Vibrational Motions in Ultrafast Electronic Relaxation of Pyrazine.

Ultrafast internal conversion via a conical intersection is ubiquitous in highly efficient photochemical reactions. Internal conversion from the 1ππ* to the 1nπ* state of pyrazine is the paradigm for this phenomenon; however, the relaxation occurs in such a short time (<20 fs) that the nuclear motion is difficult to observe in real time. The present study precisely measures the vibrational coherence transferred from the 1ππ* state to the 1nπ* state using time-resolved photoelectron spectroscopy with an unprecedented time resolution of 13.3 fs and reveals the key nuclear motions that drive the internal conversion.

Generation of sub-10-fs deep and extreme ultraviolet pulses for time-resolved photoemission spectroscopy.

We present a light source capable of generating sub-10-fs deep UV (DUV) and extreme UV (EUV) pulses for use in time-resolved photoemission spectroscopy. The fundamental output of a Ti:sapphire laser was compressed using the multi-plate method and mixed with the uncompressed second harmonic in a filamentation four-wave mixing process to generate sub-10-fs DUV pulses. Sub-10-fs EUV pulses were generated via high-order harmonic generation driven by the second harmonic pulses that were compressed using Ar gas and chirped mirrors. The minimum cross correlation time between 267 and 57 nm (corresponding to 21.7 eV) was measured to be 10.6 ± 0.4 fs.

Ultrafast Ring Closure Reaction of Gaseous cis-Stilbene from S1(ππ*).

cis-Stilbene (cis-St) is a well-known benchmark system for cis-trans photoisomerization. cis-St also produces 4a,4b-dihydrophenanthrene (DHP) in solution with a quantum yield of less than 0.19. The ring closure reaction, however, has never been identified for gaseous cis-St, and a recent computational simulation predicted the quantum yield of DHP to be only 0.04. In the present study, we identified an ultrafast ring closure reaction of gaseous cis-St for the first time using extreme ultraviolet time-resolved photoelectron spectroscopy. Surface hopping trajectory calculations at the SA3-XMS-CASPT2(2,2) level of theory reproduce the features of the observed time-resolved photoelectron spectra and predict the cis-St:DHP:trans-St branching ratio to be 0.55:0.41:0.04, in contrast with previous estimates. The results indicate that photoexcited cis-St favors ring closure over cis-trans isomerization under the isolated condition.

Formation of Long-Lived Dark States during Electronic Relaxation of Pyrimidine Nucleobases Studied Using Extreme Ultraviolet Time-Resolved Photoelectron Spectroscopy.

Ultrafast electronic relaxation of nucleobases from 1ππ* states to the ground state (S0) is considered essential for the photostability of DNA. However, transient absorption spectroscopy (TAS) has indicated that some nucleobases in aqueous solutions create long-lived 1nπ*/3ππ* dark states from the 1ππ* states with a high quantum yield of 0.4-0.5. We investigated electronic relaxation in pyrimidine nucleobases in both aqueous solutions and the gas phase using extreme ultraviolet (EUV) time-resolved photoelectron spectroscopy. Femtosecond EUV probe pulses cause ionization from all electronic states involved in the relaxation process, providing a clear overview of the electronic dynamics. The 1nπ* quantum yields for aqueous cytidine and uracil (Ura) derivatives were found to be considerably lower (<0.07) than previous estimates reported by TAS. On the other hand, aqueous thymine (Thy) and thymidine exhibited a longer 1ππ* lifetime and a higher quantum yield (0.12-0.22) for the 1nπ* state. A similar trend was found for isolated Thy and Ura in the gas phase: the 1ππ* lifetimes are 39 and 17 fs and the quantum yield for 1nπ* are 1.0 and 0.45 for Thy and Ura, respectively. The result indicates that single methylation to the C5 position hinders the out-of-plane deformation that drives the system to the conical intersection region between 1ππ* and S0, providing a large impact on the photophysics/photochemistry of a pyrimidine nucleobase. The significant reduction of 1nπ* yield in aqueous solution is ascribed to the destabilization of the 1nπ* state induced by hydrogen bonding.

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.

Ultrafast Photoisomerization of Ethylene Studied Using Time-Resolved Extreme Ultraviolet Photoelectron Spectroscopy.

The photoisomerization of isolated ethylene (ethene) was observed in real time from the Franck-Condon region in the 1ππ* state to ground-state products using time-resolved photoelectron spectroscopy with extreme ultraviolet (EUV, 21.7 eV) probe pulses. A combination of filamentation four-wave mixing and high-order harmonic generation was employed to obtain a temporal resolution of 31 ± 2 fs. The nuclear wave packet created by a 160 nm pump pulse accesses C═C twisted geometries within 10 fs, and the population transfer from the excited to the ground state occurs within the next 20-30 fs. Formation of vibrationally highly excited ground-state molecules was observed in less than 45 fs, and they decayed with two time constants of 0.87 and >5 ps. The interpretation of the photoelectron spectra is supported by vertical ionization energies calculated using XMS-CASPT2 along geodesically interpolated reaction paths from the Franck-Condon region to the products.

Effects of additives on reaction of nucleosides with UV light in presence of uric acid and salicylic acid.

Recently, we reported that uric acid and salicylic acid are photosensitizers of the reaction of nucleosides with UV light via radical formation and energy transfer, respectively. In the present study, effects of 45 biologically relevant compounds on nucleoside reactions photosensitized by uric acid and salicylic acid were examined. When a mixed solution of 2'-deoxycytidine, 2'-deoxyguanosine, thymidine, and 2'-deoxyadenosine with uric acid was irradiated with UV light of a wavelength longer than 300 nm, all the nucleosides decreased. The addition of antioxidants suppressed the consumption of nucleosides. When the UV reaction of nucleosides was conducted with salicylic acid, thymidine decreased almost exclusively. Several antioxidants such as ascorbates, thiols, catecholamines, trans-2-hexen-1-ol, penicillin G, and NaHSO3 enhanced the consumption of thymidine, although the other antioxidants suppressed it. The results suggest that antioxidants may be beneficial to protect against DNA damage by photosensitization via radical formation, but that several of them may be detrimental as they facilitate DNA damage by photo-sensitization via energy transfer.

Ultrafast Ring-Opening Reaction of 1,3-Cyclohexadiene: Identification of Nonadiabatic Pathway via Doubly Excited State.

The photoinduced ring-opening reaction of 1,3-cyclohexadiene (CHD) to produce 1,3,5-hexatriene (HT) plays an essential role in the photobiological synthesis of vitamin D3 in the skin. This reaction follows the Woodward-Hoffmann rule, and C5-C6 bond rupture via an electronically excited state occurs with conrotatory motion of the end CH2 groups. However, it is noted that the photoexcited S1(π,π*) state of CHD is not electronically correlated with the ground state of HT, and the reaction must proceed via nonadiabatic transitions. In the present study, we have clearly observed the nonadiabatic reaction pathway via the doubly excited state of CHD using ultrafast extreme UV photoelectron spectroscopy. The results indicate that the reaction occurs in only 68 fs and creates product vibrational coherence. Extensive computational simulations support the interpretation of experimental results and provide further insights into the electronic dynamics in this paradigmatic electrocyclic ring-opening reaction.

Spiers Memorial Lecture: Introduction to ultrafast spectroscopy and imaging of photochemical reactions.

A brief overview is presented on ultrafast spectroscopy and imaging of photochemical reactions by highlighting several experimental studies reported in the last five years. A particular focus is placed on new experiments performed using high-order harmonic generation, X-ray free electron lasers, and relativistic electron beams. Exploration of fundamental chemical reaction dynamics using these advanced experimental methodologies is in an early stage, and exciting new research opportunities await in this rapidly expanding and advancing research field. At the same time, there is no experimental methodology that provides all aspects of the electronic and structural dynamics in a single experiment, and investigations using different methodologies with various perspectives need to be considered in a comprehensive manner.

Femtosecond X-ray spectroscopy of haem proteins.

We discuss our recently reported femtosecond (fs) X-ray emission spectroscopy results on the ligand dissociation and recombination in nitrosylmyoglobin (MbNO) in the context of previous studies on ferrous haem proteins. We also present a preliminary account of femtosecond X-ray absorption studies on MbNO, pointing to the presence of more than one species formed upon photolysis.

Valence Photoelectron Spectra of Liquid Methanol and Ethanol Measured Using He II Radiation.

High-resolution photoelectron (PE) spectra of liquid methanol and ethanol were measured using a liquid microjet and He IIα radiation (40.813 eV). The vertical ionization energy and the ionization threshold were determined as 9.70 ± 0.07 and 8.69 ± 0.07 eV for methanol and 9.52 ± 0.07 and 8.52 ± 0.07 eV for ethanol, respectively. Individual photoemission bands observed for the liquids are well correlated with those in PE spectra of the gaseous samples also measured in the present study, except that the liquid band positions were shifted on average by -1.23 eV for methanol and -1.10 eV for ethanol as compared to the gas. The 5a' and 7a' bands of liquid methanol exhibit specifically larger broadening than other bands, for which we attempted spectral fitting with two components, similarly with the case of the 3a1 band of liquid water. PE spectra of both liquid and gaseous ethanol are congested partly due to the presence of the trans and gauche isomers; however, the overall band positions are generally in good agreement with predictions based on quantum chemical calculations. Comparison of the measured PE spectra with experimental and simulated X-ray emission spectra indicate that spectral differences in the lowest ionization band of both methanol and ethanol originate from involvement of nuclear dynamics in the X-ray emission process.

Uric Acid as a Photosensitizer in the Reaction of Deoxyribonucleosides with UV Light of Wavelength Longer than 300 nm: Identification of Products from 2'-Deoxycytidine.

DNA reacts directly with UV light with a wavelength shorter than 300 nm. Although ground surface sunlight includes little of this short-wavelength UV light due to its almost complete absorption by the atmosphere, sunlight is the primary cause of skin cancer. Photosensitization by endogenous substances must therefore be involved in skin cancer development mechanisms. Uric acid is the final metabolic product of purines in humans, and is present at relatively high concentrations in cells and fluids. When a neutral mixed solution of 2'-deoxycytidine, 2'-deoxyguanosine, thymidine, and 2'-deoxyadenosine was irradiated with UV light with a wavelength longer than 300 nm in the presence of uric acid, all the nucleosides were consumed in a uric acid dose-dependent manner. These reactions were inhibited by the addition of radical scavengers, ethanol and sodium azide. Two products from 2'-deoxycytidine were isolated and identified as N4-hydroxy-2'-deoxycytidine and N4,5-cyclic amide-2'-deoxycytidine, formed by cycloaddition of an amide group from uric acid. A 15N-labeled uric acid, uric acid-1,3-15N2, having two 14N and two 15N atoms per molecule, produced N4,5-cyclic amide-2'-deoxycytidine containing both 14N and 15N atoms from uric acid-1,3-15N2. Singlet oxygen, hydroxyl radical, peroxynitrous acid, hypochlorous acid, and hypobromous acid generated neither N4-hydroxy-2'-deoxycytidine nor N4,5-cyclic amide-2'-deoxycytidine in the presence of uric acid. These results indicate that uric acid is a photosensitizer for the reaction of nucleosides by UV light with a wavelength longer than 300 nm, and that an unidentified radical derived from uric acid with a delocalized unpaired electron may be generated.

Reactions of kynurenic acid with hypobromous acid and hypochlorous acid.

Kynurenic acid, a tryptophan metabolite, acts as antagonist or agonist of several receptors. Hypobromous acid (HOBr) and hypochlorous acid (HOCl) are generated by eosinophils and neutrophils. At inflammation sites, kynurenic acid may encounter HOBr and HOCl to generate products. When kynurenic acid was incubated with HOBr under neutral conditions, kynurenic acid generated a single product almost exclusively. This was identified as 3-bromokynurenic acid. Kynurenic acid reacted with HOCl, generating two products. The major product was identified as 3-chlorokynurenic acid with its oxidative decarboxylation product, 3-chloro-4-hydroxy-2(1H)-quinolinone as a by-product. Free amino acids suppressed the reactions of kynurenic acid with HOBr and HOCl. Taurine suppressed the HOCl reaction but not the HOBr reaction. An eosinophil peroxidase system containing H2O2, NaCl, and NaBr reacted with kynurenic acid, generating 3-bromokynurenic acid under mildly acidic conditions. Although a myeloperoxidase system containing H2O2 and NaCl reacted with kynurenic acid to generate 3-chlorokynurenic acid under mildly acidic conditions, the product was altered to 3-bromokynurenic acid by addition of NaBr to the system. These results suggest that 3-bromokynurenic acid and 3-chlorokynurenic acid may be generated from kynurenic acid at inflammation sites in humans, although their formation will be suppressed by coexistent amino acids.

Methyl substitution effects on the non-adiabatic dynamics of benzene: lifting three-state quasi-degeneracy at conical intersections.

Previously, theoretical calculations on the non-adiabatic dynamics of benzene from the S2 state have indicated that the S2/S1 and S1/S0 conical intersections (CIs) facilitate ballistic nuclear wavepacket motion from S2 to S0 (fast channel) and branching to S1 (slow channel). In this paper, we present time-resolved photoelectron spectra of benzene and its methyl-derivatives (toluene and o-xylene) measured with a vacuum-UV laser, which clearly reveal both the fast and slow channels. The extremely short propagation time of the wavepacket between the two CIs of benzene indicates that the two are in close proximity to each other, while methyl substitution extends the propagation time and decreases the branching ratio into the fast channel. The results suggest that the quasi-degeneracy of the three states in benzene is lifted by the geometrical shifts of the CIs by methyl substitution.

Surface potential of liquid microjet investigated using extreme ultraviolet photoelectron spectroscopy.

Photoelectron spectroscopy of a liquid microjet requires careful energy calibration against electrokinetic charging of the microjet. For minimizing the error from this calibration procedure, Kurahashi et al. previously suggested optimization of an electrolyte concentration in aqueous solutions [Kurahashi et al., J. Chem. Phys. 140, 174506 (2014)]. More recently, Olivieri et al. proposed an alternative method of applying a variable external voltage on the liquid microjet [Olivieri et al., Phys. Chem. Chem. Phys. 18, 29506 (2016)]. In this study, we examined these two methods of calibration using extreme ultraviolet photoelectron spectroscopy with a magnetic bottle time-of-flight photoelectron spectrometer. We confirmed that the latter method flattens the vacuum level potential around the microjet, similar to the former method, while we found that the applied voltage energy-shifts the entire spectrum. Thus, careful energy recalibration is indispensable after the application of an external voltage for accurate measurements. It is also pointed out that electric conductivity of liquid on the order of 1 mS/cm is required for stable application of an external voltage. Therefore, both methods need a similar concentration of an electrolyte. Using the calibration method proposed by Olivieri et al., Perry et al. have recently revised the vertical ionization energy of liquid water to be 11.67(15) eV [Perry et al., J. Phys. Chem. Lett. 11, 1789 (2020)], which is 0.4 eV higher than the previously estimated value. While the source of this discrepancy is still unclear, we estimate that their calibration method possibly leaves uncertainty on the order of 0.1 eV.

Reaction of thymidine and ascorbic acid induced by UV in the presence of salicylic acid.

When a neutral solution of thymidine and ascorbic acid was irradiated with UV light of wavelength longer than 300 nm in the presence of salicylic acid as a photosensitizer, six product peaks appeared in an HPLC chromatogram in addition to small amounts of thymidine dimers. The six products were identified as three pairs of diastereomers of 5-(2-deoxy-2-l-ascorbyl)-5,6-dihydrothymidine, 5-(2-l-ascorbyl)-5,6-dihydrothymidine, and 5,6-dihydrothymidine. These results suggest that novel DNA damage may be generated by ascorbic acid with salicylic acid induced by sunlight.

Probing ultrafast dynamics during and after passing through conical intersections.

Time-resolved photoelectron spectroscopy using vacuum-UV probe pulses enables observing ultrafast dynamics during and after passing through conical intersections (CIs). The ring-puckering CI plays a prominent role following the ππ* photoexcitation of furan. More than 90% of the excited molecules safely return to the original ground state, while the remaining 10% transforms into isomers after passing through the puckering CI.

Time-Resolved Photoelectron Imaging of Acetone with 9.3 eV Photoexcitation.

Ultrafast electronic relaxation following 9.3 eV photoexcitation of gaseous acetone was investigated with time-resolved photoelectron imaging spectroscopy. An intense photoionization signal due to a transition from the 41A1(π,π*) state to the D1(π-1) cationic state diminishes within 50 fs, owing to vibrational wave packet motion leaving our observation energy window. Additional photoionization signals were assigned to transitions from Rydberg states with principal quantum numbers of 3-8 to the D0(n-1) cationic state, created by strong vibronic couplings with the bright 41A1(π,π*) state. The deactivation processes of the 41A1(π,π*) and Rydberg states are discussed based on their decay profiles obtained from a time-energy map of photoelectron kinetic energy distributions.

Ultrafast photoelectron spectroscopy of aqueous solutions.

Chemical reaction dynamics in liquids and at interfaces are central themes in the materials, energy, and environmental sciences. Ultrafast photoelectron spectroscopy of liquids enables unprecedented access to the electronic dynamics of transient chemical species, providing deeper insights into nonadiabatic reaction dynamics in aqueous solutions, which are strongly coupled with solvation dynamics.

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

We report solvated electron (esolv -) 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 esolv -. 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 esolv - created by the charge-transfer-to-solvent reaction from CH3O- to liquid methanol. The solvation dynamics of esolv - 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.