Improved Chiral Photoelectron Spectroscopy via Selection of Chirality-Selective Molecular Axis Orientations: A Theoretical Analysis of Non-Negative Smooth Functions.

Photoelectron spectroscopy for chiral discrimination is routinely performed for low photoelectron kinetic energies (PKEs), whereas it is considered impossible for high PKEs. We demonstrate theoretically that chiral photoelectron spectroscopy for high PKEs is possible using chirality-selective molecular orientation. The photoelectron angular distribution associated with one-photon ionization by unpolarized light can be characterized by a single parameter, ß. We show that most other anisotropy parameters are zero when ß is 2, as is often the case in the high PKEs. Exceptionally, odd-order anisotropy parameters are increased by a factor of 20 by orientation, even for high PKEs.

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.

Ultrafast Extreme Ultraviolet Photoelectron Spectroscopy of Nonadiabatic Photodissociation of CS2 from 1B2 (1Σu+) State: Product Formation via an Intermediate Electronic State.

We studied nonadiabatic dissociation of CS2 from the 1B2 (1Σu+) state using ultrafast extreme ultraviolet photoelectron spectroscopy. A deep UV (200 nm) laser using the filamentation four-wave mixing method and an extreme UV (21.7 eV) laser using the high-order harmonic generation method were employed to achieve the pump-probe laser cross-correlation time of 48 fs. Spectra measured with a high signal-to-noise ratio revealed clear dynamical features of vibrational wave packet motion in the 1B2 state; its electronic decay to lower electronic state(s) within 630 fs; and dissociation into S(1D2), S(3PJ), and CS fragments within 300 fs. The results suggest that both singlet and triplet dissociation occur via intermediate electronic state(s) produced by electronic relaxation from the 1B2 (1Σu+) state.

Theoretical Study on the Spectroscopic Observation of Intersystem Crossing between 3B1 and 1A1 States of GeH2 Using the GeH2- (2B1) Anion.

We performed nonadiabatic quantum wave packet dynamics calculations to simulate the photodetachment spectrum of the GeH2- (2B1) anion. We developed the (4 × 4) diabatic potential energy surfaces to describe the intersystem crossing transitions between the neutral 1A1 and 3B1 states induced by spin-orbit interactions based on ab initio calculations. The spin-orbit coupling matrix elements were calculated using the Breit-Pauli Hamiltonian with the spin-free states obtained from the multireference configuration interaction method. The calculated photodetachment spectrum showed many intense peaks that could be assigned to the vibrational states mostly associated with the pure singlet or triplet spin states. However, we also found weak satellite peaks that could be assigned to vibrational states consisting of the highly excited vibrational state on the singlet surface and the low-lying vibrational state on the triplet surface.

Circular dichroism in photoionization of degenerate orbitals: Spin-polarized photoelectrons and spontaneous separation of oriented photoions.

This work investigated the circular dichroic effect on the photoionization integral cross section of molecules in conjunction with irreducible tensor theory and effective operator formalism. The results show that the dichroic effect can be non-zero for complex orbitals, but becomes zero for all real orbitals due to time-reversal symmetry, within the electric dipole and Born-Oppenheimer approximations. Calculations were performed for carbon monoxide, boric acid, and fullerene, and implications of the first-order coefficient for the spin polarization of photoelectrons and the molecular axis orientation of photoions are discussed herein. The results of this work demonstrate that the photoionization of complex orbitals can cause photoions to become oriented such that photoions originating from complex conjugate orbitals are oriented opposite to one another. Due to electron-ion recoil, the spontaneous separation of these two kinds of photoions is expected for the point groups C n , C n v , C ∞ v , C nh , and S n with n ≥ 3.

Communication: Photoionization of degenerate orbitals for randomly oriented molecules: The effect of time-reversal symmetry on recoil-ion momentum angular distributions.

The photoelectron asymmetry parameter ß, which characterizes the direction of electrons ejected from a randomly oriented molecular ensemble by linearly polarized light, is investigated for degenerate orbitals. We show that ß is totally symmetric under the symmetry operation of the point group of a molecule, and it has mixed properties under time reversal. Therefore, all degenerate molecular orbitals, except for the case of degeneracy due to time reversal, have the same ß (Wigner-Eckart theorem). The exceptions are e-type complex orbitals of the Cn, Sn, Cnh, T, and Th point groups, and calculations on boric acid (C3h symmetry) are performed as an example. However, including those point groups, all degenerate orbitals have the same ß if those orbitals are real. We discuss the implications of this operator formalism for molecular alignment and photoelectron circular dichroism.

Charge-transfer-to-solvent reactions from I(-) to water, methanol, and ethanol studied by time-resolved photoelectron spectroscopy of liquids.

The charge-transfer-to-solvent (CTTS) reactions from iodide (I(-)) to H2O, D2O, methanol, and ethanol were studied by time-resolved photoelectron spectroscopy of liquid microjets using a magnetic bottle time-of-flight spectrometer with variable pass energy. Photoexcited iodide dissociates into a weak complex (a contact pair) of a solvated electron and an iodine atom in similar reaction times, 0.3 ps in H2O and D2O and 0.5 ps in methanol and ethanol, which are much shorter than their dielectric relaxation times. The results indicate that solvated electrons are formed with minimal solvent reorganization in the long-range solvent polarization field created for I(-). The photoelectron spectra for CTTS in H2O and D2O-measured with higher accuracy than in our previous study [Y. I. Suzuki et al., Chem. Sci. 2, 1094 (2011)]-indicate that internal conversion yields from the photoexcited I(-*) (CTTS) state are less than 10%, while alcohols provide 2-3 times greater yields of internal conversion from I(-*). The overall geminate recombination yields are found to be in the order of H2O > D2O > methanol > ethanol, which is opposite to the order of the mutual diffusion rates of an iodine atom and a solvated electron. This result is consistent with the transition state theory for an adiabatic outer-sphere electron transfer process, which predicts that the recombination reaction rate has a pre-exponential factor inversely proportional to a longitudinal solvent relaxation time.

Full observation of ultrafast cascaded radiationless transitions from S2(ππ(∗)) state of pyrazine using vacuum ultraviolet photoelectron imaging.

A photoexcited molecule undergoes multiple deactivation and reaction processes simultaneously or sequentially, which have been observed by combinations of various experimental methods. However, a single experimental method that enables complete observation of the photo-induced dynamics would be of great assistance for such studies. Here we report a full observation of cascaded electronic dephasing from S2(ππ(*)) in pyrazine (C4N2H4) by time-resolved photoelectron imaging (TRPEI) using 9.3-eV vacuum ultraviolet pulses with a sub-20 fs time duration. While we previously demonstrated a real-time observation of the ultrafast S2(ππ(*)) → S1(nπ(*)) internal conversion in pyrazine using TRPEI with UV pulses, this study presents a complete observation of the dynamics including radiationless transitions from S1 to S0 (internal conversion) and T1(nπ(*)) (intersystem crossing). Also discussed are the role of (1)Au(nπ(*)) in the internal conversion and the configuration interaction of the S2(ππ(*)) electronic wave function.

Ultrafast photodynamics of pyrazine in the vacuum ultraviolet region studied by time-resolved photoelectron imaging using 7.8-eV pulses.

The ultrafast electronic dynamics of pyrazine (C4N2H4) were studied by time-resolved photoelectron imaging (TRPEI) using the third (3ω, 4.7 eV) and fifth harmonics (5ω, 7.8 eV) of a femtosecond Ti:sapphire laser (ω). Although the photoionization signals due to the 5ω - 3ω and 3ω - 5ω pulse sequences overlapped near the time origin, we have successfully extracted their individual TRPEI signals using least squares fitting of the observed electron kinetic energy distributions. When the 5ω pulses preceded the 3ω pulses, the 5ω pulses predominantly excited the S4 (ππ(*), (1)B1 u+(1)B2u) state. The photoionization signal from the S4 state generated by the time-delayed 3ω pulses was dominated by the D3((2)B2g)←S4 photoionization process and exhibited a broad electron kinetic energy distribution, which rapidly downshifted in energy within 100 fs. Also observed were the photoionization signals for the 3s, 3pz, and 3py members of the Rydberg series converging to D0((2)Ag). The Rydberg signals appeared immediately within our instrumental time resolution of 27 fs, indicating that these states are directly photoexcited from the ground state or populated from S4 within 27 fs. The 3s, 3pz, and 3py states exhibited single exponential decay with lifetimes of 94 ± 2, 89 ± 2, and 58 ± 1 fs, respectively. With the reverse pulse sequence of 3ω - 5ω, the ultrafast internal conversion (IC) from S2(ππ(*)) to S1(nπ(*)) was observed. The decay associated spectrum of S2 exhibited multiple bands ascribed to D0, D1, and D3, in agreement with the 3ω-pump and 6ω-probe experiment described in our preceding paper [T. Horio et al., J. Chem. Phys. 145, 044306 (2016)]. The electron kinetic energy and angular distributions from S1 populated by IC from S2 are also discussed.

Effective attenuation length of an electron in liquid water between 10 and 600 eV.

The absolute values of the effective attenuation length of an electron in liquid water are determined using soft x-ray O1s photoemission spectroscopy of a liquid beam of water without employing any theoretical estimation or computationally obtained value. The effective attenuation length is greater than 1 nm in the entire electron kinetic energy region and exhibits very flat energy dependence in the 10-100 eV region.

Time- and angle-resolved photoemission spectroscopy of hydrated electrons near a liquid water surface.

We present time- and angle-resolved photoemission spectroscopy of trapped electrons near liquid surfaces. Photoemission from the ground state of a hydrated electron at 260 nm is found to be isotropic, while anisotropic photoemission is observed for the excited states of 1,4-diazabicyclo[2,2,2]octane and I- in aqueous solutions. Our results indicate that surface and subsurface species create hydrated electrons in the bulk side. No signature of a surface-bound electron has been observed.

Photoelectron spectroscopy of aqueous solutions: streaming potentials of NaX (X = Cl, Br, and I) solutions and electron binding energies of liquid water and X-.

The streaming potentials of liquid beams of aqueous NaCl, NaBr, and NaI solutions are measured using soft X-ray, He(I), and laser multiphoton ionization photoelectron spectroscopy. Gaseous molecules are ionized in the vicinity of liquid beams and the photoelectron energy shifts are measured as a function of the distance between the ionization point and the liquid beam. The streaming potentials change their polarity with concentration of electrolytes, from which the singular points of concentration eliminating the streaming potentials are determined. The streaming currents measured in air also vanish at these concentrations. The electron binding energies of liquid water and I(-), Br(-), and Cl(-) anions are revisited and determined more accurately than in previous studies.

Effect of electron correlation and shape resonance on photoionization from the S1 and S2 states of pyrazine.

In a previous study [T. Horio, T. Fuji, Y.-I. Suzuki, and T. Suzuki, J. Am. Chem. Soc. 131, 10392 (2009)], we demonstrated that the time-energy map of photoelectron angular anisotropy enables unambiguous identification of ultrafast S(2)(ππ*)-S(1)(nπ*) internal conversion in pyrazine. A notable characteristic of this map is that the forbidden ionization process of D(0)(n(-1)) ← S(2)(ππ*) gives a negative photoelectron anisotropy parameter. In the present study, we elucidate the mechanism of this process by calculating the photoionization transition dipole moments and photoelectron angular distribution using the first-order configuration interaction method and the continuum multiple scattering Xα approximation; these calculations at the S(0) equilibrium geometry reproduce the observed anisotropy parameters for D(0) ← S(2) and D(0) ← S(1) ionizations, respectively. On the other hand, they do not reproduce the small difference in the photoelectron anisotropy parameters for the D(1)(π(-1)) ← S(2) and D(0) ← S(1) ionizations, both of which correspond to removal of an electron from the same π* orbital in the excited states. We show that these ionizations are affected by the ka(g) shape resonance and that the difference between their photoelectron anisotropy parameters originates from the difference in the molecular geometry in D(1) ← S(2) and D(0) ← S(1).

Time-energy mapping of photoelectron angular distribution: application to photoionization stereodynamics of nitric oxide.

The time-energy mapping of the photoionization integral cross section and laboratory-frame photoelectron angular distribution is used to study photoionization stereodynamics of a diatomic molecule. The general theoretical formalism [Y. Suzuki and T. Suzuki, Mol. Phys., 2007, 105, 1675] is simplified for application to a diatomic molecule, and a high-resolution photoelectron imaging apparatus is used to determine the transition dipole moments and phase shifts of photoelectron partial waves in near-threshold and non-dissociative photoionization of NO from the A(2)Σ(+) state. The transition dipoles and phase shifts thus determined are in reasonable agreement with those by state-to-state photoionization experiment and Schwinger variational calculations. The difference of the phase shifts from those expected from the quantum defects of Rydberg states suggests occurrence of weak hybridization of different l-waves, in addition to the well-known s-d super complex. The circular dichroism in photoelectron angular distribution is also simulated from our results.

Excited-state dynamics of CS2 studied by photoelectron imaging with a time resolution of 22 fs.

The ultrafast dynamics of CS(2) in the (1)B(2)((1)Σ(u)(+)) state was studied by photoelectron imaging with a time resolution of 22 fs. The photoelectron signal intensity exhibited clear vibrational quantum beats due to wave packet motion. The signal intensity decayed with a lifetime of about 400 fs. This decay was preceded by a lag of around 30 fs, which was considered to correspond to the time for a vibrational wave packet to propagate from the Franck-Condon region to the region where predissociation occurred. The photoelectron angular distribution remained constant when the pump-probe delay time was varied. Consequently, variation of the electronic character caused by the vibrational wave packet motion was not identified within the accuracy of our measurements.

Time-resolved photoelectron imaging of S2 → S1 internal conversion in benzene and toluene.

Ultrafast internal conversion of benzene and toluene from the S(2) states was studied by time-resolved photoelectron imaging with a time resolution of 22 fs. Time-energy maps of the photoelectron intensity and the angular anisotropy were generated from a series of photoelectron images. The photoelectron kinetic energy distribution exhibits a rapid energy shift and intensity revival, which indicates nuclear motion on the S(2) adiabatic surface, while the ultrafast evolution of the angular anisotropy revealed a change in the electronic character of the S(2) adiabatic surface. From their decay profiles of the total photoelectron intensity, the time constants of 48 ± 4 and 62 ± 4 fs were determined for the population decay from the S(2) states in benzene and toluene, respectively.

Photoelectron imaging spectroscopy of S1(1B2u π,π*) benzene via 6(1)1n (n = 0-3) levels.

We report resonance-enhanced two-photon ionization photo-electron spectroscopy of jet-cooled benzene via the 6(1)1(n) (n = 0-3) vibronic levels in S(1)((1)B(2u) π,π*) using a nanosecond UV laser and photoelectron imaging. The best energy resolution (ΔE/E) was 0.7%. The photoelectron spectrum from the S(1) 6(1)1(3) level (E(vib) = 3284 cm(-1)) in the channel three region exhibited a clear signature of intramolecular vibrational redistribution (IVR). The spectral features were consistent with picosecond zero kinetic energy photoelectron (ZEKE) spectra reported by Smith et al. [ J. Phys. Chem. 1995, 99, 1768]. The photoelectron angular anisotropy parameter ß(2) was found to be negative in ionization from the 6(1)1(n) (n = 0-3) levels with photoelectron kinetic energies up to 5000 cm(-1). No influence of a shape resonance was identified.

Ultrafast photodynamics of furan.

Ultrafast photodynamics of furan has been studied by time-resolved photoelectron imaging (TRPEI) spectroscopy with an unprecedented time resolution of 22 fs. The simulation of the time-dependent photoelectron kinetic energy distribution (PKED) has been performed with ab initio nonadiabatic dynamics "on the fly" in the frame of time-dependent density functional theory. Based on the agreement between experimental and theoretical time-dependent photoelectron signal intensity as well as on PKED, precise time scales of ultrafast internal conversion from S(2) over S(1) to the ground state S(0) of furan have been revealed for the first time. Upon initial excitation of the S(2) state which has π-π* character, a nonadiabatic transition to the S(1) state occurs within 10 fs. Subsequent dynamics invokes the excitation of the C-O stretching and C-O-C out of plane vibrations which lead to the internal conversion to the ground state after 60 fs. Thus, we demonstrate that the TRPEI combined with high level nonadiabatic dynamics calculations provide fundamental insight into ultrafast photodynamics of chemically and biologically relevant chromophores.

Time-resolved photoelectron imaging of ultrafast S2-->S1 internal conversion through conical intersection in pyrazine.

A nonadiabatic electronic transition through a conical intersection was studied by pump-probe photoelectron imaging spectroscopy with a 22 fs time resolution in the benchmark polyatomic molecule of pyrazine and deuterated pyrazine. The lifetimes of the S(2) state of pyrazine and deuterated pyrazine were determined to be 22+/-3 fs by the global fitting of the time-energy maps of photoelectron kinetic energy (PKE) distributions. The lifetime of S(3) was determined to be 40-43 fs. Two-dimensional maps of photoelectron distributions were obtained for time (t) and PKE, and individual PKE distributions upon ionization from S(2) and S(1) were extracted. Quantum beat with an approximately 50 fs period was observed after the S(2)-->S(1) internal conversion, which was attributed to the totally symmetric vibration nu(6a) in S(1).

Direct measurement of vertical binding energy of a hydrated electron.

We present the first measurement of the vertical binding energy (VBE) of a hydrated electron in bulk water by the time-resolved photoelectron spectroscopy (TRPES) of the charge-transfer-to-solvent (CTTS) reaction in aqueous NaI solution. Our best estimate of VBE is 3.27 +/- 0.10 eV for H(2)O and 3.20 +/- 0.10 eV for D(2)O.