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tokens: the uncertainties of the PFI-PE peak energies. We find that the PFI-PE energy resolution achieved in the present PFI-PE measurement is within the FWHM range of 1.5–2.0 cm −1 for the PFI-PE bands, indicating that these PFI-PE bands have minor perturbations by near resonance autoionizing Rydberg states. Table 1 Comparison of Ionization Energies (IEs) in cm −1 Obtained from the Present VUV–PFI–PE and VUV–PIE Rydberg Series Measurements and Those Calculated Using Known Spectroscopic and IE Data Notes. a Obtained from the present PFI-PE measurement. b Obtained from Page Gudeman ( 1990 ). c Calculated using known spectroscopic data and the IE value from the literature (see the text). d Obtained based on the present PIE Rydberg series measurement. The observed PFI-PE bands are also depicted in Figure 1 as the lower stick spectra. However, since the relative intensities of these PFI-PE peaks have not been measured carefully, we have arbitrarily set all four PFI-PE bands of the Co atom to have the same intensity. The ionization energy (IE) of the Co atom was previously determined to be 7.8810 eV (63,564.5 cm −1 ) by Page Gudeman ( 1990 ) by employing the double-resonance fluorescence-dip technique. The latter IE value is found to be in excellent agreement with the lowest PFI-PE peak at 63,564.0 ± 1.5 cm −1 observed in the present study. According to the NIST atomic database (Kramida et al. 2016 ), the four lowest spin–orbit levels of neutral Co are 4 F 9/2 , 4 F 7/2 , 4 F 5/2 , and 4 F 3/2 (in ascending order), and the four lowest spin–orbit levels of the Co + ion are 3 F 4 , 3 F 3 , 3 F 2 , and 5 F 5 (in ascending order). The energies of the 4 F 7/2 , 4 F 5/2 , and 4 F 3/2 excited levels of neutral Co measured with respect to the ground 4 F 9/2 state of Co have been reported to be 816.00, 1406.84, and 1809.33 cm −1 , respectively. The energies of the 3 F 3 , 3 F 2 , and 5 F 5 excited spin–orbit levels of Co + measured with respect to the ground 3 F 4 state of Co + have also been determined to be 950.51, 1597.32, and 3350.58 cm −1 , respectively. Thus, the IE of Co observed at 63,564.5 cm −1 can be assigned as the photoionization transition Co(3d 7 4s 2 4 F 9/2 ) Co + (3d 8 3 F 4 ). This IE[Co( 4 F 9/2 ) Co + ( 3 F 4 )] value, along with the known spacings of the spin–orbit states for Co and Co + , allows the calculation of the photoionization transition energies from the 4 F 9/2 , 4 F 7/2 , 4 F 5/2 , and 4 F 3/2 states of the Co atom to excited spin–orbit levels 3 F 3 , 3 F 2 , and 5 F 5 of the Co + ion. We found that such IE calculations predict IE[Co( 4 F 9/2 ) Co + ( 3 F 3 )] = 64,515 cm −1 for Co(3d 7 4s 2 4 F 9/2 ) Co + (3d 8 3 F 3 ), IE[Co( 4 F 9/2 ) Co + ( 5 F 5 )] = 66,915.1 cm −1 for Co(3d 7 4s 2 4 F 9/2 ) Co + (3d 7 4s 1 5 F 5 ), and IE[Co( 4 F 7/2 ) Co + ( 3 F 3 )] =63,699.0 cm −1 for Co(3d 7 4s 2 4 F 7/2 ) Co + (3d 8 3 F 3 ). These calculated IE values for the photoionization transitions are included in Table 1 to compare with the