Real-time dynamics of vibronic wavepackets within Rydberg and ion-pair states of molecular iodine.

Real-time dynamics of vibrationally and electronically excited I2 molecules has been investigated experimentally using the pump-probe technique. A 2-photon excitation was performed either at 269 nm or 266 nm. An electronic and vibrational wavepacket was built as coherent superposition of a few (269 nm excitation) or many (266 nm excitation) Rydberg states of the and series partly coupled with ion-pair states. The probe operated by ionisation or photodetachement. The energy and angular distribution of the resulting photoelectrons, I+ photocations and I- photoanions were monitored. During the dynamics that is turned on by the pump excitation, the wavepacket splits and explores a variety of electronic states of Rydberg and ion-pair character. The experimental results were complemented by molecular dynamics calculations. This provided invaluable information to identify wavepacket motion along ion-pair potential energy curves.

Time-resolved photoelectron spectroscopy of 4-(dimethylamino)benzethyne - an experimental and computational study.

We investigated the excited-state dynamics of 4-(dimethylamino)benzethyne (4-DMABE) in a combined theoretical and experimental study using surface-hopping simulations and time-resolved ionisation experiments. The simulations predict a decay of the initially excited S2 state into the S1 state in only a few femtoseconds, inducing a subsequent partial twist of the dimethylamino group within ∼100 fs. This leads to drastically reduced Franck-Condon factors for the ionisation transition to the cationic ground state, thus inhibiting the effective ionisation of the molecule, which leads to a vanishing photoelectron signal on a similar timescale as observed in our time-resolved photoelectron spectra. From the phototoelectron spectra, an adiabatic ionisation energy of 7.17 ± 0.02 eV was determined. The experimental decays match the theoretical predictions very well and the combination of both reveals the electronic characteristics of the molecule, namely the role of intramolecular charge transfer (ICT) states in the deactivation pathway of electronically excited 4-DMABE.

Excited state dynamics of normal dithienylethene molecules either isolated or deposited on an argon cluster.

Real-time dynamics of the electronically excited open-ring isomer of 1,2-bis(2-methylbenzo[b]thiophen-3-yl)perfluorocyclopentene (BTF6) and 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (PTF6) molecules was investigated using a set-up that associates a molecular beam, femtosecond lasers and velocity map imaging. The molecules were either free in the gas phase or bound to an argon cluster. DFT and TDDFT calculations were performed on BTF6. The calculated vertical excitation energies indicate an excitation by the pump laser towards a superposition of S5 and S6 states. The free molecule dynamics was found to follow a three wavepacket model. One describes the parallel conformer (P) of these molecules. It is unreactive with respect to the ring closure reaction which is responsible for the photochromic property of these molecules. It has no observable decay at the experiment time scale (up to 350 ps). The other two wavepackets describe the reactive antiparallel conformer (AP). They are formed by an early splitting of the wavepacket that was launched initially by the pump laser. They can be considered as generated by excitation of different, essentially uncoupled, deformation modes. They subsequently evolve along independent pathways. One is directed ballistically towards a conical intersection (CI) and decays through the CI to a potential energy surface where it can no longer be detected. The other fraction of the wavepacket decays also towards undetected states but in this case the driving mechanism is a non-adiabatic electronic relaxation within a potential well of the energy surfaces where it was launched. When BTF6 and PTF6 molecules are bound to an argon cluster, the same three wavepacket model applies. The vibronic relaxation timespan is enhanced by a factor 5 and a larger fraction of AP conformers follows this pathway. In contrast, the time constant associated with the ballistic movement is enhanced by only a factor of 2.

Reaction dynamics within a cluster environment.

This perspective article reviews experimental and theoretical works where rare gas clusters and helium nanodroplets are used as a nanoreactor to investigate chemical dynamics in a solvent environment. A historical perspective is presented first followed by specific considerations on the mobility of reactants within these reaction media. The dynamical response of pure clusters and nanodroplets to photoexcitation is shortly reviewed before examining the role of the cluster (or nanodroplet) degrees of freedom in the photodynamics of the guest atoms and molecules.

Anisotropic dynamics of two-photon ionization: An attosecond movie of photoemission.

Imaging in real time the complete dynamics of a process as fundamental as photoemission has long been out of reach because of the difficulty of combining attosecond temporal resolution with fine spectral and angular resolutions. Here, we achieve full decoding of the intricate angle-dependent dynamics of a photoemission process in helium, spectrally and anisotropically structured by two-photon transitions through intermediate bound states. Using spectrally and angularly resolved attosecond electron interferometry, we characterize the complex-valued transition probability amplitude toward the photoelectron quantum state. This allows reconstructing in space, time, and energy the complete formation of the photoionized wave packet.

Photodissociation of the trichloromethyl radical: photofragment imaging and femtosecond photoelectron spectroscopy.

Halogen-containing radicals play a key role in catalytic reactions leading to stratospheric ozone destruction, thus their photochemistry is of considerable interest. Here we investigate the photodissociation dynamics of the trichloromethyl radical, CCl3 after excitation in the ultraviolet. While the primary processes directly after light absorption are followed by femtosecond-time resolved photoionisation and photoelectron spectroscopy, the reaction products are monitored by photofragment imaging using nanosecond-lasers. The dominant reaction is loss of a Cl atom, associated with a CCl2 fragment. However, the detection of Cl atoms is of limited value, because in the pyrolysis CCl2 is formed as a side product, which in turn dissociates to CCl + Cl. We therefore additionally monitored the molecular fragments CCl2 and CCl by photoionisation at 118.2 nm and disentangled the contributions from various processes. A comparison of the CCl images with control experiments on CCl2 suggest that the dissociation to CCl + Cl2 contributes to the photochemistry of CCl3.

Time-Resolved Observation of the Solvation Dynamics of a Rydberg Excited Molecule Deposited on an Argon Cluster. II. DABCO☆ at Long Time Delays.

The real-time dynamics of DABCO-argon clusters is investigated in a femtosecond pump-probe experiment where the pump excites DABCO to the S1 state within the argon cluster. The probe operates by photoionization and documents the energy and angular distributions of the resulting photoelectrons. The present work complements a previous work from our group [Awali Phys. Chem. Chem. Phys., 2014, 16, 516-526] where this dynamics was probed at short time, up to 4 ps after the pump pulse. Here, the dynamics is followed up to 500 ps. A multiscale dynamics is observed. It includes a jump between two solvation sites (time scale 0.27 ps) followed by the relaxation of the solvation cage excess vibrational energy (time scale 14 ps) and then by that of DABCO (time scale >150 ps). Polarization anisotropy, double polarization, and angular anisotropy effects are reported also. They are interpreted (quantitatively for the former effect) in terms of decoherence of rotational alignment, driven by the overall rotation of the DABCO-argon clusters. A tomographic view of the DABCO excited orbital, provided by the double anisotropy effect, is discussed on a qualitative basis.

Excited State Dynamics of Isolated 6- and 8-Hydroxyquinoline Molecules.

The photoinduced dynamics of isolated n-hydroxyquinoline (nHQ) molecules (n=6,8) was investigated in femtosecond pump-probe experiments. A qualitative difference was found between 8HQ and 6HQ. After an initial rapid decay corresponding to the departure of the initial wavepacket out of the Franck-Condon region of the excitation, the 8HQ probe signal decays to zero in 0.37 ps whereas a much longer time constant of 10.4 ps is observed in 6HQ. This interrogates on the role played by the intramolecular H-bond N ··· HO which is at play the 8HQ molecule. Ab-initio were performed at the MCSCF/aug-cc-pVDZ level on the 8HQ molecule to help the discussion. A complex energy landscape was found, which includes a conical intersection.

Unimolecular decomposition of methyl ketene and its dimer in the gas phase: theory and experiment.

We present a combined theoretical and experimental investigation on the single photoionization and dissociative photoionization of gas-phase methyl ketene (MKE) and its neutral dimer (MKE2). The performed experiments entail the recording of photoelectron photoion coincidence (PEPICO) spectra and slow photoelectron spectra (SPES) in the energy regime 8.7-15.5 eV using linearly polarized synchrotron radiation. We observe both dimerization and trimerization of the monomer which brings about significantly complex and abstruse dissociative ionization patterns. These require the implementation of theoretical calculations to explore the potential energy surfaces of the monomer and dimer's neutral and ionized geometries. To this end, explicitly correlated quantum chemical methodologies involving the coupled cluster with single, double and perturbative triple excitations (R)CCSD(T)-F12 method, are utilized. An improvement in the adiabatic ionization energy of MKE is presented (AIE = 8.937 ± 0.020 eV) as well as appearance energies for multiple fragments formed through dissociative ionization of either the MKE monomer or dimer. In this regard, the synergy of experiment and theory is crucial to interpreting the obtained results. We discuss the potential astrochemical implications of this work in the context of recent advances in the field of astrochemistry and speculate on the potential presence and eventual fate of interstellar MKE molecules.

Exploring the Excited-State Dynamics of Hydrocarbon Radicals, Biradicals, and Carbenes Using Time-Resolved Photoelectron Spectroscopy and Field-Induced Surface Hopping Simulations.

Reactive hydrocarbon molecules like radicals, biradicals, and carbenes are not only key players in combustion processes and interstellar and atmospheric chemistry but also important intermediates in organic synthesis. These systems typically possess many low-lying, strongly coupled electronic states. After light absorption, this leads to rich photodynamics characterized by a complex interplay of nuclear and electronic motion, which is still not comprehensively understood and not easy to investigate both experimentally and theoretically. In order to elucidate trends and contribute to a more general understanding, we here review our recent work on excited-state dynamics of open-shell hydrocarbon species using time-resolved photoelectron spectroscopy and field-induced surface hopping simulations and report new results on the excited-state dynamics of the tropyl and the 1-methylallyl radical. The different dynamics are compared, and the difficulties and future directions of time-resolved photoelectron spectroscopy and excited-state dynamics simulations of open-shell hydrocarbon molecules are discussed.

Energetics and ionization dynamics of two diarylketone molecules: benzophenone and fluorenone.

Single photon ionization and subsequent unimolecular ion decomposition were studied on jet-cooled benzophenone and fluorenone separately, using VUV synchrotron radiation in a photoion/photoelectron coincidence setup. Slow PhotoElectron Spectra (SPES) were recorded in coincidence with either the parent or the fragment ions for hν < 12.5 eV. Dissociative ionization is observed for benzophenone only. The full interpretation of the measurements, including the identification of the neutral and ionic species when dissociative ionization is at play, benefits from high level ab initio computations for determining the equilibrium structures and the energetics of the neutral and ionized molecules and of their fragments. Electronically excited states of the parent molecular ions were calculated also. From this analysis, an accurate experimental determination of the energetics of the benzophenone and fluorenone ions and of their fragmentation channels is available: adiabatic ionization energies of benzophenone at 8.923 ± 0.005 eV and of fluorenone at 8.356 ± 0.007 eV; and appearance energies of benzophenone fragment ions at 11.04 ± 0.02 eV (loss of C6H5), 11.28 ± 0.02 eV (loss of H) and 11.45 ± 0.02 eV (loss of CO). The corresponding fragmentation mechanisms are explored, showing likely concerted bonds rearrangement. Possible pre-ionizing fragmentation is discussed in light of the spectra presented. The structural rigidity of fluorenone diarylketone seems to be the origin of the inhibition of the fragmentation of its cation.

Large amplitude motion within acetylene-rare gas complexes hosted in helium droplets.

Near-infrared spectroscopy of the C2H2-Ar, Kr complexes was performed in the spectral region overlapping the ν3/ν2 + ν4 + ν5 Fermi-type resonance of C2H2. The experiment was conducted along the HElium NanoDroplet Isolation (HENDI) technique in order to study the coupling dynamics between a floppy molecular system (C2H2-Ar and C2H2-Kr) and a mesoscopic quantum liquid (the droplet). Calculations were performed using a spectral element based close-coupling program and state-of-the-art 2-dimensional potential energy surfaces to determine the bound states of the C2H2-Ar and C2H2-Kr complexes and simulate the observed spectra. This furnished a quantitative basis to unravel how the superfluid and non-superfluid components of the droplet affect the rotation and the deformation dynamics of the hosted complex.

Excited state dynamics and time-resolved photoelectron spectroscopy of para-xylylene.

We investigated the excited-state dynamics of para-xylylene using a combination of field-induced surface hopping (FISH) simulations and time-resolved ionisation experiments. Our simulations predict an ultrafast decay of the initially excited bright state (S2/S3) to the S1 state on a sub-100 fs time scale, followed by return to the ground state within ∼1 ps. This is accompanied by a transient change of the biradical character of the molecule, as monitored by calculating natural orbital occupation numbers. Specifically, the initially low biradicality is increased by electronic excitation as well as by vibrational activation. Experimentally, para-xylylene was generated by pyrolysis from [2,2]paracyclophane and excited with 266 nm radiation into the S2/S3 bright state. The subsequent dynamics were followed using ionisation as the probe step, with both mass spectra and photoelectron spectra recorded as a function of pump-probe delay. The observed decay of photoelectron and photoion intensities closely matches the theoretical predictions and is consistent with the sequential mechanism found in the simulations. This mechanism exhibits characteristic signatures in both time-resolved mass and photoelectron spectra, in particular in the appearance of fragment ions that are exclusively generated from the S1 state. This allows for a separation of the S2 and S1 dynamics in the photoelectron and mass spectra. An excellent agreement between the observed and the simulated ion signal is observed.

Unveiling the complex vibronic structure of the canonical adenine cation.

Adenine, a DNA base, exists as several tautomers and isomers that are closely lying in energy and that may form a mixture upon vaporization of solid adenine. Indeed, it is challenging to bring adenine into the gas phase, especially as a unique tautomer. The experimental conditions were tuned to prepare a jet-cooled canonical adenine (9H-adenine). This isolated DNA base was ionized by single VUV photons from a synchrotron beamline and the corresponding slow photoelectron spectrum was compared to ab initio computations of the neutral and ionic species. We report the vibronic structure of the X+ 2A'' (D0), A+ 2A' (D1) and B+ 2A'' (D2) electronic states of the 9H adenine cation, from the adiabatic ionization energy (AIE) up to AIE + 1.8 eV. Accurate AIEs are derived for the 9H-adenine (X[combining tilde] 1A') + hν → 9H-adenine+ (X+ 2A'', A+ 2A', B+ 2A'') + e- transitions. Close to the AIE, we fully assign the rich vibronic structure solely to the 9H-adenine (X 1A') + hν → 9H-adenine+ (X+ 2A'') transition. Importantly, we show that the lowest cationic electronic states of canonical adenine are coupled vibronically. The present findings are important for understanding the effects of ionizing radiation and the charge distribution on this elementary building block of life, at ultrafast, short, and long timescales.

The surprisingly high ligation energy of CO to ruthenium porphyrins.

A combined theoretical and experimental approach has been used to investigate the binding energy of a ruthenium metalloporphyrin ligated with CO, ruthenium tetraphenylporphyrin [RuII TPP], in the RuII oxidation degree. Measurements performed with VUV ionization using the DESIRS beamline at Synchrotron SOLEIL led to adiabatic ionization energies of [RuII TPP] and its complex with CO, [RuII TPP-CO], of 6.48 ± 0.03 eV and 6.60 ± 0.03 eV, respectively, while the ion dissociation threshold of [RuII TPP-CO]+ is measured to be 8.36 ± 0.03 eV using the ground-state neutral complex. These experimental data are used to derive the binding energies of the CO ligand in neutral and cationic complexes (1.88 ± 0.06 eV and 1.76 ± 0.06 eV, respectively) using a Born-Haber cycle. Density functional theory calculations, in very satisfactory agreement with the experimental results, help to get insights into the metal-ligand bond. Notably, the high ligation energies can be rationalized in terms of the ruthenium orbital structure, which is singular compared to that of the iron atom. Thus, beyond indications of a strengthening of the Ru-CO bond due to the decrease in the CO vibrational frequency in the complex as compared to the Fe-CO bond, high-level calculations are essential to accurately describe the metal ligand (CO) bond and show that the Ru-CO bond energy is strongly affected by the splitting of triplet and singlet spin states in uncomplexed [Ru TPP].

Self-trapping relaxation decay investigated by time-resolved photoelectron spectroscopy.

The present work combines time-resolved photoelectron spectroscopy on isolated species with high-level data processing to address an issue which usually pertains to materials science: the electronic relaxation dynamics towards the formation of a self-trapped exciton (STE). Such excitons are common excited states in ionic crystals, silica and rare gas matrices. They are associated with a strong local deformation of the matrix. Argon clusters were taken as a model. They are excited initially to a Wannier exciton at 14 eV and their evolution towards the formation of an STE has showed an unusual type of vibronic relaxation where the electronic excitation of the cluster decreases linearly as a function of time with a 0.59 ± 0.06 eV ps-1 rate. The decay was followed for 3.0 ps, and the STE formation occurred in ∼5.1 ± 0.7 ps.

Femtosecond dynamics of the 2-methylallyl radical: A computational and experimental study.

We investigate the photodynamics of the 2-methylallyl radical by femtosecond time-resolved photoelectron imaging. The experiments are accompanied by field-induced surface hopping dynamics calculations and the simulation of time-resolved photoelectron intensities and anisotropies, giving insight into the photochemistry and nonradiative relaxation of the radical. 2-methylallyl is excited at 236 nm, 238 nm, and 240.6 nm into a 3p Rydberg state, and the subsequent dynamics is probed by multiphoton ionization using photons of 800 nm. The photoelectron image exhibits a prominent band with considerable anisotropy, which is compatible with the result of theory. The simulations show that the initially excited 3p state is rapidly depopulated to a 3s Rydberg state, from which photoelectrons of high anisotropy are produced. The 3s state then decays within several 100 fs to the D1 (nπ) state, followed by the deactivation of the D1 to the electronic ground state on the ps time scale.

Identifying Cytosine-Specific Isomers via High-Accuracy Single Photon Ionization.

Biological entities, such as DNA bases or proteins, possess numerous tautomers and isomers that lie close in energy, making the experimental characterization of a unique tautomer challenging. We apply VUV synchrotron-based experiments combined with state-of-the-art ab initio methodology to determine the adiabatic ionization energies (AIEs) of specific gas-phase cytosine tautomers produced in a molecular beam. The structures and energetics of neutral and cationic cytosine tautomers were determined using explicitly correlated methods. The experimental spectra correspond to well-resolved bands that are attributable to the specific contributions of five neutral tautomers of cytosine prior to ionization. Their AIEs are experimentally determined for the first time with an accuracy of 0.003 eV. This study also serves as an important showcase for other biological entities presenting a dense pattern of isomeric and tautomeric forms in their spectra that can be investigated to understand the charge redistribution in these species upon ionization.

Direct observation of slow intersystem crossing in an aromatic ketone, fluorenone.

Direct measurements of Single vibronic Level InterSystem Crossing (SLISC) have been performed on the fluorenone molecule in the gas phase, by time resolved photoelectron and photoion spectroscopy. Vibronic transitions above the S1 nπ* origin were excited in the 432-420 nm region and the decay of S1 and growth of T1(3)ππ* could be observed within a 10 ns time domain. The ionization potential is measured as 8.33 ± 0.04 eV. The energy of the first excited triplet state of fluorenone, T1 has been characterized directly at 18 640 ± 250 cm(-1). The internal conversion of S1 to S0 is found to amount to ∼15% of the population decay, thus ISC is the dominant electronic relaxation process. ISC, although favored by the S1(1)nπ*-T1(3)ππ* coupling scheme, is 3 orders of magnitude less efficient than in the similar molecule benzophenone. Thus, the planarity of the fluorenone molecule disfavors the exploration of the configuration space where surface crossings would create high ISC probability, which occurs in benzophenone through surface crossings. The time evolution of S1 fluorenone is well accounted for by the statistical decay of individual levels into a quasi-continuum of T1 vibronic levels.

Absorption Spectroscopy, a Tool for Probing Local Structures and the Onset of Large-Amplitude Motions in Small KAr(n) Clusters at Increasing Temperatures.

Photoabsorption spectra of KArn (n = 1-10) are simulated at temperatures ranging between 5 and 25 K. The calculations associate a Monte Carlo (MC) method to sample cluster geometries at temperature T, with a one-electron ab initio model to calculate the ground-state and excited-state energies of the cluster. The latter model replaces the K(+) core electrons and all the electrons of the Ar atoms by appropriate pseudopotentials, complemented by core polarization potentials. It also provides the necessary oscillator strengths to simulate the spectra. Global optimization by basin-hopping is used in combination with MC simulation at low temperature (5 K) to identify the most stable isomer and remarkable isomers of ground-state KArn clusters, which are stable with respect to deformations of the order of those expected with Zero Point Energy motions. The absorption spectra calculated for each of these isomers at 5 K suggest that absorption spectroscopy can probe sensitively the local environment of K atom: surface location of K with respect to a close-packed Ar moiety, number of Ar atom in close vicinity, and local symmetry about K. Simulation at increasing temperatures, up to the evaporation limit of K out of the cluster, shows the onset of large amplitude motions above 20 K, when the K atom experiences a variety of local environments.