Attosecond control of ionization by wave-packet interference

A train of attosecond pulses, synchronized to an infrared (IR) laser field, is used to create a series of electron wave packets (EWPs) that are below the ionization threshold in .helium. The ionization probability is found to strongly oscillate with the delay between the IR and attosecond fields twice per IR laser cycle. Calculations that reproduce the experimental results demonstrate that this ionization control results from interference between transiently bound EWPs created by different pulses in the train. In this way, we are able to observe, for the first time, attosecond wave-packet interference in a strongly driven atomic system.     

Attosecond transient absorption spectroscopy:Comparative study based on three-level modeling

In the present paper, the time-resolved transient absorption spectroscopy of helium atoms is investigated based on the three-level modeling. The helium atoms are subjected to an extreme ultraviolet(XUV) attosecond pulse and a time-delayed infrared(IR) few-cycle laser field. The odd excited state are populated from the ground state by the XUV pulse due to the dipole selection rule, and probed by the time-delayed IR laser. The time-resolved transient absorption spectroscopy based on the different coupling mechanism demonstrate some different features, the photoabsorption spectrum based on three-level model with rotating wave approximation(RWA) cannot repeat the fast oscillation and the sideband structure which have been observed in the previous experimental investigation. The dressing effect of IR laser pulse on the ground state can contribute new interference structures in the photoabsorption spectrum.     

Attosecond electron bunches

Electron bunches of attosecond duration may coherently interact with laser beams. We show how p-polarized ultraintense laser pulses interacting with sharp boundaries of overdense plasmas can produce such bunches. Particle-in-cell simulations demonstrate attosecond bunch generation during pulse propagation through a thin channel or in the course of grazing incidence on a plasma layer. In the plasma, due to the self-intersection of electron trajectories, electron concentration is abruptly peaked. A group of counterstream electrons is pushed away from the plasma through nulls in the electromagnetic field, having inherited a peaked electron density distribution and forming relativistic ultrashort bunches in vacuum.     

High harmonic generation via continuum wave-packet interference

High-order harmonic generation (HHG) is investigated theoretically in the over-the-barrier ionization regime revealing the strong signature of interference between two separately ionized and separately propagating free wave packets of a single electron. The interference leads to the emission of coherent light at a photon energy corresponding to the kinetic-energy difference of the two recolliding electron quantum paths, thus complementary to the well-known classical three-step picture of HHG. As will be shown by time-frequency analysis of the emitted radiation, the process entirely dominates the coherent HHG emission after the atomic ground state has been depleted by a strong field. Moreover, it can be isolated from the continuum-bound harmonics via phase matching.     

Real-time observation of phase-controlled molecular wave-packet interference

The quantum interference of two molecular wave packets has been precisely controlled in the B electronic state of the I2 molecule by using a pair of fs laser pulses whose relative phase is locked within the attosecond time scale and its real-time evolution has been observed by another fs laser pulse. It is clearly observed that the temporal evolution changes drastically as a function of the relative phase between the locked pulses, allowing us to read both amplitude and phase information stored in the wave functions of the molecular ensemble.     

原子双阱势中电子波包的非绝热操控

交叉电场和磁场中的氢原子具有双阱势能结构。本文基于量子力学模拟,采用半周期脉冲（HCP）序列开展电子波包的非绝热操控。详细研究了HCP的电场幅度、数量和时间间隔对电子波包的空间和能量分布的影响,实现了将初始电子波包非绝热转移到远离库伦中心的外势阱中的低能态,这些低能态较为稳定且显示出很大的电偶极矩。相比光激发和绝热操控,本文的方法更为快速、高效,可以应用于更为复杂的外场或分子体系。     

Attosecond electron wave packet dynamics in strong laser fields

We use a train of sub-200 attosecond extreme ultraviolet (XUV) pulses with energies just above the ionization threshold in argon to create a train of temporally localized electron wave packets. We study the energy transfer from a strong infrared (IR) laser field to the ionized electrons as a function of the delay between the XUV and IR fields. When the wave packets are born at the zero crossings of the IR field, a significant amount of energy (approximately 20 eV) is transferred from the field to the electrons. This results in dramatically enhanced above-threshold ionization in conditions where the IR field alone does not induce any significant ionization. Because both the energy and duration of the wave packets can be varied independently of the IR laser, they are valuable tools for studying and controlling strong-field processes.     

Photoinduced electron transfer of Rhodamine 6G/N,N-diethylaniline revealed by multiplex transient grating and transient absorption spectroscopies

The dynamics and spectroscopic characteristics of the ultrafast photoinduced electron transfer (ET) of Rhodamine 6G (Rh6G⁺) in N,N-diethylaniline (DEA) were studied using femtosecond time-resolved multiplex transient grating and transient absorption spectroscopies. The ultrafast photoinduced forward ET from DEA to the Rh6G⁺* cation radical excited state has a time constant of τ _FET = 219–318 fs. The much slower backward ET from the neutral radical Rh6G^· to DEA⁺ with a time constant of τ _BET = 22.76–42.31 ps occurs in the inverted region. Intramolecular vibrational relaxation of the excited state takes place in τ _IVR = 2.18–6.91 ps.     

Relaxation of siloxanes observed by FIR absorption

The FIR spectra of liquid dimethyl siloxanes of different molecular weight, from the monomer to the polymer with 1000 monomer units, have been measured and interpreted in terms of the calculated dipole correlation functions. Evidence for a fast localized motion in the siloxane chain is presented.     

高能电子与超强激光束作用产生的阿秒脉冲列

利用非线性汤姆孙散射的理论，从理论和数值模拟上研究了单电子在横向穿越高斯激光束束 腰时所辐射的x射线阿秒脉冲列的性质. 主要分析了电子以初始能量γ₀=1M eV—100M eV横向穿越激光振幅参数为a₀=1—10的高斯光束束腰获得的阿秒辐射脉冲的 时间 和空间性质. 计算表明，辐射呈现脉冲列的形式. 脉冲列的包络宽度取决于激光强度、束腰 的宽度以及入射电子能量. 电子的初始能量比激光强度对电子辐射脉冲的影响更大. 辐射脉 宽、脉冲间隔和脉冲包络宽度都正比于1/γ²₀，辐射功率正比于 γ⁶₀，辐射能 量正比于γ⁴₀. 当改变激光振幅a₀时，辐射功率正比 于a²₀、辐射包络中单 个脉冲脉宽正比于1/a₀、脉冲之间的间隔正比于a₀. 当保持激光强 度不变，而改变光束 束腰半径w₀时，辐射的脉冲数量、包络和辐射能量正比于w₀. 当 激光功率保 持不变时而改变激光强度和束腰半径时，脉冲包络宽度和最大辐射能量都基本不变. 当激光 振幅参数a₀=1，电子初始能量为10MeV时，激光束腰为两个激光波长时，电子 辐 射脉冲包络宽度只有14×10^-3τ₀（τ₀为入 射激光周期），达到几个阿秒的量级. 关键词： 阿秒脉冲 非线性汤姆孙散射 高斯激光光束     

Attosecond pump probe: exploring ultrafast electron motion inside an atom

The attosecond pump probe, in close analogy to the standard femtosecond probing technique, has been proposed and theoretically demonstrated with its application to explore ultrafast electron motions inside atoms. We have performed realistic modeling for the full dynamics of both the femtosecond pumping and the attosecond probing processes. Our simulations have illustrated that an ultrashort oscillation period of 2.0 fs can be mapped out for a wave packet in low-lying excited states of the helium atom. This opens the prospect of a wealth of similar pump probe experiments to examine ultrafast electronic or atomic motions.     

Chlorine migration under electron bombardment observed by aes

During heating, a TiAl alloy becomes covered by elemental sulphur and chlorine which remain segregated at the surface after cooling. Auger electron spectroscopy has shown that the chlorine disappears progressively with time at room temperature, while the sulphur is not removed. Experiments have been performed which show that the disappearance of chlorine is due to electron bombardment and that chlorine is expelled only locally from a zone around, and considerably larger than, the area of beam impact. We suggest that chlorine escapes from the impact area by superficial diffusion and not by electron-stimulated desorption. The influence of various parameters, such as the properties of the electron beam and the temperature of the sample, have been studied in an attempt to explain the observed electron beam effect.     

Nonlinear absorption by quantum interference in cold atoms

We report an experimental observation of third-order nonlinear absorption by quantum interference in (87)Rb atoms cooled and confined in a magneto-optic trap. A coupling laser creates electromagnetically induced transparency (EIT) in a multilevel Rb system in which the third-order nonlinear absorption is enhanced by constructive quantum interference while the linear absorption is inhibited by destructive interference. Our experiment demonstrates the EIT system proposed by Harris and Yamamoto [Phys. Rev. Lett. 81, 3611 (1998)], which absorbs two photons but not one photon in the dressed-state picture.     

Attosecond resolved electron release in two-color near-threshold photoionization of N2

We have simulated two-color photoionization of N(2) by solving the time-dependent Schr?dinger equation with a simple model accounting for the correlated vibronic dynamics of the molecule and of the ion N(2)(+). Our results, in very good agreement with recent experiments [Haessler et al., Phys. Rev. A 80, 011404 (2009)], show how a resonance embedded in the molecular continuum dramatically affects the phases of the two-photon transition amplitudes. In addition, we introduce a formal relation between these measurable phases and the photoelectron release time, opening the way to attosecond time-resolved measurements, equivalent to double-slit experiments in the time domain.     

Attosecond photoelectron spectroscopy of electron tunneling in a dissociating hydrogen molecular ion

We demonstrate the potential of intense-field pump, attosecond probe photoelectron spectroscopy to monitor electron tunneling between the two protons during dissociative ionization of the hydrogen molecule, with attosecond temporal and Angstrom-scale spatial resolution.     

Rydberg states of HCN observed by electron impact spectroscopy

The electron energy loss spectrum of HCN has been determined in the energy region 8–13.6 eV at impact energies of 100, 50 and 30 eV. It is shown that energy loss spectra of HCN at intermediate impact energies can be satisfactorily analysed unlike the diffuse unassigned optical absorption spectra that have previously been reported. Rydberg series have been assigned using term values and quantum defects together with ionization potentials obtained by photoelectron spectroscopy.     

Resonant Auger Scattering by Attosecond X-Ray Pulses

As x-ray probe pulses approach the subfemtosecond range, conventional x-ray photoelectron spectroscopy(XPS) is expected to experience a reduction in spectral resolution due to the effects of the pulse broadening.However, in the case of resonant x-ray photoemission, also known as resonant Auger scattering(RAS), the spectroscopic technique maintains spectral resolution when an x-ray pulse is precisely tuned to a core-excited state. We present theoretical simulations of XPS and RAS spectra on a sho...     

Properties of liquid silicon observed by time-resolved x-ray absorption spectroscopy

Time-resolved x-ray spectroscopy at the Si L edges is used to probe the electronic structure of an amorphous Si foil as it melts following absorption of an ultrafast laser pulse. Picosecond temporal resolution allows observation of the transient liquid phase before vaporization and before the liquid breaks up into droplets. The melting causes changes in the spectrum that match predictions of molecular dynamics and ab initio x-ray absorption codes.     

Co dimers observed by extended x-ray absorption fine structure spectroscopy

Cobalt was introduced into a silver matrix by ion implantation and observed by extended x-ray absorption fine structure (EXAFS) spectroscopy. In the range 0.10--0.70 at. % evidence of Co dimers dispersed in the matrix was found. The dimers are somewhat contracted with respect to the bulk Co nearest neighbor distance and distributed in a chainlike configuration with each dimer at 90 degrees from each other along opposite square faces of the Ag fcc lattice.