Resonance ionization: Difference between revisions
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Resonance ionization was first used in a spectroscopy experiment in 1971 at the [[Institute for Spectroscopy Russian Academy of Sciences]]; in that experiment, ground state [[rubidium]] atoms were ionized using [[Ruby laser|ruby lasers]].[[Resonance ionization#cite note-4|<sup>[4]</sup>]] In 1974, a group of researchers at the Oak Ridge National Laboratory wanted to use laser light to measure the number of singlet metastable helium, He (2<sup>1</sup>S), particles created from energetic protons <ref>{{Cite journal|last=Hurst|first=G. S.|date=1975|title=Saturated Two-Photon Resonance Ionization of|url=https://link.aps.org/doi/10.1103/PhysRevLett.35.82|journal=Physical Review Letters|volume=35|issue=2|pages=82–85|doi=10.1103/physrevlett.35.82}}</ref>. They achieved the selective ionization of the excited state of an atom at nearly 100% efficiency by using pulsed laser light to pass a beam of protons into a helium gas cell. This experiment was seminal in the journey towards using resonance ionization spectroscopy (RIS) for atomic analysis in research settings. Cesium atoms was subsequently used to show that single atoms of an element could be counted if its resonance ionization was performed in a counter in which an electron could be detected for an atom in its ground state <ref>{{Cite journal|last=Hurst, G.S.|first=|date=1984|title=Historical survey of resonance ionization spectroscopy|url=http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/16/000/16000382.pdf|journal=Second International Symposium on Resonance Ionization Spectroscopy and Its Applications|volume=|pages=|via=}}</ref>. Subsequently, advanced techniques categorized under resonance ionization mass spectrometry (RIMS) were used to generate the relative abundance of various ion types by coupling the RIS lasers to magnetic sector, quadrupole, or time-of-flight (TOF) mass spectrometers. |
Resonance ionization was first used in a spectroscopy experiment in 1971 at the [[Institute for Spectroscopy Russian Academy of Sciences]]; in that experiment, ground state [[rubidium]] atoms were ionized using [[Ruby laser|ruby lasers]].[[Resonance ionization#cite note-4|<sup>[4]</sup>]] In 1974, a group of researchers at the Oak Ridge National Laboratory wanted to use laser light to measure the number of singlet metastable helium, He (2<sup>1</sup>S), particles created from energetic protons <ref>{{Cite journal|last=Hurst|first=G. S.|date=1975|title=Saturated Two-Photon Resonance Ionization of|url=https://link.aps.org/doi/10.1103/PhysRevLett.35.82|journal=Physical Review Letters|volume=35|issue=2|pages=82–85|doi=10.1103/physrevlett.35.82}}</ref>. They achieved the selective ionization of the excited state of an atom at nearly 100% efficiency by using pulsed laser light to pass a beam of protons into a helium gas cell. This experiment was seminal in the journey towards using resonance ionization spectroscopy (RIS) for atomic analysis in research settings. Cesium atoms was subsequently used to show that single atoms of an element could be counted if its resonance ionization was performed in a counter in which an electron could be detected for an atom in its ground state <ref>{{Cite journal|last=Hurst, G.S.|first=|date=1984|title=Historical survey of resonance ionization spectroscopy|url=http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/16/000/16000382.pdf|journal=Second International Symposium on Resonance Ionization Spectroscopy and Its Applications|volume=|pages=|via=}}</ref>. Subsequently, advanced techniques categorized under resonance ionization mass spectrometry (RIMS) were used to generate the relative abundance of various ion types by coupling the RIS lasers to magnetic sector, quadrupole, or time-of-flight (TOF) mass spectrometers. |
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== Method == |
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In resonant ionization, atoms or molecules from ground state are excited to higher energy states by the resonant absorption of photons to produce ions. These ions are then monitored by appropriate detectors. In order to ensure a highly-efficient sensitivity and process saturation, the atomic or molecular beam must be formed from the ground state, the atoms should be efficiently excited and ionized, each atom should be converted by the photon field of a short-timed pulsed laser into a positive ion and a valence electron, and subsequent quantitative detection of the produced ions is ensured using relevant ion detectors <ref>{{Cite journal|last=Rimke|first=Hubertus|last2=Herrmann|first2=Günter|last3=Mang|first3=Marita|last4=Mühleck|first4=Christoph|last5=Riegel|first5=Joachim|last6=Sattelberger|first6=Peter|last7=Trautmann|first7=Norbert|last8=Ames|first8=Friedhelm|last9=Kluge|first9=Hans-Jürgen|date=1989-05-01|title=Principle and analytical applications of resonance lonization mass spectrometry|url=https://link.springer.com/article/10.1007/BF01244676|journal=Microchimica Acta|language=en|volume=99|issue=3-6|pages=223–230|doi=10.1007/bf01244676|issn=0026-3672}}</ref>. |
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In a basic RIS process, a pulsed laser beam produces photons of the right energy in order to excite an atom initially in its ground state, ''a'', to an excited level, ''b''. During the laser pulse, the ion population of state b increases at the expense of that of state a. After a few minutes, the rate of stimulated emission from the excited state will equal rate of production so that the system is in equilibrum as long as the laser intensity is kept sufficiently high during a pulse. This high laser intensity translates into a photon fluence (photons per unit of beam area) large enough so that a necessary condition for the saturation of the RIS process has been met. If, in addition, the rate of photoionization is greater than the rate of consumption of intermediates, then each selected state is converted to one electron plus one positive ion, so that the RIS process is saturated. <ref name=":3">{{Cite journal|last=Hurst|first=G. S.|date=1979|title=Resonance ionization spectroscopy and one-atom detection|url=https://link.aps.org/doi/10.1103/RevModPhys.51.767|journal=Reviews of Modern Physics|volume=51|issue=4|pages=767–819|doi=10.1103/revmodphys.51.767}}</ref> A usually efficient way to produce free atoms of an element in the ground state is to atomize the elements by ion sputtering or thermal vaporization of the element from a laser matrix under vacuum conditions or at environments with pressures very low below the normal atmospheric pressure. The resulting plume of secondary atoms is then channeled through the path of multiple tuned laser beams which are capable of exciting consecutive electronic transitions in the specified element. This promotes the atoms above their ionization potentials while interfering atoms from other elements are hardly ionized since they are generally transparent to the laser beam. This produces photoions which are extracted and directed towards an analytical facility such as a magnetic sector to be counted. This approach is extremely sensitive to atoms of the specified element so that the ionization efficiency is almost 100% and also elementally selective, due to the highly unlikely chance that other species will be resonantly ionized <ref name=":3" /><ref>{{Cite journal|last=Hurst|first=G. S.|date=1977|title=One-atom detection using resonance ionization spectroscopy|url=https://link.aps.org/doi/10.1103/PhysRevA.15.2283|journal=Physical Review A|volume=15|issue=6|pages=2283–2292|doi=10.1103/physreva.15.2283}}</ref> |
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To achieve high ionization efficiencies, monochromatic lasers with high instantaneous spectral power are used. Typical lasers being used include continuous-wave lasers with extremely high spectral purity and pulsed lasers for analyses involving limited atoms <ref>{{Cite journal|last=Wendt|first=Klaus|last2=Trautmann|first2=Norbert|title=Recent developments in isotope ratio measurements by resonance ionization mass spectrometry|url=http://linkinghub.elsevier.com/retrieve/pii/S1387380604004683|journal=International Journal of Mass Spectrometry|volume=242|issue=2-3|pages=161–168|doi=10.1016/j.ijms.2004.11.008}}</ref> Continuous-wave lasers however are often preferred to pulsed lasers due to the latter’s relatively low duty cycle since they can only produce photoions during the brief later pulses, and the difficulty in reproducing results due to pulse-to-pulse jitter, laser beam drifting, and wavelength variations <ref>{{Cite journal|last=Levine|first=Jonathan|last2=Savina|first2=Michael R.|last3=Stephan|first3=Thomas|last4=Dauphas|first4=Nicolas|last5=Davis|first5=Andrew M.|last6=Knight|first6=Kim B.|last7=Pellin|first7=Michael J.|title=Resonance ionization mass spectrometry for precise measurements of isotope ratios|url=http://linkinghub.elsevier.com/retrieve/pii/S1387380609002553|journal=International Journal of Mass Spectrometry|volume=288|issue=1-3|pages=36–43|doi=10.1016/j.ijms.2009.07.013}}</ref>. Moderate laser powers, if high enough to affect the desired transition states, can be used since the non-resonant photoionization cross section is then low which implies a negligible ionization efficiency of unwanted atoms. The influence of the laser matrix to be used for the sample can also be reduced by separating evaporation and ionization processes both in time and in space. |
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Another factor that could affect the selectivity and sensitivity of the ionization process is the presence of contaminants caused by surface or impact ionization. This can be reduced up to appreciable orders of magnitude by using mass analysis so that isotopic compositions of the desired element can be determined. Most of the elements of the Periodic Table can be ionized by one of the several ionization schemes available. The appropriate ionization scheme is chosen based on the level scheme of the element’s atonm, its ionization energy, required selectivity and sensitivity, and the wavelengths and power levels of the laser beams in use <ref>{{Cite journal|last=Rimke|first=Hubertus|last2=Herrmann|first2=Günter|last3=Mang|first3=Marita|last4=Mühleck|first4=Christoph|last5=Riegel|first5=Joachim|last6=Sattelberger|first6=Peter|last7=Trautmann|first7=Norbert|last8=Ames|first8=Friedhelm|last9=Kluge|first9=Hans-Jürgen|date=1989-05-01|title=Principle and analytical applications of resonance lonization mass spectrometry|url=https://link.springer.com/article/10.1007/BF01244676|journal=Microchimica Acta|language=en|volume=99|issue=3-6|pages=223–230|doi=10.1007/bf01244676|issn=0026-3672}}</ref>. A representative resonance ionization mass spectrometry set-up consists of a laser system (consisting of multiple lasers), sample from which the atoms are derived, a suitable mass spectrometer for mass analysis, and a detector or data collection system. |
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==Applications== |
==Applications== |
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Revision as of 17:29, 7 March 2018
Resonance ionization is a selective mode of ionization that uses pulsed laser light to promote an atom or molecule beyond its ionization potential in order to form an ion. [1] This ionization method differs from other spectrometric techniques in that it uses a monochromatic tunable laser to form its ions in a highly-efficient and species-selective manner. [2] In resonance ionization, an ion gun creates a cloud of atoms and molecules from a gas-phase sample surface and the tunable laser is used to fire a beam of photons at the cloud of particles emanating from the sample (analyte). An initial photon from this beam is absorbed by one of the sample atoms, exciting one of the atom's electrons to an intermediate excited state. A second photon then ionizes the atom from the intermediate state such that its high energy level causes it to be ejected from its orbital; the result is a packet of positively charged ions which are then delivered to a mass analyzer. [3] This method of ionizing atoms is analogous to REMPI; the difference being that resonance ionization is used for an atomic (elemental) analyte, whereas REMPI is used for a molecular analyte.[4]
The analytical technique used to detect these ions is known as Resonance Ionization Mass Spectrometry (RIMS). RIMS is derived from the original method, Resonance Ionization Spectroscopy (RIS), which was initially being used to detect single atoms with better time resolution. [5] RIMS has proved useful in the investigation of radioactive isotopes (such as for studying rare fleeting isotopes produced in high-energy collisions), trace analysis (such as for discovering impurities in highly pure materials), atomic spectroscopy (such as for detecting low-content materials in biological samples), and for applications in which high levels of sensitivity and elemental selectivity are desired.
History
Resonance ionization was first used in a spectroscopy experiment in 1971 at the Institute for Spectroscopy Russian Academy of Sciences; in that experiment, ground state rubidium atoms were ionized using ruby lasers.[4] In 1974, a group of researchers at the Oak Ridge National Laboratory wanted to use laser light to measure the number of singlet metastable helium, He (21S), particles created from energetic protons [6]. They achieved the selective ionization of the excited state of an atom at nearly 100% efficiency by using pulsed laser light to pass a beam of protons into a helium gas cell. This experiment was seminal in the journey towards using resonance ionization spectroscopy (RIS) for atomic analysis in research settings. Cesium atoms was subsequently used to show that single atoms of an element could be counted if its resonance ionization was performed in a counter in which an electron could be detected for an atom in its ground state [7]. Subsequently, advanced techniques categorized under resonance ionization mass spectrometry (RIMS) were used to generate the relative abundance of various ion types by coupling the RIS lasers to magnetic sector, quadrupole, or time-of-flight (TOF) mass spectrometers.
Method
In resonant ionization, atoms or molecules from ground state are excited to higher energy states by the resonant absorption of photons to produce ions. These ions are then monitored by appropriate detectors. In order to ensure a highly-efficient sensitivity and process saturation, the atomic or molecular beam must be formed from the ground state, the atoms should be efficiently excited and ionized, each atom should be converted by the photon field of a short-timed pulsed laser into a positive ion and a valence electron, and subsequent quantitative detection of the produced ions is ensured using relevant ion detectors [8].
In a basic RIS process, a pulsed laser beam produces photons of the right energy in order to excite an atom initially in its ground state, a, to an excited level, b. During the laser pulse, the ion population of state b increases at the expense of that of state a. After a few minutes, the rate of stimulated emission from the excited state will equal rate of production so that the system is in equilibrum as long as the laser intensity is kept sufficiently high during a pulse. This high laser intensity translates into a photon fluence (photons per unit of beam area) large enough so that a necessary condition for the saturation of the RIS process has been met. If, in addition, the rate of photoionization is greater than the rate of consumption of intermediates, then each selected state is converted to one electron plus one positive ion, so that the RIS process is saturated. [9] A usually efficient way to produce free atoms of an element in the ground state is to atomize the elements by ion sputtering or thermal vaporization of the element from a laser matrix under vacuum conditions or at environments with pressures very low below the normal atmospheric pressure. The resulting plume of secondary atoms is then channeled through the path of multiple tuned laser beams which are capable of exciting consecutive electronic transitions in the specified element. This promotes the atoms above their ionization potentials while interfering atoms from other elements are hardly ionized since they are generally transparent to the laser beam. This produces photoions which are extracted and directed towards an analytical facility such as a magnetic sector to be counted. This approach is extremely sensitive to atoms of the specified element so that the ionization efficiency is almost 100% and also elementally selective, due to the highly unlikely chance that other species will be resonantly ionized [9][10]
To achieve high ionization efficiencies, monochromatic lasers with high instantaneous spectral power are used. Typical lasers being used include continuous-wave lasers with extremely high spectral purity and pulsed lasers for analyses involving limited atoms [11] Continuous-wave lasers however are often preferred to pulsed lasers due to the latter’s relatively low duty cycle since they can only produce photoions during the brief later pulses, and the difficulty in reproducing results due to pulse-to-pulse jitter, laser beam drifting, and wavelength variations [12]. Moderate laser powers, if high enough to affect the desired transition states, can be used since the non-resonant photoionization cross section is then low which implies a negligible ionization efficiency of unwanted atoms. The influence of the laser matrix to be used for the sample can also be reduced by separating evaporation and ionization processes both in time and in space.
Another factor that could affect the selectivity and sensitivity of the ionization process is the presence of contaminants caused by surface or impact ionization. This can be reduced up to appreciable orders of magnitude by using mass analysis so that isotopic compositions of the desired element can be determined. Most of the elements of the Periodic Table can be ionized by one of the several ionization schemes available. The appropriate ionization scheme is chosen based on the level scheme of the element’s atonm, its ionization energy, required selectivity and sensitivity, and the wavelengths and power levels of the laser beams in use [13]. A representative resonance ionization mass spectrometry set-up consists of a laser system (consisting of multiple lasers), sample from which the atoms are derived, a suitable mass spectrometer for mass analysis, and a detector or data collection system.
Applications
A major advantage of using resonance ionization lies in the fact that it is highly selective ionization mode; it is able to target a single type of atom amongst a background of many types of atoms, even when said background atoms are much more abundant than the target atoms.[14] In addition, resonance ionization incorporates the high selectivity that is desired in spectroscopy methods with ultrasensitivity. This makes resonance ionization useful when analyzing complex samples with many atomic components.[15]
Due to the high sensitivity and its ability to target a single atom, Resonance Ionization coupled with mass spectrometry is a great technique for cosmochemistry which works with trace amounts of samples[16].
See also
References
- ^ Samuel Hurst, G.; Letokhov, Vladilen S. (1994). "Resonance Ionization Spectroscopy". Physics Today. 47 (10): 38–45. doi:10.1063/1.881420. ISSN 0031-9228.
- ^ Fassett, J.D.; Travis, J.C. (1988). "Analytical applications of resonance ionization mass spectrometry (RIMS)". Spectrochimica Acta Part B: Atomic Spectroscopy. 43 (12): 1409–1422. doi:10.1016/0584-8547(88)80180-0. ISSN 0584-8547.
- ^ Hurst, G. S.; Kutschera, W.; Oeschger, H.; Korschinck, G.; Donahue, D. S.; Litherland, A. E.; Ledingham, K.; Henning, W. (1987). "Detection of Single Atoms by Resonance Ionization Spectroscopy [and Discussion]". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 323 (1569): 155–170. doi:10.1098/rsta.1987.0079. ISSN 1364-503X.
- ^ Dass, Chhabil (2007). "Chapter 7: Inorganic Mass Spectrometry". In Desiderio, Dominic M.; Nibbering, Nico M. (eds.). Fundamentals of Contemporary Mass Spectrometry (1st ed.). John Wiley & Sons, Inc. pp. 273–275. ISBN 978-0471682295.
- ^ Young, J. P.; Shaw, R. W.; Smith, D. H. (2008). "Resonance ionization mass spectrometry". Analytical Chemistry. 61 (22): 1271A–1279A. doi:10.1021/ac00197a002. ISSN 0003-2700.
- ^ Hurst, G. S. (1975). "Saturated Two-Photon Resonance Ionization of". Physical Review Letters. 35 (2): 82–85. doi:10.1103/physrevlett.35.82.
- ^ Hurst, G.S. (1984). "Historical survey of resonance ionization spectroscopy" (PDF). Second International Symposium on Resonance Ionization Spectroscopy and Its Applications.
- ^ Rimke, Hubertus; Herrmann, Günter; Mang, Marita; Mühleck, Christoph; Riegel, Joachim; Sattelberger, Peter; Trautmann, Norbert; Ames, Friedhelm; Kluge, Hans-Jürgen (1989-05-01). "Principle and analytical applications of resonance lonization mass spectrometry". Microchimica Acta. 99 (3–6): 223–230. doi:10.1007/bf01244676. ISSN 0026-3672.
- ^ a b Hurst, G. S. (1979). "Resonance ionization spectroscopy and one-atom detection". Reviews of Modern Physics. 51 (4): 767–819. doi:10.1103/revmodphys.51.767.
- ^ Hurst, G. S. (1977). "One-atom detection using resonance ionization spectroscopy". Physical Review A. 15 (6): 2283–2292. doi:10.1103/physreva.15.2283.
- ^ Wendt, Klaus; Trautmann, Norbert. "Recent developments in isotope ratio measurements by resonance ionization mass spectrometry". International Journal of Mass Spectrometry. 242 (2–3): 161–168. doi:10.1016/j.ijms.2004.11.008.
- ^ Levine, Jonathan; Savina, Michael R.; Stephan, Thomas; Dauphas, Nicolas; Davis, Andrew M.; Knight, Kim B.; Pellin, Michael J. "Resonance ionization mass spectrometry for precise measurements of isotope ratios". International Journal of Mass Spectrometry. 288 (1–3): 36–43. doi:10.1016/j.ijms.2009.07.013.
- ^ Rimke, Hubertus; Herrmann, Günter; Mang, Marita; Mühleck, Christoph; Riegel, Joachim; Sattelberger, Peter; Trautmann, Norbert; Ames, Friedhelm; Kluge, Hans-Jürgen (1989-05-01). "Principle and analytical applications of resonance lonization mass spectrometry". Microchimica Acta. 99 (3–6): 223–230. doi:10.1007/bf01244676. ISSN 0026-3672.
- ^ Beekman, D. W.; Callcott, T. A. (June 1980). "Resonance ionization source for mass spectroscopy" (PDF). International Journal of Mass Spectrometry and Ion Physics. 34 (1–2): 89–97. Retrieved 6 April 2017.
- ^ Hurst, G. S. (December 1984). Payne, M. G. (ed.). Resonance ionization spectroscopy 1984: invited papers from the Second International Symposium on Resonance Ionization Spectroscopy and its Applications held in Knoxville, Tennessee, USA, on 16-20 April 1984 (1984 ed.). Adam Hilger. ISBN 0854981624.
- ^ "Resonance ionization mass spectrometry for precise measurements of isotope ratios". International Journal of Mass Spectrometry. 288 (1–3): 36–43. 2009-11-01. doi:10.1016/j.ijms.2009.07.013. ISSN 1387-3806.
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