Dynamics of electron ejection on photoionization of trans-stilbene and biphenyl in acetonitrile as observed with femtosecond time-resolved near-IR absorption spectroscopy
Photoionization in solution is a basic but complex phenomenon involving a solute, an ejected electron and surrounding solvent molecules. It may seem obvious that an electron is released immediately after the parent molecule is excited to an electronic state that directly leads to the electron dissociation. However, it has been reported that the radical cations are formed in 17 ps, 24 ps, and 38 ps for trans-stilbene and 20 ps for biphenyl, based on time-resolved Raman and visible absorption measurements. For understanding this intriguing phenomenon, we observe the solvation process of electrons ejected from trans-stilbene and biphenyl with femtosecond time-resolved near-IR spectroscopy covering 900 to 1550 nm. We find that the near-IR absorption signals of the ejected electrons rise in 0.28 ± 0.01 ps for trans-stilbene and 0.33 ± 0.04 ps for biphenyl. The parent molecules release electrons in about 0.3 ps, not in a few tens of picoseconds, after the photoirradiation. The delayed appearance of the radical cation signals strongly suggests that the radical cation is formed initially in a highly excited state, electronically and vibrationally, that would not give a clear signal of Raman or absorption transitions. It then relaxes to the radical ground state in a few tens of picoseconds. We clarify the electron dissociation process associated with the photoionization of aromatic molecules with fast time-resolved spectroscopy.