Highly Sensitive Optical Spectroscopy of Supported Chiral Molecules and Clusters

The most common way to exploit the properties of small metal clusters and nanoparticles is to support them onto a surface. Due to cluster-support interactions, it is not possible to extract the properties of supported metal clusters from gas phase or matrix-embedded cluster studies. Investigating optical properties of supported size-selected metal clusters is essential for the understanding of the electronic structure of such systems. Particularly, catalytic and photochemical properties of cluster-assembled materials are affected by the electronic structure of supported clusters.  

In the study of supported size-selected metal clusters, it is necessary to employ highly sensitive spectroscopic methods because firstly, the absorption cross section of metal clusters is very small and secondly, the surface coverage should be kept very low (~1% ML) in order to hinder agglomeration.

 We have developed two complementary methods:

 

  • In surface Cavity Ring-down Spectroscopy (s-CRDS), the rate of absorption rather than the magnitude of the absorption of a light pulse confined in an optical cavity is measured. The sample is placed inside a high-finesse optical resonator consisting of two highly reflective mirrors. A short laser pulse is coupled into the cavity where it is reflected back and forth. A small fraction of light leaks out of the cavity. Instead of measuring the total intensity of the light that exits the cavity, the decay time of this leakage is determined. The more the sample absorbs the shorter the measured decay time. By recording the decay times as a function of wavelength, a spectrum of either the cluster (in the UV/Vis regime) or the adsorbates (in the IR) can be obtained. s-CRDS is a direct linear method with a much higher sensitivity compared to conventional methods[1].

 

  • In surface Second Harmonic Generation Spectroscopy(s-SHGS), nonlinear surface absorption processes are traced by measuring the intensity of photons emitted at doubled frequency. In a centrosymmetric medium, second harmonic generation is dipole forbidden, but this does not hold at the surface where symmetry is always broken. The signal is therefore intrinsically surface-specific. Moreover, this process is enhanced when in resonance with electronic or vibronic transitions of clusters and cluster-adsorbate complexes. By tuning the wavelength of an incident laser beam and recording the frequency-doubled signal, information on the electronic structure of the clusters can be obtained. s-SHGS is a nonlinear method.

Although s-CRDS and s-SHGS are strong surface spectroscopic techniques on their own, their combination is particularly successful in revealing the optical properties of supported metal clusters and adsorbates[2, 3].

  • Second Harmonic Generation Optical Rotatory Dispersion (SHG-ORD) is the nonlinear analogue of linear optical rotation. Linearly p-polarized fundamental light is used as an excitation source while the polarization of the detected s-SHG light is measured. The major advantage of this background free technique is the combination of the high surface sensitivity of s-SHG and the chiral sensitive effect of optical rotation. Furthermore, compared to linear processes, chiroptical effects are highly enhanced in the nonlinear processes[4]

  • Second Harmonic Generation Circular Dichroism (SHG-CD) is the nonlinear analogue of the more common circular dichroism. In this technique the s-SHG of left circularly polarized (LCP) and right circularly polarized light (RCP) are compared for adsorbed chiral molecules. The nonlinear enhancement of chiroptical effects is the main advantage of SHG-CD over its linear counterpart.

 

M. Sc. Alexander von Weber

References

[1] A. Kartouzian, M. Thämer, T. Soini, et al., J. Appl. Phys., 104,2008
[2] M. Thämer, A. Kartouzian, P. Heister, et al., JPC C, 116,2012
[3] M. Thämer, A. Kartouzian, P. Heister, et al., Small, 10,2014
[4] P. Heister, T. Lünskens, M. Thämer, A. Kartouzian, et al., PCCP, 16,2014

Project Funding

  • Deutsche Forschungsgemeinschaft (DFG)
  • European Research Council (ERC) - Advanced Grant