Looking into temporal dynamics of the reactive flux that is precisely located at the well-characterized conical intersection has been one of chemists' longstanding goals. We report here real-time nonadiabatic bifurcation dynamics in the S-CH bond predissociation of thioanisole (CHSCH) in the first electronically excited state (S). It is found that two distinct adiabatic and nonadiabatic reaction pathways are activated simultaneously only when the vibronic state near the first conical intersection is optically accessed. Our time-resolved measurement of the product state distribution could separate two different dynamic channels unambiguously, unraveling the detailed dynamic mechanism of the nonadiabatic reaction taking place in the vicinity of the conical intersection. The nonadiabatic channel, where the reactive flux funnels through two consecutive conical intersections along the reaction coordinate, is found to be significantly faster than the adiabatic channel along the minimum energy reaction pathway. The kinetic energy release ratio and the nonadiabatic transition probability are found to be much higher for the nonadiabatic channel than those of the adiabatic channel, giving insights into the bifurcation dynamics occurring at the conical intersection.
The excess electron in the dipole-bound state (DBS) of the anion is found to be recaptured into the excited valence orbital localized at the positive end of the dipole, leading to the chemical bond cleavage of the anion. In the DBS of the 4-iodophenoxide anion, the extremely loosely bound electron (binding energy of 53 cm −1 ) is recaptured into the πσ* valence orbital, which is repulsive along the C−I bond extension coordinate, leading to the iodide (I − ) and phenoxyl diradical (•C 6 H 4 O•) channel at the asymptotic limit. This is the first real-time observation of the state-specific relaxation (other than autodetachment) dynamics of the DBS and subsequent chemical reaction. The lifetime of the 4-iodophenoxide DBS at its zero-point energy (ZPE), which is measured for the cryogenically cooled trapped anion using the picosecond laser pump−probe scheme, has been estimated to be ∼9.5 ± 0.3 ps. Quantum mechanical calculations support the efficient transition from the DBS (below the detachment threshold) to the low-lying πσ* valence orbital of the first excited state of the anion. Similar experiments on 4-chlorophenoxide and 4-bromophenoxide anions indicate that the electron recaptures into excited valence orbitals hardly occur in the DBS of those anions, giving the long lifetimes (≫ns) at ZPE, suggesting that the internal conversion to S 0 may be the major relaxation pathway for those anions.
The autodetachment dynamics of vibrational
Feshbach resonances
of the quadrupole-bound state (QBS) for the first time has been investigated
in real time for the first excited state of the 4-cyanophenoxide (4-CP)
anion. Individual vibrational resonances of the cryogenically cooled
4-CP QBS have been unambiguously identified, and their autodetachment
rates state-specifically measured using the picosecond time-resolved
pump–probe technique employing the photoelectron velocity-map
imaging method. The autodetachment lifetime (τ) is found to
be strongly dependent on mode, giving τ values of ∼56,
∼27, and ≤2.8 ps for the 12′1 (E
vib = 406 cm–1), 12′2 (E
vib = 806 cm–1), and 21′1 (E
vib =
220 cm–1) modes, respectively. The striking mode-specific
behavior of the QBS lifetime has been invoked by the physical model
in which the loosely bound electron falls off by the dynamic wobbling
of the three-dimensional quadrupole moment ellipsoid associated with
the corresponding vibrational motion in the autodetachment process.
Real-time autodetachment dynamics of the loosely-bound excess electron from the vibrational Feshbach resonances of the dipole-bound states (DBS) of 4-bromophonoxide (4-BrPhO-) and 4-chlorophenoxide (4-ClPhO-) anions have been thoroughly investigated. The...
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