Rydberg and pulsed field ionization-zero electron kinetic energy spectra of YO

A spectroscopic study of the Rydberg states of YO accessed from particular rotational levels of the $A 2Π1/2,$ $v=0$ state has been combined with a pulsed field ionization, zero electron kinetic energy (PFI-ZEKE) investigation. The results provide accurate values of the ionization energy of YO, ionization energy I.E.(YO)=49 304.316(31) cm⁻¹ [6.112 958(4) eV], and of the rotational constant (and bond length) of the $YO+$ cation in its $X 1Σ+,$ $v=0$ ground state, $B0+=0.4078(3) cm−1$ $[r0=1.7463(6) Å].$ The improved value of I.E.(YO) combined with the known ionization energy of atomic yttrium then leads to the result $D00(Y−O)−D00(Y−O)=0.1041±0.0001 eV.$ Combining this result with the value of $D00(Y+−O)$ obtained from guided ion beam mass spectrometry yields an improved value of $D00(Y−O)=7.14±0.18 eV.$ The PFI-ZEKE spectra display an interesting channel-coupling effect so that all rotational levels with $J+⩽J′(A)+0.5$ are observed with high intensity, where $J+$ is the angular momentum of the $YO+$ cation that is produced and $J′(A)$ is the angular momentum of the $A 2Π1/2$ state that is reached when the first photon is absorbed. This is thought to result from the interaction between the dipole moment of the rotating $YO+$ core and the Rydberg electron, which can induce changes in l and $J+$ subject to the dipolar coupling matrix element selection rule, $ΔJ+=±1,$ Δl=±1. The channel-coupling mechanism also appears to induce an inverse autoionization process in which an unbound electron with a low value of l is captured either by its low-$J+$ $YO+$ cation or by a second $YO+$ cation with the same value of $J+.$ This inverse autoionization process is extremely sensitive to the electron kinetic energy, leading to narrow peaks in the PFI-ZEKE spectrum which are only slightly broader than the laser linewidth employed for this study (0.25 cm⁻¹).