Ultrafast processes in matter, such as the electron emission after light absorption, can now be studied using ultrashort light pulses of attosecond duration (10−18 seconds) in the extreme ultraviolet ...spectral range. The lack of spectral resolution due to the use of short light pulses has raised issues in the interpretation of the experimental results and the comparison with theoretical calculations. We determine photoionization time delays in neon atoms over a 40–electron volt energy range with an interferometric technique combining high temporal and spectral resolution. We spectrally disentangle direct ionization from ionization with shake-up, in which a second electron is left in an excited state, and obtain excellent agreement with theoretical calculations, thereby solving a puzzle raised by 7-year-old measurements.
Electron dynamics induced by resonant absorption of light is of fundamental importance in nature and has been the subject of countless studies in many scientific areas. Above the ionization threshold ...of atomic or molecular systems, the presence of discrete states leads to autoionization, which is an interference between two quantum paths: direct ionization and excitation of the discrete state coupled to the continuum. Traditionally studied with synchrotron radiation, the probability for autoionization exhibits a universal Fano intensity profile as a function of excitation energy. However, without additional phase information, the full temporal dynamics cannot be recovered. Here we use tunable attosecond pulses combined with weak infrared radiation in an interferometric setup to measure not only the intensity but also the phase variation of the photoionization amplitude across an autoionization resonance in argon. The phase variation can be used as a fingerprint of the interactions between the discrete state and the ionization continua, indicating a new route towards monitoring electron correlations in time.
Display omitted
► We find the phase for laser-assisted XUV ionization transition matrix elements (M). ► The phase of M is simply: the sum of the scattering phase of the intermediate continuum state ...and an universal phase. ► The universal phase is independent of the initial state and it leads to a delay observed in attosecond time-delay experiments.
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. The observed time-delay is a sum of two components: a one-photon Wigner-like delay and a universal delay that originates from the probing process itself.
We study resonant two-color two-photon ionization of helium via the 1s3p (1)P(1) state. The first color is the 15th harmonic of a tunable Ti:sapphire laser, while the second color is the fundamental ...laser radiation. Our method uses phase-locked high-order harmonics to determine the phase of the two-photon process by interferometry. The measurement of the two-photon ionization phase variation as a function of detuning from the resonance and intensity of the dressing field allows us to determine the intensity dependence of the transition energy.
High-order harmonic generation (HHG) in gases has been established as an important technique for the generation of coherent extreme ultraviolet (XUV) pulses at ultrashort time scales. Its main ...drawback, however, is the low conversion efficiency, setting limits for many applications, such as ultrafast coherent imaging, nonlinear processes in the XUV range, or seeded free electron lasers. Here we introduce a novel scheme based on using below-threshold harmonics, generated in a "seeding cell", to boost the HHG process in a "generation cell", placed further downstream in the focused laser beam. By modifying the fundamental driving field, these low-order harmonics alter the ionization step of the nonlinear HHG process. Our dual-cell scheme enhances the conversion efficiency of HHG, opening the path for the realization of robust intense attosecond XUV sources.
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.
Highlights ► We identified 2608 narcolepsy cases in 6 EU countries between 2000 and 2010. ► After pandemic vaccination, incidence rates increased in Finland, Sweden and Denmark. ► In Denmark, the ...increase did not follow high pandemic vaccination coverage. ► In Italy, the Netherlands and the United Kingdom no increased rates were seen.
Engaging in competitive sports as a youth can have many health benefits, but recent studies also report a high risk for injury. The long‐term purpose of this Swedish research program is to develop a ...framework for safe track and field training for young athletes (aged 12‐15 years). The aim of this study was to establish what is perceived to contribute and cause injuries in youth track and field by compiling the best available experiential knowledge about the underlying factors and use this knowledge to identify appropriate areas to handle these in practical ways. Nine focus group interviews with in total 74 participants and confirming interviews with five individuals were performed in seven Swedish regions. Qualitative research methods were used for data analysis. Injuries in youth athletes were not considered to be strictly the result of individual factors but rather the result of the interactions between factors at different levels. Three major factors emerged as follows: Insufficient knowledge for athletic development in daily practice; shortsighted communities of practice and sports policies not adjusted to youth; and societal health behaviors. The experiential knowledge in the national sporting community suggests that if effective and sustainable injury prevention processes are to be implemented for youth track and field, an ecological (holistic‐developmental) approach to injury prevention is needed. Such an approach allows a longitudinal development‐focused strategy for prevention that spans an athlete's entire career.
PDF
