Ab Initio Transition Dipole Moments Research Articles

The A2Π–X2Σ+ Band System of YO

YO bands are conspicuous in the spectra of S stars. The laboratory spectrum of the A2Π–X2Σ+ electronic transition has been recorded with a Fourier transform spectrometer using a composite-wall hollow cathode lamp source. The A2Π–X2Σ+ transition with v′≤4 and v″ ≤ 4 has been rotationally analyzed using the modern spectral fitting program PGOPHER. Vibronic band strengths were calculated using an ab initio transition dipole moment function. A line list for the A2Π–X2Σ+ transition is provided and can be utilized in the modeling of S stars.

ExoMol line lists – LV: hyperfine-resolved molecular line list for vanadium monoxide (51V16O)

ABSTRACT The new hyperfine-resolved HyVO line lists for vanadium monoxide (51V16O) are presented. The new line list considers 15 electronic states and comprises almost 59 billion transitions between 3.4 million energy levels, up to 45 000 cm−1. Empirical energy levels obtained through the Marvel procedure are used to update calculated values to experimental accuracy, making the line list well suited for high-resolution experiments. Machine learning is utilized to refine the energies of levels in important bands, where empirical values are not known. These updates allow for the accurate prediction of line positions in regions of astrophysical interest, particularly in strong rovibronic bands in the infrared and visible. Previously published ab initio transition dipole moment curves are adjusted based on experimental lifetime measurements, resulting in increased intensities in key regions. The line list covers levels up to F = 150 and is complete up to 5400 K, making it well suited for studies of ultra-hot Jupiter and M-type stellar atmospheres where VO is believed to be a strong source of opacities. The full line list is made available via www.exomol.com.

S-type Stars: Line List for the A2Π–X2Σ+ Band System of LaO

LaO bands are found in the spectra of cool S-type stars. The bands of the A2Π–X2Σ+ transition with and v″ ≤ 4 are rotationally analyzed, providing spectroscopic constants for the A2Π state. Line strengths are calculated using an ab initio transition dipole moment function, and radiative lifetimes for the A2Π state have also been computed. A line list for the A2Π–X2Σ+ transition of LaO is provided and can be used to determine LaO stellar abundances.

ExoMol line lists – XLVII. Rovibronic molecular line list of the calcium monohydroxide radical (CaOH)

ABSTRACT Any future detection of the calcium monohydroxide radical (CaOH) in stellar and exoplanetary atmospheres will rely on accurate molecular opacity data. Here, we present the first comprehensive molecular line list of CaOH covering the $\tilde{A}\, ^2\Pi$–$\tilde{X}\, ^2\Sigma ^+$ rotation-vibration-electronic and $\tilde{X}\, ^2\Sigma ^+$–$\tilde{X}\, ^2\Sigma ^+$ rotation-vibration bands. The newly computed OYT6 line list contains over 24.2 billion transitions between 3.2 million energy levels with rotational excitation up to J = 175.5. It is applicable to temperatures up to T = 3000 K and covers the 0–35 000 cm−1 range (wavelengths λ > 0.29 μm) for rotational, rotation-vibration and the $\tilde{A}\, ^2\Pi$–$\tilde{X}\, ^2\Sigma ^+$ electronic transitions. The strong band around 16 000 cm−1 (λ = 0.63 μm) is likely to be of interest in future astronomical observations, particularly in hot rocky exoplanets where temperatures can become extremely high. The OYT6 line list has been generated using empirically refined $\tilde{X}\, ^2\Sigma ^+$ and $\tilde{A}\, ^2\Pi$ state potential energy surfaces, high-level ab initio transition dipole moment surfaces, and a rigorous treatment of both Renner–Teller and spin-orbit coupling effects, which are necessary for correctly modelling the CaOH spectrum. Post-processing of the CaOH line list has been performed so as to tailor it to high-resolution applications, i.e. by replacing calculated energy levels with more accurate empirically derived values (where available), hence improving the accuracy of the predicted line positions in certain regions. The OYT6 line list is available from the ExoMol data base at www.exomol.com and the CDS astronomical data base.

ExoMol line lists – XLV. Rovibronic molecular line lists of calcium monohydride (CaH) and magnesium monohydride (MgH)

ABSTRACT New molecular line lists for calcium monohydride (40Ca1H) and magnesium monohydride (24Mg1H) and its minor isotopologues (25Mg1H and 26Mg1H) are presented. The rotation-vibration-electronic (rovibronic) line lists, named XAB, consider transitions involving the $X\, {}^{2}\Sigma ^{+}$, $A\, {}^{2}\Pi$, and $B/B^{\prime }\, {}^{2}\Sigma ^{+}$ electronic states in the 0–30 000 cm−1 region (wavelengths λ > 0.33 μm) and are suitable for temperatures up to 5000 K. A comprehensive analysis of the published spectroscopic literature on CaH and MgH is used to obtain new extensive data sets of accurate rovibronic energy levels with measurement uncertainties and consistent quantum number labelling. These data sets are used to produce new spectroscopic models for CaH and MgH, composed of newly empirically refined potential energy curves and couplings in/between the different electronic states (e.g. spin-orbit, electronic angular momentum, Born-Oppenheimer breakdown, spin-rotation, Λ-doubling), and previously published ab initio transition dipole moment curves. Along with Einstein A coefficients, state lifetimes and Landé g-factors are provided, the latter being particularly useful as CaH and MgH can be used to probe stellar magnetic fields. Computed energy levels have been replaced with the more accurate empirical values (if available) when post-processing the line lists, thus tailoring the line lists to high-resolution applications. The XAB line lists are available from the ExoMol database at www.exomol.com and the CDS astronomical database.

Visible Opacity of M Dwarfs and Hot Jupiters: The TiO B 3Π−X 3Δ Band System

Abstract The TiO B 3Π − X 3Δ electronic transition ( γ ′ system) is an important opacity source in the atmospheres of M dwarfs and hot Jupiter exoplanets. The 0–0, 1–0, and 2–1 bands of the B 3Π − X 3Δ band system have been analyzed using a TiO emission spectrum recorded at the McMath-Pierce Solar Telescope, operated by the National Solar Observatory at Kitt Peak, Arizona. Improved spectroscopic and equilibrium constants were determined. Line strengths were calculated from an ab initio transition-dipole moment function scaled using an experimental lifetime. A new line list for v ′ = 0 – 2 and v″ = 0–4 of the B 3Π − X 3Δ band system is provided.

ExoMol line lists – XLIV. Infrared and ultraviolet line list for silicon monoxide (28Si16O)

ABSTRACT A new silicon monoxide (28Si16O) line list covering infrared, visible, and ultraviolet regions called SiOUVenIR is presented. This line list extends the infrared EBJT ExoMol line list by including vibronic transitions to the $A\, {}^{1}\Pi$ and $E\, {}^{1}\Sigma ^{+}$ electronic states. Strong perturbations to the $A\, {}^{1}\Pi$ band system are accurately modelled through the treatment of six dark electronic states: $C\, {}^{1}\Sigma ^{-}$, $D\, {}^{1}\Delta$, $a\, {}^{3}\Sigma ^{+}$, $b\, {}^{3}\Pi$, $e\, {}^{3}\Sigma ^{-}$, and $d\, {}^{3}\Delta$. Along with the $X\, {}^{1}\Sigma ^{+}$ ground state, these nine electronic states were used to build a comprehensive spectroscopic model of SiO using a combination of empirical and ab initio curves, including the potential energy (PE), spin–orbit, electronic angular momentum, and (transition) dipole moment curves. The ab initio PE and coupling curves, computed at the multireference configuration interaction level of theory, were refined by fitting their analytical representations to 2617 experimentally derived SiO energy levels determined from 97 vibronic bands belonging to the X–X, E–X, and A–X electronic systems through the MARVEL (Measured Active Rotational–Vibrational Energy Levels) procedure. 112 observed forbidden transitions from the C–X, D–X, e–X, and d–X bands were assigned using our predictions, and these could be fed back into the MARVEL procedure. The SiOUVenIR line list was computed using published ab initio transition dipole moments for the E–X and A–X bands; the line list is suitable for temperatures up to 10 000 K and for wavelengths longer than 140 nm. SiOUVenIR is available from www.exomol.com and the CDS data base.

Spectral transition properties of the A1Π-X1Σ+ system for PN

Transition properties for the A1Π-X1Σ+ system of PN have been investigated. Three sets of the potential energy curves (PECs) for the X1Σ+ and A1Π states are studied by the ab initio calculation, the Rydberg-Klein-Rees method and an empirical fitting to the experimental energies. Transition probabilities are then computed utilizing the empirical PECs and a high-level ab initio transition dipole moment. The partition functions for temperatures up to 10000 K are generated by employing the hybrid empirical and ab initio energies of eight low-lying states. Finally, the line list of PN A1Π-X1Σ+ transition is presented.

A new line list for the A[formula omitted] electronic transition of OH

A new line list for the A2Σ+−X2Π electronic transition of OH has been calculated. Line positions have been taken from the literature and refitted with Western’s PGOPHER program. Line intensities were calculated using a new ab initio Transition Dipole Moment Function (TDMF) obtained with Molpro 2012. The new TDMF and the potential functions from LeRoy’s RKR program have been used as input to LeRoy’s LEVEL program in order to calculate Transition Dipole Moment Matrix Elements (TDMMEs). These matrix elements were transformed from Hund’s case (b) to Hund’s case (a) as required for the PGOPHER program using the method of Brooke et al. [JQSRT 2016; 168: 142–157] The line list was calculated with PGOPHER for bands with v′=0−4 in the A2Σ+ state and v″=0−9 for the X2Π state.

Ab initio transition dipole moments and potential energy curves for the low-lying electronic states of CaH

Ab initio potential energy curves have been calculated for the X2Σ+, A2Π, B2Σ+, 12Δ, E2Π and D2Σ+ states of CaH using the multi-reference configuration interaction method with large active space and basis sets. Transition dipole moments were calculated at Ca–H distances from 2.0 a0 to 14.0 a0, and excited state lifetimes were obtained. Our theoretical transition dipole moments can be combined with the available experimental data on the X2Σ+, A2Π and B2Σ+ states to calculate Einstein A coefficients for all rovibronic transitions of CaH appearing in solar and stellar spectra.

Einstein A coefficients for rovibronic lines of the A 2 Π → X 2Σ+ and B′ 2Σ+→ X 2 Σ+ transitions of MgH

Einstein A coefficients have been calculated for rovibronic lines of the A 2 � → X 2 � + and B � 2 � + → X 2 � + transitions of 24 MgH. Using the experimentally determined analytic potential energy curve for the X 2 � + state, and the Rydberg–Klein–Rees potentials for the A 2 � and B � 2 � + states, combined with the most recent ab initio transition dipole moments, rovibronic transition moments were computed. The rovibronic transition frequencies were obtained accurately by subtracting the published term values of the X 2 � + , A 2 � and B � 2 � + states. Using the rovibronic transition dipole moments, transition frequencies and the H¨ onl–London factors, the Einstein A coefficients were calculated individually for ∼30 000 rovibronic lines of the A 2 � → X 2 � + and B � 2 � + → X 2 � + transitions of 24 MgH.

Theoretical radiative properties between states of the triplet manifold of NH radical

Ab initio transition dipole moments between states of the triplet manifold of NH radical are determined at the complete active space self-consistent field, followed by the internally contracted multireference singles plus doubles configuration interaction level of theory with a modified aug-cc-pVTZ basis set that accounts for valence-Rydberg interactions. This enables the computation of various radiative characteristics such as Einstein coefficients, radiative lifetimes, and oscillator strengths. These properties concern as well valence and Rydberg states. For the valence states, only the (0, 0) band of the A 3 Pi-X 3 Sigma(-) transition has received some important amount of attention. Data for the other transitions are rather scarce and sometimes inexistent. The results obtained in this work show good agreement with the available experimental data in comparison to other theoretical numbers reported in the literature.

Potential Energy Curves for X1Σ+ and A1Π States of CO: The A1Π(v′=1–23)←X1Σ+(v″=0, 1) Transitions

We report new potential energy curves for the X1Σ+ and A1Π states of the CO molecule by employing the realistic extended Hartree–Fock approximate correlation energy model for diatomics with inclusion of the united-atom limit (EHFACE2U). Special care has been taken to describe the A1Π potential maximum, since this affects the vibrational wave functions of the highest vibrational levels. The corresponding vibronic spectroscopy and Franck–Condon factors (v″=0, 1) have also been calculated using accurate ab initio transition dipole moments. Comparison has been made with recent theoretical and experimental data.

Analysis and transition probabilities of the B1Πu → X1Σg + system of Na 2 excited by the 5017 Å line of the argon ion laser

The fluorescence spectrum of Na 2 induced by the 5017.16 Å line of an argon ion laser has been analyzed. This work extends previous observations on Na 2 by using this excitation line. We have observed seven fluorescence series for the B 1 Π u → X 1 Σ g + band system corresponding to the excitation transitions: ν′ = 5, J′ = 37 ← ν″ = 6, J″ = 38; ν′ = 5, J′ = 88 ← ν″ = 5, J″ = 87; ν′ = 5, J′ = 124 ← ν″ = 3, J″ = 125; ν′ = 0, J′ = 42 ← ν″ = 2, J″ = 43; ν′ = 10, J′ = 78 ← ν″ = 9, J″ = 77; ν′ = 14, J′ = 68 ← ν″ = 12, J″ = 68 and ν′ = 15, J′ = 58 ← ν″ = 13, J″ = 58. The latter four series are reported for the first time. The radiative transition probabilities for the observed fluorescence series are calculated by using hybrid Perturbed-Morse-Oscillator (PMO)-Rydberg-Klein-Rees (RKR)-long range potential energy curves for the B 1 Π u and X 1 Σ g + states and ab initio transition dipole moment function. Radiative lifetimes of the rovibrational levels of the upper state pumped by the laser line have also been calculated. The transition probabilities and lifetimes are in good agreement with the corresponding observed measurements usually within the experimental uncertainty. From the rotational satellite structure of some bands, collision-induced transition rates and average cross sections have been determined.

Photodissociation of CH2. IV. Predicted resonance Raman emission spectrum of photodissociating 3CH2

Predicted resonance Raman emission spectra of photodissociating 3CH2 excited in its first absorption band are presented. In the three-dimensional wave packet calculations, we used ab initio potential energy surfaces for both the 3B1 ground state and the first excited 3A1 state, and an ab initio transition dipole moment function. The resonance Raman emission of CH2 is similar to that of à state H2O in that it shows a strong progression in the symmetric stretch vibration. It differs from that of à state H2O in that it also shows a fairly strong progression in the bending mode, which results from the much larger anisotropy of the excited state potential of CH2 in the exit channel. Because the stretching vibrational wave functions of CH2 are much more normal modelike than those of H2O the ratio of the intensity of emission into the (vs=2, vb=0, va=0 state to the intensity of emission into the (vs=0, vb=0, va=2) state is larger for CH2 by more than an order of magnitude. The measurement of resonance Raman emission spectra of CH2 would yield the energies of the symmetric stretch fundamental and its overtones, which are unknown at present, and present the first empirical evidence concerning the first excited triplet state.

Photodissociation of CH2. II. Three-dimensional wave packet calculations on dissociation through the first excited triplet state

Quantitative results on photodissociation of CH2(X̃ 3B1) through the first excited (1 3A1) triplet state, producing CH (X 2Π)+H(2S), are presented. A three-dimensional time dependent quantum mechanical method was adopted to perform the dynamics using ab initio potential energy surfaces and an ab initio transition dipole moment function. The calculations were performed for J=0, where J is the angular momentum associated with the overall rotation of the nuclei. Comparison with calculations in which the bending angle was kept fixed at its ground state equilibrium value shows that a two-dimensional treatment suffices for obtaining the absorption spectrum. On the other hand, a three-dimensional calculation is necessary for correctly predicting the final rotational state distribution of the CH fragment.

Theoretical study of the radiative properties of the triplet states of the NH radical: Transition dipole moments, radiative lifetimes, photodissociation cross sections

Ab initio transition dipole moments between the X 3Σ− and the A 3Π states of NH and the 2 3Σ− and 2 3Π dissociative states have been computed. These transition dipole moments have been used to compute photodissociation cross sections and interstellar photodissociation rates for NH. Photodissociation rates for NH in the interstellar radiation field range from 1.9 to 4.7×10−10 s−1 depending on the field used. Direct dissociation via the 2 3Σ− and 2 3Π states is found to be the only important pathway for photodestruction of NH in diffuse interstellar clouds. A large photodissociation cross section is found for the A 3Π–2 3Σ− transition and a method for photolyzing NH in the laboratory is suggested. Einstein A coefficients and radiative lifetimes have also been computed for the A–X transition and are compared with recent theoretical and experimental work.