Comparison study of atomic and molecular single ionization in the multiphoton ionization regime

In this Letter, we report, for the first time in the multiphoton ionization regime, a comparison study of single-electron ionization of diatomic molecules versus rare gas atoms with virtually the same ionization potentials. In comparing N2+ to Ar+, a higher ion signal is seen in N2+ compared to Ar+ for linear polarization but the difference vanishes in circularly polarized light. In comparing O2+ to Xe+, we observe a suppression in O2+ compared to Xe+ for both linear and circular polarization but this suppression exhibits an intensity dependence; i.e., there is little suppression for O2+ at the lowest intensity range, but the suppression becomes increasingly stronger as the laser intensity increases. The multielectron screening model is used to discuss possible mechanisms of this intensity dependent suppression in O2+ in the multiphoton ionization regime.     

硫化氢分子的共振增强多光子电离谱

利用激光质谱法在 4 2 3～ 4 62nm范围内获得了环境污染气体硫化氢分子的共振增强多光子电离 (REMPI)谱 ,在其REMPI谱中共观察到 8个谱峰 ,其中有两个谱峰是新发现的 ,并对它们进行了试探性标识。另外还从多个角度对硫化氢分子的多光子电离解离 (MPID)机理进行了分析 ,推论出其MPID过程应属于母体离子碎裂模式。     

Controlling attosecond double ionization dynamics via molecular alignment

We investigate the dynamics of double ionization in aligned nitrogen molecules. An ultrashort, weak laser pulse creates an aligned ensemble of molecules that is ionized with a subsequent, strong probe pulse. We find that the two electrons involved in nonsequential double ionization more likely exit the molecule in the same direction if it is parallel to the probe laser polarization, indicating that they are ejected within a few hundred attoseconds of each other. Double ionization is less probable and takes longer for perpendicular molecules.     

Strong-field molecular ionization: determination of ionization probabilities calibrated with field-free alignment

We report an original optical method providing the probability of molecular ionization induced by femtosecond laser pulses. The approach consists of exploiting molecular alignment to extract reliable information about ionization. The cross defocusing technique implemented for this purpose reveals a sensitivity with respect to postpulse alignment, as well as to the free electron density induced by the ultrashort laser pulse. The analysis of the resulting signal thus gives access to absolute single-ionization probabilities calibrated through the degree of alignment, provided that free electrons are produced mainly by single ionization. The relevance of the method is assessed in N2.     

Isotope-selective IR multiphoton dissociation of CHClF2 in the presence of NO2

2 to the CHClF₂/He mixture irradiated by a Q-switched CO₂ laser leads to oxidation of the dissociation product according to the reaction: CF₂+NO₂→COF₂+NO. The resulting COF₂ with a ¹³C content near 50% is easy to convert to CO₂ or CO for further enrichment by a nonlaser process. We measured the dependence of the fraction of dimerised CF₂ on NO₂ pressure p_NO2 and the amount of NO₂ required to suppress dimerisation on the dissociation yield. Both agree with a kinetic model using known rate constants. For the range of the dissociation parameters (¹³CF₂ yield of 10% per pulse, isotope selectivity of 130) of practical interest, 95% of the CF₂ produced is oxidized at p_NO2≈1/2p_CHClF2. In the absence of NO₂, major (20%–35%) losses of CF₂ at the metal walls of the irradiation system were observed. Addition of NO₂ suppresses them. For comparison, we also used O₂ as a scavenger in CHClF₂ dissociation. NO₂ is by orders of magnitude more efficient. Received: 21 January 1997/Revised version: 23 March 1997     

Isotope-selective infrared multiphoton dissociation of CF3Br in a supersonic free jet

The carbon-isotope selectivity in the multiphoton dissociation of CF₃Br is studied in the collisional region of supersonic free jet. The isotopic abundance of¹²C and¹³C in C₂F₆ formed by recombination of the dissociation products is measured with a quadrupole mass spectrometer. An enrichmet factor of 9.4 is obtained for¹²C with the 9R(30)CO₂ laser line while the factor of 6.9 is obtained for¹³C with the 9P(16) line.     

Phase-dependent excitation and ionization in the multiphoton ionization regime

We theoretically study the dependence of atomic excitation and ionization on the carrier envelope phase of few-cycle laser pulses in the multiphoton ionization regime. Our theoretical results for the hydrogen atom based on the solution of the 3D time-dependent Schr?dinger equation show that the strong phase dependence can be seen in not only total ionization, but also bound-state population under the weak laser intensity regime.     

Electron and multiphoton ionization of xenon

A method for determining photoionization cross sections based on comparison of the characteristics of electron- and photon-impact processes is described, discussed, and demonstrated to have many advantageous features.     

Resonant multiphoton ionization of Li

We present calculations of three photon ionization of Li produced by 3-ps laser pulses within a single electron local model potential using the dressed state picture. The laser frequency ranges from 15 000 to 18 400 cm⁻¹. We have found that the measured ionization signal as a function of photon frequency results from ionization processes in a region where the laser intensity is not homogeneous. We assume a Gaussian shape for the light pulse in the interaction volume. Our results are shown to be in good agreement with experiment. We propose that free electrons are submitted to a ponderomotif potential to interpret experimental results. This revised version was published online in August 2006 with corrections to the Cover Date.     

Saturation and rabi oscillations in resonant multiphoton ionization

We present an analysis of saturation in resonant two-photon ionization which includes bound and continuum energy shifts, induced widths and bound-bound nutational Rabi frequencies at extreme saturation.     

Few cycle dynamics of multiphoton double ionization

In intense field ionization, an electron removed from the atomic core oscillates in the combined fields of the laser and the parent ion. This oscillation forces repeated revivals of its spatial correlation with the bound electrons. The total probability of double ionization depends on the number of returns and therefore on the number of optical periods in the laser pulse. We observed the yield of Ne(2+) relative to Ne(+) with 12 fs pulses to be clearly less compared to 50 fs pulses in qualitative agreement with our theoretical model.     

Localized and dispersive states in the multiphoton ionization processes

We introduce the notion of localized and dispersive states in the problem of multiphoton ionization of atoms by a laser beam. The connection is made with the ionization probability.     

NH_3多光子电离和裂解动力学过程分析

本文分析了在XeCl准分子激光作用下NH_3分子的多光子电离质谱形成机理。所建立的动力学模型包括主要由经过中间态(6)和(1)的(2+1)共振多光子电离,生成NH_3~+;离子-分子反应形成大量的NH_4~+;经(6)态的(2+2)共振多光子电离,可能产生低产额的NH_2~+。这个模型的速率方程解与实验测量符合得很好,并在离子流强度对激光脉冲宽度的依赖关系中预示直接电离或间接电离机制的信息。     

Energy and angular distributions of photoelectrons in multiphoton ionization

The energy and angular distributions of photoelectrons in the photoionization of an atom or ion by powerful laser radiation are calculated. The results obtained are valid for all values of the Keldysh parameter γ. Linear and circular polarizations of the light are considered. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 493–497 (25 October 1999)