{"created":"2023-06-20T13:21:08.727523+00:00","id":1252,"links":{},"metadata":{"_buckets":{"deposit":"9d40b712-cf82-43c4-8229-ec2e789ba73b"},"_deposit":{"created_by":1,"id":"1252","owners":[1],"pid":{"revision_id":0,"type":"depid","value":"1252"},"status":"published"},"_oai":{"id":"oai:ir.soken.ac.jp:00001252","sets":["2:431:24"]},"author_link":["0","0","0"],"item_1_creator_2":{"attribute_name":"著者名","attribute_type":"creator","attribute_value_mlt":[{"creatorNames":[{"creatorName":"冬木, 正紀"}],"nameIdentifiers":[{}]}]},"item_1_creator_3":{"attribute_name":"フリガナ","attribute_type":"creator","attribute_value_mlt":[{"creatorNames":[{"creatorName":"フユキ, マサノリ"}],"nameIdentifiers":[{}]}]},"item_1_date_granted_11":{"attribute_name":"学位授与年月日","attribute_value_mlt":[{"subitem_dategranted":"2007-03-23"}]},"item_1_degree_grantor_5":{"attribute_name":"学位授与機関","attribute_value_mlt":[{"subitem_degreegrantor":[{"subitem_degreegrantor_name":"総合研究大学院大学"}]}]},"item_1_degree_name_6":{"attribute_name":"学位名","attribute_value_mlt":[{"subitem_degreename":"博士（理学）"}]},"item_1_description_12":{"attribute_name":"要旨","attribute_value_mlt":[{"subitem_description":"
Since nuclear motions of adsorbates strongly couple to electron motions in
metals due to electron-hole pair creation, strong nonadiabatic couplings between
electrons and phonons are essential also in various processes on metal surfaces. For a
deeper understanding of nonadiabatic couplings, it is vital to observe nuclear motions of
adsorbates under electronic excitation directly in the time domain. However, these
measurements are still very rare.
A laser pulse with duration shorter than the oscillation periods of surface
phonons can create coherent surface phonons, i.e., a lattice mode with a large number of
phonons in one mode with a constant phase-lattice relation. In this thesis, the
excitation and decay mechanisms of coherent vibration of adsorbates were investigated
for a deeper understanding of electron-phonon couplings. Since alkali-metal
adsorption systems are a paradigm for chemisorption, their geometric and electronic
structures have been extensively studied. Thus, they are suitable to explore
nonadiabatic couplings at metal surfaces.
For photo-induced processes at metal surfaces, the relevant electronic
transitions are categorized into two types: electronic transition between surface states
and that in bulk metals. We clarified which electronic excitation is responsible for
generating the coherent vibration of adsorbates by investigating its pump photon energy
dependence and pump polarization dependence.
In general, decay of a coherent vibration of adsorbates is contributed by phase
relaxation (pure dephasing) and population decay. The population decay channels are
insensitive to surface temperature as long as the vibrational mode can be treated as a
harmonic oscillator. Thus, total decay rates do not strongly depend on surface
temperature, unless pure dephasing is effective. To clarify the decay mechanism of
coherent phonons at alkali-metal covered metal surfaces, the pump power dependence
of decay time was investigated as a function of transient surface temperature that
increases with pump power.
Sample preparations were conducted in an ultrahigh vacuum chamber. The
coverage and superstructure of alkali-metal adsorbates on contamination-free metal
surfaces were determined by low energy electron diffraction, Auger electron
spectroscopy, and x-ray photoelectron spectroscopy. Coherent surface phonons at
alkali-metal adsorption surfaces were monitored with time-resolved second harmonic
generation (TRSHG) spectroscopy. In this method, surface phonons were excited
coherently by the irradiation of an ultrashort pump laser pulse and the evolution of
coherent surface phonons was probed by monitoring intensity modulations in the second
harmonic intensity of probe pulses as a function of pump-probe delay. Two sets of
home-built non-collinear optical parametric amplifiers pumped by a Ti:sapphire
regenerative amplifier supplied ultrashort pulses (25 fs) independently tunable from 2.0
to 2.5 eV, which were used as pump and probe pulses in TRSHG spectroscopy.
Three different alkali-metal covered surfaces were chosen: potassium on
platinum (111), sodium on copper (111), and potassium on copper (111).
Experimental results indicated that the excitation and dynamics of coherent surface
phonons strongly depend on the combination of alkali-metal adatoms and metal
substrate.
At K-covered Pt(111) surfaces, five coherently excited phonon modes were
observed. K coverage dependence revealed that they are attributed to a K-Pt stretching
vibrational mode and four Pt surface phonon modes. The frequency of the K-Pt
stretching phonon mode depends on the superstructure of K: 5.0-5.3 and 4.5-4.8 THz
for (2×2) and (√3×√3)R30° superstructures, respectively. Comparison of the
frequencies of the Pt surface phonon modes (2.7-3.8 THz) with those at a clean Pt(111)
surface suggests that the K-Pt stretching vibrational mode is weakly coupled to the Pt
surface phonon modes.
At a fu11 monolayer K-covered Pt(111) surface (0.38 ML, 1 ML = 1.51×1015
atoms/cm2), the excitation mechanism and dynamics of the coherent surface phonons
extensively investigated. When the photon energy of a pump pulse was varied
from 2.0 to 2.4 eV, the initial amplitude of the K-Pt stretching mode was enhanced by a
factor of 2 at a photon energy resonant to the transition from the K-induced surface
occupied state to the second lowest image potential state. Modulation signals of
TRSHG traces disappeared when the polarization of the pump laser was changed from
p- to s-polarization. The photon energy and polarization dependences indicate that the
electronic transition between the K-induced surface occupied state and the image
potential state is responsible for the generation of the coherent K-Pt stretching vibration.
The decay time of the K-Pt stretching mode became shorter and its frequency redshifted
as the absorbed fluence of the pump pulse increased. This fluence dependence was
interpreted to be due to anharmonic coupling between the K-Pt stretching and lateral modes.
At Na-covered Cu(l11) surfaces, two coherently excited phonon modes were
observed. Na coverage dependence revealed that they are attributed to strong Na-Cu
stretching resonances coupled with Cu surface phonon modes such as a surface
Rayleigh phonon mode. The higher frequency phonon mode showed a redshift from
6.2 to 5.5 THz with increase of coverage from 0.14 to 0.44 ML (1 ML = 1.76×1015
atoms/cm2). While the lower frequency phonon mode appeared at 0.44 ML with a Na
(3/2×3/2) superstructure, this mode disappeared at 0.14 ML.
At a fu11 monolayer Na-covered Cu(111) surface (0.44 ML), the excitation and
dynamics of coherent surface phonons were extensively investigated. When the
photon energy of the pump laser was varied from 2.0 to 2.5 eV, the initial amplitude of
the Na-Cu stretching mode was not enhanced by the resonant electronic transition
between surface states but increased with the absorbance of bulk Cu. This result
clearly indicates that the electronic transition in the Cu substrate is responsible for the
generation of the coherent Na-Cu stretching vibrational modes rather than the electronic
transition between surface states. This conclusion is in stark contrast to the case of
K/Pt(111).
The decay time of the Na-Cu stretching mode with the frequency of 5.5 THz
was 0.3 ps fu11 monolayer coverage, which was much shorter than those of Cs-Pt
(1.9 ps) and K-Pt (1.1 ps). at the As stated earlier, the Na-Cu stretching vibrational phonon
mode is strongly coupled with bulk phonon modes and becomes a surface resonance
mode. Thus, the fast decay of the Na-Cu stretching mode is caused by both effective
population decay and pure dephasing associated with coupling to Cu bulk phonon
modes. The decay time of the Na-Cu stretching mode became shorter and its
frequency redshifted as the absorbed fluence of the pump pulse increased. This
fluence dependence was interpreted to be due to anharmonic coupling between the
Na-Cu stretching and other phonon modes.
For K-covered Cu(111) surfaces, an abrupt frequency jump of the coherent
K-Cu stretching vibrational mode was observed: from 3.0 to 5.5 THz at 0.28-0.30 ML
(1 ML = 1.76×1015 atoms/cm2). With increase of K coverage, the decay time
decreased dramatically from 0.9 to 0.4 ps at 0.28-0.35 ML. The abrupt changes in
frequency and decay time occurred at around 0.30 ML. At 0.30 ML, the compression
of a (2×2) superstructure of K is completed and the growth manner of K overlayer
changes. The abrupt changes in frequency and decay time are associated with the
changes in the adsorption site of K that influence strongly the deformation potential
with respect to a K-Cu bond as well as the nonadiabatic coupling between electrons and
phonons.
On metal surfaces, electrons and phonons are nonadiabatically coupled via
electron-hole pair creation. This thesis made it clear that there are two kinds of
electronic transitions that drive the coherent stretching vibration between alkali-metal
adsorbate and metal substrate: the electronic transition between surface states and that in
the substrate. This thesis also clarified that the pure dephasing as well as energetic
relaxation to electrons or other phonons is significant for the decay process. The most
effective pathway for decay depended on adsorption system, alkali-metal coverage, and
pump absorbed fluence. The comparative study in this thesis indicates that the
couplings between surface phonon modes significantly characterize the nonadiabatic
couplings between electrons and phonons at alkali-metal covered metal surfaces.
","subitem_description_type":"Other"}]},"item_1_description_7":{"attribute_name":"学位記番号","attribute_value_mlt":[{"subitem_description":"総研大甲第1081号","subitem_description_type":"Other"}]},"item_1_select_14":{"attribute_name":"所蔵","attribute_value_mlt":[{"subitem_select_item":"有"}]},"item_1_select_8":{"attribute_name":"研究科","attribute_value_mlt":[{"subitem_select_item":"先導科学研究科"}]},"item_1_select_9":{"attribute_name":"専攻","attribute_value_mlt":[{"subitem_select_item":"22 光科学専攻"}]},"item_1_text_10":{"attribute_name":"学位授与年度","attribute_value_mlt":[{"subitem_text_value":"2006"}]},"item_creator":{"attribute_name":"著者","attribute_type":"creator","attribute_value_mlt":[{"creatorNames":[{"creatorName":"FUYUKI, Masanori","creatorNameLang":"en"}],"nameIdentifiers":[{}]}]},"item_files":{"attribute_name":"ファイル情報","attribute_type":"file","attribute_value_mlt":[{"accessrole":"open_date","date":[{"dateType":"Available","dateValue":"2016-02-17"}],"displaytype":"simple","filename":"甲1081_要旨.pdf","filesize":[{"value":"418.2 kB"}],"format":"application/pdf","licensetype":"license_11","mimetype":"application/pdf","url":{"label":"要旨・審査要旨","url":"https://ir.soken.ac.jp/record/1252/files/甲1081_要旨.pdf"},"version_id":"450c9f29-4066-40a6-8b63-b1ed6a208e1f"}]},"item_language":{"attribute_name":"言語","attribute_value_mlt":[{"subitem_language":"eng"}]},"item_resource_type":{"attribute_name":"資源タイプ","attribute_value_mlt":[{"resourcetype":"thesis","resourceuri":"http://purl.org/coar/resource_type/c_46ec"}]},"item_title":"Coherent surface phonon dynamics at alkali metal-covered metal surfaces","item_titles":{"attribute_name":"タイトル","attribute_value_mlt":[{"subitem_title":"Coherent surface phonon dynamics at alkali metal-covered metal surfaces"},{"subitem_title":"Coherent surface phonon dynamics at alkali metal-covered metal surfaces","subitem_title_language":"en"}]},"item_type_id":"1","owner":"1","path":["24"],"pubdate":{"attribute_name":"公開日","attribute_value":"2010-02-22"},"publish_date":"2010-02-22","publish_status":"0","recid":"1252","relation_version_is_last":true,"title":["Coherent surface phonon dynamics at alkali metal-covered metal surfaces"],"weko_creator_id":"1","weko_shared_id":-1},"updated":"2023-06-20T16:06:37.569795+00:00"}