Two-photon detachment cross-section of atoms in the extended asymptotic model

Summary A previously proposed one-electron model for photoionization, referred to as extended asymptotic model [J. Mol. Struct. (Teochem) 166, 369 (1988)], is further developed to cope with two-photon ionization processes. A simple application of the approach to the case of the negative ion H⁻ is investigated and discussed. The authors of this paper have agreed to not receive the proofs for correction.     

Kroll-Watson-type theorem for potential scattering in a bichromatic laser field

Summary We show that for potential scattering of electrons in a bichromatic laser field a Kroll-Watson type of theorem can be derived in which the scatteringT-matrix element is composed of a renormalized on-shell matrix element for scattering without a field times a generalized Bessel function factor. The radiation field has two components of frequency ω₁ and sω₁ (s=2,3,…) and both components are out of phase by an angle ϕ. Our paper is a generalization of earlier investigations which were performed in the first-order Born approximation.     

Bremsstrahlung and harmonic generation in laser-assisted electron-nucleus collision

Summary The spectrum of radiation emitted by an electron colliding with a nucleus in the presence of a monochromatic laser field is calculated. The Coulomb potential is treated at all orders and the radiation is seen as spontaneous transition between two free states. The kinetic energy of the electron isT∈100−3000 eV and the laser intensityI∈10¹⁰−10¹³ W/cm²; in this condition we see strong enhancement in thebremsstrahlung cross-section when the emitted frequency is an integral multiple of the laser frequency.     

Feedback effects in a He−Ne long tube at 3391 nm

Summary In this paper some experiments are presented on the 3391 generation in a He−Ne long tube and, in particular, on its sensitivity to its own radiation reflected back into the tube: the self-stimulation or feedback effect. The smallest feedback, experimentally estimated as 2.5·10⁻⁶, corresponds to a signal gain not smaller than 4·10⁵. Reflecting the output of one tube end back into the tube causes the output to be reduced in a ratio not smaller than 5:1. Also some preliminary observations of fluctuations of the 3391 generation are presented. The work was supported by CPBR 8.14.     

Simultaneous multiphoton ionization by two radiation fields. Effects of the statistical properties of the radiation

Summary We report on calculations of differential and total ionization cross-sections of hydrogen atoms irradiated by two radiation fields with different properties. One of the fields is of low intensity and relatively high frequency, the other is of low frequency and high intensity. In particular, we show that the inclusion of the multimode structure of the low-frequency laser field modifies considerably the shape of the angular distribution of the photoelectrons and the rates of ionization into the different channels characterized by the number of low-frequency photons exchanged. Further, we find that the average energy exchanged between the photoelectrons and the low-frequency radiation field is independent of the statistical properties of the low-frequency laser field.     

An approach to the study of strong-field laser-induced autoionization

Summary An approach is presented to the study of laser-induced autoionization which usescomplex dressed states as a basis, instead of the commonly used Fano eigenstates. The complex dressed states are the eigenstates of an effective Hamiltonian governing the equations of motion for the atomic probability amplitudes in the broad background assumption and in the rotating frame at the laser frequency. The electron and photon spectra are described in terms of the eigenvalues associated with the complex dressed states.

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Riassunto Viene presentato un approccio allo studio dell'autoionizzazione indotta da laser che, al posto degli autostati di Fano, utilizza quale base una coppia distati vestiti complessi. Tali stati sono gli autostati di una hamiltoniana effettiva che governa le equazioni di moto per le ampiezze di probabilità atomiche in un sistema di riferimento ruotante alla frequenza del laser e nell'ipotesi di continuo largo. Gli spettri di elettroni e fotoni sono descritti in termini degli autovalori associati agli stati vestiti complessi.

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Резюме Предлагается подход к исследованию автоионизации, индуцированной лазерным сильным полем, в котором используются, как, базис, комплексные “одетые” состояния, вместо обычных собственных состояний Фано. Комплексные “одетые” состояния представляют собственные состояния эффективного гамильтониана, определяющего уравнения движения для амплитуды вероятности атома во вращвющейся системе отсчета с лазерной частотой и в предположении широкого континуума. Электронные и фотонные спектры описываются в терминах собственных значений, связанных с комплексными “одетыми” состояниями.

     

On the 3391 superstimulation and its competition with the 633 laser oscillations

Summary Competition between the 3391 radiation emitted by a long He−Ne tube and the 633 laser oscillations was investigated experimentally. the results were analysed by means of the balance equations of populations. The absolute values of populations of the 5s′[1/2] ₁ ⁰ and 4p′[3/2]₂ levels, the excitation rates of the levels and the rates of induced transitions were computed. A new experiment on the 3391 radiation generated by the long He−Ne tube showed a negligible value of the radiation in the middle of the tube.

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Riassunto Si è studiata sperimentalmente la competizione tra la radiazione 3391 emessa da un lungo tubo a He−Ne e l'oscillazione dellaser 633. I risultati sono stati analizzati per mezzo dell'equazioni di bilanciamento delle popolazioni. Sono stati calcolati i valori assoluti delle popolazioni dei livelli 5s′[1/2] ₁ ⁰ e 4p′[3/2]₂, i valori di eccitazione dei livelli e i valori delle transizioni indotte. Un nuovo esperimento sulla radiazione 3391 generata dal lungo tubo a He−Ne ha mostrato un valore trascurabile della radiazione nel centro del tubo.

     

3391 and 7699 laser generations and questions on the chaos

Summary In this report investigations are presented of laser generations at 3391 nm and 3391 and 7699 nm, or rather of changes induced by them in populations of the 5s′ [1/2] ₁ ⁰ , 4p′[3/2]₂ and 3d′[5/2] ₃ ⁰ levels, depending upon the angle of the tilted mirror of the He−Ne laser system. This writer has discovered that the above laser generations develop at a strongly tilted mirror, when the beam reflected from one mirror, in order to reach the other, has to be reflected many times on the inside walls of the discharge tube. The work was supported by CPBR 8.14.     

Light-induced ejection of calcium atoms from polymer surfaces

Laser-induced fluorescence (LIF) of calcium atoms at room temperature has been observed in a polydimethylsiloxane (PDMS) coated cell when the walls are illuminated with non resonant visible light. Ca atomic density in the gas phase, monitored by the LIF, is much higher than normal room-temperature vapour pressure of calcium. In past years photon-stimulated desorption (PSD) was observed for several alkali metals that adsorbed to solid films of PDMS polymers. High yields of photo-desorbed atoms (and molecules in the case of sodium) can be induced, at room temperature and below, by weak intensity radiation. The desorption is characterised by a frequency threshold, whereas any power threshold is undetectable. The calcium photo-ejection is characterised both by a frequency threshold (about 18 500 cm^-1) and by an observable power threshold (whose value becomes lower when the photo-ejecting light wavelength decreases).     

Open-channel fluorescence imaging of atoms in high-gradient magnetic fields

A method of imaging distributions of cold atoms under the presence of large trapping-field-induced level shifts is investigated. By utilizing a probe laser tuned to an open transition, the fluorescence yield per atom is largely fixed throughout the trap volume, independent of the trapping field. This enables a reliable conversion of fluorescence images into atomic-density profiles. The method is applied to measure distributions of ⁸⁷Rb atoms in a high-gradient (2.7 kG/cm) magnetic atom guide. We characterize the parameters for which the open-channel imaging method performs best. Results of quantum Monte Carlo simulations verify the underlying assumptions of the method.     

Magneto-optical trapping of Stark-slowed metastable He atoms

We report on the first successful loading of a magneto-optical trap (MOT) with metastable He atoms from a Stark-slower. Thereby, deceleration of the atoms relies on laser-atom interaction in an inhomogeneous electric field. We show that the results obtained are comparable with early results from other groups achieved with a Zeeman slower. The Stark slower, which is able to fully control the final velocity of the atomic He beam, is the first step in achieving complete spin independent kinematic control based solely on electric fields. Received 2 October 2002 / Received in final form 20 January 2003 Published online 29 April 2003 RID="a" ID="a"e-mail: eichmann@mbi-berlin.de     

Theoretical study of collisional redistribution of light near the resonance of the Ba, Sr and Mg atoms perturbed by He and Ne

Absorption coefficient and polarization of collisionally redistributed fluorescence light in a range of detunings around the atomic resonance have been calculated for Ba, Sr and Mg perturbed by He and Ne. Results are obtained from fully-quantum mechanical coupled-channels calculations including the relevant ground and two excited and molecular states for each diatomic. Close-coupling calculations are carried out based on the theoretical potential curves obtained by means of a pseudopotential + valence configuration-interaction (CI) technique. For accurate comparison with experiment the calculated absorption coefficients and polarizations have been thermally averaged over the collision energy. The theoretical absorption profiles and linear polarization ratios agree, in general, quite well with the available experimental data. Received: 19 February 1998 / Accepted: 9 April 1998     

碱金属原子的光激发与光电离

本文研究了碱金属原子在三步激光脉冲作用下的光激发和光电离过程的动力学特性, 重点关注和比较了锂和铯原子的异同. 针对多种激发模式, 本文不但建立了其原子布居数在各个跃迁态的速率方程组, 还给出了各相关态的光激发和光电离过程的解析解. 通过精心设计并选择了特殊情况, 显著简化了解析解的数学表达式, 从而凸显和讨论了其物理内涵. 通过自行编程, 系统地计算和观察了各种激发模式对锂原子的光激发和光电离过程的可能影响, 研究和讨论了电离率随激光参数的变化规律. 在相同激发模式下, 比较和分析了采用两种不同激发路径所导致的各态原子布居率的变化, 凸显了改变原子参数所产生的作用. 探讨了锂和铯原子在类似的激发条件下在电离率方面的差别. 最后, 基于本文的研究结果, 本文指出了优化电离率的多种途径.     

Non-relativistic exact transition matrix elements for hydrogenic atoms

Summary By using a general formula due to Moses (Phys. Rev. A,8, 1710 (1973)) exact matrix elements for different transitions in non-relativistic amomic hydrogen are evaluated explicitly.     

Implementation of a CNOT gate in two cold Rydberg atoms by the nonholonomic control technique

We present a demonstrative application of the nonholonomic control method to a real physical system composed of two cold Cesium atoms. In particular, we show how to implement a CNOT quantum gate in this system by means of a controlled Stark field.     

A simple non-perturbative approach of atom ionisation by intense and ultra-short laser pulses

A simple theoretical approach based on Coulomb-Volkov states is introduced to predict ionisation of atoms by intense laser pulses in cases where the effective interaction time does not exceed one or two optical cycles [M. Nisoli et al., Opt. Lett. 22, 522 (1997)]. Under these conditions, the energy distributions of ejected electrons predicted by this non-perturbative approach are in very good agreement with “exact" results obtained by a full numerical treatment. The agreement is all the better that the principal quantum number of the initial state is high. For very strong fields, most electrons are ejected at an energy which is close to the classical kinetic energy that would be transferred to free electrons by the electromagnetic field during the pulse. The power of the present approach appears when keV. In this region, full numerical treatments become very lengthy and finally do not converge. However, the present Coulomb-Volkov theory still makes reliable predictions in very short computer times. Received 19 November 1999 and Received in final form 19 January 2000     

ac Stark effect for a two-level atom in a squeezed vacuum via a two-photon process

The nondegenerate two-photon interaction of a two-level atom with a broadband multimode squeezed vacuum is investigated. We find that in the two-photon process the squeezed vacuum has a driving effect on the atom which can lead to an ac Stark effect when the average photon number of the squeezed vacuum is larger than a critical value. Received: 2 February 1999 / Received in final form: 20 April 1999     

Theoretical calculation of electron-impact resonance excitation of alkali atoms

Total cross-section for electron impact excitation of the resonance levels in Li, Na and K are calculated using the method due to Crothers and McCarroll. The results are compared with other available theoretical and experimental data.     

Entangling atoms in photonic crystals

We propose a method for entangling a system of two-level atoms in photonic crystals. The atoms are assumed to move in void regions of a photonic crystal. The interaction between the atoms is mediated either via a defect mode or via a resonant dipole-dipole interaction. We show that these interactions can produce pure entangled atomic states. We analyze the problem with parameters typical for currently existing photonic crystals and Rydberg atoms and we show that the atoms can emerge from photonic crystals in entangled states. Depending on the linear dimensions of the crystal we estimate that a pair of atoms entangled in a photonic crystal can be separated by tens of centimeters. Receive 11 June 1999 and Received in final form 4 October 1999