Disentanglement of electron dynamics and space-charge effects in time-resolved photoemission fromh-BN/Ni(111)

Abstract

We observe time-resolved and polarization dependent two-, three-, and four-photon transitions from a Ni(111) surface covered with a monolayer of hexagonal boron nitride. The spectra show features due to transitions involving two unoccupied intermediate states: the n = 1 image potential state of Ni(111) and a boron nitriderelated interface state. We use these transitions in order to track the effects of space-charge on the spectra in view of pump-probe experiments, at high excitations densities. A simple model is presented, which reproduces the measured energy shifts and broadenings in great detail, allowing the ultrafast dynamics and the space-charge effects to be disentangled. Moreover, owing to the high excitation densities an additional particular three-photon transition via both the interface state and a virtual intermediate state could be measured and identified. The polarization-dependent transient of this transition has successfully been simulated, allowing the lifetime of the involved intermediate state to be extracted from the data.

Excitation of Coherent Phonons in the One-Dimensional Bi(114) Surface

“…We excite the sample with a linearly p-polarized infrared pump pulse (h 1 ¼ 1:55 eV) [16]. In the measurements shown here, the absorbed pump fluence was set to 0:57 mJ=cm 2 , corresponding to an excitation density of n % 0:34% of the valence electron density [14].…”

Full characterization and optimization of a femtosecond ultraviolet laser source for time and angle-resolved photoemission on solid surfaces

“…This effect is well known and has been observed in several experiments 8,11 with femtosecond laser pulses. Here, we quantify the importance of space charge and attempt to determine an upper limit on the acceptable number of photons/pulse.…”

“…Indeed, for femtosecond pulses, the density of photoelectrons can be considerable and space charge can shift, broaden, and distort the electronic bands. [8][9][10][11] Thus, in order to avoid the space charge effect while maintaining a high photon flux, it is desirable to work with a high repetition laser source (>100 kHz).…”

Time-Resolved Photoelectron Spectroscopies Using Synchrotron Radiation: Past, Present, and Future

“…76,77) Especially when the pulse-width is femtoseconds-scale, the effect is critically enhanced due to significant increase of electron density by the ultrashort time duration. Figure 12 (left) shows W 4f core-level photoemission spectra measured at FLASH as a function of pulse intensity.…”