Chemical Physics Letters 418 (2006) 569–575
www.elsevier.com/locate/cplett
Theoretical CASPT2 study of the excited state double proton
transfer reaction in the 7-azaindole dimer
Luis Serrano-Andrés *, Manuela Merchán
Instituto de Ciencia Molecular, Universitat de València, Dr. Moliner 50, Burjassot, ES-46100 Valencia, Spain
Received 10 October 2005; in final form 2 November 2005
Available online 1 December 2005
Abstract
Accurate CASPT2 calculations on the excited state double proton transfer reaction in the 7-azaindole dimer have been performed on
different symmetric and asymmetric pathways along the protons interchange. The presence of a conical intersection connecting the initially photoinduced singlet excited state with a charge transfer state leading both to asymmetric ionic and neutral structures promotes a
step-wise mechanism, probably taking place through the ionic intermediate. The concerted path is computed slightly higher in energy in
the gas phase.
Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction
The 7-azaindole dimer (AD) undergoes a photoinduced
tautomerization process in which the hydrogens attached
to the pyrrolic units are transferred to the pyridinic units
leading to the 7-azaindole tautomer (TD). The reaction
has been described to yield double fluorescence attributed
to the locally excited states of both tautomers before and
after the photoreaction, and has been since long known
both in condensed phases [1,2] and in the vapor [3] (see
Fig. 1). Femtosecond experiments [4,5] suggested that the
reaction proceeds in the excited state through an ionic
asymmetric intermediate by means of the two-step mechanism displayed in Fig. 1, with measured lifetimes in molecular beams 0.65 and 3.3 ps [4]. Those conclusions were
strongly questioned, and the impossibility of a symmetry
breaking along the reaction path has been postulated as
an argument in favor of a symmetric, concerted process
[6]. The elucidation of the precise reaction mechanism
has been a matter of an agitated and fierce debate defending either the concerted [6–9] or the asymmetric, step-wise
*
Corresponding author. Fax: +34 963543156.
E-mail address: [email protected] (L. Serrano-Andrés).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.11.041
[10–14] pathways. In the present contribution, we describe
CASSCF/CASPT2 calculations on the tautomerization
mechanism through three different paths: a symmetric evolution representing the one-step concerted reaction, and
two asymmetric pathways related to the two-step mechanisms in which one proton is first transferred to one of
the 7-azaindole monomers, while the second is transferred
back to the opposed moiety in a subsequent step. Special
emphasis has been put along the years on the need of accurate theoretical descriptions. We will focus here on the theoretical aspects of the reaction, computed in the present
Letter at a level of theory much more reliable than previous
calculations [6,14].
2. Methods
The quantum-chemical methodology employed in the
present work comprises the well-established and accurate
[15–17] complete active space self-consistent field
(CASSCF) method [18], combined with a multi-state second-order perturbation approach, the PMCAS-CI/MSCASPT2 procedure [15,19]. Geometries of the ground
and excited states of the 7-azaindole dimer (AD), the 7azaindole dimer tautomer, 7-H-pyrrolo[2,3-b]pyridine,
(TD), and different reaction intermediates (DI), including
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L. Serrano-Andrés, M. Merchán / Chemical Physics Letters 418 (2006) 569–575
N2
H
N3
N1
H
DITS
N4
Symmetric
C2h Path
N2
H
ADS1
N3
N1
N2
H
DII
N4
Asymmetric
Cs Path
Ionic
H
H
N3
N1
N4
N2
H
N3
hν
F1
N2
Asymmetric
Cs Path
Neutral
N1
H
ADS0
N2
H
N3
TDS1
H
N4
N1
DIN
H
F2
N4
Dimer Reaction
Intermediates
DI
N2
H
N3
H
N3
N1
N1
TDS0
H
N4
N4
Tautomer Dimer
TD
7-Azaindole Dimer
AD
Fig. 1. Scheme of the explored paths in the excited state double proton transfer tautomerization of 7-azaindole dimer.
energy minima, transition states, and conical intersections,
have been optimized at the CASSCF level using a 631G(dp) basis set and an active space of 12 active electrons
in 12 active orbitals (12, 12), including four rr* and eight
pp* orbitals and electrons. At the optimized geometries, a
larger active space adding two inner p orbitals, (16, 14),
and a larger (ANO) type [20] one-electron basis set used
previously [21], including C,N(14s9p4d)/H(8s4p3d) (1518)
primitive gaussians contracted to C,N[3s2p1d]/H[2s1p]
(312) functions, were used to generate the final wave functions for five computed roots. The geometry optimizations
have been restricted to planarity, that is, the system keeps
the Cs symmetry, except for the symmetric intermediate,
DITS, in which the C2h symmetry was employed. Oscillator
strengths and radiative lifetimes were obtained as described
elsewhere [22]. The calculations employed the MOLCAS-5
[23] and GAUSSIAN98 [24] suits of programs.
3. Results and discussion
3.1. Geometries
Fig. 2 contains the main bond distances and angles for
the structures optimized in the present study. Six of them
have been computed as minima, corresponding to the following cases: ADS0 and TDS0 represent the 7-azaindole
dimer and tautomer ground state structures (becoming
C2h), respectively; ADS1 and TDS1 are the corresponding
L. Serrano-Andrés, M. Merchán / Chemical Physics Letters 418 (2006) 569–575
571
Fig. 2. Bond lengths and labeling for the computed structures.
excited singlet S1 states Cs minima; DII(S1), an ionic (with
monomers charged positive and negative, see Fig. 1) dimer
reaction intermediate obtained as the minimum of the third
SA-CASSCF root (S2, becoming S1 at the MS-CASPT2
level), and, finally, DIN(S1), a neutral reaction intermediate
also obtained as a low-energy minimum in the S1 hypersurface. Those two intermediates have an asymmetric Cs
structure. A minimum energy crossing point (MECP) or
lowest-lying conical intersection was computed connecting
the S1 and S2 hypersurfaces, DICI(S1/S2). Finally, the DITS
conformation was obtained as a transition state structure
in the S1 (11Bu) hypersurface. As mentioned, the optimization was in this case restricted to the C2h symmetry, and the
resultant imaginary frequency corresponding to the transition state path was that representing the symmetric interchange of protons between N1 and N4 and N3 and N2.
Apart from the mentioned structures, in a previous study
[21] we reported the excited singlet S1 states minima for
AD and TD computed within the C2h point group, hereafter ADS1C2h and TDS1C2h, respectively.
3.2. Reaction path: initial and final steps
Table 1 compiles the energy differences, oscillator
strengths, and radiative lifetimes for several low-lying singlet excited states of the azaindole dimer (AD) and its tautomer (TD) in the region representing the beginning
(reactive) and end (product) of the excited state tautomerization reaction. Unlike in a previous study [21], where C2h
restrictions were imposed and a localized picture of the
excitations was found, we have computed here excited
states minima within Cs symmetry, allowing delocalization.
As before, two pairs of almost degenerated singlet states
are obtained: 11Bu–21Ag and 21Bu–31Ag, in increasing
order of energies. The analysis of their wave functions
and the comparison with the states of the monomer can
be found elsewhere [21,25] and indicates that the states of
the dimer correspond to a combination of the monomer
two La- and two Lb-type (PlattÕs nomenclature [21]),
respectively. Their wave functions can be considered as linear combinations of two HOMO ! LUMO (La) configurations and two HOMO 1 ! LUMO plus two
HOMO ! LUMO 1 (Lb) one-electron promotions. For
symmetry reasons, the system has pairs of almost degenerate orbitals (two HOMOs, two LUMOs, etc.). When the
symmetry is released to Cs the S1 states minima lie near
0.1 eV below the C2h minima. The nature of the electronic
transition is basically maintained both in Cs or C2h: while
the La (11Bu–21Ag) states display a noticeable transfer of
charge from the pyrrole to the pyridine ring, for Lb
(21Bu–31Ag) the excitation is maintained within the pyridine ring. The difference is that for the Cs structure the
excitation localizes in one of the monomers, as well as
the geometrical changes, leading to a localized picture.
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L. Serrano-Andrés, M. Merchán / Chemical Physics Letters 418 (2006) 569–575
Table 1
Computed and experimental energy differences (eV), oscillator strengths (f), and radiative lifetimes (srad, ns) related to the absorption and emission
processes in the 7-azaindole dimer (AD) and its tautomer dimer (TD)a
Stateb
Absorptionb
Emissionb
Experiment
EVA
f
Te
EVE
Azaindole dimer (AD)
11Bu
21Ag
21Bu
31Ag
4.02
4.06
4.66
4.69
0.389
forb.
0.101
forb.
3.87
3.68
5.0
4.28c, 4.00d, 3.5e, 0.13f, 13–38g
4.00d
0.048f
Tautomer dimer (TD)
11Bu
21Ag
21Bu
31Ag
2.85
4.06
4.66
4.69
0.037
forb.
0.101
forb.
2.79
2.15
106.5
2.95h, 2.86i, 2.58j, 0.023f, 160g
a
b
c
d
e
f
g
h
i
j
srad
Optimized CASSCF geometries. EVA, Te, and EVE, energy differences. See Fig. 4.
C2h geometries in absorption; Cs in emission. C2h state labels employed.
Absorption band maximum for AD in solution [30].
Band origins for jet-cooled AD in the one- (11Bu) and two-photon (21Ag) spectra [3,27].
Fluorescence (F1) band maximum in different environments [30,32].
Estimated oscillator strengths in hexane [29].
Estimated radiative lifetimes in hexane [29].
Band origin for F2 measured in a supersonic jet [3].
Highest-energy band for F2 measured in nonpolar solvents [1,2].
Tautomer fluorescence (F2) band maximum in different environments [30,32].
Those minima, ADS1 and TDS1 correspond to locally
excited (LE) structures of the dimer from which the double
emission will take place. The localized picture can be
observed in Fig. 2, where geometry changes in S1 are
reported restricted to one of the monomers and in Fig. 3,
which displays the asymmetric charge density differences.
Fig. 3. Differential electron density for the main singlet–singlet valence transitions in 7-azaindole dimer, tautomer, and intermediates. The electron density
is shifted upon light-induced excitation from darker to lighter regions.
L. Serrano-Andrés, M. Merchán / Chemical Physics Letters 418 (2006) 569–575
It is also important to consider that, in S1 a net flow of
charge (0.1–0.2 e) takes place from pyrrole to pyridine in
AD and vice versa for TD. This effect, also reported in previous calculations [21,26], is already present, in a minor
extent, in the delocalized C2h picture and in both monomers. The increase of charge in the pyridine nitrogen atoms
can be considered the driving force triggering the proton
transfer process. For AD in its S1 state the pyrrole unit
behaves as a proton donor, while pyridine as a proton
acceptor, and the opposite holds true for TD. Within the
C2h hypersurface this process leads to a concerted, symmetric, proton interchange, whereas in the Cs hypersurface to
the stepwise, asymmetric, proton (not hydrogen) transfer.
As an evidence of the accuracy of the present calculations, we can compare the computed and experimental values for absorption and emission. Fuke et al. [27] estimated
a band origin for the lowest dimer (AD) transition in the
gas phase at 4.0 eV, approximately 0.3 eV red-shifted as
compared to the origin of the monomer absorption. The
maximum of the band varies from 4.1 to 4.4 eV depending
on the experimental conditions [28]. The computed vertical
absorption (11Bu), 4.02 eV, in agreement with the experimental findings in the gas phase. Both experiment [29]
and theory obtain large oscillator strengths for the La-type
transitions. A two-photon allowed band was reported only
573
0.0045 eV below the 4.0 eV origin [27], that, in the light of
the present and previous [21] results, it can be assigned to
the one-photon forbidden 21Ag state, computed closely
degenerated with 11Bu (see Table 1). Regarding the tautomer (TD) absorption bands, both one-photon allowed
11Bu and 21Bu transitions are predicted very weak, computed at energies 2.85 and 4.66 eV with low oscillator
strengths, 0.037 and 0.101, respectively, probably explaining the lack of clear experimental information extracted
from the optical spectra [2,28,30]. Lowest energy band origins (Te) were calculated (11Bu states) at 3.87 and 2.79 eV
for AD and TD, respectively, while corresponding vertical
emissions were computed at 3.68 and 2.15 eV. Those results
can be compared to the observed fluorescence bands. The
so-called violet fluorescence (F1), attributed to AD, is
reported starting at 4.00 eV for jet-cooled samples in both
the one- and two-photon spectra [3,27], with band maxima
near 3.5 eV in different solvents [2,31,32]. For TD, the
green fluorescence (F2) band is reported starting at
2.86 eV [3] with maximum at 2.58 eV [3,30].
3.3. Reaction path: neutral and ionic intermediates
Fig. 4 displays the three computed paths connecting the
initial and final tautomerization steps, and Table 2 contains
Fig. 4. Scheme of the potential energy profiles computed in the 7-azaindole dimer excited state tautomerization process. Unless specified, energy
differences in kcal mol1 referred to ADS1 minimum.
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L. Serrano-Andrés, M. Merchán / Chemical Physics Letters 418 (2006) 569–575
Table 2
Relative energies, charge distributions of the two moieties, and dipole moments for the three lowest singlet states of the 7-azaindole dimer at the different
structures computed at the PMCAS-CI/MS-CASPT2 levela
ADS0
ADS1
DICIb
DIIb
DINb
TDS1
TDS0
Ec
M1d
M2d
le
0.00
0.00
0.00
0.00
0.18
0.00
0.00
0.03
1.76
0.93
+0.93
10.55
1.27
0.95
+0.95
8.69
2.08
1.04
+1.04
15.62
1.44
0.00
0.00
1.22
1.00
0.00
0.00
0.00
S1 state
Ec
M1d
M2d
le
4.02
0.00
0.00
0.00
3.87
0.00
0.00
0.47
4.37
0.92
+0.92
9.95
4.27
0.97
+0.97
12.13
3.27
0.04
+0.04
17.29
3.59
0.00
0.00
4.24
3.89
0.00
0.00
0.00
S2 state
Ec
M1d
M2d
le
4.06
0.00
0.00
0.00
4.36
0.00
0.00
0.49
4.43
+0.06
0.06
15.37
4.47
+0.05
0.05
10.68
4.73
0.03
+0.03
10.91
3.59
0.00
0.00
4.97
3.89
0.00
0.00
0.00
Structure
S0 state
a
b
c
d
e
Structures as labeled in Figs. 1 and 2.
M1 (upper) and M2 (lower) moieties have accumulated atomic numbers 61 and 63, respectively, at these geometries (Fig. 2).
MS-CASPT2 energy differences (eV) with respect to S0 at the ADS0 structure.
Charge differences for upper (M1) and lower (M2) monomers in Fig. 2.
Dipole moments (D) at the PMCAS-CI level.
the relative energy differences, charge distributions, and
dipole moments for all the studied situations. Three intermediate species have been computed with totally different
character. A minimum structure lying 13.88 kcal mol1
well below the reference AD(S1) minimum has been located
corresponding to a neutral intermediate asymmetric structure, coined DIN. Charge density difference plots in Fig. 3
show that, as compared to the ground state, DIN represents
a charge transfer state exciting one electron from the proton-donating towards the proton-accepting monomer.
Higher in energy (near 23 kcal mol1 from the latter),
another minimum is found describing an ionic asymmetric
structure, DII, in which the proton-donating monomer has
one excess negative charge while the opposite monomer has
the corresponding positive charge. The excitation can be
here characterized (see Fig. 3) by an intramonomer (proton-accepting moiety) transition. It is noticeable that the
intermonomer distance is much shorter (up to 0.7 Å) in
the ionic, DII, than in the neutral, DIN, intermediate,
because of the larger electrostatic attraction between the
two moieties in the former. Finally, a neutral symmetric
(C2h) structure has been computed at near 15 kcal mol1
from the reference structure as a transition state, DITS,
connecting the symmetric hypersurfaces which represent
the suggested concerted path. All the low-lying excited
states computed here have been found with pp* character.
Reactives, products, and reaction intermediates were qualitatively described by previous CIS results [14,33] in a similar way as those obtained here.
Three different pathways can be therefore be envisaged.
The process through the neutral asymmetric intermediate,
DIN, represents a two step procedure which proceeds via
the conical intersection DICI (S2/S1). At that point, S1 con-
nects adiabatically with the LE (11Bu) state near the FC
region, leading to a proton transfer situation from one of
the pyrrolic nitrogen atoms to a pyridinic nitrogen atom
of the opposite monomer, and diabatically, with the DII
ionic intermediate. On the other hand, S2 is in the FC
region a high-lying charge transfer (CT) state in which
one electron is excited from one to the other monomer.
At the CI, geometrically placed near DII, we have an effective proton-transfer situation in which already the asymmetrically transferred proton belongs to one of the
monomers, and the switch to the CT state leading to the
DIN neutral intermediate represents the excitation of an
electron from the deprotonated monomer to the protonated moiety. The initial transfer through the neutral intermediate has been criticised [6] based on simple valence
theory, assuming that the process takes place with an initial
H-atom transfer, prevented by the expected repulsion
between the nitrogen lone-pairs and the H radical. Considering the model presented here based on our computed CI,
the criticism cannot be sustained, because the process ultimately proceeds via a proton-transfer mechanism, which is,
along the path, compensated by an intermonomer charge
transfer leading to the final neutral intermediate structure.
From the CI, however, the potential energy profile seems a
steep-descendent path towards the Cs minimum, DIN, in
which the two monomer separates near 0.7 Å from the
structure of the CI. The minimum seems too deep to represent a path prone to tautomerization. Two other possibilities can be put forward: a further dissociation of the two
weakly bound asymmetric moieties in a process with high
relaxation lifetimes or a relaxation towards the ground
state through a corresponding CI. Such mechanism has
been described for the aminopyridine dimer [34] and the
L. Serrano-Andrés, M. Merchán / Chemical Physics Letters 418 (2006) 569–575
guanine–cytosine pair [33,35], which are well-known nonfluorescent and photostable systems, unlike AD/TD, where
the presence of an accessible CI with the ground state is less
probable. Further studies at this respect are however
required.
A second possibility for the suggested two-step proton
transfer mechanism relates the ionic intermediate, DII, adiabatically connecting with the initially promoted ADS1
state. Depending on the topology of the conical intersection, the energy can proceed towards the latter structure
by the same proton-transfer mechanism described above,
and continue in a second step towards the final products.
Finally, the concerted path was explored by computing
the symmetric transition state structure, DITS, which is
near 3.5 kcal mol1 higher than the calculated CI. The differences can however be expected to be larger in polar solvents because of the large dipole moments exhibited by the
asymmetric structures. Probably, the concerted mechanism
could only participate in the process at high excitation
energies. In any case, it will be most probably a subtle balance of experimental conditions: excess energy, solvation,
and tunneling effects which will lead the process towards
one or the other path. To conclude, we consider that with
the present results at hand based on accurate quantum
chemical ab initio methods, the discussion on the double
proton transfer tautomerization mechanism in the 7-azaindole dimer can continue with the support of a solid theoretical basis. Further insight on theoretical grounds could be
only obtained by considering the dynamics of the system.
Acknowledgements
The present research has been supported by Projects
CTQ2004-01739 of the Spanish MEC and GV04B-228 of
the Generalitat Valenciana.
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