Evidence for the phantom state in photoinduced cis–trans isomerization of stilbene

Transient absorption spectra of cis-stilbene in solution, recorded in the range 275–690 nm with 80 fs instrument response, reveal two successive evolving bands. One at 630 nm decays on a subpicosecond scale; it corresponds to vertically-excited nearly planar S1 state. The other, growing and decaying band at 340 nm, is shown to be consistent with theoretically predicted phantom state p∗ of perpendicular molecular geometry. The lifetime of p∗ is determined to be τ p ∗ = 0.4 ps in acetonitrile and 0.3 ps in hexane.