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 Title: SPIN-SPIN AND SPIN-ROTATION FINE STRUCTURE OF THE METASTABLE a 3Σu+ STATES OF MOLECULAR HELIUM Author(s): Jansen, Paul Contributor(s): Merkt, Frederic; Semeria, Luca Subject(s): (Hyper)fine structure, tunneling Abstract: In a recent series of experimentsfootnote{P. Jansen, L. Semeria, L. Esteban Hofer, S. Scheidegger, J.,A. Agner, H. Schmutz, and F. Merkt, textit{Phys. Rev. Lett.} textbf{114}, 133202 (2015).}textsuperscript{,}footnote{L. Semeria, P. Jansen, and F. Merkt, textit{J. Chem. Phys.} textbf{145}, 204301 (2016).}, we have determined term values of all rotational levels of the $X^+,^2Sigma_u^+,(nu^+=0)$ ground vibronic state of $^4$He$_2^+$ with rotational quantum number $N^+leq 19$ at an accuracy of 25 MHz using MQDT-assisted Rydberg-series extrapolation of metastable helium molecules in the $a,^3Sigma_u^+$ state. The precison of these experiments was limited by the 150,MHz linewidth of the pulsed laser system employed. In order to improve our resolution and possibly observe the spin-rotation splitting in the He$_2^+$ ion, we have replaced the pulsed laser by a CW laser system with a bandwidth of 1.5,MHz. This system was used to measure the spin-spin and spin-rotation fine structure of metastable He$_2$ in the $a,^3Sigma_u^+$ $(nu''=0)$ state. Metastable helium molecules were produced by striking a discharge in an expansion of neat helium gas. By cooling the source to a temperature of 10,K, the velocity of the molecular beam was reduced to 500,m/s and an experimental Doppler-limited linewidth of 25,MHz was observed. Fine-structure splittings for all rotational levels with $N''leq 27$ have been measured at an accuracy of 5,MHz and, when possible, have been compared to the values reported in earlier investigations.footnote{_x000d_ W. Lichten, M.,V. McCusker, and T.,L. Vierima, textit{J. Chem. Phys.} textbf{61}, 2200 (1974).}textsuperscript{,}footnote{W. Lichten and T. Wik, textit{J. Chem. Phys.} textbf{69}, 98 (1978).}textsuperscript{,}footnote{M. Kristensen and N. Bjerre, textit{J. Chem. Phys.} textbf{93}, 983 (1990).}textsuperscript{,}footnote{I Hazell, A. Norregaard, and N. Bjerre, textit{J. Mol. Spectrosc.} textbf{172}, 135 (1995).} This comparison revealed a discrepancy that increased with increasing values of $N''$. To verify our results, we have recently constructed a variaton of a classical molecular-beam magnetic-resonance setup that uses a multistage Zeeman decelerator and a RF stripline for de- and repopulation of the $F_2$ spin-rotational components with $J''=N''$, respectively. Issue Date: 6/23/2017 Publisher: International Symposium on Molecular Spectroscopy Citation Info: APS Genre: CONFERENCE PAPER/PRESENTATION Type: Text Language: English URI: http://hdl.handle.net/2142/96940 DOI: 10.15278/isms.2017.FB10 Date Available in IDEALS: 2017-07-27
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