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\title{THE NEWLY SET-UP SPECTROSCOPIC MOLECULAR BEAM APPARATUS: \\ APPLICATIONS TOWARDS MULTIPHOTON RESONANCE}

\author{\underline{\FirstName J. \LastName JAKOB}, \underline{\FirstName V. \LastName GRIGORIAN}, \underline{\FirstName S. \LastName SCHWETJE}, {\FirstName M. A. \LastName KAUFMANN}, {\FirstName A. A. \LastName BREIER}, {\FirstName G. W. \LastName FUCHS} and {\FirstName T. F. \LastName GIESEN}}
\address{Laborastrophysik, Universit{\"a}t Kassel, Heinrich-Plett-Stra{\ss}e 40, 34132 Kassel, Germany}


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\fancyhead[C]{{\footnotesize \textit{HRMS, 28th Colloquium, Dijon, France,
  Aug. 28 - Sept. 1, 2023}}}
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\index{REMPI}
\index{Mass-selective}
\index{Double-resonance}
\index{optical}
\index{UV}
\index{Mid infrared}
\index{Microwave}
\index{Millimetre Wave}
\index{Chirality}
\index{Three-Wave Mixing}



Reconciling high spectral resolution and high sensitivity simultaneously is a daunting task. Employing the double resonance technique using a combination of microwave (MW) and infrared (IR) radiation, we have previously shown that we can transfer the precision of the MW radiation into vibrationally excited states and therefore improve the spectral resolution by up to three orders of magnitude.\footnote{J. Jakob, ``High-Resolution Infrared-Microwave Two-Photon Spectroscopy'', Master's Thesis, Laboratory Astrophysics - Universit{\"a}t Kassel (2021)}

Now, we present a new molecular beam apparatus that allows high-resolution infrared and optical spectroscopy of cold neutral molecules, detected via resonant multiphoton ionization time-of-flight (REMPI-ToF) mass spectroscopy. A pulsed nozzle produces cold molecules in an adiabatically expanding supersonic jet. Two skimmers reduce the molecular flow to a molecular beam. We use a high-power (up to 1.5~W) continuous-wave optical parametric oscillator (cwOPO) from \textit{Qioptiq} for a narrowband excitation ahead of the REMPI-ToF detection. The narrow linewidth of the cwOPO (10~-~20~kHz) in the mid-infrared (2.7~-~4~$\mu$m) and it's frequency-stability ($\pm$~3~MHz) ensures for ro-vibrational resolution. The subsequent detection with REMPI provides excellent detection sensitivity even for extremely small samples in addition to mass selectivity. 

In a future perspective, multiphoton excitation via IR light and MW radiation in the form of three-wave mixing (3WM) schemes\footnote{\href{https://doi.org/10.1063/1.5097406}{doi:10.1063/1.5097406}, M. Leibscher; T. F. Giesen; C. P. Koch, ``Principles of enantio-selective excitation in three-wave mixing spectroscopy of chiral molecules'', J. Chem. Phys., \textbf{151}, 014302 (2019)} or by photo-electron circular dichroism (PECD)\footnote{\href{https://doi.org/10.1002/cphc.201402643}{doi:10.1002/cphc.201402643}, C. Lux; M. Wollenhaupt; C. Sarpe; T. Baumert, ``Photoelectron Circular Dichroism of Bicyclic Ketones from Multiphoton Ionization with Femtosecond Laser Pulses'', ChemPhysChem, \textbf{16}, 115-137 (2014)}, will allow to distinguish the enantiomers of chiral molecules in a cold molecular beam.  









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