Diffractive imaging of dissociation and ground state dynamics in a complex molecule

We have investigated the structural dynamics in photoexcited 1,2-diiodotetrafluoroethane molecules (C2F4I2) in the gas phase experimentally using ultrafast electron diffraction and theoretically using FOMO-CASCI excited state dynamics simulations. The molecules are excited by an ultra-violet femtosecond laser pulse to a state characterized by a transition from the iodine 5p orbital to a mixed 5p|| hole and CF2 antibonding orbital, which results in the cleavage of one of the carbon-iodine bonds. We have observed, with sub-Angstrom resolution, the motion of the nuclear wavepacket of the dissociating iodine atom followed by coherent vibrations in the electronic ground state of the C2F4I radical. The radical reaches a stable classical (non-bridged) structure in less than 200 fs.Comment: 13 pages, 11 figure

Excited-state Forces within a First-principles Green's Function Formalism

We present a new first-principles formalism for calculating forces for optically excited electronic states using the interacting Green's function approach with the GW-Bethe Salpeter Equation method. This advance allows for efficient computation of gradients of the excited-state Born-Oppenheimer energy, allowing for the study of relaxation, molecular dynamics, and photoluminescence of excited states. The approach is tested on photoexcited carbon dioxide and ammonia molecules, and the calculations accurately describe the excitation energies and photoinduced structural deformations.Comment: 2 figures and 2 table

Ultrafast dynamics in light-driven molecular rotary motors probed by femtosecond stimulated raman spectroscopy

Photochemical isomerization in sterically crowded chiral alkenes is the driving force for molecular rotary motors in nanoscale machines. Here the excited state dynamics and structural evolution of the prototypical light driven rotary motor are followed on the ultrafast timescale by femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption (TA). TA reveals a sub 100 fs blue shift and decay of the Franck-Condon bright state arising from relaxation along the reactive potential energy surface. The decay is accompanied by coherently excited vibrational dynamics which survive the excited state structural evolution. The ultrafast Franck-Condon bright state relaxation is to a dark excited state, which FSRS reveals to have a rich spectrum compared to the electronic ground state, with the most intense Raman active modes shifted to significantly lower wavenumber. This is discussed in terms of a reduced bond order of the central bridging bond and overall weakening of bonds in the dark state, which is supported by electronic structure calculations. The observed evolution in the FSRS spectrum is assigned to vibrational cooling accompanied by partitioning of the dark state between the product isomer and the original ground state. Formation of the product isomer is observed in real time by FSRS. It is formed vibrationally hot and cools over several picoseconds, completing the characterization of the light driven half of the photocycle

DNA/RNA: Building Blocks of Life Under UV Irradiation

International audienceDuring the last 10 years, intense experimental and theoretical work has proven the existence of ultrafast nonradiative decay routes for UV-excited monomeric nucleic acid bases, accounting for their high photostability. This mechanism has been explained by the occurrence of easily accessible conical intersections connecting the first excited ππ* state with the ground state. However, recent studies of substituent and solvent effects indicate that the situation is more complicated than what was initially thought, notably by the presence of dark excited states. Moreover, the actual shape of the excited-state potential energy surface may induce nonexponential dynamics. Further efforts are needed in order to clarify how various environmental factors affect the structural and dynamical aspects of the nucleic acid base excited states

The photoinduced transition in magnetoresistive manganites: a comprehensive view

We use femtosecond x-ray diffraction to study the structural response of charge and orbitally ordered Pr$_{1-x}$Ca$_x$MnO$_3$ thin films across a phase transition induced by 800 nm laser pulses. By investigating the dynamics of both superlattice reflections and regular Bragg peaks, we disentangle the different structural contributions and analyze their relevant time-scales. The dynamics of the structural and charge order response are qualitatively different when excited above and below a critical fluence $f_c$. For excitations below $f_c$ the charge order and the superlattice is only partially suppressed and the ground state recovers within a few tens of nanosecond via diffusive cooling. When exciting above the critical fluence the superlattice vanishes within approximately half a picosecond followed by a change of the unit cell parameters on a 10 picoseconds time-scale. At this point all memory from the symmetry breaking is lost and the recovery time increases by many order of magnitudes due to the first order character of the structural phase transition

Ultrafast Excited State Relaxation of a Metalloporphyrin Revealed by Femtosecond X-ray Absorption Spectroscopy

Photoexcited Nickel­(II) tetramesitylporphyrin (NiTMP), like many open-shell metalloporphyrins, relaxes rapidly through multiple electronic states following an initial porphyrin-based excitation, some involving metal centered electronic configuration changes that could be harnessed catalytically before excited state relaxation. While a NiTMP excited state present at 100 ps was previously identified by X-ray transient absorption (XTA) spectroscopy at a synchrotron source as a relaxed (d,d) state, the lowest energy excited state (<i>J. Am. Chem. Soc.</i>, <b>2007</b>, <i>129</i>, 9616 and <i>Chem. Sci.</i>, <b>2010</b>, <i>1</i>, 642), structural dynamics before thermalization were not resolved due to the ∼100 ps duration of the available X-ray probe pulse. Using the femtosecond (fs) X-ray pulses of the Linac Coherent Light Source (LCLS), the Ni center electronic configuration from the initial excited state to the relaxed (d,d) state has been obtained via ultrafast Ni K-edge XANES (X-ray absorption near edge structure) on a time scale from hundreds of femtoseconds to 100 ps. This enabled the identification of a short-lived Ni­(I) species aided by time-dependent density functional theory (TDDFT) methods. Computed electronic and nuclear structure for critical excited electronic states in the relaxation pathway characterize the dependence of the complex’s geometry on the electron occupation of the 3d orbitals. Calculated XANES transitions for these excited states assign a short-lived transient signal to the spectroscopic signature of the Ni­(I) species, resulting from intramolecular charge transfer on a time scale that has eluded previous synchrotron studies. These combined results enable us to examine the excited state structural dynamics of NiTMP prior to thermal relaxation and to capture intermediates of potential photocatalytic significance

Adaptive control of CO2_2 bending vibration: deciphering field-system dynamics

We combined adaptive closed-loop optimization, phase-shaping with a restricted search space and imaging to control dynamics and decipher the optimal pulse. The approach was applied to controlling the amplitude of CO$_2$ bending vibration during strong-field Coulomb explosion. The search space was constrained by expressing the spectral phase as a Taylor series, which generated pulses with characteristics commensurate with the natural physical features of this problem. Optimal pulses were obtained that enhanced bending by up to 56% relative to what is observed with comparably intense, transform limited pulses. We show that (1) this judicious choice of a reduced parameter set made unwrapping the dynamics more transparent and (2) the enhancement is consistent with field-induced structural changes to a bent excited state of CO$_2^{2+}$, which theoretical simulations have identified as the state from which the explosion originates.Comment: 4 pages, 3 figures, 1 table, added reference

Imaging of Alignment, Deformation and Dissociation of CS2 Molecules using Ultrafast Electron Diffraction

Imaging the structure of molecules in transient excited states remains a challenge due to the extreme requirements for spatial and temporal resolution. Ultrafast electron diffraction from aligned molecules (UEDAM) provides atomic resolution and allows for the retrieval of structural information without the need to rely on theoretical models. Here we use UEDAM and femtosecond laser mass spectrometry (FLMS) to investigate the dynamics in carbon disulfide (CS2) following the interaction with an intense femtosecond laser pulse. We have retrieved images of ground state and excited molecules with 0.03 {\AA} precision. We have observed that the degree of alignment reaches an upper limit at laser intensities below the ionization threshold, and found evidence of structural deformation, dissociation, and ionization at higher laser intensities