The coherent interaction with ultrashort light pulses is a powerful strategy for monitoring and controlling the dynamics of wave packets in all states of matter. As light presents an oscillation period of a few femtoseconds (T = 2.6 fs in the near infrared spectral range), an external optical field can induce changes in a medium on the sub-cycle timescale, i.e. in a few hundred attoseconds. In this work, we resolve the dynamics of autoionizing states on the femtosecond timescale and observe the sub-cycle evolution of a coherent electronic wave packet in a diatomic molecule, exploiting a tunable ultrashort extreme ultraviolet pulse and a synchronized infrared field. The experimental observations are based on measuring the variations of the extreme ultraviolet radiation transmitted through the molecular gas. The different mechanisms contributing to the wave packet dynamics are investigated through theoretical simulations and a simple three level model. The method is general and can be extended to the investigation of more complex systems.

We study theoretically the dipole radiation of a hydrogen atom driven by an intense sub-cycle pulse. The time-dependent Schrodinger equation for the system is solved by ab initio calculation to obtain the dipole response. Remarkably, a narrowband emission lasting longer than the driving pulse appears at a frequency just above the ionization threshold. An additional calculation using the strong field approximation also recovers this emission, which suggests that it corresponds to the oscillation of nearly bound electrons that behave similarly to Rydberg electrons. The predicted phenomenon is unique to ultrashort driving pulses but not specific to any particular atomic structure.

Isolated attosecond pulses (IAPs), with durations approaching the natural timescale of electrons in materials, are tremendously valuable light sources for fundamental research and applications. An IAP is typically extracted by complicated gating methods from an attosecond pulse train that is produced by the high-harmonic generation (HHG) process [1]. Recently it was demonstrated that an intense 35-fs NIR pulse could be compressed down to sub-cycle duration in a kagome-style hollow-core photonic crystal fiber (HC-PCF) filled with argon [2], where the on-axis intensity and duration of the compressed pulse are suitable for direct IAP generation. The high-frequency features of HHG by ultrashort driving fields have previously been theoretically studied [3,4]; here we investigate the more general features over the complete spectral range in the single-cycle limit.

We review the theoretical investigations of the autoionzing wave packet excited by an isolated attosecond pulse and dressed by a time-delayed intense laser pulse. The few-level model is described and the applications in photoemission and photoabsorption are given. For the three-level, resonantly coupled system, the main features are explained by the Rabi oscillation modulated in the dressing field. For such a system, by precisely controlling the intensity and the time delay of the dressing pulse, we show the shaping of the attosecond pulse when propagating in a gas medium. A more sophisticated multi-level system with coupling terms involving continuum states is also developed, in which the importance of the continuum-continuum coupling is evaluated with the help of an ab initio calculation.

High-harmonic generation driven by 2-fs self-compressed pulses from an argon-filled kagome photonic-crystal fiber is investigated with the 1D atom model. The input carrier-envelope phase strongly modulates the simulated attosecond-pulse emission.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text M. Reduzzi, W. -. Chu, C. Feng, A. Dubrouil, J. Hummert, F. Calegari, F. Frassetto, L. Poletto, O. Kornilov, M. Nisoli, C. -. Lin, and G. Sansone, "Attosecond dynamics of autoionizing states in electronic molecular wave packets," in 19th International Conference on Ultrafast Phenomena, OSA Technical Digest (online) (Optica Publishing Group, 2014), paper 08.Tue.A.2. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

We propose a general technique to retrieve the information of dipole-forbidden resonances in the autoionizing region. In the simulation, a helium atom is pumped by an isolated attosecond pulse in the extreme ultraviolet (EUV) combined with a few-femtosecond laser pulse. The excited wave packet consists of the $^1S$, $^1P$, and $^1D$ states, including the background continua, near the $2s2p(^1P)$ doubly excited state. The resultant electron spectra with various laser intensities and time delays between the EUV and laser pulses are obtained by a multilevel model and an ab initio time-dependent Schr\"odinger equation calculation. By taking the ab initio calculation as a "virtual measurement", the dipole-forbidden resonances are characterized by the multilevel model. We found that in contrast to the common assumption, the nonresonant coupling between the continua plays a significant role in the time-delayed electron spectra, which shows the correlation effect between photoelectrons before they leave the core. This technique takes the advantages of ultrashort pulses uniquely and would be a timely test for the current attosecond technology.

The time evolution of an autoionizing atomic system is studied theoretically in the presence of a moderately intense dressing laser pulse. We first examine how an autoionizing wave packet evolves in time in the absence of an external field, and take the single 2pns(1 P) resonances in beryllium as examples. Alternatively, we study the electron dynamics where an attosecond extreme ultraviolet (XUV) pulse excites two autoionizing states in the presence of a strong time-delayed coupling infrared (IR) laser pulse. The IR can be viewed as a probe to extract or a control to modify the autoionization dynamics. The photoelectron and photoabsorption spectra are calculated for various time delays between the XUV and the IR pulses, and the results are compared with the available experiments. Finally, simulation of the coupled 2s2p(1 P) and 2s 2(1 S) resonances in helium shows substantial spectral modifications by the dressing field parameters. Its analogy to electromagnetically induced transparency in the time domain is discussed.

An extreme ultraviolet (EUV) single attosecond pulse passing through a laser-dressed dense gas is studied theoretically. The weak EUV pulse pumps the helium gas from the ground state to the 2s2p(P-1) autoionizing state, which is coupled to the 2s(2)(S-1) autoionizing state by a femtosecond infrared laser with the intensity in the order of 10(12) W/cm(2). The simulation shows how the transient absorption and emission of the EUV are modified by the coupling laser. A simple analytical expression for the atomic response derived for delta-function pulses reveals the strong modification of the Fano lineshape in the spectra, where these features are quite universal and remain valid for realistic pulse conditions. We further account for the propagation of pulses in the medium and show that the EUV signal at the atomic resonance can be enhanced in the gaseous medium by more than 50% for specifically adjusted laser parameters, and that this enhancement persists as the EUV propagates in the gaseous medium. Our result demonstrates the high-level control of nonlinear optical effects that are achievable with attosecond pulses. DOI: 10.1103/PhysRevA.87.013415

The spectroscopy of light absorption is an essential tool for uncovering the microscopic structure of a material. The observed spectral line positions reveal the energy levels of the excited quantum states, whereas the line shapes are determined by how the material relaxes after light is absorbed. In the optical frequency regime, the absorption profile has a symmetric shape. By coupling the material to an intense optical laser, however, the absorption can be controlled, leading to many interesting phenomena such as electromagnetically induced transparency (EIT) ( 1 ), slow and stopped light ( 2 ), and others. Extending such manipulations to extreme ultraviolet (XUV) and soft x-ray frequencies has presented a challenge. With the advent of intense ultrafast few-femtosecond infrared lasers in recent years, as reported on page 716 of this issue, Ott et al. ( 3 ) demonstrate that such manipulations are now possible.

We study theoretically the photoabsorption spectra of an attosecond extreme ultraviolet (XUV) pulse by a laser-dressed atomic system. A weak XUV excites an autoionizing state which is strongly coupled to another autoionizing state by a laser. The theory was applied to explain two recent experiments [Loh, Greene, and Leone, Chem. Phys. 350, 7 (2008); Wang, Chini, Chen, Zhang, Cheng, He, Cheng, Wu, Thumm, and Chang, Phys. Rev. Lett. 105, 143002 (2010)] where the absorption spectra of the XUV lights were measured against the time delay between the laser and the XUV. In another example, we study an attosecond pulse exciting the 2s2p(P-1) resonance of helium which is resonantly coupled to the 2s(2)(S-1) resonance by a moderately intense 540-nm laser. The relation between the photoabsorption spectra and the photoelectron spectra and the modification of the transmitted lights in such an experiment are analyzed. The role of Rabi flopping between the two autoionizing states within their lifetimes is investigated with respect to the laser intensity and detuning.

An optical coherent control scheme has been proposed and theoretically investigated where an extreme ultraviolet single attosecond pulse (SAP) propagates through a dense helium gas dressed by a time-delayed femtosecond laser pulse. The laser pulse couples the 2s2p(^1P) and 2s^2(^1S) autoionizing states when the SAP excites the 2s2p state. After going through the gas, the spectral and temporal profiles of the SAP are strongly distorted. A narrowed but enhanced spike in the spectrum shows up for specific intensities and time delays of the laser, which exemplifies the control of a broadband photon wave packet by an ultrashort dressing field for the first time. We analyze the photon and electron dynamics and conclude on the dressing condition that maximizes this enhancement. The result demonstrates new possibilities of attosecond optical control.

The strong coupling between two autoionizing states in helium is studied theoretically with the pump-probe scheme. An isolated 100-as XUV pulse is used to excite helium near the 2s2p((1) P) resonance state in the presence of an intense infrared (IR) laser. The laser field introduces strong coupling between 2s2p((1) P) and 2p(2)((1) S) states. The IR also can ionize helium from both autoionizing states. By changing the time delay between the XUV and the IR pulses, we investigated the photoelectron spectra near the two resonances. The results are used to explain the recent experiment by Gilbertson et al. [Phys. Rev. Lett. 105, 263003 (2010)]. Using the same isolated attosecond pulse and a 540-nm laser, we also investigate the strong coupling between 2s2p((1) P) and 2s(2)((1) S) by examining how the photoelectron spectra are modified versus the time delay and the possibility of observing Autler-Townes doublet in such experiments.

Near-threshold resonances have been studied for Be-like ions with a focus on overlapping resonances among Rydberg series converging to different thresholds. The behavior of the overlapping as a function of Z and the approach to the limit of infinite Z are investigated. The 4s4p resonance is shown and discussed in detail as an example.

We study atomic autoionization processes in the time domain. With the emerging attosecond extreme vacuum ultraviolet and soft x-ray pulses, we first address how to characterize the time evolution of the decay of a discrete state into a degenerate continuum. A short pump beam generates a number of resonance states in a series and the nearby background continuum, and the resultant wave packet evolves with time until the full decay of the bound states. Taking the 2pns({sup 1}P{sup o}) resonance series embedded in the 2s{epsilon}p({sup 1}P{sup o}) continuum in a beryllium atom as an example, the time evolution of the autoionizing wave packet in the energy domain and in coordinate space is calculated and analyzed, where Fano profiles build up in the photoelectron energy during the process. A proposed pump-probe scheme assumes that the probe beam ionizes the 2s inner electron in the wave packet. The lifetimes of the resonances and the photoelectron energy distribution can be obtained from the ionization yield vs. the time delay of the probe.

Photoionization cross sections from the ground and metastable states of Be-like ions have been calculated using the Breit-Pauli R-matrix method. The resonances were identified and characterized with energy levels, widths and effective quantum numbers using the QB program. The general behavior of the resonances was analyzed along the sequence. Relativistic effects are identified by comparing the Breit-Pauli result with our calculated LS R-matrix result.

The photoionization of the beryllium-like isoelectronic series has been studied. The bound state wave functions of the target ions were built with CIV3 program. The relativistic Breit-Pauli R-matrix method was used to calculate the cross sections in the photon energy range between the ionization threshold and 1s4f7/2 threshold for each ion. For the total cross sections of Be, B, C, N, and O, our results match experiment well. The comparison between the present work and other theoretical works are also discussed. We show the comparison with our LS results as it indicates the importance of relativistic effects on different ions. In the analysis, the resonances converging to 1s2lj and 1s 3lj were identified and characterized with quantum defects, energies and widths using the eigenphase sum methodology. We summarize the general appearance of resonances along the resonance series and along the isoelectronic sequence. Partial cross sections are also reported systematically along the sequence. All calculations were performed on the NERSC system. INDEX WORDS: Photoionization, R-matrix, Cross section, Beryllium-like ion, Resonance PHOTOIONIZATION OF THE Be ISOELECTRONIC SEQUENCE: RELATIVISTIC AND NONRELATIVISTIC R-MATRIX CALCULATIONS

The photoionization of the four-electron beryllium-like isoelectronic series from the neutral to Fe(+22) has been studied for ground (1)S and metastable (3)P initial states. The wavefunctions of the final-state (target) ions were built using the CIV3 code. Both nonrelativistic LS-coupling R-matrix and relativistic Breit-Pauli (BP) R-matrix methods were used to calculate the cross sections in the photon-energy range between the first ionization threshold and the 1s(2)4f(7/2) threshold for each ion. Our total cross sections compare well with experiment which is available for Be, B(+), C(+2), N(+3) and O(+4). The agreement between the present work and previous calculations is discussed in detail. The importance of relativistic effects is seen by the comparison between the LS and the BP results.