States of B 2 symmetry, on the other hand, do not align well with the experiment. The first such peak, close to the 12.64 eV mark in Figure , is assigned to the A 1 (0,0,1) state. This is a vibronic state that gains intensity owing to the coupling to the electronic 2 B 2 state. This vibronic peak lies in the part of the spectrum where bands of oxygen (which is an impurity in the gas sample) are visible in the experiment. The origin of the 2 B 2 state is very similar to the uncoupled B 2 (0,0,0) state, but it is located in an empty area of the experimental spectrum. We use the difference in the energy between this peak and the origin peak to report the adiabatic energy gap between the two cationic states, which, according to our simulation, is T 00 =1,368 cm -1 . This value is reported with an error estimate equal to the error estimate for the vertical excitation energy gap (65 cm -1 ), which is listed Table .