Search results for "Ground state"
showing 10 items of 928 documents
Dimers of the Higher-Energy Conformer of Formic Acid: Experimental Observation
2012
We report on the first experimental observation of formic acid dimers composed of two molecules of the higher-energy cis conformer. The cis–cis formic acid dimers are prepared in an argon matrix by selective vibrational excitation of the ground state trans conformer (deuterated form HCOOD) combined with thermal annealing of the matrix at about 30 K. Five cis–cis formic acid dimers are predicted by ab initio calculations (interaction energies from −16.9 to −27.2 kJ molˉ¹), and these structures are used for the assignment of the experimental spectra. Selective vibrational excitation of the obtained cis–cis dimers leads to the formation of several trans–cis dimers, which supports the proposed …
Magnetic hyperfine spectrum of isolated (199Hg)+ Ions
1978
The (199Hg)+ ions were stored in an RF quadrupole electric field and their ground state hyperfine populations pumped optically using the ion resonance radiation at 1942 A from a (202Hg) isotope enriched lamp. A microwave field at 40.5 GHz was phase-locked to the 1350th harmonic of the output of a frequency synthesizer, referenced to a Cs-beam frequency standard. Using a digital averager, a high background suppression optical system monitored the fluorescence light from the ions. Linewidths on the order of 3 Hz were observed for the “field-independent” 0-0 transition, and a value of 40, 507, 348, 051±50 Hz was deduced for the magnetic hyperfine interval, corrected to zero magnetic field.
Quantum gap and spin-wave excitations in the Kitaev model on a triangular lattice
2017
We study the effects of quantum fluctuations on the dynamical generation of a gap and on the evolution of the spin-wave spectra of a frustrated magnet on a triangular lattice with bond-dependent Ising couplings, analog of the Kitaev honeycomb model. The quantum fluctuations lift the subextensive degeneracy of the classical ground-state manifold by a quantum order-by-disorder mechanism. Nearest-neighbor chains remain decoupled and the surviving discrete degeneracy of the ground state is protected by a hidden model symmetry. We show how the four-spin interaction, emergent from the fluctuations, generates a spin gap shifting the nodal lines of the linear spin-wave spectrum to finite energies.
Oligothienoacenes versus oligothiophenes: Impact of ring fusion on the optical properties
2010
The impact of backbone rigidity on the optical properties of thiophene-based compounds is studied by analyzing in detail the geometrical, electronic, optical and vibronic features of a family of oligothienoacenes (nnTAs) in comparison to non-fused α-oligothiophenes (nnTs) by means of quantum-chemical calculations. Ring fusion in nnTAs provokes a greater conjugation in the ground state. However, the change in the bond length alternation upon electronic excitation is very similar in both systems, which is also reflected in a similar evolution of the first optical transition energy with increasing oligomer size. Larger transition energies in nnTAsvs.nnTs arise from an electronic effect rather …
Mid-infrared intersubband absorption in lattice-matched AlInN/GaN multiple-quantum wells
2005
We report the observation of midinfrared intersubband (ISB) absorption in nearly lattice-matched AlInNGaN multiple-quantum-wells. A clear absorption peak is observed around 3 μm involving transitions from the conduction band ground state to the first excited state. In addition to ISB absorption, photoluminescence experiments were carried out on lattice- matched AlInNGaN single quantum wells in order to determine the spontaneous polarization discontinuity between GaN and Al0.82 In0.18 N compounds. The experimental value is in good agreement with theoretical predictions. Our results demonstrate that the AlInNGaN system is very promising to achieve crack-free and low dislocation density struct…
Ultrafast internal conversion of excited cytosine via the lowest pipi electronic singlet state.
2003
Computational evidence at the CASPT2 level supports that the lowest excited state pipi* contributes to the S1/S0 crossing responsible for the ultrafast decay of singlet excited cytosine. The computed radiative lifetime, 33 ns, is consistent with the experimentally derived value, 40 ns. The nOpi* state does not play a direct role in the rapid repopulation of the ground state; it is involved in a S2/S1 crossing. Alternative mechanisms through excited states pisigma* or nNpi* are not competitive in cytosine.
Quantum chemical modelling of polarons and perovskite solid solutions
2001
Abstract Following our previous study [J. Phys.: Condens. Matter 10 (1998) 6271] of a single Nb impurity and Nb clusters in KTaO 3 , we present results of the calculations for a series of perovskite KNb x Ta 1− x O 3 (KTN) solid solutions ( x =0, 0.125, 0.25, 0.75, 1). The quantum chemical method of the intermediate neglect of the differential overlap (INDO) combined with the large unit cell (LUC) periodic model is used. According to the INDO calculations, Nb impurity becomes off-center in KTaO 3 already at the lowest studied concentrations ( x =0.125), in a good agreement with XAFS measurements. We compare our results with previous ab initio FP-LMTO calculations. Quantum chemical calculati…
55-Atom clusters of silver and gold: Symmetry breaking by relativistic effects
2006
Abstract Anionic 55-atom clusters of gold and silver are studied using density functional theory, scalar relativistic ab initio pseudopotentials and self-consistent generalized gradient corrections. An almost perfect icosahedron is found to be the clear ground state of Ag 55 - , and its electronic density of states agrees almost perfectly with recently measured high-resolution photoelectron spectra, up to the magnitude of the splitting of the highest free-electron shells by the Ih crystal field. A comparison between theory and a recent experiment allows one to assign icosahedral-based structures also for the Ag 57 - cluster. On the other hand, the Au 55 - cluster has several close-lying low…
Oxidation of small gas phase Pd clusters: A density functional study
2006
The adsorption sites of O2 on neutral PdN clusters (N = 1–4) were studied using spin density functional theory. Only for Pd1O2 molecular adsorption is found to be favorable. For Pd2–4O2 dissociative adsorption with the oxygen sitting on Pd bridge sites is preferred. Most Pd clusters remain in the same high spin states found for pure gas phase Pd clusters. Only the ground state of Pd4O2 increase its spin from a triplet to a quintet state. For molecular adsorption the O–O bond gets activated to a superoxo-like state.
Strong Instability of Ground States to a Fourth Order Schrödinger Equation
2019
Abstract In this note, we prove the instability by blow-up of the ground state solutions for a class of fourth order Schrödinger equations. This extends the first rigorous results on blowing-up solutions for the biharmonic nonlinear Schrödinger due to Boulenger and Lenzmann [8] and confirm numerical conjectures from [1–3, 11].