Direct time-domain observation of ultrafast dephasing in adsorbate-substrate vibration under the influence of a hot electron bath: Cs adatoms on Pt(111)

Nanoscale coherent phonon spectroscopy

Coherent phonon spectroscopy can provide microscopic insight into ultrafast lattice dynamics and its coupling to other degrees of freedom under nonequilibrium conditions. Ultrafast optical spectroscopy is a well-established method to study coherent phonons, but the diffraction limit has hampered observing their local dynamics directly. Here, we demonstrate nanoscale coherent phonon spectroscopy using ultrafast laser-induced scanning tunneling microscopy in a plasmonic junction. Coherent phonons are locally excited in ultrathin zinc oxide films by the tightly confined plasmonic field and are probed via the photoinduced tunneling current through an electronic resonance of the zinc oxide film. Concurrently performed tip-enhanced Raman spectroscopy allows us to identify the involved phonon modes. In contrast to the Raman spectra, the phonon dynamics observed in coherent phonon spectroscopy exhibit strong nanoscale spatial variations that are correlated with the distribution of the electronic local density of states resolved by scanning tunneling spectroscopy.

Applications of time-domain spectroscopy to electron-phonon coupling dynamics at surfaces

Photochemistry is one of the most important branches in chemistry to promote and control chemical reactions. In particular, there has been growing interest in photoinduced processes at solid surfaces and interfaces with liquids such as water for developing efficient solar energy conversion. For example, photoinduced charge transfer between adsorbates and semiconductor substrates at the surfaces of metal oxides induced by photogenerated holes and electrons is a core process in photovoltaics and photocatalysis. In these photoinduced processes, electron-phonon coupling plays a central role. This paper describes how time-domain spectroscopy is applied to elucidate electron-phonon coupling dynamics at metal and semiconductor surfaces. Because nuclear dynamics induced by electronic excitation through electron-phonon coupling take place in the femtosecond time domain, the pump-and-probe method with ultrashort pulses used in time-domain spectroscopy is a natural choice for elucidating the electron-phonon coupling at metal and semiconductor surfaces. Starting with a phenomenological theory of coherent phonons generated by impulsive electronic excitation, this paper describes a couple of illustrative examples of the applications of linear and nonlinear time-domain spectroscopy to a simple adsorption system, alkali metal on Cu(111), and more complex photocatalyst systems.

Interaction of coherent phonons with defects and elementary excitations

We present an overview of the feasibility of using coherent phonon spectroscopy to study interaction dynamics of excited lattice vibrations with their environments. By exploiting the features of coherent phonons with a pump-probe technique, one can study lattice motions in a sub-picosecond time range. The dephasing properties tell us not only about interaction dynamics with carriers (electrons and holes) or thermal phonons but also about point defects in crystals. Modulations of the coherent phonon amplitude by more than two modes are closely related to phonon-carrier or phonon-phonon interferences. Related to this phenomenon, formation of coherent phonons at higher harmonics gives direct evidence for phonon-phonon couplings. A combined study of coherent phonons and ultrafast carrier response can be useful for understanding phonon-carrier interaction dynamics. For metals like zinc, nonequilibrium electrons may dominate the dynamics of both relaxation and generation of coherent phonons. The frequency chirp of coherent phonons can be a direct measure of how and when phonon-phonon and phonon-carrier couplings occur. Carbon nanotubes show some complicated behavior due to the existence of many modes with different symmetries, resulting in superposition or interference. To illustrate one of the most interesting applications, the selective excitation of specific phonon modes through the use of a pulse train technique is shown.

Wave-Packet and Coherent Control Dynamics

This review summarizes progress in coherent control as well as relevant recent achievements, highlighting, among several different schemes of coherent control, wave-packet interferometry (WPI). WPI is a fundamental and versatile scenario used to control a variety of quantum systems with a sequence of short laser pulses whose relative phase is finely adjusted to control the interference of electronic or nuclear wave packets (WPs). It is also useful in retrieving quantum information such as the amplitudes and phases of eigenfunctions superposed to generate a WP. Experimental and theoretical efforts to retrieve both the amplitude and phase information are recounted. This review also discusses information processing based on the eigenfunctions of atoms and molecules as one of the modern and future applications of coherent control. The ultrafast coherent control of ultracold atoms and molecules and the coherent control of complex systems are briefly discussed as future perspectives.

Direct measurement of core-level relaxation dynamics on a surface-adsorbate system

The coupling between electronic states in a surface-adsorbate system is fundamental to the understanding of many surface interactions. In this Letter, we present the first direct time-resolved observations of the lifetime of core-excited states of an atom adsorbed onto a surface. By comparing laser-assisted photoemission from a substrate with a delayed Auger decay process from an adsorbate, we measure the lifetime of the 4d(-1) core level of xenon on Pt(111) to be 7.1+/-1.1 fs. This result opens up time-domain measurements of surface dynamics where energy-resolved measurements may provide incomplete information.

An instrument to investigate femtochemistry on metal surfaces in real space

A newly established combination of a femtosecond laser with a low temperature scanning tunneling microscope is described, which facilitates one to analyze femtochemistry on metal surfaces in real space. The combined instrument enables focusing the laser to some tens of micrometers and guiding it reproducibly into the tunneling gap with the aid of in situ movable mirrors. Furthermore, a method to determine the focus size on the sample is presented. The focus size is used to calculate the electron and phonon temperatures at the surface. Despite the additional noise introduced by laser operation the vertical resolution of the microscope lies below 1 pm. The potential of the instrument is demonstrated on para-chloronitrobenzene clusters adsorbed on Au(111). Single chloronitrobenzene molecules diffuse upon femtosecond laser irradiation; some smaller clusters rotate by multiples of 30 degrees ; clusters of less compact form rearrange to close-packed clusters.

Interface-Specific χ(4) Coherent Raman Spectroscopy in the Frequency Domain

We demonstrate interface-specific fourth-order (chi(4)) coherent Raman spectroscopy in the frequency domain for the first time. Because the chi(4) Raman spectroscopy uses only visible (vis) or near-IR light, it is expected to be a potential alternative to the widely utilized IR-vis sum frequency generation spectroscopy that cannot be applied to interfaces buried in thick IR absorbers such as water. We present the vibrational absolute value(chi(4))2 spectrum of rhodamine 800 at the air/water interface in a wide spectral range 100-3600 cm(-1). Comparison of the absolute value(chi(4))2 spectrum with the absolute value(chi(3))2 spectrum leads us to conclude that the present chi(4) spectroscopy successfully probes the interface distinguished clearly from the bulk.

Mode-selective excitation of coherent surface phonons on alkali-covered metal surfaces

We demonstrate the mode-selective excitation of coherent phonons at Pt(111) surfaces covered with submonolayer caesium atoms. A burst of 150 fs laser pulses with the repetition rate of 2.0-2.9 THz was synthesized by using a spatial-light modulator, and used for the coherent surface phonon excitation. The coherent nuclear motion was monitored by time-resolved second harmonic generation. By tuning the repetition rate, we succeeded in controlling the relative amplitude of the vibrational coherence of the Cs-Pt stretching mode (2.3-2.4 THz) to that of the Pt surface Rayleigh phonon mode (2.6 or 2.9 THz, depending on the Cs coverage).

Real-Space Observation of Molecular Motion Induced by Femtosecond Laser Pulses

Femtosecond laser irradiation is used to excite adsorbed CO molecules on a Cu110 surface; the ensuing motion of individual molecules across the surface is characterized on a site-to-site basis by in situ scanning tunneling microscopy. Adsorbate motion both along and perpendicular to the rows of the Cu110 surface occurs readily, in marked contrast to the behavior seen for equilibrium diffusion processes. The experimental findings for the probability and direction of the molecular motion can be understood as a manifestation of strong coupling between the adsorbates' lateral degrees of freedom and the substrate electronic excitation produced by the femtosecond laser radiation.