calculated using a time-domain formalism based on Fourier transforms of the Lax’s
autocorrelation function within the double harmonic parallel-mode approximation
[48]. Geometry parameters and vibrational frequencies in the ground electronic
states of the anion and neutral radical were found using the PBE0/(aug)-cc-pVDZ
functional in the frame of the Firefly quantum chemistry package [49]. As the
Frank-Condon envelope only determines the shape of the electron energy distribution and not the absolute position in energy, the calculated curve was shifted to
match the experimental data. Indeed, good correspondence between experiment
and theory can be made. The resulting VDE is determined to be 2.68 Æ 0.1 eV,
0.04 eV lower than obtained from the peak position of the experimental data in
Fig. 5.13. This value is slightly lower than the recently reported value of 2.85 Æ 0.1
eV [34] and 2.8 Æ 0.1 eV [33], but within the combined error bars.
Combining the information from the time-resolved action spectra and the photoelectron spectra, we conclude that the S 1 anionic state is a bound excited state
with a binding energy of only ~0.1 eV. Photodetachment in the spectral region of
this state proceeds by indirect (resonant) excitation to S 1 followed by vibrational
autodetachment into the continuum of the neutral D 0 ground state. Internal energy
residing in the chromophore makes electron emission possible even below the
vertical detachment energy, as evident from the action spectra at long wavelengths.
In regions without excited anion states, detachment proceeds directly into the
electronic continuum. At the wavelength of maximum action (482 nm) we estimate
that the branching ratio for internal conversion is about an order of magnitude
higher than that for electron emission. At first sight this might seem surprising as
electronic dynamics normally proceeds faster than nuclear dynamics. The reason
for the ‘slow’ electron emission is that it requires a non-adiabatic coupling with the
nuclear motion. In the next sections we will look into the nature of the nuclearelectron dynamics, which is operative in the GFP chromophore anion.
5.4
Theoretical Account of Photo-initiated Dynamics
5.4.1 Coupling to the Electronic Continuum
Negative ions hold a special position in atomic and molecular physics, where
electron correlation plays a crucial role for their stability. This gives this class of
objects special properties like low electronic binding energies. Provided that the
first excited state of the GFP chromophore anion is very close to the electronic
continuum, it is of particular interest to access the nature of the coupling between
them. The S 0 ! S 1 transition has a well-established π À π
∗ character. The nature
of the first excited state with respect to the corresponding detachment threshold is of
utmost importance, since it predefines the fate and the lifetimes of the chromophore
upon photoexcitation, and until recently it has been largely debated [33–35, 50]. To
address this problem theoretically, highly accurate and highly-correlated quantum
chemistry methods along with extended basis sets are required.
86
A.V. Bochenkova and L.H. Andersen
autocorrelation function within the double harmonic parallel-mode approximation
[48]. Geometry parameters and vibrational frequencies in the ground electronic
states of the anion and neutral radical were found using the PBE0/(aug)-cc-pVDZ
functional in the frame of the Firefly quantum chemistry package [49]. As the
Frank-Condon envelope only determines the shape of the electron energy distribution and not the absolute position in energy, the calculated curve was shifted to
match the experimental data. Indeed, good correspondence between experiment
and theory can be made. The resulting VDE is determined to be 2.68 Æ 0.1 eV,
0.04 eV lower than obtained from the peak position of the experimental data in
Fig. 5.13. This value is slightly lower than the recently reported value of 2.85 Æ 0.1
eV [34] and 2.8 Æ 0.1 eV [33], but within the combined error bars.
Combining the information from the time-resolved action spectra and the photoelectron spectra, we conclude that the S 1 anionic state is a bound excited state
with a binding energy of only ~0.1 eV. Photodetachment in the spectral region of
this state proceeds by indirect (resonant) excitation to S 1 followed by vibrational
autodetachment into the continuum of the neutral D 0 ground state. Internal energy
residing in the chromophore makes electron emission possible even below the
vertical detachment energy, as evident from the action spectra at long wavelengths.
In regions without excited anion states, detachment proceeds directly into the
electronic continuum. At the wavelength of maximum action (482 nm) we estimate
that the branching ratio for internal conversion is about an order of magnitude
higher than that for electron emission. At first sight this might seem surprising as
electronic dynamics normally proceeds faster than nuclear dynamics. The reason
for the ‘slow’ electron emission is that it requires a non-adiabatic coupling with the
nuclear motion. In the next sections we will look into the nature of the nuclearelectron dynamics, which is operative in the GFP chromophore anion.
5.4
Theoretical Account of Photo-initiated Dynamics
5.4.1 Coupling to the Electronic Continuum
Negative ions hold a special position in atomic and molecular physics, where
electron correlation plays a crucial role for their stability. This gives this class of
objects special properties like low electronic binding energies. Provided that the
first excited state of the GFP chromophore anion is very close to the electronic
continuum, it is of particular interest to access the nature of the coupling between
them. The S 0 ! S 1 transition has a well-established π À π
∗ character. The nature
of the first excited state with respect to the corresponding detachment threshold is of
utmost importance, since it predefines the fate and the lifetimes of the chromophore
upon photoexcitation, and until recently it has been largely debated [33–35, 50]. To
address this problem theoretically, highly accurate and highly-correlated quantum
chemistry methods along with extended basis sets are required.
86
A.V. Bochenkova and L.H. Andersen