Landé g J factor measurements in the 6snd 1,3 D 2 sequences of Yb I

The Landé g _J factors in the 6snd ^1,3 D ₂ sequences as well as states from the perturbing configurations of neutral Ytterbium have been measured by timeresolved fluorescence spectroscopy. Strong singlet-triplet mixing effects were observed. The g _J factors were measured using the Zeeman quantum-beat technique following step-wise laser excitation. The experiment provided new data for checking the Multi-channel Quantum Defect Theory (MQDT) used in energy-level and lifetime analysis.     

4pnp J=0e-2e autoionizing series of calcium: experimental and theoretical analysis

The even parity 4pnp J=0, 1, 2 doubly excited autoionizing states of neutral calcium in an atomic beam are investigated by a two-step isolated core excitation (ICE) method using two different combinations of polarization of the laser beams. The different excited energy levels are assigned to nine autoionizing Rydberg series 4p_{1/2, 3/2}np J=0, 1, 2 for ≤ n ≤ 22. The theoretical interpretation is achieved by a combination of the eigenchannel R-matrix theory and the multichannel quantum defect (MQDT) method. Two, five and six closed interacting channels are introduced for the J=0, J=1 and J=2 series respectively. Theoretical energy level positions, autoionization widths and excitation profiles are compared with the experimental data, confirming the identification of the observed structures and providing evidence of extended mixing between the 4p_1/2np and 4p_3/2np series.     

The 5dnd and 5dng J=4, 3 and 2 and 5dns J=2 series in barium: linewidths and linestrengths

The 5dnd and 5dng J=2, 3 and 4 as well as the 5dns J=2 autoionising series of barium have been studied with two-step pulsed laser excitations in an atomic beam. Dipole moments and the interactions with the 6s∈l continua were fitted starting from existing Multi-channel Quantum Defect Theory (MQDT) analyses. The interactions of the 5dnd J=4 and 2, and 5dns J=2 series with 6sεg J=4 and 6sεd J=2 continua could be analysed succesfully.     

High-resolution millimeter wave spectroscopy and multichannel quantum defect theory of the hyperfine structure in high Rydberg states of molecular hydrogen H2

Experimental and theoretical methodologies have been developed to determine the hyperfine structure of molecular ions from detailed studies of the Rydberg spectrum and have been tested on molecular hydrogen. The hyperfine structure in l=0-3 Rydberg states of H2 located below the X 2Sigmag+(v+=0,N+=1) ground state of ortho H2+ has been measured in the range of principal quantum number n=50-65 at sub-MHz resolution by millimeter wave spectroscopy following laser excitation to np and nd Rydberg states using a variety of single-photon and multiphoton excitation sequences. The np1(1), nd1(1), and the nf1(0-3) Rydberg states were found to be metastable and to have lifetimes of more than 5 micros beyond n=50. Members of other series, such as the nd1(2), nd1(3), and the np1(0) series, were found to have lifetimes of more than 1 mus. Local perturbations induced by low-n Rydberg states belonging to series converging on rovibrationally excited levels of H2+ reduce the lifetimes in narrow ranges of n values. The hyperfine structure is strongly dependent on the value of the orbital angular momentum l. In the penetrating s and p states at n approximately 50 the exchange interaction dominates over the hyperfine interaction and the levels can be labeled by the total electron spin angular momentum quantum number S (S=0 or 1). In the less penetrating d and f Rydberg states, the hyperfine interaction between the core nuclear and electron spins is larger than the exchange interaction and the Rydberg states are of mixed singlet and triplet character. A procedure based on the Stark effect and on the systematic analysis of selection rules and combination differences was developed to determine the orbital and the total angular momentum quantum numbers l and F and to construct an energy map of p and f Rydberg levels between n=54 and 64 with relative positions of an accuracy of better than 1 MHz. Multichannel quantum defect theory (MQDT) was extended to treat the hyperfine structure in molecular Rydberg states and was used to analyze the observed hyperfine structure of the p and f Rydberg states of H2. The frame transformation between the Born-Oppenheimer channels described by the angular momentum coupling scheme (abetaJ) and the asymptotic channels described by the (e[bbetaS+]) coupling scheme was derived and enables an elegant treatment of all intermediate coupling cases. Purely ab initio quantum defect theory reproduced the experimentally determined positions to within 40 MHz for the p levels and 13 MHz for the f levels. By slight adjustments of the quantum defect functions and their energy dependences and by consideration of the p-f interaction, of the singlet-triplet splittings of the f levels, and of the departure of the ionic levels from pure coupling case (bbetaS+), the agreement between theory and experiment could be improved to 600 kHz. By comparing the results of MQDT calculations of the hyperfine structure of f Rydberg levels with those of coupled equations calculations, the frame transformation approximation of MQDT was shown to be accurate to within 300 kHz. The extrapolated ionic hyperfine structure of the X 2Sigmag+(v+=0,N+=1) ionic level corresponds to the ab initio prediciton of Babb and Dalgarno [Phys. Rev. A 46, R5317 (1992)] within the experimental error.     

Multiconfigurational perturbation theory (CASPT2) applied to the study of the low-lying singlet and triplet excited states of cyclopropene

The electronic spectrum of cyclopropene has been studied using multiconfigurational second-order perturbation theory (CASPT2) with extended ANO-type basis sets. The calculation comprises two valence states and the 3s, 3p, 3d members of the Rydberg series converging to the π and σ ionization limits. A total of twenty singlet and twenty triplet excited states have been analyzed. The results confirm the valence nature of the lowest energy singlet-singlet band and yield a conclusive assignment: the first dipole-allowed transition in cyclcopropene is due to absorption to a (σ → π*) state. The (π → π*) (V) state is interleaved among a number of Rydberg states in the most intense band of the system. The remaining spectral bands are due to Rydberg transitions of higher energy. The two lowest singlet-triplet transitions involve the same valence states. The results are in agreement with available experimental data and provide a number of new assignments of the experimental spectra.     

3dnl J=3e autoionizing levels of calcium: observation by laser optogalvanic spectroscopy and theoretical analysis

The even parityJ=3 autoionizing spectra of calcium were investigated below the 3d threshold by a two-step laser excitation from the 3d4s metastables through the 3d4p ³ P ₂,¹ F ₃ intermediate states. The 3d4s were populated by electronic collisions in a d.c. glow dis-charge through a Ca heat-pipe. More than a hundred resonance transitions have been measured with an accuracy of ～0.2 cm^?1 for the narrow ones using standard laser calibration techniques. Their upper levels are assigned to all expected nine autoionizing 3dns, 3dnd and 3dng Rydberg series and the 4p 5p ³ D ₃ perturber. The theoretical interpretation is achieved by a combination of the eigenchannelR-matrix and mulitchannel quantum defect (MQDT) methods. Ten closed and two openJ=3 interacting channesl are used. Theoretical energy level positions and excitation profiles are compared with the experimental data confirming the identification of the observed structures. Strong mixing between the 3d _3/2 nd _5/2 and 3d _5/2 nd _3/2 series is found, while the 3d _3/2 nd _3/2 and 3d _5/2 nd _5/2 series are almost purejj-coupled. The calculations were particularly successful in reproducing the spectrum in the region of the 4p 5p ³ D ₃ perturber, where strong chancel mixing appears leading to interference effects in the excitation cross sections.     

Interference effects in Stark spectra of weakly autoionising 5dnf states of barium

Weakly autoionising 5d _3/2 nf Rydberg states of barium around n = 60 have been studied in the presence of a static electric field. The experiment has been carried out in a CW laser-atomic-beam setup. In between the overlapping n = 60 and 61 angular momentum manifolds broad 5d63d resonances interact with the manifold states resulting in pronounced interferences. These interferences (anti-crossings) have been analysed by a direct diagonalisation procedure neglecting interactions with the continuum, and by a Multichannel Quantum Defect Theory (MQDT) analysis including continuum interactions.     

Odd Rydberg spectrum of20NeI: High resolution laser spectroscopy and multichannel quantum defect theory analysis

We have carried out a high resolution study of odd²⁰Ne(ns, nd J=0, 1, 2, 3, 4) Rydberg states, using transverse resonant two photon laser excitation of metastable²⁰Ne(3s, 1s ₅) atoms in a highly collimated beam. Transition energies of more than 500 levels with principal quantum numbers up ton=80 have been determined with an accuracy of better than 100 MHz relative to the metastable level. Energy independent MQDT-parameters have been extracted from fits to the experimental data near the²⁰Ne⁺(² P _3/2)-threshold. The simultaneous analysis of Rydberg series with different total angular momenta provides new insight into the Coulomb and exchange interactions governing the spectrum of neon.     

A precise determination of the first ionization potential of benzene

The first ionization potentials of benzene and benzene-d₆ have been precisely determined by the extrapolation of three-photon resonant Rydberg states in the four-photon ionization spectrum of the jet-cooled molecule. The convergence of resolved transitions in two Rydberg series for principal quantum numbers as high as 14 (-h₆) and 15 (-d₆) establish adiabatic thtesholds of 74573.0 ± 2.0 cm^?1, and 74592.5 = ± 1.2 cm^?1, respectively. These results are crucial for the understanding of the many excited states of benzene in terms of quantum defect theory. Precise quantum defects have been obtained for several Rydberg series and their variation with principal quantum number is reported. The results strongly suggest that the R? series of Wilkinson is derived from aπ(e_1g)→ nf±1 Rydberg excitation.     

Collision induced mixing of two light beams in Na2 molecules

We have studied the signal of two wave mixing in Na₂ molecules under high buffer gas pressure (10–100 Torr of He). The laser light was resonant to the Na₂ (X(v=3,J=43)→B(v′=6,J′=43) transition. The mixing signal appeared under high absorption conditions (370°–640 °C) and was collision induced. In the mixing process the transmitted intensity of the weaker beam was amplified at the cost of the stronger beam intensity. The influence of the dynamics of the ground state pumping process (depopulation by light absorption and repopulation by collisional relaxation) on the mixing process was analyzed.     

Quantum-mechanical phase interference and optical polarization in low-energy Na+Hg inelastic collisions

Cross sections for production of Hg(6³P₁) and Na(3²P) have been measured for low energy (?3 keV) Na⁺Hg collisions. An antiphase oscillatory structure in the energy dependence has been observed, and attributed to phase interference between charge-exchange and direct excitation channels. From the measurement of polarization of the Hg-resonance line, another pronounced antiphase oscillatory behaviour is found for the two cross sections for production of the magnetic sublevels, m_J=0 and m_J = ± 1, of the Hg(6³P₁) state.     

Microwave spectrum of P14N and P15N: Spectroscopic constants and molecular structure

In the present work the J+1←J, with J=1–3, 6, 7, 10–16, rotational transitions of P¹⁴N and the J+1←J, with J=1, 2, 7, 8, 10–14, rotational transitions of P¹⁵N have been observed in the millimeter- and submillimeter-wave region. These measurements allowed us to improve the ground state rotational parameters of P¹⁴N as well as provide those of P¹⁵N for the first time. The present measurements have been combined with rotational and vibrorotational transitions available in the literature to yield the Dunham coefficients. The equilibrium structure has also been determined to a high accuracy. The experimental investigation has been supported by highly accurate ab initio computations.     

Autoionization in diatomic molecules: An example of electrostatic autoionization in CO

A review is presented of the different types of autoionization which have been studied theoretically in diatomic molecules: electronic (or electrostatic), rotational, spin-orbit and vibrational autoionizations. An example involving a large number of vibrational channels, is treated. It concerns the electrostatic autoionization in the 730–708 Å wave-length region which appears in the CO photoionization spectrum. The structure observed at 721 Å is explained by the enhancement in intensity of numerous levels of Rydberg series converging to excited vibrational levels of theA ² II state caused by two autoionized Rydberg levels with aB ²Σ⁺ core.     

The photodissociation of molecules near the ionisation threshold

The photodissociation (PD) of the moleculeXY near the ionisation threshold is considered within the framework of the multichannel quantum defect theory. The general analytical dependence, determining the relation between the PD amplitude, the adiabatic amplitudes of dipole transitions and elements of the collisions matrix describing the associative ionisation (X+Y→e ^?+XY ⁺) and resonance scattering of atoms (X+Y→X′+Y′), has been obtained. The collisions matrix is determined by the system of algebraic equations and is expressed via the parameters which can be reconstructed from the molecule terms adiabatic picture. The obtained expressions can be used in case of a random number of Rydberg and dissociative channels. The role of the resonance mechanism of the molecules intermediate predissociative and autoionisational states population has been investigated. The peculiarities of the PD spectra structure for different physical situations have been studied. It has been shown that the Rydberg continuum states population increases the process general efficiency (the PD cross-section averaged over resonances increases). The NO molecule near-threshold PD, proceeding through thenpπ Rydberg states is calculated.     

Measurements on the quantum defect of the2 S Rydberg series and fine structure of the2 D Rydberg series in the gallium I spectrum

Level positions of members betweenn=25 andn=45 of the² S Rydberg series in Ga I were measured with high accuracy. The quantum defect of this series turns out to be constant over the region observed, indicating an unperturbed series. The value of the quantum defect is 2.791(2). The fine structure in the² D series was measured by using direct excitation. A qualitative explanation is given.     

The collision cross sections for the fine-structure mixing of caesium 6P levels induced by collisions with potassium atoms

The cross section for the fine-structure excitation transfer Cs(6P _1/2) → Cs(6P _3/2), induced by collisions with the ground state potassium atoms, has been measured by resonant Doppler-free two-photon spectroscopy. The population densities of caesium 6P _J (J=1/2, 3/2) levels were probed by thermionic detection of the collisionally ionized caesium atoms from the Cs(6P _J ) → Cs(10S _1/2) excitation channel. The cross section for the transfer process at the temperatureT=503 K has been found to be σ(1/2 → 3/2)=45 Ų ± 20%. The result is compared with previously published experimental cross sections for fine-structure transfer in resonance states of other alkali elements perturbed by potassium and a thoeretical value of the Li(2P _J )-K system calculated in a simple approach.     

Exchange interactions in a trigonal chromium (III) dimer. Optical spectroscopy of tris-(μ-hydroxo)-bis-[(1,4,7-trimethyl-1, 4,7-triazacyclononane) chromium(III)] triperchlorate

High-resolution absorption, luminescence, Zeeman and MCD experiments were performed on single crystals of the title compound. The singlet-triplet and triplet-quintet exchange splittings in the ground state are 138 and 267 cm^?1, respectively. The corresponding exchange parameters are 2J= ?128 cm^?1 and j = 1.6 cm^?1. The zero-field splitting in the ground level is ?2.25 cm^?1. The lowest-energy emitting state is a ³E state, split into four spinor components with a total spread of 8 cm^?1.     

Study of singlet-triplet coupling in glyoxal by level anticrossing spectroscopy. II. Theory of the positions of double resonances near a singlet-triplet anticrossing. Application to the precise measurement of singlet-triplet coupling and of fine and hyperfine structure parameters of the triplet of glyoxal

We set the theory for the position of double resonance signals near an anticrossing point. We use this theory to interpret our experimental results for anticrossings between the (K ≈ 6, J = 13) rotational level of the vibrationless 0° ¹A_u state of glyoxal and five sample rovibrational triplet levels, leading to very precise (up to a few 10^?4 on relative value) measurements of singlet-triplet coupling and fine and hyperfine structure parameters. The zero field singlet-triplet coupling matrix elements measured vary from 11.4±0.14 to 300.0±0.8 MHz. The hyperfine structure constant (82 MHz) is constant to within ±2% for the five triplet levels. The two fine structure parameters to the contrary vary by a factor of roughly two.     

Two-color multiphoton ionization spectra of jet-cooled p-difluorobenzene - s and d Rydberg states

Two-color multiphoton ionization spectra of jet-cooled p-difluorobenzene due to the transitions from S₁ state to highly excited Rydberg states have been observed. At least six different Rydberg series of s and d characters were found. The results indicate a reduction of molecular symmetry in the Rydberg state to C_2h. The Rydberg states belonging to different series exhibit different ionization behavior.     

Exchange interaction via hydrogen bridges in dimeric copper(II) complexes. A theoretical analysis

The singlet—triplet splitting (E_ST=2J₁₂) is calculated in dimeric [Cu(H₂O)(NH₃)₂(OH)]⁺ where the copper atoms are joined by two O-H…O bridges. This molecule acts as a model for a well investigated copper(II) complex (O…O 2.32(2) Å) which shows an antiferromagnetic spin coupling (2J₁₂ = −94 cm⁻¹). Calculations with different oxygen-oxygen distances (2.34–2.84 Å) show a nearly linear dependence of the singlet-triplet splitting which is far from being negligible even for the greater O-O distances (2.64–2.84 Å).