Optimization of the focused flux of high harmonics

Following the theoretical predictions [1], the observation of two-photon processes by interaction of vacuum ultraviolet (VUV) radiation with inner-shell levels of atoms requires focused intensities in the 10¹³-10¹⁴ W/cm² range. Our aim is to reach this regime in order to study non-linear optics at these wavelengths. We first optimized the high harmonic conversion efficiency in argon by studying the best experimental conditions for phase-matching, concentrating on focus geometry related to laser energy, cell length and position relative to the focus. We then studied the resulting harmonic beam focusability by a toroidal mirror (f=10 cm) and made an image of the harmonic focus. We conclude with an evaluation of the focused intensity that we are able to reach experimentally.Received: 28 January 2003, Published online: 24 April 2003PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation     

Photoionization of helium with ultrashort XUV laser pulses

Coupled-channel calculations for multiphoton ionization probabilities of helium through interaction with intensive short laser pulses are presented. Besides Slater-like orbitals we use regular Coulomb wavepackets in our configurational interaction basis to describe the continuum. Linearly polarized laser pulses of 3.8 fs duration and 2.96 x 10¹⁴ Wcm^-2 peak intensity have been used for frequencies between 0.2-1.2 a.u. The results are compared with other ab initio calculations.Received: 8 April 2003, Published online: 9 September 2003PACS: 32.80.Fb Photoionization of atoms and ions - 32.80.Wr Other multiphoton processes     

Coherent phase control in ionization of magnesium by a bichromatic laser field of frequencies \(\mathsf\omega \) and 2 \(\mathsf\omega \)

We have investigated the coherent phase control on the 3p² autoionizing state (AIS) resonantly coupled with the ground state for Mg through a two- and a four-photon transition simultaneously, using a bichromatic linearly polarized laser field. The frequency is chosen such that the lasers are tunable around resonance with the transition , which implies eV and eV. We are interested in the modification of autoionizing (AI) line shape through the relative phase and laser intensities. A strong phase dependence on the total ionization yield and ionization rate is found. We also performed a time-dependent calculation which takes into consideration all the resonant states of the process.Received: 5 November 2003, Published online: 9 March 2004PACS: 32.80.Qk Coherent control of atomic interactions with photons - 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Dz AutoionizationG. Buic-Zloh: Permanent address: Institute for Space Sciences, P.O. Box MG-23, 77125 Bucharest-Mgurele, Romania.     

Investigation of highly excited states of calcium by three-photon ionization

The three-photon ionization in Ca from S₀ ground state is studied. The two-photon process is a near -- resonance process with one of the following bound states: 4s4d ¹D₂, 4p P₂, 4s6s ¹S₀, 4 D₂ and 4 S₀ while the third photon reach either directly the continuum or one of the autoionizing states. The succession of bound states as well as the transitions above the ionization limit are discussed. The dynamics of the multiphoton excitation processes is also discussed and radiative decay of 4 S₀ Ca state with two-photon excitation as well as (the measured) decay times of the Ca autoionizing states using the proper line profiles for different quantum numbers has been determined.Received: 29 September 2003, Published online: 18 May 2004PACS: 32.70.-n Intensities and shapes of atomic spectral lines - 32.70.Cs Oscillator strengths, lifetimes, transition moments - 32.80.Fb Photoionization of atoms and ions - 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)     

Efficient three-step,two-color ionization of plutonium using a resonance enhanced 2-photon transition into an autoionizing state

Resonance ionization mass spectrometry (RIMS) has proven to be a powerful method for isotope selective ultra-trace analysis of long-lived radioisotopes. For plutonium detection limits of to atoms have been achieved for various types of samples. So far a three-step, three-color laser excitation scheme was applied for efficient ionization. In this work, a two-photon transition from an excited state into a high-lying autoionizing state, will be presented, yielding a similar overall efficiency as the three-step, three-color ionization scheme. In this way, only two tunable lasers are needed, while the advantages of a three-step, three-color excitation (high selectivity, good efficiency and low non-resonant background) are preserved. The two-photon transition has been characterized with respect to saturation behavior and line width. The three-step, two-color ionization is a possibility for an improved RIMS procedure.Received: 6 January 2004, Published online: 24 February 2004PACS: 82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI) - 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Dz Autoionization     

低能He2+-He碰撞体系转移电离机理的实验研究

利用冷靶反冲离子动量谱仪,对低能He²⁺-He碰撞反应中产生的反冲靶离子和炮弹离子进行了符合测量,根据反冲靶离子的动量,研究了转移电离过程中的电荷转移机理.实验结果表明：在20—40 keV能量范围内,靶原子上的一个电子俘获到炮弹离子的基态,另一个电子直接发射到靶的连续态的直接电离及另一个电子俘获到炮弹离子的连续态的过程(ECC)是最主要的转移电离机理,且ECC过程主要发生在大碰撞参数条件下;炮弹离子俘获两个电子处在双激发态的自电离过程的贡献很小. 关键词： 冷靶反冲离子动量谱仪 转移电离机理 离子原子碰撞     

Coherent Population Trapping and Partial Decoherence in the Stochastic Limit

We use the stochastic limit technique to predict a new phenomenon concerning a two-level atom with degenerate ground state interacting with a quantum field. We show, that the field drives the state of the atom to a stationary state, which is non-unique, but depends on the initial state of the system through some conserved quantities. This non uniqueness follows from the degeneracy of the ground state of the atom, and when the ground subspace is two-dimensional, the family of stationary states will depend on a one-dimensional parameter. Only one of the stationary states in this family is a pure state and it coincides with the known trapped state. This means that by controlling the initial state (input) we can control the final state (output). The quantum Markov semigroup obtained in the limit admits an invariant pure state, but it is not true that all the extremal invariant states are pure. This is an interesting phenomenon also from mathematical point of view and its meaning will be discussed in a future paper. PACS numbers: 31.15.-p, 31.15.Gy, 32.80.Pj, 32.80.Qk     

ATI of complex systems: Ar and C<Subscript>60</Subscript>

Above threshold ionization of two structurally different systems is presented namely a rare gas such as argon and the more complex C₆₀ fullerene. We show that the ionization dynamics is different and is dominated by the presence of high-lying Rydberg states in Ar and low-lying bound states in C₆₀. The study is based on a theoretical (solving the time dependent Schrödinger equation) and/or experimental (using measurements from a photoelectron imaging spectrometer) aspect.Received: 20 December 2002, Published online: 24 April 2003PACS: 33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Fb Photoionization of atoms and ions - 36.40.-c Atomic and molecular clusters - 33.60.-q Photoelectron spectra - 61.48.+c Fullerenes and fullerene-related materials     

Nonsequential two-photon double ionization of atoms: identifying the mechanism

We develop an approximate model for the process of direct (nonsequential) two-photon double ionization of atoms. Employing the model, we calculate (generalized) total cross sections as well as energy-resolved differential cross sections of helium for photon energies ranging from 39 to 54 eV. A comparison with results of ab initio calculations reveals that the agreement is at a quantitative level. We thus demonstrate that this complex ionization process can be described by the simple model, providing insight into the underlying physical mechanism. Finally, we use the model to calculate generalized cross sections for the two-photon double ionization of neon in the nonsequential regime.     

Prospects for precision measurements of atomic helium using direct frequency comb spectroscopy

We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral region via harmonic generation, it offers the prospect of greatly improved accuracy for UV and far-UV transitions. To take advantage of this accuracy an ultracold helium sample is needed. For measurements of the triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap metastable 2³S state atoms. We analyze schemes for measuring the two-photon 2³S →4³S interval, and for resonant two-photon excitation to high Rydberg states, 2³S →3³P →n³S, D. We also analyze experiments on the singlet-state spectrum. To accomplish this we propose schemes for producing and trapping ultracold helium in the 1¹S or 2¹S state via intercombination transitions. A particularly intriguing scenario is the possibility of measuring the 1¹S →2¹S transition with extremely high accuracy by use of two-photon excitation in a magic wavelength trap that operates identically for both states. We predict a “triple magic wavelength” at 412 nm that could facilitate numerous experiments on trapped helium atoms, because here the polarizabilities of the 1¹S, 2¹S and 2³S states are all similar, small, and positive.     

Decay,interference, and chaos

We establish a close quantitative analogy between the excitation and ionization process of highly excited one electron Rydberg states under microwave driving and charge transport across disordered 1D lattices. Our results open a new arena for Anderson localization -- a disorder induced effect -- in a large class of perfectly deterministic, decaying atomic systems.Received: 15 November 2002, Published online: 8 July 2003PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 05.45.Mt Quantum chaos; semiclassical methods - 72.15.Rn Localization effects (Anderson or weak localization)     

Formation of ultracold RbCs molecules by photoassociation

The formation of ultracold metastable RbCs molecules is observed in a double species magneto-optical trap through photoassociation below the ⁸⁵Rb(5S_1/2) + ¹³³Cs(6P_3/2) dissociation limit followed by spontaneous emission. The molecules are detected by resonance enhanced two-photon ionization. Using accurate quantum chemistry calculations of the potential energy curves and transition dipole moment, we interpret the observed photoassociation process as occurring at short internuclear distance, in contrast with most previous cold atom photoassociation studies. The vibrational levels excited by photoassociation belong to the 5th 0⁺ or the 4th 0^? electronic states correlated to the Rb(5P_{1/2, 3/2}) + Cs(6S_1/2) dissociation limit. The computed vibrational distribution of the produced molecules shows that they are stabilized in deeply bound vibrational states of the lowest triplet state. We also predict that a noticeable fraction of molecules is produced in the lowest level of the electronic ground state.     

ELECTRON IMPACT IONIZATION OF HELIUM FROM ITS GROUND AND METASTABLE STATES

The triple differential cross sections for the ionization of helium from its ground state 1¹S and metastable states 2¹S and 2³S in coplanar asymmetric geometry by 150, 250 and 400 eV electrons have been calculated in the BBK model. The present results are compared with the experimental data and/or the other theoretical ones. It has been found that the structures for the metastable states differ markedly from those for the ground state. The collision mechanisms for the new structures appearing in the cross sections for the ionization from metastable states have been analysed. And it has been shown how the intensity of recoil peak changes due to the size of the electron orbital. In addition, the optimal kinematical situations for the cross sections are explored for future experiment.     

Nonlinear atomic response to intense ultrashort x rays

The nonlinear absorption mechanisms of neon atoms to intense, femtosecond kilovolt x rays are investigated. The production of Ne(9+) is observed at x-ray frequencies below the Ne(8+), 1s(2) absorption edge and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of the two-photon ionization of Ne(8+) ground state and a high-order sequential process involving single-photon production and ionization of transient excited states on a time scale faster than the Auger decay. We find that the nonlinear direct two-photon ionization cross section is orders of magnitude higher than expected from previous calculations.     

Quantum control by selective population of dressed states using intense chirped femtosecond laser pulses

The physical mechanism of strong field quantum control using chirped femtosecond laser pulses is investigated. Dressed state control is exerted by making explicit use of the temporal phase changes of the pulse. In our experiment, the dressed state population is mapped by photoelectron spectra from simultaneous excitation and ionization of potassium atoms as a function of the chirp parameter. We show that chirped pulses can be used to selectively steer ground state atoms temporarily into a single dressed state realizing transient Selective Population of Dressed States (SPODS). PACS 32.80.Qk; 32.80.Rm; 33.80.Rv     

Experimental demonstration of selective coherent population transfer via a continuum

We report the first experimental demonstration of coherent population transfer, induced by stimulated Raman adiabatic passage, via continuum states. Population is transferred from the metastable state 2s(1)S(0) to the excited state 4s(1)S(0) in helium atoms in a two-photon process mediated by coherent interaction with the ionization continuum. While incoherent techniques usually do not permit any population transfer in such a process, we show that stimulated Raman adiabatic passage allows significant population transfer to take place also via ultrafast decay channels.     

Atomic and molecular ions in intense ultra-fast laser fields

The interaction of a 60 fs 790 nm laser pulse with beams of Ar⁺, C⁺, H₂ ⁺, HD⁺ and D₂ ⁺ are discussed. Intensities up to 10¹⁶ Wcm^-2 are employed. An experimental z-scanning technique is used to resolve the intensity dependent processes in the confocal volume.Received: 6 January 2003, Published online: 15 July 2003PACS: 32.80.Fb Photoionization of atoms and ions - 33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift     

Two-photon double ionization of helium: investigating the importance of correlation in the final state

In this paper, we present theoretical results for the process of non-sequential two-photon double ionization of helium at the photon energy 42 eV. Our approach is based on solving the time-dependent Schrödinger equation in a B-spline based numerical framework. Information about the process is obtained by extracting the double-ionized component by means of uncorrelated final states. The total (generalized) cross section for the process is extracted, as well as differential cross sections resolved in electron energies and ejection angles. We focus on the impact the final-state correlation has on the accuracy of the cross sections.     

Photoassociation experiments with ultracold metastable helium

The present article gives a review of various photoassociation (PA) experiments performed at ENS with a gas of ultracold atoms of metastable helium in the 2³S₁ state, using a PA laser beam red-detuned from the 2³S₁-2³P transitions. Molecular spectra close to the D₂ atomic line (2³S P₂) are presented. All the measured lines are identified as a signature of molecular bound states having a strong (if not pure) quintet spin character at short interatomic distance. Close to the D₀ atomic line (2³S P₀), giant helium dimers can be produced [see Phys. Rev. Lett. 91, 073203 (2003)]. A laser set-up improved recently allows us to measure very accurately the binding energy of the ro-vibrational ground state of the 0 _u ⁺ purely long-range potential and the agreement with the theory published previously is excellent. Finally, preliminary results on 2 photon PA spectroscopy are given.Received: 1 July 2004, Published online: 26 October 2004PACS: 32.80.Pj Optical cooling of atoms trapping - 33.20.Kf Visible molecular spectra - 34.50.Gb Electronic excitation and ionization of molecules - 34.20.Cf Interatomic potentials and forcesJ. Léonard: Present address: Van der Waals-Zeeman Institute, Universiteit van Amsterdam, The Netherlands.M. Walhout: Permanent address: Calvin College, Grand Rapids, MI, USA.     

A tentative approach to entanglement measures for a system of a three-level atom interacting with a quantized cavity-field

In this paper, an entanglement measure due to quasi-mutual entropy from initially entangled mixed states of a three-level atom interacting with a single cavity field is introduced. Detailed analytical and explicit expressions are given taking into account an arbitrary form of the intensity-dependent coupling. Despite its simplicity the model exhibits a very broad range of intricate physical effects and it is widely used in quantized theories of laser. We show that quantum revivals are possible for a broad continuous range of physical parameters in the case of initial coherent states. Entanglement degree effects are shown to be very sensitive to the initial state of the system. Numerical calculations under current experimental conditions are taken into account and it is found that the intensity-dependent coupling changes the general features dramatically.Received: 2 June 2003, Published online: 26 August 2003PACS: 03.65.Ud Entanglement and quantum nonlocality (e.g. EPR paradox, Bells inequalities, GHZ states, etc.) - 03.67.Hk Quantum communication - 42.65.-k Nonlinear optics - 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)