Electronic structure, adsorption geometry, and photoswitchability of azobenzene layers adsorbed on layered crystals.

Mono- and multilayers of the molecular photoswitch azobenzene were adsorbed on two layered transition-metal dichalcogenides, semiconducting HfS2 and metallic TiTe2, at temperatures of 80-120 K and investigated in situ using valence-band and core-level photoelectron spectroscopy as well as near-edge X-ray absorption fine structure spectroscopy. The spectroscopic results indicate similar growth modes on the two substrates. In the monolayer systems, the azobenzene molecules tend to lie flat on the surface with average tilt angles of <15°, whereas the multilayer systems show a larger average tilt angle of 35-45°, depending on substrate surface conditions. The chemical environment of azobenzene, as investigated by XPS, does not change significantly from mono- to multilayers suggesting weak adsorbate-substrate coupling for the molecular layer that forms the interface with the substrate. Irradiation with ultraviolet light with a wavelength of 365 nm leads to a partial rearrangement of the adsorbed azobenzene molecules with a trans-to-cis conversion of up to 35%.

Collapse of long-range charge order tracked by time-resolved photoemission at high momenta.

Intense femtosecond (10(-15) s) light pulses can be used to transform electronic, magnetic and structural order in condensed-matter systems on timescales of electronic and atomic motion. This technique is particularly useful in the study and in the control of materials whose physical properties are governed by the interactions between multiple degrees of freedom. Time- and angle-resolved photoemission spectroscopy is in this context a direct and comprehensive, energy- and momentum-selective probe of the ultrafast processes that couple to the electronic degrees of freedom. Previously, the capability of such studies to access electron momentum space away from zero momentum was, however, restricted owing to limitations of the available probing photon energy. Here, using femtosecond extreme-ultraviolet pulses delivered by a high-harmonic-generation source, we use time- and angle-resolved photoemission spectroscopy to measure the photoinduced vaporization of a charge-ordered state in the potential excitonic insulator 1T-TiSe(2 )(refs 12, 13). By way of stroboscopic imaging of electronic band dispersions at large momentum, in the vicinity of the edge of the first Brillouin zone, we reveal that the collapse of atomic-scale periodic long-range order happens on a timescale as short as 20 femtoseconds. The surprisingly fast response of the system is assigned to screening by the transient generation of free charge carriers. Similar screening scenarios are likely to be relevant in other photoinduced solid-state transitions and may generally determine the response times. Moreover, as electron states with large momenta govern fundamental electronic properties in condensed matter systems, we anticipate that the experimental advance represented by the present study will be useful to study the ultrafast dynamics and microscopic mechanisms of electronic phenomena in a wide range of materials.