Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the ...light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution.
It is well established that electrons can escape from atoms through tunneling under the influence of strong laser fields, but the timing of the process has been controversial and far too rapid to ...probe in detail. We used attosecond angular streaking to place an upper limit of 34 attoseconds and an intensity-averaged upper limit of 12 attoseconds on the tunneling delay time in strong field ionization of a helium atom. The ionization field derives from 5.5-femtosecond-long near-infrared laser pulses with peak intensities ranging from 2.3 x 10¹⁴ to 3.5 x 10¹⁴ watts per square centimeter (corresponding to a Keldysh parameter variation from 1.45 to 1.17, associated with the onset of efficient tunneling). The technique relies on establishing an absolute reference point in the laboratory frame by elliptical polarization of the laser pulse, from which field-induced momentum shifts of the emergent electron can be assigned to a temporal delay on the basis of the known oscillation of the field vector.
Modelling atomic processes in intense laser fields often relies on solving the time-dependent Schrödinger equation (TDSE). For processes involving ionisation, such as above-threshold ionisation (ATI) ...and high-harmonic generation (HHG), this is a formidable task even if only one electron is active. Several powerful ideas for efficient implementation of atomic TDSE were introduced by H.G. Muller some time ago (Muller, 1999), including: separation of Hamiltonian terms into tri-diagonal parts; implicit representation of the spatial derivatives; and use of a rotating reference frame. Here, we extend these techniques to allow for non-uniform radial grids, arbitrary laser field polarisation, and non-Hermitian terms in the Hamiltonian due to the implicit form of the derivatives (previously neglected). We implement the resulting propagator in a parallel Fortran program, adapted for multi-core execution. Cost of TDSE propagation scales linearly with the problem size, enabling full-dimensional calculations of strong-field ATI and HHG spectra for arbitrary field polarisations on a standard desktop PC.
Program title: SCID-TDSE: Time-dependent solution of 1-electron atomic Schrödinger equation in strong laser fields.
Catalogue identifier: AEYM_v1_0
Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEYM_v1_0.html
Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland
Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html
No. of lines in distributed program, including test data, etc.: 334254
No. of bytes in distributed program, including test data, etc.: 20005596
Distribution format: tar.gz
Programming language: Fortran-2003 with OpenMP extensions.
Computer: Portable code. Tested on x86_64 Linux.
Operating system: Portable code. Tested on x86_64 Linux.
RAM: Memory requirements depend on the input parameters. Time propagation of a given initial state requires O(32∗NR∗(LMAX+1)∗(MMAX−MMIN+1)) bytes of memory (double precision), where NR is the number of the radial grid points; LMAX is the maximum desired angular momentum; MMIN and MMAX are respectively the smallest and largest desired angular momentum projections. Preparation of the initial atomic states and analysis of the final wavefunction in terms of the field-free atomic states may require O(64∗NR2∗NCPU) bytes of RAM (double precision), where NCPU is the number of processing threads used.
Classification: 2.5, 4.3.
External routines: BLAS and LAPACK (required); libhugetlbfs (optional), DGEFA and DGEDI (LINPACK); routines included with the code.
Nature of problem: Time propagation of non-relativistic 1-electron Schrödinger equation for a central potential, under the influence of a long-wavelength laser field treated in the velocity-gauge dipole approximation.
Solution method: The propagator is constructed by separating the Hamiltonian into a large number of non-commuting terms, where each term can be handled simply and computationally efficiently (linear scaling). Time-reversibility of the propagator is ensured by combining the individual terms symmetrically around time midpoint (See ref. 1 and the text). The numerical accuracy is achieved through implicit representation of derivatives (accurate to O(d4) for a uniform grid), combined with variable grid spacing.
Restrictions: Ill-conditioned Hamiltonians can occur for some choices or radial grids. The propagator is only approximately norm-conserving; small time steps may be necessary to achieve stable propagation.
Unusual features: Due to the implicit representation of the spatial derivative operators, the overall Hamiltonian is not Hermitian. As the result, the left wavefunction is no longer given by a complex conjugate of the right wavefunction, and must be propagated explicitly.
The code makes no assumptions on the accuracy of numerical types, and can be built for any real or integer kinds supported by the compiler. Detailed instructions for building the code in single, double, and quadruple precision are included.
Running time: Running time is input dependent. Time propagation of the Schrödinger equation scales as O(NR∗(LMAX+1)∗(MMAX−MMIN+1)) per time step. Preparation of the initial atomic state and analysis of the results in terms of the field-free atomic states may require O(NR3) and O(NR3∗(LMAX+1)) work, respectively. On a 3.6 GHz core i7 desktop with 4 CPU cores available, run times are from 2 s (probability of a perturbative bound- to-bound transition in hydrogen atom) to 1 h (high-harmonic spectrum of a hydrogen atom driven by intense elliptically-polarised IR field).
References:1H.G. Muller, Laser Physics 9 (1999), 138–148.
In 1964 Keldysh helped lay the foundations of strong-field physics by introducing a theoretical framework that characterized atomic ionization as a process that evolves with the intensity and ...wavelength of the fundamental field. Within this context, experiments have examined the intensity-dependent ionization but, except for a few cases, technological limitations have confined the majority to wavelengths below 1 μm. The development of intense, ultrafast laser sources in the mid-infrared (1 μm<λ<5 μm) region enables exploration of the wavelength scaling of the Keldysh picture while enabling new opportunities in strong-field physics, control of electronic motion and attosecond science. Here we report a systematic experimental investigation of the wavelength scaling in this region by concurrently analysing the production of energetic electrons and photons emitted by argon atoms interacting with few-cycle, mid-infrared fields. The results support the implicit predictions contained in Keldysh's work, and pave the way to the realization of brighter and shorter attosecond pulsed light sources using longer-wavelength driving fields.
Time-Resolved Holography with Photoelectrons Huismans, Y; Rouzée, A; Gijsbertsen, A ...
Science (American Association for the Advancement of Science),
01/2011, Letnik:
331, Številka:
6013
Journal Article
Recenzirano
Odprti dostop
Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow the ...measurement of electron motion in real time. We present experiments in which metastable xenon atoms were ionized with intense 7-micrometer laser pulses from a free-electron laser. Holographic structures were observed that record underlying electron dynamics on a sublaser-cycle time scale, enabling photoelectron spectroscopy with a time resolution of almost two orders of magnitude higher than the duration of the ionizing pulse.
Background. Clinical data on the association of diabetes mellitus with common infections are virtually lacking, not conclusive, and often biased. We intended to determine the relative risks of common ...infections in patients with type 1 and type 2 diabetes mellitus (DM1 and DM2, respectively). Methods. In a 12-month prospective cohort study conducted as part of the Second Dutch National Survey of General Practice, we compared 705 adult patients who had DM1 and 6712 adult patients who had DM2 with 18,911 control patients who had hypertension without diabetes. Outcome measures were medically attended episodes of infection of the respiratory tract, urinary tract, and skin and mucous membranes. We applied multivariable and polytomous logistic regression analysis to determine independent risks of infections and their recurrences in patients with diabetes, compared with control patients. Results. Upper respiratory infections were equally common among patients with diabetes and control patients. Patients with diabetes had a greater risk of lower respiratory tract infection (for patients with DM1: adjusted odds ratio AOR, 1.42 95% confidence interval {CI}, 0.96–2.08; for patients with DM2: AOR, 1.32 95% CI, 1.13–1.53), urinary tract infection (for patients with DM1: AOR, 1.96 95% CI, 1.49–2.58; for patients with DM2: AOR, 1.24 95% CI, 1.10–1.39), bacterial skin and mucous membrane infection (for patients with DM1: AOR, 1.59 95% CI, 1.12–2.24; for patients with DM2: AOR, 1.33 95% CI, 1.15–1.54), and mycotic skin and mucous membrane infection (for patients with DM1: AOR, 1.34 95% CI, 0.97–1.84; for patients with DM2: AOR, 1.44 95% CI, 1.27–1.63). Risks increased with recurrences of common infections. Conclusions. Patients with DM1 and DM2 are at increased risk for lower respiratory tract infection, urinary tract infection, and skin and mucous membrane infection. Studies are warranted into management of such infections in patients with diabetes.
The results of the PROCTOR-SCRIPT trial could not demonstrate a significant benefit of adjuvant chemotherapy with fluoropyrimidine monotherapy regarding overall survival, disease-free survival, and ...recurrence rates after preoperative (chemo)radiotherapy and TME surgery in yp stage II and III rectal cancer patients.
The discussion on the role of adjuvant chemotherapy for rectal cancer patients treated according to current guidelines is still ongoing. A multicentre, randomized phase III trial, PROCTOR-SCRIPT, was conducted to compare adjuvant chemotherapy with observation for rectal cancer patients treated with preoperative (chemo)radiotherapy and total mesorectal excision (TME).
The PROCTOR-SCRIPT trial recruited patients from 52 hospitals. Patients with histologically proven stage II or III rectal adenocarcinoma were randomly assigned (1:1) to observation or adjuvant chemotherapy after preoperative (chemo)radiotherapy and TME. Radiotherapy consisted of 5 × 5 Gy. Chemoradiotherapy consisted of 25 × 1.8–2 Gy combined with 5-FU-based chemotherapy. Adjuvant chemotherapy consisted of 5-FU/LV (PROCTOR) or eight courses capecitabine (SCRIPT). Randomization was based on permuted blocks of six, stratified according to centre, residual tumour, time between last irradiation and surgery, and preoperative treatment. The primary end point was overall survival.
Of 470 enrolled patients, 437 were eligible. The trial closed prematurely because of slow patient accrual. Patients were randomly assigned to observation (n = 221) or adjuvant chemotherapy (n = 216). After a median follow-up of 5.0 years, 5-year overall survival was 79.2% in the observation group and 80.4% in the chemotherapy group hazard ratio (HR) 0.93, 95% confidence interval (CI) 0.62–1.39;P = 0.73. The HR for disease-free survival was 0.80 (95% CI 0.60–1.07;P = 0.13). Five-year cumulative incidence for locoregional recurrences was 7.8% in both groups. Five-year cumulative incidence for distant recurrences was 38.5% and 34.7%, respectively (P = 0.39).
The PROCTOR-SCRIPT trial could not demonstrate a significant benefit of adjuvant chemotherapy with fluoropyrimidine monotherapy after preoperative (chemo)radiotherapy and TME on overall survival, disease-free survival, and recurrence rate. However, this trial did not complete planned accrual.
Dutch Colorectal Cancer group, CKTO 2003-16, ISRCTN36266738.
Attosecond Synchronization of High-Harmonic Soft X-rays Mairesse, Y.; de Bohan, A.; Frasinski, L. J. ...
Science (American Association for the Advancement of Science),
11/2003, Letnik:
302, Številka:
5650
Journal Article
Recenzirano
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.
Over the past thirty years, extensive studies of strong-field photoionization of atoms have revealed both quantum and classical aspects including above-threshold ionization, electron wave-packet ...drift, quiver and rescattering motions. Increasingly sophisticated spectroscopic techniques and sculpted laser pulses coupled with theoretical advances have led to a seemingly complete picture of this fundamental laser-atom interaction. Here, we describe an effect that seems to have escaped observation: the photoelectron energy distribution manifests an unexpected characteristic spike-like structure at low energy, which becomes prominent using mid-infrared laser wavelengths (λ>1.0 μm). The low-energy structure is observed in all atoms and molecules investigated and thus seems to be universal. The structure is qualitatively reproduced by numerical solutions of the time-dependent Schrödinger equation but its physical origin is not yet identified.
In principle, the temporal beating of superposed high harmonics obtained by focusing a femtosecond laser pulse in a gas jet can produce a train of very short intensity spikes, depending on the ...relative phases of the harmonics. We present a method to measure such phases through two-photon, two-color photoionization. We found that the harmonics are locked in phase and form a train of 250-attosecond pulses in the time domain. Harmonic generation may be a promising source for attosecond time-resolved measurements.
PDF
