14 research outputs found
Post-Hartree-Fock studies of the He/Mg(0001) interaction: Anti-corrugation, screening, and pairwise additivity
12 pĂĄgs.; 6 figs.; 5 tabs.The adsorption of noble gases on metallic surfaces represents a paradigmatic case of van-der-Waals (vdW) interaction due to the role of screening effects on the corrugation of the interaction potential [J. L. F. Da Silva et al., Phys. Rev. Lett. 90, 066104 (2003)]. The extremely small adsorption energy of He atoms on the Mg(0001) surface (below 3 meV) and the delocalized nature and mobility of the surface electrons make the He/Mg(0001) system particularly challenging, even for state-of-the-art vdW-corrected density functional-based (vdW-DFT) approaches [M. P. de Lara-Castells et al., J. Chem. Phys. 143, 194701 (2015)]. In this work, we meet this challenge by applying two different procedures. First, the dispersion-corrected second-order Möller-Plesset perturbation theory (MP2C) approach is adopted, using bare metal clusters of increasing size. Second, the method of increments [H. Stoll, J. Chem. Phys. 97, 8449 (1992)] is applied at coupled cluster singles and doubles and perturbative triples level, using embedded cluster models of the metal surface. Both approaches provide clear evidences of the anti-corrugation of the interaction potential: the He atom prefers on-top sites, instead of the expected hollow sites. This is interpreted as a signature of the screening of the He atom by the metal for the on-top configuration. The strong screening in the metal is clearly reflected in the relative contribution of successively deeper surface layers to the main dispersion contribution. Aimed to assist future dynamical simulations, a pairwise potential model for the He/surface interaction as a sum of effective HeâMg pair potentials is also presented, as an improvement of the approximation using isolated HeâMg pairs. Published by AIP Publishing.This work has been partly supported by the COST
Action No. CM1405 âMolecules in Motion (MOLIM),â Grant
Nos. FIS2011-29596-C02-01 and MAT2012-33633 from the
Spanish DirecciĂłn General de InvestigaciĂłn CientĂfica y
TĂ©cnica, and the German Research Foundation (DFG) through
Project No. VO 1711/2-1. The Cesga Super-Computer Center
(Galicia), and the Centro TĂ©cnico de InformĂĄtica (CTI, CSIC)
are acknowledged for allocating computer time.Peer Reviewe
Multiphoton Excited Spectroscopy with Plasmonic and Composite Nanostructures
Ziel dieser Arbeit ist es, das VerstĂ€ndnis der durch plasmonische und Komposit-Nanomaterialien verursachten VerstĂ€rkung der Hyper-Raman Streuung zu vertiefen. Diese Nanostrukturen werden in oberflĂ€chenverstĂ€rkten Hyper-Raman-Streuung (surface enhanced hyper Raman scattering, SEHRS) Experimenten, die durch den nichtlinearen parametrischen Prozess der Frequenzverdopplung (SHG) und der oberflĂ€chenverstĂ€rkten Raman-Streuung (SERS) ergĂ€nzt werden, zur umfassenden Untersuchung organischer MolekĂŒle und Materialien angewendet. Die SEHRS-VerstĂ€rkung von Goldnanopartikeln unterschiedlicher Form und GröĂe sowie von Metallfilmen bestehend aus periodisch angeordneten HohlrĂ€umen (Nanovoids) wurde in Experimenten mit dem Farbstoff Kristallviolett bei einer AnregungswellenlĂ€nge von 1064 nm und durch numerische Simulationen untersucht. Die Ergebnisse zeigen, dass Aggregate von groĂen kugelförmigen Goldnanopartikeln und NanostĂ€bchen in Lösung eine sehr hohe elektromagnetische SEHRS-VerstĂ€rkung bewirken. DarĂŒber hinaus können die HomogenitĂ€t des Signals, die Reproduzierbarkeit in Bezug auf die Herstellung und die SubstratstabilitĂ€t im Vergleich zu frĂŒheren AnsĂ€tzen durch Verwendung von Nanovoids signifikant verbessert werden. Die Weiterentwicklung von Nanostrukturen fĂŒr die multimodale Mehrphotonen-Spektroskopie ist hier anhand der Synthese und der optischen Charakterisierung von plasmonischen Bariumtitanat-Nanokompositen demonstriert. Eine systematische Studie der Wechselwirkung von AminosĂ€uren und aromatischen Thiolen mit Gold- und Silbernanopartikeln wurde mit SEHRS bei einer AnregungswellenlĂ€nge von 1064 nm und mit SERS bei AnregungswellenlĂ€ngen im sichtbaren Spektralbereich durchgefĂŒhrt. Zusammenfassend wurde in dieser Arbeit gezeigt, dass ein tieferes VerstĂ€ndnis und ein rationales Design verbesserter plasmonischer Nanostrukturen ermöglichen, SEHRS mit anderen Mehrphotonen-angeregten Effekten zu kombinieren und diese in der analytischen Chemie und Biophysik einzusetzen.The aim of this work is to extend the understanding of the enhancement in surface enhanced hyper Raman scattering (SEHRS) generated by plasmonic and composite nanomaterials, and to apply these nanostructures in SEHRS experiments complemented by the non-linear parametric process of second harmonic generation (SHG) and by surface enhanced Raman scattering (SERS), for the comprehensive probing of organic molecules and materials. The enhancement from gold nanoparticles with different sizes and shapes as well as from metal films comprised of periodically arranged voids was investigated in SEHRS experiments at 1064 nm excitation using the crystal violet dye and by numerical simulations. The results indicate that aggregates of large spherical gold nanoparticles and nanorods in solution provide very strong electromagnetic enhancement of HRS. Moreover, the homogeneity of the signal, reproducibility in terms of fabrication, and substrate stability can be significantly improved compared to previous approaches by using nanovoid arrays. Further developments of enhancing nanostructures towards multimodal multiphoton spectroscopic applications are demonstrated here by the synthesis and optical characterization of plasmonic-barium titanate nanocomposites. A systematic study on the interaction of amino acids and aromatic thiols with gold and silver nanoparticles was conducted with 1064 nm-excited SEHRS and SERS excited in the visible spectral range. In conclusion, this work underlines that a better understanding and a rational design of improved plasmonic nanostructures allow to combine SEHRS and other multiphoton excited effects, and to use them in analytical chemistry and biophysics
Excitation Conditions for Surface-Enhanced Hyper Raman Scattering With Biocompatible Gold Nanosubstrates
Surface enhanced hyper Raman scattering (SEHRS) can provide many advantages to probing of biological samples due to unique surface sensitivity and vibrational information complementary to surface-enhanced Raman scattering (SERS). To explore the conditions for an optimum electromagnetic enhancement of SEHRS by dimers of biocompatible gold nanospheres and gold nanorods, finite-difference time-domain (FDTD) simulations were carried out for a broad range of excitation wavelengths from the visible through the short-wave infrared (SWIR). The results confirm an important contribution by the enhancement of the intensity of the laser field, due to the two-photon, non-linear excitation of the effect. For excitation laser wavelengths above 1,000 nm, the hyper Raman scattering (HRS) field determines the enhancement in SEHRS significantly, despite its linear contribution, due to resonances of the HRS light with plasmon modes of the gold nanodimers. The high robustness of the SEHRS enhancement across the SWIR wavelength range can compensate for variations in the optical properties of gold nanostructures in real biological environments.Peer Reviewe
Bioâprobing with nonresonant surfaceâenhanced hyperâRaman scattering excited at 1,550 nm
The twoâphoton excited process of surfaceâenhanced hyperâRaman scattering (SEHRS) provides advantages for studies of complex biological samples, yet suitable SEHRS nanoprobes and labels, as well as experimental conditions, must be established. Here, SEHRS spectra of the four reporter molecules (2ânaphthalenethiol [2âNAT], paraâaminothiophenol (pATP), paraânitrothiophenol (pNTP), and crystal violet), as well as the two antidepressant drugs (desipramine and imipramine) were obtained at an excitation wavelength of 1,550 nm using different citrateâstabilized gold nanoparticles and silver nanoparticles, and under conditions that permit experiments with living cultured cells. As the results suggest, the shortâwave infrared laser excitation and the hyperâRaman scattered light (corresponding to wavelengths > 775 nm) match the plasmonic properties of the employed gold and silver nanoaggregates, as well as the requirements regarding the viability of the cells. The twoâphoton excited spectra of three types of SEHRS labels containing 2âNAT as reporter inside macrophage cells show that molecules in the cellular environment can also be observed. The possibility to use shortâwave infrared excitation with gold nanostructures has important implications for the utilization of SEHRS in bioâprobing.Fonds der Chemischen IndustrieDeutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659FP7 Ideas: European Research Council
http://dx.doi.org/10.13039/100011199Peer Reviewe
Plasmon Enhanced Two-Photon Probing with Gold and Silver Nanovoid Structures
Nonlinear optical signals benefit greatly from the enhanced local optical fields in the vicinity of plasmonic nanostructures. Gold and silver nanovoid arrays of varying size and thickness, fabricated by electrochemical deposition are shown here to act as stable plasmonic nanostructures and to enhance the weak, incoherent twoâphoton excited process of surfaceâenhanced hyper Raman scattering (SEHRS) with high microscopic homogeneity and reproducibility that typical SEHRS experiments have not been addressing so far. Silver nanovoids yield stronger enhancement than gold voids, but gold nanovoid arrays show improved stability at high laser excitation intensities. Combined screening experiments using SEHRS and secondâharmonic generation (SHG) reveal a dependence of the enhancement of both signals on void structural parameters and similar optimum geometries for both twoâphoton processes. The results confirm the suggested important role for the enhancement of the nearâinfrared excitation field in SEHRS and suggest SHG as a fast screening tool to identify nanostructures that can support high SEHRS enhancement.Peer Reviewe
Aqueous pyruvate partly dissociates under deep ultraviolet irradiation but is resilient to near ultraviolet excitation
Abstract The deep ultraviolet photochemistry of aqueous pyruvate is believed to have been essential to the origin of life, and near ultraviolet excitation of pyruvate in aqueous aerosols is assumed to contribute significantly to the photochemistry of the Earthâs atmosphere. However, the primary photochemistry of aqueous pyruvate is unknown. Here we study the susceptibility of aqueous pyruvate to photodissociation by deep ultraviolet and near ultraviolet irradiation with femtosecond spectroscopy supported by density functional theory calculations. The primary photo-dynamics of the aqueous pyruvate show that upon deep-UV excitation at 200ânm, about one in five excited pyruvate anions have dissociated by decarboxylation 100âps after the excitation, while the rest of the pyruvate anions return to the ground state. Upon near-UV photoexcitation at a wavelength of 340ânm, the dissociation yield of aqueous pyruvate 200âps after the excitation is insignificant and no products are observed. The experimental results are explained by our calculations, which show that aqueous pyruvate anions excited at 200ânm have sufficient excess energy for decarboxylation, whereas excitation at 340ânm provides the aqueous pyruvate anions with insufficient energy to overcome the decarboxylation barrier
Surface Enhanced Hyper-Raman Scattering of the Amino Acids Tryptophan, Histidine, Phenylalanine, and Tyrosine
In this work, we
report nonresonant surface-enhanced hyper-Raman
(SEHRS) spectra of the amino acids tryptophan, histidine, phenylalanine,
and tyrosine using silver nanoparticles. The spectra are obtained
at an excitation wavelength of 1064 nm and compared to the corresponding
surface-enhanced Raman scattering (SERS) spectra measured at 532 nm
excitation. The majority of the bands in the SEHRS spectra are assigned.
Important hallmarks of the spectra include strongly diminished or
absent bands from the ring breathing modes. SEHRS and SERS spectra
obtained from histidine and tyrosine indicate changes at slightly
varied amino acid concentration. Small changes in the SEHRS spectra
were more pronounced than variation in the corresponding SERS data,
supporting the high sensitivity of the SEHRS spectra with respect
to structural changes due to small variations in surface environment.
The possibility to measure nonresonant SEHRS spectra of amino acids
in solution and the complementary information obtained from the spectra
demonstrates the potential of this method for future investigations
of proteins and more complicated biological structures and their interaction
with nanostructures
Surface-Enhanced Hyper-Raman Spectra of Adenine, Guanine, Cytosine, Thymine, and Uracil
Using
picosecond excitation at 1064 nm, surface-enhanced hyper-Raman
scattering (SEHRS) spectra of the nucleobases adenine, guanine, cytosine,
thymine, and uracil with two different types of silver nanoparticles
were obtained. Comparing the SEHRS spectra with SERS data from the
identical samples excited at 532 nm and with known infrared spectra,
the major bands in the spectra are assigned. Due to the different
selection rules for the one- and two-photon excited Raman scattering,
we observe strong variation in relative signal strengths of many molecular
vibrations obtained in SEHRS and SERS spectra. The two-photon excited
spectra of the nucleobases are found to be very sensitive with respect
to moleculeânanoparticle interactions. Using both the SEHRS
and SERS data, a comprehensive vibrational characterization of the
interaction of nucleobases with silver nanostructures can be achieved
Goldâ and SilverâCoated Barium Titanate Nanocomposites as Probes for TwoâPhoton Multimodal Microspectroscopy
Improved multiphotonâexcited imaging and microspectroscopy require nanoprobes that can give different nonlinear optical signals. Here, composite nanostructures with a barium titanate core and a plasmonic moiety at their surface are synthesized and characterized. It is found that the core provides a high secondâorder nonlinear susceptibility for sensitive second harmonic generation (SHG) imaging in living cells. As a second function in the twoâphoton regime, the plasmonic part yields high local fields for resonant and nonresonant surface enhanced hyper Raman scattering (SEHRS). SEHRS complements the oneâphoton surface enhanced Raman scattering (SERS) spectra that are also enhanced by the plasmonic shells. Barium titanate silver coreâshell (Ag@BaTiO3) composites are specifically suited for SEHRS and SHG excited at 1064 nm, while gold at barium titanate (Au@BaTiO3) nanoparticles can be useful in a combination of SHG and SERS at lower wavelengths, here at 785 nm and 850 nm. The theoretical models show that the optical properties of the BaTiO3 dielectric core depend on probing frequency, shape, size, and plasmonic properties of the surrounding gold nanoparticles so that they can be optimized for a particular type of experiment. These versatile, tunable probes give new opportunities for combined multiphoton probing of morphological structure and chemical properties of biosystems.Peer Reviewe