Laser Spectroscopy
Finished Projects

Semiconductor elements such as silicon, as well as its heavier relative tin, have played and will continue to play an important role within the systems investigated here. This is justified by the variety of industrial applications based on their optical properties. Since the size and composition of the clusters has a decisive influence on the development of many intrinsic physico-chemical quantities, photodissociation spectra of charged and neutral silicon and tin clusters were studied and compared with semi-empirical as well as quantum chemical simulations. Such advanced methods have also been applied for magnetically-doped metals such as cobalt-doped silver clusters.

Prior to the installation of the VUV laser, a monochromator flash lamp setup was used in ionization spectroscopy to determine the ionization potentials of atoms and clusters. This setup has the advantage of being both very compact, inexpensive to purchase, and also capable of scanning a wide continuous wavelength range (550-150nm).

Fig. 1: Measured photoionization curves (black dots) and their fit (red). The blue curves represent the theoretically calculated curves of the adjacent geometries.
Fig. 1: Measured photoionization curves (black dots) and their fit (red). The blue curves represent the theoretically calculated curves of the adjacent geometries.

The measured photoionization curves show very good agreement with the theoretically calculated curves. It is also evident that the reduction of the bandwidth of the monochromator at Sn10 results in a steeper slope of the curve (note scaling). It can be concluded that the steepness of the curve is still limited by the experimental setup and not by the vibrational excitation of the clusters.

In addition, for the Sn12 cluster, another slope could be found after the first ionization potential (inflection point of the curve). This suggests another isomer could be present in the experiment. This theory is supported by the fact that quantum chemical calculations also predict a second isomer with 0.3 eV higher IP.

Literature

A. Macion, R. Schäfer, Rev. Sci. Instrum. 2023, 94, 063101

Small metal clusters also represent interesting research objects which can be examined for their absorption behaviour within this experiment. In addition to the structural discrimination by comparison with results of time-dependent quantum chemistry, this routine is also suitable for elucidating energetically-competing spin states in the case of magnetically-relevant systems. Moreover, due to their special magnetic position, these systems in particular exhibit unexpected phenomena that also affect their optical properties.

Using the example of Ag1-13Co+ clusters, it has been shown that most species favor unpaired electronic configurations in the ground state and show intense absorption bands at about 4 eV, which are mainly due to Co 3d → Ag 5s charge-transfer transitions. In the Ag10Co+ cluster an increasing hybridization between itinerant Ag and strongly localized Co states is observed (cf. Fig. 1), which is related to an effective quenching of the unpaired spin at the impurity position and results in the formation of a singlet state. This leads to a fundamental similarity to the Kondo effect in clusters known from solid-state physics, which is still an exciting and much-discussed field of physics in the context of the theory of magnetic moment formation.

Fig. 1: Hartree-Fock-projected electronic states, partial and total density of states (DOS) and molecular (red/blue) and natural transition orbitals (black/white) of the Ag10Co+ cluster in the singlet configuration. The intense absorption band at 4.57 eV is due to a Co 3d → Ag 5s charge-transfer transition which shows similarity to the HOMO-LUMO orbitals. The hybridization of these orbitals, indicated by the red arrow, is one of the reasons for the formation of the quenched singlet state.
Fig. 1: Hartree-Fock-projected electronic states, partial and total density of states (DOS) and molecular (red/blue) and natural transition orbitals (black/white) of the Ag10Co+ cluster in the singlet configuration. The intense absorption band at 4.57 eV is due to a Co 3d → Ag 5s charge-transfer transition which shows similarity to the HOMO-LUMO orbitals. The hybridization of these orbitals, indicated by the red arrow, is one of the reasons for the formation of the quenched singlet state.

Literature

A. Lehr, Masterthesis, Technische Universität Darmstadt, 2018.

E. Janssens, S. Neukermans, H. M. T. Nguyen, M. T. Nguyen, P. Lievens, Phys. Rev. Lett. 2005, 94, 113401.

L. Kouwenhoven, L. Glazman, Revival of the Kondo Effect, Physics World 2001.

Recently, photoabsorption spectra of isolated cationic silicon clusters Si6-100+ in the range of 1.9 to 5.4 eV were recorded and examined with respect to their optical properties. The project aimed at bridging the gap between small clusters and the well-studied regime of nanoparticles and solids. The effective optical and fundamental band gaps were investigated and structural transitions as well as quantum confinement effects could be demonstrated by comparison with solid-state properties predicted by Mie theory. Furthermore, for the first time the influence of the dissociation threshold was critically considered in the interpretation.

Fig. 1: Absorption spectra of the Si+6-100 silicon clusters also indicating the integrated fractions of the absorption cross section relative to all Si 4n valence electrons.
Fig. 1: Absorption spectra of the Si+6-100 silicon clusters also indicating the integrated fractions of the absorption cross section relative to all Si 4n valence electrons.

A measure for the absorption strength in the range up to 5.4 eV can be obtained by the percentage of the absorption cross section relative to all 4n valence electrons of the silicon cluster, if integrated over the spectra shown in Fig. 1. The spectra of all cluster species also show a blue shift of the fundamental band gap compared to the indirect semiconductor silicon, which is due to a stronger direct character of the absorption bands and is consistent with observations on silicon nanoparticles. The optical band gaps were used to verify dissociation thresholds obtained from collision-induced experiments.

Based on the fundamental band gap, an earlier increase in the absorption cross section for medium-sized silicon clusters can also be observed. Already with the help of Mie theory this could be explained by the existence of prolate instead of molecular and spherical geometries (cf. Fig. 2), which is also supported by a large number of experiments and quantum chemical calculations. The transition to the indirectly semiconducting solid is indicated by the comparison with theoretical data for nanoparticles.

Fig. 2: Comparison of the fundamental band gaps of the clusters obtained in the experiment with theoretical data for nanoparticles and the indirect semiconductor silicon.
Fig. 2: Comparison of the fundamental band gaps of the clusters obtained in the experiment with theoretical data for nanoparticles and the indirect semiconductor silicon.

Literature

A. Lehr, M. Jäger, R. Schäfer, J. Phys. Chem. C 2020, 124, 1070-1076.

M. F. Jarrold, E. C. Honea, J. Phys. Chem. 1991, 95, 9181-9185.

V. Kecevski, O. Eriksson, J. Rusz, Phys. Rev. B 2013, 87, 245401.

K. A. Jackson, M. Horoi, I. Chaudhuri, T. Frauenheim, A. A. Shvartsburg, Phys. Rev. Lett. 2004, 93, 013401.

K.-M. Ho, A. A. Shvartsburg, B. Pan, Z.-Y. Lu, C.-Z. Wang, J. G. Wacker, J. L. Fye, M. F. Jarrold, Nature 1998, 392, 582-585.

Small tin clusters show similar growth patterns to silicon clusters, exhibit equally interesting and controllable optical properties and have a much more favorable material value in comparison. Since it can be easily vaporized and the clusters have an ionization energy attainable with a Xe flash lamp or also an F2 excimer laser, neutral species could be examined for the first time in the present photodissociation experiment with regard to their absorption behavior.

More information following soon…