Three photon resonant ionization in atomic potassium via S,P, D and F series Rydberg states

Single colour three photon resonant ionization (2 + 1) is observed in atomic potassium vapour in a heat pipe oven using an excimer laser pumped dye laser. Using wavelengths between 570 nm and 603 nm various²S and²D Rydberg states are populated by two photon excitation. Third photon of the same wavelength ionizes the atoms. Rydberg states up ton ⋍ 50 are observed. Electric field as low as 1 V/cm causes extensive Stark mixing of the states. This results in progressively higher three photon ionization signals via the perturbed²P and²F Rydberg states. The three photon ionization process is studied using both linearly and circularly polarized incident light. The experiment shows qualitatively that the²P Rydberg states are perturbed primarily by the²D states in the prescence of an external electric field and to a much smaller extent by²S states. This is also explained theoretically by calculating the Stark mixing coefficients under the Bates and Daamgard (1949) approximation. Implication for a similar effect in other alkali elements is discussed.     

强激光场阈上电离理论

本文提出一种统一的阈上电离理论。根据该理论,可以定量描述较真实的原子阈上电离过程并能清楚地阐明实验观测的阈上电离现象。该现象可理解为前后相继的两个过程:(1)先从无场的初态吸收N个光子而到场缀末态的量子跃迁过程。(2)欲定量描述实验测量,还需考虑光电子逃离光场时与光场相互作用的“后作用效应”。该“后作用效应”是由非均匀激光场所形成的无质动力所引起的经典物理过程。 关键词：     

Multiphoton ionization of the hydrogen atom by a circularly polarized electromagnetic field

This paper examines the multiphoton ionization of the ground state of the hydrogen atom in the field of a circularly polarized intense electromagnetic wave. To describe the states of photoelectrons, quasiclassical wave functions are introduced that partially allow for the effect of an intense electromagnetic wave and that of the Coulomb potential. Expressions are derived for the angular and energy distributions of photoelectrons with energies much lower than the ionization potential of an unperturbed atom. It is found that, due to allowance for the Coulomb potential in the wave function of the final electron states, the transition probability near the ionization threshold tends to a finite value. In addition, the well-known selection rules for multiphoton transitions in a circularly polarized electromagnetic field are derived in a natural way. Finally, the results are compared with those obtained in the Keldysh-Faisal-Reiss approximation. Zh. éksp. Teor. Fiz. 116, 807–820 (September 1999)     

Molecular frame photoelectron emission in the presence of autoionizing resonances

We have measured the angular distribution of valence-shell photoelectrons excited by circularly polarized light from fixed-in-space N2O molecules, near to and on top of resonances due to Rydberg states embedded in the ionization continuum. The sign of the circular dichroism for ionization into the N2O+ (B2Pi, (1pi)-1) state is reversed on top of the lowest dominant resonances. Measured angular distributions are well predicted by state-of-the-art multichannel configuration interaction calculations. The change in sign of the circular dichroism at the peak of the resonance is the result of a rapid change in the phases of resonant dipole matrix elements by a factor of 2pi as the energy is scanned across the resonance.     

Double ionization of helium interacting with elliptically polarized laser pulse by classical ensemble simulations

This paper uses the classical ensemble method to study the double ionization of a 2-dimensional (2D) model helium atom interacting with an elliptically polarized laser pulse. The classical ensemble calculation demonstrates that the ratio of double to single ionization decreases with the increasing ellipticity of the driving field. The classical scenario shows that there are hardly any e--e recollisions with the circularly polarized laser pulse. The double ionization probability is studied for linearly and circularly polarized laser pulses. The classical numerical results are consistent with the semiclassical rescattering mechanism and in agreement with the experimental results and the quantum calculations qualitatively.     

Helicity-dependent time delays in multiphoton ionization by two-color circularly polarized laser fields

By numerically solving the three-dimensional time-dependent Schrödinger equation, we have invest-tigated multiphoton ionization of hydrogen atom in the two-color circularly polarized (TCCP) laser fields consisting of a strong 400 nm and a much weaker 800 nm pulses. Due to the presence of perturb-bative 800 nm laser pulse, sideband peaks emerge between the above-threshold ionization rings in the photoelectron momentum distributions. Our numerical results show that the sideband peaks exhibit one-lobe structure in the co-rotating TCCP laser fields, while it displays the three-lobe structure in the counter-rotating TCCP laser fields. Moreover, the photoelectron yield of sidebands in the co-rotating TCCP fields is much higher than those of the counter-rotating TCCP fields. These phenomena could be well explained from the perspective of the photon-absorption channels via the selection rules. In-terestingly, an obvious phase shift between the sidebands of different orders from the co-rotating and counter-rotating TCCP fields is observed. This shift indicates the helicity-dependent time delay in the one-photon continuum-continuum transition process.     

Observation of polarization switching: D1 line of sodium in a Fabry-Perot

We report observation of the complete polarization switching sequence for a gas of atoms with a J = $\text{1}\text{2}$ to J = $\text{1}\text{2}$ transition, exposed to resonant or near resonant, linearly polarized laser excitation in an optical cavity. The sequence is from linearly polarized cavity output at low input power to predominantly circularly polarized output at intermediate power and back to linearly polarized output at high input power. The switching is abrupt, exhibits hysteresis and, when hyperfine optical pumping effects are minimized, is phenomenologically in accord with predictions from theory.     

Partitioning of the linear photon momentum in multiphoton ionization

The balance of the linear photon momentum in multiphoton ionization is studied experimentally. In the experiment argon and neon atoms are singly ionized by circularly polarized laser pulses with a wavelength of 800 and 1400 nm in the intensity range of 10(14)-10(15) W/cm2. The photoelectrons are measured using velocity map imaging. We find that the photoelectrons carry linear momentum corresponding to the photons absorbed above the field free ionization threshold. Our finding has implications for concurrent models of the generation of terahertz radiation in filaments.     

Multiphoton ionization of the hydrogen atom exposed to circularly or linearly polarized laser pulses

This paper studies the multiphoton ionization of the hydrogen atom exposed to the linearly or circularly polarized laser pulses by solving the time-dependent Schr?dinger equation. It finds that the ratio of the ionization probabilities by linearly and circularly polarized laser pulses varies with the numbers of absorbing photons. With the same laser intensity, the circularly polarized laser pulse favors to ionize the atom with more ease than the linearly polarized laser pulse if only two or three photons are necessary to be absorbed. For the higher order multiphoton ionization, the linearly polarized laser pulse has the advantage over circularly polarized laser pulse to ionize the atom.     

Wakefield generation by elliptically polarized femtosecond laserpulse in ionizing gases

Wakefield generation by a femtosecond laser pulse is described in the frame of the slowly varying amplitudes approximation. The amplitude of the wakefield A, is studied as a function of laser pulse and background gas parameters, and is compared with well-known results for preformed, completely ionized plasma A_p,i. It is found that the ionization processes can increase A_p as compared to A_p,i at comparatively high laser peak intensities. It is shown that the increase of the wakefield amplitude due to gas ionization is more pronounced for circularly polarized laser pulses than for linearly polarized laser pulses. The strongest enhancement of A_p in comparison with A_p,i takes place for longer laser pulses with a duration in excess of the plasma wave period when the resonant conditions for ponderomotive excitation of the wakefield are not matched. Thus, ionization processes can expand the region of parameters for efficient generation of the laser wakefields     

Correlation of momenta of electrons and the nucleus in atoms

The process of resonant multiphoton ionization of a hydrogen atom in the ground 1s state is studied by direct numerical integration of the nonstationary Schrödinger equation for a quantum system in an electromagnetic field. The dependence of photoionization probability on the radiation intensity is found to be nonmonotonic. It is established that the minima of ionization correspond to multiphoton resonances between the ground state and one of the excited (Rydberg) atomic states perturbed by the laser field. It is shown that ionization is suppressed due to rearrangement of Rydberg states in a strong electromagnetic field and is accompanied by efficient Raman Λ transitions, which connect a set of closely lying Rydberg states via the continuum.     

Elastic scattering of electrons by metastable hydrogen atoms in the presence of a resonant laser field

Within the framework of the rotating wave approximation the elastic scattering of electrons by metastable 2s state of hydrogen atoms is studied in the presence of a resonant laser field. The frequency of the circularly polarized laser field is chosen to match the 2s-3p transition frequency in the hydrogen atom. Variation of the cross section with laser intensity and with incident electron energy (50-150 eV) is investigated. Received: 18 July 1997 / Received in final form: 5 December 1997 / Accepted: 19 January 1998     

Rescattering double ionization of D2 and H2 by intense laser pulses

We have measured momentum spectra and branching ratios of charged ionic fragments emitted in the double ionization of D2 (and H2) molecules by short intense laser pulses. We find high-energy coincident D+ (and H+) ion pairs with kinetic energy releases between 8 and 19 eV which appear for linearly polarized light but are absent for circularly polarized light. The dependence on the polarization, the energy distributions of the ions, and the dependence on laser intensity of yield ratios lead us to interpret these ion pairs as due to a rescattering mechanism for the double ionization. A quantitative model is presented which accounts for the major features of the data.     

Effect of Time-Dependent Ionization on Propagation of a Few-Cycle Circularly Polarized Laser Pulse in Two-Level Medium

Influence of multiphoton ionization on the propagation and spectrum of few-cycle circularly （elliptically） polarized laser pulses in an open two-level medium （two-level plus continuum model） is investigated based on the conventional two-level model proposed by Slavcheva and Hess （Phys. Rev. A 72 （2005） 053804）, and the propagation dynamics of an arbitrary elliptically polarized laser pulse is reduced into that of right and left circularly polarized laser pulses. When the laser intensity is high enough to cause ionization, there are significant impacts of ionization on the pulse reshaping and on the higher order spectral components, and the impacts for the open two-level model are different from those for the dosed two-level model.     

Alignment of molecules by strong laser pulses

We review how moderately intense laser fields offer an approach to alignment of molecules [1]. In particular, molecules can be aligned along a given space fixed axis, forced to a plane, or their rotations about all three possible axes can be eliminated by choosing a linearly polarized, a circularly polarized, or an elliptically polarized alignment field, respectively. We show how molecules in the gas phase can be aligned by turning on the laser field either slowly (a few nanoseconds) or fast (a few picoseconds) with respect to the rotational period of the molecules. The role of the intensity of the laser field and the rotational temperature of the molecules is discussed. Before concluding we describe how aligned molecules enables control and selectivity in the interaction between polarized light and molecules.Received: 15 November 2002, Published online: 18 March 2003PACS: 33.15.Bh General molecular conformation and symmetry; stereochemistry - 32.80.Lg Mechanical effects of light on atoms, molecules, and ions - 33.80.Gj Diffuse spectra; predissociation, photodissociation - 33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g. Rydberg states) - 34.50.Lf Chemical reactions, energy disposal, and angular distribution, as studied by atomic and molecular beams     

Extreme-ultraviolet harmonic generation near 13 nm with a two-color elliptically polarized laser field

Theoretical results for high-harmonic generation by a two-color elliptically polarized laser field are presented. Special emphasis is placed on coherent radiation near 13 nm in connection with the development of extreme-ultraviolet lithography. Linearly polarized radiation at this wavelength can be obtained with a linearly polarized bichromatic laser field, whereas circularly polarized radiation can be generated with a bichromatic circularly polarized field with counterrotating coplanar components. In both cases the harmonic emission efficiencies at the saturation intensity are more than 1 order of magnitude larger than for harmonic generation with a monochromatic linearly polarized field.     

利用原子的塞曼光谱对半导体激光器进行稳频

中性原子的超精细能级在磁场中产生塞曼分裂 ,另外 ,左旋和右旋圆偏振光激发下原子的跃迁选择定则不同 ,因此 ,原子在超精细塞曼能级间的吸收谱峰相对无磁场条件下的吸收谱峰有一定的移动。利用这一点 ,验证了一种简单、灵活的方法对半导体激光器进行稳频 ,使激光器的线宽稳到小于 1MHz。通过对实验结果的分析 ,发现由左旋和右旋圆偏振光激发引起的原子吸收谱峰移动之和与饱和吸收峰半高宽相等时 ,稳频效果最好     

On the Possibility to Observe Collective Tunneling in Ionization of Atoms by Intense Laser Fields

JETP Letters - The shape of momentum distributions of doubly charged ions emerging in the process of ionization of atoms in an intense low-frequency circularly polarized laser field is discussed....     

Demonstration of a Ni-like Kr optical-field-ionization collisional soft x-ray laser at 32.8 nm

We report the first experimental demonstration of a Ni-like optical-field ionization collisional soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 760 mJ, 30 fs, 10-Hz Ti:sapphire laser beam in a few mm cell filled with krypton. We have measured a gain coefficient of 78 cm(-1) on the 3d(9)4d 1S0-3d(9)4p(1)P1 transition at 32.8 nm, which is here amplified for the first time. This radiation source represents the shortest wavelength optical-field ionization collisional soft x-ray laser ever produced. The influence of the gas pressure and the pumping energy on the lasing output are also presented.     

Molecular above-threshold ionization with a circularly polarized laser field

Molecular above-threshold ionization by a circularly polarized laser field is investigated. Our theoretical approach is based on the modified molecular strong-field approximation. Various gauge-dependent versions of this approximation are considered and homonuclear and heteronuclear molecular species characterized by different symmetries are used as targets. As in the case of a linearly polarized field, the imprint of the molecular multicenter structure can be observed in the above-threshold ionization spectra. It manifests itself as minima in the rate of the ionized electrons as a function of their energy. The locations of these minima are strongly influenced by the symmetry of the corresponding highest occupied molecular orbital as well as the internuclear separation. Analyzing the interference structures of the electron spectra one can obtain information about the molecular symmetry.