The photoionization spectrum of diphenylamine provides an unusual and interesting illustration of the Franck–Condon principle. Diphenylamine (DPA), illustrated in Figure 17.1, is a relatively large molecule to study by gas phase spectroscopy and it might be thought that the vibrational structure in its electronic spectra would be highly congested and difficult to interpret. After all, this is a molecule with 66 vibrational modes! However, it was shown in Section 7.2.3 that only totally symmetric modes generally need to be considered in interpreting electronic spectra. Also, there is the further simplification that not all of the totally symmetric modes need be Franck–Condon active, i.e. will give a significant progression. DPA is an excellent example of this, with the main structure arising from a single vibrational mode.

Before spectra are considered, the experimental procedure, carried out by Boogaarts and co-workers [1], will be outlined. Mass-analysed threshold ionization (MATI) spectroscopy was employed. This technique, which was briefly described in Section 12.6, is essentially the same as ZEKE spectroscopy but employs ion rather than electron detection. It has the advantage over ZEKE spectroscopy in that ions can be separated according to their mass, which in most cases enables the spectral carrier to be determined with confidence. By analogy with ZEKE spectroscopy, a cation ← neutral molecule electronic absorption spectrum is effectively obtained.