Key-Words: Instable Molecules, Molecular Structure, Microwave Spectroscopy, Rotational Spectroscopy, Vibrational Spectroscopy, Ketenes, Cyclopentadienone, Cyclohexadienone, Cyclopentenone, Iodine Azide, Thioformaldehyde, Fluorocarbone, Glyoxal, Microwave-optical double resonance, Two photon spectroscopy, Birefringence
Spectroscopy of instable compounds. One group is concerned with the production and the spectroscopic identification of instable molecules which cannot exist in condensed phases but only in the highly diluted gas phase.They are produced in a flow system under high vacuum conditions immediately at the front end of the absorption cell of a spectrograph by pyrolysis, discharge, or chemical reaction. Since these procedures do not allow to maintain stationary concentrations time averaging methods have to be used. Rotational spectroscopy is especially suited for averaging and leads to extensive information about molecules like structure, dipole moment, nuclear quadrupole coupling constants and potential barriers. So far some ketenes with electronegative substituents have been investigated, also the barely known cyclopentadienone. Comparable studies have been carried out with the cyclohexadienone system, because this molecule is tautomeric to phenol and converts very rapidly. Since FT-IR spectroscopy is a repetitive scanning method too, it can also be used provided sufficient ground state data is available for assigning a rotation-vibration spectrum. Such measurements have been carried out with the high resolution spectrograph IFS 120 HR at Gießen with some ketenes and are presently analysed. To obtain the isotopomers of cyclopentadienone the isotopes have to be introduced at the stage of cyclopentenone. Therefore we also measured and assigned the rotational spectra of six isotopomers of the latter. In addition the rotational spectrum of the highly explosive iodine azide is presently under investigation.
Microwave optical Double Resonance (MODR). The method combines the excellent sensitivity of fluorescence detection with the fortunate small line width of pure rotational transitions. Thus, rovibrational levels of excited electronic as well as ground states can be investigated in high resolution. Using a dye laser we have observed the Zeeman effects of various MODR signals leading to electronic properties of excited states and to the localization of highly excited states of the ground state of the H2CS molecule. Furthermore, the field strengths of so-called avoided crossings have been measured and analysed resulting in the determination of hitherto unknown triplet state rovibrational levels. HCF has also been investigated by MODR. This molecule lives approximately 1 ms and could barely be detected by conventional rotational spectroscopy.
Doppler free two-photon Zeeman Spectroscopy in atoms. This method has been used to measure excited state atomic susceptibilities for the first time. The Na atom was investigated so far in a superconducting magnet. Our approach avoids the normally dominating highly field sensitive first-order spin and orbit effects, and suggests itself for measuring ultra large magnetic fields.
Field induced birefringence in gases and vapours. Combining these methods with spectroscopical ones we have determined the polarizability tensor of the ethene molecule. In addition, a semi-empirical model based on the addition of tensorial increments has been developed for estimating electric and magnetic tensor anisotropies.
Project Funding:
DFG, HBFG, Fonds der Chemischen Industrie, state funds
Project Partners:
Prof. Dr. T. M. Klapötke, Department of Chemistry, The University,
Glasgow G12 8QQ
Microwave spectrum and structure of iodine azide
Prof. Dr. Z. Meic, Ruder Boskovic Institute, 10001 Zagreb
cis-trans-potential of glyoxal
Dr. D. A. Ramsay, National Research Council of Canada, Ottawa,
K1A OR6
Analysis of avoided crossings in MODR-Zeeman-effekts of thioformaldehyde
Doctorate Degrees:
1993 Reißenauer, Bernhard:
Analysis of molar Cotton-Mouton- and Kerr-constants of gaseous
alkanes by semiempirical calculations of susceptibilities
1994 Hutter, Wolfgang:
The structure of the cyclohexadienylidene-system
1994 Frank, Thomas:
High resolution Fourier-transform infrared spectroscopy with ketenes
1994 Weber, Thomas:
Laser spectroscopic characterization of rovibronic states in the
molecules formaldehyde und thio-
formaldehyde
1995 Majer, Werner:
The rotational Zeeman effect in the mm-wave range
Awards:
1994 Hüttner, Wolfgang:
Merckle Forschungspreis 1994, 14.11.1994
Visiting Scientists:
Dr. Donald A. Ramsay, Canada, 6 months
Prof. Dr. Zlatko Meic, Kroatia, 2 months
Key-Words: analytical chemistry, frequency standards, time-of-flight mass spectrometry, induced dipole moments, diode laser spectroscopy, metal hydrides, molecular beams, sodium-vapor laser, REMPI-TOF-spectroscopy, side band spectroscopy, sub-Doppler resolution, uv-ionization spectroscopy, ZEKE photo-electron spectroscopy
The goal is the precise determination of the physical and chemical properties of free molecules and of molecular clusters using laser-based methods. In addition, new lasers and new laser methods are being developed.
Projects.
1) Pseudo-rotation and vibrationally induced effects in phosphorus pentafluoride and trifluoro ammonia. Sub-Doppler laser spectroscopy and measurements on molecular clusters in a molecular beam using CO2 side-band lasers
2) Pseudo-rotation in phosphorus pentafluoride and other symmetric molecules -
Laser side-band spectroscopy with the goal of developing frequency standards in the infrared
3) Molecular frequency standards using vibronic transitions in the visible and uv.
Visible spectrum of iodine and extension of the iodine frequency standards
4) Side-band double modulation - infrared spectroscopy with a new sub-Doppler method, which enables kinematically induced dipole moments to be measured
5) Laser side-band spectroscopy to measure calibration spectra and to observe splittings in arsine and phosphine
6) Precision measurements on diatomic metal hydrides using infrared diode laser spectroscopy and zero kinetic energy (ZEKE) photo-electron spectroscopy
7) The Investigation of molecular clusters and intermediate products using uv-ionization spectroscopy combined with time-of-flight mass spectrometry (REMPI-TOF-Spectroscopy)
8) The development of an ultra-sensitive, mass-selective form of infrared spectroscopy.
9) Application of REMPI-TOF Spectroscopy to analytical chemistry
10) Investigation of the sodium-vapor/hydrogen laser and the development of new metal vapor lasers
Publications:
Project Funding:
Deutsche Forschungsgemeinschaft, Japan Society for the Promotion of Science
Project Partners:
Dr. G. Fraser, National Institute for Standards and Technology,
Maryland, USA
Sideband spectroscopy of molecular clusters
Prof. Dr. Moret-Bailly, Laboratoire de Physique de l'Université
de Bourgogne, Frankreich
Molecular Stark effect of methane type molecules
Dr Haratoshi Takeo, Director Cluster Science Division, National
Institute for Advanced Interdisciplinary Research, Tsukuba, Japan,.
Investigation of molecular clusters
Dr. G. Taubmann, University of Ulm, Abteilung Theoretische Chemie
Doctorate Degrees:
1993 Bahnmaier, Albert Helmut:
"Entwicklung neuer Lasersysteme: a) IR-REMOI-DR-Spektroskopie,
b) Na-Dampf-Wasserstoff-Laser"
1995 Schmid, Reiner Peter:
"Photo-Ionisationspektroskopische Untersuchungen an Molekülcluster
und Metallatomen"
1995 Essig, Kay Stephan:
"Hochaufgelöste spektroskopische Untersuchungen an kurzlebigen
zweiatomigen Moleküle"
Awards:
1993 Bahnmaier, Albert Helmut:
Doctorate prize of the University of Ulm
Visiting Scholars:
Regular visits by scientists from the Cluster Science Division of NAIR, Tsukuba, Japan as part of an on-going collaboration.
Key-Words: Molecular Structure, Rotational Spectrum, Isotopomers, Regression, Dicyanides
Determination of Molecular Structure from Rotational Spectra. Long-term goal is the development or improvement of methods which permit the very accurate determination of the structure (bond distances and angles) of small to moderately-sized free molecules (< 20 atoms) from their rotational spectra in the microwave region.
The rotational spectrum of a molecule of this type is dominated by the three principal inertial moments of the molecule as a rotating body. The principal inertial moments can be found from the analysis of the rotational spectrum in the vibrational ground state, they are functions of the molecular geometry and the known atomic masses. Molecular isotopomers have (practically) identical geometries but different sets of principal inertial moments due to the different masses of one or more atoms. When the sets of principal inertial moments of a sufficient number of isotopomers of a molecule are collected by the analysis of their spectra, the data can be used to calculate (preferably by regression methods) a quite accurate geometry of this molecule because all atomic masses are known. Nonetheless, this method can usually not take full advantage of the very high precision of microwave spectroscopy, because the inertial moments taken from the spectra are contaminated by a multitude of small but unknown interactions between molecular rotation and vibration, even in the vibrational ground state of the molecule. These interactions could be experimentally determined only then when for each isotopomer of the molecule under investigation the rotational spectra could be measured not only in the vibrational ground state but also in the excited states of all normal vibrations, which is feasible only for very small molecules (max. 4 atoms).
Recent research, also of this group, has attempted to take into account the vibrational perturbations of the three inertial moments of each of the isotopomers by simple models, e.g., by three isotopomer-independent constants. Significant improvements of the structures determined could be obtained as well as better insight into previously favoured evaluation techniques.
Two program packages coded for this purpose were further optimised and made available to groups at Lille, Kiel, and Vancouver. Members of the former Division of Physical Chemistry of the University of Ulm participated in joint work coordinated by the Lille group to compare the molecular structure of some simple dicyanides obtained from the rotational spectra by the above methods with quantum-chemical calculations (J. Mol. Structure, accepted). Cooperation with the Lille group (Prof. Demaison) has started to include iteratively determined weights into the fitting routines of the program packages with practical applications in mind.
The intentions and results of the above research, also of own work, have been presented in invited contributions to a report of advances and an encyclopedia:
Project Funding:
Fonds der Chemie des Verbands der Chemischen Industrie,
CNRS (for French partner)
Project Partner:
Prof. J. Demaison,
Université des Sciences et Techniques de Lille, U.F.R.
de Physique, F-59655 Villeneuve d'Ascq
"Practical Determination of Reliable Structures from Rotational
Spectroscopy"