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The relative phase of first ( ω 1 ) and third harmonics ( ω 3 ) extreme ultraviolet light pulses was varied to control the population of the 2 s 2 state in helium through the interference of ω 1 + ω 1 and ω 3 − ω 1 two-photon excitation paths. The population was monitored by observing the total electron yield due to the 2 s 2 autoionization decay. Maximum yield occurs when the relative phase of the two harmonics matches the phase difference of complex atomic amplitudes governing the two excitation paths. The calculated trend of atomic phase differences agrees well with the measured data in the spectral region of the resonance, provided that time-reversed − ω 1 + ω 3 path is also taken into account. These results open the way to accessing phase differences of two-photon ionization paths involving energetically distant intermediate states and to perform interferometry in the extreme ultraviolet range by monitoring final state populations.

We report on the observation of time-resolved quantum beats in the helium fluorescence from the transition 1s3p -> 1s2s, where the initial state is excited by XUV free-electron laser radiation. The quantum beats originate from the Zeeman splitting of the magnetic substates due to an external magnetic field. We perform a systematic study of this effect and discuss the possibilities of studying this phenomenon in the XUV and x-ray regime., 7 pages, 4 figures

We report an electron scattering experiment on argon gas where a keV electron beam is used as a probe and electrons are collected with a magnetic bottle spectrometer. For this purpose, we have built a thermionic gun that produces electron pulses with nanosecond duration by sweeping the beam across a small aperture. To reach the target, electrons must pass through the hole in an axially symmetric arrangement of strong permanent magnets required to operate the magnetic bottle. From the recorded multi-hit sequence of electron arrival times on the microchannel plate detector, a kinetic energy spectrum is built that allows an analysis of the elastic and inelastic electron scattering channels by means of the coincidence technique. After a description of the instrumental configuration and discussion of suitable working parameters, the results of an angle-integrated (e, 2e) experiment are presented for 800 eV electron scattering on argon atoms.

We present a theoretical approach for describing collective x-ray emission processes in extended multilevel targets, based on a combination of propagation equations for the quantum correlation functions and the semiclassical Maxwell-Bloch equations. Such a description overcomes the key deficiencies of the two constituent sets of equations, which have until now been independently used to describe these phenomena. The model developed is employed to study the spectral properties of superfluorescence of $K\ensuremath{\alpha}$ emission in zinc at $\ensuremath{\sim}8630\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ after inner-shell photoionization with intense attosecond free-electron laser pulses. At high pump intensities, the numerical simulations predict a splitting of the $K{\ensuremath{\alpha}}_{1}$ emission line due to a self-induced Autler-Townes effect, which could readily be observed with standard high-resolution x-ray spectrometers. As short duration of the pump pulse is one of the crucial parameters for the manifestation of this phenomenon in the hard-x-ray regime, experimental verification could be possible due to the most recent developments of free-electron laser sources.

Photons have fixed spin and unbounded orbital angular momentum (OAM). While the former is manifested in the polarization of light, the latter corresponds to the spatial phase distribution of its wavefront1. The distinctive way in which the photon spin dictates the electron motion upon light–matter interaction is the basis for numerous well-established spectroscopies. By contrast, imprinting OAM on a matter wave, specifically on a propagating electron, is generally considered very challenging and the anticipated effect undetectable2. In refs. 3,4, the authors provided evidence of OAM-dependent absorption of light by a bound electron. Here, we seek to observe an OAM-dependent dichroic photoelectric effect, using a sample of He atoms. Surprisingly, we find that the OAM of an optical field can be imprinted coherently onto a propagating electron wave. Our results reveal new aspects of light–matter interaction and point to a new kind of single-photon electron spectroscopy. The findings that the spatial distribution of an optical field with vortex phase profile can be imprinted coherently onto a propagating electron wave reveal new aspects of light–matter interactions and will help develop future single-photon electron spectroscopy.

We present a method for measuring a complete frequency map of laser-dressed states with femtosecond lifetimes. It is based on collecting decay products as a function of the frequency of the excitation pulse and its delay with respect to the linearly chirped laser pulse, centered at the resonant transition frequency. The method has been tested on the case of laser-coupled 2s2p (1) P-0 and 2p(2) S-1(e) autoionizing resonances in He and is applicable to other quantum systems with short-lived states. When the laser coupling effects are known, the excitation pulse properties can be extracted from the map.

We present a theoretical study of self-amplification of extreme ultraviolet light in a helium gas target. Resonant excitation to the $3a\phantom{\rule{0.16em}{0ex}}^{1}P^{o}$ doubly excited state below the $N=2$ threshold by $100\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$ free-electron laser pulses is followed by fluorescence at $40.7\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ photon energy. The process is modeled by a three-level atomic system coupled to the continuum and described by the Maxwell-Bloch equations. The evolution of state populations and radiation fields is studied over a wide range of pump intensities and target pressures, clearly showing the regions of spontaneous emission, amplification, and saturation of light emitted in the forward direction. The treatment includes both coherent and SASE pump pulses. In both cases the amplified pulses are temporally coherent and exhibit a duration similar to that of the pump pulses.

We present a theoretical study of He $2\phantom{\rule{0.16em}{0ex}}^{1}P--2\phantom{\rule{0.16em}{0ex}}^{1}S$ emission from a helium gas, where the upper atomic state is prepared by a 100 fs laser pulse resonant with the $1\phantom{\rule{0.16em}{0ex}}^{1}S--2\phantom{\rule{0.16em}{0ex}}^{1}P$ transition. The fluorescence signal emitted in the forward direction at 2059 nm and the transmitted signal at $58.4\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ are followed in time starting with the low pump intensity regime where the single-atom spontaneous emission results in a characteristic exponential decay. By increasing the gas density and pump intensity, collective effects such as self-amplified spontaneous emission and superfluorescence modify the temporal dependence of the radiation field by amplifying the radiation pattern as well as changing its delay and intensity distribution. The extended Maxwell-Bloch simulation of the corresponding three-level $\mathrm{\ensuremath{\Lambda}}$ system in one spatial dimension covers emission delays of up to 2.4 ns, reaching beyond the natural lifetime of the $2\phantom{\rule{0.16em}{0ex}}^{1}P$ state.

According to the Beer-Lambert law, the spectral density of weak electromagnetic radiation is attenuated exponentially, with the characteristic length depending on atomic density and the frequency-dependent photoabsorption cross section of the target. We discuss an analog of the Beer-Lambert law in the time domain, showing that the temporal profile of the initial light pulse enters explicitly into each term of expansion of the exponential function as a multiconvolution with the corresponding temporal response of an atom. The form invokes a description of pulse propagation in terms of the temporal base functions, which may be precalculated for a chosen initial pulse shape, allowing us to obtain the attenuated profile at a given target depth simply by summing up a sufficient number of weighted base functions. The variant with a 100-fs initial Gaussian pulse is worked out in detail, with the atomic response exhibiting either pure exponential or mixed prompt-exponential decay, characteristic of a photoexcited bound state or a Fano resonance. The method is shown to be valid for an arbitrary temporal profile of the initial electric field and is illustrated for pulses resonantly tuned to the low-lying doubly excited states of the He atom.

Synopsis We have observed the autoionization of the laser-coupled 2s2p 1 Po and 2p 2 1 Se resonances in helium. The ions were collected while varying the frequency and delay of the extreme-ultraviolet (EUV) excitation pulse with respect to the linearly chirped visible (VIS) laser pulse. From the measured frequency-delay map the Autler-Townes splitting, the EUV-VIS cross-correlation and the linear chirp parameter were extracted.