Evolution of photoelectron angular distributions in a train of identical,circularly polarized few-cycle laser pulses

Photoelectron angular distribution (PAD) of atoms irradiated by a train of identical, circularly polarized few-cycle laser pulses is studied in the frame of a nonperturbative scattering theory. Our study shows that the PADs vary with the kinetic energy of photoelectron, the carrier-envelope phase, and the pulse duration. We find that along with increasing of the kinetic energy of photoelectron or with decreasing of the pulse duration or the both, the original one maximum of PAD splits into two maxima; the newly produced two maxima evolve to the opposite pole of the symmetric axis, and finally incorporate as a new maximum located in the symmetric axis.Received: 26 February 2004, Published online: 10 August 2004PACS: 32.80.Rm 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 - 42.65.Re Ultrafast processes; optical pulse generation and pulse compression     

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     

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     

Electronic transfer processes studied at different time scales by selective resonant core hole excitation of adsorbed molecules

Core hole spectroscopy has been shown to provide information on ultrafast charge transfer processes on a time scale of the core hole lifetime. In this paper we present high-resolution autoionization studies of SF₆ and thiophene molecules adsorbed on a Ru(001) surface to further illustrate the potential of the so-called core hole clock method. Resonance states with very different core hole lifetimes can be excited with these sulfur-containing molecules. The selection of a specific resonant core hole excitation enables a variation of the time scale for probing electronic as well as nuclear dynamics. PACS 32.80.Dz; 32.80.Hd; 73.20.Jc     

Carrier-envelope phase measurement using a non phase stable laser

We demonstrate that the phase between the carrier and the pulse envelope of a few-cycle laser pulse can be retrieved from non phase stable laser systems, provided that such laser pulses are about 5 fs long and the repetition rate is in the order of 1 kHz. Our approach is based on online determination of the phase using f-2f interferometry. By a comparison of the self referencing interferometric signal with the photoelectron current emitted into a 7 degree solid angle parallel to the laser polarization, we obtain the absolute value of the carrier envelope phase. This is provided that a Coulomb correction for electron energies below 10 eV can be correctly taken into account. PACS 42.50.Hz; 42.65.Re; 32.80.Rm     

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     

Matter-wave cavity gravimeter

We propose a gravimeter based on a matter-wave resonant cavity loaded with a Bose–Einstein condensate and closed with a sequence of periodic Raman pulses. The gravimeter sensitivity increases quickly with the number of cycles experienced by the condensate inside the cavity. The matter wave is refocused thanks to a spherical wave-front of the Raman pulses. This provides a transverse confinement of the condensate which is discussed in terms of a stability analysis. We develop the analogy of this device with a resonator in momentum space for matter waves. PACS 06.30.Gv; 06.30.Ft; 03.75.-b; 03.75.Dg; 32.80.Lg; 32.80.-t; 32.80.Pj     

An all-optical ion-loading technique for scalable microtrap architectures

An experimental demonstration of a novel all-optical technique for loading ion traps, which has particular application to microtrap architectures, is presented. The technique is based on photoionisation of an atomic beam created by pulsed laser ablation of a calcium target, and provides improved temporal control compared to traditional trap loading methods. Ion loading rates as high as 125 ions per second have so far been observed. Also described are observations of trap loading where Rydberg state atoms are photoionised by the ion Doppler cooling laser. PACS 32.80.Fb; 32.80.Dz; 39.10.+j; 52.38.Mf     

Coherent control of high-order harmonics generated with intense femtosecond laser pulses

Experimental results and theoretical analysis on the coherent control of high-order harmonics with chirped femtosecond laser pulses are presented. The coherent control of high-order harmonic generation resulted in sharp harmonic spectra by compensating for induced harmonic chirp with the control of applied laser chirp and it was found to be crucial also in producing sharp and bright harmonics.Received: 18 November 2002, Published online: 8 July 2003PACS: 42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation - 32.80.-t Photon interactions with atoms - 42.65.Re Ultrafast processes; optical pulse generation and pulse compression     

Predicting intense-field photoionization of atoms and molecules from their linear photoabsorption spectra in the ionization continuum

We report a new approach to intense-field photoionization that is based on the ad hoc assumption that m photons of energy arriving within a typical electronic response time are effectively equivalent to a single photon of energy . The heuristic model contains no adjustable parameters and unifies apparent multiphoton and field aspects. Moreover, nonsequential, suppressed and above-threshold ionization phenomena become readily understandable. Predicted ionization intensities are in satisfactory agreement with available experimental data ranging from C₆H₆ to Ne^{3 +} , from femtosecond to nanosecond laser pulses, and from ultraviolet to infrared laser radiation.Received: 20 January 2004, Published online: 17 August 2004PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Wr Other multiphoton processes - 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift     

An intense cold beam of metastable helium

We have produced and characterised a slow, bright and intense atomic beam of metastable helium atoms, suitable for atomic physics experiments. The maximum continuous flux attained was 2×10¹⁰ atoms/s, while a typical longitudinal peak velocity of the beam was ∼26 m/s with a divergence in the range of 15 mrad to 30 mrad. PACS 32.80.Pj; 32.80.Lg; 39.10.+j     

Cavity cooling of translational and ro-vibrational motion of molecules: ab initio-based simulations for OH and NO

We present detailed calculations on the basis of our recent proposal for simultaneous cooling of the rotational, vibrational and external molecular degrees of freedom [1]. In this method, the molecular ro-vibronic states are coupled by an intense laser and an optical cavity via coherent Raman processes enhanced by the strong coupling with the cavity modes. For a prototype system, OH, we showed that the translational motion is cooled to a few μK and the molecule is brought to the internal ground state in about a second. Here, we investigate numerically the dependence of the cooling scheme on the molecular polarizability, selecting NO as a second example. Furthermore, we demonstrate the general applicability of the proposed cooling scheme to initially vibrationally and rotationally hot molecular systems. PACS 33.80.Ps; 32.80.Lg; 42.50.Pq     

Influence of elastic-scattering processes on an X-ray photoelectron spectroscopy signal: Effect of the underlying surface

The boundary-value problem for the transfer equation with internal sources and without them is solved using the invariant embedding method. The influence of processes of photoelectron elastic scattering on the energy spectra of photoelectron emission is analyzed. The focus of attention is on the analysis of variations in the intensity and the spectral composition of the ascending flow of the photoelectron emission of a layer when underlying layers with different compositions appear. The physical fundamentals of the influence of elastic scattering on the formation of X-ray photoelectron spectra are demonstrated. The peak intensities for homogeneous Au 4s _1/2 and Si 2s _1/2 targets are calculated with and without the inclusion of reflection processes. The error appearing as a result of neglecting elastic scattering processes, which transform the descending motion into ascending type, is determined.     

A variable angle photoelectron spectrometer using a position-sensitive detector

A variable angle photoelectron spectrometer utilizing a position-sensitive multidetector is described. Photoelectron spectra of N₂ obtained using a synchrotron radiation source are presented. The data are acquired using a new technique in which the photoelectron yield is obtained as a function of both wavelength and photoelectron energy, and this gives detailed information on autoionization processes.     

Two-photon interference spectra at the sum-frequency generation of a bichromatic field interacting with a three-level atom

We have calculated the spectral functions for a number of induced excitations near the sum-frequency generation of a bichromatic field interacting with a three-level atom in the cascade configuration. Although the two-photon spontaneous emission is electric dipole forbidden, there are induced peaks whose subnatural line widths and spectral heights depend on the intensities of the bichromatic field. At low intensities of both laser fields, the maximum heights of the induced peaks take positive, zero, and negative values, indicating that the physical processes of absorption (attenuation), dark resonance (nonabsorbing state), and stimulated emission (amplification) are likely to occur at the sum-frequency generation of the two laser fields, respectively. The derived results are graphically presented and discussed. PACS 32.80.-t; 42.50.Hz     

Probing electron-electron correlation with attosecond pulses

We study two-photon double ionization of helium in its ground state at sufficiently low laser intensities so that three and more photon absorptions are negligible. In the regime where sequential ionization dominates, the two-photon double ionization one-electron energy spectrum exhibits a well defined double peak structure directly related to the electron-electron correlation in the ground state. We demonstrate that when helium is exposed to subfemtosecond or attosecond pulses, both peaks move and their displacement is a signature of the time needed by the He⁺ orbital to relax after the ejection of the first electron. This result rests on the numerical solution of the corresponding non-relativistic time-dependent Schrödinger equation.Received: 17 January 2003, Published online: 18 March 2003PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Dz Autoionization     

Electronic structure of cerium heavy-fermion systems

A number of cerium-based systems are investigated using x-ray photoelectron spectroscopy. It is revealed that there is a correlation between the parameters of the x-ray photoelectron spectra and the heavy-fermion state. This correlation makes it possible to investigate the relaxation processes, anomalously strong reorganization of orbitals, profound collective effects induced by a vacancy field, and hybridization of localized unoccupied f states with delocalized occupied states.     

Time-dependent spectrum of thermionic emission from hot C60

Experimental measurements of time-dependent photoelectron spectra observed in thermionic emission of hot C₆₀ excited by multiphoton absorption are presented. Time resolved velocity-map imaging is used to record photoelectron spectra and to disentangle direct and delayed processes. The evolution of the kinetic energy distribution of thermal electrons as a function of the delay after multiphoton excitation is described within the general formalism of the detailed balance theory. Experimental spectra obtained in the near-UV are in excellent agreement with the assumption of thermal equilibrium.     

Fluorescence from doubly driven four-level atoms

The unusually narrow features in the fluorescence from ⁸⁵Rb driven by two laser fields L1 and L2, reported in [1], are explained on the basis of a four-level density matrix calculation. The L2 laser enables atom transfer to the fluorescing levels connected by the strong L1 laser. In turn the L1 laser causes the Autler-Townes splitting of the upper levels connected by L2 laser. These two effects are shown to maximise fluorescence within a narrow spectral range of the scanned L2 laser due to velocity selection of atoms from co-propagating and counter propagating L1 and L2 lasers. The analysis reveals the existence of narrow spectral features from a collection of atoms at room temperature even in the absence of induced coherences between the levels.Received: 2 July 2004, Published online: 21 September 2004PACS: 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift - 32.80.Bx Level crossing and optical pumping     

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