The effect of the bromine atom on the transitions related to the benzene ring is small, as can be seen by comparing the 1sC→π* transition in the neutral Bz and BrBz x-ray spectra, which are similar. The same conclusion can be drawn from fluorinated benzene x-ray spectrum as mentioned previously, as a change in the number of the fluorine atoms had no effect on the energy of the 1sC→π* transition. Therefore, the major effect of the bromine atom should be related to the C*-Br bond, in particular, on the transition to the orbitals related to this bond, for example 1sC*→σ*(C*-Br). Above 287 eV, the experimental spectrum of the cation is dominated by the split-band J1, J2. According to our calculations, the most intense transition in this region is due to 1sC*→σ*(C*-Br), which in neutral BrBz corresponds to band C. We note that after ionization a major change of the cation's ground-state structure relative to the neutral ground state BrBz appears to be the C*-Br bond length, which changes from 1.9189 Å to 1.8439 Å (according to our calculated geometries obtained at the EOM-CCSD/aug-cc-pVTZ level, see in our supplementary file). This could be one of the reasons for the dramatic change in transition intensities for peaks J1, J2 compared to peak C and D in neutral BrBz. Moreover, the transition to a σ*(C*-Br) orbital may be affected by vibrational motion of the C*-Br bond. Two-photon excitation imparts vibrational excitation to the cation, even though the exact determination of the excited vibrational state is difficult due to the low spectral resolution of the experimental system, the spectral width of the UV pulses, and due to the high density of the vibrational states at 9.24 eV. However, since the two-photon ionization process is sensitive to the vibrational mode(s) of the (intermediate) electronic excited state of neutral BrBz reached by the first photon excitation, 1 A1 → 1 B2 (ππ*), where the density of possible vibrational states is lower (in the region reached by one photon), it is possible to predict which vibrational modes of the cation are activated. The transition from the electronic ground state to the ground vibrational state of 1 B2 is 4.586 eV. With the first photon (4.64 ± 0.08 eV) it is thus possible to reach the vibrational ground state and excite the C6H5-Br stretching mode (v6a = 0.03645 eV) of the electronic intermediate state ( 1 B2). It is likely that excited vibrational states of the cation involve the C6H5-Br stretching mode (v6a at 0.047 eV for BrBz + in the electronic ground state) alone or in combination with other modes, as shown in One-Photon Mass-Analyzed Threshold Ionization (MATI) spectra. The excited C*-Br stretching mode may influence the x-ray spectrum by shifting 1sC*→σ*(C*-Br) transition energies and their Preparation of BrBz + in the electronic ground state and exclusion of possible contributions due to both one-photon excitation of neutral BrBz and photodissociation processes, as well as three-photon electronically-excited cations, are expected to be small enough compared to the discussed observations of this work, similar to our previous study with benzene. The cross section for the intermediate electronic resonance state [ 1 A1 → 1 B2 (ππ*), 4.586 eV] of BrBz with one photon (266.8 nm) is 3 x 10 -19 (cm 2 /molecule), 11 slightly greater than in Bz, (10 -20 -10 -21 cm 2 /molecule), but the major process, driven by fs pulses around 266 nm, is REMPI (1+1). Preparation of excited electronic states of BrBz + may be possible only by transferring an electron from a lower orbital and requiring 3 photons, as shown in the schematic orbital diagram of neutral BrBz in Figure . Moreover, excitation from the lower orbital with a 267 nm photon is below the first resonance (excitation to first electronic state) and therefore expected to be a significantly less probable process. From this we conclude that the major probed molecules by the x-ray flux are bromobenzene cations in the ground electronic state.