Abstract
In graphene the relaxation in unoccupied electronic states is accompanied by electron momentum changes and phonon generation. Theoretical calculations and experimental observations show an initial ultrafast relaxation of hot electrons accompanied by a population of optical phonons which decay on a picosecond time scale. Here the speaker presents the results of excited electron relaxation dynamics in graphene on Ni(111) without and with gold intercalation measured by time-resolved IR/XUV two-photon photoemission.
Narrow 7-armchair graphene nanoribbons (a-GNRs) develop a band gap of up to several electron volts. Growing the nanoribbons from 10,10’-dibromo-9,9’-bianthryl precursor molecules on high-index single crystals, like Au(788), allows the preparation of spatially aligned nanoribbons with a width of about 0.74 nm and a mean length of about 23 nm. This in turn permits the study of momentum-resolved electron dynamics by angle-resolved measurements along the axes of the nanoribbons. Employing UPS and IPE the bandgap was determined to about Eg ~ (2.8 ± 0.3) eV. Time-resolved photoelectron spectroscopy on such 7-aGNRs on Au(788) was carried out under ultra-high vacuum conditions utilizing a time-of-flight electron spectrometer and a multi-anode detector. The sample was excited by the frequency doubled output of a femtosecond Ti:sapphire laser amplifier at λ = 395 nm (hν = 3.1 eV). Recompressing the pulses in a quartz prism compressor yields pulse durations of about 20 fs. With this set-up we identified unoccupied states at energies of E1 = 0.6 eV and E2/3 = 3.7 eV above the Fermi energy, respectively. The energetic positions of these states are in agreement with IPE measurements we performed previously on this system. From the dependence of the signal on the polarization of the incident light we found that the states at 3.7 eV obeys a strict selection rule, indicating their π-symmetry. The electronic dynamics has been measured by time-resolved 3-photon photoemission spectroscopy with cross-polarized laser pulses, yielding electronic lifetimes of τ ~ 75 and 110 fs.
About the speaker
Prof Helmut Zacharias received his PhD in Physics from Bielefeld University in 1978, and was postdoctoral fellow there until 1981. He had been faculty at the German Research Foundation and the University of Duisburg-Essen. He joined the University of Münster in 1996, and is currently Professor of Physics.
Prof Zacharias’ research interests are concerned with physical and chemical processes upon, within and underneath surfaces and interfaces. One of the goals of his research group is to contribute to the understanding of catalytic surface reactions. In order to investigate the dynamics of such reactions the systems under investigation are optically excited and the contributions of the internal states (rotation and vibration) of the different molecules as well as their spatial alignment are probed via the interaction with pulsed, tunable laser radiation. For resolving the dynamics of these very fast processes, laser pulses with extreme short (femtosecond) duration are required. Latest laser technologies permit the generation of laser pulses with pulse durations within the range of few femtoseconds in the wavelength range from the mid infrared (MIR) to the extreme ultra-violet spectral region (XUV). To supplement the scientific goals attention is paid to the development of laser sources for ultrashort light pulses in the extreme ultraviolet (XUV) and X-ray range.
Prof Zacharias is a Member of the Optical Society of America and the German Physical Society.
|
