脉冲激光激发的原子共振荧光

本文从含有弛豫项的光与二能级原子相互作用的布洛赫(Block)方程出发,利用计算机进行数值求解,在强共振激发条件下,给出了几种不同的激光脉冲(包括常见的高斯脉冲)激发的原子共振荧光谱,获得了原子在瞬态辐射过程中的一些新现象。     

Perturbation of the excited states of the hydrogen atom and hydrogen-like ions by a neutral atom

Owing to the degeneracy of the energy levels, the wavefunction of the electron in the excited states of the hydrogen atom and hydrogen-like ions perturbed by a neutral atom B is significantly different from the wavefunction of the unperturbed state. The perturbed function has a wide high maximum in the region of atom B, which is explained by multiple collisions of the electron with atom B, because the classical trajectories in the Coulomb field are closed and the size of atom B is much smaller than the size of the excited-state orbit. The radiative lifetimes of the excited states are much larger than those of unperturbed states. The orbital angular momentum L of the excited electron is strongly changed in collisions with atom B owing to the quantum interference or mixing of the temporal phases of adiabatic wavefunctions. The cross sections for such a change in the orbital angular momentum are several orders of magnitude larger than the cross sections found in early investigations in the approximation of the single collision of the electron with atom B.     

Ionization of a single hydrogen-like atom by laser pulse of near-atomic strength

The dynamics of high-harmonic generation and atom ionization by a strong and superstrong laser field are studied. In contrast to many earlier works, the present theory does not impose limitations on the laser field’s strength. We solve the nonrelativistic problem of a single hydrogen-like atom’s ionization from the ground state by a short laser pulse of subatomic, atomic, and superatomic field strength. Within the framework of the proposed method, we investigated the matrix elements of the ionization transition and revealed its substantially nonlinear dependence on the laser field strength. Both ionization and recombination processes are taken into account. The proposed method enables us to take into account the arbitrary order multiphoton ionization processes.     

Ionization of two-electron atom in ultrashort laser pulse

The method of numerical integration of the classical equations of motion was used to study interaction of a model two-electron atom with ultrashort laser pulses. Mechanisms and specific features of the ionization process were analyzed in a broad range of laser-field parameters.     

Stimulated emission of a laser photon by the excited states of a three-level atom

We study the stimulated emission spectra arising from the emission of a laser photon by two excited states of a three-level atom interacting with a laser field at low intensities. The lifetimes of the stimulated photons emitted by the two excited states are much longer than those emitted spontaneously, while the intensities of the induced peaks take negative values indicating that amplification occurs at low frequencies. The ratio of the intensities of the light emitted by the excited states |3 > and |2 > of the atom is proportional to (₃/₂)^1/2, where ₃ and ₂ are the radiative decay rates of the spontaneously emitted photons by the excited states |3 > and |2 > into the ground state |1 > of the atom, respectively. An absorption spectrum is induced into the ground state of the atom by the laser field. The competition between induced absorption and stimulated emission at low frequencies without population inversion is considered in the low-intensity limit of the laser field. It is shown that for values of ₃/₂ > 1 the relative intensity (height) of the induced peak takes positive values implying that the process of the induced absorption dominates. As the ratio ₃/₂ increases, the height of the induced peak decreases and vanishes for values of ₃/₂ < 400. For values of ₃/₂ > 400, the height of the induced peak becomes negative indicating that the process of the stimulated emission (amplification) is likely to occur at low frequencies. The computed spectra are graphically presented and discussed.Issued as NRCC No. 39088     

序列脉冲激光中的原子共振辐射

本文详细研究了序列脉冲激光与二能级原子共振作用的Bloch方程的解。在此基础上,结合数值计算,求出了序列脉冲作用下二能级原子的共振辐射谱;并得出如下结论:对满足K_1/K=1的原子(如仅受自然展宽的气体原子),序列脉冲差不多等效于一单色激光场;而对满足K_1/K(?)=1的原子(比如一些固体原子)序列脉冲场中边频成份也表现出较强的作用,辐射谱呈现出多边峰现象。     

正交非均匀电场中氢原子的高激发态

用数学软件Mathematica研究正交非均匀电场中氢原子两个高激发态(n=4和5)的能级和波函数.讨论能级分裂,并绘制零级近似下氢原子的电子概率角分布图.     

Ionization of a hydrogen atom from its ground state by a coherent bichromatic laser field

We study the “coherent phase control” between the three-photon ionization by a fundamental laser field and the one-photon ionization by its third harmonic for a hydrogen atom in its ground state. The relative phase δ of the harmonic field with respect to the fundamental laser radiation “modulates” the interference between the two ionization channels, which is important near the crossing points between the ionization rates of the two individual processes. Numerical results for the total ionization rate and for the angular distribution of the photoelectrons as a function of the phase δ are presented for frequencies located in the vicinity of the atomic resonances corresponding to the absorption of two laser photons. Received 31 August 2000 and Received in final form 6 February 2001     

Ionisation of Rydberg hydrogen atom near a metal surface by short pulse laser

In the ionisation of Rydberg hydrogen atoms near a metal surface,the electron will escape from the nucleus and arrive at the detector in a time sequence.This probability flux train relies on the initial electron wave packet irradiated by the laser pulse.For simplicity,the laser pulse is usually simplified to a delta function in energy domain,resulting in a sharp initial arrival time with an exponentially decaying tail at the detector.Actually and semiclassically,the initial outgoing wave should be modeled as an ensemble of trajectories propagating away from the atomic core in all directions with a range of launch times and a range of energies.In this case,each pulse in the pulse train is averaged out rather than a sharp profile.We examine how energy and time averaging of the electron wave packet affects the resolution of escaping electron pulses and study the energy dependence of the arrival time for each pulse in the ionisation train.An optimization condition for the laser pulse shape to generate narrow ionisation electron pulse in the train is obtained.The ionisation rates with various excitation energy are calculated also,which show the excitation to higher N Rydberg states will narrow the electron pulse as well.     

Ionization of a multilevel atom by ultrashort laser pulses

Specific features of ionization of single atoms by laser fields of a near-atomic strength are investigated. Calculations are performed for silver atoms interacting with femtosecond laser pulses with wavelengths λ = 800 nm (Ti:Sapphire) and λ = 1.064 μm (Nd:YAG). The dependences of the probability of ionization and of the form of the photoelectron energy spectra on the field of laser pulses for various values of their duration are considered. It is shown that the behavior of the probability of ionization in the range of subatomic laser pulse fields is in good agreement with the Keldysh formula. However, when the field strength attains values close to the atomic field strength, the discrepancies in these dependences manifested in a decrease in the ionization rate (ionization stabilization effect) or in its increase (accelerated ionization) are observed. These discrepancies are associated with the dependence of the population dynamics of excited discrete energy levels of the atom on the laser pulse field amplitude.     

Ultrafast dynamics in the excited hydrogen atom transfer states of ammonia clusters

Neutral ammonia clusters (NH₃)_m are photo-excited to the electronic state by a deep UV femtosecond laser pump pulse. Within a few hundred femtoseconds a significant fraction of the clusters rearrange to form an H-transfer state (NH₃)_m-2NH₄(3s)NH₂ with the subunit NH₄ in its 3s electronic ground state. This state is then electronically excited by a time-delayed infrared control pulse of variable wavelength. Finally, a third (probe) pulse in the UV ionizes the clusters for detection. The lifetime of the excited (NH₃)_m-2NH₄(3p)NH₂ states is found to vary between 2.7 and 0.13 ps depending on cluster size and excitation energy. It increases drastically upon deuteration. The corresponding cluster size-dependent photoelectron spectra allow us to disentangle the underlying energetics of the excitation and ionization process and reveal additional processes, such as nonresonant ionization or dissociative ionization. The experimental findings suggest that the excited H-transfer ammonia complexes with m > 2 are deactivated by an internal conversion process back to the electronically lowest H-transfer state followed by fast dissociation. Received 22 September 2001 and Received in final form 31 January 2002     

A study of energy transfer between electronically excited states using a picosecond laser pulse

By means of a novel picosecond polarization spectrometer we have determined that under conditions of normal pumping of organic systems a substantial annihilation of excited states occurs before the 6–8 ps duration of the pumping pulse. The effect is exposed by noting that prior to 6 ps the optical absorption of the S₁ transient produced by 353 nm pumping of 1,2-benzanthracene is caused to be depolarized at higher concentrations. In another case (xanthone) where no long range interactions can occur the depolarization of the spectra only occurred at low concentrations due to saturation effects.     

Isolated attosecond pulse generation from atom radiated by a three-color laser pulse

We theoretically investigate high-order harmonic and attosecond pulse generation from helium atom in a three-color laser field, which is synthesized by 10 fs/800 nm Ti-sapphire laser and a two-color field consisting of 30 fs/532 nm and30 fs/1330 nm pulses. Compared with harmonic spectrum generated by a monochromatic field, the harmonics generated from the synthesized three-color field show a supercontinuum spectrum with a bandwidth of 235 eV, ranging from the 154 th to the 306 th order harmonic. This phenomenon can be attributed to the fact that the ionization of atoms as well as motion of ionized electron can be effectively controlled in the three-color field. Therefore, an isolated 46-as pulse can be generated by superposing supercontinuum from the 160 th to the 210 th order harmonics.     

利用三色组合脉冲激光获得孤立阿秒脉冲发射

孤立阿秒脉冲因可以跟踪和控制原子及分子内电子的运动过程而备受关注.本文从理论上开展了氦原子在3束飞秒脉冲激光组合场辐照下产生的高次谐波和阿秒脉冲辐射的研究.组合激光场由16 fs/1600 nm,15 fs/1100 nm和5.3 fs/800 nm的钛宝石脉冲构成.与前两束脉冲合成的双色场产生谐波谱相比,附加钛宝石脉冲的三色场产生的高次谐波发射谱呈现出高转换效率及宽带超连续特性,超连续谱范围覆盖从230—690次谐波,傅里叶变换后实现了128 as高强度孤立短脉冲的产生.该结果归因于合成的三色场呈现出高功率及少周期的中红外飞秒脉冲激光特性,可以有效控制原子电离以及复合发生在中红外飞秒脉冲的一个有效光学周期内.     

Perturbation of highly excited states of an atom by the field of a neutral particle

Rydberg atom A** states perturbed by the force field of a neutral atom B are examined. The problem is reduced to studying the wave functions of the composite system and elucidating their fundamental differences from the Rydberg wave functions. The wave function of the Rydberg atom A**-atom B system is constructed using the finite-radius potential method with consideration for the short- and long-range interactions of the weakly bound Rydberg electron with the perturbing particle. The Na**-He system is considered as an example.