Ground State Resonance Research Articles

Correlated Ground State and Giant Resonances Built on It

It is demonstrated that a slight modification of the time-dependent density-matrix theory (TDDM) which was originally formulated to describe nuclear dynamics in terms of a mean field and two-body correlations provides us with a method for obtaining a correlated ground state. In the modified TDDM the residual interaction is gradually turned on starting with the Hartree-Fock (HF) ground state. The ground state correlations and El and E2 giant resonances in 40Ca are studied using the modified TDDM. Special emphasis is given to the comparison of the energy spectra of the giant resonances built on the correlated ground state with those built on the HF ground state. It is found that spurious components seen in the E2 giant resonance built on the HF ground state can be eliminated through a consistent treatment of the ground state. Ground state correlations and giant resonances in 220 are also studied in the same framework to elucidate the role of two-body correlations in such unstable nuclei. It is found that the effects of two-body correlations are not so important in such a neutron rich nucleus as in the closed-shell nuclei.

Spectroscopic Studies in InGaN Quantum Wells

AbstractFundamental electronic modulations in strained wurtzite If-nitride, in particular InxGa1-xN, quantum wells (QWs) were treated to explore the reason why practical InGaN devices emit bright luminescences in spite of the large threading dislocation (TD) density. The emission mechanisms were shown to vary depending on the well thickness L and InN molar fraction x. The electric field across the QW plane, F, which is a sum of the fields due to spontaneous and piezoelectric polarization and the pn junction field, causes the redshift of the ground state resonance energy through the quantum confined Stark effect (QCSE). The absorption spectrum is modulated by QCSE, quantum-confined Franz-Keldysh effect (QCFK), and Franz-Keldysh (FK) effect from the barrires when, for the first approximation, potential drop across the well (F/L) exceeds the valence band discontinuity, δEv. Under large F/L, holes are confined in the triangular potential well formed at one side of the well. This produces apparent Stokes-like shift in addition to the in-plane net Stokes shift on the absorption spectrum. The QCFK and FK further modulate the electronic structure of the wells with L greater than the three dimensional (3D) free exciton (FE) Bohr radius, aB. When F/L exceeds ΔEc, both electron (e) and hole (h) confined levels drop into the triangular potential wells at opposite sides of the wells, which reduces the wavefunction overlap. Doping of Si in the barriers partially screens the F resulting in a smaller Stokes-like shift, shorter recombination decay time, and higher emission efficiency. Finally, the use of InGaN was found to overcome the field-induced oscillator strength lowering due to the spontaneous and piezoelectric polarization. Effective in-plane localization of the QW excitons (confined excitons, or quantized excitons) in quantum disk (Q-disk) size potential minima, which are produced by nonrandom alloy potential fluctuation enhanced by the large bowing parameter and F, produces confined e-h pairs whose wavefunctions are still overlapped when L<aB. Their Coulomb interaction is more pronounced for F L<ΔEv.

TDHF study of the He+ collision on atomic He targets at the8Be ground state energy

Experimentally the8Be ground state resonance has been studied in He+ collisions on atomic He atoms. The nuclear resonance manifests itself by satellite resonance lines corresponding to different electron configurations of the Be ion. Experimentally a large probability for the emission of one electron has been deduced. We study the atomic He++He collision within a model in which the evolution of the electron wavefunction is treated dynamically in the TDHF scheme, and the motion of the nuclei is treated classically. In agreement with experiment we find a large probability for one electron to be emitted into the continuum during the lifetime of the8Be ground state resonance.

Correlated Ground State and E2 Giant Resonance Built on It

Correlated Ground State and <i>E</i>2 Giant Resonance Built on It

Correlated Ground State and E2 Giant Resonance Built on It

Correlated Ground State and E2 Giant Resonance Built on It

Thermal effects on isoscalar giant resonance energies in hot nuclei.

The thermal effects on the energies of the isoscalar giant multipole resonances of hot nuclei are discussed and an approximate formula for the energy as a function of temperature is derived via a hydrodynamic theory. The energy difference between the isoscalar giant multipole resonance of a hot nucleus and its ground-state resonance depends on the competition between the volume expansion and the increase of the average kinetic energy per nucleon of hot nuclei, which lower and raise the resonance energy, respectively, and nearly counteract each other in magnitude. The variaiton of the isoscalar giant resonance energy with temperature is very small.

Stimulated emission pumping spectroscopy via two-color resonant four-wave mixing

We present a combined theoretical and experimental study of the application of two-color resonant four-wave mixing (RFWM) to stimulated emission pumping (SEP) spectroscopy. The theoretical approach employs time-independent, diagrammatic perturbation theory and a spherical tensor analysis in an extension of a recent treatment of degenerate four-wave mixing [Williams, Zare, and Rahn, J. Chem. Phys. 101, 1072 (1994)]. The resulting signal expression for two-color RFWM separates the molecular properties from purely laboratory-frame factors determined by the polarizations of the input beams and the rotational branch types of the SEP PUMP and DUMP transitions. This expression is valid in the limit of weak fields and for molecules in which the total angular momentum (omitting nuclear spin) is a good quantum number. In addition, we demonstrate that the spectral response for tuning the DUMP laser is a simple Lorentzian in free-jet experiments. We test our theoretical results and demonstrate the applicability of RFWM-SEP to jet-cooled, transient species in experiments on C3 and HCO. Using the well-studied à 1Πu–X̃ 1Σ+g system of C3, we illustrate and compare the two possible schemes for RFWM-SEP. These are defined as ω1=ω2 (PUMP) and ω3=ω4 (DUMP) or ω1=ω4 (PUMP) and ω2=ω3 (DUMP), where ω1, ω2, and ω3 are the input frequencies and ω4 is the signal frequency. Using the B̃ 2A′–X̃ 2A′ system of HCO, we obtain RFWM-SEP spectra that probe ground-state vibrational resonances lying above the low threshold for dissociation to H+CO. Varying the polarization of the input beams or PUMP rotational branch produce dramatic effects in the relative intensities of rotational lines in the RFWM-SEP spectra of HCO; these effects are well-described by our theoretical analysis. Finally, RFWM-SEP spectra of HCO resonances that are homogeneously broadened by dissociation are consistent with the theoretically predicted Lorentzian line shape; the full widths for these levels are in good agreement with those determined via unsaturated fluorescence depletion SEP.

The ultraviolet spectrum of the gravitational lens candidate UM 425 = QSO 1120+019: Evidence for broad absorption line (BAL) structure

The UV line profile structure of high-ionization resonance lines found with the International Ultraviolet Explorer (IUE) in the brightest of four multiply imaged sources (image-A) in the `candidate gravitational lens UM 425 = QSO 1120+019 indicates broad absorption line (BAL) structure. The deep-broad trough associated with the O VI line extends to velocities ~-12,000 km s^-1^, and contains discrete features that suggest multicomponent velocity structure. This structure may include contributions from C IV absorption from the early-type galaxy that is believed to lens UM 425. A strong absorption feature in the blue wing of the Lyman-α λ1216 emission line may be a Lyman-α absorption system at a z_Lya_ = 1.437 +/- 0.003, or it may be formed by the superposition of the broad N V λλ1238, 1242 absorption trough on the extended blue emission wing of the QSO Lyman-α line. We obtained a redshift of z_QSO_ = 1.471 +/- 0.003 from Lyman-α λ1215, consistent with the redshift found by Meylan and Djorgovski in the optical. The Lyman-α line appears unusually weak due to the presence of N V λ1240 BAL absorption. A Lyman-limit absorption system at λ912 was not observed in the QSO rest frame. The detection of BAL structure in the other weaker ground-state resonance lines of N II(1) and S IV (1) was not found, suggesting these lines are formed in a region that is distinct from the BAL component. Detection of BAL structure in the other fainter images in this system with HST instrumentation, similar to structure observed here in image A, could provide evidence that UM 425 is a gravitational lens.

Magnetopolarons on the surface of helium films

The energy spectrum of the surface electrons on liquid helium films in the presence of a perpendicular applied magnetic field is determined in different approaches within second order perturbation theory. The influence of the electron-ripplon interaction on the Landau levels are studied for arbitrary strength of the magnetic field and several values of the coupling constant. Current theories of the polaron ground-state and cyclotron resonance are compared to the exact weak-coupling results obtained. We found a shift down in the cyclotron resonance frequency due to polaron effects.

Iron Boltzmann factor LIDAR: proposed new remote-sensing technique for mesospheric temperature

We describe a new LIDAR technique for middle atmospheric temperature measurement. The proposed LIDAR exploits the Fe layer in the 80-100-km altitude region. Absolute temperatures are inferred by the use of the Maxwell-Boltzmann relationship from the ratio of LIDAR returns from mesospheric Fe atoms excited at 372 and 374 nm, corresponding to the ground-state resonance line and a thermally populated resonance line, respectively. The wavelengths of the new LIDAR are favorable for capturing Rayleigh signals from the middle atmosphere. A simulation indicates that a complete temperature profile from 30 to 100 km can be acquired with the proposed LIDAR by monitoring simultaneously the Rayleigh signals and the Fe fluorescence returns excited by the same transmitter pulse.

Does the ground-state resonance of 10Li overlap neutron threshold?

Recent measurements suggest that the ground state of 10Li is a resonance which may well be wide enough to overlap the (n + 9Li) threshold. In this context we recall some of the curious properties of resonances located near threshold and entered from a non-decay channel, including their asymmetry and the fact that the peak observed in the cross section occurs at neither the R-matrix nor the S-matrix energy, but rather between the two. Because of these and other complications, it does not seem likely that either the ifℓ-value of the resonance or the energy of the corresponding state can accurately be determined from the shape of the resonance peak alone.

Thermally desactivated resonant current in high peak to valley current ratio (69:1) GaAs/GaAlAs resonant tunneling structures: A spectroscopic view of the emitter density of state

We describe the realization of very high peak to valley current ratio double barrier resonant tunneling structures by molecular beam epitaxy. We find that the resonant current through the ground state resonance decreases as the temperature is raised. This unique behavior is explained within the Esaki-Tsu formalism and originates in the Fermi-Dirac statistics.

Energy-independent effective interactions: Analytic expressions for the required G-matrix energy derivatives

In a recent paper it has been shown that the successful application of the method of Suzuki and Lee for calculating an energy-independent effective interaction requires that the energy derivatives of the G-matrix be calculated accurately. In that work methods by which these derivatives may be calculated analytically in problems where P, the projection operator which defines the active space, can be written in the center-of-mass and relative coordinates were presented. In this work we extend the applicability of the theory by presenting analytic expressions for calculating the energy derivatives of the G-matrix elements obtained by the method of Barrett et al. Convergence properties of the iterative processes by which the effective interaction is calculated are investigated and guidelines for choosing the starting energy are given. As an example of its use, the theory is applied to the ground state and low-lying resonances of 4He.

All-optical microwave frequency standard: a proposal

A simple method to produce a clock transition with purely optical means by modulated pumping is described. The field-independent ground state resonance of /sup 87/Rb atoms using sinusoidal modulation of the injection current of an AlGaAs laser diode emitting at 780 nm (FM modulation) is observed. The 6.835 GHz resonance with a subharmonic modulation frequency of 1.139 GHz is detected. A high-contrast resonance peak is observed and a condition for zero light shift is found. The linewidth is 3 kHz (at 6.835 GHz) in this preliminary experiment, due to the small size of the light beam ( approximately 2 mm diameter) and the low buffer gas pressure (680 Pa) that was used. A theoretical model that explains the main features of the experiment is described. >

Tunneling through Single AlGaAs Barriers

AbstractTunneling transfer through Al0.35Ga0.65As barriers is studied in asymmetric double quantum well structures by time‐resolved photoluminescence measurements in the pico‐ and femtosecond regime. A large variety of electron and hole resonances is detected when electric fields of both signs are externally applied. The ground state resonance shifts, when the electrons tunnel in the reverse direction, revealing the importance of excitonic effects. Longitudinal optical phonon assisted tunneling plays a minor role for narrow quantum wells in comparison to impurity or interface roughness assisted transfer. Resonant electron tunneling times depend exponentially on the integrated square root of the tunneling barrier height and are an order of magnitude faster than resonant hole tunneling times. The n = 2 to n = 1 electronic intersubband scattering time in a 10 nm quantum well is determined to be 550 fs measuring the transfer time through a thin barrier.

Resonant multiwave-mixing spectra of gas-phase sodium: Nonperturbative calculations.

We perform nonperturbative calculations of resonant multiwave mixing of two high-intensity laser beams of different frequencies both for a general three-level system and for the 3 $^{2}$${\mathit{S}}_{1/2}$--3 $^{2}$${\mathit{P}}_{1/2}$ electronic resonance of the sodium atom. Our calculations proceed by direct numerical integration of the density-matrix equations and subsequent Fourier analysis of the calculated time-dependent density-matrix elements. We examine the case where two nearly degenerate input beams are each detuned many collisional linewidths from electronic resonance, and the frequency difference between the input beams is tuned through a ground-state resonance. Unlike previous high-intensity analyses, each input beam can be resonant with numerous transitions simultaneously; for a three-level system, we refer to this as an ``M-type'' system, in analogy with \ensuremath{\Lambda}- and V-type systems. Calculated four-, six-, and eight-wave-mixing spectra exhibit subharmonic resonances of the type observed experimentally by Trebino and Rahn [Opt. Lett. 12, 912 (1987)]. The three-level system is studied to gain insight into the physics of these subharmonic resonances. For more quantitative comparison with experiment, we use a model that includes the 16 Zeeman and hyperfine states in the 3 $^{2}$${\mathit{S}}_{1/2}$ and the 3 $^{2}$${\mathit{P}}_{1/2}$ levels, and accounts for arbitrary linear polarization of the input laser radiation. The relative intensities of the resonance features in the calculated multiwave-mixing spectra show good agreement with experiment. These resonance features broaden and shift as laser intensity increases, as predicted for four-wave-mixing processes in previous, less general high-intensity analyses. Both theory and experiment also show additional resonant features in the eight-wave-mixing spectra that appear at very high intensity.

Resonances in tunnelling between quantum wells

Time-resolved photoluminescence measurements in the picosecond regime on GaAs/Al0.35Ga0.65As asymmetric double quantum well structures with perpendicular electric fields applied in both directions are reported. A large number of electron and hole resonances are detected. A splitting of the ground state resonances reveals the importance of excitonic effects. Longitudinal optical phonon assisted tunnelling plays a minor role for narrow quantum wells in comparison with impurity or interface roughness assisted transfer. Resonant electron tunnelling times depend exponentially on the square root of the effective barrier height and are an order of magnitude shorter than resonant hole tunnelling times.

Dynamics in Biological Molecules Probed by Raman Saturation Spectroscopy.

We have developed a new technique to monitor photophysical relaxation precesses with the resolution inherent in vibrational Raman scattering. We examine the ground-state ultraviolet resonance Raman saturation behavior of tryptophan derivatives. We derive an expression for Raman saturation in a low laser pulse energy flux regime. We demonstrate the utility of the new technique for studying tryptophan and tyrosine excited-state relaxation rates in proteins and for investigating the interaction of dyes with DNA.

Resonance Raman spectra of 4a,4b-dihydrophenanthrene, the photocyclization product of cis-stilbene

Resonance Raman spectra 4a,4b-dihydrophenanthrene have been obtained by irradiating degassed samples of cis-stilbene in cyclohexane and in methanol at 266 nm and subsequently exciting the Raman spectrum within the absorption band of the dihydrophenanthrene product at 436 and 477 nm. We believe that these are the first published vibrational spectra of this species. The Raman spectra at both wavelengths are dominated by a strong band at 1474 cm −1 which is presumably the mostly in-phase stretch of the double bonds in the conjugated polyene chain, shifted down in frequency by about 100 cm −1 from the corresponding all-trans linear polyene. Other weaker vibrations are observed at 769, 1035, 1080, 1147, 1201, 1298, 1534, and 1555 cm −1, and some assignments are suggested based on comparison with QCFF/PI calculations. The nonresonantly enhanced Raman lines of the other cis-stilbene photoproduct, trans-stilbene, relative to dihydrophenanthrene are much stronger in methanol than in cyclohexane under the same irradiation conditions. This observation, together with the results of simple laser photolysis experiments in which the 436 nm absorbance is used to monitor the rates of dihydrophenanthrene formation and destruction, indicate a large solvent effect on the quantum yield for photocyclization of cis-stilbene and little or no solvent effect on the yield for photochemical ring opening of dihydrophenanthrene. The ground state resonance Raman intensities of cis-stilbene excited at 266 nm indicate slightly faster distortion of the vertically excited state along torsional coordinates in methanol relative to cyclohexane solution.

Vibrational spectroscopy of excited electronic states in carotenoids in vivo. Picosecond time-resolved resonance Raman scattering

The vibrational spectroscopy and population dynamics of excited singlet (2(1)Ag), excited triplet (3B u), and the ground (1Ag) electronic states of carotenoids in chromatophores of Chromatium vinosum (mainly spirilloxanthin and rhodopin) and of the same carotenoids in benzene solutions are examined by picosecond time-resolved resonance Raman scattering. Coherent Stokes Raman scattering from the ground states of carotenoids in chromatophores also is observed. Resonance Raman spectra of in vitro rhodopin and spirilloxanthin when compared with in vivo data demonstrate that scattering from spirilloxanthin dominates the in vivo spectrum. Comparisons of the time-dependent intensities of 2(1)Ag and 1Ag resonance Raman bands from both in vitro and in vivo carotenoids suggest that vibrationally excited levels in 1Ag are populated directly by the decay of the 2(1)Ag state and that these levels relax into a thermalized distribution in less than 50 ps. The appearance of asymmetrically broadened, ground-state resonance Raman bands supports this conclusion. Formation of the 3Bu state is observed for carotenoids in chromatophores, but not for in vitro spirilloxanthin indicating that the 3Bu state is formed by fission processes originating from the spatial organization of pigments within chromatophores. The rate at which the intensities of 2(1)Ag resonance Raman bands decay is faster for the carotenoids in vivo than for those in vitro thereby indicating that additional relaxation channels (e.g., energy transfer to bacteriochlorophylls) are present in the chromatophore. The similarity of the in vivo and in vitro 2(1)Ag resonance Raman spectra shows that no significant modifications in the vibronic coupling has been caused by the chromatophore environment.