| People involved: | René Wugt Larsen, Anders Engdahl, Bengt Nelander |
This project is related to the following Fields, Subjects and Techniques:
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The work in the molecular spectroscopy group consists of six different projects. Two of these are carried out at the Chemical Center using a combination of matrix isolation in noble gas matrices and infrared spectroscopy. (see: Setup) Two of the other projects need high resolution spectroscopy and are carried out with the FTIR spectrometer at Max-lab. The remaining projects are for practical reasons carried out at Max-lab.
One of the matrix isolation projects is a study of intermolecular vibrations of hydrogen bonded molecular complexes. The second is an investigation of the interactions between free radicals and stable molecules. At Max-lab, we do high resolution infrared spectroscopy of stable molecules and of weak molecular complexes in collaboration with Flemming Hegelund in Århus. In addition exploratory studies are performed in order to develop new experiments at Max-lab. We have recently started a study of infrared spectra of water aggregates in solid para hydrogen in collaboration with Vilnius University.
Intermolecular vibrations of molecular complexes are difficult to study. Molecular beam studies have been carried out in the low energy end of the spectrum, below 150 cm-1, where tunneling motions between different minima are found. Gas phase studies on equilibrium systems, where one component is water are difficult since the low vapor pressure of water makes it almost impossible to obtain sufficient concentrations of complexes. The matrix isolation method allows one to prepare samples which contain sufficient numbers of complexes for infrared spectroscopic studies. In the mid infrared, where bands are generally sharp there is rarely any difficulty to observe complex formation and assign the observed bands. In the far infrared part of the spectrum, where the intermolecular vibrations are found, bands tend to be broadened by interactions with the phonon band of the matrix. Since the wavelengths of the infrared radiation in this part of the spectrum are comparable to the matrix thickness, it is difficult to establish a good base line. In order to overcome these difficulties we use laser irradiation to modify the complex we are investigating. In a few cases, we can use the 3rd Stokes shifted radiation from a dye laser to irradiate infrared bands of the complex. In these cases, we obtain a complete assignment of the complex, since all its infrared bands change in the same way under irradiation. Unfortunately, the Raman shifting process is highly non-linear with intensity variations of much more than a factor of two between different pulses. The method therefore only works when the lifetime of the modified complex is an order of magnitude longer than the time between pulses (50 ms).
The work during the last few years has concentrated on hydrogen peroxide complexes. We have been able to show that the hydrogen peroxide dimer is cyclic, with two equivalent components [Eng01]. We have also studied the water hydrogen peroxide complex [Eng00], and the ozone hydrogen peroxide complex.
We have developed a very effective synthesis of matrix isolated peroxy radical. We have used this method to study the peroxy radical complexes of water [Nel97], ammonia [Eng99], carbon monoxide[Sve00], nitrogen [Sve00], carbon dioxide [Sve01] sulphur dioxide [Sve01] and hydrogen peroxide (manuscript in preparation). The assignment has been facilitated by the fast photolysis of the complexes, compared to the peroxyradical decomposition. The cage effect often makes the photodissociation of small molecules to a very slow process. The peroxy radical is decomposed to a hydroxyl radical and a singlet D oxygen atom. In the complexes we have studied, the oxygen atom has added to the stable molecule. In some cases, we have evidence for the formation of hydroxyl radical complexes.
Photodecomposition of the carbon monoxide peroxy radical complex produced the hydroxyl radical carbon dioxide complex [Sve01]. We have also recently been able to investigate the water hydroxyl radical complex, which we form by irradiating a molecular hydrogen peroxy radical complex [Eng03].
As additional results of these studies, we have identified the HOOO radical water complex and H2O3 [Eng02].
Using a small He-bath cryostat at 2.7 K, we have been able to prepare millimeter thick matrices of solid para-hydrogen. Para-hydrogen has been prepared from normal hydrogen using a iron oxide catalyst at temperatures around 15K. We have studied infrared spectra of the matrices from 10 cm-1 to 5000 cm-1. The water monomer rotates almost freely in these matrices. Residual ortho-hydrogen molecules perturb the rotation of a small subset of the water. We have seen both vibration-rotation transitions and pure rotation transitions of water. We have also been able to identify bands due to the water dimer, trimer and tetramer, both in the mid infrared (intra molecular bands) and in the far infrared (intermolecular bands).
We have measured the HCl libration band of the OC-HCl complex, a manuscript is in preparation. Work is in progress on the hydrogen cyanide dimer and the hydrogen cyanide complexes of ammonia and hydrogen chloride.
High resolution spectra of glyoxal, glyoxal-d2 and 13C-glyoxal have been measured and analyzed. By combining the measured data with vibration rotation interaction constants from ab initio calculations, we have obtained the first gas phase structure of glyoxal (manuscript in preparation).
Test experiments using the storage ring as a source for infrared microscopy have been performed [Joh00].