Showing papers on "Stark effect published in 2012"

Terahertz quantum cascade lasers operating up to ∼ 200 K with optimized oscillator strength and improved injection tunneling

Abstract: A new temperature performance record of 199.5 K for terahertz quantum cascade lasers is achieved by optimizing the lasing transition oscillator strength of the resonant phonon based three-well design. The optimum oscillator strength of 0.58 was found to be larger than that of the previous record (0.41) by Kumar et al. [Appl. Phys. Lett. 94, 131105 (2009)]. The choice of tunneling barrier thicknesses was determined with a simplified density matrix model, which converged towards higher tunneling coupling strengths than previously explored and nearly perfect alignment of the states across the injection and extraction barriers at the design electric field. At 8 K, the device showed a threshold current density of 1 kA/cm2, with a peak output power of ∼ 38 mW, and lasing frequency blue-shifting from 2.6 THz to 2.85 THz with increasing bias. The wavelength blue-shifted to 3.22 THz closer to the maximum operating temperature of 199.5 K, which corresponds to ∼ 1.28ħω/κB. The voltage dependence of laser frequency is related to the Stark effect of two intersubband transitions and is compared with the simulated gain spectra obtained by a Monte Carlo approach.

Persistent Spectral Hole-Burning: Science and Applications

Abstract: 1. Introduction.- 1.1 Fundamental Requirements for Persistent Spectral Hole-Burning.- 1.2 Significance for Science and Applications.- 1.3 Historical Overview and Survey of Mechanisms.- 1.4 Synopsis of the Book.- References.- 2. Basic Principles and Methods of Persistent Spectral Hole-Burning.- 2.1 Background.- 2.2 Homogeneous Spectrum of an Electron-Vibrational Transition.- 2.2.1 Integrated Intensities of Purely Electronic Lines and Phonon Sidebands, Electron-Phonon Interactions and Temperature Dependence.- 2.2.2 Relative Width (Q-factor) of PEL. Peak Intensities.- 2.2.3 Role of Local Modes.- 2.3 Inhomogeneous Broadening of the Vibronic Spectrum.- 2.3.1 Inhomogeneous Broadening of Purely Electronic Lines Inhomogeneous Distribution Function.- 2.3.2 Selectivity of the Spectral Response of an Inhomogeneous Absorption Band.- 2.3.3 Inhomogeneous Distribution Function Under Monochromatic Laser Excitation. Site-Selection Spectroscopy.- 2.4 Persistent Spectral Hole-Burning.- 2.4.1 Burning of Spectral Holes in the Inhomogeneous Distribution Function.- 2.4.2 Early Observations of Persistent Spectral Hole-Burning.- 2.5 Kinetics of Persistent Spectral Hole-Burning.- 2.6 Spectroscopic Applications.- 2.6.1 Homogeneous Zero-Phonon Line Broadening and Dephasing in Crystals.- 2.6.2 Photochemical Hole-Burning in Glassy Matrices.- 2.6.3 Homogeneous Linewidths of Vibronic Transitions and Relaxation.- 2.6.4 Off-Resonance Hole-Burning and Non-Correlation Effects.- 2.6.5 Hole-Burning in the Spectra of Chlorophyll-like Molecules.- 2.7 Special Methods of Hole-Burning and Detection.- 2.7.1 Detection of Holes by Doppler Scanning.- 2.7.2 Holographic Detection of Spectral Holes.- 2.7.3 Creation of Sharp Antiholes.- 2.8 Hole-Burning Time-and-Space-Domain Holography.- 2.8.1 Hole-Burning by Picosecond Pulses.- 2.8.2 Theory of Time-and-Space-Domain Holographic Recording and Playback.- 2.8.3 Experimental Results and Discussion.- 2.9 Concluding Remarks.- References.- 3. Photochemical Hole-Burning in Electronic Transitions.- 3.1 Photochemical, Photophysical, and Spin Hole-Burning.- 3.1.1 Historic Survey.- 3.1.2 Radiation-Induced Saturation Versus Chemical Depletion.- a) Transient Saturation.- b) Chemical Depletion.- 3.1.3 Photochemical Systems and Mechanisms.- 3.2 Spectroscopic Analysis of Hole-Burning Experiments.- 3.2.1 General Remarks.- 3.2.2 Fast Relaxation Processes and Excited State Dephasing.- a) Lineshape Analysis.- b) Temperature Dependence of the "Homogeneous" Linewidth.- 3.2.3 Spectral Diffusion in Glasses.- a) TLS Parameters and Tunnelling Rates.- b) Spectroscopic Parameters.- 3.3 Field Effects in Hole-Burning Spectroscopy.- 3.3.1 Introduction: The Site Memory Function.- 3.3.2 Electric-Field Effects.- a) Stark Effect for Molecules with Inversion Symmetry.- b) Stark Effect for Molecules Without Inversion Symmetry.- 3.3.3 Strain-Field Effects.- References.- 4. Persistent Spectral Hole-Burning in Inorganic Materials.- 4.1 Introduction.- 4.2 Hole-Burning Mechanisms.- 4.3 Color Centers.- 4.4 Rare Earth Compounds.- 4.4.1 Trivalent Rare Earth Ions in Glasses.- 4.4.2 Divalent Rare Earth Ions in Crystals.- a) CaF2:Sm2+.- b) SrF2:Sm2+.- c) BaClF:Sm2+.- 4.5 Transition Metal Ions.- 4.5.1 LiGa5 O8:Co2+.- 4.5.2 Y3Al5O12:Ti3+.- 4.6 Conclusion.- References.- 5. Two-Level-System Relaxation in Amorphous Solids as Probed by Nonphotochemical Hole-Burning in Electronic Transitions.- 5.1 Background.- 5.2 Survey of NPHB Systems.- 5.2.1 Hydrogen-Bonded Crystals.- 5.2.2 Molecules in Amorphous Polyacene Films.- 5.2.3 Molecules in Organic Glasses.- 5.2.4 Molecules in Polymers.- 5.2.5 Rare-Earth Ions in Glasses and Polymers.- 5.3 Optical Linewidths and Dephasing in Amorphous Solids.- 5.3.1 Single-Impurity Single-TLS System Hamiltonian.- 5.3.2 Optical Dephasing due to Off-Diagonal Modulation.- 5.3.3 Recent Experiments.- 5.3.4 New Theories.- 5.3.5 Comparison of Theories and Experimental Data.- 5.3.6 Hole Widths and TLS Relaxation Processes in Organic Systems.- 5.4 Density of States Functions for TLS.- 5.5 Laser-Induced Hole Filling.- 5.5.1 Rhodamine 640 in Poly(vinylalcohol).- 5.5.2 Nd3+ and Pr3+ in Poly(vinylalcohol).- 5.5.3 A Tentative Model for LIHF.- 5.6 Recent Developments.- 5.7 Concluding Remarks.- References.- 6. Persistent Infrared Spectral Hole-Burning for Impurity Vibrational Modes in Solids.- 6.1 Introduction.- 6.1.1 Matrix-Isolated Molecules in Van der Waals and Ionic Solids.- 6.1.2 Persistent IR Hole-Burning in Vibrational Modes.- 6.2 Molecules in Van der Waals Matrices.- 6.2.1 1,2-Difluorethane (DFE).- a) Diode Laser Measurements.- b) CO2 Laser Measurements.- 6.2.2 Interpretation of Persistence.- 6.2.3 Molecular Aggregates of Methyl Nitrite or Methanol.- 6.3 ReO4? in Alkali Halide Crystals.- 6.3.1 Background and Spectroscopic Information.- 6.3.2 Measurements of Relaxation Times T1 and T2.- 6.3.3 Persistent Spectral Holes for ReO4? in Alkali Halides.- a) Summary of Characteristics.- b) Model for the PIRSH Process.- 6.3.4 Persistent Spectral Pegs.- 6.3.5 Ultrasonic Studies of Multiple Ground State Configurations.- 6.3.6 Conclusions on the ReO4? System.- 6.4 Persistent Spectral Hole-Burning for CN? Molecules in Alkali Halide Crystals.- 6.4.1 Background Information on Matrix-Isolated CN?.- 6.4.2 High-Resolution FTIR Spectroscopy in the CN? Stretch Region.- 6.4.3 Hole-Burning in the CN? Stretch Mode Region.- 6.4.4 A Study of the CN?:Na+ Center Dynamics.- a) Fluorescence.- b) Hole-Burning and ?l Center Geometry.- 6.4.5 Other CN? Complexes.- 6.5 Conclusion.- 6.5.1 Comparison of the Three Types of Vibrational Hole-Burning Systems.- 6.5.2 Systems Which do not Exhibit PIRSH Formation.- a) Derivatives of the CN? Molecule.- b) Spherical-Top Molecules Which Contain Hydrogen.- 6.5.3 Future Prospects.- a) NO2? in Alkali Halides.- b) Disordered Solids.- References.- 7. Frequency Domain Optical Storage and Other Applications of Persistent Spectral Hole-Burning.- 7.1 Introduction.- 7.2 Systems Issues for Frequency Domain Optical Storage.- 7.2.1 General Remarks.- 7.2.2 Engineering Studies.- 7.3 Materials Research for Frequency Domain Optical Storage.- 7.3.1 General Materials Requirements.- 7.3.2 Limitations of Single-Photon Recording Mechanisms.- 7.3.3 Photon-Gated Mechanisms.- 7.3.4 Limitations on Storage Density.- 7.4 Alternative Data-Storage Configurations.- 7.4.1 Time Domain Storage.- 7.4.2 Electric-Field Readout.- 7.4.3 Holographic Readout.- 7.5 Other Applications of Persistent Spectral Hole-Burning.- 7.5.1 General Remarks.- 7.5.2 Laser Pulse Shaping Based on Fourier Synthesis.- 7.5.3 Laser Pulse Shaping Based on Voltage Modulation.- 7.5.4 Frequency Multiplexed Optical Spatial Filters.- 7.6 Summary and Future Prospects.- References.

High-Accuracy Optical Clock Based on the Octupole Transition in Yb + 171

Abstract: We experimentally investigate an optical frequency standard based on the 467 nm (642 THz) electric-octupole reference transition (2)S(1/2)(F=0)→(2)F(7/2)(F=3) in a single trapped (171)Yb(+) ion. The extraordinary features of this transition result from the long natural lifetime and from the 4f(13)6s(2) configuration of the upper state. The electric-quadrupole moment of the (2)F(7/2) state is measured as -0.041(5)ea(0)(2), where e is the elementary charge and a(0) the Bohr radius. We also obtain information on the differential scalar and tensorial components of the static polarizability and of the probe-light-induced ac Stark shift of the octupole transition. With a real-time extrapolation scheme that eliminates this shift, the unperturbed transition frequency is realized with a fractional uncertainty of 7.1×10(-17). The frequency is measured as 642 121 496 772 645.15(52) Hz.

Dynamic Stabilization of the Optical Resonances of Single Nitrogen-Vacancy Centers in Diamond

Abstract: We report electrical tuning by the Stark effect of the excited-state structure of single nitrogen-vacancy (NV) centers located ≲ 100 nm from the diamond surface. The zero-phonon line (ZPL) emission frequency is controllably varied over a range of 300 GHz. Using high-resolution emission spectroscopy, we observe electrical tuning of the strengths of both cycling and spin-altering transitions. Under resonant excitation, we apply dynamic feedback to stabilize the ZPL frequency. The transition is locked over several minutes and drifts of the peak position on timescales ≳ 100 ms are reduced to a fraction of the single-scan linewidth, with standard deviation as low as 16 MHz (obtained for an NV in bulk, ultrapure diamond). These techniques should improve the entanglement success probability in quantum communications protocols.

Subcycle ac stark shift of helium excited states probed with isolated attosecond pulses.

Abstract: Recent advances in attosecond science have relied upon the nearly instantaneous response of free electrons to an external field. However, it is still not clear whether bound electrons are able to rearrange on sublaser cycle time scales. Here, we probe the optical Stark shifts induced by a few-cycle near infrared laser field in helium bound states using isolated attosecond pulses in a transient absorption scheme and uncover a subcycle laser-induced energy level shift of the laser-dressed $1s3p$ state.

Surface-enhanced Raman trajectories on a nano-dumbbell: transition from field to charge transfer plasmons as the spheres fuse.

Abstract: By taking advantage of the tensor nature of surface-enhanced Raman scattering (SERS), we track trajectories of the linker molecule and a CO molecule chemisorbed at the hot spot of a nano-dumbbell consisting of dibenzyldithio-linked silver nanospheres. The linear Stark shift of CO serves as an absolute gauge of the local field, while the polyatomic spectra characterize the vector components of the local field. We identify surface-enhanced Raman optical activity due to a transient asperity in the nanojunction in an otherwise uneventful SERS trajectory. During fusion of the spheres, we observe sequential evolution of the enhanced spectra from dipole-coupled Raman to quadrupole- and magnetic dipole-coupled Raman, followed by a transition from line spectra to band spectra, and the full reversal of the sequence. From the spectrum of CO, the sequence can be understood to track the evolution of the junction plasmon resonance from dipolar to quadrupolar to charge transfer as a function of intersphere separation, which evolves at a speed of ∼1 A/min. The crossover to the conduction limit is marked by the transition of line spectra to Stark-broadened and shifted band spectra. As the junction closes on CO, the local field reaches 1 V/A, limited to a current of 1 electron per vibrational cycle passing through the molecule, with associated Raman enhancement factor via the charge transfer plasmon resonance of 10(12). The local field identifies that a sharp protrusion is responsible for room-temperature chemisorption of CO on silver. The asymmetric phototunneling junction, Ag-CO-Ag, driven by the frequency-tunable charge transfer plasmon of the dumbbell antenna, combines the design elements of an ideal rectifying photocollector.

Single Molecule Quantum-Confined Stark Effect Measurements of Semiconductor Nanoparticles at Room Temperature

Abstract: We measured the quantum-confined Stark effect (QCSE) of several types of fluorescent colloidal semiconductor quantum dots and nanorods at the single molecule level at room temperature. These measurements demonstrate the possible utility of these nanoparticles for local electric field (voltage) sensing on the nanoscale. Here we show that charge separation across one (or more) heterostructure interface(s) with type-II band alignment (and the associated induced dipole) is crucial for an enhanced QCSE. To further gain insight into the experimental results, we numerically solved the Schrodinger and Poisson equations under self-consistent field approximation, including dielectric inhomogeneities. Both calculations and experiments suggest that the degree of initial charge separation (and the associated exciton binding energy) determines the magnitude of the QCSE in these structures.

Characterization of an atmospheric helium plasma jet by relative and absolute optical emission spectroscopy

Abstract: The characteristics of plasma temperatures (gas temperature and electron excitation temperature) and electron density in a pulsed-dc excited atmospheric helium plasma jet are studied by relative and absolute optical emission spectroscopy (OES). High-resolution OES is performed for the helium and hydrogen lines for the determination of electron density through the Stark broadening mechanism. A superposition fitting method composed of two component profiles corresponding to two different electron densities is developed to fit the investigated lines. Electron densities of the orders of magnitude of 1021 and 1020 m−3 are characterized for the center and edge regions in the jet discharge when the applied voltage is higher than 13.0 kV. The atomic state distribution function (ASDF) of helium demonstrates that the discharge deviates from the Boltzmann–Saha equilibrium state, especially for the helium lower levels, which are significantly overpopulated. Local electron excitation temperatures T13 and Tspec corresponding to the lower and upper parts of the helium ASDF are defined and found to range from 1.2 eV to 1.4 eV and 0.2 eV to 0.3 eV, respectively. A comparative analysis shows that the Saha balance is valid in the discharge for helium atoms at high excited states.

Dynamical polarizability of atoms in arbitrary light fields: general theory and application to cesium

Abstract: We present a systematic derivation of the dynamical polarizability and the ac Stark shift of the ground and excited states of atoms interacting with a far-off-resonance light field of arbitrary polarization. We calculate the scalar, vector, and tensor polarizabilities of atomic cesium using resonance wavelengths and reduced matrix elements for a large number of transitions. We analyze the properties of the fictitious magnetic field produced by the vector polarizability in conjunction with the ellipticity of the polarization of the light field.

Experimental Quantification of Electrostatics in X–H···π Hydrogen Bonds

Abstract: Hydrogen bonds are ubiquitous in chemistry and biology. The physical forces that govern hydrogen-bonding interactions have been heavily debated, with much of the discussion focused on the relative contributions of electrostatic vs quantum mechanical effects. In principle, the vibrational Stark effect, the response of a vibrational mode to electric field, can provide an experimental method for parsing such interactions into their electrostatic and nonelectrostatic components. In a previous study we showed that, in the case of relatively weak O–H···π hydrogen bonds, the O–H bond displays a linear response to an electric field, and we exploited this response to demonstrate that the interactions are dominated by electrostatics (Saggu, M.; Levinson, N. M.; Boxer, S. G. J. Am. Chem. Soc.2011, 133, 17414–17419). Here we extend this work to other X–H···π interactions. We find that the response of the X–H vibrational probe to electric field appears to become increasingly nonlinear in the order O–H < N–H < S–H. The...

Multiorbital tunneling ionization of the CO molecule.

Abstract: We coincidently measure the molecular-frame photoelectron angular distribution and the ion sum-momentum distribution of single and double ionization of CO molecules by using circularly and elliptically polarized femtosecond laser pulses, respectively. The orientation dependent ionization rates for various kinetic energy releases allow us to individually identify the ionizations of multiple orbitals, ranging from the highest occupied to the next two lower-lying molecular orbitals for various channels observed in our experiments. Not only the emission of a single electron, but also the sequential tunneling dynamics of two electrons from multiple orbitals are traced step by step. Our results confirm that the shape of the ionizing orbitals determine the strong laser field tunneling ionization in the CO molecule, whereas the linear Stark effect plays a minor role.

Solvent-Induced Infrared Frequency Shifts in Aromatic Nitriles Are Quantitatively Described by the Vibrational Stark Effect

Abstract: The physical properties of solvents strongly affect the spectra of dissolved solutes, and this phenomenon can be exploited to gain insight into the solvent–solute interaction. The large solvatochromic shifts observed for many dye molecules in polar solvents are due to variations in the solvent reaction field, and these shifts are widely used to estimate the change in the dye’s dipole moment upon photoexcitation, which is typically on the order of ∼1–10 D. In contrast, the change in dipole moment for vibrational transitions is approximately 2 orders of magnitude smaller. Nonetheless, vibrational chromophores display significant solvatochromism, and the relative contributions of specific chemical interactions and electrostatic interactions are debated, complicating the interpretation of vibrational frequency shifts in complex systems such as proteins. Here we present a series of substituted benzonitriles that display widely varying degrees of vibrational solvatochromism. In most cases, this variation can be...

Ionization in elliptically polarized pulses: Multielectron polarization effects and asymmetry of photoelectron momentum distributions

Abstract: In the tunneling regime we present a semiclassical model of above-threshold ionization with inclusion of the Stark shift of the initial state, the Coulomb potential, and a polarization induced dipole potential. The model is used for the investigation of the photoelectron momentum distributions in close to circularly polarized light, and it is validated by comparison with ab initio results and experiments. The momentum distributions are shown to be highly sensitive to the tunneling exit point, the Coulomb force, and the dipole potential from the induced dipole in the atomic core. This multielectron potential affects both the exit point and the dynamics, as illustrated by calculations on Ar and Mg. Analytical estimates for the position of the maximum in the photoelectron distribution are presented, and the model is compared with other semiclassical approaches.

Spin-orbit-coupled dipolar Bose-Einstein condensates.

Abstract: We propose an experimental scheme to create spin-orbit coupling in spin-3 Cr atoms using Raman processes. By employing the linear Zeeman effect and optical Stark shift, two spin states within the ground electronic manifold are selected, which results in a pseudospin-1/2 model. We further study the ground state structures of a spin-orbit-coupled Cr condensate. We show that, in addition to the stripe structures induced by the spin-orbit coupling, the magnetic dipole-dipole interaction gives rise to the vortex phase, in which a spontaneous spin vortex is formed.

Dynamic interference of photoelectrons produced by high-frequency laser pulses.

Abstract: The ionization of an atom by a high-frequency intense laser pulse, where the energy of a single photon is sufficient to ionize the system, is investigated from first principles. It is shown that as a consequence of an ac Stark effect in the continuum, the energy of the photoelectron follows the envelope of the laser pulse. This is demonstrated to result in strong dynamic interference of the photoelectrons of the same kinetic energy emitted at different times. Numerically exact computations on the hydrogen atom demonstrate that the dynamic interference spectacularly modifies the photoionization process and is prominently manifested in the photoelectron spectrum by the appearance of a distinct multipeak pattern. The general theory is well approximated by explicit analytical expressions that allow for a transparent understanding of the discovered phenomena and for making predictions on the dependence of the measured spectrum on the pulse.

Electron Spin Resonance of Paramagnetic Crystals

Abstract: One. Paramagnetic Ions of Transition Elements.- 1. Paramagnetism of incomplete electronic shells.- 2. Level splitting of free paramagnetic ions. The Zeeman effect.- 3. Level splitting of free atoms in an external electric field. The Stark effect.- Two. Levels of Paramagnetic Ions in Crystal Lattices and Magnetic Fields.- 1. Macroscopic description of crystal systems. Symmetry of crystals.- 2. Symmetry point groups.- 3. Representations of groups.- 4. Group-theoretical classification of levels in fields of various symmetries.- 5. Application of the interaction Hamiltonian to the calculation of initial splitting.- 6. Paramagnetic ions in crystal fields. Splitting of energy levels.- 7. Qualitative picture of level splitting in a static magnetic field.- 8. Application of spin Hamiltonians. Angular dependence of EPR spectra.- Three. Electron Paramagnetic Resonance.- 1. Phenomenological treatment of EPR.- 2. Dynamic theory of paramagnetic resonance.- Four. EPR Line Shapes and line Widths.- 1. Dipole-dipole interactions.- 2. Exchange interactions.- 3. Application of the method of moments to analysis of EPR line shapes.- Five. Relaxation Processes in Paramagnetic Crystals.- 1. Spin-lattice relaxation.- 2. Relaxation associated with spin-spin interactions.- 3. Multiple cross-relaxation transitions and harmonic cross-relaxation.- 4. Level population changes due to cross-relaxation processes.- 5. Spin-lattice relaxation of ions of the 4f-and 3d-transition groups.- Six. Paramagnetic Single Crystals as Active Elements in Quantum Paramagnetic Amplifiers (QPA).- 1. Operating principles of QPA.- 2. Requirements from paramagnetic crystals in QPA.- 3. Formation of local symmetry centers in the synthesis of paramagnetic single crystals.

Observation and cancellation of a perturbing dc stark shift in strontium optical lattice clocks

Abstract: We report on the observation of a dc Stark frequency shift at the 10-13 level by comparing two strontium optical lattice clocks. This frequency shift arises from the presence of electric charges trapped on dielectric surfaces placed under vacuum close to the atomic sample. We show that these charges can be eliminated by shining UV light on the dielectric surfaces, and characterize the residual dc Stark frequency shift on the clock transition at the 10-18 level by applying an external electric field. This study shows that the dc Stark shift can play an important role in the accuracy budget of lattice clocks, and should be duly taken into account.

Electric-field-dependent photoconductivity in CdS nanowires and nanobelts: exciton ionization, Franz-Keldysh, and Stark effects.

Abstract: We report on the electric-field-dependent photoconductivity (PC) near the band-edge region of individual CdS nanowires and nanobelts. The quasi-periodic oscillations above the band edge in nanowires and nanobelts have been attributed to a Franz–Keldesh effect. The exciton peaks in PC spectra of the nanowires and thinner nanobelts show pronounced red-shifting due to the Stark effect as the electric field increases, while the exciton ionization is mainly facilitated by strong electron–longitudinal optical (LO) phonon coupling. However, the band-edge transition of thick nanobelts blue-shifts due to the field-enhanced exciton ionization, suggesting partial exciton ionization as the electron–LO phonon coupling is suppressed in the thicker belts. Large Stark shifts, up to 48 meV in the nanowire and 12 meV in the thinner nanobelts, have been achieved with a moderate electric field on the order of kV/cm, indicating a strong size and dimensionality implication due to confinement and surface depletion.

Electrostatic spin crossover in a molecular junction of a single-molecule magnet Fe{2}.

Abstract: Spin crossover by means of an electric bias is investigated by spin-polarized density-functional theory calculations combined with the Keldysh nonequilibrium Green's technique in a molecular junction, where an individual single-molecule magnet ${\mathrm{Fe}}_{2}(\mathrm{acpybutO})({\mathrm{O}}_{2}\mathrm{CMe})(\mathrm{NCS}{)}_{2}$ is sandwiched between two infinite Au(100) nanoelectrodes. Our study demonstrates that the spin crossover, based on the Stark effect, is achieved in this molecular junction under an electric bias but not in the isolated molecule under external electric fields. The main reason is that the polarizability of the molecular junction has an opposite sign to that of the isolated molecule, and thus from the Stark effect the condition for the spin crossover in the molecular junction is contrary to that in the isolated molecule.

Scattering resonances in slow NH3-He collisions

Abstract: We theoretically study slow collisions of NH3 molecules with He atoms, where we focus in particular on the observation of scattering resonances. We calculate state-to-state integral and differential cross sections for collision energies ranging from 10−4 cm−1 to 130 cm−1, using fully converged quantum close-coupling calculations. To describe the interaction between the NH3 molecules and the He atoms, we present a four-dimensional potential energy surface, based on an accurate fit of 4180 ab initio points. Prior to collision, we consider the ammonia molecules to be in their antisymmetric umbrella state with angular momentum j = 1 and projection k = 1, which is a suitable state for Stark deceleration. We find pronounced shape and Feshbach resonances, especially for inelastic collisions into the symmetric umbrella state with j = k = 1. We analyze the observed resonant structures in detail by looking at scattering wavefunctions, phase shifts, and lifetimes. Finally, we discuss the prospects for observing the ...

Orientation-dependent ionization yields from strong-field ionization of fixed-in-space linear and asymmetric top molecules

Abstract: The yield of strong-field ionization, by a linearly polarized probe pulse, is studied experimentally and theoretically as a function of the relative orientation between the laser field and the molecule. Experimentally, carbonyl sulphide (OCS), benzonitrile and naphthalene molecules are aligned in one or three dimensions before being singly ionized by a 30 fs laser pulse centred at 800 nm. Theoretically, we address the behaviour of these three molecules. We consider the degree of alignment and orientation and model the angular dependence of the total ionization yield by molecular tunnelling theory accounting for the Stark shift of the energy level of the ionizing orbital. For naphthalene and benzonitrile, the orientational dependence of the ionization yield agrees well with the calculated results, in particular, we observe that ionization is maximized when the probe laser is polarized along the most polarizable axis. For OCS the observation of the maximum ionization yield when the probe is perpendicular to the internuclear axis contrasts the theoretical results.

Modulating the bandgaps of graphdiyne nanoribbons by transverse electric fields.

Abstract: The effect of external transverse electric fields on the bandgaps of graphdiyne nanoribbons is investigated from first-principles calculations. The giant Stark effect is observed in the ribbons. When the field is applied, the valence and conduction band edge states are found to be strongly localized at low and high potential edges of the ribbon, respectively. Due to the wavefunction localization, the bandgap decreases with increasing field strength, and a semiconductor–metal transition occurs below a threshold field value. It is also shown that the bandgap decreasing rate depends linearly on the ribbon width. The tunable bandgap of a graphdiyne nanoribbon under an electric field would be helpful for practical applications.

Vibrational stark effect of the electric-field reporter 4-mercaptobenzonitrile as a tool for investigating electrostatics at electrode/SAM/solution interfaces.

Abstract: 4-mercaptobenzonitrile (MBN) in self-assembled monolayers (SAMs) on Au and Ag electrodes was studied by surface enhanced infrared absorption and Raman spectroscopy, to correlate the nitrile stretching frequency with the local electric field exploiting the vibrational Stark effect (VSE). Using MBN SAMs in different metal/SAM interfaces, we sorted out the main factors controlling the nitrile stretching frequency, which comprise, in addition to external electric fields, the metal-MBN bond, the surface potential, and hydrogen bond interactions. On the basis of the linear relationships between the nitrile stretching and the electrode potential, an electrostatic description of the interfacial potential distribution is presented that allows for determining the electric field strengths on the SAM surface, as well as the effective potential of zero-charge of the SAM-coated metal. Comparing this latter quantity with calculated values derived from literature data, we note a very good agreement for Au/MBN but distinct deviations for Ag/MBN which may reflect either the approximations and simplifications of the model or the uncertainty in reported structural parameters for Ag/MBN. The present electrostatic model consistently explains the electric field strengths for MBN SAMs on Ag and Au as well as for thiophenol and mercaptohexanoic acid SAMs with MBN incorporated as a VSE reporter.

Measurement of Plasma Parameters in Laser-Induced Breakdown Spectroscopy Using Si-Lines

Abstract: The electron density and temperature of the laser induced silicon plasma were measured using two different methods. The plasma was produced via the interaction of high peak power Nd-YAG laser at the fundamental wavelength of 1064 nm with a plane solid iron target contain small traces of silicon as an element of minor concentration. The lines from the Si I at 288.15 nm and Si II-ionic lines at 413.08 and 634.71 nm were utilized to evaluate the plasma parameters. The reference plasma parameters were measured utilizing the Hα-line at 656.27 nm appeared in the spectra under the same condition. The electron density was measured utilizing the Stark broadening of the silicon lines and the temperature from the standard Saha-Boltzmann plot method. The comparison between electron densities from different silicon lines to that from the Hα-line reveals that the Si I-line at 288.15 nm contain some optical thickness while the Si II-ionic lines were found to be free from this effect. The measurements were repeated at different delay times between the laser and the camera in the range from 1 - 5 μsec. The electron density was found decreases from 2 × 1018 down to 4 × 1017 cm–3. After correcting the spectral intensity at the Si I-line at 288.15 nm, the temperatures evaluated from the different methods were found in an excellent agreement and decreases from 1.25 down to 0.95 eV with delay time.

Ultrafast dynamics of the indoline dye D149 on electrodeposited ZnO and sintered ZrO2 and TiO2 thin films

Abstract: The ultrafast photoinjection and subsequent relaxation steps of the indoline dye D149 were investigated in detail for a mesoporous electrodeposited ZnO thin film and compared with experiments on sintered TiO2 and ZrO2 thin films, all in contact with air, using pump–supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370–770 nm. D149 efficiently injects electrons into the ZnO surface with time constants from ≤70 fs (time-resolution-limited) up to 250 fs, without the presence of slower components. Subsequent spectral dynamics with a time constant of 20 ps and no accompanying change in the oscillator strength are assigned to a transient Stark shift of the electronic absorption spectrum of D149 molecules in the electronic ground state due to the local electric field exerted by the D149˙+ radical cations and conduction band electrons in ZnO. This interpretation is consistent with the shape of the relaxed PSCP spectrum at long times, which resembles the first derivative of the inverted steady-state absorption spectrum of D149. In addition, steady-state difference absorption spectra of D149˙+ in solution from spectroelectrochemistry display a bleach band with distinctly different position, because no first-order Stark effect is present in that case. Interference features in the PSCP spectra probably arise from a change of the refractive index of ZnO caused by the injected electrons. The 20 ps component in the PSCP spectra is likely a manifestation of the transition from an initially formed bound D149˙+–electron complex to isolated D149˙+ and mobile electrons in the ZnO conduction band (which changes the external electric field experienced by D149) and possibly also reorientational motion of D149 molecules in response to the electric field. We identify additional spectral dynamics on a similar timescale, arising from vibrational relaxation of D149˙+ by interactions with ZnO. TiO2 exhibits similar dynamics to ZnO. In the case of ZrO2, electron injection accesses trap states, which exhibit a substantial probability for charge recombination. No Stark shift is observed in this case. In addition, the spectroelectrochemical experiments for D149˙+ in dichloromethane and acetonitrile, which cover the spectral range up to 2000 nm, provide for the first time access to its complete D0 → D1 absorption band, with the peak located at 1250 and 1055 nm, respectively. Good agreement is obtained with results from DFT/TDDFT calculations of the D149˙+ spectrum employing the MPW1K functional.

Dynamic Stark Effect in Strongly Coupled Microcavity Exciton Polaritons

Abstract: We present experimental observations of a nonresonant dynamic Stark shift in strongly coupled microcavity quantum well exciton polaritons--a system which provides a rich variety of solid-state collective phenomena. The Stark effect is demonstrated in a GaAs/AlGaAs system at 10 K by femtosecond pump-probe measurements, with the blueshift approaching the meV scale for a pump fluence of 2 mJ cm(-2) and 50 meV red detuning, in good agreement with theory. The energy level structure of the strongly coupled polariton Rabi doublet remains unaffected by the blueshift. The demonstrated effect should allow generation of ultrafast density-independent potentials and imprinting well-defined phase profiles on polariton condensates, providing a powerful tool for manipulation of these condensates, similar to dipole potentials in cold-atom systems.

Quantum-confined stark effect in localized luminescent centers within InGaN/GaN quantum-well based light emitting diodes

Abstract: The nature of the polarization-field in disorder induced nanoscale potential fluctuations (radiative traps) within (In,Ga)N based quantum-well (QW) heterostructures remains ambiguous. Spectrally resolved photoluminescence microscopy has been utilized to probe the local polarization field by monitoring the extent of quantum-confined Stark effect (QCSE) in radiative trap centers spontaneously formed within an (In,Ga)N QW based light emitting diode. Interestingly, two distinct categories of nanoscale radiative domains, which arise from indium compositional and interface-morphology related fluctuations of the active layers, are found to have very different degree of built-in polarization fields. Screening of QCSE in indium-rich emission centers results in blue-shift of transition energies by up to 400 meV, significantly higher than that reported previously for group III-nitride based semiconductor heterostructures. A lack of correlation between the extent of QCSE and local indium mole-fractions suggests that ...

Electrostatic hexapole state-selection of the asymmetric-top molecule propylene oxide: Rotational and orientational distributions

Abstract: This theoretical study is complementary to previous experimental work (see D.-C. Che, F. Palazzetti, Y. Okuno, V. Aquilanti, T. Kasai, J. Phys. Chem. A 114 (2010) 3280) on the orientation and rotational state-selection of supersonic molecular beams of the asymmetric-top molecule propylene oxide, both pure and seeded (in He and in Ar) by using an 85-cm length hexapole state-selector. One objective is to obtain an accurate distribution of the rotational states after hexapole selection for the three molecular beams, the most relevant feature consisting in the evaluation of the variation in energy of the manifold of rotational states when an electric field is applied (the Stark forces). Previously, the Stark effect on the effective dipole moment was considered through second order for all rotational states, while in this work the energy derivatives of the rotational states with respect to the applied electric field, are obtained accurately by diagonalizing the very large Stark matrices, whose elements depend on the dipole moment components of the molecule. The Stark energies and the corresponding forces were calculated for values of the electric field between 0 and 80 kV cm −1 in steps of 0.5 kV cm −1 and then linearly interpolated, covering the whole experimental range of the hexapole. A treatment is given for the intricate pattern of avoided crossings among derivatives of the rotational levels and two limiting cases were considered, corresponding to transitions occurring either adiabatically or diabatically. The two treatments lead to slightly different distributions of the rotational states for the pure and Ar seeded molecular beams, while for the He seeded molecular beam the two distributions are substantially the same. This experimental arrangement is a perspective tool for experiments of photochemistry and scattering of oriented molecules and clusters, and therefore we calculated the orientational distributions in a configuration where a uniform electric field is placed between the hexapole field and the detector.

Measurement of the temporal evolution of electron density in a nanosecond pulsed argon microplasma: using both Stark broadening and an OES line-ratio method

Abstract: The temporal evolution of electron density in a nanosecond pulsed argon microplasma is measured using a combination of Stark broadening and the optical emission line-ratio method. In the initial discharge period (0?100?ns), the electron density can reach as high as ?1018?cm?3. It decreases to ?1017?1016?cm?3 in the early afterglow period (100?ns?1??s after the ignition) and ?1016?1013?cm?3 in the late afterglow period (1?20??s). It is demonstrated that the optical emission spectroscopy (OES) line-ratio method can obtain the electron density in the range 1013?1016?cm?3, while in the range 1016?1018?cm?3, the Stark broadening technique with argon 2p?1s lines (in Paschen's notation) is a better choice. These results are in good agreement with those from the Stark broadening technique with hydrogen Balmer lines. Finally, a possible mechanism for such a density evolution is briefly discussed.