Time-resolved photoelectron spectroscopy of molecules and clusters

Dynamics of Anions: From Bound to Unbound States and Everything In Between

Gas-phase anions present an ideal playground for the exploration of excited-state dynamics. They offer control in terms of the mass, extent of solvation, internal temperature, and conformation. The application of a range of ion sources has opened the field to a vast array of anionic systems whose dynamics are important in areas ranging from biology to star formation. Here, we review recent experimental developments in the field of anion photodynamics, demonstrating the detailed insight into photodynamical and electron-capture processes that can be uncovered. We consider the electronic and nuclear ultrafast dynamics of electronically bound excited states along entire reaction coordinates; electronically unbound states showing that photochemical concepts, such as chromophores and Kasha's rule, are transferable to electron-driven chemistry; and nonvalence states that straddle the interface between bound and unbound states. Finally, we consider likely developments that are sure to keep the field of anion dynamics buoyant and impactful.

On the multiphoton ionisation photoelectron spectra of phenol

The phenol molecule is a prototype for non-adiabatic dynamics and the excited-state photochemistry of biomolecules. In this article, we report a joint theoretical and experimental investigation on the resonance enhanced multiphoton ionisation photoelectron (REMPI) spectra of the two lowest ionisation bands of phenol. The focus is on the theoretical interpretation of the measured spectra using quantum dynamics simulations. These were performed by numerically solving the time-dependent Schrödinger equation using the multi-layer variant of the multiconfiguration time-dependent Hartree algorithm together with a vibronic coupling Hamiltonian model. The ionising laser pulse is modelled explicitly within the ionisation continuum model to simulate experimental femtosecond 1+1 REMPI photoelectron spectra. These measured spectra are sensitive to very short lived electronically excited states, providing a rigorous benchmark for our theoretical methods. The match between experiment and theory allows for an interpretation of the features of the spectra at different wavelengths and shows that there are features due to both 'direct' and 'indirect' ionisation, resulting from non-resonant and resonant excitation by the pump pulse.

Time-Resolved X-ray Photoelectron Spectroscopy: Ultrafast Dynamics in CS2 Probed at the S 2p Edge

Recent developments in X-ray free-electron lasers have enabled a novel site-selective probe of coupled nuclear and electronic dynamics in photoexcited molecules, time-resolved X-ray photoelectron spectroscopy (TRXPS). We present results from a joint experimental and theoretical TRXPS study of the well-characterized ultraviolet photodissociation of CS2, a prototypical system for understanding non-adiabatic dynamics. These results demonstrate that the sulfur 2p binding energy is sensitive to changes in the nuclear structure following photoexcitation, which ultimately leads to dissociation into CS and S photoproducts. We are able to assign the main X-ray spectroscopic features to the CS and S products via comparison to a first-principles determination of the TRXPS based on ab initio multiple-spawning simulations. Our results demonstrate the use of TRXPS as a local probe of complex ultrafast photodissociation dynamics involving multimodal vibrational coupling, nonradiative transitions between electronic states, and multiple final product channels.

Rydberg state dynamics and fragmentation mechanism of N,N,N',N'-tetramethylmethylenediamine

The non-adiabatic relaxation processes and the fragmentation dynamics of Rydberg-excited N,N,N',N'-tetramethylmethylenediamine (TMMDA) are investigated using femtosecond time-resolved photoelectron imaging and time-resolved mass spectroscopy. Excitation at 208 nm populates TMMDA in a charge-localized 3p state. Rapid internal conversion (IC) to 3s produces two charge-delocalized conformers with independent time constants and distinct population ratios. As the system explores the 3s potential surface, the structural evolution continues on a 1.55 ps timescale, followed by a slower (12.1 ps) relaxation to the ground state. A thorough comparison of the time-dependent mass and photoelectron spectra suggests that ionization out of the 3p state ends up with the parent ion, the vibrational energy of which is insufficient for the bond cleavage. On the contrary, by virtue of the additional energy acquired by IC from 3p, the internal energy deposited in 3s is available to break the C-N bond, leading to the fragment ion. The fragmentation is found to occur on the ion surface instead of the Rydberg surface.

Ultrafast X-Ray Probes of Elementary Molecular Events

Elementary events that determine photochemical outcomes and molecular functionalities happen on the femtosecond and subfemtosecond timescales. Among the most ubiquitous events are the nonadiabatic dynamics taking place at conical intersections. These facilitate ultrafast, nonradiative transitions between electronic states in molecules that can outcompete slower relaxation mechanisms such as fluorescence. The rise of ultrafast X-ray sources, which provide intense light pulses with ever-shorter durations and larger observation bandwidths, has fundamentally revolutionized our spectroscopic capabilities to detect conical intersections. Recent theoretical studies have demonstrated an entirely new signature emerging once a molecule traverses a conical intersection, giving detailed insights into the coupled nuclear and electronic motions that underlie, facilitate, and ultimately determine the ultrafast molecular dynamics. Following a summary of current sources and experiments, we survey these techniques and provide a unified overview of their capabilities. We discuss their potential to dramatically increase our understanding of ultrafast photochemistry.

Effects of Autoionizing Resonances on Wave-Packet Dynamics Studied by Time-Resolved Photoelectron Spectroscopy

We report a combined experimental and theoretical study on the effect of autoionizing resonances in time-resolved photoelectron spectroscopy. The coherent excitation of N_{2} by ∼14.15  eV extreme-ultraviolet photons prepares a superposition of three dominant adjacent vibrational levels (v^{'}=14-16) in the valence b^{'} ^{1}Σ_{u}^{+} state, which are probed by the absorption of two or three near-infrared photons (800 nm). The superposition manifests itself as coherent oscillations in the measured photoelectron spectra. A quantum-mechanical simulation confirms that two autoionizing Rydberg states converging to the excited A ^{2}Π_{u} and B ^{2}Σ_{u}^{+} N_{2}^{+} cores are accessed by the resonant absorption of near-infrared photons. We show that these resonances apply different filters to the observation of the vibrational wave packet, which results in different phases and amplitudes of the oscillating photoelectron signal depending on the nature of the autoionizing resonance. This work clarifies the importance of resonances in time-resolved photoelectron spectroscopy and particularly reveals the phase of vibrational quantum beats as a powerful observable for characterizing the properties of such resonances.

Femtosecond Time-Resolved Observation of Relaxation and Wave Packet Dynamics of the S1 State in Electronically Excited o-Fluoroaniline

Quantum beat frequency is the basis for understanding interference effects and vibrational wave packet dynamics and has important applications. Using femtosecond time-resolved mass spectrometry and femtosecond time-resolved photoelectron image combined with theoretical calculations, we study the electronic excited-state relaxation of o-fluoraniline molecule and the time-dependent evolution of vibrational wave packets between different eigenstates. After the molecule absorbs a photon of 288.3 nm and is excited to the S₁ state, intramolecular vibrational redistribution first occurs on the time scale τ₁ = 349 fs, and then the transition to the triplet state occurs through the intersystem crossing on the time scale τ₂ = 583 ps, and finally, the triplet state occurs decays slowly through the time scale τ₃ = 2074 ps. We find the intramolecular vibrational redistribution is caused by the 0⁰, 10b¹ and 16a¹ vibrational modes of the S_l state origin. That is, the 288.3 nm femtosecond laser excites the molecule to the S₁ state, and the continuous flow of the vibrational wave packet prepares a coherent superposition state of three vibrational modes. Through extracting the oscillation of different peak intensities in the photoelectron spectrum, we observe reversible changes caused by mutual interference of the S₁ 0⁰, S₁ 10b¹ and S₁ 16a¹ states when the wave packets flow. When the pump pulse is 280 nm, the beat frequency disappears completely. This is explained in terms of increases in the vibrational field density and characteristic period of oscillation, and statistical averaging makes the quantum effect smooth and indistinguishable. In addition, the Rydberg component of the S₁ state is more clearly resolved by combining experiment and theory.

Metal cluster plasmons analyzed by energy-resolved photoemission

The optical response of size-selected metal clusters is studied by wavelength-dependent photoemission and energy-resolved photoelectron detection. Relative photodetachment cross sections giving information on the plasmon are determined for the example of closed-shell Ag₉₁^-. Notably, the peak energy of this anion (3.74 eV) is higher than the small particle limit in Mie theory of 3.5 eV. Different methods to extract cross sections from the spectra are applied. In particular, we compare the results obtained by integrating the full electron yields to analyses based on evaluating specified binding energy windows. The approach opens up new possibilities to conduct studies on Landau fragmentation as a result of multielectron excitations.

UV Photoelectron Spectroscopy of Aqueous Solutions

Knowledge of the electronic structure of an aqueous solution is a prerequisite to understanding its chemical and biological reactivity and its response to light. One of the most direct ways of determining electronic structure is to use photoelectron spectroscopy to measure electron binding energies. Initially, photoelectron spectroscopy was restricted to the gas or solid phases due to the requirement for high vacuum to minimize inelastic scattering of the emitted electrons. The introduction of liquid-jets and their combination with intense X-ray sources at synchrotrons in the late 1990s expanded the scope of photoelectron spectroscopy to include liquids. Liquid-jet photoelectron spectroscopy is now an active research field involving a growing number of research groups. A limitation of X-ray photoelectron spectroscopy of aqueous solutions is the requirement to use solutes with reasonably high concentrations in order to obtain photoelectron spectra with adequate signal-to-noise after subtracting the spectrum of water. This has excluded most studies of organic molecules, which tend to be only weakly soluble. A solution to this problem is to use resonance-enhanced photoelectron spectroscopy with ultraviolet (UV) light pulses (hν ≲ 6 eV). However, the development of UV liquid-jet photoelectron spectroscopy has been hampered by a lack of quantitative understanding of inelastic scattering of low kinetic energy electrons (≲5 eV) and the impact on spectral lineshapes and positions.In this Account, we describe the key steps involved in the measurement of UV photoelectron spectra of aqueous solutions: photoionization/detachment, electron transport of low kinetic energy electrons through the conduction band, transmission through the water-vacuum interface, and transport through the spectrometer. We also explain the steps we take to record accurate UV photoelectron spectra of liquids with excellent signal-to-noise. We then describe how we have combined Monte Carlo simulations of electron scattering and spectral inversion with molecular dynamics simulations of depth profiles of organic solutes in aqueous solution to develop an efficient and widely applicable method for retrieving true UV photoelectron spectra of aqueous solutions. The huge potential of our experimental and spectral retrieval methods is illustrated using three examples. The first is a measurement of the vertical detachment energy of the green fluorescent protein chromophore, a sparingly soluble organic anion whose electronic structure underpins its fluorescence and photooxidation properties. The second is a measurement of the vertical ionization energy of liquid water, which has been the subject of discussion since the first X-ray photoelectron spectroscopy measurement in 1997. The third is a UV photoelectron spectroscopy study of the vertical ionization energy of aqueous phenol which demonstrates the possibility of retrieving true photoelectron spectra from measurements with contributions from components with different concentration profiles.

Different timescales during ultrafast stilbene isomerization in the gas and liquid phases revealed using time-resolved photoelectron spectroscopy

Directly contrasting ultrafast excited-state dynamics in the gas and liquid phases is crucial to understanding the influence of complex environments. Previous studies have often relied on different spectroscopic observables, rendering direct comparisons challenging. Here, we apply extreme-ultraviolet time-resolved photoelectron spectroscopy to both gaseous and liquid cis-stilbene, revealing the coupled electronic and nuclear dynamics that underlie its isomerization. Our measurements track the excited-state wave packets from excitation along the complete reaction path to the final products. We observe coherent excited-state vibrational dynamics in both phases of matter that persist to the final products, enabling the characterization of the branching space of the S₁-S₀ conical intersection. We observe a systematic lengthening of the relaxation timescales in the liquid phase and a red shift of the measured excited-state frequencies that is most pronounced for the complex reaction coordinate. These results characterize in detail the influence of the liquid environment on both electronic and structural dynamics during a complete photochemical transformation.

Electronic coherences in nonadiabatic molecular photophysics revealed by time-resolved photoelectron spectroscopy

Time-resolved photoelectron spectroscopy (TRPES) is a promising technique for the study of ultrafast molecular processes, such as the nonadiabatic dynamics taking place at conical intersections. Directly accessing the evolution of the coherences generated at the conical intersection should provide most valuable dynamical information. However, the signals are dominated by background contributions due to state populations, and most theoretical treatments completely neglect the role of the coherences. Here we show that distinguishable signatures of molecular coherences appear in TRPES. These can be recorded using currently available ultrashort pulses and unambiguously extracted at the postprocessing stage. The technique thus provides direct access to nonadiabatic coherence dynamics.

Isomer-Selective Spectroscopy and Dynamics of Phenol-Arn (n ≤ 5) Clusters

Structures and ionization-induced solvation dynamics of phenol-(argon)_n clusters, PhOH-Ar_n (n ≤ 5), were studied by using a variety of isomer-selective photoionization and vibrational spectroscopic techniques. Several higher-energy isomers were found and assigned for the first time by systematically controlling the experimental conditions of the supersonic expansion. This behavior is also confirmed for the PhOH-Kr₂ cluster. Solvation structures are elucidated by evaluating systematic shifts in the S₁ ← S₀ origin and ionization energies obtained by resonance-enhanced photoionization, in addition to the OH stretching frequency obtained by IR photodissociation. Isomer-selective picosecond time-resolved IR spectroscopy for the n = 2 clusters revealed that the dynamics for the ionization-induced intermolecular π → H site-switching reaction strongly depends on the initial isomeric structure. In particular, the reaction time for the (1|1) isomer is 7 ps, while that for (2|0) is <3 ps. This difference shows that the switching time is determined by the distance of the reaction coordinate between the initial π-site and the final OH-site.

Unravelling the role of charge transfer state during ultrafast intersystem crossing in compact organic chromophores

When organic electron donor (D) and acceptor (A) chromophores are linked together, an electron transfer (ET) state can take place. When a short bridge such as one Sigma bond is used to link the donor and the acceptor, complete charge separation is difficult to access and one usually observes an intramolecular charge transfer (CT) state instead. Due to the inevitable coupling between the donor and the acceptor in compact organic chromophores, the most common decay pathway for the CT state is charge recombination, which may lead to a distinct longer wavelength fluorescence emission or non-radiative dissipation of the excited state energy. However, recent studies have shown that unique excited state dynamics can be observed when the CT state is involved during both forward and backward intersystem crossing (ISC) from singlet excited states to triplet excited states in organic chromophores. Analysis of the mechanism for ISC involving the CT state has received much attention over the last decade. In this perspective, we present a collection of molecular design rationales, spectroscopy and theoretical investigations that provide insights into the mechanism of the ISC involving the CT state in compact organic chromophores. We hope that this perspective will prove beneficial for researchers to design novel compact organic chromophores with a predictable ISC property for future biochemical and optoelectronic applications.

Modeling the Photoelectron Angular Distributions of Molecular Anions: Roles of the Basis Set, Orbital Choice, and Geometry

A study investigating the effect of the basis set, orbital choice, and geometry on the modeling of photoelectron angular distributions (PADs) of molecular anions is presented. Experimental and modeled PADs for a number of molecular anions, including both closed- and open-shell systems, are considered. Guidelines are suggested for chemists who wish to design calculations to capture the correct chemical physics of the anisotropy of photodetachment, while balancing the computational cost associated with larger molecular anions.

Signatures of Conical Intersection Dynamics in the Time-Resolved Photoelectron Spectrum of Furan: Theoretical Modeling with an Ensemble Density Functional Theory Method

The non-adiabatic dynamics of furan excited in the ππ* state (S2 in the Franck-Condon geometry) was studied using non-adiabatic molecular dynamics simulations in connection with an ensemble density functional method. The time-resolved photoelectron spectra were theoretically simulated in a wide range of electron binding energies that covered the valence as well as the core electrons. The dynamics of the decay (rise) of the photoelectron signal were compared with the excited-state population dynamics. It was observed that the photoelectron signal decay parameters at certain electron binding energies displayed a good correlation with the events occurring during the excited-state dynamics. Thus, the time profile of the photoelectron intensity of the K-shell electrons of oxygen (decay constant of 34 ± 3 fs) showed a reasonable correlation with the time of passage through conical intersections with the ground state (47 ± 2 fs). The ground-state recovery constant of the photoelectron signal (121 ± 30 fs) was in good agreement with the theoretically obtained excited-state lifetime (93 ± 9 fs), as well as with the experimentally estimated recovery time constant (ca. 110 fs). Hence, it is proposed to complement the traditional TRPES observations with the trXPS (or trNEXAFS) measurements to obtain more reliable estimates of the most mechanistically important events during the excited-state dynamics.

Diradical Interactions in Ring-Open Isoxazole

Electron capture by the σ* LUMO of isoxazole triggers the dissociation of the O-N bond and the opening of the ring. Photodetachment of the resulting anion accesses a neutral structure, in which the O· and ·N bond fragments interact through the intact remainder of the molecular ring and via a 3 Å gap created by the bond dissociation. These through-bond and through-space interactions result in a dense manifold of diradical states, including (in the order of increasing energy) a triplet, an open-shell singlet, a closed-shell singlet, and another triplet state. We report photoelectron spectra that reflect partially resolved signatures of these states. Remarkably, the structure of the isoxazole diradical manifold is qualitatively different from that of the analogous system in oxazole. The distinct properties of the two manifolds are explained by using a coupled-fragments molecular-orbital model. Consistent with the past conclusions [Culberson et al. Phys. Chem. Chem. Phys. 2014, 16, 3964-3972], the lingering through-space interactions between the O· and ·C bond fragments in ring-open oxazole are responsible for the relative stabilization of the closed-shell singlet state, which correlates with the ground-state cyclic structure. In contrast, the placement of the N atom in the terminal position within the ring-open structure of isoxazole is the key factor leading to the near degeneracy of the π and σ* orbitals, favoring a triplet-state configuration.

Improved insights in time-resolved photoelectron imaging

We review new light source developments and data analysis considerations relevant to the time-resolved photoelectron imaging technique. Case studies illustrate how these themes may enhance understanding in studies of excited state molecular dynamics.

Computation of Molecular Electron Affinities Using an Ensemble Density Functional Theory Method

The computation of electron attachment energies (electron affinities) was implemented in connection with an ensemble density functional theory method, the state-interaction state-averaged spin-restricted ensemble-referenced Kohn-Sham (SI-SA-REKS or SSR) method. With the use of the extended Koopmans' theorem, the electron affinities and the respective Dyson orbitals are obtained directly for the neutral molecule, thus avoiding the necessity to compute the ionized system. Together with the EKT-SSR (extended Koopmans' theorem-SSR) method for ionization potentials, which was developed earlier, EKT-SSR for electron affinities completes the implementation of the EKT-SSR formalism, which can now be used for obtaining electron detachment as well as the electron attachment energies of molecules in the ground and excited electronic states. The extended EKT-SSR method was tested in the calculation of several closed-shell molecules. For the molecules in the ground states, the EKT-SSR energies of Dyson's orbitals are virtually identical to the energies of the unoccupied orbitals in the usual single-reference spin-restricted Kohn-Sham calculations. For the molecules in the excited states, EKT-SSR predicts an increase of the most positive electron affinity by approximately the amount of the vertical excitation energy. The electron affinities of a number of diradicals were calculated with EKT-SSR and compared with the available experimental data. With the use of a standard density functional (BH&HLYP), the EKT-SSR electron affinities deviate on average by ca. 0.2 eV from the experimental data. It is expected that the agreement with the experiment can be improved by designing density functionals parametrized for ionization energies.

Excited state dynamics of cis,cis-1,3-cyclooctadiene: UV pump VUV probe time-resolved photoelectron spectroscopy

We present UV pump, vacuum ultraviolet probe time-resolved photoelectron spectroscopy measurements of the excited state dynamics of cis,cis-1,3-cyclooctadiene. A 4.75 eV deep UV pump pulse launches a vibrational wave packet on the first electronically excited state, and the ensuing dynamics are probed via ionization using a 7.92 eV probe pulse. The experimental results indicate that the wave packet undergoes rapid internal conversion to the ground state in under 100 fs. Comparing the measurements with electronic structure and trajectory surface hopping calculations, we are able to interpret the features in the measured photoelectron spectra in terms of ionization to several states of the molecular cation.

Structural or population dynamics: what is revealed by the time-resolved photoelectron spectroscopy of 1,3-cyclohexadiene? A study with an ensemble density functional theory method

Time-resolved photoelectron spectra during the photochemical ring-opening reaction of 1,3-cyclohexadiene (CHD) are modeled by an ensemble density functional theory (eDFT) method. The computational methodology employed in this work is capable of correctly describing the multi-reference effects arising in the ground and excited electronic states of molecules, which is important for the correct description of the ring-opening reaction of CHD. The geometries of molecular species along the non-adiabatic molecular dynamics (NAMD) trajectories reported in a previous study of the CHD photochemical ring-opening were used in this work to calculate the ionization energies and the respective Dyson orbitals for all possible ionization channels. The obtained theoretical time-resolved spectra display decay characteristics in a reasonable agreement with the experimental observations; i.e., the decay (and rise) of the most mechanistically significant signals occurs on the timescale of 100-150 fs. This is very different from the excited state population decay characteristics (τ_S1 = 234 ± 8 fs) obtained in the previous NAMD study. The difference between the population decay and the decay of the photoelectron signal intensity is traced back to the geometric transformation that the molecule undergoes during the photoreaction. This demonstrates the importance of including the geometric information in interpretation of the experimental observations.

Computation of Molecular Ionization Energies Using an Ensemble Density Functional Theory Method

Computation of the ionization energies and of the respective Dyson orbitals based on the use of the extended Koopmans theorem (EKT) is implemented in connection with an ensemble density functional theory (eDFT) method, the state-interaction state-averaged spin-restricted ensemble-referenced Kohn-Sham (SI-SA-REKS or SSR) method. The new methodology enables fast computation of the ionization energies and evaluation of the respective Dyson orbitals, the square norms of which are related with the ionization probabilities, in the ground and excited electronic states of molecules. As the application of EKT recycles the intermediate quantities from the SSR analytical energy gradient, evaluation of the ionization energies and probabilities can be carried out on-the-fly during the nonadiabatic molecular dynamics simulations. This opens up a perspective for fast theoretical simulation of the time-resolved photoelectron spectroscopy observations. In the present work, the new methodology is tested in the computation of the ionization energies and Dyson orbitals of several molecules in the ground and excited electronic states, including strongly correlated species, such as the ozone molecule, dissociating chemical bonds, and conical intersections.

Ab Initio Linear and Pump-Probe Spectroscopy of Excitons in Molecular Crystals

Linear and nonlinear spectroscopies are powerful tools used to investigate the energetics and dynamics of electronic excited states of both molecules and crystals. While highly accurate ab initio calculations of molecular spectra can be performed relatively routinely, extending these calculations to periodic systems is challenging. Here, we present calculations of the linear absorption spectrum and pump-probe two-photon photoemission spectra of the naphthalene crystal using equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD). Molecular acene crystals are of interest due to the low-energy multiexciton singlet states they exhibit, which have been studied extensively as intermediates involved in singlet fission. Our linear absorption spectrum is in good agreement with experiment, predicting a first exciton absorption peak at 4.4 eV, and our two-photon photoemission spectra capture the qualitative behavior of multiexciton states, whose double-excitation character cannot be captured by current methods. The simulated pump-probe spectra provide support for existing interpretations of two-photon photoemission experiments in closely related acene crystals such as tetracene and pentacene.

Time-resolved near-edge X-ray absorption fine structure of pyrazine from electronic structure and nuclear wave packet dynamics simulations

As a demonstration of the analysis of the electronic structure and the nuclear dynamics from time-resolved near-edge X-ray absorption fine structure (TR-NEXAFS), we present the TR-NEXAFS spectra of pyrazine following the excitation to the ¹B_2u(ππ^*) state. The spectra are calculated combining the frozen-core/core-valence separated equation-of-motion coupled cluster singles and doubles approach for the spectral signatures and the multiconfiguration time-dependent Hartree method for the wave packet propagation. The population decay from the ¹B_2u(ππ^*) state to the ¹B_3u(nπ^*) and ¹A_u(nπ^*) states, followed by oscillatory flow of population between the ¹B_3u(nπ^*) and ¹A_u(nπ^*) states, is interpreted by means of visualization of the potential energy curves and the reduced nuclear densities. By examining the electronic structure of the three valence-excited states and the final core-excited states, we observe that the population dynamics is explicitly reflected in the TR-NEXAFS spectra, especially when the heteroatoms are selected as the X-ray absorption sites. This work illustrates the feasibility of extracting fine details of molecular photophysical processes from TR-NEXAFS spectra by using currently available theoretical methods.

Femtosecond real-time probing of the excited-state intramolecular proton transfer reaction in methyl salicylate

The excited-state intramolecular proton transfer (ESIPT) process and subsequent electronic relaxation dynamics in methyl salicylate have been investigated using femtosecond time-resolved ion yield spectroscopy combined with time-resolved photoelectron imaging. Excitation with a tunable pump pulse populates the keto tautomer in the first excited electronic state S₁(ππ^*). As a hydrogen atom transfers from the phenolic group to the carbonyl group within 100 fs, the molecular geometry changes gradually, leading to a variation in the electronic photoionization channel. By virtue of the accidental resonance with some intermediate Rydberg states, the time-dependent photoelectron spectra provide a direct mapping of the ESIPT reaction from the initially populated keto tautomer to the proton-transferred enol tautomer. Subsequently, the population around the enol configuration undergoes intramolecular vibrational redistribution on a subpicosecond time scale, followed by internal conversion to the ground state with a wavelength-dependent lifetime in the picosecond range. Furthermore, the excitation energies of several Rydberg states in methyl salicylate are determined experimentally.

Time-resolved radiation chemistry: femtosecond photoelectron spectroscopy of electron attachment and photodissociation dynamics in iodide-nucleobase clusters

Iodide-nucleobase (I-·N) clusters studied by time-resolved photoelectron spectroscopy (TRPES) are an opportune model system for examining radiative damage of DNA induced by low-energy electrons. By initiating charge transfer from iodide to the nucleobase and following the dynamics of the resulting transient negative ions (TNIs) with femtosecond time resolution, TRPES provides a novel window into the chemistry triggered by the attachment of low-energy electrons to nucleobases. In this Perspective, we examine and compare the dynamics of electron attachment, autodetachment, and photodissociation in a variety of I-·N clusters, including iodide-uracil (I-·U), iodide-thymine (I-·T), iodide-uracil-water (I-·U·H2O), and iodide-adenine (I-·A), to develop a more unified representation of our understanding of nucleobase TNIs. The experiments probe whether dipole-bound or valence-bound TNIs are formed initially and the subsequent time evolution of these species. We also provide an outlook for forthcoming applications of TRPES to larger iodide-containing complexes to enable the further investigation of microhydration dynamics in nucleobases, as well as electron attachment and photodissociation in more complex nucleic acid constituents.

Imaging Reversible and Irreversible Structural Evolution in Photoexcited 2,4-Difluoroaniline

Here, we demonstrate the capability of femtosecond time-resolved photoelectron imaging to visualize the reversible and irreversible structural evolution in electronically excited 2,4-difluoroaniline. As a coherent superposition of out-of-plane vibrational motions is created following 299.8 nm excitation, the molecular geometry alters periodically, thus modulating the photoionization channel. The reversible nuclear motion between distinct geometries is directly observed as the reverse beats in the time-dependent photoelectron spectra. Moreover, with the molecule highly vibrationally excited at 289.0 nm, the exponential decrease/increase changes in photoelectron signal provide a physically intuitive and complete picture of the irreversible geometry rearrangement away from a nonplanar geometry in the vertical Franck-Condon region toward the planar minimum.

Imaging large amplitude out-of-plane motion in photoexcited pentafluorobenzene using time-resolved photoelectron spectroscopy: a computational study

Excited-state dynamics of pentafluorobenzene is studied in detail for a quartic vibronic coupling model including the six b1 vibrational modes of the molecule and the two lowest excited electronic states. The study analyzes the influence of the large-amplitude out-of-plane vibrational motion on the electronic dynamics and extends to the simulation of the emerging time-resolved photoelectron spectra. The mapping of coherent non-separable electron-nuclear motion into oscillatory photoelectron signals is discussed.

Conical-intersection dynamics and ground-state chemistry probed by extreme-ultraviolet time-resolved photoelectron spectroscopy

Time-resolved photoelectron spectroscopy (TRPES) is a useful approach to elucidate the coupled electronic-nuclear quantum dynamics underlying chemical processes, but has remained limited by the use of low photon energies. Here, we demonstrate the general advantages of XUV-TRPES through an application to NO₂, one of the simplest species displaying the complexity of a non-adiabatic photochemical process. The high photon energy enables ionization from the entire geometrical configuration space, giving access to the true dynamics of the system. Specifically, the technique reveals dynamics through a conical intersection, large-amplitude motion and photodissociation in the electronic ground state. XUV-TRPES simultaneously projects the excited-state wave packet onto many final states, offering a multi-dimensional view of the coupled electronic and nuclear dynamics. Our interpretations are supported by ab initio wavepacket calculations on new global potential-energy surfaces. The presented results contribute to establish XUV-TRPES as a powerful technique providing a complete picture of ultrafast chemical dynamics from photoexcitation to the final products.

Real-time observation of the photoionization-induced water rearrangement dynamics in the 5-hydroxyindole-water cluster by time-resolved IR spectroscopy

Solvation plays an essential role in controlling the mechanism and dynamics of chemical reactions in solution. The present study reveals that changes in the local solute-solvent interaction have a great impact on the timescale of solvent rearrangement dynamics. Time-resolved IR spectroscopy has been applied to a hydration rearrangement reaction in the monohydrated 5-hydroxyindole-water cluster induced by photoionization of the solute molecule. The water molecule changes the stable hydration site from the indolic NH site to the substituent OH site, both of which provide a strongly attractive potential for hydration. The rearrangement time constant amounts to 8 ± 2 ns, and is further slowed down by a factor of more than five at lower excess energy. These rearrangement times are slower by about three orders of magnitude than those reported for related systems where the water molecule is repelled from a repulsive part of the interaction potential toward an attractive well. The excess energy dependence of the time constant is well reproduced by RRKM theory. Differences in the reaction mechanism are discussed on the basis of energy relaxation dynamics.

Femtosecond time-resolved observation of butterfly vibration in electronically excited o-fluorophenol

The butterfly vibration during the hydrogen tunneling process in electronically excited o-fluorophenol has been visualized in real time by femtosecond time-resolved ion yield spectroscopy coupled with time-resolved photoelectron imaging technique. A coherent superposition of out-of-plane C–F butterfly motions is prepared in the first excited electronic state (S₁). As the C–F bond vibrates with respect to the aromatic ring, the nuclear geometry varies periodically, leading to the corresponding variation in the photoionization channel. By virtue of the more favorable ionization probability from the nonplanar minimum via resonance with the Rydberg states, the evolution of the vibrational wave packet is manifested as a superimposed beat in the parent-ion transient. Moreover, time-resolved photoelectron spectra offer a direct mapping of the oscillating butterfly vibration between the planar geometry and nonplanar minimum. The beats for the photoelectron peaks originating from the planar geometry are out of phase with those from the nonplanar minimum. Our results provide a physically intuitive and complete picture of the oscillatory flow of energy responsible for the coherent vibrational motion on the excited state surface.

An apparatus for quantitative high-harmonic generation spectroscopy in molecular vapours

We present an apparatus for performing gas phase high-harmonic generation spectroscopy of molecules primarily found in the liquid phase. Liquid molecular samples are heated in a temperature controlled bath and their vapour is used to back a continuous flow gas jet, with vapour pressures of over 1 bar possible. In order to demonstrate the system, we perform high harmonic spectroscopy experiments in benzene with a 1.8 μm driving field. Using the unique capabilities of the system, we obtain spectra that are nearly free from the effects of longitudinal phase-matching, amenable to comparison with advanced numerical modelling.

Deciphering environment effects in peptide bond solvation dynamics by experiment and theory

Probing solvation dynamics at the molecular level: different water migration pathways around a peptide bond.

A proposed new scheme for vibronically resolved time-dependent photoelectron spectroscopy: pump-repump-continuous wave-photoelectron spectroscopy (prp-cw-pes)

We propose a new scheme for time-resolved photoelectron spectroscopy denoted as pump-repump-continuous wave-photoelectron spectroscopy (prp-cw-pes). This scheme is comprised of two femtosecond laser (pump) pulses under cw illumination (probe). By changing the time-delay between pump and repump lasers one can manipulate the populations of vibronic levels in electronic excited states. The cw laser acts on for a long time and establishes resonance between excited states and the continuum photo-ionized states. Sharp spectra can be obtained from the resonance condition [formula: see text]. The intensities in the spectra are sensitive to the time-delay between the pump-repump pulses, but only depend on the populations of excited states, not the phase relations (coherence). As a result, each time-delayed snapshot spectrum is a weighted sum of so-called fingerprints, where a fingerprint is the vibrationally resolved photoelectron spectrum for a single vibronic excited state. The latter information can potentially be simulated reliably using vibronic models and wave packet propagation methods. In the easiest application of the experiment, different time-delays produce different spectra, for a single molecular system. This wealth of experimental data can be fitted to an, ideally small, set of theoretical fingerprints by adjusting the populations as fitting parameters. This technique might be able to distinguish between closely related molecular species. Adopting a different viewpoint, the proposed scheme can also be employed to monitor the time-dependent dynamics by changing the phase relationship between the pump and repump laser that can be viewed as a "control mechanism" employed in wave packet interferometry. Simplifications arise as the change in the spectra is due to the changing populations, not because of the coherence. In this paper, we outline the ideas behind the scheme and illustrate the ideas theoretically using simple model systems.

Excited-state dynamics of furan studied by sub-20-fs time-resolved photoelectron imaging using 159-nm pulses

The excited-state dynamics of furan were studied by time-resolved photoelectron imaging using a sub-20-fs deep UV (198 nm) and vacuum UV (159 nm) light source. The 198- and 159-nm pulses produce photoionization signals in both pump-probe and probe-pump pulse sequences. When the 198-nm pulse precedes the 159-nm pulse, it creates the (1)A2(3s) Rydberg and (1)B2(ππ(∗)) valence states, and the former decays exponentially with a time constant of about 20 fs whereas the latter exhibits more complex wave-packet dynamics. When the 159-nm pulse precedes the 198-nm pulse, a wave packet is created on the (1)A1(ππ(∗)) valence state, which rapidly disappears from the observation window owing to structural deformation. The 159-nm photoexcitation also creates the 3s and 3px,y Rydberg states non-adiabatically.