Excited States Of Molecules Research Articles

Quantum State-Dependent Fragmentation Dynamics of D2S Molecules Following Excitation at Wavelengths ∼ 129.1 and ∼ 139.1 nm.

The first high-resolution translational spectroscopy studies of D atom photoproducts following excitation to the Rydberg states of D2S are reported. Excitation at wavelengths λ ∼ 139.1 nm reveals an unusual 'inverse' isotope effect; the 1B1(3da1←2b1) Rydberg state of D2S predissociates much faster than its counterpart in H2S. This is attributed to accidental near resonance with a vibrationally excited level of a lower-lying, more heavily predissociated Rydberg state of D2S that boosts the probability of nonadiabatic coupling to the dissociation continuum with 1A″ symmetry. Excitation at λ ∼ 129.1 nm populates the 1B1(4da1←2b1) Rydberg state, which predissociates more slowly and allows the study of ways in which the branching into different quantum states of the SD products varies with the choice of parent excited (JKaKc) level. All excited parent levels yield both ground (X) and electronically excited (A) state SD fragments. The former are distributed over a wide range of rovibrational (v″, N″) levels, while the population of levels with low v' and high N' is favored in the latter. These trends reflect the topographies of the dissociative 1A″ (1A') potential energy surfaces that correlate with the respective dissociation limits. Rotational motion about the b-inertial axis in the excited state molecule increases the relative yield of SD(A) products, consistent with dissociation by rotationally (Coriolis-) induced coupling from the photoexcited Rydberg level to the 1A' continuum. Molecules excited to the rotationless (JKaKc = 000) level also yield some SD(A) products, however, confirming the operation of a rival fragmentation pathway wherein photoexcited molecules decay by initial vibronic coupling to the 1A″ continuum, with subsequent nonadiabatic coupling between the 1A″ and 1A' continua enabling access to the D + SD(A) limit.

Time-resolved Auger-Meitner spectroscopy of the photodissociation dynamics of CS2

Abstract The photodissociation dynamics of UV excited CS2 are investigated using time-resolved Auger-Meitner spectroscopy. Auger-Meitner decay is initiated by inner-shell ionisation with a femtosecond duration X-ray (179.9 eV) probe generated by the FERMI free electron laser. The time-delayed X-ray probe removes an electron from the S(2p) orbital leading to secondary emission of a high energy electron through Auger-Meitner decay. We monitor the electron kinetic energy of the Auger-Meitner emission as a function of pump-probe delay and observe time-dependent changes in the spectrum that correlate with the formation of bound, excited-state CS2 molecules at early times, and CS + S fragments on the picosecond timescale. The results are analysed based on a simplified kinetic scheme that provides a time constant for dissociation of approximately 1.2 ps, in agreement with previous time-resolved X-ray photoelectron spectroscopy measurements (Gabalski, et al. J. Phys. Chem. Lett. 2023, 14, 7126–7133).

Counterion Effects in [Ru(bpy)3](X)2-Photocatalyzed Energy Transfer Reactions.

Photocatalysis that uses the energy of light to promote chemical transformations by exploiting the reactivity of excited-state molecules is at the heart of a virtuous dynamic within the chemical community. Visible-light metal-based photosensitizers are most prominent in organic synthesis, thanks to their versatile ligand structure tunability allowing to adjust photocatalytic properties toward specific applications. Nevertheless, a large majority of these photocatalysts are cationic species whose counterion effects remain underestimated and overlooked. In this report, we show that modification of the X counterions constitutive of [Ru(bpy)3](X)2 photocatalysts modulates their catalytic activities in intermolecular [2 + 2] cycloaddition reactions operating through triplet-triplet energy transfer (TTEnT). Particularly noteworthy is the dramatic impact observed in low-dielectric constant solvent over the excited-state quenching coefficient, which varies by two orders of magnitude depending on whether X is a large weakly bound (BArF 4 -) or a tightly bound (TsO-) anion. In addition, the counterion identity also greatly affects the photophysical properties of the cationic ruthenium complex, with [Ru(bpy)3](BArF 4)2 exhibiting the shortest 3MLCT excited-state lifetime, highest excited state energy, and highest photostability, enabling remarkably enhanced performance (up to >1000 TON at a low 500 ppm catalyst loading) in TTEnT photocatalysis. These findings supported by density functional theory-based calculations demonstrate that counterions have a critical role in modulating cationic transition metal-based photocatalyst potency, a parameter that should be taken into consideration also when developing energy transfer-triggered processes.

Non-adiabatic direct quantum dynamics using force fields: Toward solvation.

Quantum dynamics simulations are becoming a powerful tool for understanding photo-excited molecules. Their poor scaling, however, means that it is hard to study molecules with more than a few atoms accurately, and a major challenge at the moment is the inclusion of the molecular environment. Here, we present a proof of principle for a way to break the two bottlenecks preventing large but accurate simulations. First, the problem of providing the potential energy surfaces for a general system is addressed by parameterizing a standard force field to reproduce the potential surfaces of the molecule's excited-states, including the all-important vibronic coupling. While not shown here, this would trivially enable the use of an explicit solvent. Second, to help the scaling of the nuclear dynamics propagation, a hierarchy of approximations is introduced to the variational multi-configurational Gaussian method that retains the variational quantum wavepacket description of the key quantum degrees of freedom and uses classical trajectories for the remaining in a quantum mechanics/molecular mechanics like approach. The method is referred to as force field quantum dynamics (FF-QD), and a two-state ππ*/nπ* model of uracil, excited to its lowest bright ππ* state, is used as a test case.

GPAW: An open Python package for electronic structure calculations.

We review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for the implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE), providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe-Salpeter Equation, variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn-Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support for graphics processing unit (GPU) acceleration has been achieved with minor modifications to the GPAW code thanks to the CuPy library. We end the review with an outlook, describing some future plans for GPAW.

Measuring Intermolecular Excited State Geometry for Favorable Singlet Fission in Tetracene.

Singlet fission (SF) is the process of converting an excited singlet to a pair of excited triplets. Harvesting two charges from a single photon has the potential to increase photovoltaic device efficiencies. Acenes, such as tetracene and pentacene, are model molecules for studying SF. Despite SF being an endoergic process for tetracene and exoergic for pentacene, both acenes exhibit near unity SF quantum efficiencies, raising questions about how tetracene can overcome the energy barrier. Here, we use recently developed instrumentation to measure inelastic neutron scattering (INS) while optically exciting the model molecules using two different excitation energies. The spectroscopic results reveal intermolecular structural relaxation due to the presence of a triplet excited state. The structural dynamics of the combined excited state molecule and surrounding tetracene molecules are further studied using time-dependent density functional theory (TD-DFT), which shows that the singlet and triplet levels shift due to the excited state geometry, reducing the uphill energy barrier for SF to within kT.

Electron collision cross section data in plasma etching modeling

Semiconductor chips are the cornerstone of the information age, which play a vital role in the rapid development of emerging technologies such as big data, machine learning, and artificial intelligence. Driven by the growing demand for computing power, the chip manufacturing industry has been committed to pursuing higher level of integration and smaller device volumes. As a critical step in the chip manufacturing processes, the etching process therefore faces great challenges. Dry etching (or plasma etching) process based on the low-temperature plasma science and technology is the preferred solution for etching the high-precision circuit pattern. In the low-temperature plasma, electrons obtain energy from the external electromagnetic field and transfer the energy to other particles through collision process. After a series of complex physical and chemical reactions, a large number of active particles such as electrons, ions, atoms and molecules in excited states, and radicals are finally generated, providing the material conditions for etching the substrate. Dry etching chamber is a nonlinear system with multiple space-time dimensions, multiple reaction levels and high complexity. Facing such a complex system, only by fully understanding the basic physical and chemical reaction of the etching process can we optimize the process parameters and improve the etching conditions, so as to achieve precision machining of the semiconductor and meet the growing demand of the chip industry for etching rate and yield. In the early days, the process conditions of dry etching were determined through the trial-and-error method, which is characterized by high cost and low yield. However, with the help of plasma simulation, nowadays people have been able to narrow the scope of experiment to a large extent, and find out efficiently the optimal process conditions in a large number of parameters. In this review, we first introduce the basic theory of the mostly used models for plasma simulation including kinetic, fluid dynamic, hybrid and global models, in which the electron collision cross sections are the key input parameters. Since the formation of the low-temperature plasma is driven by the electron-heavy particle collision processes, and the active species for plasma etching are generated in the reactions induced by electron impact, the accuracy and completeness of the cross-section data greatly affect the reliability of the simulation results. Then, the theoretical and experimental methods of obtaining the cross-section data of etching gases are summarized. Finally, the research status of the electron collision cross sections of etching atoms and molecules is summarized, and the future research prospect is discussed.

Excited state dynamics of bay and peri benzothienyl perylene to understand the excimer formation and its dissociation

Many planar acenes such as pyrene, perylene etc. are known to form dimeric complex of their ground and excited state (excimer formation). Relaxation of excimer, carrier mobility & exciton diffusion in the solid state, and even singlet fission depends on the interaction between ground and excited state molecules in excimer. The role of excimer in triplet formation can be understood by: S0↑↓+S1↑↓⇄{S1S01↑↓↑↓}⇄{T1T11↑↑↓↓}⇄T1↑↑+T1↓↓. Intermediate excimer (T1T1)1 formation i.e., (SnS0)1 → (T1T1)1, and its decay to triplets have been studied by several groups, however, reports on the dissociation of excimer are very few. In this work, we aimed to study the dynamics of excimer formation and its further dissociation using perylene derivatives. We synthesized two positional isomers of benzothienyl substituted perylene i.e., peri-BT and bay-BT, and studied their photophysical properties in solution, nanoaggregates, and thermally evaporated thin films. Excited state dynamics in thin films suggest the dissociation of 1(excimer) to a monomeric singlet excited state in a microsecond timescale. Transient absorption spectral characteristics suggest the formation of a charge transfer state within picoseconds, followed by decay to a long-lived (few microseconds) excimer state. The formation of CT is promoted by strong intermolecular interaction in the thin film.

Stochastic Real-Time Second-Order Green's Function Theory for Neutral Excitations in Molecules and Nanostructures.

We present a real-time second-order Green's function (GF) method for computing excited states in molecules and nanostructures, with a computational scaling of O(Ne3), where Ne is the number of electrons. The cubic scaling is achieved by adopting the stochastic resolution of the identity to decouple the 4-index electron repulsion integrals. To improve the time propagation and the spectral resolution, we adopt the dynamic mode decomposition technique and assess the accuracy and efficiency of the combined approach for a chain of hydrogen dimer molecules of different lengths. We find that the stochastic implementation accurately reproduces the deterministic results for the electronic dynamics and excitation energies. Furthermore, we provide a detailed analysis of the statistical errors, bias, and long-time extrapolation. Overall, the approach offers an efficient route to investigate excited states in extended systems with open or closed boundary conditions.

Chemical applications of hybridized light-matter states (a review)

Interactions between light and matter are a fundamental part of chemical sciences responsible for basic photophysical processes such as phosphorescence and fluorescence. However, these photophysical phenomena occur in the "weak" limit of interaction between light and matter in which the photon and molecule interact with each other without the former fundamentally changing the physical properties of the latter. By constructing a Fabry-Perot cavity, which traps light of a certain frequency, then placing a molecule in a cavity that undergoes a molecular electron transition at the frequency of the trapped light, scientists can force strong light-matter interaction. This interaction occurs if the exchange between the light of the cavity mode and the molecule's excited state is faster than the decay rate of either state, forming a hybrid light-matter state known as a polariton. The photophysical properties of these polariton states have been of interest to scientists due to the possibility that they can allow for the modification of the reactivity of molecules without the addition of functional groups or modification of the surrounding environment. Of particular interest is the ability of polaritons to influence the potential energy surface of molecules, with polaritons showing the ability to both, suppress the photochemical reaction in molecules such as spiropyran and stilbene, while also enhancing the nonradiative relaxation rate of porphyrins. Due to their photonic nature, polaritons have also shown the ability to facilitate long range energy transfer processes in organic dye molecules. This review focuses on discussing these recent advances in a chemistry context as well as the optical design of cavities required to sustain polaritons.

Direct measurement of high-lying vibrational repumping transitions for molecular laser cooling

Molecular laser cooling and trapping requires addressing all spontaneous decays to excited vibrational states that occur at the $\gtrsim 10^{-4} - 10^{-5}$ level, which is accomplished by driving repumping transitions out of these states. However, the transitions must first be identified spectroscopically at high-resolution. A typical approach is to prepare molecules in excited vibrational states via optical cycling and pumping, which requires multiple high-power lasers. Here, we demonstrate a general method to perform this spectroscopy without the need for optical cycling. We produce molecules in excited vibrational states by using optically-driven chemical reactions in a cryogenic buffer gas cell, and implement frequency-modulated absorption to perform direct, sensitive, high-resolution spectroscopy. We demonstrate this technique by measuring the spectrum of the $\tilde{A}^2\Pi_{1/2}(1,0,0)-\tilde{X}^2\Sigma^+(3,0,0)$ band in $^{174}$YbOH. We identify the specific vibrational repump transitions needed for photon cycling, and combine our data with previous measurements of the $\tilde{A}^2\Pi_{1/2}(1,0,0)-\tilde{X}^2\Sigma^+(0,0,0)$ band to determine all of the relevant spectral constants of the $\tilde{X}^2\Sigma^+(3,0,0)$ state. This technique achieves high signal-to-noise, can be further improved to measure increasingly high-lying vibrational states, and is applicable to other molecular species favorable for laser cooling.

Solute − solvent repulsion effects on the absorption spectra of anthracene in n-hexane investigated under high pressure

The band positions in the UV–VIS absorption spectra of compressed solution of anthracene in n-hexane significantly depend not only on the dispersive but also on the repulsive solute–solvent interactions, what has so far been omitted. Their strength is determined not only by the solvent polarity but also by Onsager cavity radius changing with pressure. The results obtained for anthracene show that repulsive interactions should be included in the interpretation of barochromic and solvatochromic results of aromatic compounds. We show that the barochromic studies in the liquid solvent can be an alternative to solvatochromic studies, e.g. to determine the polarizability of organic molecules in the electronic excited state. The pressure-induced polarity change in n-hexane exceeds that induced by the exchange of n-alkane solvents between n-pentane and n-hexadecane.

Mapping extended reaction coordinates in photochemical dynamics

Laser-based experiments facilitate numerous strategies for interrogating the complex non-adiabatic dynamics operating in the excited states of molecules. Measurements may be broadly separated into frequency- and time-resolved variants, with a combination of different approaches (with different associated observables) typically being required to reveal a complete mechanistic picture. In the former category, quantum state-resolved information may often be obtained using narrow linewidth lasers. This provides detailed information relating to the starting point on the photochemical reaction coordinate (via the absorption spectrum) and the asymptotic endpoints (i.e. the photoproducts). Direct observation of the intermediate pathways connecting these two limits is often not possible, however, due to the inherently long temporal duration of the laser pulses relative to the typical timescales of non-adiabatic energy redistribution processes. It is therefore desirable to obtain complementary information that monitors real-time evolution along the reaction coordinate as excited state population traverses the potential energy landscape. This may be achieved in time-resolved pump–probe experiments conducted using ultrafast (i.e. sub-picosecond) laser pulses. The use of valence state photoionization for the probe step is a commonly employed methodology and has proved highly instructive in revealing subtle mechanistic details of key energy redistribution pathways operating in many different molecular systems. One frequent limitation here, however, is the restricted “view” along the reaction coordinate(s) connecting the initially prepared excited states to various photoproducts. Guided by examples drawn from recent work using time-resolved photoelectron imaging, this review will discuss such issues in detail and highlight some strategies that potentially help overcome them – with particular emphasis placed on the advantages of projecting as deeply as possible into the ionization continuum. The role of complementary measurements using other spectroscopic techniques and the importance of high-level supporting theory to guide data interpretation will also be reinforced.

Geometric Phase Effect in Attosecond Stimulated X-ray Raman Spectroscopy

Conical intersections (CIs) are diabolical points in the potential energy surfaces generally caused by point-wise degeneracy of different electronic states, and give rise to the geometric phases (GPs) of molecular wave functions. Here we theoretically propose and demonstrate that the transient redistribution of ultrafast electronic coherence in attosecond Raman signal (TRUECARS) spectroscopy is capable of detecting the GP effect in excited state molecules by applying two probe pulses including an attosecond and a femtosecond X-ray pulse. The mechanism is based on a set of symmetry selection rules in the presence of nontrivial GPs. The model of this work can be realized for probing the geometric phase effect in the excited state dynamics of complex molecules with appropriate symmetries, using attosecond light sources such as free-electron X-ray lasers.

Using synthesis to steer excited states and their properties and functions

The nature, properties and reactivities of ground-state molecules have been studied widely. By contrast, the parallel manipulation and knowledge of molecules in their excited states are not well controlled or understood. Here we have investigated the manipulation and control of excited-state molecules, with the generation of libraries of molecules. The power and ability of light to drive chemical reactions, conformational changes, motion, luminescence and energy conversions in nature have inspired researchers to harness excited states to achieve the desired control of manipulation of molecules in their excited state to tackle challenges in materials and energy research and sustainability. To this end, synthesis can be used to harness excited states and molecular functional materials. Furthermore, supramolecular chemistry can provide an additional dimension of control to the excited states and expand the library of excited-state functional materials. By mastering the design of excited states, an in-depth understanding and discovery of excited states with desirable properties and controllable transformation by design may open up a new area of research, resulting in libraries of excited-state molecules and chemistry that parallel ground-state chemistry and, in doing so, offer unlimited opportunities.

Synthesis of Naphtho-[2,3]-furan derivatives for investigative insights into solvatochromic behaviour

The paper delineates the synthesis of naphtho-[2,3]-furan derivatives (viz. a viz. naphtho-[2,3]-furan-3-yl acetate (NF), 9-nitronaphtho-[2,3]-furan-3-yl acetate (NNF) and 9-bromonaphtho-[2,3-]-furan-3-yl acetate (BNF)) using a multistep approach. The products were obtained in high yields and characterized by FT-IR, 1H NMR, 13C NMR, and HRMS spectroscopy. The absorption and fluorescence spectra of these derivatives were recorded in the solvent of different polarities. Solvatochromic correlations were employed to estimate ground state (μg) and excited state (μe) dipole moments. The results suggested higher stability of molecules in the singlet excited state than in the ground state. Theoretical investigations indicated that the ΔEHUMO-LUMO gap varied as ΔENF>ΔEBNF >ΔENNF. The thermogravimetric analysis (TGA) confirmed the higher stability of NNF as compared to BNF and NF.

Mechanism of fluorescence enhancement of HClO detected by excited-state intramolecular proton transfer based HBT-OMe molecule

The molecule with excited-state intramolecular proton transfer (ESIPT) has wide applications in fluorescent probe, biology imaging, light-emitting materials, etc. Biologically active oxygen hypochlorite (HClO) exists widely in the biological and chemical environment, which can pose a great threat to human health. Design of HClO-sensitive molecules in solvents is very important. Recently, Wu et al. [Wu L L, Yang Q Y, Liu L Y, et al. <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1039/C8CC03717E">2018 <i>Chem. Commun.</i> <b>54</b> 8522</ext-link>] designed an ESIPT-based HBT-OMe probe molecule, which can detect HClO due to its methoxy-hydroxy-benzothiazole. They found that the fluorescence intensity of the system gradually increases with HClO increasing. However, the microscopic mechanism of this highly efficient fluorescent probe is not well understood. Therefore, in this work, we theoretically investigate the ESIPT mechanism of the HBT-Ome and its product molecule by using density functional theory and time-dependent density functional theory. Based on polarizable continuum model (PCM) with the integral equation formalism variant (IEFPCM) and Becke’s three-parameter hybrid exchange function with the Lee-Yang-Parr gradient-corrected functional (B3LYP) as well as the TZVP basis, the optimized structures are obtained. The structures show that the HBT-Ome product molecules tend to undergo proton transfer in the excited state but HBT-OMe molecules cannot undergo the proton transfer process. The analysis of frontier molecular orbitals not only explains the reason why the fluorescence of the HBT-Ome product is enhanced, but also demonstrates that the HBT-Ome fluorescence intensity is diminished owing to twisted intramolecular charge transfer in the excited state. It is twisted intramolecular charge transfer that leads smaller charge density to be overlapped and the fluorescence intensity of HBT-OMe molecule to be further weakened. Infrared vibrational spectrum shows the enhancement of intramolecular hydrogen bond of O—H, which indicates the tendency of proton transfer. The molecular covalent interaction analysis shows that the intramolecular interactions of HBT-OMe remain largely unchanged clearly. The intramolecular O—H bonding interaction is weakened, and the N—H bonding interaction is increased for HBT-OMe product molecule. The enhancement of intramolecular hydrogen bond of N—H further illustrates the trend of proton transfer. The calculated potential energy curve provides direct evidence for the occurrence of ESIPT in the HBT-Ome product molecule. Our work is of great significance in designing and synthesizing the HClO fluorescent probes based on ESIPT molecules.

Adiabatic theory for high Rydberg molecules

AbstractMolecules in high Rydberg states, or high Rydberg molecules, are special types of excited species. Classically, at least one of the valence electrons of such molecules is excited to a high‐energy near‐threshold orbit with a negative eccentricity. Therefore, the electron spends most of its orbiting time on roaming slowly far away from the parent ionic core. With this classical picture in mind, we develop a general adiabatic theory termed the inverse Born‐Oppenheimer approximation, in contrast to the conventional Born‐Oppenheimer adiabatic theory for molecules in ground or lower excited states, to describe high Rydberg molecules. A thorough theoretical formulation starting from the non‐relativistic molecular Hamiltonian and Schrödinger equation is presented with emphasis on the mathematical rigorousness of this theory. Quantitative calculations of the transition rate constants for many radiative and nonradiative processes involved in high Rydberg molecules are shown explicitly.

Insights from computational analysis: Excited-state hydrogen-bonding interactions and ESIPT processes in phenothiazine derivatives

Organic materials with Mechanofluorochromism (MFC) properties have potential application value. Phenothiazine derivatives are a class of substances with MFC properties that have been synthesized and reported in experiments (Dyes and Pigments 172 (2020) 107835). Dual fluorescence of a series of phenothiazine derivatives is observed in the experiment, which proved that the ESIPT process is carried out. In this work, we choose phenothiazine derivatives (C2PAHN, C4PAHN, C8PAHN) as models to theoretically analyze the influence of different alkyl chain lengths on the excited state intramolecular proton transfer (ESIPT). In addition, the shift value of fluorescence spectrum is related to the length of alkyl chain. The fluorescence shift of C2PAHN is the largest (6.31 nm), and that of C8PAHN is the smallest (2.40 nm). The theory of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) are adopted to simulate the molecular dynamics in the ground state and excited state. The analysis of the optimized molecular geometry parameters and infrared vibrational spectroscopy (IR) illustrate the stronger hydrogen bonding of the excited state molecules, which is favorable for the progress of ESIPT. Fluorescence spectroscopy reveals that the appropriate increase or decrease of alkyl chains would change the photophysical properties of the molecules. Frontier molecular orbitals (FMOs) indicate that the rearrangement of electron density from electronic level to is the driving force of the ESIPT process. Reduction density gradient (RDG) surfaces and Natural Population Analysis (NPA) tentatively lead to the conclusion that alkyl chain length is inversely proportional to hydrogen bond strength. Finally, the data are qualitatively analyzed by scanning the potential energy curves, and it is concluded that the longer the alkyl chain, the weaker the hydrogen bonding effect and the more unfavorable the ESIPT process.

Time-Dependent Second-Order Green’s Function Theory for Neutral Excitations

We develop a time-dependent second-order Green's function theory (GF2) for calculating neutral excited states in molecules. The equation of motion for the lesser Green's function (GF) is derived within the adiabatic approximation to the Kadanoff-Baym (KB) equation, using the second-order Born approximation for the self-energy. In the linear response regime, we recast the time-dependent KB equation into a Bethe-Salpeter-like equation (GF2-BSE), with a kernel approximated by the second-order Coulomb self-energy. We then apply our GF2-BSE to a set of molecules and atoms and find that GF2-BSE is superior to configuration interaction with singles (CIS) and/or time-dependent Hartree-Fock (TDHF), particularly for charge-transfer excitations, and is comparable to CIS with perturbative doubles (CIS(D)) in most cases.