Experimental Term Values Research Articles

A coupled-channel deperturbation treatment of the [formula omitted] complex of the CN radical towards spectroscopic accuracy

A global deperturbation analysis of the experimental rovibronic term values of the X2Σ+, A2Π and B2Σ+ states of the 12C14N isotopomer has been performed. The inverse spectroscopic problem was directly solved in the framework of the reduced 4×4 coupled-channel (RCC) deperturbation model based on potential energy curves (PECs) as well as the spin-orbit and L-uncoupling electronic matrix elements between the X2Σ+, A2Π, and B2Σ+ states. Regular perturbations of the X∼A∼B complex by the remote doublet states manifold were taken into account by introducing of the fine-structure parameters as explicit functions of interatomic distance. The optimized PECs and non-adiabatic coupling functions describe the vast majority (5600–6570) of the empirical term values, attributed to locally and regularly perturbed levels of the complex, with a root-mean-squared deviation of 0.015-0.05 cm−1, depending on a particular set of the experimental term values included in the fitting procedure. The resulting mass-invariant RCC deperturbation parameters can straightforwardly extend a line-list of all CN isotopomers into a wide region of vibrational and rotational quantum numbers.

The [formula omitted] state of KCs revisited: Hyperfine structure analysis and potential refinement

Laser-induced fluorescence spectra of the c3Σ+(vc,Jc=Nc)→a3Σ+(va,Na=Jc±1) transitions excited from the ground X1Σ+ state of 39K133Cs molecule were recorded with Fourier-transform spectrometer IFS125-HR (Bruker) at the highest achievable spectral resolution of 0.0063 cm−1. Systematic study of the hyperfine structure (HFS) of the a3Σ+ state for levels with va∈[0,27] and Na∈[24,90] shows that the splitting monotonically increases with va. The spectroscopic study was supported by ab initio calculations of the magnetic hyperfine interaction in X1Σ+ and a3Σ+ states. The discovered variation of the electronic matrix elements with the internuclear distance R is in a good agreement with the observed va-dependencies of the HFS. Overall set of available experimental data on the a3Σ+ state was used to improve the potential energy curve particularly near a bottom, providing the refined dissociation energy De=267.21(1) cm−1. The ab initio HFS matrix elements, combined with the empirical X1Σ+ and a3Σ+ PECs in the framework of the invented coupled-channel deperturbation model, reproduce the experimental term values of both ground states within 0.003 cm−1 accuracy up to their common dissociation limit.

Fourier-transform spectroscopy and relativistic electronic structure calculation on the [formula omitted] state of KCs

The Ti:Saphire laser operated within 13800 - 11800 cm−1 range was used to excite the c3Σ+ state of KCs molecule directly from the ground X1Σ+ state. The laser-induced fluorescence (LIF) spectra of the c3Σ+→a3Σ+ transition were recorded with Fourier-transform spectrometer within 8000 to 10000 cm−1 range. Overall 673 rovibronic term values belonging to both e/f-components of the c3Σ+(Ω=1±) state of 39KCs, covering vibrational levels from v = 0 to about 45, and rotational levels J∈[11,149] were determined with the accuracy of about 0.01 cm−1; among them 7 values for 41KCs. The experimental term values with v∈[0,22] were involved in a direct point-wise potential reconstruction for the c3Σ+(Ω=1±) state, which takes into account the Ω-doubling effect caused by the spin-rotational interaction with the nearby c3Σ+(Ω=0−) state. The analysis and interpretation were facilitated by the fully-relativistic coupled cluster calculation of the potential energy curves for the B1Π, c3Σ+, and b3Π states, as well as of spin-forbidden c−X and spin-allowed c−a transition dipole moments; radiative lifetimes and vibronic branching ratios were calculated. A comparison of relative intensity distributions measured in vibrational c→a LIF progressions with their theoretical counterparts unambiguously confirms the vibrational assignment suggested in [J. Szczepkowski, et al., JQSRT, 204, 133–137 (2018)].

A coupled–channels model describing the low-lying [formula omitted], [formula omitted] and [formula omitted] electronic states of nickel monohydride with experimental accuracy

A detailed study of the three interacting low-lying electronic states 2Δ, 2Σ+ and 2Π of NiH –the so called “supermultiplet” – is presented. A coupled-channels model reproduces the experimental term values of 58NiH, 60NiH and 62NiH with accuracy very close to the estimated uncertainty of 0.01 cm−1. The model is based on a set of Hund’s case (a) potential curves and R-dependent coupling functions. In addition to the expected spin-orbit and various rotational couplings between the zero-order states, second-order effects are found to be important. The spin-orbit interaction is large compared to the separations between these electronic states, so that most of the observed rovibrational levels are strong mixtures of the Ω components of the multiplet. The fitting procedure proved difficult because there were no perturbation-free data to determine the starting values for the model functions. For the potential curves we were guided by previous effective Hamiltonian models; ab initio predictions supplied starting values for the spin-orbit and the rotational coupling functions. We believe that this model may be reliably extrapolated to higher rotational levels, with potential applications in the simulation of high temperature spectra, for example in the context of stellar atmospheres.

High-resolution Fourier-transform spectroscopy and deperturbation analysis of the A1Π(v = 1) level in 12C18O

The A1Π(v = 1) level of the 12C18O isotopologue was precisely reinvestigated with two complementary spectroscopic techniques. High resolution B1Σ+ → A1Π(0, 1), (1, 1) and C1Σ+ → A1Π(0, 1) emission bands were recorded in the visible region, 20,700 – 26,100 cm−1, with a 1.71 m Fourier-transform spectrometer (Bruker IFS 125-HR) installed at the University of Rzeszów. The resulting line centre accuracy of isolated and medium to strong lines is 0.005 cm−1. In addition, high-resolution spectra of the A1Π ⟵ X1Σ+(1, 0), B1Σ+⟵ X1Σ+(0, 0) and (1, 0) as well as C1Σ+⟵ X1Σ+(0, 0) bands were recorded between 66,200 and 95,250 cm−1 using the vacuum-ultraviolet Fourier-transform spectrometer installed at the DESIRS beamline of the SOLEIL synchrotron. The wavenumber accuracy for isolated and strong spectral lines is 0.01 cm−1. A data set of 626 spectral lines belonging to seven bands was incorporated into a global deperturbation analysis. Significantly improved deperturbed molecular constants for the A1Π(v = 1), a´3Σ+(v = 10), D1Δ(v = 1), and I1Σ−(v = 2) levels, term values of the B1Σ+(v = 0, 1) and C1Σ+(v = 0) Rydberg states as well as the accompanying spin-orbit and rotation-electronic (L-uncoupling) interaction parameters were obtained. The experimental ro-vibrational term values of the A1Π(v = 1) level and its perturbers were also determined. The mixed composition of interacting states is expressed in terms of their 1Π percentage character.

Fourier-transform spectroscopy, relativistic electronic structure calculation, and coupled-channel deperturbation analysis of the fully mixed A1Σu+ and b3Πu states of Cs2

The 4503 rovibronic term values belonging to the mutually perturbed $A^1\Sigma^+_u$ and $b^3\Pi_u$ states of Cs$_2$ were extracted from laser induced fluorescence (LIF) $A\sim b\rightarrow X^1\Sigma^+_g$ Fourier transform spectra with the 0.01 cm$^{-1}$ uncertainty. The experimental term values of the $A^1\Sigma^+_u\sim b^3\Pi_u$ complex covering the rotational levels $J\in [4,395]$ in the excitation energy range $[9655,13630]$ cm$^{-1}$ were involved into coupled-channel (CC) deperturbation analysis. The deperturbation model takes explicitly into account spin-orbit coupling of the $A^1\Sigma^+_u(A0^+_u)$ and $b^3\Pi^+_{0_u}(b0^+_u)$ states as well as spin-rotational interaction between the $\Omega=0$, $1$ and $2$ components of the $b^3\Pi^+_{\Omega_u}$ state. The \emph{ab initio} relativistic calculations on the low-lying electronic states of Cs$_2$ were accomplished in the framework of Fock space relativistic coupled cluster (FSRCC) approach to provide the interatomic potentials of the interacting $A0^+_u$ and $b0^+_u$ states as well as the relevant $A\sim b$ spin-orbit coupling function. To validate the present CC deperturbation analysis solely obtained by energy-based data, the $A\sim b \to X(v^{\prime\prime}_X)$ LIF intensity distributions were measured and compared with their theoretical counterparts obtained by means of the non-adiabatic vibrational wave functions of the $A\sim b$ complex and the FSRCC $A\sim b \to X$ transition dipole moments calculated by the finite-field method.

The spin-orbit coupling of the [formula omitted] and 43Π states in KCs: Observation and deperturbation

Energies of 1655 rovibrational levels of the mutually perturbed 61Σ+ and 43Π0+ states in KCs were measured in a series of experiments employing polarization labelling spectroscopy technique. The experimental term values of the 61Σ+∼43Π0+ complex in KCs were used in coupled-channels deperturbation analysis which took explicitly into account the spin-orbit coupling of the 61Σ+ and 43Π0+ states. The empirical deperturbation treatment was supported by ab initio electronic structure calculation of adiabatic potentials of the interacting states and of the corresponding spin-orbit coupling matrix element as a function of internuclear distance.

Fourier-transform spectroscopy, direct potential fit, and electronic structure calculations on the entirely perturbed (4)1Πstate of RbCs

We perform a high-resolution Fourier-transform spectroscopic study of the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state of the RbCs molecule by applying two-step $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}\ensuremath{\leftarrow}A{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}\ensuremath{\sim}b{\phantom{\rule{0.16em}{0ex}}}^{3}\mathrm{\ensuremath{\Pi}}\ensuremath{\leftarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ optical excitation followed by observation of the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}\ensuremath{\rightarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ laser-induced fluorescence (LIF) spectra. In many LIF progressions the collision-induced satellite rotational lines are observed, thus increasing the amount of term values and allowing us to estimate the $\mathrm{\ensuremath{\Lambda}}$-doubling effect in the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state. The direct potential fit (DPF) of experimental term values of 777 rovibronic levels of both $^{85}\mathrm{RbCs}$ and $^{87}\mathrm{RbCs}$ isotopologues is performed by means of the robust weighted nonlinear least-squares method. The DPF analysis based on the adiabatic approximation and analytical expanded Morse oscillator potential reveals numerous regular shifts in the measured level positions. The spectroscopic studies of the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state are supported by the electronic structure calculations including the potential energy curves of the singlet- and triplet-state manifold and spin-allowed transition dipole moments. The subsequent estimates of radiative lifetimes and corresponding vibronic branching ratios elucidate a dominant contribution of the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}\ensuremath{\rightarrow}A\ensuremath{\sim}b$ channel into the total radiative decay of the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state. The relative intensity distributions simulated for $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}\ensuremath{\rightarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ LIF progressions agree well with their observed counterparts even for the profoundly shifted levels of the entirely perturbed $(4){\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ state. To get insight into the origin of the intramolecular perturbations, the relevant spin-orbit- and $L$-uncoupling electronic matrix elements are evaluated.

Fundamental vibration frequency and rotational structure of the first excited vibrational level of the molecular helium ion ( ).

The term values of the rotational levels of the first excited vibrational state of the electronic ground state of with a rotational quantum number N + ≤ 13 have been determined with an accuracy of 1.2 × 10-3 cm-1 (∼35 MHz) by multichannel-quantum-defect-theory-assisted Rydberg spectroscopy of metastable He2. Comparison of the experimental term values with the most accurate ab initio results for available in the literature [W.-C. Tung, M. Pavanello, and L. Adamowicz, J. Chem. Phys. 136, 104309 (2012)] reveals inconsistencies between the theoretical and experimental results that increase with increasing rotational quantum numbers. The fundamental vibrational wavenumber of was determined to be 1628.3832(12) cm-1 by fitting effective molecular constants to the obtained term values.

New observations and analyses of highly excited bands of the fourth-positive [formula omitted] band system in [formula omitted

In the emission spectrum of the CO molecule thirteen bands of the fourth-positive (A1Π→X1Σ+) band system were photographed and analysed. Under high resolution and by using conventional photographic spectroscopy, the following excited bands: 7–16, 7–17, 8–18, 8–19, 9–19, 9–20, 10–20, 10–21, 11–21, 11–22, 12–22, 13–24, 14–26 and approximately 1340 molecular lines were recorded. A rotational analysis of the bands: 7–17, 8–18, 8–19, 9–20, and 11–22 were carried out for the first time, and the bands 11–21 and 14–26 were identified and analysed for the first time. After detailed rotational analysis of the bands, experimental rovibronic term values of the A1Π (v=9–14) levels were determined for the first time. Individual and equilibrium molecular constants of the A1Π state as well as the Franck–Condon factors were determined for the analysed bands. Moreover, a region of analysed perturbations of the A1Π (v=7–14) levels was extended to the J⩾30 rotational levels. Also, most of theoretically predicted perturbations of the vA=13 and 14 levels were identified for the first time.

Direct coupled-channels deperturbation analysis of the A(1)Σ(+) ∼ b(3)Π complex in LiCs with experimental accuracy.

We have carried out the direct deperturbation analysis of about 780 rovibronic term values of the strongly spin-orbit (SO) coupled A(1)Σ(+) and b(3)Π states of the (7)Li(133)Cs molecule recorded by polarization labelling spectroscopy technique. The explicit A(1)Σ(+) ∼ b(3)ΠΩ=0,1,2 coupled-channels treatment allowed us to reproduce 95% experimental term values with a standard deviation of 0.05 cm(-1) which is close to the accuracy of the present experiment. The initial potential energy curves (PECs) of the mutually perturbed states and SO matrix elements were ab initio evaluated in the basis of the spin-averaged wave functions. The empirically refined PECs and SO functions, along with the theoretical transition dipole moments, were used to predict energy and radiative properties of the A ∼ b complex for low J levels of both (7)Li(133)Cs and (6)Li(133)Cs isotopologues. The reasonable candidates for the stimulated Raman transitions between initial Feshbach resonance states, the mixed levels of the A ∼ b complex, and absolute ground X(1)Σ(+) (v = 0 and J = 0) state were identified.

Fourier-transform spectroscopy and potential construction of the (2)(1)Π state in KCs.

The paper presents an empirical pointwise potential energy curve (PEC) of the (2)(1)Π state of the KCs molecule constructed by applying the Inverted Perturbation Approach routine. The experimental term values in the energy range E(v', J') ∈ [15 407; 16 579] cm(-1) involved in the fit were based on Fourier-Transform spectroscopy data obtained with 0.01 cm(-1) accuracy from the laser-induced (2)(1)Π → X(1)Σ(+) fluorescence spectra. Buffer gas Ar was used to facilitate the appearance of rotation relaxation lines in the spectra, thus enlarging the (2)(1)Π data set and allowing determination of the Λ-splitting constants. The data set included vibrational v' ∈ [0, 28] and rotational J' ∈ [7, 274] quantum numbers covering about 67% of the potential well. The present PEC reproduces the overall set of data included in the fit with a standard deviation of 0.5 cm(-1). The obtained value of the Λ-doubling constant q = + 1.8 × 10(-6) cm(-1) for J' > 50 and v' ∈ [0, 6] is in an excellent agreement with q = + 1.84 × 10(-6) cm(-1) reported in Kim, Lee, and Stolyarov [J. Mol. Spectrosc. 256, 57-67 (2009)].

Extended Fourier-transform spectroscopy studies and deperturbation analysis of the spin-orbit coupled A1Σ+ and b3Π states in RbCs.

The article presents a study of the strongly spin-orbit coupled singlet A(1)Σ(+) and triplet b(3)Π states of the RbCs molecule, which provide an efficient optical path to transfer ultracold molecules to their rovibrational ground state. Fourier-transform A(1)Σ(+) - b(3)Π → X(1)Σ(+) and (4)(1)Σ(+) → A(1)Σ(+) - b(3)Π laser-induced fluorescence (LIF) spectra were recorded for the natural mixture of the (85)Rb(133)Cs and (87)Rb(133)Cs isotopologues produced in a heat pipe oven. Overall 8730 rovibronic term values of A(1)Σ(+) and b(3)Π states were determined with an uncertainty of 0.01 cm(-1) in the energy range [9012, 14087] cm(-1), covering rotational quantum numbers J ∈ [6, 324]. An energy-based deperturbation analysis performed in the framework of the four A(1)Σ(+) - b(3)Π(Ω = 0, 1, 2) coupled-channels approach reproduces 97% of the experimental term values of both isotopologues with a standard deviation of 0.0036 cm(-1). The reliability of the deperturbed mass-invariant potentials and spin-orbit coupling functions of the interacting A(1)Σ(+) and b(3)Π states is additionally proved by a good reproduction of the A - b → X and (4)(1)Σ(+) → A - b relative intensity distributions. The achieved accuracy of the A - b complex description allowed us to use the latter to assign the observed (5)(1)Σ(+) → A - b and (3)(1)Π → A - b transitions. As is demonstrated, LIF to the A - b complex becomes as informative as to the ground X(1)Σ(+) state, which is confirmed by comparing the results of (4)(1)Σ(+) state analysis based on (4)(1)Σ(+) → A - b LIF with the data from V. Zuters et al. [Phys. Rev. A 87, 022504 (2013)] based on (4)(1)Σ(+) → X LIF.

Potential construction of the B(1)1 Π state in KCs based on Fourier-Transform spectroscopy data

The paper presents an empirical pointwise potential energy curve (PEC) of the extensively perturbed B(1)1Π state of the KCs molecule constructed by applying an Inverted Perturbation Approach routine. The experimental term values in the energy range E(v′,J′)∈[14071;15502]cm−1 involved in the fit were based on Fourier-Transform spectroscopy data obtained with 0.01cm−1 accuracy from laser-induced B(1)1Π→X1Σ+ fluorescence spectra in the present work (654 term values) combined with 520 term values from Birzniece et al. (2012) . The data set included vibrational v′∈[0,35] and rotational J′∈[7,233] quantum numbers covering about 85% of the potential well. The present fit reproduces the data included in the fit with a standard deviation 0.94cm−1. The empirical PEC contains an inflection at about R=4.1 Åthat reflects the avoided crossing of two Ω=1 states corresponding to B(1)1Π and c(2)3Σ+ states as predicted by ab initio calculations.

High-resolution spectroscopy and quantum-defect model for the gerade triplet np and nf Rydberg states of He2

Photoionization spectra and Rydberg-state-resolved threshold-ionization spectra of the gerade triplet np Rydberg states of (4)He2 located in the vicinity of the X(+) (2)Σ(u)(+) (ν(+) = 0) ionization threshold were recorded from the 2sσ a (3)Σ(u)(+) metastable state. An accuracy of 0.01 cm(-1) was achieved for the experimental term values of the observed Rydberg states. The data were combined with spectroscopic data on low-lying triplet np and nf Rydberg states from the literature to derive energy- and internuclear-distance-dependent eigenquantum-defect parameters of multichannel quantum-defect theory (MQDT). The MQDT calculations reproduce the experimental data within their experimental uncertainties and enabled the derivation of potential-energy curves for the lowest triplet p Rydberg states (n = 2-5) of He2. The eigenquantum-defect parameters describing the p -f interaction were found to be larger than 0.002 at the energies corresponding to the high-n Rydberg states, so that the p -f interaction plays an important role in the autoionization dynamics of np Rydberg states with v(+) = 0. By extrapolating the experimental term values of triplet np Rydberg states of (4)He2 in the range of principal quantum number n between 87 and 110, the positions of the (v(+) = 0, N(+) = 3) and (v(+) = 0, N(+) = 5) levels of the ground state of (4)He(+)(2) were determined to lie 70.937(3) cm(-1) and 198.369(6) cm(-1), respectively, above the (v(+) = 0, N(+) = 1) ground rotational level.

Fourier-transform spectroscopy of (4)1Σ+ → A 1Σ+ − b 3Π, A 1Σ+ − b 3Π → X1Σ+, and $(1)^3\Delta _1\rightarrow b{\,}^3\Pi _{0^\pm }$(1)3Δ1→bΠ±3 transitions in KCs and deperturbation treatment of A 1Σ+ and b 3Π states

High resolution Fourier-transform spectroscopy data of term values in the spin-orbit (SO) coupled first excited A 1Σ+ and b 3Π states in KCs were obtained from (4)1Σ+ → A 1Σ+ − b 3Π, A 1Σ+ − b 3Π → X1Σ+, and $(1)^3\Delta _1\rightarrow b{\,}^3\Pi _{0^\pm }$(1)3Δ1→bΠ0±3 spectra of laser-induced fluorescence (LIF). About 3000 new rovibronic term values of the A 1Σ+ and $b{\,}^3\Pi _\Omega$b3ΠΩ states were obtained with an uncertainty about 0.01 cm−1 and added to the previously obtained 3439 term values in Kruzins et al. [Phys. Rev. A 81, 042509 (2010)] and 30 term values of the $b{\,}^3\Pi _{0^+}$b3Π0+ state levels below the A 1Σ+ state in Tamanis et al. [Phys. Rev. A 82, 032506 (2010)]. The data field was extended considerably, going down to vibrational level vb = 0 and up in energy to 13 814 cm−1, as compared to previously achieved vb = 14 and E = 13 250 cm−1. Overall 6431 e-symmetry term values of 39K133Cs were included in 4 × 4 coupled-channel deperturbation analysis. The analytical Morse-Long-Range (MLR) function yielded empirical diabatic potentials for the A 1Σ+ and $b{\,}^3\Pi _{0^+}$b3Π0+ states while the morphing of the SO ab initio points [J. T. Kim et al., J. Mol. Spectrosc. 256, 57 (2009)] provided the empirical diagonal and off-diagonal SO functions. Overall 98.5% of the fitted term values were reproduced with a rms (root mean square) uncertainty of 0.004 cm−1. The reliability of the model is proved by a good agreement of predicted and measured term values of the 41K133Cs isotopologue, as well as of measured and calculated intensities of (4)1Σ+ → A 1Σ+ − b 3Π LIF progressions. Direct-potential-fit of low-lying vb levels of the $b{\,}^3\Pi _{0^-}$b3Π0− component yielded the MLR potential which represents the 204 f-symmetry experimental term values with a rms uncertainty of 0.002 cm−1. The Ω-doubling of the b 3Π0 sub-state demonstrates a pronounced vb-dependent increase.

First observations and analyses of the fourth-positive (A1Π → X1Σ+) band system in 14C16O

In the isotopic 14C16O molecule eight bands of the fourth-positive (A1Π→X1Σ+) system were investigated for the first time. Under high resolution, using conventional photographic emission spectroscopy technique, highly excited 7–19, 8–20, 9–21, 10–21, 11–22, 12–23, 12–24 and 13–24 bands were recorded and spectroscopically analyzed. First individual constants of the rovibronic structure: Bv′ and Dv′ of both A1Π and X1Σ+ states as well as the equilibrium constants: Be, αe, γe, ωe, ωexe, and ωeye of the upper A1Π state were obtained. First values of band origins for all studied bands were also computed. On the basis of these parameters and the ground X1Σ+ state constants recalculated isotopically from Le Floch [1] spectroscopic constants, the first experimental rovibronic term values of the A1Π state vA=7–13 levels were calculated. Numerous observed rotational perturbations of the A1Π state were identified and compared with the theoretically determined ones.

Density functional theory study of yttrium monohalides

The molecular properties of the ground state and some low-lying excited states of the yttrium monohalides, YX (X = F, Cl, Br, and I), have been calculated using the B3P86 method of density functional theory (DFT). For each state, the equilibrium bond lengths of re , the harmonic vibrational frequencies of ωe , the dissociation energies of D0 , the term values of Te , the dipole moments of μ, for all the yttrium monohalides were obtained analytically and compared with the experimental and other theoretical values. Compared with the experimental results, the hybrid density functional B3P86 is superior to or comparable with the other theoretical results and predicts well all the spectroscopic parameters such as equilibrium bond lengths and the harmonic vibrational frequencies, but underestimates the term values. By comparing the computed results for B3P86 and CCSD(T), a reasonable explanation has been given for the discrepancy between the computed and experimental term values of YX. In addition, the effect of the halogens on the spectroscopic parameters for the group of YX has been discussed with the help of the molecular orbital diagram.

Towards efficient refinement of molecular potential energy surfaces: Ammonia as a case study

In order to approach experimental accuracy in ro-vibrational calculations for polyatomic molecules one needs to empirically refine even a high accuracy ab initio potential energy surface (PES). This is most efficiently done through a least-squares fitting of theoretical energies to the available experimental data by varying potential parameters in a given analytical representation. The PES resulting from such a fitting is then referred to as a ‘spectroscopic’ PES. In the present work we report a new approach to the construction of ‘spectroscopic’ PESs of polyatomic molecules. We represent the refinement as a perturbation to the initial PES, which is diagonalized in a basis of eigenfunctions of the unperturbed Hamiltonian. We apply this method to construct a new ‘spectroscopic’ PES for 14NH 3 using literature values for observed spectroscopic data for J ⩽ 8 and covering the energy range below 10 300 cm −1. We impose the constraint that the resulting PES remain close to the ab initio surface. The new ‘spectroscopic’ PES of NH 3 (called NH3-Y2010) reproduces the selected experimental term values with a root-mean-square deviation of 0.2 cm −1.

Fourier transform spectroscopy and direct potential fit of a shelflike state: Application to E(4)1Σ+ KCs

The paper presents high-resolution experimental study and a direct potential construction of a shelflike state E(4)(1)Σ(+) of the KCs molecule converging to K(4(2)S) + Cs(5(2)D) atomic limit; such data are of interest for selecting optical paths for producing and monitoring cold polar diatomics. The collisionally enhanced laser induced fluorescence (LIF) spectra corresponding to both spin-allowed E(4)(1)Σ(+) → X(1)(1)Σ(+) and spin-forbidden E(4)(1)Σ(+) → a(1)(3)Σ(+) transitions of KCs were recorded in visible region by Fourier transform spectrometer with resolution of 0.03 cm(-1). Overall about 1650 rovibronic term values of the E(4)(1)Σ(+) state of (39)K(133)Cs and (41)K(133)Cs isotopologues nonuniformly covering the energy range [16987, 18445] cm(-1) above the minimum of the ground X-state were determined with the uncertainty of 0.01 cm(-1). Experimental data field is limited by vibrational levels v' ∈ [2, 74] with rotational quantum numbers J' ∈ [1, 188]. The closed analytical form for potential energy curve (PEC) based on Chebyshev polynomial expansion (CPE) was implemented to a direct potential fit (DPF) of the experimental term values of the most abundant (39)K(133)Cs isotopologue. Besides analyticity, regularity, correct long-range behavior, and nice convergence properties, the CPE form demonstrated optimal balance on flexibility and constraint for the DPF of a shelflike state aggravated by a limited data set. The mass-invariant properties of the CPE PEC were tested by the prediction of rovibronic term values of the (41)K(133)Cs isotopomer which coincided with their experimental counterparts with standard deviation of 0.0048 cm(-1). The CPE modeling is compared with the highly flexible pointwise inverted perturbation approach model, as well as with conventional Dunham analysis of restricted data set v' ≤ 50. Reliability of the empirical PEC is additionally confirmed by good agreement between the calculated and experimental relative intensity distributions in the long E(v') → X(v") LIF progressions.