IR spectroscopy of first-row transition metal clusters and their complexes with simple molecules FELIX facility, Radboud University Nijmegen, the Netherlands Anton Pannekoek Astronomical Institute, University of Amsterdam, the Netherlands Denis M. Kiawi

[ mbar] K [ mbar] K Relative atomic abundance (H=1) in dense molecular cloud Williams & Herbst, Surf. Sci. 500, 823 (2002 Depletion of elemental species in Interstellar Medium

In what form is Fe present in the ISM? Hypothesis: FeS formation Fe → Fe n Fe n + S→ Fe n S m nFe + mS → Fe n S m If true:  Efficient conversion of Fe to FeS Williams & Herbst, Surf. Sci. 500, 823 (2002) Clue: cometary fragments contain FeS grains

Get an insight in interaction between Fe and S on the scale of clusters by studying:  Fe n  Fe n S m  Fe n -SX (S-containing ligands) Goals of my PhD project Possible outcome  Insight in mechanism of FeS formation in ISM  Compare lab results with astronomical observations  Identification of IR spectrum of Fe/FeS species in the ISM Methods:  IR spectroscopy geometric structure  DFT calculations

Why clusters ?  Model systems for Fe particles in ISM  Isolated species in gas phase  Size range: n=3-100  Transition between atomic and bulk properties  Strong size-dependence of properties

TiC Clusters detected in ISM Von Helden et al, Science 288, 14 (2000) Observed spectrum Lab spectrum of TiC

Experimental setup  Laser ablation of Fe rod  Cluster formation in helium carrier gas  Gaseous molecules injected downstream  Interaction with IR & UV laser  Mass analysis in RETOF MS

IR laser source: FELIX Inter-Cavity Spectral range:  m cm -1 Pulse enrgy: up to 100 mJ (5-50  m operational, rest soon!)

Test experiment: Co n + with water Relative ease of production Cobalt clusters Co as analogue for Fe clusters H 2 O as analogue for H 2 S

Felix region Cluster – water vibrations Free water modes IR Spectroscopy of Co + -H 2 O clusters Free water molecule  Asymmetric stretch: 3756 cm-1  Symmetric stretch: 3657 cm-1  Bend: 1595 cm-1 PBE/TZP ADF package S=7 Co + -H 2 O calculated:  Libration mode: 345 cm -1  Twisting mode550 cm -1  Stretching mode475 cm -1

IR spectroscopy of Co n + -H 2 O clusters Clear resonances Water bending vibration  intact Cluster-water vibrations Small shift in bending, other modes do shift

IR spectroscopy of Co n + -H 2 O clusters Wave number (cm-1) Free water bending mode: 1595 cm -1 Extra water attached: hydrogen bonding leads to a red shift of ca. 30 cm -1

Signal-to-noise ratio poorer due to low Fe cluster production Water binds intact on Fe cluster surface Clear shift from Free water bending and stretching mode region IR spectroscopy of Fe n + -H 2 O clusters

Conclusions Astrophysical motivation to study Fe and FeS clusters Laboratory IR spectra of Co n + -H 2 O and Fe n + -H 2 O clusters H 2 O bending mode proves that water is bonded molecularly to the cluster Clear size dependence of cluster-molecule vibrational frequencies Outlook: DFT of Co n + -H 2 O and Fe n + -H 2 O IR spectroscopy of  Fe n -H 2 S  Fe n S m  Fe n

Knickelbein et al, J.Chem.Phys. 93,1533 (1990) Teaser: IR ion dip spectrum of neutral Fe 13 UV laser

Acknowledgement Nijmegen Jos Oomens Joost Bakker + entire FELIX Team University of Amsterdam Rens Waters Wybren-Jan Buma Free University Amsterdam Luuk Visscher $:

Motivation Source:

Outlook: IR Spectroscopy of Al n O m + clusters

IR Spectroscopy of Co-H 2 O clusters Wave number (cm-1)

Laser ablation technique Second pulsed valve Liquid Nitrogen cooling Aperture for metal rod Pulsed valve Initial design by André Fielicke

Origin of Iron hubblesite.org/gallery/album/star/pr b/large_web M< 8M o → no Fe M> 8M o → Fe is formed  Fe once formed, sits in the center of the star

Mass spectrum of Fe +

Selecting mass distribution by deflection TOFMS RMJordan company Deflection plates in flight path mass30 mass 70

Why is FEL is needed for such experiments ?

Why action spectroscopy as detection method ? Gas Phase: Number densities of ions with same charge does not exceed 10 7 molecules/cm 3. Classical absorption spectroscopy For direct absorption experiments number densities ~ molecules/cm 3  Size selective detection of clusters

Theory DFT geometry optimization Evaluate many spin states! Functionals -PBE -TPSS Basis set -TZP Analytical frequency calculations Commercial package ADF Amsterdam Density Functional DFT Calculations Outcome Binding energy of various cluster geometries IR spectra

DFT Calculations Co 13 + Co 9 + Co 4 + Co 6 +

DFT Calculations J. Jalink in preparation

IR spectrum of Fe 13 Depletion quite narrow in UV wavelength Possible mechanism:

Teaser # 2: size-dependent IR spectra of Al n O m + +Formation of dust grains in AGB stars