An Atomic-Level View of Melting Using Femtosecond Electron Diffraction
21 Nov 2003-Science (American Association for the Advancement of Science)-Vol. 302, Iss: 5649, pp 1382-1385
TL;DR: Observations of the structural evolution of aluminum as it underwent an ultrafast laser–induced solid-liquid phase transition provide an atomic-level description of the melting process, which is best understood as a thermal phase transition under strongly driven conditions.
Abstract: We used 600-femtosecond electron pulses to study the structural evolution of aluminum as it underwent an ultrafast laser–induced solid-liquid phase transition. Real-time observations showed the loss of long-range order that was present in the crystalline phase and the emergence of the liquid structure where only short-range atomic correlations were present; this transition occurred in 3.5picoseconds for thin-film aluminum with an excitation fluence of 70 millijoules per square centimeter. The sensitivity and time resolution were sufficient to capture the time-dependent pair correlation function as the system evolved from the solid to the liquid state. These observations provide an atomic-level description of the melting process, in which the dynamics are best understood as a thermal phase transition under strongly driven conditions.
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TL;DR: In this paper, an attosecond "oscilloscope" was used to visualize the oscillating electric field of visible light with an oscillator and probe multi-electron dynamics in atoms, molecules and solids.
Abstract: Summary form only given. Fundamental processes in atoms, molecules, as well as condensed matter are triggered or mediated by the motion of electrons inside or between atoms. Electronic dynamics on atomic length scales tends to unfold within tens to thousands of attoseconds (1 attosecond [as] = 10-18 s). Recent breakthroughs in laser science are now opening the door to watching and controlling these hitherto inaccessible microscopic dynamics. The key to accessing the attosecond time domain is the control of the electric field of (visible) light, which varies its strength and direction within less than a femtosecond (1 femtosecond = 1000 attoseconds). Atoms exposed to a few oscillations cycles of intense laser light are able to emit a single extreme ultraviolet (XUV) burst lasting less than one femtosecond. Full control of the evolution of the electromagnetic field in laser pulses comprising a few wave cycles have recently allowed the reproducible generation and measurement of isolated sub-femtosecond XUV pulses, demonstrating the control of microscopic processes (electron motion and photon emission) on an attosecond time scale. These tools have enabled us to visualize the oscillating electric field of visible light with an attosecond "oscilloscope", to control single-electron and probe multi-electron dynamics in atoms, molecules and solids. Recent experiments hold promise for the development of an attosecond X-ray source, which may pave the way towards 4D electron imaging with sub-atomic resolution in space and time.
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TL;DR: It is shown that the momentum distribution of the extracted electron carries the fingerprint of the highest occupied molecular orbital, whereas the elastically scattered electrons reveal the position of the nuclear components of the molecule.
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TL;DR: By photoexciting the monoclinic semiconducting phase, the authors were able to induce a transition to a metastable state that retained the periodic lattice distortion characteristic of the semiconductor but also acquired metal-like mid-infrared optical properties.
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497 citations
References
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Book•
01 Jan 1954
TL;DR: In this paper, Bragg et al. describe the Optik der R6ntgenstrahlen in Raumgitter and leitet damit fiber zu den experimentellen Methoden and Ergebn]ssen, die den folgenden B~nden vorbehalten bleiben.
Abstract: W~hrend W. L. Bragg im ersten, 1933 erschienenen Bande dieses grossangelegten Werkes eine allgemeine ()-bersicht fiber das ganze Gebiet der RSntgenstrahibeugung an Kristallen gab und dabei besonderes Gewicht auf die Atom-Theorie des Kristallbaus legte, behandelt Band I I die Optik der R6ntgenstrahlen im Raumgitter und leitet damit fiber zu den experimentellen Methoden und Ergebn]ssen, die den folgenden B~nden vorbehalten bleiben. Der Verfasser, wohlbekannt durch seine Untersuchungen fiber Atomformfaktoren und fiber den Einfluss der Mosaikstruktur und der Temperatur auf die Intensitar der Interferenzmaxima, hat die Arbeit an dem Buch schon in England begonnen. Seine Uebersiedelung nach Kapstadt, mehr noch der bald darauf ausbrechende Krieg, haben Fertigstellung und Drucklegung bis 1948 verzSgert. Aber, wie das Sprichwort sagt, 'was lange w~hrt, wird endlich gut ' . Das nunmehr vorliegende Werk ist bewundernswert wegen der Vielseitigkeit des behandelten Stoffs, der umfassenden Berficksichtigung der Literatur aus allen Teilen der Erde, der Vollst~ndigkeit in allen theoretischen Einzelheiten und der Kunst der Darstellung im Allgemeinen. Nach der experimentellen Seite hin l~sst tier Autor Knappheit walten; die sp~teren B~nde werden ja die Erg~nzungen bringen. Und doch spfirt man in seiner Darstellung fiberall die 'Lebensn~he', die einer vielseitigen experimentellen Bet~tigung entspringt. Der Verfasser folgt insofern der historischen Entwicklung, als er mit der Kinematischen Theorie der RSntgenstrahlinterferenzen beginnt, in der die gegenseitigen Zustrahlungen zwischen den Atomen des Kristalls vernachl~ssigt werden. Er braucht dabei yon vornherein das Verfahren des reziproken Gitters, das ja der Benutzung des Braggschen Spiegelungsgesetzes weniger bei den regul~ren Maxima eines ungestSrten Raumgitters vorzuziehen ist, als bei den durch GitterstSrungen .aller Art hervorgerufenen Nebenmaxima. Zu solchen StSrungen ist auch die endliche Begrenzung der KristaUe zu z~hlen; auf den Kristallformfaktor und seine Analogie bei der Beugung sichtbaren Lichts geht das letzte Kapitel mit erfreulicher Griindlichkeit ein, obwohl meines Wissens dieser Teil der Theorie bisher weniger fiir die RSntgen-, als fiir die Elektronen-Strahlen in Betracht kommt. Von grundlegender Bedeutung in der kinematischen Theorie ist der Atomformfaktor. Kapitel I I I und IV berechnen ihn sowohl ffir Frequenzen weir oberhalb aller Absorptionskanten, als auch ffir Spektralbereiche in deren N~he, und zwar zun~chst nach der kIassischen Theorie, dann nach der Quantentheorie. Die Formeln der letzteren lassen sich n~mlich auf diese Art leicht anschaulich deuten. Eingehender als jedes andere mir bekannte Buch, geht dieses auf die Darstellung der Elektronendichte durch Fourierreihen und auf ihre experimentelle Ermitt lung ein. Und fast noch wertvoller erscheint dem Referenten der Bericht fiber die Nebenmaxima, welche tells bei Mischkristallen, tells als Folge der W~rmeschwingungen auch bei reinen Substanzen auftreten und erst verh~ltnism~issig sp~t erforscht wurden. Hier kommen besonders die schSnen Arbeiten yon K. Lonsdale und ihren Mitarbeitern in das rechte Licht, um so mehr, als der Text von einer grossen Zahl photographischer Reproduktionen unterstfitzt wird, wie denn auch die anderen Kapitel damit vortrefflich ausgestattet sind. Es entspringt wohl teils weiser Beschr~fl~ung des Autors, tells unfiberwundenen theoretischen Schwierigkeiten, dass das Buch die Diskussion dieser vielgestaltigen Erscheinungen nur soweit durchffihrt, dass aus den Beobachtungen die zugehSrige Intensit~tsverteilung im Raume des reziproken f i t t e r s angegeben wird, womit die Zuf~lligkeiten der Einfallsrichtung und der Wellenl~uge abgestreift werden. Die weitergehende Aufgabe, diese Intensit~tsverteilung aus den Gitterschwingungen zu deuten, geht fiber die 0p t ik hinaus und ist fiberhaupt nur in ganz wenigen F~llen bisher einigermassen gelSst. Aber auch die dynamische Theorie der RSntgenstrahlinterferenzen kommt zu Wort. Das Buch bringt sowohl die Ueberlegungen, durch welche C. G. Darwin schon 1914 zu ihr fiberleitete, als auch die einige Jahre sp~tere und die weir jiingere, yon Referenten gegebene, Form dieser Theorie. Der Reziprozit~tssatz der Optik gestattet dann den Uebergang zu den von Kossel entdeckten Interferenzen, bei denen die Strahiungsquelle im Inneren des Kristalls liegt. Die letzten Kapitel gehen fiber die auf Kristalle bezfiglichen TJ-berlegungen insofern hinaus, als sie neben der Beugungen besonders ldeinen Kristalliten und Festk6rpern mit Faserstruktur auch die Beugung an Gasmolekiilen, an Fliissigkeiten und Gl~sern in Betrachb ziehen. Die ~4Jmlichkeit der dabei benutzten Denkmethoden mit denen der Kristalltheorie und die grosse Bedeutung dieser Erscheinungen in der heutigen Physik und Chemie rechtfertigen diese ~-berschreitung des eigentlichen Themas. Man muss diesem Buche die weiteste Verbreitung wfinschen. Es ist mehr als ein Hilfsmittel ffir den auf dem Sondergebiete der R6ntgenstrahlbeugung arbcitenden Wissenschaftler. Es hat weitgehenden Wert fiir die wissenschaftHche Allgemeinbfldung der Physiker.
2,363 citations
TL;DR: In this paper, the electron structure factor for forward scattering for a crystal containing ionized atoms is derived and a parametric fit to these is given in the range of sin θ/λ from 0.0 to 2.0 A−1.
Abstract: Kinematic X-ray and electron scattering factors, found with the use of relativistic Hartree–Fock atomic fields, are tabulated for 76 atoms and ions. Parametric fits to these are given in the range of sin θ/λ from 0.0 to 2.0 A−1. A method is developed to obtain the electron structure factor for forward scattering for a crystal containing ionized atoms.
1,613 citations
Book•
01 Jan 1963
TL;DR: In the formalism of Newman-Penrose, a family of exact solutions of the Einsteirr-Maxwell equations of the type of Bertotti-Robinson is obtained with a cosmological term belonging to the degenerate type D in the algebraic classification of Petrov.
Abstract: It has been noted that the family of plane electromagnetic waves and the "electromagnetic universe" of Bertotti--Robinson exhaust the entire class of conformally flat Einsteirr-Maxwell spaces. In the formalism of Newman--Penrose a family of exact solutions of the Einstein--Maxwell equations of the type of Bertotti--Robinson is obtained with a cosmological term belonging to the degenerate type D in the algebraic classification of Petrov and describing the space--time generated by a covariantly constant, nonisotropic electromagnetic field.
1,380 citations
TL;DR: Measurements of the characteristic melting time of InSb are presented with a recently developed technique of ultrafast time-resolved X-ray diffraction that, in contrast to optical spectroscopy, provides a direct probe of the changing atomic structure.
Abstract: Ultrafast time-resolved optical spectroscopy has revealed new classes of physical, chemical and biological reactions, in which directed, deterministic motions of atoms have a key role. This contrasts with the random, diffusive motion of atoms across activation barriers that typically determines kinetic rates on slower timescales. An example of these new processes is the ultrafast melting of semiconductors, which is believed to arise from a strong modification of the inter-atomic forces owing to laser-induced promotion of a large fraction (10% or more) of the valence electrons to the conduction band. The atoms immediately begin to move and rapidly gain sufficient kinetic energy to induce melting—much faster than the several picoseconds required to convert the electronic energy into thermal motions. Here we present measurements of the characteristic melting time of InSb with a recently developed technique of ultrafast time-resolved X-ray diffraction that, in contrast to optical spectroscopy, provides a direct probe of the changing atomic structure. The data establish unambiguously a loss of long-range order up to 900 A inside the crystal, with time constants as short as 350 femtoseconds. This ability to obtain the quantitative structural characterization of non-thermal processes should find widespread application in the study of ultrafast dynamics in other physical, chemical and biological systems.
635 citations
TL;DR: This direct imaging of reactions was achieved using the third-generation apparatus equipped with an electron pulse, a charge-coupled device camera, and a mass spectrometer to demonstrate the vastly improved sensitivity, resolution, and versatility of UED for studying ultrafast structural dynamics in complex molecular systems.
Abstract: Ultrafast electron diffraction (UED) has been developed to study transient structures in complex chemical reactions initiated with femtosecond laser pulses. This direct imaging of reactions was achieved using our third-generation apparatus equipped with an electron pulse (1.07 ± 0.27 picoseconds) source, a charge-coupled device camera, and a mass spectrometer. Two prototypical gas-phase reactions were studied: the nonconcerted elimination reaction of a haloethane, wherein the structure of the intermediate was determined, and the ring opening of a cyclic hydrocarbon containing no heavy atoms. These results demonstrate the vastly improved sensitivity, resolution, and versatility of UED for studying ultrafast structural dynamics in complex molecular systems.
484 citations