Internal Conversion In Pyrazine Research Articles

Cavity Control of Molecular Spectroscopy and Photophysics.

ConspectusOptical cavities have been established as a powerful platform for manipulating the spectroscopy and photophysics of molecules. Molecules placed inside an optical cavity will interact with the cavity field, even if the cavity is in the vacuum state with no photons. When the coupling strength between matter excitations, either electronic or vibrational, and a cavity photon mode surpasses all decay rates in the system, hybrid light-matter excitations known as cavity polaritons emerge. Originally studied in atomic systems, there has been growing interest in studying polaritons in molecules. Numerous studies, both experimental and theoretical, have demonstrated that the formation of molecular polaritons can significantly alter the optical, electronic, and chemical properties of molecules in a noninvasive manner.This Account focuses on novel studies that reveal how optical cavities can be employed to control electronic excitations, both valence and core, in molecules and the spectroscopic signatures of molecular polaritons. We first discuss the capacity of optical cavities to manipulate and control the intrinsic conical intersection dynamics in polyatomic molecules. Since conical intersections are responsible for a wide range of photochemical and photophysical processes such as internal conversion, photoisomerization, and singlet fission, this provides a practical strategy to control molecular photodynamics. Two examples are given for the internal conversion in pyrazine and singlet fission in a pentacene dimer. We further show how X-ray cavities can be exploited to control the core-level excitations of molecules. Core polaritons can be created from inequivalent core orbitals by exchanging X-ray cavity photons. The core polaritons can also alter the selection rules in nonlinear spectroscopy.Polaritonic states and dynamics can be monitored by nonlinear spectroscopy. Quantum light spectroscopy is a frontier in nonlinear spectroscopy that exploits the quantum-mechanical properties of light, such as entanglement and squeezing, to extract matter information inaccessible by classical light. We discuss how quantum spectroscopic techniques can be employed for probing polaritonic systems. In multimolecule polaritonic systems, there exist two-polariton states that are dark in the two-photon absorption spectrum due to destructive interference between transition pathways. We show that a time-frequency entangled photon pair can manipulate the interference between transition pathways in the two-photon absorption signal and thus capture classically dark two-polariton states. Finally, we discuss cooperative effects among molecules in spectroscopy and possibly in chemistry. When many molecules are involved in forming the polaritons, while the cooperative effects clearly manifest in the dependence of the Rabi splitting on the number of molecules, whether they can show up in chemical reactivity, which is intrinsically local, is an open question. We explore the cooperative nature of the charge migration process in a cavity and show that, unlike spectroscopy, polaritonic charge dynamics is intrinsically local and does not show collective many-molecule effects.

Combined Surface-Hopping, Dyson Orbital, and B-Spline Approach for the Computation of Time-Resolved Photoelectron Spectroscopy Signals: The Internal Conversion in Pyrazine.

A computational protocol for simulating time-resolved photoelectron signals of medium-sized molecules is presented. The procedure is based on a trajectory surface-hopping description of the excited-state dynamics and a combined Dyson orbital and multicenter B-spline approach for the computation of cross sections and asymmetry parameters. The accuracy of the procedure has been illustrated for the case of ultrafast internal conversion of gas-phase pyrazine excited to the 1B2u(ππ*) state. The simulated spectra and the asymmetry map are compared to the experimental data, and a very good agreement was obtained without applying any energy-dependent rescaling or broadening. An interesting side result of this work is the finding that the signature of the 1Au(nπ*) state is indistinguishable from that of the 1B3u(nπ*) state in the time-resolved photoelectron spectrum. By locating four symmetrically equivalent minima on the lowest-excited (S1) adiabatic potential energy surface of pyrazine, we revealed the strong vibronic coupling of the 1Au(nπ*) and 1B3u(nπ*) states near the S1 ← S0 band origin.

Identification of an ultrafast internal conversion pathway of pyrazine by time-resolved vacuum ultraviolet photoelectron spectrum simulations.

The internal conversion from the optically bright S2 (1B2u, ππ*) state to the dark S1 (1B3u, nπ*) state in pyrazine is a standard benchmark for experimental and theoretical studies on ultrafast radiationless decay. Since 2008, a few theoretical groups have suggested significant contributions of other dark states S3 (1Au, nπ*) and S4 (1B2g, nπ*) to the decay of S2. We have previously reported the results of nuclear wave packet simulations [Kanno et al., Phys. Chem. Chem. Phys. 17, 2012 (2015)] and photoelectron spectrum calculations [Mignolet et al., Chem. Phys. 515, 704 (2018)] that support the conventional two-state picture. In this article, the two different approaches, i.e., wave packet simulation and photoelectron spectrum calculation, are combined: We computed the time-resolved vacuum ultraviolet photoelectron spectrum and photoelectron angular distribution for the ionization of the wave packet transferred from S2 to S1. The present results reproduce almost all the characteristic features of the corresponding experimental time-resolved spectrum [Horio et al., J. Chem. Phys. 145, 044306 (2016)], such as a rapid change from a three-band to two-band structure. This further supports the existence and character of the widely accepted pathway (S2 → S1) of ultrafast internal conversion in pyrazine.

Assessing the performance of trajectory surface hopping methods: Ultrafast internal conversion in pyrazine.

Trajectory surface hopping (TSH) methods have been widely used to study photoinduced nonadiabatic processes. In the present study, nonadiabatic dynamics simulations with the widely used Tully's fewest switches surface hopping (FSSH) algorithm and a Landau-Zener-type TSH (LZSH) algorithm have been performed for the internal conversion dynamics of pyrazine. The accuracy of the two TSH algorithms has been critically evaluated by a direct comparison with exact quantum dynamics calculations for a model of pyrazine. The model comprises the three lowest excited electronic states (B3u(nπ*), A1u(nπ*), and B2u(ππ*)) and the nine most relevant vibrational degrees of freedom. Considering photoexcitation to the diabatic B2u(ππ*) state, we examined the time-dependent diabatic and adiabatic electronic population dynamics. It is found that the diabatic populations obtained with both TSH methods are in good agreement with the exact quantum results. Fast population oscillations between the B3u(nπ*) and A1u(nπ*) states, which reflect nonadiabatic electronic transitions driven by coherent dynamics in the normal mode Q8a, are qualitatively reproduced by both TSH methods. In addition to the model study, the TSH methods have been interfaced with the second-order algebraic diagrammatic construction ab initio electronic-structure method to perform full-dimensional on-the-fly nonadiabatic dynamics simulations for pyrazine. It is found that the electronic population dynamics obtained with the LZSH method is in excellent agreement with that obtained by the FSSH method using a local diabatization algorithm. Moreover, the electronic populations of the full-dimensional on-the-fly calculations are in excellent agreement with the populations of the three-state nine-mode model, which confirms that the internal conversion dynamics of pyrazine is accurately represented by this reduced-dimensional model on the time scale under consideration (200 fs). The original FSSH method, in which the electronic wave function is propagated in the adiabatic representation, yields less accurate results. The oscillations in the populations of the diabatic B3u(nπ*) and A1u(nπ*) states driven by the mode Q8a are also observed in the full-dimensional dynamics simulations.

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.

Coherent phase control of internal conversion in pyrazine.

Shaped ultrafast laser pulses were used to study and control the ionization dynamics of electronically excited pyrazine in a pump and probe experiment. For pump pulses created without feedback from the product signal, the ion growth curve (the parent ion signal as a function of pump/probe delay) was described quantitatively by the classical rate equations for internal conversion of the S2 and S1 states. Very different, non-classical behavior was observed when a genetic algorithm (GA) employing phase-only modulation was used to minimize the ion signal at some pre-determined target time, T. Two qualitatively different control mechanisms were identified for early (T < 1.5 ps) and late (T > 1.5 ps) target times. In the former case, the ion signal was largely suppressed for t < T, while for t ≫ T, the ion signal produced by the GA-optimized pulse and a transform limited (TL) pulse coalesced. In contrast, for T > 1.5 ps, the ion growth curve followed the classical rate equations for t < T, while for t ≫ T, the quantum yield for the GA-optimized pulse was much smaller than for a TL pulse. We interpret the first type of behavior as an indication that the wave packet produced by the pump laser is localized in a region of the S2 potential energy surface where the vertical ionization energy exceeds the probe photon energy, whereas the second type of behavior may be described by a reduced absorption cross section for S0 → S2 followed by incoherent decay of the excited molecules. Amplitude modulation observed in the spectrum of the shaped pulse may have contributed to the control mechanism, although this possibility is mitigated by the very small focal volume of the probe laser.

Ultrafast Internal Conversion of Aromatic Molecules Studied by Photoelectron Spectroscopy using Sub-20 fs Laser Pulses

This article describes our recent experimental studies on internal conversion via a conical intersection using photoelectron spectroscopy. Ultrafast S2(ππ*)–S1(nπ*) internal conversion in pyrazine is observed in real time using sub-20 fs deep ultraviolet pulses (264 and 198 nm). While the photoelectron kinetic energy distribution does not exhibit a clear signature of internal conversion, the photoelectron angular anisotropy unambiguously reveals the sudden change of electron configuration upon internal conversion. An explanation is presented as to why these two observables have different sensitivities to internal conversion. The 198 nm probe photon energy is insufficient for covering the entire Franck-Condon envelopes upon photoionization from S2/S1 to D1/D0. A vacuum ultraviolet free electron laser (SCSS) producing 161 nm radiation is employed to solve this problem, while its pulse-to-pulse timing jitter limits the time resolution to about 1 ps. The S2–S1 internal conversion is revisited using the sub-20 fs 159 nm pulse created by filamentation four-wave mixing. Conical intersections between D1(π−1) and D0(n−1) and also between the Rydberg state with a D1 ion core and that with a D0 ion core of pyrazine are studied by He(I) photoelectron spectroscopy, pulsed field ionization photoelectron spectroscopy and one-color resonance-enhanced multiphoton ionization spectroscopy. Finally, ultrafast S2(ππ*)–S1(ππ*) internal conversion in benzene and toluene are compared with pyrazine.

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).

Overlapping resonances interference-induced transparency: The S → S2/S1 photoexcitation spectrum of pyrazine

The phenomenon of "overlapping resonances interference-induced transparency" (ORIT) is introduced and studied in detail for the S(0) → S(2)/S(1) photoexcitation of cold pyrazine (C(4)H(4)N(2)). In ORIT, a molecule becomes transparent at specific wavelengths due to interferences between envelopes of spectral lines displaying overlapping resonances. An example is the S(2) ↔ S(1) internal conversion in pyrazine where destructive interference between overlapping resonances causes the S(0) → S(2)/S(1) light absorption to disappear at certain wavelengths. ORIT may be of practical importance in multi-component mixtures where it would allow for the selective excitation of some molecules in preference to others. Interference-induced cross section enhancement is also shown.

Simulation of time resolved photoelectron spectra with Stieltjes imaging illustrated on ultrafast internal conversion in pyrazine

We present an approach for the simulation of time resolved photoelectron spectra based on the combination of the ab initio nonadiabatic molecular dynamics "on the fly" with the Stieltjes imaging method utilizing discrete neutral states above the ionization limit for the approximate description of the ionization continuum. Our approach has been implemented in the framework of the time-dependent density functional theory and has been applied to interrogate the ultrafast internal conversion between the S(2) and S(1) states in pyrazine. The simulations reveal that, parallel to the S(2)-->S(1) internal conversion, a change in the dominant ionization process (S(2)-->D(1) versus S(1)-->D(0)) occurs on the time scale of 20 fs such that no significant change in the photoelectron kinetic energy distribution is observed. The presented results are in full agreement with the experimental results presented in the accompanying paper [Suzuki et al., J. Chem. Phys. 132, 174302 (2010)] and provide an insight into the interplay between the nonradiative relaxation and the photoionization process in pyrazine as reflected in the time resolved photoelectron spectrum. Our approach represents a general tool for the investigation of ultrafast photoionization processes in complex systems and thus can be used to investigate the ultrafast femtochemistry of complex molecular systems including all degrees of freedom.

Probing Ultrafast Internal Conversion through Conical Intersection via Time-Energy Map of Photoelectron Angular Anisotropy

The ultrafast S(2) --> S(1) internal conversion of pyrazine through a conical intersection of the potential energy surfaces was clearly observed by ultrafast photoelectron imaging. The 2D time-energy map of the photoelectron angular anisotropy revealed a clear signature of the internal conversion (the color change of green-orange-green in the horizontal direction) at approximately 0.9 eV. The orange color at approximately 20 fs and 0.9 eV was found to correspond to the non-Koopmans ionization process from S(2)(pi,pi*) to D(0)(n(-1)).

Overlapping resonances in the coherent control of radiationless transitions: Internal conversion in pyrazine

Coherent control of radiationless transitions is developed and applied to internal conversion. Conditions for active versus passive control are described and overlapping resonances are shown necessary for the phase control of radiationless transitions in molecular systems. Applications to pyrazine show the possibility of extensive control via optimized state preparation, as well as the significant role of overlapping resonances, even in the evolution of single vibrational states in S2.

Dynamics of radiationless transitions in large molecular systems: A Franck–Condon-based method accounting for displacements and rotations of all the normal coordinates

An efficient method to study the dynamics of radiationless transition in large molecular systems is proposed. It is based on the use of the whole set of normal coordinates of vibration and allows for taking properly into account both the displacements and the mix of the normal modes upon transition between two electronic states. The Hamiltonian matrix elements are written in terms of generalized Franck–Condon integrals and are analytically evaluated by recursion formulas. Applications to the S2→S1 internal conversion in pyrazine and to long-range electron transfer between quinones in photosynthetic reaction centers are given.

S 1-S 2 Conical intersection and ultrafast S 2→S 1 Internal conversion in pyrazine

A low-lying conical intersection of the S 1 and S 2 potential energy surfaces of pyrazine is identified and modelled by a two-state three-mode vibronic coupling Hamiltonian. The quantum dynamics of this model is studied. The results explain qualitatively the essentially structureless bandshape of the S 2 absorption spectrum and reveal details about the femtosecond population decay of the S 2 state.

An “exact model” study of vibronic interactions in radiationless transitions with applications to internal conversion in pyrazine

An “exact model” study of vibronic interactions and the “proximity effect” in radiationless transitions is reported. A comparison is made between the “exact” results and those obtained employing the crude adiabatic Born—Oppenheimer scheme and the adiabatic Born—Oppenheimer scheme. It is shown that the computed radiationless decay rates are very sensitive to the scheme adopted. The exact solutions of the linear Herzberg—Teller vibronic coupling model are employed in a theoretical investigation of the “proximity effect” and internal conversion in pyrazine. The presented results suggest that the deuterium isotope effect on the internal conversion rate in pyrazine may be very small, although the vibrational energy distribution in S 0 may be dramatically altered.