O. Yenen

TWO-STAGE PARALLEL-PLATE ENERGY ANALYZER FOR SIMULTANEOUS DETECTION OF POSITIVE, NEGATIVE, AND NEUTRAL PARTICLES

We describe a unique apparatus that simultaneously measures the laboratory energy and angles of several charged particles formed in a single beam. The ability to separate and detect particles of opposite polarity allows one to measure them in coincidence. Equations for the trajectories of the particles are derived and discussed in detail. The expressions for the corresponding resolution of the detected particles are also presented. Data produced in recent experiments illustrate some of the analyzer’s more important features.

Alignment and orientation of Ar+ in He+-Ar collisions

We have measured the alignment and orientation parameters of the2F72/0 and2F52/0 states of Ar+ formed in the two-electron process; He++Ar→He(1s2)+Ar+(3p44p′). These have been measured at a collision energy of 0.25 keV/amu and for scattering angles ranging from 0.94° to 3.75°. First, by comparing the orientation parameter for the Ar+[(3p4[1D]4p′2F72/0] and the Ar+[(3p4[1D]4p′2F52/0] states, we have experimentally determined the importance of the spin-dependent interactions for the present collision system, by testing the Percival-Seaton hypothesis of spin independence. If the Percival-Seaton hypothesis holds for this system, the orientation parameter should beJ-independent. Secondly, the magnitude of the orientation parameter can be interpreted as resulting from the collective circulation of the unexcited 3p4 electrons and the excited 4p electron. The direction of this collective circulation is compared to the propensity rule for colliding di-atom systems.

Covalent-ionic problem: H+ −H − ion pairs from doubly excited (H2)∗∗ formed by transfer-excitation of H+2 during collisions

Abstract We measured the laboratory energy distribution of H + −H − pairs in coincidence, at 0° with respect to the beam direction, produced by 4.0 keV H + 2 colliding with Xe and He. The transformation of the laboratory energy distribution of H + −H − pairs in coincidence from the laboratory frame to the center-of-mass frame shows that, contrary to the earlier measurements, the charge transfer to the dissociative excited H 2 state releases a substantial amount of maximum kinetic energy ( ∼ 4 eV/ion) which suggests that H + −H − pairs result from the dissociation of doubly excited states of H 2 . These doubly excited states of H 2 ∗∗ are formed by an electron transfer-excitation process which is studied in molecules for the first time. The energies of these doubly excited states have been calculated by Guberman [J. Chem. Phys. 78 (1983) 1404] and are consistent with our measurements of the H + −H − energies. The dissociation process proceeds from covalent doubly excited states through states of mixed doubly and singly excited configurations, to states of mixed ionic and covalent configurations, to a final pure ionic state. The details of this transformation are still open to conjecture and await further theory and calculations.

Use of partial-wave decomposition to identify resonant interference effects in the photoionization?excitation of argon

We have studied simultaneous photoionization and excitation of Ar in the range of incident photon energies between 36.00 and 36.36 eV, where the resonant production of doubly excited neutral Ar states imbedded in the ionization continuum is dominant. By measuring the relative Stokes parameters of the fluorescence from residual Ar +∗ (3p 4 [ 3 P] 4p) ions ( 2 P1/2, 465.8 nm transition; 2 P3/2, 476.5 nm; 2 D3/2, 472.7 nm; 2 D5/2, 488.0 nm; 4 P5/2, 480.6 nm; 4 D5/2, 514.5 nm) we demonstrate a technique for determining individual partial-wave cross sections in photoionizing collisions. This procedure is shown to be important in sorting out competing dynamical ionization mechanisms, particularly with regard to resonant production of intermediate doubly excited autoionizing states. Comparison with theoretical photoionization cross sections demonstrates that spin–orbit coupling between different states of Ar II needs to be accounted for in the calculations. (Some figures in this article are in colour only in the electronic version)

Measured correlated motion in the continuum of three Coulomb-interacting particles, H+,H−,H+

We have measured the center of mass motion of the three massive Coulomb-interacting particles H+,H−,H+ in the continuum. Starting with excited H3+ we have experimentally studied the dissociation of this three-body system by determining the final-state energy sharing configurations and mutual H+,H+ correlation angles. Each measured triple dissociation event was mapped onto a Dalitz Plot that elucidated the various dynamic processes. In addition to the direct dissociation of excited H3+ into three Coulomb-interacting particles, we have identified intermediate compound states that decay into the observed final-state channel.

Experimental determination of orbital and spin contributions to density matrices of excited atomic states formed in transfer-excitation

For a quantum system, the density matrix contains all the statistical information that can be obtained from measurements. For excited atomic states, the coefficients in a spherical basis expansion of the density matrix, up to rank two, are proportional to Alignment and Orientation Parameters given by Fano and Macek and are called the state multipoles or multipole moments. They can be experimentally determined from the measurements of the Stokes parameters of the emitted photon. To obtain maximum possible information, one needs to break the axial symmetry of the experiment by specifying a reflection plane determined through a coincidence measurement. For LS coupled atomic states, if one can spectroscopically resolve J-multiplets, and if the process is spin dependent, information about the contributions to the state multipoles due to orbital and spin angular momenta can be extracted. In the process, it is also possible to determine the octupole moments of the excited state due to orbital and spin angular mo...

Alignment and orientation measurements of (Ar+)∗ formed in transfer-excitation collisions with He+

Abstract We have measured the total alignment parameter, A0, for several states of Ar+ formed in 1–7 keV collisions of He+ with Ar. Significant alignment has been found. We have also measured alignment and orientation parameters [U. Fano and J. Macek, Rev. Mod. Phys. 45 (1973) 553] for scattering angles from 0.5° to 3°.

Two- and three-electron processes in H+3-He collisions

Abstract We have measured the energy spectrum of electrons emitted at 160° laboratory angle for 50–110 keV H + 3 -He collisions. The spectra show the well-known autoionizing states of He and the He − (1s2s 2 ) Feshbach resonance. The triplet autoionizing states result from three-electron processes, whereas the singlets can be formed by either two- or three-electron processes. The Feshbach resonance is formed by a transfer and excitation mechanism, a two-electron process. The dependence of these states on the energy of the incoming beam is presented.

Collisional dissociation spectroscopy of (H3+)∗ and (He2+)∗ using small accelerators

Abstract The laboratory energy distributions, at 0° scattering, of H− and He+ resulting from the collisional excitation and dissociation of keV H3+ and He2+ have been measured. From the H− distribution we have determined an approximate c.m. energy distribution resulting from the three particle breakup of ( H 3 + ) ∗ → H + + H − + H + . From the He+ distribution we have found possible evidence for an electronically excited, metastable state of (He2+) at about 25 eV above the dissociation limit of He+ + He.