Nonadiabatic Pathways Research Articles

Photochromic reaction pathways of 1,1′-azobis-1,2,3-triazole: A CASSCF and spin-flip DFT study

This study employed multiconfigurational CASSCF and MS-CASPT2 methods to investigate the photoinduced isomerization mechanism of 1,1′-azobis-1,2,3-triazole. The MS-CASPT2//CASSCF computational results indicate that the photoisomerization reaction of 1,1′-azobis-1,2,3-triazole involves a non-adiabatic transition pathway through the S2 state. After photoexcitation to the S2 state, the molecule undergoes internal conversion to reach S1-min, followed by a non-radiative transition back to the ground state. The S1/S0-CI and S1-min have similar structures and close energies, which facilitates unidirectional rotation. The weak coupling between the ground state and excited states may be due to the strong electron-withdrawing nature of the N8 chain. Additionally, using the MS-CASPT2//CASSCF computational results as a reference, we compared the differences in describing the photoisomerization process of this compound using the SF-DFT method, both qualitatively and quantitatively. The results demonstrate that the SF-DFT method significantly underestimates the energy of the S1 state. These findings are expected to deepen the understanding of non-adiabatic transitions in the photoinduced rotation of high-nitrogen compounds and provide theoretical insights for other high-nitrogen compounds that may exhibit photochromism.

Competing Nonadiabatic Relaxation Pathways for Near-UV Excited ortho-Nitrophenol in Aqueous Solution.

Nitrophenols are atmospheric pollutants found in brown carbon aerosols produced by biomass burning. Absorption of solar radiation by these nitrophenols contributes to atmospheric radiative forcing, but quantifying this climate impact requires better understanding of their photochemical pathways. Here, the photochemistry of near-UV (λ = 350 nm) excited ortho-nitrophenol in aqueous solution is investigated using transient absorption spectroscopy and time-resolved infrared spectroscopy over the fs to μs time scale to characterize the excited states, intermediates, and photoproducts. Interpretation of the transient spectroscopy data is supported by quantum chemical calculations using linear-response time-dependent density functional theory (LR-TDDFT). Our results indicate efficient nonradiative decay via an S1(ππ*)/S0 conical intersection leading to hot ground state ortho-nitrophenol which vibrationally cools in solution. A previously unreported minor pathway involves intersystem crossing near an S1(nπ*) minimum, with decay of the resulting triplet ortho-nitrophenol facilitated by deprotonation. These efficient relaxation pathways account for the low quantum yields of photodegradation.

Collective rovibronic dynamics of a diatomic gas coupled by cavity

We consider an ensemble of homonuclear diatomic molecules coupled to the two polarization directions of a Fabry-Pérot cavity via fully quantum simulations. Accompanied by analytical results, we identify a coupling mechanism mediated simultaneously by the two perpendicular polarizations, and inducing polaritonic relaxation towards molecular rotations. This mechanism is related to the concept of light-induced conical intersections (LICIs). However, unlike LICIs, these nonadiabatic pathways are of a collective nature, since they depend on the intermolecular orientation of all electronic transition dipoles in the polarization plane. Notably, this rotational mechanism directly couples the bright upper and lower polaritonic states, and it stays in direct competition with the collective relaxation towards dark states. Published by the American Physical Society 2024

Molecular structures in the non-adiabatic relaxaiton processes of excited states of pentafluoropyridine

In this work, the molecular structure and energy of some critical points in nonradiative relaxation process of the excited state of pentafluoropyridine are studied through quantum chemistry calculation. The structures and the vertical excitation energies and adiabatic excitation energies of the ground state and two lowest exited states are calculated. The geometry of the ground state is a planar structure with C<sub>2v</sub> symmetry, while the geometries of the two lowest excited states are half-boat structures with out-of-plane distortions. Furthermore, the topology structures and energy of the conical intersections of S<sub>2</sub>/S<sub>1</sub>, S<sub>1</sub>/S<sub>0</sub> and S<sub>2</sub>/S<sub>0</sub> are determined. The topology structures of the conical intersections S<sub>2</sub>/S<sub>1</sub>, S<sub>1</sub>/S<sub>0</sub> and S<sub>2</sub>/S<sub>0</sub> in the branching space are all peaked with asymmetric structures, and are determined to be structure of boat, half-boat, and chair, respectively. Their corresponding energy values are estimated at 6.39, 5.16 and 8.51 eV, respectively. The results show that the primary non-adiabatic relaxation pathway is the wavepacket of the S<sub>2</sub> state rapidly evolving into the S<sub>1</sub> state via the S<sub>2</sub>/S<sub>1</sub>, and then directly relaxing to the ground state via the S<sub>1</sub>/S<sub>0</sub>. In addition, the probability of directly relaxing to the ground state through S<sub>2</sub>/S<sub>0</sub> is smaller.

Formation Mechanisms of Electronically Excited Nitrogen Molecules from N + N2 and N + N + N Collisions Revealed by Full-Dimensional Potential Energy Surfaces.

This work reports six new full-dimensional adiabatic potential energy surfaces (PESs) of the N3 system (four 4A″ states and two 2A″ states) at the MRCI + Q/AVQZ level of theory that correlated to N2(X1Σg+) + N(4S), N2(X1Σg+) + N(2D), N2(A3Σu+) + N(4S), N2(B3Πg) + N(4S), N2(W3Δu) + N(4S), and N(4S) + N(4S) + N(4S) channels. The neural networks with a proper account of the nuclear permutation invariant symmetry of N3 were employed to fit the PESs based on about 4000 ab initio points. The accuracy of the PESs was validated by excellent agreement on the equilibrium bond length, vertical excitation energy, and dissociation energy with experimental values. Two possible mechanisms of the formation of N2(A) were found. One is that the collision occurs between N2(X) and N(4S) in the 14A″ state, followed by a nonadiabatic transition through the conical intersection with the 24A″ PES, resulting in the formation of the N2(A) + N(4S) product. The other takes place in the collision among three N(4S) atoms in the adiabatic 24A″ state, and then, N2(A) + N(4S) is formed. This is the first systematical research of the N3 system focusing on the formation of the excited states of N2 via both adiabatic and nonadiabatic pathways.

Spectroscopy and Photochemistry of OAlNO and Implications for New Metal Chemistry in the Atmosphere.

A new aluminum-bearing species, OAlNO, which has the potential to impact the chemistry of the Earth's upper atmosphere, is characterized via high-level, ab initio, spectroscopic methods. Meteor-ablated aluminum atoms are quickly oxidized to aluminum oxide (AlO) in the mesosphere and lower thermosphere (MLT), where a steady-state layer of AlO then builds up. Concurrent formation of nitric oxide (NO) in the same region of the atmosphere will lead to the bimolecular formation of the OAlNO molecule. Molecular orbital analysis provides fundamental insights into the chemical bonding and energetic arrangement of the triplet (1 3A″) ground state and singlet (1 1A') excited-state species of OAlNO. Additionally, unpaired electrons on the terminal oxygen atom of triplet (1 3A″) OAlNO cause it to be reactive to atmospheric species, potentially impacting climate science and high-altitude chemistry. The triplet (1 3A″) ground-state species exhibits a large permanent dipole moment useful for rotational spectroscopic detection; however, similar rotational constants to the singlet (1 1A') excited-state species will hamper differentiation in a spectrum. Strong infrared intensities will assist in detection and discrimination of the different spin states and isomers. Repulsive electronic excited states of OAlNO will lead to photolysis of the Al-N bond and formation of various electronic states of AlO + NO through nonadiabatic pathways. Reaction through the OAlNO intermediate represents a means for the production of electronically excited AlO, leading to new chemistry in the atmosphere. Excitation to higher-lying electronic states will lead to fluorescence with a minor Stokes shift, useful for laboratory investigation. Such physical properties of this molecule will allow for new, unexplored chemical pathways in the MLT to be considered.

Driving forces for ultrafast laser-induced sp2 to sp3 structural transformation in graphite

Understanding the microscopic mechanism of photoinduced sp2-to-sp3 structural transformation in graphite is a scientific challenge with great importance. Here, the ultrafast dynamics and characteristics of laser-induced structural transformation in graphite are revealed by non-adiabatic quantum dynamic simulations. Under laser irradiation, graphite undergoes an interlayer compression and sliding stage, followed by a key period of intralayer buckling and interlayer bonding to form an intermediate sp2-sp3 hybrid structure, before completing the full transformation to hexagonal diamond. The process is driven by the cooperation of charge carrier multiplication and selective phonon excitations through electron-phonon interactions, in which photoexcited hot electrons scattered into unoccupied high-energy conduction bands play a key role in the introduction of in-plane instability in graphite. This work identifies a photoinduced non-adiabatic transition pathway from graphite to diamond and shows far-reaching implications for designing optically controlled structural phase transition in materials.

Spin-dependent reactivity and spin-flipping dynamics in oxygen atom scattering from graphite

The formation of two-electron chemical bonds requires the alignment of spins. Hence, it is well established for gas-phase reactions that changing a molecule’s electronic spin state can dramatically alter its reactivity. For reactions occurring at surfaces, which are of great interest during, among other processes, heterogeneous catalysis, there is an absence of definitive state-to-state experiments capable of observing spin conservation and therefore the role of electronic spin in surface chemistry remains controversial. Here we use an incoming/outgoing correlation ion imaging technique to perform scattering experiments for O(3P) and O(1D) atoms colliding with a graphite surface, in which the initial spin-state distribution is controlled and the final spin states determined. We demonstrate that O(1D) is more reactive with graphite than O(3P). We also identify electronically nonadiabatic pathways whereby incident O(1D) is quenched to O(3P), which departs from the surface. With the help of molecular dynamics simulations carried out on high-dimensional machine-learning-assisted first-principles potential energy surfaces, we obtain a mechanistic understanding for this system: spin-forbidden transitions do occur, but with low probabilities.

Vacuum ultraviolet photodissociation dynamics of OCS via the F Rydberg state: The O (3PJ=2,1,0) product channels.

We study the vacuum ultraviolet (VUV) photodissociation dynamics of carbonyl sulfide (OCS) by using the time sliced velocity map ion imaging technique. Experimental images of the dissociative O (3PJ=0,1,2) products were acquired at five VUV photolysis wavelengths from 133.26 to 139.96nm that correspond to the F Rydberg state of OCS. High vibrational states of the carbon monosulfide (CS) co-products are partially resolved in the images. The product total kinetic energy releases, angular distributions, and the product state branching ratios were derived from the experimental images. Notably, it is found that the anisotropic parameters change systematically with the photolysis wavelength. The anisotropic parameters and the product state branching ratios are significantly sensitive to the J quantum number of the O (3PJ) products. The phenomenon indicates that multiple nonadiabatic pathways are strongly involved in the photodissociation processes.

Photodissociation study of CO2 on the formation of state-correlated CO(X1Σ+, v) with O(3P2) photoproducts in the low energy band centered at 148 nm.

The spin-forbidden O(3P2) + CO(X1Σ+, v) channel formed from the photodissociation of CO2 in the low energy band centered at 148nm is investigated by using the time-sliced velocity-mapped ion imaging technique. The vibrational-resolved images of the O(3P2) photoproducts measured in the photolysis wavelength range of 144.62-150.45nm are analyzed to give the total kinetic energy releases (TKER) spectra, CO(X1Σ+) vibrational state distributions, and anisotropy parameters (β). The TKER spectra reveal the formation of correlated CO(X1Σ+) with well resolved v = 0-10 (or 11) vibrational bands. Several high vibrational bands that were observed in the low TKER region for each studied photolysis wavelength exhibit a bimodal structure. The CO(X1Σ+, v) vibrational distributions all present inverted characteristics, and the most populated vibrational state changes from a low vibrational state to a relatively higher vibrational state with a change in the photolysis wavelength from 150.45 to 144.62nm. However, the vibrational-state specific β-values for different photolysis wavelengths present a similar variation trend. The measured β-values show a significant bulge at the higher vibrational levels, in addition to the overall slow decreasing trend. The observed bimodal structures with mutational β-values for the high vibrational excited state CO(1Σ+) photoproducts suggest the existence of more than one nonadiabatic pathway with different anisotropies in the formation of O(3P2) + CO(X1Σ+, v) photoproducts across the low energy band.

Two state reactivity (TSR) and hydrogen tunneling reaction kinetics measured in the Co+ mediated decomposition of CH3CHO.

The electronic structure of a transition metal atom allows it to act as a catalytic active site by providing lower energy alternative pathways in chemical transformations. We have identified and kinetically characterized three such pathways in the title reaction. One is an adiabatic pathway that occurs on a single potential energy surface described within the Born-Oppenheimer approximation. A second pathway opens microseconds into the reaction as a portion of the reacting population competitively transitions from triplet to singlet multiplicity to circumvent energetic barriers on the triplet surface. These pathways are single- and two-state reactive (SSR and TSR) where the Co+ cation mediates an oxidative addition/reductive elimination sequence of the CH3CHO molecule. The third observed reaction pathway is the aldehyde hydrogen tunneling through an Eyring barrier to form high-spin products. First-order rate constants for the adiabatic and nonadiabatic energy lowered pathways, and the hydrogen tunneling pathway, are each measured using the single photon initiated dissociative rearrangement reaction (SPIDRR) experimental technique. We believe that this is the first experimental study where such disparate dynamic features (SSR, TSR, and H-tunneling) are disentangled in a system's chemistry, attributing specific rate constant values to each effect and quantifying the various competitions. Moreover, multi-reference CASSCF/CASPT2 calculations indicate that structures with covalent Co-H bonds are present exclusively along the excited singlet surface. This phenomenon significantly reduces these structures' energy relative to their triplet counterparts, thus enabling the surface crossing and spin inversion that cause the observed two-state reactivity.

Multistate electronic quenching: Nonadiabatic pathways in NOA2Σ+ + O2X 3Σg - scattering.

The quenching of NOA2Σ+ with O2 as a collisional partner is important for combustion and atmospheric processes. There is still a lack of theoretical understanding of this event, especially concerning the nature of the different quenching pathways. In this work, we provide potential energy surfaces (PESs) of 20 electronic states of this system. We computed the spin-doublet and spin-quartet PESs using SA-CASSCF and XMS-CASPT2. We find two potential quenching pathways. The first one (Q1) is a two-step orientation-specific process. The system first undergoes an electron transfer (NO+X1Σ+ + O2 -X 2Πg) at short distances, before crossing to lower neutral states, such as NOX2Π + O2a 1Δg, O2b 1Σg +, O2X 3Σg -, or even 2 O(3P). The second quenching pathway (Q2) is less orientation-dependent and should be sudden without requiring the proximity conditioning Q1. The Q2 cross section will be enhanced with increasing initial vibrational level in both O2 and NO. It is responsible for the production of NOX2Π with higher O2 excited states, such as O2c 1Σu -, A'3Δu, or A 3Σu +. Overall, this work provides a first detailed theoretical investigation of the quenching of NOA2Σ+ by O2X 3Σg - as well as introduces a weighting scheme generally applicable to multireference, open-shell bimolecular systems. The effect of spin-multiplicity on the different quenching pathways is also discussed.

Observation of Competitive Nonadiabatic Photodissociation Dynamics of H2S+ Cations.

A comprehensive understanding of dissociation mechanisms is of fundamental importance in the photochemistry of small molecules. Here, we investigated the detailed photodissociation dynamics of H2S+ near 337 nm by using the velocity map ion imaging technique together with the theoretical characterizations by developing global full-dimensional potential energy surfaces (PESs). Rotational state resolved images were acquired for the S+(4S) + H2 product channel. Significant changes in product total kinetic energy release distributions and angular distributions have been observed within a small excitation photon energy range of 5 wavenumbers. Analysis based on the full-dimensional PESs reveals that two nonadiabatic pathways determined by the transition state connecting two minima on the 12A' state are responsible for the dramatic variation of observed product distributions. The current study has directly witnessed the competitive photodissociation mechanisms controlled by a critical energy point on the PES, thereby providing in-depth insight into the nonadiabatic dynamics in photochemistry.

The Photoionization Dynamics, Electronic Spectroscopy, and Excited State Photochemistry of AlCO and AlOC

The high cosmic abundance of carbon monoxide (CO) and the ubiquitous nature of aluminum-coated dust grains sets the stage for the production of weakly bound triatomic molecules AlCO (X 2Π) and AlOC (X 2Π) in circumstellar envelopes of evolved stars. Following desorption of cold AlCO and AlOC from the dust grain surface, incoming stellar radiation in the 2–9 eV wavelength range (visible to vacuum ultraviolet) will drive various photochemical processes. Ionization to the singlet cation state will cause an immediate Al–X (X = C, O) bond dissociation to form Al+ (1S) and CO (X 1Σ+) coproducts, whereas ionization to the higher-lying triplet states will lead to stabilization of AlCO+ (X 3Π) and AlOC+(X 3Π) in deep potential wells. In competition with ionization is electronic excitation. Excitation to the spectroscopically bright 1 2Π and 2 2Σ+ states will lead to either highly Stokes-shifted fluorescence, or photodissociation to yield Al (2D) + CO (X 1Σ+) products via nonadiabatic pathways, making AlCO and AlOC good candidates for electronic experimental studies. These many photoinduced pathways spanning orders of magnitude of the electromagnetic spectrum will lead to the depletion of AlCO and AlOC in astronomical environments, potentially explaining the lack of observational detection of these molecules. Furthermore, these results indicate new catalytic pathways to the freeing of aluminum atoms trapped in solid aluminum dust grains. Additionally, the results herein implicate an ion–neutral reaction as a possible important pathway in [Al, C, O] cation formation.

First-Principles Insights into Adiabatic and Nonadiabatic Vibrational Energy-Transfer Dynamics during Molecular Scattering from Metal Surfaces: The Importance of Surface Reactivity.

Energy transfer is ubiquitous during molecular collisions and reactions at gas-surface interfaces. Of particular importance is vibrational energy transfer because of its relevance to bond forming and breaking. In this Perspective, we review recent first-principles studies on vibrational energy-transfer dynamics during molecular scattering from metal surfaces at the state-to-state level. Taking several representative systems as examples, we highlight the intrinsic correlation between vibrational energy transfer in nonreactive scattering and surface reactivity and how it operates in both electronically adiabatic and nonadiabatic pathways. Adiabatically, the presence of a dissociation barrier softens a bond in the impinging molecule and increases its couplings with other molecular modes and surface phonons. In the meantime, the stronger interaction between the molecule and the surface also changes the electronic structure at the barrier, resulting in an increase of nonadiabatic effects. We further discuss future prospects toward a more quantitative understanding of this important surface dynamical process.

Photodissociation dynamics of CO2 + hv → CO(X1Σ+) + O(1D2) via the 3P1Πu state.

The vacuum ultraviolet (VUV) photodissociation of CO2 is important to understand the primary photochemical processes of CO2 induced by solar VUV excitation in the Earth's atmosphere. Here, we report a detailed study of vibrational-state-specific photodissociation dynamics of the CO(X1Σ+) + O(1D2) channel via the 3P1Πu state by using the time-sliced velocity-mapped ion imaging apparatus combined with the single VUV photoionization detection scheme. By recording the sliced images of the O(1D2) photoproducts formed by VUV photoexcitation of CO2 to the individual vibrational structure of the 3P1Πu state, both the vibrational state distributions of the counterpart CO(X1Σ+) photoproducts and the vibrational-state-specific product anisotropy parameters (β) are determined. The experimental results show that photodissociation of CO2 at 108.22, 107.50, 106.10, and 104.76nm yields less anisotropic (β > 0) and inverted distributed CO(X1Σ+, v) photoproducts. The possible dissociation mechanism for the CO(X1Σ+) + O(1D2) channel may involve the non-adiabatic transition of excited CO2* from the initially prepared state to the 31A' state with potential energy barriers. While at 108.82 and 107.35nm, the vibrational distributions are found to have the population peaked at a low vibrational state, and the anisotropy parameters turn out to be negative. Such variation indicates the possibility of another non-adiabatic dissociation pathway that may involve Coriolis-type coupling to the low-lying dissociative state. These observations show sclear evidence of the influence of the initially vibrational excitations on the photodissociation dynamics of CO2 via the 3P1Πu state.

Optical Control of Multistage Phase Transition via Phonon Coupling in MoTe_{2}.

The temporal characters of laser-driven phase transition from 2H to 1T^{'} has been investigated in the prototype MoTe_{2} monolayer. This process is found to be induced by fundamental electron-phonon interactions, with an unexpected phonon excitation and coupling pathway closely related to the nonequilibrium relaxation of photoexcited electrons. The order-to-order phase transformation is dissected into three substages, involving energy and momentum scattering processes from optical (A_{1}^{'} and E^{'}) to acoustic phonon modes [LA(M)] in subpicosecond timescale. An intermediate metallic state along the nonadiabatic transition pathway is also identified. These results have profound implications on nonequilibrium phase engineering strategies.

Adiabatic potential energy surfaces and photodissociation mechanisms for highly excited states of H2O

Full-dimensional adiabatic potential energy surfaces of the electronic ground state X̃ and nine excited states Ã, Ĩ, B̃, C̃, D̃, D̃′, D̃″, Ẽ′ and F̃ of H2O molecule are developed at the level of internally contracted multireference configuration interaction with the Davidson correction. The potential energy surfaces are fitted by using Gaussian process regression combining permutation invariant polynomials. With a large selected active space and extra diffuse basis set to describe these Rydberg states, the calculated vertical excited energies and equilibrium geometries are in good agreement with the previous theoretical and experimental values. Compared with the well-investigated photodissociation of the first three low-lying states, both theoretical and experimental studies on higher states are still limited. In this work, we focus on all the three channels of the highly excited state, which are directly involved in the vacuum ultraviolet photodissociation of water. In particular, some conical intersections of D̃–Ẽ′, Ẽ′-F̃, ÖĨ and Ĩ–C̃ states are clearly illustrated for the first time based on the newly developed potential energy surfaces (PESs). The nonadiabatic dissociation pathways for these excited states are discussed in detail, which may shed light on the photodissociation mechanisms for these highly excited states.

Photodissociation Dynamics of Methyl Hydroperoxide at 193 nm: ATrajectory Surface-Hopping Study.

The photodissociation of methyl hydroperoxide (CH3OOH) at 193 nm has been studied using a direct dynamics trajectory surface-hopping (TSH) method. The potential energies, energy gradients, and nonadiabatic couplings are calculated on the fly at the MRCIS(6,7)/aug-cc-pVDZ level of theory. The hopping of a trajectory from one electronic state to another is decided on the basis of Tully's fewest switches algorithm. An analysis of the trajectories reveals that the cleavage of the weakest O-O bond leads to major products CH3O(2E) + OH(2Π), contributing about 72.7% of the overall product formation. This OH elimination was completed in the ground degenerate product state where both the ground singlet (S0) and first excited singlet (S1) states become degenerate. The O-H bond dissociation of CH3OOH is a minor channel contributing about 27.3% to product formation, resulting in products CH3OO + H. An inspection of the trajectories indicates that unlike the major channel OH elimination, the H-atom elimination channel makes a significant contribution (∼3% of the overall product formation) through the nonadiabatic pathway via conical intersection S1/S0 leading to ground-state products CH3OO(X 2A″) + H(2S) in addition to adiabatic dissociation in the first excited singlet state, S1, correlating to products CH3OO(1 2A') + H(2S). The computed translational energy of the majority of the OH products is found to be high, distributed in the range of 70 to 100 kcal/mol, indicating that the dissociation takes place on a strong repulsive potential energy surface. This finding is consistent with the nature of the experimentally derived translational energy distribution of OH with an average translational energy of 67 kcal/mol after the excitation of CH3OOH at 193 nm.

Non-adiabatic quantum dynamics studies of the Mg+(3p) + D2 → MgD+ + D reaction

Non-adiabatic quantum dynamics study of the Mg + (3p) + D 2 reaction was carried out by using the time-dependent wave packet method. The results show that there is the triatomic complex supported by the well along the reaction path, which is manifested as metastable resonances on the probability curves. The MgD + products are obtained via a non-adiabatic pathway, and the reaction is more efficient than the quenching progress at the selected collision energies. The product angular distributions present obvious forward scattering bias, and the weak backward scatterings are mainly contributed by the new partial waves that are opened as the collision energy increases. Further rovibrational state-resolved results reflect that the predominant forward scattering correlates with the internally hot products and the backward-scattered products occupy the colder vibrational and rotational states.