Our publications – QUEM-CHEM

Publications in 2021

Chemical Enhancement vs Molecule–Substrate Geometry in Plasmon-Enhanced Spectroscopy

Rodriguez, R. D.; Villagómez, C. J.; Khodadadi, A.; Kupfer, S.; Averkiev, A.; Dedelaite, L.; Tang, F.; Khaywah, M. Y.; Kolchuzhin, V.; Ramanavicius, A.; Adam, P.-M.; Gräfe, S.; Sheremet, E. ACS Photonics 2021, 8 (8), 2243–2255. DOI: https://doi.org/10.1021/acsphotonics.1c00001.

Light interaction with metal nanostructures exposes exciting phenomena such as strong amplification and localization of electromagnetic fields. In surface-enhanced Raman spectroscopy (SERS), the strong signal amplification is attributed to two fundamental mechanisms, electromagnetic and chemical enhancement (EM and CM, respectively). While the EM mechanism is accepted as the main responsible for signal amplification, a long-standing controversy on the CM mechanism’s role still prevails. The CM contribution can be evidenced when compared to the nonenhanced (or bulk) Raman signal as a change in intensity ratios, peak shifts, or appearance of new Raman modes. However, it is also possible to induce similar spectral variations by changing the relative orientation between the electric field and molecule or when a high electric field gradient is achieved. Therefore, in this work, we show specific spectral changes in SERS affected by the molecular orientation, while changes in other modes can be attributed to chemical enhancement. On the basis of our experimental and quantum chemical results for cobalt phthalocyanine, we identify low-frequency Raman modes (LFMs) sensitive to charge-transfer compared to high-frequency modes (HFMs) that are rather sensitive to geometrical effects and temperature changes. These results provide new evidence on the role of molecule excitation/polarization that comes now as a more general and dominant effect than the chemical enhancement mechanism so far attributed to charge-transfer processes. These findings make it possible to engineer multifunctional Raman molecular probes with selective sensitivity to the local environment (HFMs) and charge-transfer processes (LFMs).

Light-matter quantum dynamics of complex laser-driven systems

Gonoskov, I.; Gräfe, S. J. Chem. Phys. 2021, 154 (23), 234106. DOI: https://doi.org/10.1063/5.0048930. ( PDF)

Ubiquitous to most molecular scattering methods is the challenge to retrieve bond distance and angle from the scattering signals since this requires convergence of pattern matching algorithms or fitting methods. This problem is typically exacerbated when imaging larger molecules or for dynamic systems with little a priori knowledge. Here, we employ laser-induced electron diffraction (LIED) which is a powerful means to determine the precise atomic configuration of an isolated gas-phase molecule with picometre spatial and attosecond temporal precision. We introduce a simple molecular retrieval method, which is based only on the identification of critical points in the oscillating molecular interference scattering signal that is extracted directly from the laboratory-frame photoelectron spectrum. The method is compared with a Fourier-based retrieval method, and we show that both methods correctly retrieve the asymmetrically stretched and bent field-dressed configuration of the asymmetric top molecule carbonyl sulfide (OCS), which is confirmed by our quantum-classical calculations.

Molecular structure retrieval directly from laboratory-frame photoelectron spectra in laser-induced electron diffraction

Sanchez, A.; Amini, K.; Wang, S.-J.; Steinle, T.; Belsa, B.; Danek, J.; Le, A. T.; Liu, X.; Moshammer, R.; Pfeifer, T.; Richter, M.; Ullrich, J.; Gräfe, S.; Lin, C. D.; Biegert, J. Nat Commun 2021, 12 (1), 1520. DOI: https://doi.org/10.1038/s41467-021-21855-4. ( PDF)

Spatially Resolving the Enhancement Effect in Surface-Enhanced Coherent Anti-Stokes Raman Scattering by Plasmonic Doppler Gratings

Ouyang, L.; Meyer-Zedler, T.; See, K.-M.; Chen, W.-L.; Lin, F.-C.; Akimov, D.; Ehtesabi, S.; Richter, M.; Schmitt, M.; Chang, Y.-M.; Gräfe, S.; Popp, J.; Huang, J.-S. ACS Nano 2021, 15 (1), 809–818. DOI: https://doi.org/10.1021/acsnano.0c07198. ( PDF)

Well-designed plasmonic nanostructures can mediate far and near optical ﬁelds and thereby enhance light−matter interactions. To obtain the best overall enhancement, structural parameters need to be carefully tuned to obtain the largest enhancement at the input and output frequencies. This is, however, challenging for nonlinear light−matter interactions involving multiple frequencies because obtaining the full picture of structure-dependent enhancement at individual frequencies is not easy. In this work, we introduce the platform of plasmonic Doppler grating (PDG) to experimentally investigate the enhancement eﬀect of plasmonic gratings in the input and output beams of nonlinear surface-enhanced coherent anti-Stokes Raman scattering (SECARS). PDGs are designable azimuthally chirped gratings that provide broadband and spatially dispersed plasmonic enhancement. Therefore, they oﬀer the opportunity to observe and compare the overall enhancement from diﬀerent combinations of enhancement in individual input and output beams. We ﬁrst conﬁrm PDG’s capability of spatially separating the input and output enhancement in linear surface-enhanced ﬂuorescence and Raman scattering. We then investigate spatially resolved enhancement in nonlinear SECARS, where coherent interaction of the pump, Stokes, and anti-Stokes beams is enhanced by the plasmonic gratings. By mapping the SECARS signal and analyzing the azimuthal angle-dependent intensity, we characterize the enhancement at individual frequencies. Together with theoretical analysis, we show that while simultaneous enhancement in the input and output beams is important for SECARS, the enhancement in the pump and anti-Stokes beams plays a more critical role in the overall enhancement than that in the Stokes beam. This work provides an insight into the enhancement mechanism of plasmon-enhanced spectroscopy, which is important for the design and optimization of plasmonic gratings. The PDG platform may also be applied to study enhancement mechanisms in other nonlinear light−matter interactions or the impact of plasmonic gratings on the ﬂuorescence lifetime.

Are charged tips driving TERS-resolution? A full quantum chemical approach

Fiederling, K.; Kupfer, S.; Gräfe, S. J. Chem. Phys. 2021, 154 (3), 034106. DOI: https://doi.org/10.1063/5.0031763. ( PDF)

Experimental evidence suggests an extremely high, possibly even sub-molecular, spatial resolution of tip-enhanced Raman spectroscopy (TERS). While the underlying mechanism is currently still under discussion, two main contributions are considered: The involved plasmonic particles are able to highly confine light to small spatial regions in the near-field, i.e., the electromagnetic effect and the chemical effect due to altered molecular properties of the sample in close proximity to the plasmonic tip. Significant theoretical effort is put into the modeling of the electromagnetic contribution by various groups. In contrast, we previously introduced a computational protocol that allows for the investigation of the local chemical effect—including non-resonant, resonant, and charge transfer contributions—on a plasmonic hybrid system by mapping the sample molecule with a metallic tip model at the (time-dependent) density functional level of theory. In the present contribution, we evaluate the impact of static charges localized on the tip’s frontmost atom, possibly induced by the tip geometry in the vicinity of the apex, on the TERS signal and the lateral resolution. To this aim, an immobilized molecule, i.e., tin(II) phthalocyanine (SnPc), is mapped by the plasmonic tip modeled by a single positively vs negatively charged silver atom. The performed quantum chemical simulations reveal a pronounced enhancement of the Raman intensity under non-resonant and resonant conditions with respect to the uncharged reference system, while the contribution of charge transfer phenomena and of locally excited states of SnPc is highly dependent on the tip’s charge.

Laser-induced electron diffraction of the ultrafast umbrella motion in ammonia

Belsa, B.; Amini, K.; Liu, X.; Sanchez, A.; Steinle, T.; Steinmetzer, J.; Le, A. T.; Moshammer, R.; Pfeifer, T.; Ullrich, J.; Moszynski, R.; Lin, C. D.; Gräfe, S.; Biegert, J. Structural Dynamics 2021, 8 (1), 014301. DOI: https://doi.org/10.1063/4.0000046. ( PDF)

Visualizing molecular transformations in real-time requires a structural retrieval method with Angström spatial and femtosecond temporal atomic resolution. Imaging of hydrogen-containing molecules additionally requires an imaging method sensitive to the atomic positions of hydrogen nuclei, with most methods possessing relatively low sensitivity to hydrogen scattering. Laser-induced electron diffraction (LIED) is a table-top technique that can image ultrafast structural changes of gas-phase polyatomic molecules with sub-Angström and femtosecond spatiotemporal resolution together with relatively high sensitivity to hydrogen scattering. Here, we image the umbrella motion of an isolated ammonia molecule (NH₃) following its strong-field ionization. Upon ionization of a neutral ammonia molecule, the ammonia cation (NH₃⁺) undergoes an ultrafast geometrical transformation from a pyramidal (Φ_HNH = 107°) to planar (Φ_HNH = 120°) structure in approximately 8 femtoseconds. Using LIED, we retrieve a near-planar (Φ_HNH = 117 ± 5°) field-dressed NH₃ molecular structure 7.8-9.8 femtoseconds after ionization. Our measured field-dressed NH₃⁺ structure is in excellent agreement with our calculated equilibrium field-dressed structure using quantum chemical ab initio calculations.

Publications in 2020

The impact of electron–electron correlation in ultrafast attosecond single ionization dynamics

Fröbel, F. G.; Ziems, K. M.; Peschel, U.; Gräfe, S.; Schubert, A. J. Phys. B: At. Mol. Opt. Phys. 2020, 53 (14), 144005. DOI: https://doi.org/10.1088/1361-6455/ab8c21. ( PDF)

The attosecond ultrafast ionization dynamics of correlated two- or many-electron systems have, so far, been mainly addressed investigating atomic systems. In the case of single ionization, it is well known that electron–electron correlation modifies the ionization dynamics and observables beyond the single active electron picture, resulting in effects such as the Auger effect or shake-up/down and knock-up/down processes. Here, we extend these works by investigating the attosecond ionization of a molecular system involving correlated two-electron dynamics, as well as non-adiabatic nuclear dynamics. Employing a charge-transfer molecular model system with two differently bound electrons, a strongly and a weakly bound electron, we distinguish different pathways leading to ionization, be it direct ionization or ionization involving elastic and inelastic electron scattering processes. We find that different pathways result in a difference in the electronic population of the parent molecular ion, which, in turn, involves different subsequent (non-adiabatic) postionization dynamics on different time scales.

The chemical effect goes resonant – a full quantum mechanical approach on TERS

Fiederling, K.; Abasifard, M.; Richter, M.; Deckert, V.; Gräfe, S.; Kupfer, S. Nanoscale 2020, 12 (11), 6346–6359. DOI: https://doi.org/10.1039/C9NR09814C. ( PDF)

Lately, experimental evidence of unexpectedly extremely high spatial resolution of tip-enhanced Raman scattering (TERS) has been demonstrated. Theoretically, two different contributions are discussed: an electromagnetic effect, leading to a spatially confined near field due to plasmonic excitations; and the so-called chemical effect originating from the locally modified electronic structure of the molecule due to the close proximity of the plasmonic system. Most of the theoretical efforts have concentrated on the electromagnetic contribution or the chemical effect in case of non-resonant excitation. In this work, we present a fully quantum mechanical description including non-resonant and resonant chemical contributions as well as charge-transfer phenomena of these molecular-plasmonic hybrid systems at the density functional and the time-dependent density functional level of theory. We consider a surface-immobilized tin(II) phthalocyanine molecule as the molecular system, which is minutely scanned by a plasmonic tip, modeled by a single silver atom. These different relative positions of the Ag atom to the molecule lead to pronounced alterations of the Raman spectra. These Raman spectra vary substantially, both in peak positions and several orders of magnitude in the intensity patterns under non-resonant and resonant conditions, and also, depending on, which electronic states are addressed. Our computational approach reveals that unique – non-resonant and resonant – chemical interactions among the tip and the molecule significantly alter the TERS spectra and are mainly responsible for the high, possibly sub-Angstrom spatial resolution.

Publications in 2019

Imaging an isolated water molecule using a single electron wave packet

Liu, X.; Amini, K.; Steinle, T.; Sanchez, A.; Shaikh, M.; Belsa, B.; Steinmetzer, J.; Le, A.-T.; Moshammer, R.; Pfeifer, T.; Ullrich, J.; Moszynski, R.; Lin, C. D.; Gräfe, S.; Biegert, J. The Journal of Chemical Physics 2019, 151 (2), 024306, DOI: 10.1063/1.5100520. ( PDF)

Observing changes in molecular structure requires atomic-scale Ångstrom and femtosecond spatio-temporal resolution. We use the Fourier transform (FT) variant of laser-induced electron diffraction (LIED), FT-LIED, to directly retrieve the molecular structure of H₂O⁺ with picometer and femtosecond resolution without a priori knowledge of the molecular structure nor the use of retrieval algorithms or ab initio calculations. We identify a symmetrically stretched H₂O⁺ field-dressed structure that is most likely in the ground electronic state. We subsequently study the nuclear response of an isolated water molecule to an external laser field at four different field strengths. We show that upon increasing the laser field strength from 2.5 to 3.8 V/Å, the O–H bond is further stretched and the molecule slightly bends. The observed ultrafast structural changes lead to an increase in the dipole moment of water and, in turn, a stronger dipole interaction between the nuclear framework of the molecule and the intense laser field. Our results provide important insights into the coupling of the nuclear framework to a laser field as the molecular geometry of H₂O⁺ is altered in the presence of an external field.

Imaging the Renner–Teller effect using laser-induced electron diffraction

Amini, K.; Sclafani, M.; Steinle, T.; Le, A.-T.; Sanchez, A.; Müller, C.; Steinmetzer, J.; Yue, L.; Saavedra, J. R. M.; Hemmer, M.; Lewenstein, M.; Moshammer, R.; Pfeifer, T.; Pullen, M. G.; Ullrich, J.; Wolter, B.; Moszynski, R.; Abajo, F. J. G. D.; Lin, C. D.; Gräfe, S.; Biegert, J. Proceedings of the National Academy of Sciences 2019, 116 (17), 8173–8177, DOI: 10.1073/pnas.1817465116. ( PDF)

Laser-induced electron diffraction is a molecular-scale electron microscopy that captures clean snapshots of a molecule’s geometry with subatomic picometer and attosecond spatiotemporal resolution. We induce and unambiguously identify the stretching and bending of a linear triatomic molecule following the excitation of the molecule to an excited electronic state with a bent and stretched geometry. We show that we can directly retrieve the structure of electronically excited molecules that is otherwise possible through indirect retrieval methods such as pump-probe and rotational spectroscopy measurements.