Femtosecond Pump-probe Photoelectron Spectroscopy Research Articles

Local Ionization Dynamics Traced by Photoassisted Scanning Tunneling Microscopy: A Theoretical Approach.

For tracing the spatiotemporal evolution of electronic systems, we suggest and analyze theoretically a setup that exploits the excellent spatial resolution based on scanning tunneling microscopy techniques combined with the temporal resolution of femtosecond pump-probe photoelectron spectroscopy. As an example, we consider the laser-induced, local vibrational dynamics of a surface-adsorbed molecule. The photoelectrons released by a laser pulse can be collected by the scanning tip and utilized to access the spatiotemporal dynamics. Our proof-of-principle calculations are based on the solution of the time-dependent Schrödinger equation supported by the ab initio computation of the matrix elements determining the dynamics.

Resonant tunneling through the repulsive Coulomb barrier of a quadruply charged molecular anion

Multiply charged anions possess a repulsive Coulomb barrier (RCB) against electron emission, thus allowing for long-lived metastable species with negative electron binding energies. For the prototypical multianion, bisdisulizole tetra-anion, we demonstrate that electronically excited states supported by the RCB can undergo resonant tunneling. The dynamics of this process was investigated by one-photon photoelectron imaging and femtosecond pump-probe photoelectron spectroscopy and confirmed by theoretical calculations. Efficient resonant tunneling emission of electrons from the excited states of multianions may be common for systems with sufficiently large RCB. This may provide new opportunities to study electron emission dynamics in complex systems.

Femtosecond time-resolved photoelectron spectroscopy with a vacuum-ultraviolet photon source based on laser high-order harmonic generation

A laser-based tabletop approach to femtosecond time-resolved photoelectron spectroscopy with photons in the vacuum-ultraviolet (VUV) energy range is described. The femtosecond VUV pulses are produced by high-order harmonic generation (HHG) of an amplified femtosecond Ti:sapphire laser system. Two generations of the same setup and results from photoelectron spectroscopy in the gas phase are discussed. In both generations, a toroidal grating monochromator was used to select one harmonic in the photon energy range of 20-30 eV. The first generation of the setup was used to perform photoelectron spectroscopy in the gas phase to determine the bandwidth of the source. We find that our HHG source has a bandwidth of 140 ± 40 meV. The second and current generation is optimized for femtosecond pump-probe photoelectron spectroscopy with high flux and a small spot size at the sample of the femtosecond probe pulses. The VUV radiation is focused into the interaction region with a toroidal mirror to a spot smaller than 100 × 100 μm(2) and the flux amounts to 10(10) photons/s at the sample at a repetition rate of 1 kHz. The duration of the monochromatized VUV pulses is determined to be 120 fs resulting in an overall pump-probe time resolution of 135 ± 5 fs. We show how this setup can be used to map the transient valence electronic structure in molecular dissociation.

Ultrafast dynamics in thiophene investigated by femtosecond pump probe photoelectron spectroscopy and theory

A hybrid of a time-of-flight mass spectrometer and a time-of-flight "magnetic-bottle type" photoelectron (PE) spectrometer is used for fs pump-probe investigations of the excited state dynamics of thiophene. A resonant two-photon ionization spectrum of the onset of the excited states has been recorded with a tunable UV laser of 190 fs pulse width. With the pump laser set to the first intense transition we find by UV probe ionization first a small time shift of the maxima in the PE spectrum and then a fast decay to a low constant intensity level. The fitted time constants are 80+/-10 fs, and 25+/-10 fs, respectively. Theoretical calculations show that upon geometry relaxation the electronic state order changes and conical intersections between excited states exist. We use the vertical state order S1, S2, S3 to define the terms S1, S2, and S3 for the characterization of the electron configuration of these states. On the basis of our theoretical result we discuss the electronic state order in the UV spectra and identify in the photoelectron spectrum the origin of the first cation excited state D1. The fast excited state dynamics agrees best with a vibrational dynamics in the photo-excited S1 (80+/-10 fs) and an ultrafast decay via a conical intersection, presumably a ring opening to the S3 state (25+/-10 fs). The subsequently observed weak constant signal is taken as an indication, that in the gas phase the ring-closure to S0 is slower than 50 ps. An ultrafast equilibrium between S1 and S2 before ring opening is not supported by our data.

In situ photovoltage measurements using femtosecond pump-probe photoelectron spectroscopy and its application to metal–HfO2–Si structures

We report in situ photovoltage measurements of metal-oxide-semiconductor (MOS) structures using femtosecond pump-probe photoelectron spectroscopy. This technique, which employs a single femtosecond laser, is a noncontact noninvasive measurement method for extracting the magnitude and direction of the band bending in Si substrates covered with high-k dielectric stacks and thin metal layers. We studied MOS structures consisting of thin metal layers of both high and low work functions deposited atop HfO2 grown on Si (100) substrates during various phases of processing. Excitation of the sample by a pulse of laser light flattens the bands of the Si substrate, which can be monitored as a rigid shift in the observed photoelectron spectrum. Particular attention is given to the potential effects of electron-hole recombination and metallic screening on the magnitude of the shift in the photoelectron spectra. We find that while as-deposited metals follow the metal-induced gap state model, thermal annealing of these structures drives the interface silicon Fermi energy to midgap. Charged oxygen-vacancy related defects at or near the HfO2/metal interface contribute significantly to the Fermi energy shift.

Oxygen defects and Fermi level location in metal-hafnium oxide-silicon structures

We describe an in situ method for measuring the band bending of Si substrates in complex metal-oxide-semiconductor systems using femtosecond pump-probe photoelectron spectroscopy. Following deposition of metal layers (Pt, Re, or Re oxide) on the high-k dielectric HfO2, measurement of the band bending in the underlying Si provides a direct determination of the location of the Fermi level within the Si band gap at the Si-dielectric interface. Changes in the Fermi level with post-deposition anneals and oxygen exposure were correlated with valence and core photoelectron spectroscopy as well as capacitance-voltage measurements. These studies illuminate the roles that gate metal work function, modified by metal induced gap states and defects within the oxide, such as oxygen vacancies, play in defining the location of the Fermi level in metal-oxide-semiconductor structures.

Pump-probe photoionization study of the passage and bifurcation of a quantum wave packet across an avoided crossing.

The application of femtosecond pump-probe photoelectron spectroscopy to directly observe vibrational wave packets passing through an avoided crossing is investigated using quantum wave packet dynamics calculations. Transfer of the vibrational wave packet between diabatic electronic surfaces, bifurcation of the wave packet, and wave packet construction via nonadiabatic mixing are shown to be observable as time-dependent splittings of peaks in the photoelectron spectra.

Femtosecond pump–probe photoelectron spectroscopy on Na2: a tool to study basic coherent control schemes

Femtosecond pump–probe photoelectron spectroscopy has become a common tool in ultrafast gas-phase science because of its sensitivity both to structural and electronic changes in a molecule upon excitation. Here a summary and extended discussion of our experiments is presented. We focus on the potential of this method to study basic femtosecond coherent control schemes. Multi-photon excitation of the Na2 molecule with femtosecond laser pulses leads to preparation of vibrational wave packets. Mapping of the vibrational wave-packet dynamics by time-resolved photoelectron spectroscopy offers the high spatial and temporal resolution required to investigate a variety of laser-control parameters in great detail. Besides an illustration of the Tannor–Kosloff–Rice scheme we demonstrate electronic transitions at Franck–Condon forbidden internuclear distances due to the intensity of the applied laser pulses. Further we discuss the influence of simple phase-modulated (linearly chirped) laser pulses on a molecular multi-photon process. An enhanced population transfer is derived due to synchronization of the wave-packet motion on an electronic potential to an appropriate chirp of the laser pulses. In addition, the influence of the temporal profile on the population transfer and the role of the pulse duration are studied.

Femtosecond pump-probe photoelectron spectroscopy: Mapping of vibrational wave-packet motion.

Energy-resolved photoelectron spectroscopy in combination with femtosecond pump-probe and molecular beam techniques are used to map molecular dynamics along the internuclear coordinate. The capabilities of this method are demonstrated on the one-dimensional wave-packet motion on electronically excited states of the sodium dimer: by recording transient photoelectron spectra in a pump-probe experiment we are able to follow the molecular wave-packet motion on neutral electronic states along all energetically allowed internuclear distances simultaneously. \textcopyright{} 1996 The American Physical Society.