It is worth noting that a valence-type anion is formed initially upon photoexcitation. However, a large dipole moment of the neutral core, which is greater than the
critical value, 2.5 D [56], may also support the so-called dipole-bound states, which
are governed solely by the long-range electrostatic interactions. A remarkable
feature of the GFP chromophore is that its radical neutral core has a very large
dipole moment. Our estimates based on the extended multiconfigurational quasidegenerate perturbation theory [57] give a value of 7 D. Therefore, such bound
states might also occur in the case of the GFP chromophore anion. Being close to
the detachment threshold, the dipole-bound states reside in the same energy domain
as the S 1 state. Experimentally, a direct population of these states is unlikely due to
a negligible oscillator strength for such transitions compared to that of the bright
S 0 !S 1 transition. However, these states may serve as doorways into the PD
channel by bridging the valence-type bound S 1 state to the neutral D 0 plus an
unbound electron state. Importantly, the ion-dipole interaction also has a crucial
impact on the threshold behavior of PD cross-sections. One might remember that
even the leading term of the threshold function alters from the well-known form of a
Wigner law [58], which presumes that final-state forces are of a finite range
vanishing faster than r
À2 . Therefore, the dipole potential has to be taken into
account while describing and simulating VAD spectral profiles [59].
5.4.2 Internal Conversion
In photochemistry of polyatomic molecules, nuclear relaxation through conical
intersections plays a crucial role in their fast electronic de-excitation [60, 61].
Upon such non-adiabatic transitions, the particular nuclear motion induces a coupling between two adiabatic surfaces that eventually cross. The intersecting
hypersurfaces constitute a dynamical funnel for the most efficient electron-tonuclei energy transfer in radiationless decays. In light-sensitive proteins, such a
coupling makes the primary step in vision the fastest known biological process in
nature, where nuclear rearrangements along the reaction pathway occur within
200 fs [62].
In the case of the GFP chromophore anion, femtosecond time-resolved photoelectron spectroscopy has been used to study the excited-state dynamics following
excitation of the S 1 state at 500 nm (2.48 eV) and probing it with 800 nm (1.55 eV)
[63]. The excited-state population is found to decay bi-exponentially, with the
characteristic lifetimes of (sub)picoseconds. Such an ultrafast decay, attributed to
internal conversion that is a dominant channel at low excitation energies [36],
indicates that conical intersection(s) might be involved in radiationless de-excitation.
Indeed, high-level ab initio calculations within the XMCQDPT2 multistate
multi-reference perturbation theory [57] reveals two non-equivalent pathways in
IC, where the S 1 and S 0 surfaces approach each other upon twisting in the central
bridge moiety of the chromophore. Remarkably, only one of them directly leads to a
conical intersection seam, energetically lying below the planar minimum in S 1 . It is
mediated by twisting about the bond adjacent to the heterocyclic ring and formally
90
A.V. Bochenkova and L.H. Andersen
critical value, 2.5 D [56], may also support the so-called dipole-bound states, which
are governed solely by the long-range electrostatic interactions. A remarkable
feature of the GFP chromophore is that its radical neutral core has a very large
dipole moment. Our estimates based on the extended multiconfigurational quasidegenerate perturbation theory [57] give a value of 7 D. Therefore, such bound
states might also occur in the case of the GFP chromophore anion. Being close to
the detachment threshold, the dipole-bound states reside in the same energy domain
as the S 1 state. Experimentally, a direct population of these states is unlikely due to
a negligible oscillator strength for such transitions compared to that of the bright
S 0 !S 1 transition. However, these states may serve as doorways into the PD
channel by bridging the valence-type bound S 1 state to the neutral D 0 plus an
unbound electron state. Importantly, the ion-dipole interaction also has a crucial
impact on the threshold behavior of PD cross-sections. One might remember that
even the leading term of the threshold function alters from the well-known form of a
Wigner law [58], which presumes that final-state forces are of a finite range
vanishing faster than r
À2 . Therefore, the dipole potential has to be taken into
account while describing and simulating VAD spectral profiles [59].
5.4.2 Internal Conversion
In photochemistry of polyatomic molecules, nuclear relaxation through conical
intersections plays a crucial role in their fast electronic de-excitation [60, 61].
Upon such non-adiabatic transitions, the particular nuclear motion induces a coupling between two adiabatic surfaces that eventually cross. The intersecting
hypersurfaces constitute a dynamical funnel for the most efficient electron-tonuclei energy transfer in radiationless decays. In light-sensitive proteins, such a
coupling makes the primary step in vision the fastest known biological process in
nature, where nuclear rearrangements along the reaction pathway occur within
200 fs [62].
In the case of the GFP chromophore anion, femtosecond time-resolved photoelectron spectroscopy has been used to study the excited-state dynamics following
excitation of the S 1 state at 500 nm (2.48 eV) and probing it with 800 nm (1.55 eV)
[63]. The excited-state population is found to decay bi-exponentially, with the
characteristic lifetimes of (sub)picoseconds. Such an ultrafast decay, attributed to
internal conversion that is a dominant channel at low excitation energies [36],
indicates that conical intersection(s) might be involved in radiationless de-excitation.
Indeed, high-level ab initio calculations within the XMCQDPT2 multistate
multi-reference perturbation theory [57] reveals two non-equivalent pathways in
IC, where the S 1 and S 0 surfaces approach each other upon twisting in the central
bridge moiety of the chromophore. Remarkably, only one of them directly leads to a
conical intersection seam, energetically lying below the planar minimum in S 1 . It is
mediated by twisting about the bond adjacent to the heterocyclic ring and formally
90
A.V. Bochenkova and L.H. Andersen