arXiv subjects

Tamás Szidarovszky

Publications and source records attributed to Tamás Szidarovszky.

12 recordsLinked to original sources

arXiv2026

Rovibrational energy levels of H$_2$O by quantum computing

We calculate rovibrational energy levels of H$_2$O using a trapped-ion quantum computer. We first derive the qubit form of Watson's Hamiltonian, including the rovibrational coupling terms. In a second step, we employ a variant of the quantum-selected configuration-interaction method to calculate rovibrational energy levels. A truncated form of the qubit Hamiltonian is used to generate correlated rovibrational wave functions on the quantum computer by time evolution, and a basis set is selected by sampling from the measured probability distribution. The rovibrational energy levels are obtained by constructing a Hamiltonian matrix using the selected basis set, and diagonalizing the matrix using a classical computer. We show that an accuracy of a few $hc$ cm$^{-1}$ can be achieved for low-lying rovibrational energy levels.

quant-ph↗

arXiv2025

Ab initio study on the dynamics and spectroscopy of collective rovibrational polaritons

Accurate rovibrational molecular models are employed to gain insight in high-resolution into the collective effects and intermolecular processes arising when molecules in the gas phase are interacting with a resonant infrared (IR) radiation mode. An efficient theoretical approach is detailed and numerical results are presented for the HCl, H$_2$O and CH$_4$ molecules confined in an IR cavity. It is shown that by employing a rotationally resolved model for the molecules, revealing the various cavity-mediated interactions between the field-free molecular eigenstates, it is possible to obtain a detailed understanding of the physical processes governing the energy level structure, absorbtion spectra, and dynamic behavior of the confined systems. Collective effects, arising due to the cavity-mediated interaction between molecules, are identified in energy level shifts, in intensity borrowing effects in the absorbtion spectra, and in the intermolecular energy transfer occurring during Hermitian or non-Hermitian time propagation.

physics.chem-ph↗

arXiv2023

Pauli principle in polaritonic chemistry

The consequences of enforcing permutational symmetry, as required by the Pauli principle (spin-statistical theorem), on the state space of molecular ensembles interacting with the quantized radiation mode of a cavity are discussed. The Pauli-allowed collective states are obtained by means of group theory, i.e., by projecting the state space onto the appropriate irreducible representations of the permutation group of the indistinguishable molecules. It is shown that with increasing number of molecules the ratio of Pauli-allowed collective states decreases very rapidly. Bosonic states are more abundant than fermionic states, and the brightness of Pauli-allowed state space (the contribution from photon excited states) increases(decreases) with increasing fine structure in the energy levels of the material ground(excited) state manifold. Numerical results are shown for the realistic example of rovibrating H$_2$O molecules interacting with an infrared cavity mode.

quant-ph↗

arXiv2023

An efficient and flexible approach for computing rovibrational polaritons from first principles

A theoretical framework is presented for the computation of rovibrational polaritonic states of a molecule in a lossless infrared (IR) microcavity. In the proposed approach the quantum treatment of the rotational and vibrational motion of the molecule can be formulated using arbitrary approximations. The cavity-induced changes in electronic structure are treated perturbatively, which allows using the existing polished tools of standard quantum chemistry for determining electronic molecular properties. As a case study, the rovibrational polaritons and related thermodynamic properties of H$_2$O in an IR microcavity are computed for varying cavity parameters and applying various approximations to describe the molecular degrees of freedom. The self-dipole interaction is found to be significant for nearly all light-matter coupling strengths investigated, and the molecular polarizability proved to be important for the correct qualitative behavior of the energy level shifts induced by the cavity. On the other hand, the magnitude of polarization remains small, justifying the perturbative approach for the cavity-induced changes in electronic structure. Comparing results obtained using a high-accuracy variational molecular model with those obtained utilizing the rigid rotor and harmonic oscillator approximations revealed that as long as the rovibrational model is appropriate for describing the field-free molecule, the computed rovibropolaritonic properties can be expected to be accurate as well. Strong light-matter coupling between the radiation mode of an IR cavity and the rovibrational states of H$_2$O lead to minor changes in the thermodynamic properties of the system, and these changes seem to be dominated by non-resonant interactions between the quantum light and matter.

physics.chem-ph↗

arXiv2021

A quantum-chemical perspective on the laser-induced alignment and orientation dynamics of the CH$_3$X (X = F, Cl, Br, I) molecules

The laser-induced alignment-and-orientation (A&O) dynamics of the prolate symmetric top CH$_3$X (X = F, Cl, Br, I) molecules is investigated, with particular emphasis on the effect of halogen substitution on the rotational constants, dipole moments, and polarizabilities of these species, as these quantities determine the A&O dynamics. Insight into possible control schemes for preferred A&O dynamics of halogenated molecules and best practices for A&O simulations are provided, as well. It is shown that for accurate A&O-dynamics simulations it is necessary to employ large basis sets and high levels of electron correlation when computing the rotational constants, dipole moments, and polarizabilities. The benchmark-quality values of these molecular parameters, corresponding to the equilibrium, as well as the vibrationally averaged structures are obtained with the help of the focal-point analysis (FPA) technique and explicit electronic-structure computations utilizing the gold-standard CCSD(T) approach, basis sets up to quintuple-zeta quality, core-correlation contributions and, in particular, relativistic effects for CH$_3$Br and CH$_3$I. It is shown that the different A&O behavior of the CH$_3$X molecules in the optical regime is mostly caused by the differences in their polarizability anisotropy, in other terms, the size of the halogen atom. In contrast, the A&O dynamics of the CH$_3$X series induced by an intense few-cycle THz pulse is mostly governed by changes in the rotational constants, due to the similar dipole moments of the CH$_3$X molecules. The A&O dynamics is most sensitive to the $B$ rotational constant: even the difference between its equilibrium and vibrationally-averaged values results in noticeably different A&O dynamics. The contribution of rotational states having different symmetry, weighted by nuclear-spin statistics, to the A&O dynamics is also studied.

physics.chem-ph↗

arXiv2021

Nonadiabatic phenomena in molecular vibrational polaritons

Nonadiabatic phenomena are investigated in the rovibrational motion of molecules confined in an infrared cavity. Conical intersections (CIs) between vibrational polaritons, similar to CIs between electronic polaritonic surfaces, are found. The spectral, topological, and dynamic properties of the vibrational polaritons show clear fingerprints of nonadiabatic couplings between molecular vibration, rotation and the cavity photonic mode. Furthermore, it is found that for the investigated system, composed of two rovibrating HCl molecules and the cavity mode, breaking the molecular permutational symmetry, by changing $^{35}$Cl to $^{37}$Cl in one of the HCl molecules, the polaritonic surfaces, nonadiabatic couplings, and related spectral, topological, and dynamic properties can deviate substantially. This implies that the natural occurrence of different molecular isotopologues needs to be considered when modeling realistic polaritonic systems.

quant-ph↗

arXiv2020

Three-player polaritons: nonadiabatic fingerprints in an entangled atom-molecule-photon system

A quantum system composed of a molecule and an atomic ensemble, confined in a microscopic cavity, is investigated theoretically. The indirect coupling between atoms and the molecule, realized by their interaction with the cavity radiation mode, leads to a coherent mixing of atomic and molecular states, and at strong enough cavity field strengths hybrid atom-molecule-photon polaritons are formed. It is shown for the Na$_2$ molecule that by changing the cavity wavelength and the atomic transition frequency, the potential energy landscape of the polaritonic states and the corresponding spectrum could be changed significantly. Moreover, an unforeseen intensity borrowing effect, which can be seen as a strong nonadiabatic fingerprint, is identified in the atomic transition peak, originating from the contamination of the atomic excited state with excited molecular rovibronic states.

physics.chem-ph↗

arXiv2019

Rovibronic spectra of molecules dressed by light fields

The theory of rovibronic spectroscopy of light-dressed molecules is presented within the framework of quantum mechanically treated molecules interacting with classical light fields. Numerical applications are demonstrated for the homonuclear diatomic molecule Na$_2$, for which the general formulae can be simplified considerably and the physical processes leading to the light-dressed spectra can be understood straightforwardly. The physical origin of different peaks in the light-dressed spectrum of Na$_2$ is given and the light-dressed spectrum is investigated in terms of its dependence on the dressing field's intensity and wavelength, the turn-on time of the dressing field, and the temperature. The important implications of light-dressed spectroscopy on deriving field-free spectroscopic quantities are also discussed.

physics.chem-ph↗

arXiv2019

Robust field-dressed spectra of diatomics in an optical lattice

The absorption spectra of the cold Na2 molecule dressed by a linearly polarized standing laser wave is investigated. In the studied scenario the rotational motion of the molecules is frozen while the vibrational and translational degrees of freedom are accounted for as dynamical variables. In such a situation a light-induced conical intersection (LICI) can be formed. To measure the spectra a weak field is used whose propagation direction is perpendicular to the direction of the dressing field but has identical polarization direction. Although LICIs are present in our model, the simulations demonstrate a very robust absorption spectrum, which is insensitive to the intensity and the wavelength of the dressing field and which does not reflect clear signatures of light-induced nonadiabatic phenomena related to the strong mixing between the electronic, vibration and translational motions. However, by widening artificially the very narrow translational energy level gaps, the fingerprint of the LICI appears to some extent in the spectrum.

physics.chem-ph↗

arXiv2018

Conical Intersections Induced by Quantum Light: Field-Dressed Spectra from the Weak to the Ultrastrong Coupling Regimes

A fundamental theoretical framework is formulated for the investigation of rovibronic spectra resulting from the coupling of molecules to one mode of the radiation field in an optical cavity. The approach involves the computation of (1) cavity-field-dressed rovibronic states, which are hybrid light-matter eigenstates of the `molecule + cavity radiation field' system, and (2) the transition amplitudes between these field-dressed states with respect to a weak probe pulse. The predictions of the theory are shown for the homonuclear Na$_2$ molecule. The field-dressed rovibronic spectrum demonstrates undoubtedly that the Born--Oppenheimer approximation breaks down in the presence of the cavity radiation field. A clear fingerprint of the strong nonadiabaticity is found, which can only emerge in the close vicinity of conical intersections. In this work, the conical intersection is induced by the quantized radiation field, and it is thus called a "light-induced conical intersection" (LICI). Dependence of the cavity-field-dressed spectrum on the cavity-mode wavelength as well as on the light-matter coupling strength is investigated. Essential changes are identified in the spectra from the weak to the ultrastrong coupling regimes.

physics.atm-clus↗

arXiv2016

Definitive ideal-gas thermochemical functions of the H$_2$$^{16}$O molecule

$Q_{\rm int}$($T$), of the H$_2$$^{16}$O molecule is reported for temperatures between 0 and 6000 K. Determination of $Q_{\rm int}$($T$) is principally based on the direct summation technique involving all accurate experimental energy levels known for H$_2$$^{16}$O (almost 20~000 rovibrational energies including an almost complete list up to a relative energy of 7500 \cm), augmented with a less accurate but complete list of first-principles computed rovibrational energy levels up to the first dissociation limit, about 41~000 \cm\ (the latter list includes close to one million bound rovibrational energy levels up to $J = 69$, where $J$ is the rotational quantum number). Partition functions are developed for {\it ortho}- and {\it para}-H$_2$$^{16}$O as well as for their equilibrium mixture. Unbound rovibrational states above the first dissociation limit are considered using an approximate model treatment. The effect of the excited electronic states on the thermochemical functions is neglected, as their contribution to the thermochemical functions is negligible even at the highest temperatures considered. Based on the high-accuracy $Q_{\rm int}$($T$) and its first two moments, definitive results, in 1~K increments, are obtained for the following thermochemical functions: Gibbs energy, standardized enthalpy, entropy, and isobaric heat capacity. Reliable approximately two standard deviation uncertainties, as a function of temperature, are estimated for each quantity determined. These uncertainties emphasize that the present results are the most accurate ideal-gas thermochemical functions ever produced for H$_2$$^{16}$O. It is recommended that the new value determined for the standard molar enthalpy increment at 298.15~K, $9.90404 \pm 0.00001$ kJ~mol$^{-1}$, should replace the old CODATA datum, $9.905 \pm 0.005$ kJ~mol$^{-1}$.

physics.chem-ph↗

arXiv2016

Fragmentation of long-lived hydrocarbons after strong field ionization

We experimentally and theoretically investigated the deprotonation process on nanosecond to microsecond timescale in ethylene and acetylene molecules, following their double ionization by a strong femtosecond laser field. In our experiments we utilized coincidence detection with the reaction microscope technique, and found that both the lifetime of the long-lived ethylene dication leading to the delayed deprotonation and the relative channel strength of the delayed deprotonation compared to the prompt one have no evident dependence on the laser pulse duration and the laser peak intensity. Quantum chemical simulations suggest that such delayed fragmentation originates from the tunneling of near-dissociation-threshold vibrational states through a dissociation barrier on a dication electronic state along C--H stretching. Such vibrational states can be populated through strong field double ionization induced vibrational excitation on an electronically excited state in the case of ethylene, and through intersystem crossing from electronically excited states to the electronic ground state in the case of acetylene.

physics.chem-ph↗