Gradient- and Sensitivity-Enhanced TOCSY Experiments

A pulsed field gradient version of the sensitivity-enhanced 2D TOCSY experiment is proposed which yields high-quality spectra with improved sensitivity and a minimum of two scans pert₁increment. For rapid acquisition of 1D TOCSY spectra, the 1D DPFGSE–TOCSY experiment was modified to include phase-encoded multiple-selective excitation followed by a simple spectral editing. Combination of these two building blocks is used in a sensitivity-enhanced 2D analog of the 3D TOCSY–TOCSY experiment which provides an efficient tool for resolving severely overlapped signals of oligomers in short experimental time.     

Resonant widths, line intensities and lineshapes for MQDT models with two or more open channels

Approximate analytical formulae describing the energy variation of line intensities, autoionization widths and lineshape asymmetries, are derived for a Phase-Shifted Multichannel Quantum Defect Theory model composed of two closed interacting channels coupled to two effective continua. This is accomplished by putting the two compatibility equation solutions, for the common phase shifts of the two open channels, in such a form so the resonant behavior is attributed to one of them, the other accounting for an energy dependent background. Then, the well-known procedures for the simpler case where only one continuum is considered are applied, using only the resonant solution. The method is quite general and applicable to any MQDT model with two or more open channels. The resulting analytical formulae are tested on experimental spectra of Sr, Ba and Cu and it is shown that they are valid as long as: i) The resonances are non-overlapping, ii) The direct closed channel coupling is much stronger than the indirect one through the continua and (when excitation matrix elements are involved) iii) The open channels excitation strength is smaller or at least comparable to the closed channels one. Received: 26 May 1998 / Accepted: 1st July 1998     

Temporal coherent control induced by wave packet interferences in one and two photon atomic transitions

The interaction of a sequence of two identical ultrashort laser pulses with an atomic system results in quantum interferences as in Ramsey fringes experiments. These interferences allow achievement of temporal coherent control of the excitation probability. We present the results of a temporal coherent control experiment on two different atomic systems: one-photon absorption in K (4s-4p) and two-photon absorption in Cs (6s-7d). In K, the quantum interferences between the two excitation paths associated with the laser pulses are revealed through rapid oscillations of the excitation probability as a function of the time delay between the two pulses. These oscillations take place at the transition frequency (period T = 2.56 fs). The interferences are modulated by beats (at about 580 fs) resulting from the doublet structure of the excited state (4p (² P _1/2 , ² P _3/2 )). Three complementary interpretations of this experiment are presented: in terms of beats of quantum interferences, of variation in the spectrum intensity, and of wave packet interferences. Whenever the two laser pulses are temporally overlapped, optical interferences are superimposed on to the quantum interferences. The distinction between these two types of interference is clearly revealed in the two-photon excitation scheme performed on Cs (6s-7d (² D _3/2 , ² D _5/2 )) because quantum interferences occur at twice the frequency of the optical interferences. Received: 30 December 1997 / Revised: 28 February 1998 / Accepted: 4 March 1998     

Constructive and Destructive Two-Pulse Excitation Investigated with a White-Light Michelson Interferometer

Linear propagation of two pulses through methanol solution of aluminum phthalocyanine chloride is investigated using a modified white-light Michelson interferometer. The observed coherence time of the white light is 6 fs, and the separation between the two-excitation pulses is set to about 10 fs. The excitation is dependent on the phase-relation between the two pulses. We have observed an enhancement of the excitation when the two pulses are in-phase and strong suppression of the excitation when the two pulses are out of phase by π.     

Measurement of the transverse kinetic energies of argon recoil ions produced in 120 MeV -Ar collisions

Measurements were carried out to deduce the transverse kinetic energies of highly charged argon recoil ions produced in single collisions of 120 MeV ions with argon atoms in which the post collision charge states of the projectiles were not determined. A time of flight spectrometer was designed and fabricated to detect the charge states of recoils. Experimental procedures for optimizing the spectrometer for extraction, transmission and detection of recoils are described. A simple approach for determining the transverse kinetic energy of the recoil ions from FWHM of the peaks is reported. This method is shown to be independent of the choice of collision partners and requires only the knowledge of the physical values of “optimized parameters” of time-of-flight spectrometer used in the experiment. The transverse kinetic energy of the recoil ions determined from the present approach is found to vary from 0.03 eV for to 4.02 eV for Ar¹⁰⁺. These values are compared with the results reported by earlier workers and are shown to follow a q²-behaviour up to a charge state q =8+ of the recoil ions. Received: 5 February 1998 / Revised: 8 June 1998 / Accepted: 11 June 1998     

Total cross-sections for positron scattering by a series of molecules

The additivity rule is employed to obtain the total (elastic+inelastic) cross-sections for positron scattering from molecules including a number of diatomic, polyatomic molecules (H₂, N₂, HCl, CO₂, NH₃, SF₆, CH₄, C₂H₄ and C₃H₈) over an incident energy range of 10-1000 eV. The total cross-sections (TCS) of the constituent atoms of molecules are obtained by employing a complex optical model potential (composed of static, polarization and absorption potential). The present results are compared with experimental data and other theoretical calculations, good agreement is obtained in intermediate- and high-energy region. Received: 11 November 1997 / Revised: 23 March 1998 / Accepted: 16 June 1998     

Elastic scattering of low-energy electrons by NO

We report elastic integral, momentum transfer and differential cross sections for electron scattering by N₂O for energies up to 50 eV. These results were obtained at the static-exchange approximation with the Schwinger Multichannel Method with Pseudopotentials [M.H.F. Bettega, L.G. Ferreira and M.A.P. Lima, Phys. Rev. A 47, 1111 (1993)]. In general our results show good agreement with experimental data and with other theoretical results but some discrepancies are found. We have also found a shape resonance around 4 eV in agreement with previous calculations using the R-matrix Method of Sarpal et al. [J. Phys. B 29, 857 (1996)]. On the other hand, the existence of a resonance at about 13 eV, clearly seen by the Schwinger Variational Iterative Method [Michelin et al., J. Phys. B 29, 2115 (1996)], can not be confirmed by our calculations. At this energy, our cross sections show a broad bump with no clear resonant behavior given by the eigenphase sum. Received: 13 November 1997 / Revised: 13 March 1998 / Accepted: 9 April 1998     

Triple differential cross section of electron impact ionisation of Na(3s, 3p, 3d)

The fivefold differential cross section (5DCS) of the ionisation by electron impact of atomic sodium is determined theoretically for its fundamental 3s(² S) state and the excited 3p(² P) and 3d(² D) states by a procedure which employs in the transition matrix element of the first order Born approximation, the correlated double continuum (3C) wave function. This permits us to determine the statistical M-state population and the orientation and alignment tensors in (e,2e) detection. It is also shown that, the use of Gamow correlation term, in the independent particle (2C) model, reproduces, only in some situations, the shape of the angular distribution of the 5DCS obtained by the (3C) wave function. Received: 17 November 1997 / Received in final form: 16 March 1998 / Accepted: 21 March 1998     

On phase-coherence in rescattering in multiphoton ionization and in higher-order harmonic generation

Summary  The recently discovered plateau and rings in high-order multiphoton ionization apparently have their origin in rescattering. Similarly, rescattering is involved in higher-order harmonic generation. Using a simple one-dimensional model, it is shown that the phase-coherence in this rescattering process is of considerable importance. The authors of this paper have agreed to not receive the proofs for correction. This work has been supported by the East-West Program of the Austrian Academy of Sciences and by the Austrian Ministry of Science and Transportation under project no. 45.372/2-IV/6/97. One of us (JZK) acknowledges the support of the Polish Committee of Scientific Research (grant KBN 2 P03B 007 13).     

氢分子与氦原子碰撞转动激发截面理论计算

当入射氦原子能量为０．０５ｅＶ时，计算了氦原子与氢分子碰撞的弹性散射和第一转动激发截面，用密耦方法计算了５０个分波，得到的分波截面收敛得很好。