Entanglement of polar symmetric top molecules as candidate qubits

Proposals for quantum computing using rotational states of polar molecules as qubits have previously considered only diatomic molecules. For these the Stark effect is second-order, so a sizable external electric field is required to produce the requisite dipole moments in the laboratory frame. Here we consider use of polar symmetric top molecules. These offer advantages resulting from a first-order Stark effect, which renders the effective dipole moments nearly independent of the field strength. That permits use of much lower external field strengths for addressing sites. Moreover, for a particular choice of qubits, the electric dipole interactions become isomorphous with NMR systems for which many techniques enhancing logic gate operations have been developed. Also inviting is the wider chemical scope, since many symmetric top organic molecules provide options for auxiliary storage qubits in spin and hyperfine structure or in internal rotation states.     

Electric susceptibility of unsolvated glycine-based peptides

The DC electric susceptibilities of unsolvated glycine-based peptides, WGn (W = tryptophan and G = glycine) with n = 1-5, have been measured by deflection of a molecular beam in an electric field. These are the first electric deflection measurements performed on peptides. At 300 K the susceptibilities are in the range of 200-400 A(3). By far the largest contribution to the susceptibilities is from the permanent dipole moment of the peptides. The results indicate that the peptides do not have rigid conformations with fixed dipoles. Instead the dipole is averaged as the peptides explore their energy landscape. For a given WGn peptide, all molecules have almost the same average dipole, which suggests that they all explore a similar energy landscape on the microsecond time scale of the measurement. The measured susceptibilities are in good overall agreement with values calculated from the average dipole moment deduced from Monte Carlo simulations.     

Electric deflection studies on lead clusters

The dielectric response to an inhomogeneous electric field has been investigated for Pb(N) clusters (N=7-38) within a molecular beam experiment. The experiments give clear evidence that lead clusters with 12, 14, and 18 atoms possess permanent dipole moments. For these cluster sizes, the permanent electric dipole moments strongly determine the response to the electric field, leading to a significantly increased apparent polarizability. An adiabatic polarization mechanism allows a semiquantitative explanation of the observed susceptibility anomalies. The beam profiles of most of the lead clusters with N not equal12, 14, and 18 also display a small broadening induced by the electric field, indicating permanent dipole moments of about (0.01-0.02) D/atom. Nearly constant dipole moments per atom for larger lead clusters (N>20) manifest in a linear increase in the polarizability per atom. Also, for lead clusters such as Pb(25), which do exhibit almost no measurable beam broadening, the polarizabilties are increased compared to the bulk value. This could be partially explained by the electronic structure of the lead clusters but might be also a consequence of quenched permanent dipole moments because for highly flexible clusters only an increased beam deflection, but no broadening, will be observed.     

Orientation of dipole molecules and clusters upon adiabatic entry into an external field

The induced polarization of a beam of polar clusters or molecules passing through an electric or magnetic field region differs from the textbook Langevin-Debye susceptibility. This distinction, which is important for the interpretation of deflection and focusing experiments, arises because instead of acquiring thermal equilibrium in the field region, the beam ensemble typically enters the field adiabatically, i.e., with a previously fixed distribution of rotational states. We discuss the orientation of rigid symmetric top systems with a body-fixed electric or magnetic dipole moment. The analytical expression for their "adiabatic-entry" orientation is elucidated and compared with exact numerical results for a range of parameters. The differences between the polarization of thermodynamic and "adiabatic-entry" ensembles of prolate and oblate tops, and of symmetric top and linear rotators, are illustrated and identified.     

Asymmetric top rotors in electric fields. II. Influence of internal torsions in molecular beam deflection experiments

We report on electric deflection experiments of aminobenzonitrile and (dimethylamino)benzonitrile molecules. They are used as prototypes to study the influence of the asymmetry and rotation-vibration couplings in deflection experiments. Experimental deflection profiles are compared to results of ab initio calculations in the frame of the rigid rotor Stark effect and of the statistical linear response. The change in symmetry and the introduction of methyl groups lead to a transition from the rigid rotor response to the linear response. From the experimental results, a total dipole of mu = 6.2 +/- 0.6 D has been deduced for the m-(dimethylamino)benzonitrile molecule (MDMABN).     

The electric dipole moment of iridium monofluoride, IrF

The P(5) branch features of the A (3)Φ(4)←X (3)Φ(4) (1,0) band near 628.2 nm of a molecular beam of iridium monofluoride, (193)IrF, were recorded field free and in the presence of a static electric field. The (193)Ir (I(1)=3/2) and (19)F (I(2)=1/2) hyperfine interactions in the A (3)Φ(4) (υ=1) and X (3)Φ(4) (υ=0) states were analyzed. The permanent electric dipole moments, μ(el), for the A (3)Φ(4) (υ=1) and X (3)Φ(4) (υ=0) states were determined to be 1.88(5) and 2.82(6) D, respectively, from the analysis of the observed Stark shifts. A comparison of the electric dipole moments for IrC, IrN, and CoF is presented.     

Stark-selected beam of ground-state OCS molecules characterized by revivals of impulsive alignment

We make use of an inhomogeneous electrostatic dipole field to impart a quantum-state-dependent deflection to a pulsed beam of OCS molecules, and show that those molecules residing in the absolute ground state, X(1)Σ(+), |00(0)0>, J = 0, can be separated out by selecting the most deflected part of the molecular beam. Past the deflector, we irradiate the molecular beam by a linearly polarized pulsed nonresonant laser beam that impulsively aligns the OCS molecules. Their alignment, monitored via velocity-map imaging, is measured as a function of time, and the time dependence of the alignment is used to determine the quantum state composition of the beam. We find significant enhancements of the alignment ( = 0.84) and of state purity (>92%) for a state-selected, deflected beam compared with an undeflected beam.     

Matrix-induced Linear Stark Effect of Single Dibenzoterrylene Molecules in 2,3-Dibromonaphthalene Crystal

Absorption and fluorescence from single molecules can be tuned by applying an external electric field – a phenomenon known as the Stark effect. A linear Stark effect is associated to a lack of centrosymmetry of the guest in the host matrix. Centrosymmetric guests can display a linear Stark effect in disordered matrices, but the response of individual guest molecules is often relatively weak and non-uniform, with a broad distribution of the Stark coefficients. Here we introduce a novel single-molecule host-guest system, dibenzoterrylene (DBT) in 2,3-dibromonaphthalene (DBN) crystal. Fluorescent DBT molecules show excellent spectral stability with a large linear Stark effect, of the order of 1.5 GHz/kVcm⁻¹, corresponding to an electric dipole moment change of around 2 D. Remarkably, when the electric field is aligned with the a crystal axis, nearly all DBT molecules show either positive or negative Stark shifts with similar absolute values. These results are consistent with quantum chemistry calculations. Those indicate that DBT substitutes three DBN molecules along the a-axis, giving rise to eight equivalent embedding sites, related by the three glide planes of the orthorhombic crystal. The static dipole moment of DBT molecules is created by host-induced breaking of the inversion symmetry. This new host–guest system is promising for applications that require a high sensitivity of fluorescent emitters to electric fields, for example to probe weak electric fields.     

Thermally induced polarizabilities and dipole moments of small tin clusters

We study the influence of thermal excitation on the electric susceptibilities for Sn(6) and Sn(7) clusters by molecular beam electric deflection and Monte-Carlo simulations in conjunction with quantum-chemical calculations. At low temperatures (40 K), no field-induced broadening of the Sn(6) and Sn(7) cluster beams are observed, in agreement with vanishing permanent electric dipole moments due to their centro-symmetrical ground states. The electric polarizabilities of Sn(6) and Sn(7), as inferred from the field-induced molecular beam deflection, are in good agreement with the quantum-chemical predictions. At elevated temperatures of 50-100 K, increased polarizabilities of about 2-3 ?(3) are obtained. Also, we found indications of a field-induced beam broadening which points to the existence of permanent dipole moments of about 0.01-0.02 D per atom at higher temperatures. These results cannot be explained by thermal excitations within a harmonic oscillator model, which would yield a temperature-independent polarizability and fluxional, but not permanent, dipole moments. We analyze this behavior by Monte-Carlo simulations in order to compute average temperature-induced electric dipole moments. For that purpose, we developed a novel technique for predicting observables sampled on the quantum-chemical potential energy surface by an umbrella sampling correction of Monte-Carlo results obtained from simulations utilizing an empirical potential. The calculated, fluxional dipole moments are in tune with the observed beam broadenings. The cluster dynamics underlying the polarizability appear to be intermediate between rigid and floppy molecules which leads to the conclusion that the rotational, not the vibrational temperature seems to be the key parameter that determines the temperature dependence of the polarizability.     

Producing translationally cold, ground-state CO molecules

Carbon monoxide molecules in their electronic, vibrational, and rotational ground state are highly attractive for trapping experiments. The optical or ac electric traps that can be envisioned for these molecules will be very shallow, however, with depths in the sub-milliKelvin range. Here, we outline that the required samples of translationally cold CO (X(1)Σ(+), v' = 0, N' = 0) molecules can be produced after Stark deceleration of a beam of laser-prepared metastable CO (a(3)Π(1)) molecules followed by optical transfer of the metastable species to the ground state via perturbed levels in the A(1)Π state. The optical transfer scheme is experimentally demonstrated and the radiative lifetimes and the electric dipole moments of the intermediate levels are determined.     

Stark effect,polarizabilities and the electric dipole moment of heteronuclear diatomic molecules in 1Σ states

The theory of second-order Stark effect in ¹Σ states of heteronuclear diatomic molecules is thoroughly reviewed. The rigorous treatment given demonstrates that by introducing rotational, vibrational and electronic branch polarizabilities, the intrinsic character of the second-order Stark effect in diatomic molecules can be shown to be related more closely to polarizabilities than to dipole moments. The well-known expression for the Stark shift in ¹Σ levels which is dominated by the square of the dipole moment is only a crude, though sufficient approximation whenever large dipole moments are involved. For small dipole moments, however, this approximation is likely to fail, leading to an erroneous determination of such dipole moments. In the limiting case of negligible influence of the molecular rotation on the vibronic matrix elements, the arithmetic mean of the electronic branch polarizabilities turns out to be equal to the well-known static electronic polarizabilities α_∥ and α_⊥. The results are applied to the interpretation of the Stark splitting in the A¹Σ⁺, υ′ = 5, J′ = 1 level of ⁷LiH, recently determined by Stark quantum-beat spectroscopy.     

Rydberg atoms in crossed external magnetic and electric fields: Experimental observation of an outer potential minimum

Rydberg atoms in crossed magnetic and electric fields have been investigated. The existence of field induced bound states with a large electric dipole moment was demonstrated. For this purpose the highly excited atoms were deflected by means of an inhomogeneous electric field.     

Electric deflection studies of rhodium clusters

The static electric dipole polarizabilities of rhodium clusters Rhn, n=5-28, have been measured via a molecular beam deflection method. Uniform high-field beam deflections, indicative of induced polarization, were observed for all Rhn except Rh7 and Rh10 which by contrast exhibited beam broadening and anomalously high effective polarizabilities. Analysis of the beam deflection profile of Rh7 indicates that it possesses a permanent dipole moment of 0.24+/-0.02 D. Unlike the other clusters in the n=5-28 size range, the polarizability of Rh10 is observed to decrease with increasing source temperature. We attribute this temperature dependence to paraelectric behavior, suggesting that Rh10 is a fluxional molecule possessing a dipole moment that spatially fluctuates, uncorrelated with overall rotation.     

Asymmetric top rotors in electric fields: influence of chaos and collisions in molecular beam deflection experiments

We report on electric deflection experiments of structural isomers of aminobenzonitrile. They are used as models to study the influence of the asymmetry of a molecule on the molecule-electric field interaction. Experimental deflection profiles are compared to Stark effect calculations. We found increasing deviations from the calculated Stark effect behavior with increasing asymmetry. This deviation is induced by interactions with other particles and is directly related to the chaotic behavior of the rotational motion of asymmetric rotors in a static electric field.     

Electric deflection of rotating molecules

We provide a theory of the deflection of polar and nonpolar rotating molecules by inhomogeneous static electric field. Rainbowlike features in the angular distribution of the scattered molecules are analyzed in detail. Furthermore, we demonstrate that one may efficiently control the deflection process with the help of short and strong femtosecond laser pulses. In particular, the deflection process may be turned off by a proper excitation, and the angular dispersion of the deflected molecules can be substantially reduced. We study the problem both classically and quantum mechanically, taking into account the effects of strong deflecting field on the molecular rotations. In both treatments we arrive at the same conclusions. The suggested control scheme paves the way for many applications involving molecular focusing, guiding, and trapping by inhomogeneous fields.     

Stark quantum beat spectroscopy: The vibrational dependence of the dipole moment in the A1Σ+ state of BaO

The method of quantum beat spectroscopy following pulsed dye laser excitation is applied to measure electric field splittings in excited states of the ¹³⁸Ba¹⁶O molecule. Stark quantum beats were observed in the fluorescent decay of the A¹Σ⁺ (ν′ = 0, 1, 2, 3, J′ = 1) states. From the observed beat frequencies values of electric dipole moments in different vibrational states were derived. The results are: μ(ν′ = 0) = 2.98(7) D, μ(ν′ = 1) = 2.66(7) D, μ(ν′ = 2)3.15(8) D and μ(ν′ = 3) = 3.18(8) D.     

Electric field effects on water clusters (n = 3-5): systematic ab initio study of structures, energetics, and transition states

The structures, energetics, and transition states of water clusters (trimer to pentamer, n = 3-5) are investigated as a function of electric field by using ab initio calculations. With an increasing strength of the field, the most stable cyclic structures of trimer, tetramer, and pentamer open up to align their dipole moments along the direction of the field. For the lower strength (below 0.3 V/angstroms) of the electric field, the dipole moment of each water monomer is along the same direction with the field, while it retains the cyclic structure. For the higher strength of the field, to have a higher dipole moment for the cluster along the field direction, each cyclic structure opens up to form a linear chain or "water wire." We have investigated the transition state structures between the cyclic and linear forms for the field strengths of 0.3-0.4 V/angstroms where both cyclic and linear forms are energetically comparable.     

A molecular-beam optical Stark study of lines in the (1,0) band of the F4delta(7/2)-X4delta(7/2) transition of iron monohydride, FeH

A supersonic molecular beam of iron monohydride, FeH, has been generated using a laser ablation/chemical reaction scheme and probed at near-natural linewidth resolution by optical Stark spectroscopy utilizing laser-induced fluorescence detection. The observed Stark splitting in Q(3.5) and R(3.5) lines of the F4delta(7/2) <-- X4delta(7/2) (1,0) transition were analyzed to determine values for the magnitudes of the permanent electric dipole moments, absolute value(mu), which were found to be 2.63(3) and 1.29(3) D for the X4delta (v = 0) and F4delta (v = 1) states, respectively. A comparison with ab initio theoretical predictions is made. The lambda doubling in the low-J levels of the F4delta(7/2) (v = 1) state is also modeled.     

Stark beats of Lyman-α emission of foil-excited hydrogen atom

The Stark beats of Lyman-α emission due ton=1 ? 2 transition of hydrogen atom have been studied by the beam-foil method. After passage through a thin carbon foil, the static electric field of 500 V/cm was applied to the beam in the direction either parallel to or anti-parallel to the the beam velocity. The linearly polarized emission was measured by using a toroidal mirror at a Brewster's angle reflection. When the direction of the applied electric field is reversed, an appreciable phase shift was observed. The analysis of the data leads to the complete determination of the density matrix of the H(n=2) atoms at the time of their production.     

Dipolar and nonpolar altitudinal molecular rotors mounted on an Au(111) surface

We describe the preparation of a compound whose molecules consist of two metal sandwich stands carrying tentacles with affinity to metal surfaces and holding an axle that carries a dipolar or a nonpolar rotator. The dipolar rotor exists as three pairs of enantiomers, rapidly interconverting at room temperature. When mounted on a gold surface, each molecule represents a chiral altitudinal rotor, with the rotator axle parallel to the surface. The surface-mounted rotor molecules are characterized by several spectroscopic and imaging techniques. At any one time, in about one-third of the dipolar rotors the rotator is free to turn and the direction of its dipole can be flipped by the electric field applied by an STM tip, as revealed by differential barrier height imaging. Molecular dynamics calculations suggest that electric field normal to the surface causes members of one pair of enantiomers to rotate unidirectionally.