Dynamics of electron solvation in methanol: Excited state relaxation and generation by charge-transfer-to-solvent

Extreme ultraviolet time-resolved photoelectron spectroscopy of adenine, adenosine and adenosine monophosphate in a liquid flat jet

Time-resolved photoelectron spectroscopy using an extreme-ultraviolet (XUV) probe pulse was used to investigate the UV photoinduced dynamics of adenine (Ade), adenosine (Ado), and adenosine-5-monophosphate (AMP) in a liquid water jet. In contrast to previous studies using UV probe pulses, the XUV pulse at 21.7 eV can photoionize all excited states of a molecule, allowing for full relaxation pathways to be addressed after excitation at 4.66 eV. This work was carried out using a gas-dynamic flat liquid jet, resulting in considerably enhanced signal compared to a cylindrical jet. All three species decay on multiple time scales that are assigned based on their decay associated spectra; the fastest decay of ∼100 fs is assigned to ππ* decay to the ground state, while a smaller component with a lifetime of ∼500 fs is attributed to the nπ* state. An additional slower channel in Ade is assigned to the 7H Ade conformer, as seen previously. This work demonstrates the capability of XUV-TRPES to disentangle non-adiabatic dynamics in an aqueous solution in a state-specific manner and represents the first identification of the nπ* state in the relaxation dynamics of adenine and its derivatives.

Distortion Correction of Low-Energy Photoelectron Spectra of Liquids Using Spectroscopic Data for Solvated Electrons

Time-resolved photoelectron spectroscopy (TRPES) enables real-time observation of ultrafast electronic dynamics in solutions. When extreme ultraviolet (EUV) probe pulses are employed, they can ionize solutes from all electronic states involved in the dynamics. However, EUV pulses also produce a strong ionization signal from a solvent that is typically 6 orders of magnitude greater than the pump-probe photoelectron signal of solutes. Alternatively, UV probe pulses enable highly sensitive and selective observation of photoexcited solutes because typical solvents such as water are transparent to UV radiation. An obstacle in such UV-TRPES measurements is spectral distortion caused by electron scattering and a yet to be identified mechanism in liquids. We have previously proposed the spectral retrieval (SR) method as an a posteriori approach to removing the distortion and overcoming this difficulty in UV-TRPES; however, its accuracy has not yet been verified by comparison with EUV-TRPES results. In the present study, we perform EUV-TRPES for charge transfer reactions in water, methanol, and ethanol, and verify SR analysis of UV-TRPES. We also estimate a previously undetermined energy-dependent intensity factor and expand the basis sets for SR analysis. The refined SR method is employed for reanalyzing the UV-TRPES data for the formation and relaxation dynamics of solvated electrons in various systems. The electron binding energy distributions for solvated electrons in liquid water, methanol, and ethanol are confirmed to be Gaussian centered at 3.78, 3.39, and 3.25 eV, respectively, in agreement with Nishitani et al. [ Sci. Adv. 2019, 5(8), eaaw6896]. An effective energy gap between the conduction band and the vacuum level at the gas-liquid interface is estimated to be 0.2 eV for liquid water and 0.1 eV for methanol and ethanol.

Charge Transfer Reactions from I- to Polar Protic Solvents Studied Using Ultrafast Extreme Ultraviolet Photoelectron Spectroscopy

Charge transfer reactions from I^- to solvent water, methanol, and ethanol were studied using extreme ultraviolet time-resolved photoelectron spectroscopy (EUV-TRPES). This technique eliminates spectral broadening, previously seen in UV-TRPES, caused by electron inelastic scattering in liquids, and enables clear observation of the temporal evolution of the spectral shape. The peak position, width, and intensity of the electron binding energy distribution indicate electron detachment and subsequent solvation and thermalization processes. Geminate recombination between detached electrons and iodine atoms is discussed using a diffusion equation and a global fitting analysis based on a kinetics model.

Nonadiabatic Dynamics Studied by Liquid-Jet Time-Resolved Photoelectron Spectroscopy

The development of the liquid microjet technique by Faubel and co-workers has enabled the investigation of high vapor pressure liquids and solutions utilizing high-vacuum methods. One such method is photoelectron spectroscopy (PES), which allows one to probe the electronic properties of a sample through ionization in a state-specific manner. Liquid microjets consisting of pure solvents and solute-solvent systems have been studied with great success utilizing PES and, more recently, time-resolved PES (TRPES). Here, we discuss progress made over recent years in understanding the solvation and excited state dynamics of the solvated electron and nucleic acid constituents (NACs) using these methods, as well as the prospect for their future.The solvated electron is of particular interest in liquid microjet experiments as it represents the simplest solute system. Despite this simplicity, there were still many unresolved questions about its binding energy and excited state relaxation dynamics that are ideal problems for liquid microjet PES. In the work discussed in this Account, accurate binding energies were measured for the solvated electron in multiple high vapor pressure solvents. The advantages of liquid jet PES were further highlighted in the femtosecond excited state relaxation studies on the solvated electron in water where a 75 ± 20 fs lifetime attributable to internal conversion from the excited p-state to a hot ground state was measured, supporting a nonadiabatic relaxation mechanism.Nucleic acid constituents represent a class of important solutes with several unresolved questions that the liquid microjet PES method is uniquely suited to address. As TRPES is capable of tracking dynamics with state-specificity, it is ideal for instances where there are multiple excited states potentially involved in the dynamics. Time-resolved studies of NAC relaxation after excitation using ultraviolet light identified relaxation lifetimes from multiple excited states. The state-specific nature of the TRPES method allowed us to identify the lack of any signal attributable to the ¹nπ* state in thymine derived NACs. The femtosecond time resolution of the technique also aided in identifying differences between the excited state lifetimes of thymidine and thymidine monophosphate. These have been interpreted, aided by molecular dynamics simulations, as an influence of conformational differences leading to a longer excited state lifetime in thymidine monophosphate.Finally, we discuss advances in tabletop light sources extending into the extreme ultraviolet and soft X-ray regimes that allow expansion of liquid jet TRPES to full valence band and potentially core level studies of solutes and pure liquids in liquid microjets. As most solutes have ground state binding energies in the range of 10 eV, observation of both excited state decay and ground state recovery using ultraviolet pump-ultraviolet probe TRPES has been intractable. With high-harmonic generation light sources, it will be possible to not only observe complete relaxation pathways for valence level dynamics but to also track dynamics with element specificity by probing core levels of the solute of interest.

Design and characterization of a magnetic bottle electron spectrometer for time-resolved extreme UV and X-ray photoemission spectroscopy of liquid microjets

We describe a magnetic bottle time-of-flight electron spectrometer designed for time-resolved photoemission spectroscopy of a liquid microjet using extreme UV and X-ray radiation. The spectrometer can be easily reconfigured depending on experimental requirements and the energy range of interest. To improve the energy resolution at high electron kinetic energy, a retarding potential can be applied either via a stack of electrodes or retarding mesh grids, and a flight-tube extension can be attached to increase the flight time. A gated electron detector was developed to reject intense parasitic signal from light scattered off the surface of the cylindrically shaped liquid microjet. This detector features a two-stage multiplication with a microchannel plate plus a fast-response scintillator followed by an image-intensified photon detector. The performance of the spectrometer was tested at SPring-8 and SACLA, and time-resolved photoelectron spectra were measured for an ultrafast charge transfer to solvent reaction in an aqueous NaI solution with a 200 nm UV pump pulses from a table-top ultrafast laser and the 5.5 keV hard X-ray probe pulses from SACLA.

Photoemission from non-polar aromatic molecules in the gas and liquid phase

The photoelectron spectra of both liquid and gas phase aromatic molecules are reported. The spectra were obtained using a 34.1 eV source produced by high harmonic generation and analysed with the help of high-level ab initio simulations using the reflection principle combined with path integral molecular dynamics simulations accounting for nuclear quantum effects for the gas phase. We demonstrate the suitability of three trimethylbenzenes (1,3,5-trimethylbenzene, 1,2,3-trimethylbenzene and 1,2,4-trimethylbenzene) as a solvent for liquid photoelectron spectroscopy of solute species. We also discuss the electrokinetic charging of a non-polar liquid jet.

Relaxation Dynamics of Hydrated Thymine, Thymidine, and Thymidine Monophosphate Probed by Liquid Jet Time-Resolved Photoelectron Spectroscopy

The relaxation dynamics of thymine and its derivatives thymidine and thymidine monophosphate are studied using time-resolved photoelectron spectroscopy applied to a water microjet. Two absorption bands are studied; the first is a bright ππ* state which is populated using tunable-ultraviolet light in the range 4.74-5.17 eV and probed using a 6.20 eV probe pulse. By reversing the order of these pulses, a band containing multiple ππ* states is populated by the 6.20 eV pulse and the lower energy pulse serves as the probe. The lower lying ππ* state is found to decay in ∼400 fs in both thymine and thymidine independent of pump photon energy, while thymidine monophosphate decays vary from 670 to 840 fs with some pump energy dependence. The application of a computational quantum mechanical/molecular mechanical scheme at the XMS-CASPT2//CASSCF/AMBER level of theory suggests that conformational differences existing between thymidine and thymidine monophosphate in solution account for this difference. The higher lying ππ* band is found to decay in ∼600 fs in all three cases, but it is only able to be characterized when the 5.17 eV probe pulse is used. Notably, no long-lived signal from an nπ* state can be identified in either experiment on any of the three molecules.

Solvated electron formation from the conduction band of liquid methanol: Transformation from a shallow to deep trap state

We report solvated electron (e_solv ^-) formation dynamics from the conduction band of liquid methanol studied using femtosecond time-resolved photoelectron spectroscopy. Liquid methanol is excited with vacuum UV (9.3 eV) pump pulses, and the subsequent electron dynamics are probed with UV pulses. The photoelectron signal exhibits a short-lived component (τ = 85 fs) without spectral evolution followed by a long-lived component with continuous spectral evolution over tens of picoseconds. We ascribe the former to a superexcited state, most likely the Wannier exciton, and the latter to the ground electronic state of e_solv ^-. In order to extract accurate energetics from the observed photoelectron spectra, we employ a spectral retrieval method to account for spectral broadening and shifting due to inelastic scattering of photoelectrons in the liquid. The electron binding energy (eBE) of the initial trap state of an electron is determined to be about 1.5 eV, and its biexponential increase up to 3.4 eV is observed with time constants of 2 and 31 ps, which are greater than 0.27 and 13 ps observed for e_solv ^- created by the charge-transfer-to-solvent reaction from CH₃O^- to liquid methanol. The solvation dynamics of e_solv ^- created by the electron trapping exhibit a pseudoisosbestic point at a pump-probe delay time of around 15 ps, and the peak energy of the eBE distribution rapidly changes around that time. These results indicate that there exist two trap states, both of which exhibit increasing eBE with time; however, the eBE of the shallow trap state increases only up to 2.1 eV, and transformation to a deep trap state at 25 ps occurs to reach an eBE of 3.4 eV.

Ultrafast Internal Conversion and Solvation of Electrons in Water, Methanol, and Ethanol

Ultrafast internal conversion from the first excited state of a solvated electron in water, methanol, and ethanol is investigated using time-resolved photoelectron spectroscopy of liquid microjets and a spectral retrieval method. Photoelectron spectra corrected for inelastic scattering clearly reveal well-separated signals from the excited and ground states, and the latter enables us to analyze the solvation dynamics in the ground state after internal conversion. Measurements with 25 fs time resolution identify a rapid increase in the vertical electron binding energy of the solvated electron owing to nuclear wave packet motions in the excited state and allow us to precisely determine the internal conversion time.

Design and characterization of a recirculating liquid-microjet photoelectron spectrometer for multiphoton ultraviolet photoelectron spectroscopy

A new recirculating liquid-microjet photoelectron spectrometer for multiphoton ultraviolet photoelectron spectroscopy is described. A recirculating system is essential for studying samples that are only available in relatively small quantities. The reduction in background pressure when using the recirculating system compared to a liquid-nitrogen cold-trap results in a significant improvement in the quality of the photoelectron spectra. Moreover, the recirculating system results in a negligible streaming potential. The instrument design, operation, and characterization are described in detail, and its performance is illustrated by comparing a photoelectron spectrum of aqueous phenol recorded using the recirculating system with one recorded using a liquid nitrogen cold-trap.

Time-resolved photoelectron spectroscopy of adenosine and adenosine monophosphate photodeactivation dynamics in water microjets

The excited state relaxation dynamics of adenosine and adenosine monophosphate were studied at multiple excitation energies using femtosecond time-resolved photoelectron spectroscopy in a liquid water microjet. At pump energies of 4.69-4.97 eV, the lowest ππ^* excited state, S₁, was accessed and its decay dynamics were probed via ionization at 6.20 eV. By reversing the role of the pump and probe lasers, a higher-lying ππ^* state was excited at 6.20 eV and its time-evolving photoelectron spectrum was monitored at probe energies of 4.69-4.97 eV. The S₁ ππ^* excited state was found to decay with a lifetime ranging from ∼210 to 250 fs in adenosine and ∼220 to 250 fs in adenosine monophosphate. This lifetime drops with increasing pump photon energy. Signal from the higher-lying ππ^* excited state decayed on a time scale of ∼320 fs and was measureable only in adenosine monophosphate.

Ab initio molecular dynamics study of solvated electrons in methanol clusters

Stable surface excess electronic states in small methanol cluster anions were identified and characterized in ab initio molecular dynamics simulations.

Excess electrons in methanol clusters: Beyond the one-electron picture

We performed a series of comparative quantum chemical calculations on various size negatively charged methanol clusters, CH₃OH_n^-. The clusters are examined in their optimized geometries (n = 2-4), and in geometries taken from mixed quantum-classical molecular dynamics simulations at finite temperature (n = 2-128). These latter structures model potential electron binding sites in methanol clusters and in bulk methanol. In particular, we compute the vertical detachment energy (VDE) of an excess electron from increasing size methanol cluster anions using quantum chemical computations at various levels of theory including a one-electron pseudopotential model, several density functional theory (DFT) based methods, MP2 and coupled-cluster CCSD(T) calculations. The results suggest that at least four methanol molecules are needed to bind an excess electron on a hydrogen bonded methanol chain in a dipole bound state. Larger methanol clusters are able to form stronger interactions with an excess electron. The two simulated excess electron binding motifs in methanol clusters, interior and surface states, correlate well with distinct, experimentally found VDE tendencies with size. Interior states in a solvent cavity are stabilized significantly stronger than electron states on cluster surfaces. Although we find that all the examined quantum chemistry methods more or less overestimate the strength of the experimental excess electron stabilization, MP2, LC-BLYP, and BHandHLYP methods with diffuse basis sets provide a significantly better estimate of the VDE than traditional DFT methods (BLYP, B3LYP, X3LYP, PBE0). A comparison to the better performing many electron methods indicates that the examined one-electron pseudopotential can be reasonably used in simulations for systems of larger size.

Dynamics of dipole- and valence bound anions in iodide-adenine binary complexes: A time-resolved photoelectron imaging and quantum mechanical investigation

Dipole bound (DB) and valence bound (VB) anions of binary iodide-adenine complexes have been studied using one-color and time-resolved photoelectron imaging at excitation energies near the vertical detachment energy. The experiments are complemented by quantum chemical calculations. One-color spectra show evidence for two adenine tautomers, the canonical, biologically relevant A9 tautomer and the A3 tautomer. In the UV-pump/IR-probe time-resolved experiments, transient adenine anions can be formed by electron transfer from the iodide. These experiments show signals from both DB and VB states of adenine anions formed on femto- and picosecond time scales, respectively. Analysis of the spectra and comparison with calculations suggest that while both the A9 and A3 tautomers contribute to the DB signal, only the DB state of the A3 tautomer undergoes a transition to the VB anion. The VB anion of A9 is higher in energy than both the DB anion and the neutral, and the VB anion is therefore not accessible through the DB state. Experimental evidence of the metastable A9 VB anion is instead observed as a shape resonance in the one-color photoelectron spectra, as a result of UV absorption by A9 and subsequent electron transfer from iodide into the empty π-orbital. In contrast, the iodide-A3 complex constitutes an excellent example of how DB states can act as doorway state for VB anion formation when the VB state is energetically available.

Microjets and coated wheels: versatile tools for exploring collisions and reactions at gas–liquid interfaces

Scattering experiments using liquid microjets provide a window into collisions and reactions at the surfaces of high vapor pressure liquids.

Electrons, excitons and hydrogen bonding: electron-promoted desorption from molecular ice surfaces

Desorption of benzene from methanol and diethyl ether ices during irradiation with 250 eV electrons is reported and compared with our previous work on benzene/water ices to highlight the role of hydrogen bonding in excitation transport.