Post-Hartree-Fock studies of the He/Mg(0001) interaction: Anti-corrugation, screening, and pairwise additivity

12 págs.; 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 He–Mg pair potentials is also presented, as an improvement of the approximation using isolated He–Mg pairs. Published by AIP Publishing.This work has been partly supported by the COST Action No. CM1405 “Molecules in Motion (MOLIM),” Grant Nos. FIS2011-29596-C02-01 and MAT2012-33633 from the Spanish Dirección General de Investigación Científica y Técnica, and the German Research Foundation (DFG) through Project No. VO 1711/2-1. The Cesga Super-Computer Center (Galicia), and the Centro Técnico de Informática (CTI, CSIC) are acknowledged for allocating computer time.Peer Reviewe

Multiphoton Excited Spectroscopy with Plasmonic and Composite Nanostructures

Ziel dieser Arbeit ist es, das Verständnis der durch plasmonische und Komposit-Nanomaterialien verursachten Verstärkung der Hyper-Raman Streuung zu vertiefen. Diese Nanostrukturen werden in oberflächenverstärkten Hyper-Raman-Streuung (surface enhanced hyper Raman scattering, SEHRS) Experimenten, die durch den nichtlinearen parametrischen Prozess der Frequenzverdopplung (SHG) und der oberflächenverstärkten Raman-Streuung (SERS) ergänzt werden, zur umfassenden Untersuchung organischer Moleküle und Materialien angewendet. Die SEHRS-Verstärkung von Goldnanopartikeln unterschiedlicher Form und Größe sowie von Metallfilmen bestehend aus periodisch angeordneten Hohlräumen (Nanovoids) wurde in Experimenten mit dem Farbstoff Kristallviolett bei einer Anregungswellenlänge von 1064 nm und durch numerische Simulationen untersucht. Die Ergebnisse zeigen, dass Aggregate von großen kugelförmigen Goldnanopartikeln und Nanostäbchen in Lösung eine sehr hohe elektromagnetische SEHRS-Verstärkung bewirken. Darüber hinaus können die Homogenität des Signals, die Reproduzierbarkeit in Bezug auf die Herstellung und die Substratstabilität im Vergleich zu früheren Ansätzen durch Verwendung von Nanovoids signifikant verbessert werden. Die Weiterentwicklung von Nanostrukturen für die multimodale Mehrphotonen-Spektroskopie ist hier anhand der Synthese und der optischen Charakterisierung von plasmonischen Bariumtitanat-Nanokompositen demonstriert. Eine systematische Studie der Wechselwirkung von Aminosäuren und aromatischen Thiolen mit Gold- und Silbernanopartikeln wurde mit SEHRS bei einer Anregungswellenlänge von 1064 nm und mit SERS bei Anregungswellenlängen im sichtbaren Spektralbereich durchgeführt. Zusammenfassend wurde in dieser Arbeit gezeigt, dass ein tieferes Verständnis 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

Bio‐probing with nonresonant surface‐enhanced hyper‐Raman scattering excited at 1,550 nm

The two‐photon excited process of surface‐enhanced hyper‐Raman 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 (2‐naphthalenethiol [2‐NAT], para‐aminothiophenol (pATP), para‐nitrothiophenol (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 citrate‐stabilized gold nanoparticles and silver nanoparticles, and under conditions that permit experiments with living cultured cells. As the results suggest, the short‐wave infrared laser excitation and the hyper‐Raman 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 two‐photon excited spectra of three types of SEHRS labels containing 2‐NAT as reporter inside macrophage cells show that molecules in the cellular environment can also be observed. The possibility to use short‐wave infrared excitation with gold nanostructures has important implications for the utilization of SEHRS in bio‐probing.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 two‐photon excited process of surface‐enhanced 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 second‐harmonic generation (SHG) reveal a dependence of the enhancement of both signals on void structural parameters and similar optimum geometries for both two‐photon processes. The results confirm the suggested important role for the enhancement of the near‐infrared 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 Earth’s 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 200 nm, about one in five excited pyruvate anions have dissociated by decarboxylation 100 ps after the excitation, while the rest of the pyruvate anions return to the ground state. Upon near-UV photoexcitation at a wavelength of 340 nm, the dissociation yield of aqueous pyruvate 200 ps 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 200 nm have sufficient excess energy for decarboxylation, whereas excitation at 340 nm 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 molecule–nanoparticle interactions. Using both the SEHRS and SERS data, a comprehensive vibrational characterization of the interaction of nucleobases with silver nanostructures can be achieved

Gold‐ and Silver‐Coated Barium Titanate Nanocomposites as Probes for Two‐Photon Multimodal Microspectroscopy

Improved multiphoton‐excited 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 second‐order nonlinear susceptibility for sensitive second harmonic generation (SHG) imaging in living cells. As a second function in the two‐photon regime, the plasmonic part yields high local fields for resonant and nonresonant surface enhanced hyper Raman scattering (SEHRS). SEHRS complements the one‐photon surface enhanced Raman scattering (SERS) spectra that are also enhanced by the plasmonic shells. Barium titanate silver core–shell (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