arXiv subjects

Nikolai F. Zobov

Publications and source records attributed to Nikolai F. Zobov.

17 recordsLinked to original sources

arXiv2024

The 2024 release of the ExoMol database: molecular line lists for exoplanet and other hot atmospheres

The ExoMol database (www.exomol.com) provides molecular data for spectroscopic studies of hot atmospheres. These data are widely used to model atmospheres of exoplanets, cool stars and other astronomical objects, as well as a variety of terrestrial applications. The 2024 data release reports the current status of the database which contains recommended line lists for 91 molecules and 224 isotopologues giving a total of almost 10$^{12}$ individual transitions. New features of the database include extensive "MARVELization" of line lists to allow them to be used for high resolutions studies, extension of several line lists to ultraviolet wavelengths, provision of photodissociation cross sections and extended provision of broadening parameters. Some of the in-house data specifications have been rewritten in JSON and moved to conformity with other international standards. Data products, including specific heats, a database of lifetimes for plasma studies, and the ExoMolHR web app which allows exclusively high resolution data to be extracted, are discussed.

astro-ph.GA↗

arXiv2022

Sub-percent accuracy for the intensity of a near infrared water line at 10670 cm$^{-1}$: experiment and analysis

Laser measurements of the intensity of (201) 3$_{22}$ -- (000) 2$_{21}$ near-infrared water absorption line at 10670.1 \cm\ are made using three different Herriott cells. These measurements determine the line intensity with an standard deviation below of 0.3~\% by consideration of the new geometrically derived formula for the opt ical path length without approximations. This determination together with the current accepted value leads to an overall uncertainty of 0.7~\% of the experimentally assessed line intensity which is compared with previous {\it ab initio} predictions. It is found that steady improvements in the both the dipole moment surface (DMS) and potential energy surface (PES) used in the theoretical studies leads to systematic better agreement with the observation, with the most recent prediction agreeing closely with the experiment.

physics.chem-ph↗

arXiv2021

Highly accurate HF dimer ab initio potential energy surface

A very accurate, (HF)$_2$ potential energy surface (PES) is constructed based on \ai\ calculations performed using the MOLPRO package at the CCSD(T) level of theory with an aug-cc-pvQz-F12 basis set at about 161~000 points. a higher correlation correction is computed at CCSDT(Q) 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 \ai\ points with a standard deviation of 0.3 \cm. Vibration-rotation-inversion energy levels of the HF dimer are computed for this PES by variational solution of the nuclear-motion Schrödinger using 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 2 MHz. $B$ constants for excited vibrational states are reproduced several times more accurately than by previous calculations. The experimental dissociation energy of the HF dimer is reproduced \ai\ within the experimental accuracy of about 1 \cm\ for the first time. This level of accuracy is shown to extend to higher excited inter-molecular vibrational states $v$ and higher excited rotational quantum numbers $(J,K_a)$.

physics.atm-clus↗

arXiv2020

Calculated line lists for H$_{\text{2}} \mkern0.1mu ^{\text{16}}$O and H$_{\text{2}} \mkern0.1mu ^{\text{18}}$O with extensive comparisons to theoretical and experimental sources including the HITRAN2016 database

New line lists are presented for the two most abundant water isotopologues; H$_{2}$$^{16}$O and H$_{2}$$^{18}$O. The H$_{2}$$^{16}$O line list extends to 25710 cm$^{-1}$ with intensity stabilities provided via ratios of calculated intensities obtained from two different semi-empirical potential energy surfaces. The line list for H$_{2}$$^{18}$O extends to 20000 cm$^{-1}$. The minimum intensity considered for all is $10^{-30}$ cm molecule$^{-1}$ at 296~K, assuming 100\% abundance for each isotopologue. Fluctuation of calculated intensities caused by changes in the underlying potential energy are found to be significant, particularly for weak transitions. Direct comparisons are made against eighteen different sources of line intensities, both experimental and theoretical, many of which are used within the HITRAN2016 database. With some exceptions, there is excellent agreement between our line lists and the experimental intensities in HITRAN2016. In the infrared region, many H$_{2}$$^{16}$O bands which exhibit intensity differences of 5-10\% between to the most recent 'POKAZATEL' line list (Polyansky \textit{et al.}, [Mon. Not. Roy. Astron. Soc. \textbf{480}, 2597 (2018)] and observation, are now generally predicted to within 1\%. For H$_{2}$$^{18}$O, there are systematic differences in the strongest intensities calculated in this work versus those obtained from semi-empirical calculations. In the visible, computed cross sections show smaller residuals between our work and both HITRAN2016 and HITEMP2010 than POKAZATEL. While our line list accurately reproduces HITEMP2010 cross sections in the observed region, residuals produced from this comparison do however highlight the need to update line positions in the visible spectrum of HITEMP2010. These line lists will be used to update many transition intensities and line positions in the HITRAN2016 database.

astro-ph.EP↗

arXiv2018

A new spectroscopically-determined potential energy surface and \emph{ab initio} dipole moment surface for high accuracy HCN intensity calculations

Calculations of transition intensities for small molecules like H$_2$O, CO, CO$_2$ based on s high-quality potential energy surface (PES) and dipole moment surface (DMS) can nowadays reach sub-percent accuracy. An extension of this treatment to a system with more complicated internal structure -- HCN/HNC (hydrogen cyanide/hydrogen isocyanide) is presented. A highly accurate spectroscopically-determined PES is built based on a recent \aipes\ of the HCN/HNC isomerizing system. 588 levels of HCN with $J$~=~(0,~2,~5,~9,~10) are reproduced with a standard deviation from the experimental values of $σ=0.0373$ \cm\ and 101 HNC levels with $J$~=~(0,~2) are reproduced with $σ=0.37$ \cm. The dependence of the HCN rovibrational transition intensities on the PES is tested for the wavenumbers below 7200 \cm. Intensities are computed using wavefunctions generated from an \ai\ and our optimized PES. These intensities differ from each other by more than 1\%\ for about 11\% of the transitions tested, showing the need to use an optimized PES to obtain wavefunctions for high-accuracy predictions of transition intensities. An \ai\ DMS is computed for HCN geometries lying below 11~200 \cm. Intensities for HCN transitions are calculated using a new fitted PES and newly calculated DMS. The resulting intensities compare much better with experiment than previous calculations. In particular, intensities of the H--C stretching and bending fundamental transitions are predicted with the subpercent accuracy.

physics.chem-ph↗

arXiv2018

ExoMol molecular line lists XXX: a complete high-accuracy line list for water

A new line list for H$_2$$^{16}$O is presented. This line list, which is called POKAZATEL, includes transitions between rotation-vibrational energy levels up to 41000 cm$^{-1}$ in energy and is the most complete to date. The potential energy surface (PES) used for producing the line list was obtained by fitting a high-quality ab initio PES to experimental energy levels with energies of 41000 cm$^{-1}$ and for rotational excitations up to $J=5$. The final line list comprises all energy levels up to 41000 cm$^{-1}$ and rotational angular momentum $J$ up to 72. An accurate ab initio dipole moment surface (DMS) was used for the calculation of line intensities and reproduces high-precision experimental intensity data with an accuracy close to 1 %. The final line list uses empirical energy levels whenever they are available, to ensure that line positions are reproduced as accurately as possible. The POKAZATEL line list contains over 5 billion transitions and is available from the ExoMol website (www.exomol.com) and the CDS database.

astro-ph.EP↗

arXiv2018

High accuracy water potential energy surface for the calculation of infrared spectra

Transition intensities for small molecules such as water and CO$_2$ can now be computed with such high accuracy that they are being used to systematically replace measurements in standard databases. These calculations use high accuracy ab initio dipole moment surfaces and wavefunctions from spectroscopically-determined potential energy surfaces. Here an extra high accuracy potential energy surface (PES) of the water molecule (\hato) is produced starting from an ab initio PES which is then refined to empirical rovibrational energy levels. Variational nuclear motion calculations using this PES reproduce the fitted energy levels with a standard deviation of 0.011 \cm, approximately three times their stated uncertainty. Use of wavefunctions computed with this refined PES is found to improve the predicted transition intensities for selected (problematic) transitions. A new room temperature line list for H2(16)O is presented. It is suggested that the associated set of line intensities is the most accurate available to date for this species.

physics.chem-ph↗

arXiv2018

Potential energy surface, dipole moment surface and the intensity calculations for the 10 micron, 5 micron, and 3 micron bands of ozone

Monitoring ozone concentrations in the Earth's atmosphere using spectroscopic methods is a major activity which undertaken both from the ground and from space. However there are long-running issues of consistency between measurements made at infrared (IR) and ultraviolet (UV) wavelengths. In addition, key O$_3$ IR bands at 10 \muu, 5 \muu\ and 3 \muu\ also yield results which differ by a few percent when used for retrievals. These problems stem from the underlying laboratory measurements of the line intensities. Here we use quantum chemical techniques, first principles electronic structure and variational nuclear-motion calculations, to address this problem. A new high-accuracy \ai\ dipole moment surface (DMS) is computed. Several spectroscopically-determined potential energy surfaces (PESs) are constructed by fitting to empirical energy levels in the region below 7000 \cm\ starting from an \ai\ PES. Nuclear motion calculations using these new surfaces allow the unambiguous determination of the intensities of 10 \muu\ band transitions, and the computation of the intensities of 10 \muu\ and 5 \muu\ bands within their experimental error. A decrease in intensities within the 3 \muu\ is predicted which appears consistent with atmospheric retrievals. The PES and DMS form a suitable starting point both for the computation of comprehensive ozone line lists and for future calculations of electronic transition intensities

physics.ao-ph↗

arXiv2017

An experimental water line list at 1950 K in the 6250 -- 6670 \cm\ region

An absorption spectrum of H$_2$$^{16}$O at 1950 K is recorded in a premixed methane/air flat flame using a cavity-enhanced optical frequency comb-based Fourier transform spectrometer. 2417 absorption lines are identified in the 6250 -- 6670 cm region with an accuracy of about 0.01 cm. Absolute line intensities are retrieved using temperature and concentration values obtained by tunable diode laser absorption spectroscopy. Line assignments are made using a combination of empirically known energy levels and predictions from the new POKAZATEL variational line list. 2030 of the observed lines are assigned to 2937 transitions, once blends are taken into account. 126 new energy levels of H$_2$$^{16}$O are identified. The assigned transitions belong to 136 bands and span rotational states up to $J=27$.

physics.chem-ph↗

arXiv2017

High accuracy calculations of the rotation-vibration spectrum of H$_3^+$

Calculation of the rotation-vibration spectrum of H3+, as well as of its deuterated isotopologues, with near-spectroscopic accuracy requires the development of sophisticated theoretical models, methods, and codes. The present paper reviews the state-of-the-art in these fields. Computation of rovibrational states on a given potential energy surface (PES) has now become standard for triatomic molecules, at least up to intermediate energies, due to developments achieved by the present authors and others. However, highly accurate Born--Oppenheimer energies leading to highly accurate PESs are not accessible even for this two-electron system using conventional electronic structure procedures e.g., configuration-interaction or coupled-cluster techniques with extrapolation to the complete basis set limit). For this purpose highly specialized techniques must be used, e.g., those employing explicitly correlated Gaussians and nonlinear parameter optimizations. It has also become evident that a very dense grid of \ai\ points is required to obtain reliable representations of the computed points extending from the minimum to the asymptotic limits. Furthermore, adiabatic, relativistic, and QED correction terms need to be considered to achieve near-spectroscopic accuracy during calculation of the rotation-vibration spectrum of H3+. The remaining and most intractable problem is then the treatment of the effects of non-adiabatic coupling on the rovibrational energies, which, in the worst cases, may lead to corrections on the order of several \cm. A promising way of handling this difficulty is the further development of effective, motion- or even coordinate-dependent, masses and mass surfaces. Finally, the unresolved challenge of how to describe and elucidate the experimental pre-dissociation spectra of H$_3^+$ and its isotopologues is discussed.

physics.chem-ph↗

arXiv2017

ExoMol molecular line lists XX: a comprehensive line list for H$_3^+$

H$_3^+$ is a ubiquitous and important astronomical species whose spectrum has been observed in the interstellar medium, planets and tentatively in the remnants of supernova SN1897a. Its role as a cooler is important for gas giant planets and exoplanets, and possibly the early Universe. All this makes the spectral properties, cooling function and partition function of H$_3^+$ key parameters for astronomical models and analysis. A new high-accuracy, very extensive line list for H$_3^+$ called MiZATeP was computed as part of the ExoMol project alongside a temperature-dependent cooling function and partition function as well as lifetimes for %individual excited states. These data are made available in electronic form as supplementary data to this article and at http://www.exomol.com

astro-ph.GA↗

arXiv2017

ExoMol molecular line lists XIX: high accuracy computed hot line lists for H$_2$$^{18}$O and H$_2$$^{17}$O

Hot line lists for two isotopologues of water, \octo\ and \heto, are presented. The calculations employ newly constructed potential energy surfaces (PES) which take advantage of a novel method for using the large set of experimental energy levels for \hato\ to give high quality predictions for \octo\ and \heto. This procedure greatly extends the energy range for which a PES can be accurately determined, allowing accurate prediction of higher-lying energy levels than are currently known from direct laboratory measurements. This PES is combined with a high-accuracy, {\it ab initio} dipole moment surface of water in the computation of all energy levels, transition frequencies and associated Einstein A coefficients for states with rotational excitation up to $J=50$ and energies up to 30~000 \cm. The resulting HotWat78 line lists complement the well-used BT2 \hato\ line list (Barber et.al, 2006, MNRAS, {\bf 368}, 1087). Full line lists are made available in the electronic form as supplementary data to this article and at \url{www.exomol.com}.

astro-ph.EP↗

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↗

arXiv2015

High accuracy CO$_2$ line intensities determined from theory and experiment

Atmospheric CO$_2$ concentrations are being closely monitored by remote sensing experiments which rely on knowing line intensities with an uncertainty of 0.5\%\ or better. Most available laboratory measurements have uncertainties much larger than this. We report a joint experimental and theoretical study providing rotation-vibration line intensities with the required accuracy. The {\it ab initio} calculations are extendible to all atmospherically important bands of CO$_2$ and to its isotologues. As such they will form the basis for detailed CO$_2$ spectroscopic line lists for future studies.

physics.ao-ph↗

arXiv2015

Visible line intensities of the triatomic hydrogen ion from experiment and theory

The visible spectrum of H3+ is studied using high-sensitivity action spectroscopy in a cryogenic radiofrequency multipole trap. Advances are made to measure the weak ro-vibrational transitions from the lowest rotational states of H3+ up to high excitation energies providing visible line intensities and, after normalisation to an infrared calibration line, the corresponding Einstein $B$ coefficients. {\it Ab initio} predictions for the Einstein $B$ coefficients are obtained from a highly precise dipole moment surface of H3+ and found to be in excellent agreement, even in the region where states have been classified as chaotic.

physics.chem-ph↗

arXiv2014

Recommended isolated-line profile for representing high-resolution spectroscopic transitions (IUPAC Technical Report)

The report of an IUPAC Task Group, formed in 2011 on "Intensities and line shapes in high-resolution spectra of water isotopologues from experiment and theory" (Project No. 2011-022-2-100), on line profiles of isolated high-resolution rotational-vibrational transitions perturbed by neutral gas-phase molecules is presented. The well-documented inadequacies of the Voigt profile (VP), used almost universally by databases and radiative-transfer codes, to represent pressure effects and Doppler broadening in isolated vibrational-rotational and pure rotational transitions of the water molecule have resulted in the development of a variety of alternative line-profile models. These models capture more of the physics of the influence of pressure on line shapes but, in general, at the price of greater complexity. The Task Group recommends that the partially Correlated quadratic-Speed-Dependent Hard-Collision profile should be adopted as the appropriate model for high-resolution spectroscopy. For simplicity this should be called the Hartmann--Tran profile (HTP). The HTP is sophisticated enough to capture the various collisional contributions to the isolated line shape, can be computed in a straightforward and rapid manner, and reduces to simpler profiles, including the Voigt profile, under certain simplifying assumptions.

physics.optics↗

arXiv2014

QED correction for H$_3^+$

A quantum electrodynamics (QED) correction surface for the simplest polyatomic and polyelectronic system H$_3^+$ is computed using an approximate procedure. This surface is used to calculate the shifts to vibration-rotation energy levels due to QED; such shifts have a magnitude of up to 0.25 cm$^{-1}$ for vibrational levels up to 15~000 cm$^{-1}$ and are expected to have an accuracy of about 0.02 cm$^{-1}$. Combining the new H$_3^+$ QED correction surface with existing highly accurate Born-Oppenheimer (BO), relativistic and adiabatic components suggests that deviations of the resulting {\it ab initio} energy levels from observed ones are largely due to non-adiabatic effects.

physics.chem-ph↗