The nature of plasmon-driven chemical reactions is experimentally investigated using high vacuum tip-enhanced Raman spectroscopy (HV-TERS). It is revealed that the coupling between the tip and the substrate can produce intense plasmon resonance, which then decays to produce sufficient hot electrons and thus catalyses the chemical reaction. The photoelectron emission from the laser illuminated silver substrate alone cannot drive the reaction.

Electric field gradient quadrupole Raman modes are observed in plasmon-driven chemical reactions investigated with high vacuum tip-enhanced Raman spectroscopy (HV-TERS). TER spectra reveal that 4-nitrobenzenethiol (4NBT) catalytically dimerizes to dimercaptoazobenzene (DMAB) under an HV-TERS setup. More importantly, we find that the electric field gradient leads to strong enhancement of the infrared (IR)-active modes of DMAB. The observation of both the Raman-active and IR-active modes of DMAB provides spectral evidence for ultrasensitive chemical analysis.

Aims. We compute the distribution of the magnetic field and the plasma in the global corona with a self-consistent magnetohydrostatic (MHS) model. Methods. Because direct measurements of the solar coronal magnetic field and plasma are extremely difficult and inaccurate, we use a modeling approach based on observational quantities, e.g. the measured photospheric magnetic field, to reconstruct the structure of the global solar corona. We take an analytic magnetohydrostatic model to extrapolate the magnetic field in the corona from photospheric magnetic field measurement. In the model, the electric current density can be decomposed into two components: one component is aligned with the magnetic field lines, whereas the other component flows in spherical shells. The second component of the current produces finite Lorentz forces that are balanced by the pressure gradient and the gravity force. We derive the 3D distribution of the magnetic field and plasma self-consistently in one model. The boundary conditions are given by a synoptic magnetogram on the inner boundary and by a source surface model at the outer boundary. Results. The density in the model is higher in the equatorial plane than in the polar region. We compare the magnetic field distribution of our model with potential and force-free field models for the same boundary conditions and find that our model differs noticeably from both. We discuss how to apply the model and how to improve it.

Context. This paper presents a method which can be used to calculate models of the global solar corona from observational data. Aims. We present an optimization method for computing nonlinear magnetohydrostatic equilibria in spherical geometry with the aim to obtain self-consistent solutions for the coronal magnetic field, the coronal plasma density and plasma pressure using observational data as input. Methods. Our code for the self-consistent computation of the coronal magnetic fields and the coronal plasma solves the non-forcefree magnetohydrostatic equilibria using an optimization method. Previous versions of the code have been used to compute nonlinear force-free coronal magnetic fields from photospheric measurements in Cartesian and spherical geometry, and magnetostaticequilibria in Cartesian geometry. We test our code with the help of a known analytic 3D equilibrium solution of the magnetohydrostatic equations. The detailed comparison between the numerical calculations and the exact equilibrium solutions is made by using magnetic field line plots, plots of density and pressure and some of the usual quantitative numerical comparison measures. Results. We find that the method reconstructs the equilibrium accurately, with residual forces of the order of the discretisation error of the analytic solution. The correlation with the reference solution is better than 99.9% and the magnetic energy is computed accurately with an error of <0.1%. Conclusions. We applied the method so far to an analytic test case. We are planning to use this method with real observational data as input as soon as possible.