Reinterpretation of the resonant two-photon ionization spectrum of 1-methylphenylhydrazine

The resonant two-photon ionization (R2PI) spectrum of 1-metylphenylhydrazine (MPH) reported in our previous paper (J. Mol. Struct. 984 (2010) 307–315) is in fact the R2PI spectrum of N-methylaniline (MAN). One-photon VUV ionization mass spectrum shows that the peak of MPH+ ions is predominant while the peak of MAN+ ions is very weak, however, the two-photon UV ionization mass spectrum shows that the peak of MPH+ ions is very weak while the peak of MAN+ ions is very strong. The difference between the two mass spectra reveals that the MPH in the first electronically excited (S1) state may dissociate quickly, giving MAN. The photodissociation of MPH makes it impossible to scan the R2PI spectrum of MPH with the nanosecond UV laser. The UV absorption spectra of MPH and MAN in solutions show that the maximum absorption related to the S1 ← S0 transition of MPH is blue-shifted with respect to that of MAN, supporting the conclusion of the paper mentioned above that, for MPH in the S1 state, the non-planar structure with p–π conjugation is stable, but the planar structure with p–p–π conjugation can hardly exist due to the steric effect.