No abstract

The first experimental evidence for the existence of image-potential states in carbon nanotubes is presented. The observed features constitute a new class of surface image states due to their quantized centrifugal motion. Measurements of binding energies and the temporal evolution of image state electrons were performed using femtosecond time-resolved photoemission. The associated lifetimes are found to be significantly longer than those of n=1 image state on graphite, indicating a substantial difference in electron decay dynamics between tubular and planar graphene sheets.

The existence of low angular momentum image-potential states is predicted both for single-and multiwalled nanotubes. The states are confined between the self-induced potential on the vacuum side and the surface barrier, created by the central ascent in the transverse nanotube potential. Effective interactions near the surface of the nanotube are modeled with a cylindrical jelliumlike surface barrier, parameterized to ensure the correct transition into the long-range image potential. Binding energies and wave functions are calculated for (12,0), (10,10), (9,0) single-walled and ͑d = 9.48 nm͒ multiwalled nanotubes for different values of electron angular momenta. In addition, the expected relative lifetimes were calculated for the case of zero-angular momentum states of a (10,10) SWNT. The possible formation of image-potential states in nanotube bundles is briefly discussed.

The nature of low-energy excitations in multiwalled nanotubes (MWNTs) is investigated by means of two-color time-resolved photoemission. A careful analysis of the ballistic transport, secondary excitations, and band structure effects was employed in order to extract single electron lifetimes from the observed relaxation trend. It is demonstrated that in the vicinity of the Fermi level the energy dependence of e-e scattering times is inversely proportional to approximately the square of the excitation energy. This result provides strong evidence that electron transport in MWNTs exhibits a Fermi-liquid behavior, indicating that long-range e-e interaction along the tube vanishes due to screening.

This paper presents an investigation of the charge-state and energy dependences of transfer-ionization ͑TI͒ and single-electron-capture ͑SC͒ processes for fluorine ions ͑q =4+ to 9+͒ incident on a supersonic He jet target. The measurements were made for beam energies between 0.5 and 2.5 MeV/ u. A recoil ion momentum spectrometer was used to separate TI and SC based on the longitudinal momentum transfer and time of flight of the recoil ions. The cross-section ratios for TI to SC, R = TI / SC , were determined and observed to decrease monotonically with velocity. The values of R were combined with measured total transfer cross sections to deduce the cross sections for both SC and TI. Coupled-channel calculations of the energy dependence of TI and SC for F 9+ + He were compared to the experimental cross sections as well as the values of R. The calculated cross sections were found to be slightly lower and the R values slightly higher than the measured values, but with approximately the same energy dependences. A q 2 scaling of the He 2+ + He data was also compared to the present data and was found to give unexpected good agreement.