Knowledge Bank

Rotational spectrum of the FeNO radical generated by the ultraviolet photolysis of Fe(CO)$2$(NO)$2$ was measured in the millimeter-wave region. The measurements were performed in the supersonic jet expansion with a millimeter-wave multi-reflection cell. Three rotational transitions ($J=9.5-8.5$ $\sim$ $11.5-10.5$) in the $\Omega =5/2$ spin substate of the $X2\Delta i$ ground vibronic state were measured in the frequency region of 87$-$106GHz.Therotationallinesweresplitinto2components($\Delta F = 0,+1$)duetothehyperfineinteractionoftheN($I = 1$) nucleus. In the upper spin substate $\Omega =3/2$ ($ASO\approx -417cm-1$) of the electronic ground state, seven rotational transitions ($J=28.5-27.5$ $\sim$ $34.5-33.5$) were measured with a conventional absorption cell (2.7 m in length) in the room temperature. Moreover, six rotational transitions ($J=28.5-27.5$ $\sim$ $33.5-32.5$) in the $\nu 2$ vibrationally excited state (for both $P$ = 3/2 and 7/2 components) and the high-$J$ lines of $\Omega =5/2$ spin substate of the ground state were also observed with the conventional absorption cell. Rotational line intensity of the $\Omega =3/2$ substate was about one tenth of that for the $\Omega =5/2$ substate because of the large spin-orbit interaction constant $ASO$. Molecular constants, including the rotational constant $B$, centrifugal distortion constant $D$, hyperfine constant $a+bF/4+c/6$, and vibration rotation constant $\alpha 2$, were determined by a least squares fitting of the observed spectrum. The electronic ground state of FeNO was confirmed to be $X2\Delta i$ as in the case of CoCO and the unpaired electron is localized almost in the 3{\it d} orbital of Fe. The hyperfine constant of FeNO, $a+bF/4+c/6=-1.359(57)$ MHz, is much smaller than that of CoCO, 466.073(54) MHz. Rovibrational transitions were also observed by the infrared diode laser spectroscopy with the ultraviolet photolysis of Fe(CO)$2$(NO)$2$.}