Benjamin Schröder

An ultrafast nanotip electron gun triggered by grating-coupled surface plasmons

We demonstrate multiphoton photoelectron emission from gold nanotips induced by nanofocusing surface plasmons, resonantly excited on the tip shaft by a grating coupler. The tip is integrated into an electron gun assembly, which facilitates control over the spatial emission sites and allows us to disentangle direct grating emission from plasmon-triggered apex emission. The nanoscale source size of this electron gun concept enables highly coherent electron pulses with applications in ultrafast electron imaging and diffraction.

STRONG THEORETICAL SUPPORT FOR THE ASSIGNMENT OF B11244 TO l-C3H+

Highly accurate quantum chemical calculations beyond CCSD(T) have been used to study the molecular cation l-C3H+ which is the carrier of harmonically related radio lines observed in the Horsehead photodissociation region and toward Sgr B2(N). Excellent agreement with spectroscopic and radioastronomical measurements is obtained for the rotational constant, with the calculated value of B 0 = 11246.4 MHz only 1.5 MHz or 0.01% above the experimental value. The unusually large ratio of centrifugal distortion constants D 0(exp.)/De(theor.) = 1.80 is attributed to the shallow CCC bending potential of l-C3H+ and is quantitatively reproduced by variational calculations within a pseudo-triatomic model. A comparative study of centrifugal distortion constants in a series of four linear interstellar molecules (C3N–, C3O, l-C3H+, and C3) is made and some general conclusions are drawn.

Effects of defaunation on fermentation characteristics and biotin balance in an artificial rumen-simulation system (RUSITEC) receiving diets with different amounts and types of cereal

Biotin is required by rumen microbes for efficient fermentation. To evaluate the role of protozoa in ruminal biotin metabolism, five diets composed of grass hay or of grass hay/cereal grain mixtures were supplied to faunated or defaunated RUSITEC fermenters. In the mixed diets, hay was replaced to 33:67 or 67:33 w/w on an air-dried basis by either wheat or maize grain in order to simulate different cellulolytic and amylolytic fermentation conditions. Defaunation increased SCFA production, whereas NH4 concentration and the release of CH4 were reduced. Biotin input declined when cereal grain was used to replace the hay. With the exception of the high-wheat treatment, defaunated fermenters yielded higher biotin outputs than faunated fermenters. The biotin balance, calculated as the difference between the total biotin output (biotin in the solid residue contained in the nylon bags after fermentation plus the biotin in the effluent) and the biotin input with the feed, was negative for all the dietary treatments apart from fermenters supplied with the high-maize diet. It was less negative or, in the case of the high-maize diets, more positive for defaunated compared with faunated fermenters. It was concluded that, under normal faunated conditions, protozoa directly utilise or indirectly affect the bacterial synthesis and/or utilisation of biotin. With diets of a high fermentation potential, as realised with the high-wheat diet, protozoa prevent the development of a bacterial population that would utilise high or synthesise low amounts of biotin.

Challenging High-Level ab initio Rovibrational Spectroscopy: The Nitrous Oxide Molecule

Abstract The equilibrium structure and rovibrational energies of nitrous oxide (N2O) in its electronic ground state (X1Σ+) are derived from a high-level ab initio potential energy function (PEF). This PEF is based on a composite approach with the basic contribution given by explicitly correlated coupled-cluster (CC) calculations. Smaller contributions include corrections due to inner-shell correlation, scalar-relativistic effects and higher-order correlation up to iterative pentuple excitations (CCSDTQP in CC nomenclature). The high importance of higher-order correlation in order to reach the desired accuracy led to the use of an extrapolation scheme to approximately account for the effect of hextuple and some pentuple excitations. A reasoning for the soundness of the method is given in this work. The results of the rovibrational calculations are compared to those of two multi-reference (MR) based composite PEFs, where the basic contribution is given by MR configuration interaction and MR average coupled-pair functional calculations. A highly accurate electric dipole moment function is also computed by the three composite methods in excellent agreement with the experimental values available. Subtle irregularities in the intensity pattern are reproduced in great detail and several kinds of resonances are analyzed without the need to empirically adjust our best ab initio PEF. The equilibrium bond lengths were determined by a mixed experimental/theoretical approach yielding Re(NN) = 1.12695(10) Å and Re(NO) = 1.18539(5) Å.

Theoretical rovibrational spectroscopy beyond fc-CCSD(T): the cation CNC+

An accurate near-equilibrium potential energy surface (PES) for CNC+ is constructed based on a high-level composite ab initio method. By combining explicitly correlated all-electron CCSD(T)-F12b with scalar relativistic effects and higher order correlation up to coupled cluster theory with singles, doubles, triples and quadruples (CCSDTQ) we achieve convergence in the wavenumbers of the fundamentals to ca. 1 cm−1. Rovibrational energies are calculated in a variational approach and vibrational term energies and rotational constants are in excellent agreement with available experimental data. Accurate values for centrifugal distortion constants of CNC+ in different vibrational states are predicted. Especially the centrifugal distortion constants in the vibrational ground state of D0 = 0.563 · 10−6 cm−1 and H0 = 0.188 · 10−10 cm−1 should be superior to experimentally derived values. Reassignments of some experimentally observed transitions are suggested based on a comparison of experimental and calculated term differences. The bending part of the PES appears to be almost quartic and the band origin of the bending vibration is predicted at 94.2 cm−1. Absolute line intensities are calculated for various transitions in CNC+. For the bending vibration, an intensity is predicted that is three orders of magnitude smaller than for the antisymmetric stretching vibration.

High-level theoretical rovibrational spectroscopy beyond fc-CCSD(T): The C3 molecule

An accurate local (near-equilibrium) potential energy surface (PES) is reported for the C3 molecule in its electronic ground state (X̃(1)Σg (+)). Special care has been taken in the convergence of the potential relative to high-order correlation effects, core-valence correlation, basis set size, and scalar relativity. Based on the aforementioned PES, several rovibrational states of all (12)C and (13)C substituted isotopologues have been investigated, and spectroscopic parameters based on term energies up to J = 30 have been calculated. Available experimental vibrational term energies are reproduced to better than 1 cm(-1) and rotational constants show relative errors of not more than 0.01%. The equilibrium bond length has been determined in a mixed experimental/theoretical approach to be 1.294 07(10) Å in excellent agreement with the ab initio composite value of 1.293 97 Å. Theoretical band intensities based on a newly developed electric dipole moment function also suggest that the infrared active (1, 1(1), 0)←(0, 0(0), 0) combination band might be observable by high-resolution spectroscopy.

High-level theoretical spectroscopic parameters for three ions of astrochemical interest

The equilibrium geometry and rovibrational spectroscopic parameters of the three astrochemical ions l-C3H+, l-SiC2H+, and C3N− and some of their isotopologues are obtained from high-level quantum chemical calculations. A composite approach based on the explicitly correlated coupled-cluster method CCSD(T)-F12b, that further includes core correlation, scalar-relativistic effects and most importantly higher order correlation beyond CCSD(T) is used to set-up the near-equilibrium potential energy surface (PES). The spectroscopic parameters of these linear tetra-atomic ions are then extracted from these PESs by vibrational perturbation theory of second order (VPT2). Calculation of absolute intensities is also carried out for the stretching frequencies of the cations in order to identify the bands that are most likely to be detected. The importance of the accurate calculation of the rotational constants B0 and D0 for astrochemistry is discussed as well as the limits of VPT2 in this context and reasons for these limitations.

Studies on the biotin flow at the duodenum of dairy cows fed diets with different concentrate levels and types of forages.

Abstract Biotin is involved in many vital metabolic pathways and must be provided for an efficient fermentation in the rumen, as well as for the intermediary metabolism of the host animal. Factors influencing ruminal biotin metabolism and output are widely unknown at present. Therefore, dairy cows fitted with permanent cannulas in the dorsal rumen and in the proximal duodenum were fed differently composed diets, and the biotin flow at the proximal duodenum was measured. The diets (on DM basis) consisted of 8.9 kg grass hay (Diet 1), 8.9 kg corn silage plus 2.0 kg concentrate (Diet 2), or 7.3 and 7.4 kg grass silage plus 10.0 kg concentrate (Diets 3 and 4). The concentrate in Diets 3 and 4 contained 87% wheat and corn grain, respectively. The cows were pre-fed the rations for 21 days. Thereafter duodenal digesta was sampled every two h for 5 days. Cr2O3 served as a flow marker and the microbial proportion of total nitrogen at the duodenum was estimated by near infrared spectroscopy (NIRS). The duodenal flow of biotin was not related to biotin intake, but to the amount of fermented organic matter (FOM) and the amount of microbial protein (Biotin [mg/d] = 0.518 · kg FOM−0.300; r = 0.85 and biotin [mg/d] = 0.012 · g microbial protein + 1.478; r = 0.84), irrespective of the composition of the diet fed. Mean daily biotin flow was 0.48 ± 0.11 mg/kg FOM without any systematic effect of diet composition. The ruminal biotin balance, calculated as the difference between biotin flow at the duodenum and biotin intake, was positive (1.4 – 2.0 mg/d) in cows fed the mixed roughage/concentrate diets and negative (−0.71 mg/d) when the pure hay diet was fed.

Stretching our understanding of C3: Experimental and theoretical spectroscopy of highly excited nν1 + mν3 states (n ≤ 7 and m ≤ 3)

We present the high resolution infrared detection of fifteen highly vibrationally excited nν1 + mν3 combination bands (n ≤ 7 and m ≤ 3) of C3 produced in a supersonically expanding propyne plasma, of which fourteen are reported for the first time. The fully resolved spectrum, around 3 μm, is recorded using continuous wave cavity ring-down spectroscopy. A detailed analysis of the resulting spectra is provided by ro-vibrational calculations based on an accurate local ab initio potential energy surface for C3 (X̃1Σg+). The experimental results not only offer a significant extension of the available data set, extending the observed number of quanta v1 to 7 and v3 to 3, but also a vital test to the fundamental understanding of this benchmark molecule. The present variational calculations give remarkable agreement compared to experimental values with typical accuracies of ∼0.01% for the vibrational frequencies and ∼0.001% for the rotational parameters, even for high energy levels around 10 000 cm-1.

Clocking plasmon nanofocusing by THz near-field streaking

We apply terahertz (THz) near-field streaking in a nanofocusing geometry to investigate plasmon polariton propagation on the shaft of a conical nanotip. By evaluating the delay between a streaking spectrogram for plasmon-induced photoemission with a measurement for direct apex excitation, we obtain an average plasmon group velocity, which is in agreement with numerical simulations. Combining plasmon-induced photoemission with THz near-field streaking facilitates extensive control over localized photoelectron sources for time-resolved imaging and diffraction.