Excited Vibrational States Research Articles

Induced Emission on Transitions from Vibrational Excited Levels of the KrF Molecule

The paper considers the possibility of extending the spectral region of the wavelength tuning of a discharge KrF amplifier due to induced transitions from the vibrational excited states of the electronic level B. The model of the KrF amplifier on a He/Kr/F2 mixture is presented, in which the behavior of the vibrational level populations is consistent with the excitation conditions of the active medium. The simulation results show that the shift in the operating wavelength to the short-wavelength region is possible in excitation modes, when the birth rate of excimer molecules is greater than the rate of their relaxation from upper to lower vibrational levels. The theoretical dependences of gain on the wavelength for different pressures were obtained. They confirm the possibility of tuning the KrF amplifier wavelength in the range of up to 10 nm while maintaining a gain of at least 0.5 of its maximum value.

Theoretical Investigation of Rate Coefficients and Dynamical Mechanisms for N + N + N Three-Body Recombination Based on Full-Dimensional Potential Energy Surfaces

Three-body recombination reactions, in which two particles form a bound state while a third one bounces off after the collision, play significant roles in many fields, such as cold and ultracold chemistry, astrochemistry, atmospheric physics, and plasma physics. In this work, the dynamics of the recombination reaction for the N3 system over a wide temperature range (5000–20,000 K) are investigated in detail using the quasi-classical trajectory (QCT) method based on recently developed full-dimensional potential energy surfaces. The recombination products are N2(X) + N(4S) in the 14A″ state, N2(A) + N(4S) in the 24A″ state, and N2(X) + N(2D) in both the 12A″ and 22A″ states. A three-body collision recombination model involving two sets of relative translational energies and collision parameters and a time-delay parameter is adopted in the QCT calculations. The recombination process occurs after forming an intermediate with a certain lifetime, which has a great influence on the recombination probability. Recombination processes occurring through a one-step three-body collision mechanism and two distinct two-step binary collision mechanisms are found in each state. And the two-step exchange mechanism is more dominant than the two-step transfer mechanism at higher temperatures. N2(X) formed in all three related states is always the major recombination product in the temperature range from 5000 K to 20,000 K, with the relative abundance of N2(A) increasing as temperature decreases. After hyperthermal collisions, the formed N2(X/A) molecules are distributed in highly excited rotational and vibrational states, with internal energies mainly distributed near the dissociation threshold. Additionally, the rate coefficients for this three-body recombination reaction in each state are determined and exhibit a negative correlation with temperature. The dynamic insights presented in this work might be very useful to further simulate non-equilibrium dynamic processes in plasma physics involving N3 systems.

Isotopic species, vibrational states and nuclear quadrupole splitting in CH[formula omitted]Cl[formula omitted] from rotational spectroscopy at 8–18 GHz

The present work fills several significant gaps in the study of the rotational spectrum of the methylene chloride molecule. Two Fourier transform microwave spectrometers were used, providing complementary coverage of the 8–18 GHz region, at room temperature and in supersonic expansion. Hyperfine resolved measurements were made and fitted for ground states of the CH235Cl2, CH235Cl37Cl, CH237Cl2, and 13CH235Cl2 isotopic species, as well as for nine different excited vibrational states. Transitions in the v4=2, v4=3, v3=1, v9=1 states in CH235Cl2, and transitions in excited states of the ν4 mode in 35Cl37Cl and 37Cl2 isotopic species have been assigned for the first time. Vibrational and isotopic dependence of nuclear quadrupole coupling constants was identified. The v3=1 and v9=1 states in CH235Cl2 were found to be significantly coupled by c-axis Coriolis interaction, while transitions in v4=2 and v4=3 states exhibited hyperfine mediated perturbations enhancing the determination of the χab parameter and allowing assessment of its vibrational dependence.

Energy level spectrum and lattice polarization of polaron in metal halide perovskite parabolic quantum wells

In this study, polaron effect in 2D metal halide perovskite materials (TMHPMs), which naturally form quantum well (QW) structures, has been investigated. Polarization of carriers and lattice within these structures was explored based on unitary transformation and variational methods. Specifically, lattice polarization and energy level transition spectra of strong coupling polaron in parabolic potential metal halide perovskite QWs (MAPbCl3, MAPbBr3, and MAPbI3) were studied. Also, we calculated ground and first excited state energies, excitation energy, transition frequency, and vibration frequency of a TMHPM QWs. Our results indicated that parabolic potential confinement intensity affected related physical quantities, which provided insight to investigate metal halide perovskite materials and polaron effect.

Distinct vibrational motions promote disparate excited-state decay pathways in cofacial perylenediimide dimers.

A complex interplay of structural, electronic, and vibrational degrees of freedom underpins the fate of molecular excited states. Organic assemblies exhibit a myriad of excited-state decay processes, such as symmetry-breaking charge separation (SB-CS), excimer (EX) formation, singlet fission, and energy transfer. Recent studies of cofacial and slip-stacked perylene-3,4:9,10-bis(dicarboximide) (PDI) multimers demonstrate that slight variations in core substituents and H- or J-type aggregation can determine whether the system follows an SB-CS pathway or an EX one. However, questions regarding the relative importance of structural properties and molecular vibrations in driving the excited-state dynamics remain. Here, we use a combination of two-dimensional electronic spectroscopy, femtosecond stimulated Raman spectroscopy, and quantum chemistry computations to compare the photophysics of two PDI dimers. The dimer with 1,7-bis(pyrrolidin-1'-yl) substituents (5PDI2) undergoes ultrafast SB-CS from a photoexcited mixed state, while the dimer with bis-1,7-(3',5'-di-t-butylphenoxy) substituents (PPDI2) rapidly forms an EX state. Examination of their quantum beating features reveals that SB-CS in 5PDI2 is driven by the collective vibronic coupling of two or more excited-state vibrations. In contrast, we observe signatures of low-frequency vibrational coherence transfer during EX formation by PPDI2, which aligns with several previous studies. We conclude that key electronic and structural differences between 5PDI2 and PPDI2 determine their markedly different photophysics.

Vibrational-Mode-Selective Modulation of Electronic Excitation.

Vibrational-mode-selective modulation of electronic excitation is conducted with a synchronized femtosecond (fs) visible (vis) pulse and a picosecond (ps) infrared (IR) pulse. The mechanism of modulation of vibrational and vibronic relaxation behavior of excited state is investigated with ultrafast vis/IR, IR/IR, and vis-IR/IR transient spectroscopy, optical gating experiments and theoretical calculations. An organic molecule, 4'-(N,N-dimethylamino)-3-methoxyflavone (DMA3MHF) is chosen as the model system. Upon 1608 cm-1 excitation, the skeleton stretching vibration of DMA3MHF is energized, which can significantly change the shape of the absorption, facilitate the radiative decay and promote emission from vibrational excited states. As results, a remarkable enhancement and a slight blueshift in fluorescence are observed. The mode-selective modulation of electronic excitation is not limited in luminescence or photophysics. It is expected to be widely applicable in tuning many photochemical processes.

Revisiting the rovibrational (de-)excitation of molecular hydrogen by helium

Context. The collisional (de-)excitation of H2 by He plays an important role in the thermal balance and chemistry of various astro-physical environments, making accurate rate coefficients essential for interpreting observations of the interstellar medium. Aims. Our goal is to utilize a state-of-the-art potential energy surface (PES) to provide comprehensive state-to-state rate coefficients for He-induced transitions among rovibrational levels of H2. Methods. We performed quantum scattering calculations for the H2-He system. Thus, we were able to provide state-to-state rate coefficients for 1059 transitions between rovibrational levels of H2, with internal energies up to ≃15 000 cm−1, for temperatures ranging from 20 to 8000 K. Results. Our results demonstrate a good agreement with previous calculations for pure rotational transitions between low-lying rotational levels. However, we do find significant discrepancies for rovibrational processes involving highly-excited rotational and vibrational states. We attribute these differences to two key factors: (1) the broader range of intramolecular distances covered by ab initio points and (2) the superior accuracy of the PES, resulting from the utilization of the state-of-the-art quantum chemistry methods, compared to previous lower-level calculations. Conclusions. Radiative transfer calculations performed with the new collisional data indicate that the population of rotational levels in excited vibrational states experiences significant modifications, highlighting the critical need for this updated dataset in models of high-temperature astrophysical environments.

HCl trimer: HCl-stretch excited intramolecular and intermolecular vibrational states from 12D fully coupled quantum calculations employing contracted intra- and inter-molecular bases.

We present fully coupled, full-dimensional quantum calculations of the inter- and intra-molecular vibrational states of HCl trimer, a paradigmatic hydrogen-bonded molecular trimer. They are performed utilizing the recently developed methodology for the rigorous 12D quantum treatment of the vibrations of the noncovalently bound trimers of flexible diatomic molecules [Felker and Bačić, J. Chem. Phys. 158, 234109 (2023)], which was previously applied to the HF trimer by us. In this work, the many-body 12D potential energy surface (PES) of (HCl)3 [Mancini and Bowman, J. Phys. Chem. A 118, 7367 (2014)] is employed. The calculations extend to the intramolecular HCl-stretch excited vibrational states of the trimer with one- and two-quanta, together with the low-energy intermolecular vibrational states in the two excited v = 1 intramolecular vibrational manifolds. They reveal significant coupling between the intra- and inter-molecular vibrational modes. The 12D calculations also show that the frequencies of the v = 1 HCl stretching states of the HCl trimer are significantly redshifted relative to those of the isolated HCl monomer. Detailed comparison is made between the results of the 12D calculations on the two-body PES, obtained by removing the three-body term from the original 2 + 3-body PES, and those computed on the 2 + 3-body PES. It demonstrates that the three-body interactions have a strong effect on the trimer binding energy as well as on its intra- and inter-molecular vibrational energy levels. Comparison with the available spectroscopic data shows that good agreement with the experiment is achieved only if the three-body interactions are included. Some low-energy vibrational states localized in a secondary minimum of the PES are characterized as well.

The tunneling splittings of the ground state and some excited vibrational states for the inversion motion in H3C− anion and H3Si[rad] radical

Splitting of the ground state and some excited symmetric bending vibrational states due to inversion tunneling in the H3C− anion and H3Si radical is analyzed by numerically solving the vibrational Schrödinger equation of restricted (2D) dimensionality. We used the following two vibrational coordinates for the H3X structure (X = C, Si): the distance of the X atom from the plane of a regular triangle formed by three hydrogen atoms (1) and a symmetry coordinate composed of three distances between chemically non-bonded hydrogen atoms (2). The kinetic energy operator in this case takes the simplest form. The 2D potential energy surface (PES) in the given coordinates was calculated for H3C− at the CCSD(T)/aug-cc-pVTZ, CCSD(T)/aug-cc-pVQZ, CCSD(T)/aug-cc-pV5Z, CCSD(T)/CBS(TQ5), CCSD(T)/d-aug-cc-pVTZ, CCSD(T)/t-aug-cc-pVTZ, and CCSD(T)/q-aug-cc-pVTZ levels of theory, based on recommendations from recently published work [M.C. Bowman, B. Zhang, W.J. Morgan, H.F. Schaefer III, Mol.Phys., 117 (2019) 1069–1077]. The same 2D PES for the H3Si radical was calculated at the CCSD(T)/aug-cc-pVDZ CCSD(T)/aug-cc-pVTZ, CCSD(T)/aug-cc-pVQZ, and CCSD(T)/CBS(D,T,Q) as well as at the CCSD(T)/d-aug-cc-pVTZ, CCSD(T)/un-aug-cc-pVTZ, CCSD(T)/un-aug-cc-pVQZ levels of theory. The tunneling splittings for the D3C− anion and D3Si radical were calculated as well. The results of calculations demonstrate good agreement with available experimental data on umbrella vibration frequencies and inversion splittings for the title molecules.

High-resolution ro-vibrational spectrum of H[formula omitted]S in highly excited vibrational states: Re-visiting the first decade

High resolution spectra of the H232S hydrogen sulfide are re-analyzed in the region of its first decade. Thanks to the improvement of the experimental conditions (in particular, an increase in both the instrumental resolution and the effective absorption path length by 1.5 times), on the one hand, and of the use of a more correct Hamiltonian model (which additionally takes into account the interactions of the bands under study with the ν1+4ν2 and 4ν2+ν3 bands), on the other hand, the information obtained as a result of the analysis of experimental data has been significantly expanded and improved compared to the previous status. In particular, (1) the total number of assigned lines has been increased by approximately 15 percent, (2) the maximum values of quantum numbers Jmax and Kamax for the assigned lines have been increased for the studied bands in average by 20–25 percent, (3) the number of obtained ”experimental“ energy levels of the upper vibrational states (especially for the (121) and (022) vibrational states) has been significantly increased, (4) despite the increase in the number of initial vibrational–rotational energies and quantum numbers Jmax and Kamax, the value of the standard deviation drms has improved 2.7 times for the obtained upper vibrational–rotational energies.

Resonance theory of vibrational polariton chemistry at the normal incidence

Abstract We present a theory that explains the resonance effect of the vibrational strong coupling (VSC) modified reaction rate constant at the normal incidence of a Fabry–Pérot (FP) cavity. This analytic theory is based on a mechanistic hypothesis that cavity modes promote the transition from the ground state to the vibrational excited state of the reactant, which is the rate-limiting step of the reaction. This mechanism for a single molecule coupled to a single-mode cavity has been confirmed by numerically exact simulations in our recent work in [J. Chem. Phys. 159, 084104 (2023)]. Using Fermi’s golden rule (FGR), we formulate this rate constant for many molecules coupled to many cavity modes inside a FP microcavity. The theory provides a possible explanation for the resonance condition of the observed VSC effect and a plausible explanation of why only at the normal incident angle there is the resonance effect, whereas, for an oblique incidence, there is no apparent VSC effect for the rate constant even though both cases generate Rabi splitting and forming polariton states. On the other hand, the current theory cannot explain the collective effect when a large number of molecules are collectively coupled to the cavity, and future work is required to build a complete microscopic theory to explain all observed phenomena in VSC.

Quantum dynamics study of C—H stretching vibrational excitation in the F+CHD<sub>3</sub> → HF+CD<sub>3</sub> reaction

In recent decades, significant progress has been made in the precise theoretical investigation of gas-phase chemical reactions. Presently, a major challenge in the field of quantum dynamics is to develop the precise methodologies for studying chemical reactions involving more than four atoms. As a typical multi-atomic reaction system, the F+CH<sub>4</sub> reaction and its isotopic substitution reactions have attracted widespread attention from both experimental and theoretical perspectives in recent years. Experimental studies on the reaction of F+CHD<sub>3</sub> have revealed that the stretching vibration excitation of the C—H bond inhibits the bond dissociation, favoring the formation of DF+CHD<sub>2</sub> product channels. In this study, we use a seven-dimensional quantum time-dependent wave packet method to investigate the dynamics of the F+CHD<sub>3</sub> reaction in both the reactant vibrational ground state and the first stretching excited state of the C—H bond. In this work, the reaction probabilities under different vibrational conditions are analyzed, showing that when the collision energy is below 0.06 eV, the reaction probability curves exhibit numerous fast-oscillating peaks, supporting the experimentally suggested phenomena of dynamic resonance. In a collision energy range from 0.06 eV to 0.3 eV, the reaction probability for the HF product channel in the vibrational excited state is lower than that in the ground state, which is consistent with experimental observation. Through the analysis of the time-independent wave functions of product channels under low-energy collision conditions, it is found that for reactions involving vibrational ground states, the HF products in the product asymptotic region and the reaction transition state region are in the <i>v'</i> = 2 excited state and <i>v'</i> = 3 excited state of stretching vibration, respectively, which are consistent with previous experimental observations and six-dimensional quantum wave packet simulations. For reactions involving the first excited state of C—H stretching vibration, the HF products in the product asymptotic region and the reaction transition state region are both in the <i>v'</i> = 3 excited state of stretching vibration, which are consistent with the results obtained based on energy analysis. Simulation results indicate that in the case of low-energy collisions, the time-independent wave function for the C—H stretching vibrational excited state tends to be closer to the D atom side in the transition state region. This phenomenon is attributed to the more significant energy advantage of the vibrational excited state potential energy surface in the large collision angle region, explaining the inhibitory effect of stretching vibration excitation on the HF product channel. This study offers important theoretical support for explaining experimental results and contributes to a more in-depth understanding of the influence of vibrational mode excitations on the dynamical processes in poly-atomic reactions.

Predicting nucleon-nucleus scattering observables using nuclear structure theory

Developing a predictive capability for inelastic scattering will find applications in multiple areas. Experimental data for neutron-nucleus inelastic scattering is limited and thus one needs a robust theoretical framework to complement it. Charged-particle inelastic scattering can be used as a surrogate for (n, γ) reactions to predict capture cross sections for unstable nuclei. Our work uses microscopic nuclear structure calculations for spherical nuclei to obtain nucleon-nucleus scattering potentials and calculate cross sections for these processes. We implement the Jeukenne, Lejeune, Mahaux (JLM) semi-microscopic folding approach, where the medium effects on nuclear interaction are parameterized in nuclear matter to obtain the nucleon-nucleon (NN) interaction in a medium at positive energies. We solve for the nuclear ground state using the Hartree-Fock-Bogliubov (HFB) many-body method, assuming the nucleons within the nucleus interact via the Gogny-D1M potential. The vibrational excited states of the target nucleus are calculated using the quasi-particle random phase approximation (QRPA). We demonstrate our approach for spherical nuclei in the medium-mass region, showing scattering results for the 90Zr nucleus.

Gas heating by nanosecond repetitively pulsed glow discharges applied to a methane–air flame

In this work, the thermal effect induced by non-equilibrium plasma produced by nanosecond repetitively pulsed (NRP) glow discharges applied across a lean premixed methane–air flame is investigated. The flame is laminar, stationary, and axis-symmetric. The discharges are applied on the symmetry axis of the flame, crossing the fresh reactants, flame front, and burned gases. The obtained plasma-assisted flame is stable and reproducible, allowing phase-locked averaged diagnostics. The thermal effect is investigated by acquiring spatially and temporally resolved temperature measurements in the plasma discharges using optical emission spectroscopy (OES) of the second positive system of nitrogen. The results show that for an applied voltage of 10 kV, NRP glow discharges increased the gas temperature by up to 130 K within 10 ns. However, this heating was location-dependent, i.e., high near the anode tip and not detectable 0.5 mm above the anode. It was also shown that reducing the applied voltage down to 9 kV caused this ultra-fast heating to disappear. On the other hand, the reactants near the anode tip had a constant temperature of 480 K, between discharges, while the reactants were injected at 293 K. The reason for this slow heating could be due to (1) heating by the flame which is pulled down to the anode when NRP discharges are applied, (2) slow gas heating induced by relaxation of vibrational excited states of nitrogen, or (3) post-discharge oxidation chemistry. Based on temperature measurements performed on NRP corona discharges, heating by the flame could be ruled out. Zero-dimensional numerical simulations’ results suggest that post-discharge oxidation chemistry can heat the gas by 110–130 K, indicating that this is an important heating mechanism. Further investigation will be necessary to assess the significance of the vibrational–translational relaxation of nitrogen.

Detection of vibrationally excited C6H in the cold prestellar core TMC-1 with the QUIJOTE line survey

In this work, we present the detection of twelve doublets with quantum numbers of N = 12–11 to N = 17–16 of the ν11(μ2Σ) vibrationally excited state of C6H towards TMC-1. This marks the first time that an excited vibrational state of a molecule has been detected in a cold starless core. The data are part of the QUIJOTE line survey gathered with the Yebes 40m radio telescope. The line intensities have been aptly reproduced with a rotational temperature of 6.2 ± 0.4 K and a column density of (1.2 ± 0.2)×1011 cm−2. We also analysed the ground state transitions of C6H, detecting fourteen lines with quantum numbers of J = 23/2–21/2 to J = 35/2 for each of the two 2Π3/2 and 2Π1/2 ladders. It is not possible to model the intensities of all the transitions of the ground state simultaneously using a single column density. We considered the two ladders as two different species and found that the rotational temperature is the same for both ladders, Trot(2Π3/2)=Trot(2Π1/2)=6.2 ± 0.2, achieving a result that is comparable to that of the ν11(2μΣ) state. The derived column densities are N(2Π3/2) = (6.2 ± 0.3)×1012 cm−2 and N(2Π1/2) = (8.0 ± 0.4)×1010 cm−2. The fraction of C6H molecules in its 2Π3/2, 2Π1/2, and ν11(μ2Σ) states is 96.8%, 1.3%, and 1.9%, respectively. Finally, we report that this vibrational mode has also been detected towards the cold cores Lupus-1A and L1495B, as well as the low-mass star forming cores L1527 and L483, with fractions of C6H molecules in this mode of 3.8%, 4.1%, 14.8%, and 6%, respectively.

Improved Quantum-Classical Treatment of N2-N2 Inelastic Collisions: Effect of the Potentials and Complete Rate Coefficient Data Sets.

In this study, complete (i.e., including all vibrational quantum numbers in an N2 vibrational ladder) data sets of vibration-to-vibration and vibration-to-translation rate coefficients for N2-N2 collisions are explicitly computed along with transport properties (shear and bulk viscosity, thermal conductivity, and self-diffusion) in the temperature range 100-9000 K. To reach this goal, we improved a mixed quantum-classical (MQC) dynamics approach by lifting the constraint of a Morse treatment of the vibrational wave function and intramolecular potential and permitting the use of more realistic and flexible representations. The new formulation has also allowed us to separately analyze the role of intra- and intermolecular potentials on the calculated rates and properties. Ab initio intramolecular potentials are indispensable for highly excited vibrational states, though the Morse potential still gives reasonable values up to v = 20. An accurate description of the long-range interaction and the van der Waals well is a requisite for the correct reproduction of qualitative and quantitative rate coefficients, particularly at low temperatures, making physically meaningful analytical representations still the best choice compared to currently available ab initio potential energy surfaces. These settings were used to directly compute the MQC rates corresponding to a large number of initial vibrational quantum numbers, and the missing intermediate values were predicted using a machine learning technique (i.e., the Gaussian process regression approach). The obtained values are reliable in the wide temperature range employed and are therefore valuable data for many communities dealing with nonlocal thermal equilibrium conditions in different environments.

Excitation Energy-Dependent Decay Dynamics of the S1 State of N-Methyl-2-pyridone.

The UV-induced decay dynamics of N-methyl-2-pyridone is investigated using a femtosecond time-resolved photoelectron spectroscopy method. Irradiation in the wavelength range of 339.3-258.9 nm prepares N-methyl-2-pyridone molecules with very different vibrational levels of the S1(11ππ*) state. For v' = 0 (origin) and a few low-energy vibrational levels slightly above the S1 state origin, the radiative decay channel is in operation for some specific vibrations. This is revealed by the excited-state lifetime of ≫1 ns. In addition, some other nearby S1 vibronic states have a much shorter lifetime in the range of several picoseconds to a few tens of picoseconds, indicating that the radiation-less decay to the ground state (S0) via internal conversion is the dominant channel for them. As the pump wavelength slightly decreases, the radiative decay is suddenly not important at all, and the deactivation rate of the S1 state becomes faster. At shorter pump wavelengths, the lifetime of highly excited vibrational states of the S1 state further decreases with the increase in the vibrational excess energy. This study provides quantitative information about the excitation energy-dependent decay dynamics of the S1 state of N-methyl-2-pyridone. Methyl substitution effects on the excited-state dynamics of 2-pyridone are also discussed.

Applying Bayesian inference and deterministic anisotropy to retrieve the molecular structure ∣Ψ(R)∣2 distribution from gas-phase diffraction experiments

Currently, our general approach to retrieving molecular structures from ultrafast gas-phase diffraction heavily relies on complex ab initio electronic or vibrational excited state simulations to make conclusive interpretations. Without such simulations, inverting this measurement for the structural probability distribution is typically intractable. This creates a so-called inverse problem. Here we address this inverse problem by developing a broadly applicable method that approximates the molecular frame structure ∣Ψ(R, t)∣2 distribution independent of these complex simulations. We retrieve the vibronic ground state ∣Ψ(R)∣2 for both simulated stretched NO2 and measured N2O. From measured N2O, we observe 40 mÅ coordinate-space resolution from 3.75 Å−1 reciprocal space range and poor signal-to-noise, a 50X improvement over traditional Fourier transform methods. In simulated NO2 diffraction experiments, typical to high signal-to-noise levels predict 100–1000X resolution improvements, down to 0.1 mÅ. By directly measuring the width of ∣Ψ(R)∣2, we open ultrafast gas-phase diffraction capabilities to measurements beyond current analysis approaches. This method has the potential to effectively turn gas-phase ultrafast diffraction into a discovery-oriented technique to probe systems that are prohibitively difficult to simulate.

Significant non-adiabatic effects of the K(4s2S) + H2 reaction

The non-adiabatic dynamical calculations of the K(4s2S) + H2(v 0 = 1, 2, j 0 = 0) reaction are carried out using the time-dependent wave packet method. The non-adiabatic dynamics results, such as reaction probabilities and integral cross sections, are calculated and compared with previous adiabatic values. The adiabatic values are several tens of times larger than those of the non-adiabatic results. The non-adiabatic effect becomes stronger with the increase in the number of excited vibrational states. In addition, the excitation of the vibrational states of H2 can increase the reaction probability of the reaction channel. However, the KH product is still barely formed through the K(4s2S) + H2 reaction, even if the H2 molecule is excited to a high vibrational excited state, which also leads to the opposite conclusion from the adiabatic results. The forward-biased differential cross sections indicate that a direct stripping mechanism plays a dominant role in the reaction.

Fantastical excited state optimized structures and where to find them.

The quantum chemistry community has developed analytic forces for approximate electronic excited states to enable walking on excited state potential energy surfaces (PES). One can thereby computationally characterize excited state minima and saddle points. Always implicit in using this machinery is the fact that an excited state PES only exists within the realm of the Born-Oppenheimer approximation, where the nuclear and electronic degrees of freedom separate. This work demonstrates through abinitio calculations and simple nonadiabatic dynamics that some excited state minimum structures are fantastical: they appear to exist as stable configurations only as a consequence of the PES construct, rather than being physically observable. Each fantastical structure exhibits an unphysically high predicted harmonic frequency and associated force constant. This fact can serve as a valuable diagnostic of when an optimized excited state structure is non-observable. The origin of this phenomenon can be attributed to the coupling between different electronic states. As PESs approach one another, the upper surface can form a minimum that is very close to a near-touching point. The force constant, evaluated at this minimum, relates to the strength of the electronic coupling rather than to any characteristic excited state vibration. Nonadiabatic dynamics results using a Landau-Zener model illustrate that fantastical excited state structures have extremely short lifetimes on the order of a few femtoseconds. Their appearance in a calculation signals the presence of a nearby conical intersection through which the system will rapidly cross to a lower surface.