[en] 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 0.01 cm⁻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 0.94 cm⁻¹. The empirical PEC contains an inflection at about R=4.1 Å that 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. - Highlights: • An empirical potential for first singlet Pi state of the KCs molecule was constructed. • Fourier-Transform spectroscopy yielded term values with 0.01 cm⁻¹ accuracy. • Data set included vibrational levels up to v = 35 covering 85% of the potential well. • The potential reproduces the fitted data with a standard deviation 0.94 cm⁻¹. • The empirical potential contains an inflection that reflects the avoided crossing

[en] The paper presents an empirical pointwise potential energy curve (PEC) of the (2)¹Π 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)¹Π → X¹Σ⁺ fluorescence spectra. Buffer gas Ar was used to facilitate the appearance of rotation relaxation lines in the spectra, thus enlarging the (2)¹Π 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)]

[en] Laser induced fluorescence spectra of the C ¹Σ⁺-X ¹Σ⁺ system of ²³Na⁸⁵Rb and ²³Na⁸⁷Rb have allowed vibrational levels of the electronic ground state up to v^''=76, spanning 99.85% of the potential well to be observed. The ground-state term values have been fitted to a Dunham polynomial expansion, and also to a direct modified Lennard-Jones (MLJ) potential. The analytical MLJ construction allowed us to match previous measured term values for v^''≤30 with long-range behavior of the potential through the intermediate internuclear distance region covered by the present investigation

[en] Full text: In spite of the fact that the KCs molecule is among prospective objects for production of ultracold polar molecules, empirical data on its ground state potential have been obtained only very recently. There are still no experimental spectroscopic data on any of KCs excited state, and all existing information comes from ab initio calculations. We present first observation of laser induced fluorescence (LIF) for the (1, 2)¹Π states of KCs studied by Fourier transform spectroscopy using a Bruker IFS125HR, with 0.03 cm^-1 resolution. KCs molecules were formed at 280^oC in the sealed cylindrical glass cell containing K (natural isotope mixture) and Cs metals. The LIF D(2)¹Π → X¹Σ⁺ spectra have been excited by tuning within 15280 - 15140 cm^-1 a diode laser with 130 mW Mitsubishi ML101J27 laser diode. The B(1)¹ Π → X¹ Σ⁺ LIF spectra have been obtained with excitation frequencies 14400 - 14500 cm^-1 by a diode laser with 50 mW Hitachi HL6750 laser diode. Lasers were mounted in home made external cavity resonators (Littrow configuration), with a grating serving as a feedback source. Term values of the (1, 2)¹Π states in KCs have been obtained by adding transition frequencies to the respective ground state (v_X; J_X)X¹Σ⁺ term values using accurate X¹Σ⁺ state energy level data. The dependencies of the (1, 2)¹Π states term values on the factor J(J + 1) have been plotted. In particular, for the D(2)¹Π state the rotationless term values covered the energy range from ca. 15400 to 16400 cm^-1, spanning the J range from 16 to 205. Preliminary v_D identification was suggested. The dependencies have been compared with their theoretical counterparts based on calculations. The analysis has revealed a noticeable deviation from regular dependence caused, most probably, by spin-orbit interaction of the D(2)¹Π state with the (3)³Σ⁺ state and, especially, with the very closely lying (2)³Π state, as predicted. The work on vibrational assignment and potential energy curves construction of the states under study is in progress. (author)

[en] The laser-induced fluorescence (LIF) A ¹Σ⁺-b ³Π→X ¹Σ⁺ spectra of the KCs molecule were recorded in a near infrared region by a Fourier-transform spectrometer with a resolution of 0.03 cm^-1. Overall more than 200 collisionally enhanced LIF spectra were rotationally assigned to ³⁹K¹³³Cs and ⁴¹K¹³³Cs isotopomers yielding more than 3400 rovibronic term values of the strongly mixed singlet A ¹Σ⁺ and triplet b ³Π states with the uncertainty of 0.003-0.01 cm^-1. Experimental data massive starts from the lowest vibrational level v_A=0 of the singlet and nonuniformly covers the energy range E is an element of [10 040,13 250] cm^-1 with rotational quantum numbers J^' is an element of [7,225]. Besides the dominating regular A ¹Σ⁺-b ³Π_Ω=0 interactions, the weak local heterogeneous A ¹Σ⁺-b ³Π_Ω=1 perturbations have been discovered and analyzed. Coupled-channels deperturbation analysis of the experimental ³⁹K¹³³Cs e-parity term values of the A ¹Σ⁺-b ³Π_Ω=0,1,2 complex was accomplished in the framework of the phenomenological 4x4 Hamiltonian accounting implicitly for regular interactions with the remote ¹Π and ³Σ⁺ states. The diabatic potential energy curves of the A ¹Σ⁺ and b ³Π states, as well as relevant spin-orbit coupling matrix elements, were defined analytically with the expanded Morse oscillators model. The obtained parameters reproduce 95% of experimental data field of the ³⁹K¹³³Cs isotopomer with a standard deviation of 0.004 cm^-1, which is consistent with the uncertainty of the experiment. Reliability of the derived parameters was confirmed by a good agreement between the predicted and experimental term values of the ⁴¹K¹³³Cs isotopomer. The calculated relative intensity distributions in A-b→X LIF progressions are also consistent with their experimental counterparts. The deperturbation model was applied for simulation of a pump-dump optical cycle a ³Σ⁺→A ¹Σ⁺-b ³Π→X ¹Σ⁺ proposed for transformation of ultracold KCs molecules to their absolute ground state v_X=0;J_X=0.

[en] The diode-laser-induced fluorescence spectra b ³Π_Ω=0⁺→X¹Σ⁺ originated from the rovibronic levels of the 'dark' triplet b ³Π_Ω=0⁺ state of KCs dimers were recorded with a Fourier transform spectrometer with a resolution of 0.03 cm^-1. Term values of 30 rovibronic levels (v_b0*(set-membership sign)[14,18];J^'(set-membership sign)[47,134]) below the minimum of perturbing A ¹Σ⁺ state were determined with 0.003-0.01 cm^-1 uncertainty. The optimal excitation and detection lines for very weak spin-forbidden b ³Π_Ω=0⁺-X¹Σ⁺ transitions have been properly predicted in the framework of the coupled-channels deperturbation analysis recently performed for the singlet-triplet A ¹Σ⁺-b ³Π_Ω=0,1,2⁺ complex [A. Kruzins et al., Phys. Rev. A 81, 042509 (2010)]. The lowest observed triplet level has only 8.6% of singlet admixture.

[en] The article presents first experimental data on the Stark induced e-f mixing in the (4) ¹Π state of the ⁸⁵Rb¹³³Cs molecule, as well as the ab initio calculations of permanent electric dipole moments (d) in the (1,2,3,4) ¹Π states and q factors in the (2,4) ¹Π states. The appearance of the 'forbidden' lines in the laser-induced (4) ¹Π→X ¹Π⁺ fluorescence spectrum in the presence of an electric field allowed us to obtain, for the rovibronic v^'(J^')=2(82) level of the (4) ¹Π state correlating to the Rb(4d)+Cs(6s) atomic limit, the ratio q/d=0.9x10^-7 cm^-1/D with 20% experimental error. Electronic structure calculations on the ground and excited states of the RbCs molecule were performed in the framework of the Hund's a-coupling scheme by means of the many-body multipartitioning perturbation theory. The ab initio d(R) estimates revealed large values of about d≅-7 D for the (4) ¹Π state. The calculated q factor value for the (2) ¹Π state is in excellent agreement with the literature data. The theoretical estimate of the q/d ratio for the v^'(J^')=2(82) level of the (4) ¹Π state agrees satisfactory well with the present experimental value

[en] High resolution Fourier-transform spectroscopy data of term values in the spin-orbit (SO) coupled first excited A ¹Σ⁺ and b³Π_0± states in KCs were obtained from (4)¹Σ⁺ → A ¹Σ⁺ − b ³Π, A ¹Σ⁺ − b ³Π → X¹Σ⁺, and (1)³Δ₁ → b³Π_0± spectra of laser-induced fluorescence (LIF). About 3000 new rovibronic term values of the A ¹Σ⁺ and b ³Π_Ω 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 ³Π_0⁺ 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 v_b = 0 and up in energy to 13 814 cm⁻¹, as compared to previously achieved v_b = 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 ³Π_0⁺ 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 v_b levels of the b ³Π_0⁻ 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 v_b-dependent increase.

[en] Full text: Study of a³Σ⁺ states of heteronuclear alkali dimers has a strong motivation in supplying necessary spectroscopic data for producing ultra-cold molecular species in their ground state. We present the analysis of high resolution laser induced fluorescence (LIF) spectra to the a³Σ⁺ state of KCs. The fluorescence has been studied using a Bruker IFS125HR Fourier transform spectrometer, with a typical resolution of 0.03 cm^-1. KCs molecules were produced at 280^oC in a sealed cylindrical glass cell containing K and Cs metals. For excitation a single mode ring dye laser Coherent 699-21 with Rhodamine 6G dye was used as a light source. Excitation frequencies were selected between 16870 cm^-1 and 17280 cm^-1 and measured by a wavemeter (HighFinesse WS6) with about 0.015 cm^-1 accuracy. The laser frequencies were tuned until the LIF signal to the a³Σ⁺ state of KCs, as monitored by the Fourier spectrometer in the preview mode, exhibited a maximal value. LIF observed between 13700-14200 cm^-1 revealed clear hyperfine structure and therefore was attributed as fluorescence to the a³Σ⁺ state. In the same spectra LIF to the ground state was present as well. Only P and R transitions were observed in LIF to both the a³Σ⁺ and the X¹Σ⁺ states. The upper state exited in our experiments is most likely (4)¹Σ⁺ perturbed by (3)³Σ⁺ and (2)³Π states. Term values of the a³Σ⁺ state in KCs have been obtained by subtracting the transition frequencies from the respective excited state (v', J') term value. This term value was obtained by adding the energy of the corresponding level of the X¹Σ⁺ state, calculated from the analysis, to the respective (i.e. originating from the same upper state (v', J')) transition frequencies. At current research stage we have assigned 681 LIF lines in 48 LIF progressions to the a³Σ⁺ state spanning v* from 1 to 21 and J from 23 to 143. The symbol v* means the preliminary current assignment of vibrational quantum numbers. A first empirical potential of the a³Σ⁺ state is presented, obtained by the Inverted Perturbation Approach (IPA) method. The work on vibrational assignment and potential energy curve of the a³Σ⁺ state is still in progress. (author)

[en] A comprehensive study of the X ¹Σ⁺ and a ³Σ⁺ electronic states of the K and Cs atom pair is presented. Abundant spectroscopic data include 1937 transitions to the a ³Σ⁺ state and 13 724 transitions to the X ¹Σ⁺ state for ³⁹K ¹³³Cs which were obtained by Fourier-transform spectroscopy. From these data potential energy curves were constructed simultaneously for both states X ¹Σ⁺ and a ³Σ⁺ which are coupled by the hyperfine interaction. This allows us to accurately model the potential close to K(4s)+Cs(6s) atom-pair asymptote which is required to simulate cold collision processes. Dissociation energies of the X ¹Σ⁺ state as D_e^X=4069.208(40) cm^-1 and of the a ³Σ⁺ state as D_e^a=267.141(20) cm^-1 were determined. Scattering lengths are predicted for different KCs isotopologs and Feshbach resonances were calculated for magnetic fields up to 1000 G.