Fourier transform emission spectroscopy of new infrared systems of LaH and LaD

On the highest oxidation states of the actinoids in AnO4 molecules (An = Ac - Cm): A DMRG-CASSCF study

Actinoid tetroxide molecules AnO₄ (An = Ac - Cm) are investigated with the ab initio density matrix renormalization group (DMRG) approach. Natural orbital shapes are used to read out the oxidation state (OS) of the f-elements, and the atomic orbital energies and radii are used to explain the trends. The highest OSs reveal a "volcano"-type variation: For An = Ac - Np, the OSs are equal to the number of available valence electrons, that is, Ac^III , Th^IV , Pa^V , U^VI , and Np^VII . Starting with plutonium as the turning point, the highest OSs in the most stable AnO₄ isomers then decrease as Pu^V , Am^V , and Cm^III , indicating that the 5f-electrons are hard to be fully oxidized off from Pu onward. The variations are related to the actinoid contraction and to the 5f-covalency characteristics. Combined with previous work on OSs, we review their general trends throughout the periodic table, providing fundamental understanding of OS-relevant phenomena.

Monovalent lanthanide(I) in borozene complexes

Lanthanide (Ln) elements are generally found in the oxidation state +II or +III, and a few examples of +IV and +V compounds have also been reported. In contrast, monovalent Ln(+I) complexes remain scarce. Here we combine photoelectron spectroscopy and theoretical calculations to study Ln-doped octa-boron clusters (LnB₈⁻, Ln = La, Pr, Tb, Tm, Yb) with the rare +I oxidation state. The global minimum of the LnB₈⁻ species changes from C_s to C_7v symmetry accompanied by an oxidation-state change from +III to +I from the early to late lanthanides. All the C_7v-LnB₈⁻ clusters can be viewed as a monovalent Ln(I) coordinated by a η⁸-B₈²⁻ doubly aromatic ligand. The B₇³⁻, B₈²⁻, and B₉⁻ series of aromatic boron clusters are analogous to the classical aromatic hydrocarbon molecules, C₅H₅⁻, C₆H₆, and C₇H₇⁺, respectively, with similar trends of size and charge state and they are named collectively as “borozenes”. Lanthanides with variable oxidation states and magnetic properties may be formed with different borozenes.

The most common oxidation state for lanthanides is +3. Here the authors use photoelectron spectroscopy and theoretical calculations to study half-sandwich complexes where a lanthanide center in the oxidation state +1 is bound to an aromatic wheel-like B₈^2- ligand.

Theoretical calculation of the low-lying electronic states of the LaH molecule

The potential energy curves of the low-lying electronic states of the LaH molecule are reported via the CASSCF method with multireference calculations (single and double excitations with Davidson corrections). Twenty-four low-lying electronic states of the LaH molecule in the representation 2s+1Λ(+/−) below 20 000 cm−1 were investigated along with five lower electronic states in the Ω representation. The harmonic frequency ωe, the equilibrium internuclear distance Re, the rotational constants Be, and the electronic energy with respect to the ground state Te were calculated for these states. Twelve new electronic states are investigated in the present work for the first time that have not yet been observed experimentally. Using the canonical functions approach, the eigenvalues Ev, the rotational constants Bv, the centrifugal distortion constants Dv, and the abscissas of the turning points Rmin and Rmax were calculated for the investigated electronic states up to vibrational level v = 43.

Energy linkage between the singlet and triplet manifolds in LaH, and observation of new low-energy states

Wavelength-resolved fluorescence spectra of jet-cooled LaH were obtained from D1, E1, and 0(+)((3)Σ(-)) states by exciting isolated rotational levels. The observation of a(3)Δ(1) and a(3)Δ(2) states at 1259.5(5) and 1646(1) cm(-1), respectively, established the missing energy link between the singlet and triplet manifolds. The low-energy b(3)Π(0,1) and B(1)Δ(2) states predicted earlier from ab initio studies were also observed for the first time. Vibrational constants ω(e) = 1418(2) cm(-1), ω(e)x(e) = 15.6(7) cm(-1) for the ground and ΔG(1∕2) = 1326.1(7) and 1312(1) cm(-1), respectively, for the a(3)Δ(1) and b(3)Π(1) states were also determined. Vibrational frequencies were found to be in excellent agreement with earlier ab initio values. However, ab initio term energies and spin-orbit separation of (3)Δ(2)-(3)Δ(1) and (3)Π(1)-(3)Π(0) were found to be in poor agreement with the present observations. Also, the (3)Π state that was predicted to be inverted is observed to be regular.

Fourier transform emission spectroscopy of YH and YD: observation of new A1Δ and B1Π electronic states

The emission spectra of YH and YD molecules have been investigated in the 3600-12,000 cm(-1) region using a Fourier transform spectrometer. Molecules were formed in an yttrium hollow cathode lamp operated with a continuous flow of a mixture of Ne and Ar gases, and YH and YD were observed together in the same spectra. A group of bands observed near 1 μm have been identified as 0-0 and 1-1 bands of the A(1)Δ-X(1)Σ(+) and B(1)Π-X(1)Σ(+) transitions of YH and the 0-0 bands of the same two transitions for YD. The A(1)Δ and B(1)Π states of YH are separated by only about 12 cm(-1) and are involved in strong interactions. A perturbation analysis has been performed using the PGOPHER program to fit the two interacting electronic states and spectroscopic parameters for the A(1)Δ and B(1)Π states, including the interaction matrix elements, have been obtained for the first time.

All-Electron Scalar Relativistic Basis Sets for the Lanthanides

Segmented all-electron relativistically contracted (SARC) basis sets are constructed for the elements 57La-71Lu and optimized for density functional theory (DFT) applications. The basis sets are intended for use in combination with the DKH2 or ZORA scalar relativistic Hamiltonians for which individually optimized contractions are provided. Significant computational advantages can be realized owing to the loose contraction of the SARC basis sets compared to generally contracted basis sets, while their compact size allows them to replace effective core potentials for routine studies of lanthanide complexes. The new basis sets are evaluated in DFT calculations of the first four ionization energies of the lanthanides. They yield results that accurately reproduce the experimental trends, confirming a balanced treatment of different electronic configurations. The performance of the basis sets is further assessed in molecular systems with a comprehensive study of the lanthanide trihalides. Despite their compact size, the SARC basis sets demonstrate consistent, efficient, and reliable performance and will be especially useful in calculations of molecular properties that require explicit treatment of the core electrons.

Spectroscopic manifestations of the Kondo effect on single adatoms

The present topical review focuses on recent advances concerning an intriguing phenomenon in condensed matter physics, the scattering of conduction electrons at the localized spin of a magnetic impurity: the Kondo effect. Spectroscopic signatures of this effect have been observed in the past by high-resolution photoemission which, however, has the drawback of averaging over a typical surface area of 1 mm(2). By combining the atomic-scale spatial resolution of the scanning tunneling microscope (STM) with an energy resolution of a few tens of µeV achievable nowadays in scanning tunneling spectroscopy (STS), and by exposing the magnetic adatom to external magnetic fields, our understanding of the interaction of a single magnetic impurity with the conduction electrons of the nonmagnetic host has been considerably deepened. New insight has emerged by taking advantage of quantum size effects in the metallic support and by decoupling the magnetic adatom from the supporting host metal, for instance by embedding it inside a molecule or by separating it by an ultrathin insulating film from the metal surface. In this way, Kondo resonances and Kondo temperatures can be tailored and manipulated by changing the local density of states of the environment. In the weak coupling limit between a Kondo impurity and a superconductor only a convolution of tip and sample DOS is observed while for strongly coupled systems midgap states appear, indicating superconducting pair breaking. Magnetic impurities with co-adsorbed hydrogen on metallic surfaces show pseudo-Kondo resonances owing to very low-energy vibrational excitations detected by inelastic tunneling spectroscopy. One of the most recent achievements in the field has been the clarification of the role of magnetic anisotropy in the Kondo effect for localized spin systems with a spin larger than S = 1/2.

Diatomic molecular switches to enable the observation of very-low-energy vibrations

Using low-temperature scanning tunneling microscopy and spectroscopy, we found that the coadsorption of atomic hydrogen to single transition-metal and rare-earth-metal atoms on a Ag(100) surface gives rise to surprising phenomena, a bias dependent switching from a large to a small apparent size of the diatomic molecules and a concomitant appearance of very low-energy vibrational features of 3 to 7 meV in the differential conductance spectra. These phenomena, which have until now escaped observation, may be of general relevance for low-temperature adsorption.

Fourier Transform Emission Spectroscopy of ScH and ScD: The New Singlet Electronic States A 1 Delta, D 1 Pi, E 1 Delta, and F 1 Sigma-

The emission spectra of ScH and ScD have been investigated in the near infrared and visible using a Fourier transform spectrometer. The molecules were excited in a scandium hollow cathode lamp operated with neon gas and a trace of hydrogen or deuterium. Apart from the X 1 Sigma+ , B 1 Pi, C 1 Sigma+ , and G 1 Pi states reported previously [R. S. Ram and P. F. Bernath, J. Chem. Phys. 105, 2668-2674 (1996)], four additional new singlet electronic states A 1 Delta, D 1 Pi, E 1 Delta, and F 1 Sigma- have been identified below 21 000 cm-1 . The rotational analysis of several vibrational bands of the D 1 Pi-X 1 Sigma+ , D 1 Pi-A 1 Delta, E 1 Delta-B 1 Pi, E 1 Delta-A 1 Delta, and F 1 Sigma- -B 1 Pi transitions has been carried out and spectroscopic constants have been determined. The E 1 Delta state of ScH was not located because of the very weak intensity of the E 1 Delta-B 1 Pi and E 1 Delta-A 1 Delta transitions.