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\title{ACCESSING HIGHER VIBRATIONAL STATES OF He$_2^+$ THROUGH MULTI-STEP EXCITATION}

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\author{\underline{\FirstName M. \LastName HOLDENER},
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\address{Department of Chemistry and Applied Biosciences, ETH Z\"urich, Wolfgang-Pauli-Strasse 10, 8093 Z\"urich, Switzerland}

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\fancyhead[C]{{\footnotesize \textit{HRMS, 28th Colloquium, Dijon, France,
  Aug. 28 - Sept. 1, 2023}}}
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The precise measurement of highly excited vibrational states of small molecular systems such as H$_2^+$, H$_2$ and He$_2^+$ is of significant interest as benchmark to \textit{ab-initio} quantum-chemical calculations \footnote{\href{https://doi.org/10.1103/PhysRevLett.125.213001}{doi:10.1103/PhysRevLett.125.213001}, D\'{a}vid Ferenc \textit{et al.}, Phys. Rev. Lett., \textbf{125}, 213001 (2020)}. In the case of He$_2^+$, only the first few vibrational levels of the electronic ground state X$^+$ $^2\Sigma_u^+$ were precisely measured \footnote{\href{https://doi.org/10.1103/PhysRevLett.124.213001}{doi:10.1103/PhysRevLett.124.213001}, Luca Semeria \textit{et al.}, Phys. Rev. Lett., \textbf{124}, 213001 (2020)}$^,$\footnote{\href{https://doi.org/10.1063/1.5051089}{doi:10.1063/1.5051089}, Paul Jansen \textit{et al.}, J. Chem. Phys., \textbf{149}, 154302 (2018)}. In this poster presentation, we propose a method to access the higher vibrational states of He$_2^+$ X$^+$ $^2\Sigma_u^+$ using a multi-step excitation scheme. Our approach involves the production of a molecular beam of He$_2$ in the long-lived metastable a $^3\Sigma_u^+$ state \footnote{\href{https://doi.org/10.1063/1.455993}{doi:10.1063/1.455993}, Cary F. Chabalowski \textit{et al.}, J. Chem. Phys., \textbf{90}, 2504-2512 (1989)} through an electric discharge. We then utilize a high-intensity laser to promote the system to the electronic state c $^3\Sigma_g^+$ in an excited vibrational level with v in the range $3-5$. This state predominantly decays radiatively to the a $^3\Sigma_u^+$ state with $v' = v$ because of favorable Frank Condon factors. A second laser is then employed to induce a transition from the vibrationally excited metastable state a $^3\Sigma_u^+(v')$ to the ion state X$^+$ $^2\Sigma_u^+(v^+ = v')$. This method offers the prospect of studying the structure and dynamics of highly excited vibrational levels of He$_2^+$ that are typically challenging to access.



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