Atom Counting at Surfaces

Pappas, David; Hrubowchak, David M.; Ervin, Matthew H.; Winograd, Nicholas

doi:10.1126/science.243.4887.64

Cited by 74 publications

(32 citation statements)

References 8 publications

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“…In all experiments the 190 0s/ 188 0s ratio served as an internal standard for wavelength adjustment following each sample change or optical realignment. For zero detuning this procedure produces very good agreement between the (1 + a) 2 values for the different even-mass isotope pairs; the isotope ratio measurements converge to define a mean value, (1 + a) 2 = 1.016 ± 0.004. The constancy of the isotopic fractionation for all even-mass isotope pairs indicates control over laser-induced isotopic selectivity substantially exceeding that achieved in previous RIMS studies [8][9][10].…”

Section: Wavelength Dependencementioning

confidence: 97%

“…RIMS has been successfully applied to the analysis of nearly 70 elements. Very high sensitivities in trace elemental analysis have been obtained by combining resonance ionization with ion and laser microprobe techniques [1][2][3] or with thermal atomization [4][5][6]. The high efficiency and elemental selectivity [7] make laser resonance ionization a potentially important tool for applications in analytical mass spectrometry.…”

Section: Introductionmentioning

confidence: 99%

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Laser-induced isotopic selectivity in the resonance ionization of Os

Wunderlich

Hutcheon

Wasserburg

et al. 1992

International Journal of Mass Spectrometry and Ion Processes

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Isotope selective effects in resonance ionization mass spectrometry (RIMS) pose a potentially serious limitation to the application of this technique to the precise and reproducible measurement of isotope ratios. In order to identify some of the underlying causes of isotope selectivity in RIMS and to establish procedures for minimizing these effects, we investigated laser-induced isotope selectivity in the resonance ionization of Os. A single-color, one-photon resonant ionization scheme was used for several different transitions to produce Os photoions from a thermal atomization source. Variations in Os isotope ratios were studied as a function of laser parameters such as wavelength, bandwidth, power and polarization state. Isotope selectivity is strongly dependent on laser power and wavelength, even when the bandwidth of the laser radiation is much larger than the optical isotope shift. Variations in the 190 0s/ 188 0s ratio of ~ 20% for a detuning of 0.8 em_, were observed on a transition with a small oscillator strength. Large even-odd isotope selectivity with a 13% depletion of 189 0s was observed on a !'J.J = +I transition at low laser intensity; the odd mass Os isotopes are systematically depleted. For !'J.J = -I and 0 transitions the isotope selectivity was reduced by polarization scrambling and for strongly saturating conditions. A technique employing the wavelength dependence of even-even isotope selectivity as an internal wavelength standard was developed to permit accurate and reproducible wavelength adjustment of the laser radiation. This technique provides control over laser-induced isotope selectivity for single-color ionization and enabled us to obtain reproducible measurements of 192 0s/ 188 0s and 189 0s/ 190 0s ratios in the saturation regime for a !'J.J = +I transition with a precision of better than 0.5%. The application of this wavelengthtuning procedure should significantly improve the quality of RIMS isotope ratio data for many elements.

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Section: Wavelength Dependencementioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

Laser-induced isotopic selectivity in the resonance ionization of Os

Wunderlich

Hutcheon

Wasserburg

et al. 1992

International Journal of Mass Spectrometry and Ion Processes

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“…Wavelength selectivity results from the photon en- ergy being resonant with a transition to an intermediate state. Because the ionization efficiency of a resonant mechanism is greater by many orders of magnitude than nonresonant multiphoton processes (Pappas et al 1989), only those molecules with a transition in resonance with the laser photon are appreciably ionized, which allows us to achieve species-selective ionization. The chromophore we utilize is the benzene ring moiety that has a high absorption cross section at 266 nm associated with electronic excitation of the aromatic ring (π -» π*) This chromophore provides an ionization window on the class of compounds referred to as polycyclic aromatic hydrocarbons (PAHs), which are of fundamental cosmochemical interest.…”

Section: ^L 2 Ms: Methodologymentioning

confidence: 99%

Microprobe two-step laser mass spectrometry as an analytical tool for meteoritic samples

Clemett

Zare

1997

Symp. - Int. Astron. Union

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Abstract. Microprobe two-step laser mass spectrometry (/iL 2 MS) is a new mass spectrometric method in which the two essential steps of any mass spectrometric analysis, vaporization and ionization, are carried out using two independent laser sources. In the first step, the output of a pulsed infrared laser is focused on the sample to cause rapid heating in the spot area illuminated, which is typically 40 /zm by 40 /im. In the second step, the output of a pulsed ultraviolet laser causes (1+1) resonance-enhanced multiphoton ionization (REMPI) of those desorbed neutral molecules that (1) are able to absorb this UV wavelength and (2) whose ionization potential is less than the energy of two photons of this UV wavelength. The resulting ions are then mass analyzed in a reflectron time-of-flight apparatus. Under suitable conditions fragmentation can be avoided in both the vaporization and ionization steps so that μΙι 2 Μ8 can be applied to the analysis of a mixture of molecules. Applications of ^L 2 MS to meteorite samples are presented as a means of detecting trace amounts of certain organic molecules present in complex materials without prior sample preparation, extraction, purification, and separation steps. Moreover, this analysis can be carried out with micrometer spatial resolution so that in favorable cases the presence or absence of certain molecules can be correlated to mineralogical features of the sample.

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“…It has successfully been used for selective postionization of sputtered neutral species, providing powerful ultratrace analysis capabilities below atomic fractions of 10-~ [1][2][3] while removing only a few monolayers of the substrate. Results obtained using RIS with secondary neutral mass spectrometry (SNMS) are also readily quantifiable.…”

Section: Introductionmentioning

confidence: 99%

Three-color resonance ionization spectroscopy of Zr in Si

Hansen¹,

Calaway²,

Pellin³

et al. 1997

AIP Conference Proceedings

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Abstract. It has been proposed that the composition of the solar wind could be measured directly by transporting ultrapure collectors into space, exposing them to the solar wind, and returning them to earth for analysis. In a study to help assess the applicability of present and future postionization secondary neutral mass spectrometers for measuring solar wind implanted samples, measurements of Zr in Si were performed. A three-color resonant ionization scheme proved to be efficie~nt while producing a background count rate limited by secondary ion signal (5 x 10-'+ counts/laser pulse) This lowered the detection limit for these measurements to below 500 ppt for 450,000 averages. Unexpectedly, the Zr concentration in the Si was measured to be over 4 ppb, well above the detection limit of the analysis. This high concentration is thought to result from contamination during sample preparation, since a series of tests were performed that rule out memory effects during the analysis.

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Atom Counting at Surfaces

Cited by 74 publications

References 8 publications

Laser-induced isotopic selectivity in the resonance ionization of Os

Laser-induced isotopic selectivity in the resonance ionization of Os

Microprobe two-step laser mass spectrometry as an analytical tool for meteoritic samples

Three-color resonance ionization spectroscopy of Zr in Si

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