High Resolution Spectroscopy

High resolution resonance ionization of rare earth elements

High resolution resonance ionization with narrow single-frequency cw lasers is a well suitable method for spectroscopic investigations of atomic states and ultra-trace determination. Combined with a mass spectrometer for mass selection this method is called RIMS (Resonance Ionization Mass Spectrometry) and can be characterised by extremely high elemental and isotopic selectivity.
Especially rare earth elements (also called lanthanides) are of special interest for spectroscopic and analytical investigations with RIMS. This is due to the special electron configuration of these elements differing only in the electron number of an energetic lower lying 4f orbital. Thus with up to four open shells a very complicated and complex spectrum results. First measurements with RIMS started on the element Gadolinium followed by Samarium. Compared to other lanthanides they can be characterised by a high number of unpaired electrons having quite interesting applications in various fields of medizin, cosmology and environmental physics (1), (2) and (3).

Fig. 1: Isotope shifts in the first resonant transition in Samarium
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Applying multi-step resonance ionization to analytical questions makes a detailed investigation of involved energy levels (e.g.: isotope shift, hyperfine structure, transition strength) necessary. A documentation of measurements done for Gadolinium is given in (1), (4), (5). First results for Samarium are presented and discussed in (3). Figures (1) and (2) show examples of measured isotope shifts and hyperfine splittings in the first resonant transition in Samarium.

Fig. 2: Hyperfine splittings in the first resonant transition in Samarium
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Both Gadolinium and Samarium have a complex continuum structure which can be characterised by strong and narrow autoionising resonances. They are of great interest for an efficient excitation scheme. Figure 3 presents for three different excitation schemes the complete measured Gadolinium continuum structure including the spectral range between the ionization potenzial and the first excited level of the ion ground state multiplett. A detailed analysis of the data is given in (3), (6) and (7).

Fig. 3: Complete measured Gadolinium continuum structure
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A fundamental and for each atom unique property is the ionization energy (IP). Applying the high resolution method RIMS the IP could be determined from the convergence of rydberg series. In the case of Gadolinium series just below the first excited level of the ion ground state multiplett were measured and the IP was determined to IP = 49601.5143 cm-1 with a precision of 0.0008 cm-1 (8).
For Samarium series just below the IP were analysed and the IP was derived to IP = 45519.30793 (43) cm-1 with an optimized precision of four orders of magnitude compared to literature (9). The spectrum is shown in figure 4. A detailed presentation of the analytical description including perturbing resonances which do not belong to the rydberg series and an investigation of the quantum defect depending on n are given in (3) and (9).

Fig. 4: Rydberg spectrum used for determination of the
ionization energy of Samarium - click for bigger version

Literature List

(1): Blaum, K.:
Resonante Laserionisations-Massenspektrometrie an Gadolinium zur Isotopenhäufigkeitsanalyse mit geringsten Mengen
Dissertation, Institut für Physik, Universität-Mainz (2000)
(2): Blaum, K., Geppert, C., Schreiber, W.G., Hengstler, J.G., Müller, P., Nörtershäuser, W., Wendt, K., Bushaw, B.A.:
Trace determination of gadolinium in biomedical samples by diode laser-based muli-step resonance ionization mass spectrometry
Anal Bioanal Chem 372: 759-765 (2002)
(3): Schmitt, A.:
Hochauflösende Resonanzionisationsspektroskopie an Samarium und Gadolinium
Dissertation, Institut für Physik, Universität Mainz (2004)
(4): Blaum, K., Bushaw, B.A., Diel, S., Geppert, C., Kuschnick, A., Müller, P., Nörtershäuser, W., Schmitt, A. and Wendt, K.:
Hochauflösende Resonanzionisationsspektroskopie an Samarium und Gadolinium
Isotope shifts and hyperfine structure in the [Xe]4f7 5d6s2 9DJ -> [Xe]4f7 5d6s6p 9FJ+1 transition of gadolinium
(5): Blaum, K., Bushaw, B.A., Geppert, C., Müller, P., Nörtershäuser, W., Schmitt, A., Trautmann, N. and Wendt, K.:
Hochauflösende Resonanzionisationsspektroskopie an Samarium und Gadolinium
Diode-laser-based resonance Ionization mass spectrometry of gadolinium
(6): A.Schmitt:
Charakterisation of autoionizing resonances in gadolinium
(in Vorbereitung)
(7): Bushwa, B.A., Nörtershäuser, W. and Wendt, K.:
Studies of narrow autoionizing resonances in gadolinium
Spectrochim. Acta B 58, 1083 (2003)
(8): Bushaw, B.A.: Blaum, K. and Nörtershäuser, W.:
Determination of the 160Gd ionization energy
Phys. Rev. A 67, p.022508:1-5 (2003)
(9): Schmitt, A., Bushaw, B., Wendt, K.:
Determination of the 154Sm ionization energy by high-precision laser spectroscopy
J. Phys. B 37, 1633 (2004)