High-velocity limit for the ratio of helium double-to-single ionization for highly-charged, bare-ion impact

The ratio R of helium double-to-single ionization cross sections was measured for Ne"1"0"+- and Ni"2"8"+-projectiles at velocities of 0.39#<=#v_P/c#<=#0.93 intending to explore the high-velocity limit of R for highly-charged ion impact. For Ne"1"0"+-projectiles R was observed to become independent of v_P for v_P/c >or#approx# 0.73 and an asymptotic experimental value R=(2.57#+-#0.10).10"-"3 has been established. This is in good agreement with experimental results for proton, antiproton, electron and positron impact and in excellent accordance with the theoretical prediction of (2.59#+-#0.03).10"-"3. (orig.)SIGLEAvailable from TIB Hannover: RO 801(93-66) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Double ionization of helium and neon for fast heavy-ion impact: Correlated motion of electrons from bound to continuum states

Double ionization of He and Ne by charged particle impact was explored in kinematically complete experiments measuring the final momenta of the 2 emerging electrons (P_e_1"f, P_e_2"f) and of the recoiling target ion P_R"f. The momentum transfer from the 3.6 MeV/u Se"2"8"+ projectile to the electrons is found to be negligibly small. Thus, distinct patterns observed in the P_e_1"f-versus-P_e_2"f spectra sensitively reflect the details of the electrons' correlated motion. Classical calculations reasonably describe the data only when the (e-e) interaction during the ionization reaction is included. (orig.)19 refs.SIGLEAvailable from TIB Hannover: RO 801(96-16) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Atoms in extreme virtual photon fields of fast, highly charged ions

Highly-charged ions at velocities close to the speed of light generate strong (I = 10"1"7 - 10"2"0 W/cm"2), ultra-short (t = 10"-"1"9 - 10"-"1"7 s) electromagnetic pulses when passing target atoms or molecules. Single and multiple ionization occurs and can be interpreted in terms of photoionization: at small field strength ionization is due to the absorption or the scattering of a single virtual photon whereas the incoherent, ''simultaneous'' absorption of ''many'' field quanta gives rise to multiple ionization in strong fields. In this paper analogies to photoionization are investigated by comparison of kinematically complete experimental data with results of various theoretical calculations for collisions of 100 MeV/u C"6"+, 1000 MeV/u U"9"2"+ and 3.6 MeV/u Au"5"3"+ on He, Ne and Ar targets. Dynamical multiple ionization mechanisms are identified as a function of the momentum transfer. Two-electron final states are found to sensitively depend on the correlated initial state, and the many-particle Coulomb continuum is investigated for triple ionization of neon in super-strong fields. (orig.)29 refs.SIGLEAvailable from TIB Hannover: RO 801(99-29) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Double ionization of helium by fast fully stripped ions

A complete momentum-space map of the double ionisation events in the collision of 100 MeV/u C"6"+ ions with helium has been obtained. From this map the angular distribution of two ejected electrons is generated as a function of the momentum transferred by the projectile to the atom. Analysis of the angular distribution of the fragments in the plane transverse to the projectile axis (the azimuthal plane) shows a separation of events into two domains, depending on the momentum transferred by the projectile to the target. For momentum transfers smaller than 1.2 a.u., both electrons are distributed independent of the azimuthal angle. For momentum transfers larger than 1.2 a.u., the electron with larger energy is distinctly emitted along the direction of momentum transfer, and the one with smaller energy is distributed isotropically. The value of 1.2 a.u. is approximately equal to the mean value of the momenta of the bound electrons in the helium atom and demarcates indirect, soft collisions and direct, hard collisions respectively. For soft collisions, the electron angular distribution shows certain similarities with the angular distributions in photo-double ionization. (orig.)Available from TIB Hannover: RO 801(98-65) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Ultra-low energy electrons from fast heavy-ion helium collisions: the 'target Cusp'

Doubly differential cross sections d"2#sigma#/dv _p_a_r_a_l_l_e_l dv _p_e_r_p_e_n_d_i_c_u_l_a_r _t_o have been obtained by mapping the 3-dimensional velocity space of ultra-low and low-energy electrons (1.5 meV#<=# E_e#<=#100 eV) emitted in singly ionizing 3.6 MeV/u Au"5"3"+ on helium collisions. A sharp (#DELTA#E_e _p_e_r_p_e_n_d_i_c_u_l_a_r _t_o "F"W"H"M #<=# 22 meV) asymmetric peak centered at vertical stroke anti #nu# vertical stroke =0 is observed to emerge at ultra-low energies from the strongly forward shifted low-energy electron velocity distribution. The shape of this ''target cusp'', which is very sensitive on the details of the two-center potential, is in excellent accord with theoretical CTMC and CDW-EIS predictions. (orig.)Available from TIB Hannover: RO 801(98-61) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Double ionisation of helium in fast ion collisions: the role of momentum transfer

Double ionisation of helium in the perturbative regime has been explored in a kinematically complete collision experiment using 100 MeV/u C"6"+ ions. Different ionisation mechanisms are identified by inspecting the angular distribution of the electrons as a function of the momentum transfer q to the target by the projectile. For q<1.2 a.u., both electrons are distributed uniformly in the plane perpendicular to the projectile axis, and distinct similarities with photoionisation are identified. For q>1.2 a.u., the faster electron resulting from a binary encounter with the projectile is emitted along the direction of momentum transfer, while the other electron is distributed uniformly. Experimental data are compared with various model calculations based on the Bethe-Born approximation with shake-off. Surprisingly, the effect of the final state interaction is found to depend decisively on the choice of the initial state wave function. (orig.)Available from TIB Hannover: RO 801(99-02) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Low-energy electrons and their dynamical correlation with the recoil-ions for single ionization of helium by fast, heavy-ion impact

Helium single ionization by 3.6 MeV/u Ni"2"4"+ impact was explored in a kinematically complete experiment by combining a high-resolution recoil-ion momentum spectrometer with a novel 4#pi# low-energy electron analyzer. More than 90% of the ''soft electrons'' (E_e>or#approx#50 eV) are ejected in the forward direction in agreement with CTMC predictions. The electron longitudinal momentum is not balanced by the longitudinal momentum change of the projectile but mainly by the backwards recoiling He"1"+-ion. Energy losses of the 0.2 GeV projectiles as small as #DELTA#E_P/E_P=3.4.10"-"7 are observable. (orig.)Available from TIB Hannover: RO 801(94-64) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Ionization of helium in the attosecond equivalent light pulse of 1 GeV/u U"9"2"+ projectiles

Single and double ionization of helium by 1 GeV/u U"9"2"+ impact was explored in a kinematically complete experiment. The relativistic ion generates a sub-attosecond (10"-"1"8 s) superintense (I>10"1"9 W/cm"2) electromagnetic pulse, which is interpreted as a field of equivalent photons (Weizsaecker-Williams method). Cross sections, the emission characteristics of ions and electrons as well as momentum balances are quantitatively discussed in terms of photoionization of the atom in this broadband, ultra-short virtual photon field. (orig.)Available from TIB Hannover: RO 801(97-35) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Initial state dependence of low-energy electron emission in fast ion atom collisions

Single and multiple ionization of Neon and Argon atoms by 3.6 MeV/u Au"5"3"+ impact has been explored in kinematically complete experiments. Doubly differential cross sections for low-energy electron emission have been obtained for defined charge state of the recoiling target ion and the receding projectile. Observed target specific structures in the electron continuum are attributable to the nodal structure of the initial bound state momentum distribution. The experimental data are in excellent accord with CDW-EIS single ionization calculations if multiple ionization is considered appropriately. (orig.)21 refs.SIGLEAvailable from TIB Hannover: RO 801(99-20) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Electron correlations observed through intensity interferometry

Intensity interferometry was applied to study electron correlations in doubly ionizing ion-atom collisions. In this method, the probability to find two electrons emitted in the same double ionization event with a certain momentum difference is compared to the corresponding probability for two uncorrelated electrons from two independent events. The ratio of both probabilities, the so-called correlation function, is found to sensitively reveal electron correlation effects, but it is rather insensitive to the collision dynamics. (orig.)12 refs.SIGLEAvailable from TIB Hannover: RO 801(99-28) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman