Panagiotis A. Loukakos

Dynamics of ripple formation on silicon surfaces by ultrashort laser pulses in subablation conditions

An investigation of ultrashort pulsed laser induced surface modification due to conditions that result in a superheated melted liquid layer and material evaporation are considered. To describe the surface modification occurring after cooling and resolidification of the melted layer and understand the underlying physical fundamental mechanisms, a unified model is presented to account for crater and subwavelength ripple formation based on a synergy of electron excitation and capillary waves solidification. The proposed theoretical framework aims to address the laser-material interaction in sub-ablation conditions and thus minimal mass removal in combination with a hydrodynamics-based scenario of the crater creation and ripple formation following surface irradiation with single and multiple pulses, respectively. The development of the periodic structures is attributed to the interference of the incident wave with a surface plasmon wave. Details of the surface morphology attained are elaborated as a function of the imposed conditions and results are tested against experimental data.

Femtosecond dynamics of electronic states in the Mott insulator 1T-TaS2 by time resolved photoelectron spectroscopy

Photoexcitation of the Mott insulator 1T-TaS2 by an intense laser pulse leads to an ultrafast transition toward a gapless phase. Besides the collapse of the electronic gap, the sudden excitation of the charge density wave (CDW) mode results in periodic oscillations of the electronic states. We employ time resolved photoelectron spectroscopy to monitor the rich dynamics of electrons and phonons during the relaxation toward equilibrium. The qualitative difference between the oscillatory dynamics of the CDW and the monotonic recovery of the electronic gap proves that 1T-TaS2 is indeed a Mott insulator. Moreover the quasi-instantaneous build-up of mid gap states is in contrast with the retarded response expected from a Peierls insulating phase. Interestingly, the photoinduced electronic states in the midgap spectral region display a weak resonance that is reminiscent of a quasiparticle peak.

Ultrafast electron dynamics studied with time-resolved two-photon photoemission: intra- and interband scattering in C6F6/Cu(1 1 1)

The advances in femtosecond laser techniques facilitate the investigation of ultrafast electron dynamics at surfaces directly in the time-domain. We employ time-resolved two-photon-photoemission (2PPE) spectroscopy to study the electron dynamics of the unoccupied electronic states in hexafluorobenzene (C6F6) on Cu(1 1 1) serving as a model system for charge transfer across organic–metal interfaces. Our coverage-dependent study reveals a lifetime of the lowest unoccupied molecular resonance of 7 fs for a single monolayer (ML) which increases to 37 fs above 3 ML coverage. We find that the population build-up of the excited state is delayed by a characteristic time of about 10 fs with respect to the exciting laser pulse. By angle-resolved 2PPE spectroscopy, the mechanism of the delayed population rise is identified as intraband relaxation in the adsorbate band structure. The actual electron-transfer to the metal substrate occurs through interband scattering between the molecular resonance and substrate states on comparable timescales. Therefore the present study demonstrates that relaxation of hot electrons at molecule–metal interfaces include—even in the presence of strong electronic molecule–substrate interaction—also decay channels within the adlayer.

Femtosecond dynamics of electron transfer, localization, and solvation processes at the ice—metal interface

The ultrafast dynamics of excess electrons in amorphous ice layers on single-crystal metal surfaces are investigated by femtosecond time- and angle-resolved two-photon-photoemission spectroscopy. Photoexcited electrons are injected from the metal substrate into delocalized states of the conduction band of ice and localize in the ice layer within 100 fs. Subsequently, energetic stabilization of this localized species is observed on a time scale of ∼1 ps, which is attributed to electron solvation by nonadiabatic coupling to nuclear degrees of freedom of the surrounding polar molecular environment. Concomitant with this stabilization process, residual wave function overlap of the solvated electron with the metal substrate results in back-transfer by tunneling through the solvation shell. At such interfaces the correlation of electronic and molecular structure with the resulting solvation dynamics can be explored using different substrates as a template. Here we compare data on molecularly thin D 2 O ice layers grown on Cu( 111) and Ru(001). On Ru(001) both the stabilization and back-transfer proceed about three times faster compared to Cu( 111), which is attributed to different interfacial structures and the role of d-states, and projected band gaps in the electron transfer process.

Ultrafast Electron Dynamics in C6F6/Cu(111) after Localized or Delocalized Excitation

Using optical and core-hole excitation, we study relaxation dynamics of excited electrons at C6F6/Cu(111) interfaces as a function of adsorbate coverage. Differences in the coverage dependence of charge transfer times obtained by time-resolved photoemission and core-hole spectroscopy indicate different delocalization processes of the excited electron wave packet in the C6F6 resonance.

Ultrafast Electron Dynamics in C 6 F 6 /Cu(111) after a Localized or Delocalized Excitation

Comparing optical and core-hole excitation, we study relaxation dynamics of excited electrons at C6F6/Cu(111) interfaces. The pronounced tenfold difference in relaxation times is attributed to electron transfer across the interface and intra-molecular electron delocalization, respectively.