LASER SPECTROSCOPY OF THE $A^{2}\Pi- X^{2}\Sigma^{+}$ TRANSITION OF YTTERBIUM MONOACETYLIDE

The first spectroscopic identification and characterization on of ytterbium monoacety (YbCCH) and two of its isotopomers is reported. By combining resonance enhanced two photon ionization (R2P1), laser induced fluorescence (LIF), and photionization efficiency spectroscopy (PIE) with density functional calculations the $X^{2}\\Sigma^{+}$ and the $A^{2}\\pi_{1/2,3/2}$ slates of YhCCH and the $X^{1}\\Sigma^{+}$ state of $YbCCH^{+}$ have been characterized. The $A^{2}\\Pi - X^{2}\\Sigma^{+}$ system whose 0-0 band for the $A^{2}\\Pi_{1/2}$ component lies at around $16848 cm^{-1}$ for YbCCH (Aso =1411 $cm^{-1}$) has been studied at $0.3 cm^{-1}$ resolution. The excitation spectra, both R2PI and fluorescene, are characterized by progressions and sequences involving the YbCC bending mode, $\\nu_{5}$ whose wavenumber has been determined to be $93 cm^{-1}$ and $103 cm^{-1}$ for the $X^{2}\\Sigma^{+}$ and the $A^{2}\\Pi_{1/2,3/2}$ state, respectively. The dispensed fluorescence spectra show a progression in the ground state YbCC stretching mode with a wavenumber $\\omega(\\nu_{3}) = 328 cm^{-1}$. Density functional calculations confirmed the vibrational assignment and yielded a linear geometry for both the {X} - and {A}- state of YbCCH as well as for the $X^{1}\\Sigma^{+}$ state of the cation. Photoionization efficiency spectroscopy yielded an adiabatic ionization potential of $47165(10) cm^{-1} (5.8477 eV)$. Rydberg series converging to the $v^{+} =1$ and $v^{+} =2$ level of $YbCCH^{+}$ were observed and combined with the ion-appearance potentials led to $\\omega(\\nu_{5}) =97 cm^{-1}$ for the YbCC bending mode in $YbCCH^{+}$. A high resolution study of the $A^{2}\\Pi - X^{-2}\\Sigma^{+}$ system of YbCCH is currently in progress and results may be available at the time of the meeting.