The thermionic double-diode,an efficient detector in excited state laser spectroscopy

We are describing the thermionic double-diode which is a suitable instrument for excited state laser spectroscopy. In comparison to the optogalvanic technique the signal-to-noise ratio was found to be 10²?10³ times better, investigating transitions between excited Sr or Ba states. Combined with its high detection sensitivity the thermionic double-diode presents the possibility of investigating transitions between high angular momentum states. It is demonstrated by studying transitions between Rydberg levels and doubly-excited autoionizing states in Ba. Further advantages are (i) the small strength of the dc electric field and the low electron density in the laser excitation region of the double-diode commending itself for studies of Doppler-free transitions to highly excited states and (ii) the very stable working conditions allowing to vary the pressure and current conditions in the diode in a much wider range than in a discharge.     

Electronic states and transition probabilities of the Au center in KCl

The emission spectra and lifetimes of the Au^— center in KCl crystals were measured. The Au^— concentration was measured by counting the activity of a radioactive tagged sample to determine the oscillator strength of the electronic transitions. The lifetime of the A'emission is calculated from the oscillator strength and is close to the experimental value of 103 ns at 4 K. The lifetimes and intensities of the Au^— emission bands C', B', A' and were measured between 4 and 500 K. The A'emission consists of a fast and slow decaying component. Time-resolved emission spectroscopy improved the measurements of the emission. From the temperature dependence of the lifetimes and intensities it is found that the radiating states of the A' and emission ³T_1u and ³A_1u are coupled by radiationless transitions. These transitions and the radiationless transitions from the excited state of the B' emission must be described as transitions between weakly coupled states similar to the radiationless transitions in rare earth ions. The excited state of an absorption band at 236 nm is responsible for the C' emission. Both the C' emission and the 236 nm absorption are phonon-induced transitions. The most probable explanation for the C' excited state is that the C' as well as the B' excited states come from the atomic state ³P₂ which is split into ³T_2u and ³E_u by the crystal field. The splitting energy is 0.9 eV and implies that one should look for the same effect in other s² centers.     

Observation of rovibrational dephasing of molecules in parahydrogen crystals by frequency domain spectroscopy

Rotation-vibration transitions of methane molecules embedded in parahydrogen crystals were investigated through Fourier transform infrared spectroscopy. Each transition shows extremely sharp peaks with a Lorentzian line shape profile, which indicates the spectra are free from inhomogeneous broadening. The steep temperature dependence of the linewidths observed in the range between 3.7 and 8. 5 K is interpreted to be a result of the pure dephasing relaxation mechanism. A remarkable difference in population relaxation widths between stretching and bending vibrational excited states was also found.     

GaAs/Al1-xAs表面单量子阱原位光调制反射光谱研究

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Excited Electronic States of Pyrene and some Azapyrenes

Pyrene and three azapyrenes have been studied by means of linear and magnetic circular dichroic spectroscopy. As a result, an improved understanding of the lower excited states has been reached, and a simple relation has been found between structure and MCD sign of the first two transitions in the azapyrenes.     

Observation of atomiclike electronic excitations in pure 3He and 4He clusters studied by fluorescence excitation spectroscopy

The structure of the electronically excited states of 3He and 4He clusters is investigated using fluorescence excitation spectroscopy. Distinct bands are observed energetically close to atomic 1s-ns, nd, np transitions and attributed to perturbed excited He atomiclike states with different principle and orbital quantum numbers. The line shifts and widths of the bands of 3He and 4He clusters of the same size are different and correlate with the average particle density inside the clusters calculated using the density functional method.     

Collisional and radiative processes with participation of highly excited states of atoms and molecules

A number of processes in which highly excited states of atoms and molecules participate are investigated. These processes are of interest for the kinetics of a low-temperature plasma, for atomic and molecular spectroscopy, and for astrophysics. A quasiclassical theory is developed for transitions between Rydberg states with change of the principal quantum number, and also for the processes of direct and associative ionization of highly excited atoms, which result from collisions between a neutral particle and its atomic core. The state of the inner electrons of a quasimolecular (molecular) ion is not altered by transitions of the outer electrons. Specific calculations are carried out for the case of the collision of hydrogen H(n) with helium He (1s²) atoms. It is shown that the cross sections and the rate constants of these processes are determined in this case by the mechanism investigated in the paper, and not by scattering of the Rydberg electron by the neutral particle. The cross sections for dipole excitation and dissociation of molecular ions from high vibrational energy levels by electron impact is calculated in the Born-Coulomb approximation. The cross sections and the rates of dissociative and three-particle attachment of electrons to ions are determined. The processes of autoionization and autodissociation decay of Rydberg states of vibrationally excited molecules are determined. Also investigated are radiative transitions near the dissociation limit of diatomic molecular ions and neutral molecules, viz., photodissociation and radiative decay of high vibrational levels, and photodissociation and translational (inverse-bremsstrahlung) absorption in collision of atomic particles.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. Lebedeva AN SSSR, Vol. 145, pp. 80–130, 1984.     

Effect of collisional transitions on the population distributions of excited atoms in a plasma

The effect of collisional transitions on the population distribution of excited atoms has been studied by considering a plasma in which the electron density varies with time. It has been found that for highly excited states, for which the ionization rate is much larger than that of electron-density variations, the collisional transitions are not important compared with the collisional ionization and recombination processes. However, for intermediate and lower excited states, they play an important role in making the population distribution approach Saha equilibrium with the electrons.     

Terahertz electroluminescence under conditions of shallow acceptor breakdown in germanium

The spectrum of spontaneous terahertz electroluminescence was obtained near the breakdown threshold of a shallow acceptor (Ga) in germanium. The emission spectra were recorded by the Fourier spectroscopy method at a temperature of ～5.5–5.6 K. The emission spectrum exhibits narrow lines with maxima at ～1.99 THz (8.2 meV) and ～2.36 THz (9.7 meV), corresponding to the optical transitions of nonequilibrium holes from the excited impurity states to the ground state of impurity center. A broad line with a maximum at ～3.15 THz (13 meV) corresponding to the hole transitions from the valence band to the impurity ground state is also seen in the spectrum. The contribution of the hole transitions from the states of the valence band increases upon an increase in the electric-field strength. Simultaneously, the optical transitions of nonequilibrium holes between the subbands of the valence band appear in the emission spectrum. The integral terahertz-emission power is ～17 nW per 1 W of the input power.     

Using two-dimensional photon echo spectroscopy to probe the fine structure of the ground state biexciton of CdSe nanocrystals

The origin of the fine structure of the ground state single- and biexciton of CdSe nanocrystals is reviewed, along with the theoretical framework used to describe these states. Calculations were performed to determine the transition dipole moments of optically allowed transitions from the single- to biexciton fine structure states. Two-dimensional photon echo spectroscopy measurements for a sample of CdSe nanocrystals are reported. The two-dimensional electronic spectrum at a population time of 0 fs is analyzed using a simulation based on k.p theory predictions of the exciton and biexciton manifolds of states. The analysis suggests that a particular excited state absorption transition from the single- to biexciton fine structure dominates the 2D spectra. These excited state absorptions are clearly resolved in 2D spectra and the method therefore has promise for gaining clearer insights into quantum dot spectroscopy.     

Spin and polarized current from Coulomb blockaded quantum dots

We report measurements of spin transitions for GaAs quantum dots in the Coulomb blockade regime and compare ground and excited state transport spectroscopy to direct measurements of the spin polarization of emitted current. Transport spectroscopy reveals both spin-increasing and spin-decreasing transitions, as well as higher-spin ground states, and allows g factors to be measured down to a single electron. The spin of emitted current in the Coulomb blockade regime, measured using spin-sensitive electron focusing, is found to be polarized along the direction of the applied magnetic field regardless of the ground state spin transition.     

Excited level lifetime measurements following very low energy nuclear recoils

Using high resolution gamma spectroscopy it is possible to observe Doppler broadening of gamma transitions from excited states which have themselves been populated after emission of gamma photons or other low energy nuclear radiation. This allows the determination of the lifetimes of these intermediate states, providing the time dependence of the recoil velocities is known. Conversely, if the lifetime is well known, this technique may be used to study the recoil process itself (and hence the atomic interaction) at energies of only a few times the lattice binding energy.     

Excitation of lowest electronic states of thymine by slow electrons

Excitation of lowest electronic states of the thymine molecules in the gas phase is studied by elec- tron energy loss spectroscopy. In addition to dipole-allowed transitions to singlet states, transitions to the lowest triplet states were observed. The low-energy features of the spectrum at 3.66 and 4.61 eV are identified with the excitation of the first triplet states 1₃ A′ (π → π*) and 1₃ A″ (n → π*). The higher-lying features at 4.96, 5.75, 6.17, and 7.35 eV are assigned mainly to the excitation of the π → π* transitions to the singlet states of the molecule. The excitation dynamics of the lowest states is studied. It is found that the first triplet state 1³ A′(π → π*) is most efficiently excited at a residual energy close to zero, while the singlet 2₁ A′(π → π*) state is excited with almost identical efficiency at different residual energies.     

量子限制效应对δ掺杂GaAs/AlAs多量子阱中铍受主态寿命的影响

采用远红外时间分辨光谱,研究了量子限制效应对δ掺杂在GaAs/AlAs多量子阱中铍（Be）受主态寿命的影响.在低温下的远红外吸收谱中,清楚地观察到了三条主要吸收线,它们分别来源于铍受主从基态到它的三个奇宇称激发态的跃迁.实验结果表明：随着量子限制效应的增强,受主激发态寿命而减少,实验测得体材料中Be受主2p激发态的寿命是350 ps,而阱宽10 nm的多量子阱中的寿命是55 ps.量子限制效应对布里渊区折叠声学声子模的影响增强了受主带内空穴与声学声子相互作用,从而加快了受主带内空穴的弛豫过程. 关键词： 量子限制效应 受主态寿命 时间分辨光谱 δ掺杂')" href="#">δ掺杂     

The Rotational Spectrum of OCCCS Revisited

The rotational spectrum of the linear chain molecule tricarbon oxide sulfide, OCCCS, was recorded at room temperature between 118 and 179 GHz using a new commercial millimeter-wave spectrometer. The observed transitions were analyzed together with hitherto unassigned transitions from measurements between 78 and 118 GHz. About 1900 rotational transitions of OCCCS in the vibrational ground state and in 13 excited bending states could be assigned, the latter including the eighth, ninth, and tenth excited state of the low-lying bending mode nu(7) as well as the second excited state of the bending mode nu(6). Besides the determination of effective constants for these newly identified vibrational states, our data also served to considerably improve the effective constants for some of the bending states already characterized in previous publications. Copyright 2000 Academic Press.     

Calculation of interaction of the stretching and bending vibrations of HF in the hydrogen-bonded complex [F(HF)2]?

Vibrations of the [F(HF)₂]^? complex are calculated with allowance for the anharmonic interactions of the stretching vibrations of HF monomers and their rotations about the centers of gravity of HF in the plane of the complex. A four-dimensional vibrational Schr?dinger equation is solved using a potential energy surface calculated in the MP2/6-311++G(3df,3pd) approximation with the superposition of atomic functions of the monomers taken into account. The equilibrium and vibrationally averaged structures of the complex are determined. The frequencies and intensities for spectral transitions from the ground state to a number of excited vibrational states are calculated. It is shown that, due to resonances between the excited states of the stretching modes and doubly excited states of the bending modes, the overtone transitions associated with the bending modes borrow a significant part of the intensity of fundamental stretching transitions.     

Electronic transitions of fluorene, dibenzofuran, carbazole, and dibenzothiophene: From the onset of absorption to the ionization threshold

A comparative study of the electronic transitions of fluorene and its hetero-analogues dibenzofuran, carbazole, and dibenzothiophene was performed in a wide energy range. Gas phase, crystal phase, and linear dichroism electronic transmittance spectra were measured with synchrotron radiation. Electronic transitions to excited singlet states were predicted with time-dependent density functional theory, TD-B3LYP/6-31+G(d,p). Based on the experimental and theoretical results, symmetry assignments of electronic transitions in the vacuum and near-UV region are suggested. The correspondence between excited states in these molecules, similarities, and differences between their electronic spectra are discussed.     

Excited states time evolution on a laser-ablated molybdenum plume

The dynamics of the excited states on a laser-ablated Mo plume was studied, both in air and in vacuum, by emission spectroscopy along the plume expansion axis. The emission related to ionized atoms occurs in the beginning of the plume expansion, near the metal surface, and is predominantly ultraviolet emission. In the middle of the plasma plume, it takes place the electron transitions between excited states of neutral atoms, and in the end of the plume, the visible emission is to transitions to the ground state of neutral molybdenum atoms. It was possible to determine plume parameters such as plasma expansion velocity of (5.0 ± 0.7) km/s at atmospheric pressure and (4.0 ± 0.7) km/s in vacuum, and the plasma duration that was (160 ± 14) ns at atmospheric pressure and (138 ± 18) ns in vacuum.     

Changes in quantum states of atoms in a markovian thermostat

We solve the nonlinear Schrödinger equation, taking into account self-interaction through the surrounding medium. We have found that such atoms have a countable set of stable equilibrium states, coinciding with the wave eigenfunctions of an individual atom. We show that transitions of an atom from equilibrium quantum states corresponding to less excited levels to more excited levels occur in jumps, while transitions from equilibrium states corresponding to upper levels to lower levels may occur either as radiationless transitions or be accompanied by spontaneous emission at the eigenfrequency of the quantum transition. We have found the conditions for which a change in the topological structure of the atomic wavefunction is possible.     

Contactless electroreflectance spectroscopy of optical transitions in low dimensional semiconductor structures

The authors present the application of contactless electroreflectance (CER) spectroscopy to study optical transitions in low dimensional semiconductor structures including quantum wells (QWs), step-like QWs, quantum dots (QDs), quantum dashes (QDashes), QDs and QDashes embedded in a QW, and QDashes coupled with a QW. For QWs optical transitions between the ground and excited states as well as optical transitions in QW barriers and step-like barriers have been clearly observed in CER spectra. Energies of these transitions have been compared with theoretical calculations and in this way the band structure has been determined for the investigated QWs. For QD and QDash structures optical transitions in QDs and QDashes as well as optical transitions in the wetting layer have been identified. For QDs and QDashes surrounded by a QW, in addition to energies of QD and QDash transitions, energies of optical transitions in the surrounded QW have been measured and the band structure has been determined for the surrounded QW. Finally some differences, which can be observed in CER and photo-reflectance spectra, have been presented and discussed for selected QW and QD structures.