-
[show abstract]
[hide abstract]
ABSTRACT: The recently developed attosecond light sources make the investigation of ultrafast processes in matter possible with unprecedented time resolution. It has been proposed that the very mechanism underlying the attosecond emission allows the imaging of valence orbitals with Ångström space resolution. This controversial idea together with the possibility of combining attosecond and Ångström resolutions in the same measurements has become a hot topic in strong-field science. Indeed, this could provide a new way to image the evolution of the molecular electron cloud during, e.g. a chemical reaction in 'real time'. Here we review both experimental and theoretical challenges raised by the implementation of these prospects. In particular, we show how the valence orbital structure is encoded in the spectral phase of the recombination dipole moment calculated for Coulomb scattering states, which allows a tomographic reconstruction of the orbital using first-order corrections to the plane-wave approach. The possibility of disentangling multi-channel contributions to the attosecond emission is discussed as well as the necessary compromise between the temporal and spatial resolutions.
Reports on Progress in Physics 06/2012; 75(6):062401. · 14.72 Impact Factor
-
Z Diveki,
A Camper,
S Haessler,
T Auguste,
T Ruchon,
B Carré,
P Salières,
R Guichard,
J Caillat,
A Maquet, R Taïeb
[show abstract]
[hide abstract]
ABSTRACT: When generated in molecules, high-order harmonics can be emitted through different ionization channels. The coherent and ultrafast electron dynamics occurring in the ion during the generation process is directly imprinted in the harmonic signal, i.e. in its amplitude and spectral phase. In aligned N2 molecules, we find evidence for a fast variation of this phase as a function of the harmonic order when varying the driving laser intensity. Basing our analysis on a three-step model, we find that this phase variation is a signature of transitions from a single- to a multi-channel regime. In particular, we show that significant nuclear dynamics may occur in the ionization channels on the attosecond timescale, affecting both the amplitude and the phase of the harmonic signal.
New Journal of Physics 02/2012; 14(2):023062. · 4.18 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We study the temporal aspects of laser-assisted extreme ultraviolet (XUV)
photoionization using attosecond pulses of harmonic radiation. The aim of this
paper is to establish the general form of the phase of the relevant transition
amplitudes and to make the connection with the time-delays that have been
recently measured in experiments. We find that the overall phase contains two
distinct types of contributions: one is expressed in terms of the phase-shifts
of the photoelectron continuum wavefunction while the other is linked to
continuum--continuum transitions induced by the infrared (IR) laser probe. Our
formalism applies to both kinds of measurements reported so far, namely the
ones using attosecond pulse trains of XUV harmonics and the others based on the
use of isolated attosecond pulses (streaking). The connection between the
phases and the time-delays is established with the help of finite difference
approximations to the energy derivatives of the phases. This makes clear that
the observed time-delays is a sum of two components: a one-photon Wigner-like
delay and an universal delay that originates from the probing process itself.
Chemical Physics. 12/2011;
-
K Klünder,
J M Dahlström,
M Gisselbrecht,
T Fordell,
M Swoboda,
D Guénot,
P Johnsson,
J Caillat,
J Mauritsson,
A Maquet, R Taïeb,
A L'Huillier
[show abstract]
[hide abstract]
ABSTRACT: We study photoionization of argon atoms excited by attosecond pulses using an interferometric measurement technique. We measure the difference in time delays between electrons emitted from the 3s(2) and from the 3p(6) shell, at different excitation energies ranging from 32 to 42 eV. The determination of photoemission time delays requires taking into account the measurement process, involving the interaction with a probing infrared field. This contribution can be estimated using a universal formula and is found to account for a substantial fraction of the measured delay.
Physical Review Letters 04/2011; 106(14):143002. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Polarization-dependent resonant inelastic x-ray scattering (RIXS) is shown to be a new probe of molecular-field effects on molecular electronic structure. Combining experiment and theory, linear dichroism in Cl 2p RIXS following Cl 1s excitation in HCl, Cl2, and CF3Cl is ascribed to molecular-field effects, indicating polarized-RIXS provides a direct probe of spin-orbit-state populations generally applicable to all molecules.
Journal of Physics Conference Series 12/2009; 194(2):022013.
-
[show abstract]
[hide abstract]
ABSTRACT: We present a combined theoretical and experimental study of molecular field effects on molecular core levels. Polarization-dependent resonant inelastic x-ray scattering is observed experimentally after resonant K-shell excitation of CF3Cl and HCl. We explain the linear dichroism observed in spin-orbit level intensities as due to molecular field effects, including singlet-triplet exchange, and interpret this behavior in terms of population differences in the 2px,y,z inner-shell orbitals. We investigate theoretically the different factors that can affect the electronic populations and the dynamical R dependence of the spin-orbit ratio. Finally, the results obtained are used to interpret the L-shell absorption spectra of the two molecules.
Phys. Rev. A. 09/2009; 80(3).
-
S Haessler,
W Boutu,
M Stankiewicz,
L J Frasinski,
S Weber,
J Caillat, R Taïeb,
A Maquet,
P Breger,
P Monchicourt,
B Carré,
P Salières
[show abstract]
[hide abstract]
ABSTRACT: We study the spectral phase of high-order harmonic emission as an observable for probing ultrafast nuclear dynamics after the ionization of a molecule. Using a strong-field approximation theory that includes nuclear dynamics, we relate the harmonic phase to the phase of the overlap integral of the nuclear wavefunctions of the initial neutral molecule and the molecular ion after an attosecond probe delay. We determine experimentally the group delay of the high harmonic emission from D2 and H2 molecules, which allows us to verify the relation between harmonic frequency and the attosecond delay. The small difference in the harmonic phase between H2 and D2 calculated theoretically is consistent with our experimental results.
Journal of Physics B Atomic Molecular and Optical Physics 06/2009; 42(13):134002. · 1.88 Impact Factor
-
S. Haessler,
B. Fabre,
J. Higuet,
J. Caillat,
T. Ruchon,
P. Breger,
B. Carré,
E. Constant,
A. Maquet,
E. Mével,
P. Salières, R. Taïeb,
Y. Mairesse
[show abstract]
[hide abstract]
ABSTRACT: We photoionize nitrogen molecules with a train of extreme ultraviolet attosecond pulses together with a weak infrared field. We measure the phase of the two-color two-photon ionization transition (molecular phase) for different states of the ion. We observe a 0.9 shift for the electrons produced in the ionization channels leading to the X 2g+" align="middle"/>, v=1, and v=2 states. We relate this phase shift to the presence of a complex resonance in the continuum. By providing both a high spectral and temporal resolution, this general approach gives access to the evolution of extremely short-lived states, which is often not accessible otherwise.
Physical Review A (Atomic, Molecular, and Optical Physics). 01/2009; 80:011404--011404(R).
-
M Meyer,
D Cubaynes,
D Glijer,
J Dardis,
P Hayden,
P Hough,
V Richardson,
E T Kennedy,
J T Costello,
P Radcliffe,
S Düsterer,
A Azima,
W B Li,
H Redlin,
J Feldhaus, R Taïeb,
A Maquet,
A N Grum-Grzhimailo,
E V Gryzlova,
S I Strakhova
[show abstract]
[hide abstract]
ABSTRACT: Two-color multiphoton ionization of atomic helium was investigated by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. In the photoelectron spectrum, lines associated with direct ionization and above-threshold ionization show strong variations of their amplitudes as a function of both the intensity of the optical dressing field and the relative orientation of the linear polarization vectors of the two fields. The polarization dependence provides direct insight into the symmetry of the outgoing electrons in above-threshold ionization. In the high field regime, the monochromaticity of the XUV radiation enables the unperturbed observation of nonlinear processes in the optical field.
Physical Review Letters 12/2008; 101(19):193002. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Polarization-dependent resonant inelastic x-ray scattering (RIXS) is shown to be a new probe of molecular-field effects on the electronic structure of isolated molecules. A combined experimental and theoretical analysis explains the linear dichroism observed in Cl 2p RIXS following Cl 1s excitation in HCl and CF3Cl as due to molecular-field effects, including singlet-triplet exchange, indicating polarized-RIXS provides a direct probe of spin-orbit-state populations applicable to any molecule.
Physical Review Letters 10/2008; 101(13):133003. · 7.37 Impact Factor
-
O Guyétand,
M Gisselbrecht,
A Huetz,
P Agostini, R Taïeb,
A Maquet,
B Carré,
P Breger,
O Gobert,
D Garzella,
J-F Hergott,
O Tcherbakoff,
H Merdji,
M Bougeard,
H Rottke,
M Böttcher,
Z Ansari,
P Antoine
[show abstract]
[hide abstract]
ABSTRACT: Single ionization of helium by a superposition of selected XUV high harmonics and infrared radiation has been studied by a momentum imaging technique. The measured angular distributions of photoelectrons are compared to numerical time-dependent calculations, showing very good agreement after average. The calculated angular distributions appear to depend critically on the delay between harmonic and infrared pulses on the attosecond scale, and on the relative phases and intensities of the harmonics.
Journal of Physics B Atomic Molecular and Optical Physics 02/2008; 41(5):051002. · 1.88 Impact Factor
-
P. Radcliffe,
S. Düsterer,
A. Azima,
H. Redlin,
J. Feldhaus,
John Dardis,
Kevin Kavanagh,
H. Luna,
Jofre Pedregosa-Gutierrez,
Patrick Yeates,
Eugene T. Kennedy,
John T. Costello,
A. Delserieys,
C L S Lewis, R. Taïeb,
A. Maquet,
D. Cubaynes,
M Meyer
[show abstract]
[hide abstract]
ABSTRACT: Two-color above threshold ionization of helium and xenon has been used to analyze the synchronization between individual pulses of the femtosecond extreme ultraviolet (XUV) free electron laser in Hamburg and an independent intense 120 fs mode-locked Ti:sapphire laser. Characteristic sidebands appear in the photoelectron spectra when the two pulses overlap spatially and temporally. The cross-correlation curve points to a 250 fs rms jitter between the two sources at the experiment. A more precise determination of the temporal fluctuation between the XUV and infrared pulses is obtained through the analysis of the single-shot sideband intensities.
Applied Physics Letters 03/2007; · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Using a Sturmian-like expansion of the (first-order) Dirac Coulomb Green's function, the two-photon probability amplitude for the |12S1/2 → |22S1/2 transition in hydrogenic systems is calculated as a function of nuclear charge Z. A comparison between relativistic and non-relativistic treatments is made, taking care of the influence of retardation in both cases.
EPL (Europhysics Letters) 01/2007; 37(6):391. · 2.17 Impact Factor
-
Y Mairesse,
A de Bohan,
L J Frasinski,
H Merdji,
L C Dinu,
P Monchicourt,
P Breger,
M Kovacev, R Taïeb,
B Carré,
H G Muller,
P Agostini,
P Salières
[show abstract]
[hide abstract]
ABSTRACT: Subfemtosecond light pulses can be obtained by superposing several high harmonics of an intense laser pulse. Provided that the harmonics are emitted simultaneously, increasing their number should result in shorter pulses. However, we found that the high harmonics were not synchronized on an attosecond time scale, thus setting a lower limit to the achievable x-ray pulse duration. We showed that the synchronization could be improved considerably by controlling the underlying ultrafast electron dynamics, to provide pulses of 130 attoseconds in duration. We discuss the possibility of achieving even shorter pulses, which would allow us to track fast electron processes in matter.
Science 12/2003; 302(5650):1540-3. · 31.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We address the question of the importance of relativity in selected non-linear radiative processes to be observed when an
atomic system is submitted to an intense radiation field. To this end, we report on recent results obtained in the theoretical
analysis of the following processes: i) two-photon transitions in high-Z hydrogenic systems, ii) laser-assisted Mott scattering
of fast electrons, and iii) two-colour photoionisation spectra of atoms in the simultaneous presence of two ultra-intense
laser fields. In each case, the signature of relativistic effects is evidenced.
12/2001: pages 107-118;
-
[show abstract]
[hide abstract]
ABSTRACT: We address the question of the energy and angular distributions of the photoelectrons ejected from rare gas atoms submitted to ultra-intense infrared laser pulses, with peak intensities I(max) approximately 10(18) W/cm (2). Several unsolved issues regarding the angular distributions of the photoelectrons are analyzed. We believe that our results should help to trigger new investigations.
Physical Review Letters 08/2001; 87(5):053002. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The influence of laser polarization on the triple differential cross section (TDCS) for electron impact ionization of helium is analysed in the asymmetric coplanar geometry. The interaction of the laser field with the incident, and scattered electrons is treated non-perturbatively by using Volkov waves, while that of the ejected electron moving in the combined field of the residual ion He+ and of the laser is obtained by using the ansatz formulated by Joachain et al (1988 Phys. Rev. Lett. 61 165) for the case of atomic hydrogen. The remaining interactions are treated by using first-order perturbation theory. Detailed calculations of the scattering amplitudes are performed for a helium target. We discuss the influence of the laser polarization on TDCS as a function of the angle of ejection of the slow electron for selected choices of the laser frequency and number of photons exchanged between the external field and the electron-helium system.
Journal of Physics B Atomic Molecular and Optical Physics 12/1998; 32(13):3255. · 1.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We study the scattering of fast electrons by atoms in the presence of a strong laser field. Our method takes into account the `dressing' of the target states by including the laser-atom interaction to first order, while the laser-projectile interaction is treated to all orders. The interaction of the fast incident electron with the target atom is treated in the first-Born approximation. Detailed calculations are performed for the `elastic' scattering of 500 eV electrons by helium accompanied by the transfer of photons. We discuss the influence of the laser polarization on the differential cross sections as a function of the scattering angle, laser frequency and intensity.
Journal of Physics B Atomic Molecular and Optical Physics 12/1998; 31(5):1115. · 1.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The influence of linearly and circularly polarized laser fields on the dynamics of fast electron-impact excitation in atomic helium is discussed. A detailed analysis is made in the excitation of 2 1S, 3 1S and 3 1D dressed states of helium target. By using a semiperturbative treatment with the Sturmian basis expansion, we take into account the target atom distortion induced by a laser field. Important differences appear between the angular distributions depending on the different states of polarization, in particular the circular polarization presents an experimental interest. We give new features (intermediate resonances) for both polarizations, concerning the n = 2 states of helium for emission and the n = 3 for the absorption, in terms of laser frequency. Qualitative differences from the case of laser-assisted elastic collisions have been found.
Journal of Physics B Atomic Molecular and Optical Physics 12/1998; 32(12):2783. · 1.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Using a representation of the (first-order) Dirac Coulomb Green function in terms of an expansion over a Sturmian basis, we have computed two-photon probability amplitudes for transitions in hydrogenic atoms from the ground state towards states in the L and M shells. The treatment takes care of the relativistic structure of the atoms according to the Dirac theory and of the fully retarded interaction with the electromagnetic field. We present in detail the Sturmian-like expansion for the Dirac Coulomb Green function and we discuss the relative importance of relativistic and retardation effects occurring for higher nuclear charge Z. Comparison is made also with the nonrelativistic Schrödinger treatment, retardation included or not.
Phys. Rev. A. 56(1).
