Excited Vibrational States Research Articles

Relativistic ab initio study on the spectroscopic and radiative properties of the lowest states and modeling of the optical cycles for the LiFr molecule

The LiFr diatomic represents a promising candidate for indirect laser cooling that has not yet been investigated not theoretically or experimentally. The potential energy curves of the ground and low-lying excited states of the LiFr heteronuclear alkali metal dimer are calculated using the Fock-space relativistic coupled cluster theory for the first time. A number of properties such as the electronic term energies, equilibrium internuclear distances, transition and permanent dipole moments, sequences of vibrational energies, harmonic vibrational frequencies, Franck–Condon factors, and radiative lifetimes (including bound and free transitions) are predicted. The probabilities of the two-step schemes (optical cycles) for the transfer process of the LiFr molecules from high excited vibrational states to the ground vibronic state are also predicted. The data obtained would be useful for laser cooling and spectral experiments with LiFr molecules.

Vibrational excitation in the e + CO2 system: Analysis of the two-dimensional energy-loss spectrum

We present a detailed analysis of the two-dimensional electron energy-loss spectrum of ${\mathrm{CO}}_{2}$, which extends our recent Letter [Phys. Rev. Lett. 129, 013401 (2022)]. We show that our vibronic coupling model [Phys. Rev. A 105, 062821 (2022)] captures primary features of the multidimensional dynamics of the temporary molecular anion, and the calculations qualitatively reproduce the spectrum. The shape of the spectrum is given by two overlapping contributions that originate in excitation of vibrational states within ${\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ and ${\mathrm{\ensuremath{\Pi}}}_{u}$ Fermi polyads. Propensity rules in terms of scattered and vibrational wave functions are also discussed to clarify the selectivity of states from the vibrational pseudocontinuum that is responsible for the observed fine structure.

A Study of Processes Involving Adsorbed Ozone Stimulated by Resonant Excitation of Vibrational States

A Study of Processes Involving Adsorbed Ozone Stimulated by Resonant Excitation of Vibrational States

Rotational spectroscopy of urea up to 500 GHz: The ground and eight excited vibrational states

Urea, ((NH2)2CO) is an important industrial and biological molecule. The new generation of the submillimeter telescopes, in particular ALMA, allows detection of extraterrestrial molecular spectra at unprecedented spatial and spectral resolutions. Some of the data presented here facilitated the recent definitive identification of urea in the interstellar medium (Belloche et al. Astron. & Astrophys. 628 (2019) A10:1-62). Its detection yields valuable insight into the mechanisms governing formation of complex prebiotic interstellar molecules. This paper reports spectroscopic analysis of the broadband rotational spectra of urea in the 210–500 GHz spectral range for the vibrational ground state and eight lowest excited vibrational states. The lowest excited vibrational state is only 61 cm−1 above the ground state and could be a useful temperature marker for astrophysical urea. The next two lowest pairs of excited states show significant interstate coupling, which has been successfully fitted with a Coriolis coupling model. Higher vibrational states also show evidence of significant coupling. Intensities of some rotational transitions of urea display nuclear spin statistical weights arising from the equivalence of the two ammonia groups, which enabled the assignment of symmetric and antisymmetric vibrational symmetry to the six lowest studied excited states. The vibrational energies of the new states have been determined from extensive relative intensity measurements and, together with vibrational symmetry, give insight into the highly anharmonic vibrational energy level structure.

Electron collisions with ArH+ molecular ions: highly excited vibrational states and dissociative excitation

A theoretical investigation of dissociative recombination and dissociative excitation processes involving electron collisions with the argonium ion (ArH+) at energies up to 7 eV is presented. Curves and couplings obtained using R-matrix calculations are used to provide the input for molecular dynamics calculations based on the multichannel quantum defect theory. A full set of vibrationally resolved cross sections and rate coefficients is presented for the kinetic modeling of argon-containing non-equilibrium plasma.

Broadband microwave spectroscopy of cyclopentylsilane and 1,1,1-trifluorocyclopentylsilane

The broadband microwave spectra of two related organosilicon molecules, cyclopentylsilane and 1,1,1-trifluorocyclopentylsilane, have been measured and assigned. Both molecules exhibit a Cs-symmetric “envelope” structure, which is confirmed by a determination of the heavy atom backbone rs substitution structures using isotopic substitution in natural abundance, as well as the presence of strong a-type transitions, weaker c-type, and a non-detection of b-type transitions, indicative of a mirror plane perpendicular to the prolate symmetry axis of the molecule. Both molecules appear to have significant zero-point vibrational averaging of their ring twisting and puckering coordinates, which lead to deviations between the ground state experimental structure and the predicted equilibrium structures at the B3LYP-D3/def2-TZVP level of theory. Additionally, we observe an excited vibrational state associated with this large amplitude motion and we use a simple perturbation theory argument to confirm the identity of these excited state carriers.

Efficient vibrational excitation of molecular nitrogen in low-pressure plasma with ultralow electron temperature

We investigated the vibrational temperature of molecular nitrogen in the downstream of helicon-wave excited helium and argon-based plasmas. It was confirmed by optical emission spectroscopy that the major part of the helium plasma was at a recombining state and it had an ultralow electron temperature of approximately 0.1 eV. In spite of the ultralow electron temperature, the vibrational temperature of molecular nitrogen, which was added into the helium plasma, was higher than that in the argon-based plasma at an ionizing state with an electron temperature of 1.7 eV. According to the relationship between the rate coefficient of electron impact vibrational excitation and the electron temperature, the higher vibrational temperature in the helium plasma is not attributable to the more efficient vibrational excitation. Therefore, the higher vibrational temperature is owing to the less efficient destruction of vibrational excited states in the helium plasma with the ultralow electron temperature.

Pathway toward Optical Cycling and Laser Cooling of Functionalized Arenes.

Rapid and repeated photon cycling has enabled precision metrology and the development of quantum information systems using atoms and simple molecules. Extending optical cycling to structurally complex molecules would provide new capabilities in these areas, as well as in ultracold chemistry. Increased molecular complexity, however, makes realizing closed optical transitions more difficult. Building on already established strong optical cycling of diatomic, linear triatomic, and symmetric top molecules, recent work has pointed the way to cycling of larger molecules, including phenoxides. The paradigm for these systems is an optical cycling center bonded to a molecular ligand. Theory has suggested that cycling may be extended to even larger ligands, like naphthalene, pyrene, and coronene. Herein, we study optical excitation and fluorescent vibrational branching of CaO-[Formula: see text], SrO-[Formula: see text], and CaO-[Formula: see text] and find only weak decay to excited vibrational states, indicating a promising path to full quantum control and laser cooling of large arene-based molecules.

Molecule opacity study on low-lying states of CS

CS molecule, which plays a key role in atmospheric and astrophysical circumstances, has drawn great attention for long time. Owing to its large state density, the detailed information of the electronic structure of CS is still lacking. In this work, the high-level MRCI+Q method is used to compute the potential energy curves, dipole moments and transition dipole moments of singlet and triplet states correlated with the lowest dissociation limit of CS, based on which high accurate vibration–rotation levels and spectroscopic constants of bound states are evaluated. The opacity of CS relevant to atmospheric circumstance is computed at a pressure of 100 atms for different temperatures. With the increase of temperature, band systems from different transitions mingle with each other, and band boundaries become blurred, which are originated from the increased population on vibrational excited states and electronic excited states at high temperature.

Fast Near Ab Initio Potential Energy Surfaces Using Machine Learning.

A machine-learning based approach for evaluating potential energies for quantum mechanical studies of properties of the ground and excited vibrational states of small molecules is developed. This approach uses the molecular-orbital-based machine learning (MOB-ML) method to generate electronic energies with the accuracy of CCSD(T) calculations at the same cost as a Hartree-Fock calculation. To further reduce the computational cost of the potential energy evaluations without sacrificing the CCSD(T) level accuracy, GPU-accelerated Neural Network Potential Energy Surfaces (NN-PES) are trained to geometries and energies that are collected from small-scale Diffusion Monte Carlo (DMC) simulations, which are run using energies evaluated using the MOB-ML model. The combined NN+(MOB-ML) approach is used in variational calculations of the ground and low-lying vibrational excited states of water and in DMC calculations of the ground states of water, CH5+, and its deuterated analogues. For both of these molecules, comparisons are made to the results obtained using potentials that were fit to much larger sets of electronic energies than were required to train the MOB-ML models. The NN+(MOB-ML) approach is also used to obtain a potential surface for C2H5+, which is a carbocation with a nonclassical equilibrium structure for which there is currently no available potential surface. This potential is used to explore the CH stretching vibrations, focusing on those of the bridging hydrogen atom. For both CH5+ and C2H5+ the MOB-ML model is trained using geometries that were sampled from an AIMD trajectory, which was run at 350 K. By comparison, the structures sampled in the ground state calculations can have energies that are as much as ten times larger than those used to train the MOB-ML model. For water a higher temperature AIMD trajectory is needed to obtain accurate results due to the smaller thermal energy. A second MOB-ML model for C2H5+ was developed with additional higher energy structures in the training set. The two models are found to provide nearly identical descriptions of the ground state of C2H5+.

ExoMol photodissociation cross-sections – I. HCl and HF

ABSTRACT Photon initiated chemistry, i.e. the interaction of light with chemical species, is a key factor in the evolution of the atmosphere of exoplanets. For planets orbiting stars in UV-rich environments, photodissociation induced by high-energy photons dominates the atmosphere composition and dynamics. The rate of photodissociation can be highly dependent on atmospheric temperature, as increased temperature leads to increased population of vibrational excited states and the consequent lowering of the photodissociation threshold. This paper inaugurates a new series of papers presenting computed temperature-dependent photodissociation cross-sections with rates generated for different stellar fields. Cross-sections calculations are performed by solving the time-independent Schrödinger equation for each electronic state involved in the process. Here, photodissociation cross-sections for hydrogen chloride and hydrogen fluoride are computed for a grid of 34 temperatures between 0 and 10 000 K. Use of different radiation fields shows that for the Sun and cooler stars the photodissociation rate can increase exponentially for molecular temperatures above 1000 K; conversely the photodissociation rates in UV rich fields instead are almost insensitive to the temperature of the molecule. Furthermore, these rates show extreme sensitivity to the radiation model used for cool stars, suggesting that further work on these may be required. The provision of an ExoMol data base of cross-sections is discussed.

Non-Markovian vibrational relaxation dynamics at surfaces.

Vibrational dynamics of adsorbates near surfaces plays both an important role for applied surface science and as a model lab for studying fundamental problems of open quantum systems. We employ a previously developed model for the relaxation of a D-Si-Si bending mode at a D:Si(100)-(2 × 1) surface, induced by a "bath" of more than 2000 phonon modes [Lorenz and P. Saalfrank, Chem. Phys. 482, 69 (2017)], to extend previous work along various directions. First, we use a Hierarchical Effective Mode (HEM) model [Fischer et al., J. Chem. Phys. 153, 064704 (2020)] to study relaxation of higher excited vibrational states than hitherto done by solving a high-dimensional system-bath time-dependent Schrödinger equation (TDSE). In the HEM approach, (many) real bath modes are replaced by (much less) effective bath modes. Accordingly, we are able to examine scaling laws for vibrational relaxation lifetimes for a realistic surface science problem. Second, we compare the performance of the multilayer multiconfigurational time-dependent Hartree (ML-MCTDH) approach with that of the recently developed coherent-state-based multi-Davydov-D2 Ansatz [Zhou et al., J. Chem. Phys. 143, 014113 (2015)]. Both approaches work well, with some computational advantages for the latter in the presented context. Third, we apply open-system density matrix theory in comparison with basically "exact" solutions of the multi-mode TDSEs. Specifically, we use an open-system Liouville-von Neumann (LvN) equation treating vibration-phonon coupling as Markovian dissipation in Lindblad form to quantify effects beyond the Born-Markov approximation.

Gas sensing for industrial relevant nitrogen-containing compounds using a microelectronics-based absorption spectrometer in the 220–330 GHz frequency range

Gas sensing for four nitrogen-containing compounds (nitrous oxide, acetonitrile, nitric acid, and nitromethane) is explored and demonstrated using rotational absorption spectroscopy carried out with a compact terahertz-wave microelectronics-based spectrometer operating in the 220–330 GHz frequency range. The four gases investigated are important in industrial processes, as well as in chemical, combustion, environmental, and agricultural contexts. Absorption measurements were made at room temperature (297 K) and moderate pressures (33.3–2133 Pa or 0.25–16 Torr) for the characterization of spectra that are comprised of distinct fingerprint features that include isolated single transitions and bands of pressure-broadened transitions, emanating from both ground vibrational states, as well as low-lying vibrationally-excited states. The measurements demonstrate that terahertz-wave quantitative gas sensing using all-electronic miniaturized systems is possible for nitrogen-containing compounds with detection limits of the order 1012-1013 molecules cm−3 per meter pathlength and for dilute gases in air at 1 atm at concentrations of 5–1000 ppm per meter pathlength.

Highly accurate HF dimer ab initio potential energy surface.

A highly accurate, (HF)2 potential energy surface (PES) is constructed based on ab initio calculations performed at the coupled-cluster single double triple level of theory with an aug-cc-pVQZ-F12 basis set at about 152 000 points. A higher correlation correction is computed at coupled-cluster single double triple quadruple level for 2000 points and is considered alongside other more minor corrections due to relativity, core-valence correlation, and Born-Oppenheimer failure. The analytical surface constructed uses 500 constants to reproduce the ab initio points with a standard deviation of 0.3 cm-1. Vibration-rotation-inversion energy levels of the HF dimer are computed for this PES by variational solution of the nuclear-motion Schrödinger equation using the program WAVR4. Calculations over an extended range of rotationally excited states show very good agreement with the experimental data. In particular, the known empirical rotational constants B for the ground vibrational states are predicted to better than about 2MHz. B constants for excited vibrational states are reproduced several times more accurately than by previous calculations. This level of accuracy is shown to extend to higher excited inter-molecular vibrational states v and higher excited rotational quantum numbers (J, Ka).

Ultrafast control of vibrational states of polar molecules with subcycle unipolar pulses

We investigate theoretically the nonresonant excitation of vibrational levels in polar molecules by unipolar radiation pulses of duration much shorter than the characteristic period of the molecule's vibration. We consider several profiles of the potential of the interaction of atoms in a diatomic molecule and derive analytically the probabilities of the molecule's transition to excited vibrational states when driven by subcycle unipolar pulses. It is shown that the excitation efficiency is governed by the electric pulse area so that unipolar half-cycle pulses turn out to be the most efficient ones. We introduce the characteristic scale of the electric pulse area, which serves as a measure of the pulse action on the vibrational states of the molecule. The results are generalized to the interaction of excited vibrational and rotational states and it is shown that the behavior of the vibrational levels' populations versus the electric pulse area as well as the introduced characteristic scale stays valid.

Variational rovibrational calculations for tetra atomic linear molecules using Watson’s isomorphic Hamiltonian: II. The B11244 story retold

A new six-dimensional near-equilibrium potential energy function for propynylidynium (l-C3H+) is developed based on a high-level composite ab initio method. The importance of higher-order correlation contributions to the composite potential is clearly demonstrated. Variational rovibrational calculations were performed using the new C8v4 program based on Watson’s isomorphic Hamiltonian and a basis of symmetrized harmonic oscillator/rigid rotor product wave functions. The results of these calculations employing the new composite potential as well as a previously constructed quartic force field rectify long-standing discrepancies between theory and experiment. Effective Hamiltonian fits yield a ground state rotational parameter within 5 MHz of the experimental result and a quartic centrifugal distortion constant in virtual agreement with experiment. Spectroscopic parameters for low-lying excited vibrational states are presented which should provide a starting point in forthcoming experimental studies on l-C3H+. A close relationship can be established by comparing the properties of l-C3H+ with floppy C3 from which follows that l-C3H+ behaves in many respects like a “protonated”, albeit more rigid, C3.

Infrared frequency comb spectroscopy of CH2I2: Influence of hot bands and pressure broadening on the ν1 and ν6 fundamental transitions.

Direct frequency comb spectroscopy was utilized to measure the vibrational absorption spectrum of diiodomethane, CH2I2, from 2960 to 3125cm-1. The data were obtained using a CH2I2 concentration of (6.8 ± 1.3) × 1015moleculecm-3 and a total pressure of 10-300mbar with either nitrogen or argon as the bath gas. The rovibrational spectra of two fundamental transitions, ν6 and ν1, were recorded and analyzed. We suggest that a significant contribution to the observed congested spectra is due to the population in excited vibrational states of the low energy ν4 I-C-I bend, resulting in transitions 61 04n n and 11 04n n, where the integer n is the initial vibrational level v = 1-5. PGOPHER was used to fit the experimental spectrum, allowing for rotational constants and other spectral information to be reported. In addition, it was found that the peak widths for the observed transitions were limited by pressure broadening, resulting in a pressure broadening parameter of (0.143 ± 0.006) cm-1 atm-1 by N2 and (0.116 ± 0.006) cm-1 atm-1 by Ar. Further implications for other dihaloalkane infrared spectra are discussed.

Reaction Pathway Control via Reactant Vibrational Excitation and Impact on Product Vibrational Distributions: The O + HO2 → OH + O2 Atmospheric Reaction.

Chemical reactions often have multiple pathways, the control of which is of fundamental and practical importance. In this Letter, we examine the dynamics of the O + HO2 → OH + O2 reaction, which plays an important role in atmospheric chemistry, using quasi-classical trajectories on a recently developed full-dimensional potential energy surface (PES). This reaction has two pathways leading to the same products: the H abstraction pathway (Oa + HObOc → OaH + ObOc) and the O abstraction pathway (Oa + HObOc → ObH + OaOc). Under thermal conditions, the reaction is dominated by the latter channel, which is barrierless, leading to vibrational excitation of the O2 product. However, we demonstrate that excitation of the HO2 reactant in its O-H (v1) vibrational mode results in dramatic switching of the reaction pathway to the activated H abstraction channel, which leads to a highly excited OH product vibrational state distribution. The implications of such dynamical effects in the atmospheric chemistry are discussed.

Non-equilibrium simulation of energy relaxation for earth reentry utilizing a collisional-radiative model

A high-temperature air collisional-radiative model considering both vibrational and electronic excited states is established to investigate the plasma characteristics along the stagnation-line in the post-shock flow by coupling with the one-dimensional flow model. The results obtained by the model are in reasonable agreement with the experimental data and previous calculation results. The evolutions of different temperatures and energy relaxation processes are analyzed in detail for the height of 76.42 km and Mach number of 40.58. The vibrational and electronic excited modes play a critical role on the energy transfer. The vibrational excitation processes under heavy-particle impact can transfer the translational energy to the vibrational mode, then to the electrons by the vibrational de-excitation processes. The excitation of atoms by heavy-particle impact can gain energy from the translational mode, and the de-excitation processes of high-lying excited states under electron impact result in the energy transfer to electrons. The elastic collisions also play a role on the direct transfer from translational energy to electrons. The calculations are performed for a wide range of flight altitudes and Mach numbers to investigate the thermochemical state and energy relaxation. By comparing the energy transfer processes under different altitudes and Mach numbers, it is found that the energy transfer is dominated by vibrational processes for the low-Height low-Mach condition, and the contribution of electronic excited mode is negligible. The division of thermochemical state and chemical reactions in the Height-Mach number diagram is obtained, which provides the basis for the selection of thermochemical model required for atmospheric reentry calculation.

Initial state-selected scattering for the reactions H + CH4/CHD3 and F + CHD3 employing ring polymer molecular dynamics.

The inclusion of nuclear quantum effects (NQEs) in molecular dynamics simulations is one of the major obstacles for an accurate modeling of molecular scattering processes involving more than a couple of atoms. An efficient method to incorporate these effects is ring polymer molecular dynamics (RPMD). Here, we extend the scope of our recently developed method based on non-equilibrium RPMD (NE-RPMD) from triatomic chemical reactions to reactions involving more atoms. We test the robustness and accuracy of the method by computing the integral cross sections for the H/F + CH4/CHD3 reactions where the methane molecule is either initially in its vibrational ground or excited state (C-H stretch). Furthermore, we analyze the extent to which NQEs are described by NE-RPMD. The method shows significant improvement over the quasiclassical trajectory approach while remaining computationally efficient.