization Detector

The use of a resonance ionization photon detector (RID) is described for the measurement of flame temperatures with a spatial resolution of less than 100 μm. The detector, based on the two-step excitation of indium atoms, with subsequent collisional ionization, was used to record rotational excitation scans of OH in an atmospheric-pressure acetylene/air flame. The OH excitation spectra were recorded by scanning an “excitation” laser in the A2σ+ ← X2II i (1, 0) vibronic band in the wavelength range, 281–288 nm, while simultaneously illuminating the same flame region with the “detection” laser, tuned to the 6 p2 P3/2 → 10 d2. D5/2 excited-state transition of In at 786.44 nm. The excitation and detection laser beams were made orthogonal in the flame, defining the resolution to be limited by the waist of the excitation beam (100 μm), whose diameter was always smaller than the detection laser beam. A temperature profile of the flame is recorded with the use of both the RID approach and a more conventional laser-induced fluorescence (LIF) approach for comparison. A more structured temperature profile is recorded with the RID owing to its high spatial resolution, whereas the LIF method, which is inherently a line-of-sight method, produces a rather featureless temperature distribution across the flame. Anomalously high flame temperatures were recorded at the flame edge with the RID. The cause of these high flame temperatures has not been determined.

Surrogate Detection for Continuous Emission Monitoring by Resonance Ionization

ABSTRACT New measurements of resonance ionization detection limits are presented for seven species, of high thermal stability, which are potentially useful surrogates for continuous emission monitoring of incinerator effluent. Resonance ionization detection limit data are now available for 20 aliphatic and aromatic compounds; eight of these compounds have sub part-per-billion detection limits. These eight also exhibit selectivities exceeding 103 when detected in the presence of a “soup” of chemically similar interferant species likely to be present in stack gas samples. Preliminary measurements indicate that detection limits, obtained under ideal conditions in a helium carrier gas, are also approached under adverse sampling conditions tested with synthetic “soups”. The implications of these measurements on the selection of surrogates and the prospects for repetitive on-line hazardous emissions monitoring are discussed

Detection of OH in an atmospheric pressure flame via laser enhanced ionization of indium

A resonance ionization detector (RID) based on the two-step laser enhanced ionization of indium metal in an atmospheric pressure air/acetylene flame is studied and characterized. Practical utilization of the RID is shown by recording a partial excitation spectrum of the OH radical in the same air/acetylene flame used as the detector cell and measurement of the flame temperature by the Boltzmann plot method. OH transitions were excited in the X 2Π → A 2Σ +(1,0) vibronic band in the wavelength range 281–288 nm. Two possible mechanisms, fluorescence capture and collisional energy transfer, for the observation of the OH excitation spectrum are put forth and discussed. No conclusive evidence supporting either mechanism as predominant was found.

Analyses of cryogenic samples using ion-induced desorption and multiphoton resonance ionization

In this study, ion-beam-induced desorption with multiphoton resonance ionization detection of desorbed neutral molecules is used to characterize frozen aqueous solutions. This type of matrix is of particular importance since it serves as a model for biological matrices. The time-of-flight mass spectrum, obtained in this way for a millimolar tryptophan/H2O solution, is virtually identical to that for a submonolayer of tryptophan on a silicon wafer. The tryptophan signal from a frozen solution is demonstrated to have a linear dependence on concentration by using 4,4'-biphenyldiol as an internal standard. A detection limit of 2 x 10(-6) M is also demonstrated. Since our ion beam samples one layer of 0.1 cm2 and we assume 10(15) molecules/cm2 of ice, this concentration corresponds to approximately 4 x 10(6) molecules/layer. It is also shown that the signal exhibits an exponential decay with primary ion dose due to the accumulation of primary ion damage in the near-surface molecules.

Photon detection based on pulsed laser-enhanced ionization and photoionization of magnesium vapour: experimental characterization

A resonance ionization detector (RID) based on the two-step laser-enhanced ionization of Mg in a miniature air–acetylene flame is described. The detector utilizes the 285.213 nm resonance absorption of Mg as the signal transition, i.e., photons to be measured. Magnesium atoms excited by absorption of photons at 285.2 nm were further excited to higher lying energy states by absorption of photons at 571.2 or 435.2 nm, from which collisional ionization could occur, or by absorption of photons at 300.9 nm, which directly ionized the excited Mg atoms via an autoionizing level above the ionization continuum. The miniature flame used (about 2 mm in diameter) had very low noise characteristics and the minimum detectable number of photons (MDP) was limited, in some instances, by the noise of the transimpedance amplifier used. The lowest MDP, obtained for the excitation scheme 3s21S (285.2 nm)→3p 1P0(435.2 nm)→6d 1D, was experimentally determined to be 1 × 103(7 × 10–16 J). The quantum efficiency of this excitation scheme, defined as the number of electrons created per photon absorbed at the signal transition (285.2 nm), was found to be 0.75. The spectral response bandwidth of the detector in the signal transition was determined to be mostly Lorentzian in nature, owing to pressure broadening in the atmospheric pressure flame, with a stray light rejection ratio of approximately 1 × 10–5 at 100 cm–1 displacement from the absorption maximum of the RID at 285.213 nm. An application of the RID is given in the detection of weak Raman scatter of carbon tetrachloride, chloroform and dimethyl sulfoxide.

Profiling of Hydrogen in Zirconium Surfaces by Laser Ablation with Resonance Ionization

ABSTRACTElemental distributions in the bulk and metal oxide surface layers of zirconium alloys play key roles in the fracture toughness of the alloys. In particular, localized hydrogen build-up leads to hydride formation and delayed hydride cracking. Parts per million levels of H in Zr have been detected using the 1.06 μm or 355 nm output of a Nd:YAG laser for ablation followed by 2+1 resonance ionization detection of H and D. Analysis of the ablation plume has shown that it consists predominately of atomic species in thermal equilibrium between 2000 and 3600°C. Ablation of thin foils has shown that the ablation rate is on the order of mono-layers per shot and increases exponentially with increasing fluence. Laser ablation depth profiling results of H distributions in an anodically grown oxide film compare qualitatively with nuclear-reaction-analysis profiling of the same sample.

Resonance ionization detection limits for hazardous emissions

Continuous emission monitoring of the stack gases of municipal and hazardous waste incinerators requires on-line instrumentation capable of the selective detection of a wide variety of toxic emissions at part-per-billion concentrations. The capabilities of a molecular beam/resonance-enhanced multiphoton ionization/time-of-flight (MB/REMPI/TOFMS) apparatus proposed for such measurements are demonstrated. Detection limit and selectivity measurements are presented for the resonance ionization detection of thirteen hazardous species, commonly present in incinerator stack gas emissions. The species selected for study may serve as useful surrogates for monitoring wide classes of chlorinated aromatic and olefinic hydrocarbons. In particular, benzene, toluene, chlorobenzene, and tetrachloroethylene are found to be detectable in sub-ppb concentrations with virtually no interference from a wide variety of chemically similar compounds. The unique attributes of the MB/REMPI/TOFMS method for rapid, ultrasensitive, and selective detection of toxic organics make it a promising new tool for quantitative evaluation of incinerator performance.

Resonance ionization detection of neutral O(3PJ) produced from stimulated surface processes

We report the first quantum‐state specific detection of ground‐state atomic oxygen O(3PJ ) above an electron irradiated adsorbate‐covered surface using laser resonance‐enhanced multiphoton ionization spectroscopy. Electron‐stimulated desorption of O(3PJ ) is not observed from an O‐covered Pt(111) surface, but O(3PJ ) fragments are produced from electron‐stimulated dissociation of NO2(a) coadsorbed with O (θ0=0.75 ML) on Pt(111). The ∼9–10 eV threshold for O(3PJ ) formation correlates with that observed for NO2(a) dissociation. The measured O(3PJ ) spin‐orbit state distribution is (5.0):(2.5):(1.0) for J=2, 1, and 0, respectively. This is within experimental error of the 2J+1 statistical (T→∞) limit. No gas phase O(3PJ ) is detected from the stimulated dissociation of adsorbed O2 on Pt(111); however, we have utilized the O(3PJ) detection capability as an indirect probe of NO2 production from an electron‐stimulated surface reaction between coadsorbed O2 and NO on Pt(111). The NO2 is detected by laser photolysis above the surface. The ∼10‐eV NO2 production threshold correlates with dissociative ionization from the 3σg molecular bonding orbital of O2(a). Dissociative ionization can produce energetic O atoms which may then react with a coadsorbed NO, producing an NO2 desorbate.

A new, sensitive, high resolution Raman detector based on ionization

A novel detection method for weak pulsed or cw Raman fluxes is described. The detector is based upon the production of Raman scatter with a tunable pulsed or cw dye laser, collecting a large fraction of the Raman scatter and transferring it efficiently into an ionization detector containing a metal (M) vapor, such as Li. The resonance ionization detector (RID) is simultaneously illuminated by a second dye laser. When the second laser is tuned to an excited state transition of the metal vapor M and when the first laser is at such a wavelength that the Raman scatter appears at the ground state absorption transition of the metal, then a current will be produced in the RID which is proportional to the Raman scatter intensity. Both the production and collection of this current can be made very efficient (approaching 100%) and should result in improved sensitivity compared to conventional dispersive or FT Raman techniques. The new approach should be much less sensitive to scatter, should have a spectral resolution better than 0.1 cm −1 and should allow Raman scatter measurements to be made at wavenumbers below 100 cm −1 and under certain conditions to 0.01 cm −1. The approach should be especially useful in highly scattering environments like Ag-sols in surface enhanced Raman and should be useful for detection of ultratrace levels of drugs and metabolites in biological fluids. The Raman-RID approach should also be useful for resonance Raman since laser scatter and molecular fluorescence should have minimal effects.

Resonance-ionization detection of photons of sodium atoms in an argon atmosphere

The authors study the feasibility of designing and using resonance-ionization photodetectors with open low-temperature flow cells in a laser spectrometer for the spectral investigation of metal vapors. As an example they use an argon-ventilated cell containing vapors of sodium atoms. This paper describes the experimental setup, comprised of a nitrogen laser, two dye lasers, and the candidate photodetector, and discusses data on the photoionization, resonance absorption, and absorption lines of the atoms as well as the optics and performance of the system.

Energy and angle-resolved studies of neutrals desorbed from ion bombarded polycrystalline metal surfaces

Angle-resolved energy distributions and energy-resolved angle distributions for atoms desorbed from polycrystalline In and Rh foils bombarded by 5-keV Ar + ions are reported. It is possible to record these data under low dose conditions using multiphoton resonance ionization detection for the neutrals after they have left the target. The results show that angle or energy integrated data agree reasonably well with energy transport theories, an observation confirmed by many previous workers. The energy-resolved angle distributions for both In and Rh, however, exhibit a near cos θ distribution for low energy particles and a near cos 2θ distribution for the higher energy particles. The angle-resolved energy distributions also exhibit a shift in the peak position to lower values as θ is increased from 0 to 75°. Neither effects are predicted by theory. The results are discussed in terms of the anisotropy of the momentum distribution which apparently exists inside the solid during the evolution of the collison cascade.

Tunable VUV light generation for the low-level resonant ionization detection of krypton

High-power tunable VUV light pulses with energies up to 0.7 μJ were generated in the 115.7–116.9-nm region by use of a two-photon resonant four-wave mixing scheme in a Xe–Ar gas mixture. This is the highest reported pulse energy that has been produced in this wavelength region using a four-wave mixing process. Efficient detection of krypton isotopes at densities as low as 10 atoms/cm3 was demonstrated by resonantly ionizing the atom through its one-photon allowed state at the vacuum wavelength of 116.49 nm.

Detection of atomic hydrogen in flames by resonance four-photon ionization at 365 nm

The first observations of the four-photon ionization of H atoms at 364.7 nm via a three-photon resonance on the Lyman alpha transition are reported. The resonance ionization detection of H and D atoms in an H 2/D 2/O 2/Ar flame is demonstrated and H-atom density profiles are measured with good spatial resolution. A broadening of 2.2 cm −1 /GW cm 2 of the four-photon resonance by ac Stark shifting is observed.