Stark Field Research Articles

Stark Shift and Field Induced Tunneling in Doped Quantum Wells with Arbitrary Potential Profiles

ABSTRACTThe energies and resonance widths of single doped quantum wells consisting of Al-GaAs/GaAs with rectangular and annealing induced diffusion modified shapes are calculated under an uniform electric field using the stabilization method. The electronic structure is calculated without an electric field in the finite temperature density functional theory with exchange-correlation potential treated in the local density approximation. Our scheme for solving the Schrödinger and Poisson equations is based on the Fourier series method. The electric field is added to the self-consistent potential and energies are obtained as a function of the combined width of the well and barriers. This yields us the stabilization graph from which the energies and resonance widths at different field strengths are extracted using the Fermi Golden rule.

Temperature and Solvent Effects on the Luminescence Spectrum of C70: Assignment of the Lowest Singlet and Triplet States

The temperature, solvent, and concentration dependence of the fluorescence and phosphorescence spectra of C{sub 70} in glassy solutions have been examined. Spectra have been recorded over the temperature range 4-200 K. In addition, the AC Stark field modulated phosphorescence and the phosphorescence of {sup 13}C{sub 70} have been recorded and analyzed. The lowest triplet state is identified as a {sup 3}E{sub 1}` state and the vibronic structure consists primarily of Herzberg-Teller active e{sub 2}` modes. The intensity of the electronic origin is comparable to the vibronically induced intensity and is extraordinarily solvent sensitive. The solvent sensitivity exhibited by the spectra is shown to have the same origins as that observed in benzene and pyrene, but is several times greater in magnitude. Analysis of the spectra suggests that two electronic excited states contribute to the observed phosphorescence spectrum. 27 refs., 6 figs., 1 tab.

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

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.

Stark field induced perturbations in the ν1 + 3 ν3 vibrational overtone band of acetylene

The rotationally resolved, Stark split spectrum of the ν 1 + 3 ν 3 band of acetylene has been investigated with a high resolution molecular beam laser spectrometer. The results indicate a field induced coupling of the vb1030 00 0〉 upper state with the vb0040 00 0〉 state. The number of observed transitions was sufficient to allow a detailed analysis of the field induced coupling. The J-dependent polarisability and anisotropy of the polarisability of the vb1030 00 0〉 state were measured as well as the anisotropy of the polorisability of the ground state (1.071 Å 3). The coupling strength to the vb0040 00 0〉 state, and the origin of the 4 ν m3 band were determined to be 0.0696 D and 12671.50 cm −1. The latter has been compared with predictors made by local mode theory.

New room temperature CW laser at 2.82 µm:U 3+ /LiYF 4

Recently, optical studies of U doped LiYF/sub 4/(YLF) have been made. The authors show that U/YLF is a promising IR laser because the crystal field Stark level splitting of the /sub 4/I/sub 9/2/ ground multiplet (1113cm/sup -1/) is larger than in U/sup 3+//CaF (609cm/sup -1/) which makes a four level laser possible at room temperature. However the valence state 3+ is not the most stable state for the actinide ion and the absorption spectra, as well as the pale green colour, show that usually both valence states U/sup 3+/ and U/sup 4+/ are present in the crystal. This Letter reports the first observation of CW laser emission of U/sup 3+/ in LiYF/sub 4/ at room temperature at 2.82 mu m. >

Electric field screening in an adsorbed layer: CO on Pt(111)

The IR cross section and Stark tuning rate of CO on Pt(111) versus coverage are measured. (The Stark tuning rate is the variation of vibrational frequency with applied electric field.) We interpret the observations as coverage-dependent screening of applied static and IR fields at CO's adsorption site. In contrast to the standard dipole—dipole coupling model, we find that static and IR screening depend differently on coverage. We also find that the local field Stark tuning rate for CO on Pt(111) in vacuum is half that at the electrochemical double layer. Possible explanations of our results are discussed.

Transmission coefficient and Stark shifts in double-barrier quantum well structures

Using Airy functions an analytical expression is obtained for the transmission coefficient of double-barrier quantum well structures under bias. The function which determines the resonance energies follows from this result. It allows one to calculate the variations of the shift of the resonance energies with applied field (Stark effect). A number of examples are given of such variations for electrons and holes in the ground state and in the first excited state. Comparison is also made with results obtained when using some approximate expressions for the Stark shift.

Determination of water vapor by microwave spectroscopy with application to quality control of natural gas

The design and construction of a microwave spectrometer is reported. The aim of this instrument is to detect water vapor at low levels. Sinusoidal Stark field modulation is used in order to amplitude modulate the absorption of microwave energy by the water molecule, thus enhancing the sensitivity of the instrument. A semiconfocal Fabry-Perot resonator is used to hold the sample under test. The fourth harmonic of the Stark modulation frequency was used to show the quantitative presence of water in the sample gas. Both atmospheric air, and liquified natural gas (LNG) were used as carrier gases. The peak-to-peak intensity of the fourth harmonic was used to calibrate the instrument against different amounts of water present in atmospheric air. The observed limit of this prototype is around 300 p.p.m. water. No attempt was made to lower the detection limit at this point since the aim of this project was to examine the feasibility of using the fourth harmonic to determine water content reliably.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Unmodulated CO 2 laser stabilized by means of Stark and photoacoustic spectroscopy

A high-power CO 2 laser (9P(34)) for use in optically pumping a CH 3OH laser (70.5 μm) has been stabilized in the vicinity of the center of the CH 3OH saturated absorption line by means of photoacoustic spectroscopy (PAS). The stabilized pump laser beam is unmodulated because the PAS signal for stabilization was obtained by modulating the Stark field in the external PAS cell filled with CH 3OH molecules at low pressure. The unmodulated effect is to eliminate the spectrum spread due to the frequency modulation, and is demonstrated in the frequency mixing between the seventh harmonic of the CH 3OH laser and a CO 2 laser at 10.2 μm. The frequency stability has been estimated quantitatively with reference to a CO 2 laser stabilized on the Lamb dip of 4.3 μm fluorescence.

A K-band oscillator locked to the first water resonance

A Gunn diode oscillator has been locked to the rotational absorption line of water at 22235.170 MHz. The water in vapor form was obtained from atmospheric air. The air was held in a vacuum chamber which was inserted in a Fabry-Perot open semiconfocal resonator. A sinusoidal electric field, the Stark field, was impressed upon the gas in the cavity which is coupled to the Gunn oscillator, thus modulating the water vapor absorption of microwave energy. The second harmonic of the Stark Field was used to lock the Gunn oscillator. Working with the water spectral line (16/sub 16/-5/sub 23/) at 22.235 GHz, a frequency stability on the order of +or-50 kHz was achieved.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Close-coupled wave packet calculations of the scattering of oriented molecules from surfaces

We report time-dependent close-coupled wave packet calculations of transition probabilities for the direct rotationally inelastic scattering of oriented NO(X 2Π) molecules from a rigid, flat Pt surface. A comparison is made with the recent experiments of Kuipers, Tenner, Kleyn, and Stolte [Phys. Rev. Lett. 62, 2152 (1989); J. Vac. Sci. Technol. A 8, 2692 (1990)] on NO molecules which were state selected with an electrostatic hexapole lens and oriented in a Stark field. Compared with unoriented molecules, the mean rotational energy is increased (decreased) by approximately 15% when the O end (N end) of the molecule is directed towards the surface, in qualitative agreement with experiment. An analysis of the rotational excitation mechanism in this collision system shows that the steric effect is strongly influenced by the dependence of the attractive region of the gas–surface potential on molecular orientation.

Electric field effects in the photoionization of N2 near threshold

Structural features well below the usual Stark field depression of the ionization threshold are observed in the photoionization of N2 in the presence of weak (10–15 V/cm) fields. The energies of the structural features can be simulated by assuming rotational autoionization (ΔJ = −2). They are interpreted as originating from Q (J″) transitions, forbidden to spontaneously autoionize in the absence of a field. The field relaxes the rigorous autoionization selection rules by l spoilage of the Rydberg electron. The effect appears accentuated for high J, low n* states (J ∼27, 28; n* ∼ 23) and quenches at J∼11, n* ∼ 37. The quenching is attributed to the onset of the Inglis–Teller effect.

Stark-enhanced phase conjugation in shaped-microparticle suspensions.

We examine optical phase conjugation for a shaped-microparticle suspension maintained in a uniform electric field. Theory asserts that the presence of a Stark field decreases the grating response time and enhances the composite's four-wave-mixing coefficient. These changes arise from and reflect the narrow angular range in which the particles are constrained to point, giving rise to reduced rotational times and the appearance of a new coherent scattering mechanism.

The permanent dipole moment of TiN and the nuclear magnetic hyperfine structure in its X 2Σ+ and A 2Π electronic states

The permanent dipole moment of TiN in its X 2Σ+ and A 2Σ states has been determined from the complete resolution of the first- and second-order Stark splitting of the Q21(1.5)+R2(0.5) line of the (0,0) band of the A 2Π–X 2Σ+ system. Values of 3.56±0.05 D (2σ) and 4.63±0.04 D (2σ) have been derived for the X and A states respectively, from least-squares fits to plots of Stark splitting vs electric field strength. Electric fields up to 12 kV/cm have been employed avoiding voltage breakdown. The zero-field spectrum shows resolution of the nuclear magnetic hyperfine structure of the 47TiN and 49TiN isotopes. This hyperfine structure is that of the ground X 2Σ+ state only and is shown to follow closely the coupling case bβS. The value of the Fermi contact parameter is −570 MHz which implies a 4s occupation of the 9σ molecular orbital (MO) of 72%. The results are compared with calculated and available experimental values for early first-row transition metal oxides and nitrides.

Two-photon microwave transitions within a two-level system

Two-photon pure rotational transitions in the symmetric top CF3CCH have been observed with a pulsed beam Fourier transform crossed-cavity spectrometer modified to allow the application of a static Stark field. Transient one-photon emission signals at ν0=17 267 MHz for transitions between the levels (J=3,K,M) and (2,K,M) are generated for KM≠0 by the application of intense pulses at ν0/2. It has been demonstrated that the two-photon transitions occur within an effectively isolated two-level system as a result of the first order ac Stark effect. Quantitative studies of the intensities as a function of pulse length and power show that the two-photon transition probability in the microwave region is well represented by the theoretical model used by Meerts, Ozier, and Hougen [J. Chem. Phys. 90, 4681 (1989)] to treat multiphoton transitions in a two-level system whose one-photon frequency is ≲1 MHz. A description is given of the spectrometer with emphasis on the modifications made for two-photon studies.

Efficient, broadly tunable, laser-pumped Tm:YAG and Tm:YSGG cw lasers

Broadly tunable cw laser emission is reported in Tm:YAG and Tm:YSGG at room temperature with Ti:sapphire laser pumping. The Tm(3+)(3)F(4) ? (3)H(6) transition is tuned continuously over the ranges 1.87-2.16 microm in YAG and 1.85-2.14 microm in YSGG. Smooth tuning results from overlapping transitions between phonon-broadened crystal field Stark levels.

Rescattering of photodetached electrons in a Stark field.

The problem of photodetachment of a negative ion in a Stark field is treated with the account of rescattering of the electron wave reflected by the potential barrier formed by the electric field. In the case of H{sup {minus}} ions the rescattering effect is noticeable and can be observed for the fields above 1 MV/cm. The upper bound for the observation of the effect is about 4 MV/cm, since above this limit the H{sup {minus}} ions would be completely stripped by the Stark field.

Steric effects in gas-surface scattering: A quantum close-coupled study of collisions of oriented NO(X 2Π) with a Ag(111) surface

We report calculated rotational transition probabilities for oriented NO(X 2Π) molecules in the J = M = Ω = 1 2 groundrotational state scattered from Ag(111). A comparison is made with recent experiments by Kuipers et al. on NO molecules state-selected and oriented in a Stark field. Our calculations include the two adiabatic potential energy surfaces of V + and V − symmetry, required in a description of the dependence of the transition probabilities on the spin-orbit and Λ-doublet states. Compared with unoriented molecules, the mean rotational energy of the scattered molecules is increased (decreased) by approximately 30% when the O-end (N-end) of the molecule is directed toward the surface, in qualitative agreement with experiment.

Simplification of Rotational Spectra and Spectral Hole Burning by Selective Excitation Techniques in Microwave Fourier Transform Spectroscopy

Abstract Selective excitation techniques have been applied to pulsed microwave Fourier transform spectroscopy. The frequency selectivity in excitation has been reached either by a long, weak pulse, or by a DANTE multi-pulse sequence. The corresponding excitation spectra calculated from a density matrix theory are in good agreement with the experimental results. The selective excitation techniques have been combined with double resonance sequences in order to simplify the recorded one-or two-dimensional spectra. Furthermore, selective excitation techniques have been applied to burn ‘holes’ into rotational transition lines broadened by an inhomogeneous Stark field.

Measurements of reactive plasma parameters obtained from spectral profiles of particles by laser spectroscopy.

The spectral line shapes of particles (atoms, molecules, or ions) in plasmas are determined froma radiative transition probability (natural broadening), a collision frequency (pressure broadening), an electric field (Stark broadening), a magnetic field (Zeeman splitting), and a velocity distributionfunction (Doppler broadening). In addition, the intensity ratios of spectral lines are due to thepopulation densities in electronic, vibrational, or rotational levels, which are related directly totemperature. Among these effects, Stark and Doppler broadenings and intensity ratios of spectrallines have been used for diagnostics of reactive plasmas to yield respective plasma quantities. In thispaper, the mechanisms of determining these effects are reviewed briefly, and a laser and other hardwarefor the measurements of spectral line shapes are then discussed, followed by typical examplesof the measurements in reactive plasmas, with comments for future perspective.