Showing papers on "Stark effect published in 1995"

Charge-resonance-enhanced ionization of diatomic molecular ions by intense lasers.

Abstract: We study the ionization of the ${\mathrm{H}}_{2}^{+}$ molecular ion in intense, short-pulse laser fields by numerically solving the three-dimensional time-dependent Schr\"odinger equation as a function of internuclear distance $R$. Anomalously high ionization for the molecular ion at large internuclear separations is observed for orientations parallel to the linearly polarized laser field. The ionization rate is found to exhibit maxima at large $R$, exceeding the atom limit by an order of magnitude. This is attributed to transitions between pairs of chargeresonant states which are strongly coupled by the field in diatomic molecular ions. The effect is shown to also occur in higher odd-charge diatomic molecular ions and can be attributed to field-induced nonadiabatic transitions between the charge-resonant states and electron tunneling suppression by the instantaneous Stark field of the laser.

Quantitative Analysis of Aluminum Alloys by Laser-Induced Breakdown Spectroscopy and Plasma Characterization

Abstract: Laser-induced breakdown spectroscopy has been applied to perform elemental analysis of aluminum alloy targets. The plasma is generated by focusing a pulsed Nd:YAG laser on the target in air at atmospheric pressure. Such a plasma was characterized in terms of its appearance, emission spectrum, space-integrated excitation temperature, and electron density. The electron density is inferred from the Stark broadening of the profiles of ionized aluminum lines. The temperature is obtained by using Boltzmann plots of the neutral iron lines. Calibration curves for magnesium, manganese, copper, and silicon were produced. The detection limits are element-dependent but are on the order of 10 ppm.

Excited States of the Bacteriochlorophyll b Dimer of Rhodopseudomonas viridis: A QM/MM Study of the Photosynthetic Reaction Center That Includes MM Polarization

Abstract: We present a hybrid quantum mechanicaYmolecular mechanical (QM/MM) model for microscopic solvation effects that includes polarizability in the MM region (QM/MMpol). QM/MMpol treatment of both ground and excited states is presented in the formalism. We present QM/MMpol analysis of the ground and electronic excited states of the bacteriochlorophyll b dimer (P) of the photosynthetic reaction center (RC) of Rhodopseudomonas viridis using the INDO/S method. We treat P and five adjacent amino acid side chains quantum mechanically, and the remainder of the protein, cofactors, and waters of crystallization with polarizable MM (325 QM atoms embedded in the field of 20 158 polarizable MM atoms). While dimer formation alone is enough to account for the majority of the monomer BChlb to P red-shift of the lowest electronic excited state of P (QyI), we demonstrate that explicit treatment of the protein is required to properly interpret the experimental Stark effect data that describe the charge transfer asymmetry of QYl. The static-charge potential from the MM model of the RC alone causes QYI to have significantly better agreement with the Stark effect results than isolated P. However, consideration of the protein polarization potential is further required to obtain more complete agreement with Stark effect experiments. Thus, we calculate a Q,I transition energy at 10 826 cm-I with a ground to excited state change in dipole moment of 4.8 D; an absorption Stark effect angle of 43"; a net shift of 0.15 electrons from the L subunit to the M subunit of P; and a linear dichroism angle (between the transition moment of QYl and the pseudo-C* axis of the RC) of 81". These results are in good agreement with experiment. Interestingly, we find that net CT increase is greater for QYl than for the second excited state of P (Qy2), a result that we anticipated in an early model dimer study.

Vibrational Stark Effect Spectroscopy

Abstract: The effects of electric fields on vibrational spectra are of fundamental importance for understanding the anharmonicity of chemical bonds. Furthennore, the sensitivity of vibrational transitions to electric fields can provide a local probe of electrostatic fields in ordered systems such as proteins, where many infrared studies are in progress. Despite this potential utility and a significant amount of theoretical work,' there are almost no experimental data.* In this Communication we describe a method for obtaining vibrational Stark effect (VSE) spectra under quite general conditions and report the results for a simple CN stretching vibration. The basic method is essentially identical to that used for electronic Stark spectroscopy on condensed phase system^,^ modified by changing the light source and detector for measurements in the infrared. Light from a Nemst glower (Ayers Eng., CA) was chopped, passed through a 0.25 m single monochromator (grating blazed at 4.0 pm, 150 grooves/mm, Ag-coated mirrors, resolution 3.5 cm-I), passed though the sample, and detected with a liquid nitrogen-cooled InSb detector (Grasby Infrared, 1 mm2, sapphire window). An optical filter was placed before the detector to minimize light due to higher order reflections from the grating. The sample cell was constructed from two Ni-coated (-60 A thick Ni) sapphire windows separated by a 25 pm Teflon spacer. The cell is under vacuum and in contact with a liquid nitrogen-cooled cold finger. A KRS-5 grid polarizer was inserted between the monochromator and the Dewar for polarization measurements. To minimize rotation of the polarization of light by the birefringent sapphire window, the o-ray (or e-ray) axis of the window and the polarization direction of the light were always kept in the same plane (horizontal). We have chosen the CN stretching vibration of anisonitrile because it is in a convenient wavelength region, it has a moderately strong IR absorption, and the band is simple. Figure 1 shows the IR spectrum of anisonitrile in a toluene glass at 77 K (top panel) and the VSE spectrum in the middle panel. It is evident that a good signal-to-noise ratio can be obtained.

Quantum-confined Stark effects in semiconductor quantum dots

Abstract: Quantum-confined Stark effects (QCSE) on excitons, i.e., the influence of a uniform electric field on the confined excitons in semiconductor quantum dots (QD's), have been studied by using a numerical matrix-diagonalization scheme. The energy levels and the wave functions of the ground and several excited states of excitons in CdS and ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Se}}_{\mathit{x}}$ quantum dots as functions of the size of the quantum dot and the applied electric field have been obtained. The electron and hole distributions and wave function overlap inside the QD's have also been calculated for different QD sizes and electric fields. It is found that the electron and hole wave function overlap decreases under an electric field, which implies an increased exciton recombination lifetime due to QCSE. The energy level redshift and the enhancement of the exciton recombination lifetime are due to the polarization of the electron-hole pair under the applied electric field.

X‐ray spectroscopy of hot solid density plasmas produced by subpicosecond high contrast laser pulses at 1018–1019 W/cm2

Abstract: Analysis is presented of K‐shell spectra obtained from solid density plasmas produced by a high contrast (1010:1) subpicosecond laser pulse (0.5 μm) at 1018–1019 W/cm2. Stark broadening measurements of He‐like and Li‐like lines are used to infer the mean electron density at which emission takes place. The measurements indicate that there is an optimum condition to produce x‐ray emission at solid density for a given isoelectronic sequence, and that the window of optimum conditions to obtain simultaneously the shortest and the brightest x‐ray pulse at a given wavelength is relatively narrow. Lower intensity produces a short x‐ray pulse but low brightness. The x‐ray yield (and also the energy fraction in hot electrons) increases with the laser intensity, but above some laser intensity (1018 W/cm2 for Al) the plasma is overdriven: during the expansion, the plasma is still hot enough to emit, so that emission occurs at lower density and lasts much longer. Energy transport measurements indicate that approximate...

Pump-Probe Experiments with a Single Molecule: ac-Stark Effect and Nonlinear Optical Response.

Abstract: We have measured the ac-Stark effect on single terrylene molecules in $p$-terphenyl crystals at superfluid helium temperatures. With the pump beam far enough from resonance, the molecular transition shifts as expected, i.e., proportional to the pump intensity and to the inverse pump detuning. Experiments closer to resonance with stronger beams show Autler-Townes-like structures that can be well reproduced by means of Bloch equations with two intense laser fields. These experiments are a demonstration of nonlinear optical effects with the localized electronic states of a single molecule trapped in a solid.

Motional Stark effect upgrades on DIII-D

Abstract: The measurement and control of the plasma current density profile (or q profile) is critical to the advanced tokamak program on DIII‐D. A complete understanding of the stability and transport properties of advanced operating regimes requires detail poloidal field measurements over the entire plasma radius from the core to the edge. In support of this effort, we have recently completed an upgrade of the existing motional Stark effect (MSE) diagnostic, increasing the number of channels from 8 to 16. A new viewing geometry has been added to the outer edge of the plasma which improves the radial resolution in this region from 13 to ∼2 cm. This view requires the use of a reflector that has been designed to minimize polarization effects. Vacuum‐compatible polarizers have also been added to the instrument for in situ calibration. Future use of the MSE diagnostic for feedback control of the q profile will also be discussed.

Stark spectroscopy of CdSe nanocrystallites: The significance of transition linewidths

Abstract: We use Stark spectroscopy to examine the nature of the excited states of CdSe nanocrystallites. The Stark spectra we obtain are in the small coupling limit in which the changes induced by the electric field to the absorption spectrum are small compared to the transition linewidths. Within this limit, we theoretically examine the dependence of the line shape of Stark difference spectra on the linewidth of the transitions involved. For systems such as CdSe nanocrystallites, which have overlapping transitions coupled by the electric field, we find that the usual association of derivatives of absorption features with dipole moments and polarizabilities is problematic. We show that the Stark absorption spectrum of the CdSe nanocrystallites can be explained by polarizable and delocalized nonpolar excited states.

Rotational spectra and van der Waals potentials of Ne-Ar

Abstract: signal-to-noise ratio with a single polarization pulse. The ground-state rotational constants for this species yield a zero-point separation of R 0 .360.7 pm. Simple model van der Waals potentials have been fit to the microwave transitions for the various isotopomers, providing estimates of the equilibrium spacing at the well minimum of R e 5348.0~2! pm. More elaborate potentials based on ab initio calculations or on molecular-beam scattering cross sections and thermodynamic and transport properties have also been tested. The induced electric dipole moment is estimated to be m 0 57.3~1.6!310 233 Cm @0.0022~5! D# by comparison of p/2 polarization pulses with a reference molecule ~Ar‐CO2! whose dipole moment is known from Stark effect splitting measurements. Uncertainties in parentheses are one standard deviation. © 1995 American Institute of Physics.

Chromophores for second-order non-linear optic materials

Abstract: The theory of the electro-optic effect and second harmonic generation is developed in the context of the linear Stark effect. This leads to an emphasis both on the optical bandwidth of the charge-transfer transitions responsible for the hyperpolarisability, and on the relationship between the transition energies and optical frequency of an electro-optic device. A simple treatment of vibronic coupling illustrates the connection between the bond-length alternation in conjugated polyenes and the sign and magnitude of the non-linearity. A brief survey is made of current chemical strategies for optimising the non-linearity.

Refractive index modulation based on excitonic effects in GaInAs-InP coupled asymmetric quantum wells

Abstract: The effect of excitons in GaInAs-InP coupled asymmetric quantum wells on the refractive index modulation, is analyzed numerically using a model based on the effective mass approximation. It is shown that two coupled quantum wells brought in resonance by an applied electric field will, due to the reduction in the exciton oscillator strengths, have a modulation of the refractive index which is more than one order of magnitude larger than in a similar quantum well structure based on the quantum confined Stark effect, but with no coupling between the quantum wells. Calculations show that combining this strong electrorefractive effect with self-photo-induced modulation in a biased-pin-diode modulator configuration, results in an optical nonlinearity with a figure of merit of 20 cm/sup 3//J at a wavelength of 1.55 /spl mu/m. This value is large compared to optical nonlinearities originating from band edge resonance effects in III-V semiconductor materials. >

Density measurements in the edge, divertor and X‐point regions of Alcator C‐Mod from Balmer series emission

Abstract: The electron density in the edge, divertor and X‐point regions of Alcator C‐Mod [Proceedings of the IEEE 13th Symposium on Fusion Engineering (Institute of Electrical and Electronic Engineers, New York, 1990), Vol. 1, p. 13] has been measured using the Stark broadening of high‐n (n=8 through 11) transitions of the Balmer series of deuterium. These measurements have been made during typical single null, diverted operation and during gas‐puffing through capillaries located within the first wall and divertor. Electron densities up to 1.6×1021 m−3 have been measured in the X‐point region and slightly lower densities (5−7×1020 m−3) have been measured in the divertor and edge regions. These results are factors of 2 to 5 larger than the density measurements from the CO2 laser interferometer system in the main chamber (3×1020 m−3) and comparable with probe measurements in the divertor. The ratios of the widths of adjacent lines within the Balmer series have been compared with ratios calculated for the Stark broad...

Quantum cascade unipolar intersubband light emitting diodes in the 8–13 μm wavelength region

Abstract: An intersubband electroluminescent light source in the 8–13 μm spectral region is reported. The structure was grown by molecular beam epitaxy in the lattice matched AlInAs/GaInAs material system and consists of coupled‐quantum‐well active regions alternated with compositionally graded electron injecting regions. The device operates in the temperature range from 10 to 200 K with measured integrated optical powers up to 6 nW. The peak of the electroluminescent spectrum blue shifts linearly with current due to the Stark effect. From these data and straightforward electrostatic considerations we obtain the intersubband nonradiative electron lifetime, in good agreement with the calculations.

Cavity-Induced Changes of Spontaneous Emission Lifetime in One-Dimensional Semiconductor Microcavities.

Abstract: Spontaneous emission lifetime in a one-dimensional AlGaAs distributed Bragg reflector microcavity has been systematically measured as a function of emission wavelength tuned by the quantum-confined Stark effect in order to confirm the cavity-induced alteration of the SE lifetime. The result manifests a substantial cavity-induced fractional change of the exciton decay times, $\ensuremath{\sim}$30% around the on-axis resonant wavelength, despite the relatively large broadening of the excitonic emission being comparable to the cavity resonance width in the low- $Q$ cavity.

Very large Stark shift in three‐coupled‐quantum wells and their application to tunable far‐infrared photodetectors

Abstract: The quantum‐confined Stark effect in the three‐coupled‐quantum‐well (TCQW) structure is studied theoretically in this paper. The basic TCQW structures are composed of three quantum wells separated by two thin barriers. Coupled one‐dimensional Schrodinger and Poisson equations are solved self‐consistently to find the sub‐band eigenenergies and the envelope wave functions for the TCQW structures. Results indicate that the GaInAs/AlGaAs/GaAs two‐depth TCQW structure exhibits both a very large Stark shift and a high absorption coefficient for the 1→3 intersub‐band transition. By using a 1→3 intersub‐band Stark shift in the two‐depth TCQW structure, a highly sensitive tunable far‐infrared photodetector is proposed. This photodetector is ideal for device applications in the 8–14 μm atmospheric window region. The operation of this device is based on the infrared absorption by electrons in the ground state transited from the ground‐state sub‐band E1 of the TCQW to the second‐excited‐state sub‐band E3. A very larg...

Electron number density and temperature measurement in a laser-induced hydrogen plasma

Abstract: A Nd:YAG laser operated at 1064 nm and 7.5 nsec pulse duration is used to create optical breakdown in gaseous hydrogen. Time-resolved spectral measurements of the hydrogen Balmer series are reported and analyzed to characterized the electron number density and excitation temperature of the decaying plasma. The electron density is inferred from the Stark broadened Hα linewidths; excitation temperatures are estimated using Boltzmann plots of the Balmer series. In the first few microseconds following laser breakdown, electron densities are found to be in the range of 1019-1016cc-1, with corresponding excitation temperatures in the range of approx. 100,000–6600 K.

On the dynamics of high Rydberg states of large molecules

Abstract: In this paper we explore the level structure, the optical excitation modes and the dynamics of a mixed Stark manifold of very high Rydberg states (with principal quantum numbers n=80–250) of large molecules, e.g., 1,4 diaza bicyclo [2,2,2] octane (DABCO) and bis (benzene) chromium (BBC) [U. Even, R. D. Levine, and R. Bersohn, J. Phys. Chem. 98, 3472 (1994)] and of autoionizing Rydbergs of atoms [F. Merkt, J. Chem. Phys. 100, 2623 (1994)], interrogated by time‐resolved zero‐electron kinetic energy (ZEKE) spectroscopy. We pursue the formal analogy between the level structure, accessibility and decay of very high Rydbergs in an external weak (F≂0.1–1 V cm−1) electric field and intramolecular (interstate and intrastate) relaxation in a bound molecular level structure. The onset n=nM of the strong mixing (in an external field F and in the field exerted by static ions) of a doorway state, which is characterized by a low azimuthal quantum number l, a finite quantum defect δ, and a total nonradiative width Γs≂Γ0/...

Influence of disorder on the field-modulated spectra of polydiacetylene films

Abstract: Electroabsorption and electroreflectance spectra of disordered, optically isotropic polydiacetylene films (4-BCMU) have been measured with field perpendicular to the incident light. The modulated spectra are polarized reflecting the local anisotropy of \ensuremath{\pi}-conjugated polymers. Based on results on single crystals, the data are interpreted as quadratic Stark effect of excitonic transitions with the same values for polarizability and dipole moments as in single crystals, accounting only for inhomogeneous broadening and random orientation of polymer segments. Line-shape analysis reveals a large additional contribution proportional to the second derivative of the absorption spectrum. This part, absent in single crystals, is interpreted as a linear Stark effect due to permanent dipole moments arising from disorder-induced asymmetry of polymer segments. Isotropic distribution of the disorder-related dipoles leads to a quadratic dependence of the signal on field strength, different from the quadratic Stark effect only by the spectral line shape.

Ultrafast-lifetime quantum wells with sharp exciton spectra

Abstract: Sharp quantum‐confined excitons in semi‐insulating low‐temperature‐growth AlAs/GaAs quantum wells with 15 ps carrier lifetimes are demonstrated. High‐quality well‐barrier interfaces can be grown at low substrate temperatures and annealed up to temperatures of 700 °C, beyond which interface mixing broadens the exciton transitions. Electroabsorption from the quantum‐confined Stark effect in as‐grown modulators approaches 10 000 cm−1, which is comparable to traditional high‐temperature growth quantum wells. The low‐temperature growth quantum well structures eliminate the need for postgrowth processing, such as ion implantation for photorefractive quantum wells, ultrafast saturable absorption, or electro‐optic sampling applications.

Self-consistent analysis of electric field effects on Si delta -doped GaAs

Abstract: We theoretically study the subband structure of single Si delta -doped GaAs inserted in a quantum well and subject to an electric field applied along the growth direction. We use an efficient self-consistent procedure to solve simultaneously the Schrodinger and Poisson equations for different values of electric field and temperature. We thus find the confining potential, the subband energies and their corresponding envelope functions, the subband occupations, and the oscillator strength of intersubband transitions. Opposite to what is usually the case when dealing with the quantum-confined Stark effect in ordinary quantum wells, we observe an abrupt drop of the energy levels whenever the external field reaches a certain value. This critical value of the field is seen to depend only slightly on temperature. The rapid change in the energy levels is accompanied by the appearance of a secondary well in the confining potential and a strong decrease of the oscillator strength between the two lowest subbands. These results open the possibility to design devices for use as optical filters controlled by an applied electric field.

A molecular beam optical/Stark study of calcium monoacetylide

Abstract: Optical and optical Stark spectra of the 000 A 2E‐X 2A1 band system of a supersonic molecular beam sample of calcium monomethyl, CaCH3, have been recorded. Field free spectroscopic parameters were obtained on fitting the (|KR′|=1, |K′|=0 and 2)–(|KR″|=1) and the (|KR′|=0, |K′|=1)–(|KR″|=0) subbands. The branch features were reassigned and a resulting new set of spectroscopic parameters determined. The value of the a‐principal axis component of the spin–rotation parameter, eaa(A 2E), is now consistent with the assumed nature of the low‐lying excited electronic states. Dipole moments of 2.62±0.03 D and 1.69±0.02 D were determined for the X 2A1 and A 2E states, respectively. A simple electrostatic model was adapted to predict dipole moments for CaCH3 and MgCH3.

Higher-Order Stark Spectroscopy: Polarizability of Photosynthetic Pigments

Abstract: Stark spectra for nonoriented molecules with typical broad inhomogeneous line widths are obtained by applying an ac electric field and detecting the change in absorbance at the second harmonic of the field modulation frequency. The analysis of such data in terms of molecular parameters such as changes in dipole moment and polarizability is often difficult. We outline a new method, called higher-order Stark spectroscopy, in which the higher even-powered harmonics of the response to an ac field are measured. The method is applied to electronic states of photosynthetic pigments, including pure monomeric bacteriochlorophyll, the homoand heterodimer special pair primary electron donors in photosynthetic reaction centers, and the carotenoid spheroidene, both pure in an organic matrix and in the B800-850 antenna complex. It is shown that even at a qualitative level these systems divide into two groups: those where the change in dipole moment dominates the change in polarizability (pure bacteriochlorophyll, the heterodimer, and spheroidene in the antenna complex) and those where the polarizability dominates the change in dipole moment (pure spheroidene and the homodimer special pair). A quantitative analysis of the Stark effect spectrum provides information on molecular properties associated with the movement of charge such as the change in dipole moment (&), polarizability (Aa), and hyperpolarizability for an electronic or vibrational2 transition. For a uniaxially oriented system, the contributions from Ap and Aa can be readily distinguished as the former depends linearly on the field, while the latter depends quadratically on the field. However, for isotropic, immobilized samples (frozen glasses or polymer films), which are far easier to study and are often the only conditions under which samples can be studied, the contributions from Ap and Aa, as well as all other electrooptic parameters, depend on the same power of the field; consequently, the contributions can only be obtained by analyzing the Stark line shape. In the following we outline a new experimental method, called higher-order Stark spectroscopy, for obtaining more information than was previously possible about the electrooptic properties of molecules. We specifically apply this method to several photosynthetic pigments. By comparing the higher-order Stark spectra for several different types of chromophores in different environments, it is possible, even at a qualitative level. to obtain information on the relative contributions of Ap and Aa. We find directly that the first electronic excited state of the special pair primary electron donor P in photosynthetic reaction centers (RCs) has a very large polarizability, a conclusion which was previously suggested from conventional low-temperature Stark spectroscopy via a complex line shape analysi~.~ In a subsequent paper, we will show that a quantitative analysis of the higher-order Stark spectra can be used to obtain the diagonal elements of the polarizability tensor of P, and this information can be used