9 research outputs found
Ab initio calculation of charge transfer in proton collisions with N 2
Total and partial charge transfer cross sections are calculated in collisions of protons with the nitrogen molecule at energies between 0.1 and 10 keV. Ab initio potential energy curves and nonadiabatic couplings have been obtained for a number of N2 bond lengths using a multireference configuration interaction method. The influence of the anisotropy of the target molecule is investigated. Results are compared with previous experimental and theoretical dataThis work has been supported by the Project ENE2007- 62934 of the Ministerio de Ciencia e Innovación (Spain). Allocation of computational time at the CCC of the Universidad Autónoma de Madrid is gratefully acknowledge
Time-resolved X-ray Absorption and Emission Spectroscopy to Disentangle Reaction Coordinates in Photoexcited Molecules
Using Ultrafast X-ray Spectroscopy To Address Questions in Ligand-Field Theory: The Excited State Spin and Structure of [Fe(dcpp)<sub>2</sub>]<sup>2+</sup>
We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)2]2+ (where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the 5T2/3T1 crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the 5T2 state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a 5T2 state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited 5T2 state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin 1A1 ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules—in particular, transition metal complexes—that are difficult if not impossible to obtain by other means
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Theoretical
predictions show that depending on the populations
of the Fe 3d<sub><i>xy</i></sub>, 3d<sub><i>xz</i></sub>, and 3d<sub><i>yz</i></sub> orbitals two possible
quintet states can exist for the high-spin state of the photoswitchable
model system [Fe(terpy)<sub>2</sub>]<sup>2+</sup>. The differences
in the structure and molecular properties of these <sup>5</sup>B<sub>2</sub> and <sup>5</sup>E quintets are very small and pose a substantial
challenge for experiments to resolve them. Yet for a better understanding
of the physics of this system, which can lead to the design of novel
molecules with enhanced photoswitching performance, it is vital to
determine which high-spin state is reached in the transitions that
follow the light excitation. The quintet state can be prepared with
a short laser pulse and can be studied with cutting-edge time-resolved
X-ray techniques. Here we report on the application of an extended
set of X-ray spectroscopy and scattering techniques applied to investigate
the quintet state of [Fe(terpy)<sub>2</sub>]<sup>2+</sup> 80 ps after
light excitation. High-quality X-ray absorption, nonresonant emission,
and resonant emission spectra as well as X-ray diffuse scattering
data clearly reflect the formation of the high-spin state of the [Fe(terpy)<sub>2</sub>]<sup>2+</sup> molecule; moreover, extended X-ray absorption
fine structure spectroscopy resolves the Fe–ligand bond-length
variations with unprecedented bond-length accuracy in time-resolved
experiments. With <i>ab initio</i> calculations we determine
why, in contrast to most related systems, one configurational mode
is insufficient for the description of the low-spin (LS)–high-spin
(HS) transition. We identify the electronic structure origin of the
differences between the two possible quintet modes, and finally, we
unambiguously identify the formed quintet state as <sup>5</sup>E,
in agreement with our theoretical expectations