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内容記述 |
<br /> Since nuclear motions of adsorbates strongly couple to electron motions in <br /> metals due to electron-hole pair creation, strong nonadiabatic couplings between <br /> electrons and phonons are essential also in various processes on metal surfaces. For a <br /> deeper understanding of nonadiabatic couplings, it is vital to observe nuclear motions of<br /> adsorbates under electronic excitation directly in the time domain. However, these <br /> measurements are still very rare. <br /> A laser pulse with duration shorter than the oscillation periods of surface <br />phonons can create coherent surface phonons, i.e., a lattice mode with a large number of<br /> phonons in one mode with a constant phase-lattice relation. In this thesis, the <br />excitation and decay mechanisms of coherent vibration of adsorbates were investigated <br />for a deeper understanding of electron-phonon couplings. Since alkali-metal <br />adsorption systems are a paradigm for chemisorption, their geometric and electronic <br />structures have been extensively studied. Thus, they are suitable to explore <br />nonadiabatic couplings at metal surfaces. <br /> For photo-induced processes at metal surfaces, the relevant electronic <br />transitions are categorized into two types: electronic transition between surface states <br />and that in bulk metals. We clarified which electronic excitation is responsible for <br />generating the coherent vibration of adsorbates by investigating its pump photon energy<br /> dependence and pump polarization dependence.<br /> In general, decay of a coherent vibration of adsorbates is contributed by phase <br />relaxation (pure dephasing) and population decay. The population decay channels are <br />insensitive to surface temperature as long as the vibrational mode can be treated as a <br />harmonic oscillator. Thus, total decay rates do not strongly depend on surface <br />temperature, unless pure dephasing is effective. To clarify the decay mechanism of <br />coherent phonons at alkali-metal covered metal surfaces, the pump power dependence <br />of decay time was investigated as a function of transient surface temperature that <br />increases with pump power. <br /> Sample preparations were conducted in an ultrahigh vacuum chamber. The <br />coverage and superstructure of alkali-metal adsorbates on contamination-free metal <br />surfaces were determined by low energy electron diffraction, Auger electron <br /> spectroscopy, and x-ray photoelectron spectroscopy. Coherent surface phonons at <br /> alkali-metal adsorption surfaces were monitored with time-resolved second harmonic <br /> generation (TRSHG) spectroscopy. In this method, surface phonons were excited <br /> coherently by the irradiation of an ultrashort pump laser pulse and the evolution of <br /> coherent surface phonons was probed by monitoring intensity modulations in the second<br /> harmonic intensity of probe pulses as a function of pump-probe delay. Two sets of<br /> home-built non-collinear optical parametric amplifiers pumped by a Ti:sapphire <br /> regenerative amplifier supplied ultrashort pulses (25 fs) independently tunable from 2.0<br /> to 2.5 eV, which were used as pump and probe pulses in TRSHG spectroscopy. <br /> Three different alkali-metal covered surfaces were chosen: potassium on <br /> platinum (111), sodium on copper (111), and potassium on copper (111). <br /> Experimental results indicated that the excitation and dynamics of coherent surface <br /> phonons strongly depend on the combination of alkali-metal adatoms and metal <br />substrate. <br /> At K-covered Pt(111) surfaces, five coherently excited phonon modes were <br />observed. K coverage dependence revealed that they are attributed to a K-Pt stretching <br />vibrational mode and four Pt surface phonon modes. The frequency of the K-Pt <br />stretching phonon mode depends on the superstructure of K: 5.0-5.3 and 4.5-4.8 THz <br />for (2×2) and (√3×√3)R30° superstructures, respectively. Comparison of the <br />frequencies of the Pt surface phonon modes (2.7-3.8 THz) with those at a clean Pt(111) <br />surface suggests that the K-Pt stretching vibrational mode is weakly coupled to the Pt <br />surface phonon modes. <br /> At a fu11 monolayer K-covered Pt(111) surface (0.38 ML, 1 ML = 1.51×10<sup>15</sup> <br />atoms/cm<sup>2</sup>), the excitation mechanism and dynamics of the coherent surface phonons <br /> extensively investigated. When the photon energy of a pump pulse was varied<br /> from 2.0 to 2.4 eV, the initial amplitude of the K-Pt stretching mode was enhanced by a<br /> factor of 2 at a photon energy resonant to the transition from the K-induced surface<br /> occupied state to the second lowest image potential state. Modulation signals of<br /> TRSHG traces disappeared when the polarization of the pump laser was changed from<br /> p- to s-polarization. The photon energy and polarization dependences indicate that the<br /> electronic transition between the K-induced surface occupied state and the image<br /> potential state is responsible for the generation of the coherent K-Pt stretching vibration. <br />The decay time of the K-Pt stretching mode became shorter and its frequency redshifted<br />as the absorbed fluence of the pump pulse increased. This fluence dependence was<br />interpreted to be due to anharmonic coupling between the K-Pt stretching and lateral modes. <br /> At Na-covered Cu(l11) surfaces, two coherently excited phonon modes were <br />observed. Na coverage dependence revealed that they are attributed to strong Na-Cu <br />stretching resonances coupled with Cu surface phonon modes such as a surface <br />Rayleigh phonon mode. The higher frequency phonon mode showed a redshift from <br />6.2 to 5.5 THz with increase of coverage from 0.14 to 0.44 ML (1 ML = 1.76×10<sup>15</sup> <br />atoms/cm<sup>2</sup>). While the lower frequency phonon mode appeared at 0.44 ML with a Na<br /> (3/2×3/2) superstructure, this mode disappeared at 0.14 ML. <br /> At a fu11 monolayer Na-covered Cu(111) surface (0.44 ML), the excitation and <br />dynamics of coherent surface phonons were extensively investigated. When the <br />photon energy of the pump laser was varied from 2.0 to 2.5 eV, the initial amplitude of <br />the Na-Cu stretching mode was not enhanced by the resonant electronic transition <br />between surface states but increased with the absorbance of bulk Cu. This result <br />clearly indicates that the electronic transition in the Cu substrate is responsible for the <br />generation of the coherent Na-Cu stretching vibrational modes rather than the electronic<br /> transition between surface states. This conclusion is in stark contrast to the case of<br /> K/Pt(111). <br /> The decay time of the Na-Cu stretching mode with the frequency of 5.5 THz <br />was 0.3 ps fu11 monolayer coverage, which was much shorter than those of Cs-Pt<br />(1.9 ps) and K-Pt (1.1 ps). at the As stated earlier, the Na-Cu stretching vibrational phonon<br />mode is strongly coupled with bulk phonon modes and becomes a surface resonance<br /> mode. Thus, the fast decay of the Na-Cu stretching mode is caused by both effective<br />population decay and pure dephasing associated with coupling to Cu bulk phonon<br />modes. The decay time of the Na-Cu stretching mode became shorter and its <br />frequency redshifted as the absorbed fluence of the pump pulse increased. This <br />fluence dependence was interpreted to be due to anharmonic coupling between the <br />Na-Cu stretching and other phonon modes. <br /> For K-covered Cu(111) surfaces, an abrupt frequency jump of the coherent <br />K-Cu stretching vibrational mode was observed: from 3.0 to 5.5 THz at 0.28-0.30 ML <br />(1 ML = 1.76×10<sup>15</sup> atoms/cm<sup>2</sup>). With increase of K coverage, the decay time<br /> decreased dramatically from 0.9 to 0.4 ps at 0.28-0.35 ML. The abrupt changes in<br /> frequency and decay time occurred at around 0.30 ML. At 0.30 ML, the compression<br /> of a (2×2) superstructure of K is completed and the growth manner of K overlayer<br /> changes. The abrupt changes in frequency and decay time are associated with the<br /> changes in the adsorption site of K that influence strongly the deformation potential<br /> with respect to a K-Cu bond as well as the nonadiabatic coupling between electrons and<br /> phonons. <br /> On metal surfaces, electrons and phonons are nonadiabatically coupled via <br />electron-hole pair creation. This thesis made it clear that there are two kinds of <br />electronic transitions that drive the coherent stretching vibration between alkali-metal <br />adsorbate and metal substrate: the electronic transition between surface states and that in<br /> the substrate. This thesis also clarified that the pure dephasing as well as energetic<br /> relaxation to electrons or other phonons is significant for the decay process. The most<br /> effective pathway for decay depended on adsorption system, alkali-metal coverage, and<br /> pump absorbed fluence. The comparative study in this thesis indicates that the<br /> couplings between surface phonon modes significantly characterize the nonadiabatic<br /> couplings between electrons and phonons at alkali-metal covered metal surfaces.<br /> |
要旨・審査要旨 (418.2 kB)
