Abstracts Book - IMRC 2018

• SF4-O004 Invited Talk
PROBING THE STRUCTURES OF GAS-PHASE BORON CLUSTERS
USING SIZE SELECTIVE IR SPECTROSCOPY
Andre Fielicke 1
1 Fritz Haber Institute of the Max Planck Society, Molecular Physics, Germany.
Pure boron clusters are predicted to exhibit a wide range of structural motives
planar structures, double layered rings to cages. So far, most detailed insights
for their structures comes anion photoelectron spectroscopy in combination
with quantum chemical calculations.[1]
Infrared spectroscopy provides an alternative sensible probe of the molecular
structure, however, classical absorption spectroscopies are difficult to be
applied to gas-phase clusters that are typically produced in a wide size
distribution at low number densities. Therefore, in the last years techniques
have been developed that couple high sensitivity with size selectivity by
detecting the absorption of IR photons via changes in mass spectra. For cationic
boron clusters IR spectra are obtained by photodissociation of cold messenger
complexes with Kr in a cryogenic ion trap mass spectrometer.[2] By comparison
with spectra obtained density functional theory planar structure can be
unambiguously assigned to B 39 + , B 40 + , and B 41 + . While these isomers appear to
be not the global minima at low temperature, their presence can be understood
by a cluster growth at finite temperature.
IR spectra of neutral boron clusters have been obtained in the past via an IR-UV
two color ionization scheme. However, this technique is limited by the
availability of UV laser sources that need to closely match the ionization energy
of the clusters to be studied. Using an F 2 laser (hν=7.9 eV) IR spectra only of B 11 ,
B 16 , and B 17 have been obtained.[3] We are now combing a 4-wave mixing VUV
generation setup that covers the 6-9 eV range with the cluster mass
spectrometer to cover a wider mass range and in particular to study clusters
around n=40 where the presence of stable boron cages has been suggested.[4]
[1] L.-S. Wang, Int. Rev. Phys. Chem. 35 (2016) 69.
[2] M.R. Fagiani, X. Song, P. Petkov, S. Debnath, S. Gewinner, W. Schöllkopf, T.
Heine, A. Fielicke, K.R. Asmis, Angew. Chem. Int. Ed. 56 (2017) 501.
[3] C. Romanescu, D.J. Harding, A. Fielicke, L.-S. Wang, J. Chem. Phys. 137 (2012)
014317.