Robert J. Levis

Bio: Robert J. Levis is an academic researcher from Temple University. The author has contributed to research in topics: Laser & Mass spectrometry. The author has an hindex of 36, co-authored 186 publications receiving 5203 citations. Previous affiliations of Robert J. Levis include University of Washington & Pennsylvania State University.

Selective Bond Dissociation and Rearrangement with Optimally Tailored, Strong-Field Laser Pulses

Abstract: We used strong-field laser pulses that were tailored with closed-loop optimal control to govern specified chemical dissociation and reactivity channels in a series of organic molecules. Selective cleavage and rearrangement of chemical bonds having dissociation energies up to approximately 100 kilocalories per mole (about 4 electron volts) are reported for polyatomic molecules, including (CH 3 ) 2 CO (acetone), CH 3 COCF 3 (trifluoroacetone), and C 6 H 5 COCH 3 (acetophenone). Control over the formation of CH 3 CO from (CH 3 ) 2 CO, CF 3 (or CH 3 ) from CH 3 COCF 3 , and C 6 H 5 CH 3 (toluene) from C 6 H 5 COCH 3 was observed with high selectivity. Strong-field control appears to have generic applicability for manipulating molecular reactivity because the tailored intense laser fields (about 10 13 watts per square centimeter) can dynamically Stark shift many excited states into resonance, and consequently, the method is not confined by resonant spectral restrictions found in the perturbative (weak-field) regime.

A time-dependent Hartree–Fock approach for studying the electronic optical response of molecules in intense fields

Abstract: For molecules in high intensity oscillating electric fields, the time-dependent Hartree–Fock (TDHF) method is used to simulate the behavior of the electronic density prior to ionization. Since a perturbative approach is no longer valid at these intensities, the full TDHF equations are used to propagate the electronic density. A unitary transform approach is combined with the modified midpoint method to provide a stable and efficient algorithm to integrate these equations. The behavior of H2+ in an intense oscillating field computed using the TDHF method with a STO-3G basis set reproduces the analytic solution for the two-state coherent excitation model. For H2 with a 6-311++G(d,p) basis set, the TDHF results are nearly indistinguishable from calculations using the full time-dependent Schrodinger equation. In an oscillating field of 3.17 × 1013 W cm−2 and 456 nm, the molecular orbital energies, electron populations, and atomic charges of H2 follow the field adiabatically. As the field intensity is increased, the response becomes more complicated as a result of contributions from excited states. Simulations of N2 show even greater complexity, yet the average charge still follows the field adiabatically.

Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas

Abstract: High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching--the constructive addition of x-ray waves from a large number of atoms--favors longer-wavelength mid-infrared lasers. We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma. We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth-limited pulse trains of ~100 attoseconds.

Closing the Loop on Bond Selective Chemistry Using Tailored Strong Field Laser Pulses

Abstract: Strong field, closed-loop control of gas-phase photochemical reactivity is the focus of this article. The control of chemical reactivity is now possible using tailored laser pulses to circumvent previous laser bandwidth limitations. As an illustration of this capability, ketone rearrangements and dissociation reactions are considered. To introduce the experiments we discuss both optimal control theory (OCT) and optimal control experiments (OCE) with an emphasis on closed-loop control methods using near-infrared fs pulses. Because the experiments are in the strong field regime, we present the current state of the understanding of the electronic and nuclear photophysical processes that occur when polyatomic molecules are subjected to laser intensities ranging between 1013 and 1015 W cm-2. Photoelectron spectroscopy measurements are presented that begin to elucidate the control mechanisms. These delineate the order of the multiphoton process, the presence of transient shifting of excited electronic state ene...

Photoexcitation, ionization, and dissociation of molecules using intense near-infrared radiation of femtosecond duration

Abstract: The coupling mechanism between an intense (∼1013 W cm-2, 780 nm) near-infrared radiation field of duration 50−200 fs with molecules having 5−50 atoms is considered in this article. In general, the interaction of intense radiation fields with molecules can result in both electron emission and subsequent dissociation. For the laser excitation scheme employed here, intact ions are observed in addition to dissociative ionization channels for all classes of molecules investigated to date. An excitation mechanism is considered where the electric field of the laser mediates the coupling between the radiation and the molecule. This field-induced ionization is compared with the more common frequency-mediated coupling mechanism found in multiphoton processes. Measurements of intense-laser photoionization probability are presented for several series of molecules. An outline of our structure-based model is presented to enable calculation of relative tunneling rates and prediction of the laser−molecule coupling mechan...

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Attosecond Physics

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.

Extraordinary Mobility in Semiconducting Carbon Nanotubes

Abstract: Semiconducting carbon nanotube transistors with channel lengths exceeding 300 microns have been fabricated. In these long transistors, carrier transport is diffusive and the channel resistance dominates the transport. Transport characteristics are used to extract the field-effect mobility (79 000 cm2/Vs) and estimate the intrinsic mobility (>100 000 cm2/Vs) at room temperature. These values exceed those for all known semiconductors, which bodes well for application of nanotubes in high-speed transistors, single- and few-electron memories, and chemical/biochemical sensors.

Bimetallic Nanocrystals: Syntheses, Properties, and Applications

Abstract: Achieving mastery over the synthesis of metal nanocrystals has emerged as one of the foremost scientific endeavors in recent years. This intense interest stems from the fact that the composition, size, and shape of nanocrystals not only define their overall physicochemical properties but also determine their effectiveness in technologically important applications. Our aim is to present a comprehensive review of recent research activities on bimetallic nanocrystals. We begin with a brief introduction to the architectural diversity of bimetallic nanocrystals, followed by discussion of the various synthetic techniques necessary for controlling the elemental ratio and spatial arrangement. We have selected key examples from the literature that exemplify critical concepts and place a special emphasis on mechanistic understanding. We then discuss the composition-dependent properties of bimetallic nanocrystals in terms of catalysis, optics, and magnetism and conclude the Review by highlighting applications that h...