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University of Helsinki
 
Solid State Spectroscopy and Photochemistry Research Group
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Contact address:
 Laboratory of Physical Chemistry,
Department of Chemistry,
University of Helsinki
P.O. BOX 55 (A.I.Virtasen aukio 1),
FIN-00014 University of Helsinki, Finland

Fax: +358-9-19150279

Research topics of the group

 Rare-gas molecules

These studies involve experimental identification and attempts to explain the properties and formation mechanism of species containing rare gases. The UV-photochemical generation of these molecules involve atwo step process: photodissociation and photoinduced charge separation. By using IR spectroscopy it has been found that three-atomic molecular ions XHX+ (X=Ar, Kr, Xe) and their isoelectronic halogen counterparts YHY - (Y=Cl, Br, I) can act as charge traps in the solid state. Relaxation of the ionic system can lead to formation of neutral rare gas containing molecules of HRgY stoichioimetry. Up-to-now, 20 different rare-gas-containing hydrides has been made with little direct evidence of the centrosymmetric cations playing part in the formation of the neutral rare-gas-containing molecules. Instead, experimental evidence exists on the formation of neutral HRgY (Y=electronegative fragment) species from their neutral fragments H+Rg+Y. Most of the new molecules are Xe-containing species like HXeH, HXeOH, HXeCCH but species involving Kr and Ar has also been made. Computational studies on possible He- and Ne-containing species have been made but so far none of these have been experimentally prepared.

The most fascinating member of these novel rare-gas-containing hydrides is the first neutral chemical compound containing argon. The new molecule, HArF, was identified in low-temperature argon environment after photolysis of HF using VUV light and subsequent annealing. We reported the finding of HArF in Nature in August 2000 (Khriachtchev et al. Nature 406 (2000) 874) and it was chosen by the journal as the feature of the week. Up-to-now, the world's first neutral argon-containing molecule has attracted many theoretical and experimental studies. The molecule has been found to undergo an interesting site-conversion depending on the sample preparation method or annealing temperature used in experiments. The molecule has also been acting as an interesting guinea-pig for blue-shifting hydrogen bond former and extensive computational attempts.

Molecular interactions

These studies focus on molecular systems involving hydrogen bonding and van der Waals interactions. The model systems for the interaction studies include for example H2O...N2, H2O...CO, H2O2...CO, H2O2...N2, HCOOH...CO, HCN...H2O, NH3...CO etc etc. Typically the 1:1 complexes are produced photochemically from a suitable precursor and the spectral shifts due to complexation are measured for each complex present.

Theoretical methods are employed to study the structures, vibrational spectra, and electric properties of various complexes. In conjuction with high-level ab initio studies we are starting up with molecular dynamics simulations. Also, the interaction energy components are studied using the method proposed by Morokuma, which has been enlargened to accompany the second-order perturbation calculations standard for weak intermolecular interactions.

Conformational control

We are conducting photochemical studies of small molecular species in order to study their properties both on the ground state and the excited states. On the ground state the main interest is photoinduced rotamerization processes. One fascinating example of photoinduced rotamerization reaction is the characterization of cis-formic acid in solid argon. Up to now, only two relevant studies of cis-formic acid structure through microwave spectroscopy have appeared. we have reported the full vibrational spectrum of cis-formic acid. Other interesting photoinduced rotamerization reactions have been found in acetic acid, HONO, oxalyl fluoride and oxalic acid, for which also site-selective interconversions have been identified in low-temperature matrices.

On the excited states the the interest lies on the photodissociation pathways in solid rare gas matrices. The main questions are what are the photodissociation products, are there any secondary reactions, does the surrounding matrix cage have any effects on the photochemistry and so on.

Gas analysis

Collaboration with industry involves testing and searching for new applications of a gas phase analysis. GASMETTM analyzer is based on a small interferometer for the mid-infrared region. Its multicomponent analysis software is capable of handling 20 compounds simultaneously and the sensitivity is, depending on the optical path length, at best in the ppb region. Special attention is paid to demonstrate the applicability of the low-resolution FTIR-instrument to medical problems, i.e. to breath analysis.

Environmental chemistry

Special attention is payed to photochemistry of hydrogen peroxide and its sulphur analogue. One important finding is the photoproduction of H2O2 from water ground state oxygen-atom complex, a reaction which is of environmental inportance. Basic reactions of H2O2 with small molecules (SO2, NO etc.) are also being conducted.

Another interests of research of atmospheric importance are photochemical processes of weakly bound complexes and atmospheric constituents like HONO and HOSO. These studies are connected also with atomic mobilization in the solid state.

Raman and photoluminescence spectroscopy

We study light-emitting, optical, and structural properties of silicon-rich silicon oxide (SiOx, x<2) and silicon-rich silicon nitride (Si3Ny, y<4) films. Thin films on silica and silicon substrates as well as free-standing films are prepared by molecular beam deposition in Aalto University School of Science and Technology. Annealing above 1100 oC results in formation of silicon nanocrystals (Si-nc) and leads to strong 1.5 eV photoluminescence (PL). The origin of this PL has been debated for many years and typically is explained in terms of quantum confinement effect or of surface states. Our results suggest that Si grains are not the direct light-emitting phase of the annealed SiOx material and that the light-emitting centers could be located in the suboxide areas. The spectral narrowing of PL light recorded from films in the waveguiding scheme was observed and interpreted in terms of delocalization of guided modes near the cut-off condition. In this case, the positions of the PL peaks detected from the sample edge combined with transmission measurements allow to estimate the refractive index and layer thickness. The effects of laser annealing on the structural and optical properties of Si-based materials are of special interest. We are also interested in Raman spectroscopy of various carbon materials, for example, carbon nanotubes.