Experimental Physical Chemistry: Experiments

The following experiments are available in the advanced physico-chemical lab courses.

Spectroscopy (M.Che.1304)

Dipole moment

The polarizabilities and dipole moments of several organic compounds are determined by measuring the macroscopic polarization of diluted solutions. Specifically, dielectric constants and refractive indexes are measured. Dielectric constants are measured indirectly by filling solutions into a capacitor and measuring its charge/discharge time. Refractive indexes are determined by measuring the angle of total reflection with an Abbe refractometer. These quantities are connected to the polarizabilitiy and dipole moment via the Debye and Lorentz-Lorenz equations.

Fourier transform infrared spectroscopy

Rovibrational gas phase spectra recorded with a Bruker Tensor 27 FTIR spectrometer are investigated. The experiment is offered in several variants which involve different substances, e.g. HCl/DCl or Ethanol. Depending on the investigated molecule, the experiment has a somewhat different goal, but all variants include the determination of molecular properties such as vibrational frequencies and rotational constants. In addition to the measurements with the FTIR instrument, all variants are accompanied by a study of a Michelson interferometer.

Raman jet spectroscopy

A custom built Raman jet spectrometer is used to measure rotational and rovibrational spectra of nitrogen and oxygen. From the spectra, molecular properties such as vibrational frequencies and rotational constants are derived. The effect of nuclear spin statistics on the spectra is investigated. By expanding air into a vacuum chamber, a supersonic jet expansion is prepared and the resulting temperature drop in the expanded gas is measured from the intensity pattern of rovibrational lines.

Laser induced luminescence of iodine

Iodine vapor is irradiated with a 532 nm laser to induce fluorescence. The apparent orange glow due to fluorescence is investigated with a grating spectrometer, yielding vibrationally resolved spectra of iodine in the gas phase. From the spectra, molecular properties of the electronic ground state of iodine can be derived, e.g. the harmonic wavenumber and anharmonicity constant of the I-I vibration or the dissociation energy via a Birge-Sponer plot.

UV/vis absorption spectroscopy

The distinct purple colour of iodine vapor is investigated with a UV/vis absorption spectrometer, yielding vibronic spectra of iodine in the gas phase. The spectra allow the determination of properties of an electronically excited state (B 3Π0+u), such as its dissociation energy. The shapes of the (rotationally not resolved) vibrational bands allow a qualitative judgement of the change in the rotational constant upon vibronic excitation.

Tuneable diode laser absorption spectroscopy (in preparation)

The high resolution laser absorption spectrum of water vapor in air will be investigated. High resolution allows to probe the effect of pressure on the lineshape and linewidth as well as the measurement of line strengths (absorption coefficient) of single rovibrational lines. The water amount fraction in air, isotope ratios, and the temperature is also deducible.

Kinetics (M.Che.1305)

Unimolecular reaction of cyclopropane to propene in the gas phase

Unimolecular isomerization reactions in the gas phase are ideal cases for studying the interplay of reaction kinetics and the dynamical chemical equilibrium. In this experiment, consumption of cyclopropane and the formation of propene is measured at two elevated temperatures. The experimental results are analyzed referring to unimolecular rate theories.

Laser induced luminescence

In this experiment, fluorescence and phosphorescence spectra of benzil (diphenylglyoxal) are taken after excitation by a nitrogen laser. The exponential decay profiles are measured at the maximum intensity wavelength and analyzed with respect to their time constants. Important topics for discussion are the mechanism of laser radiation, laser types and the Jablonski diagrams for fluorescence and phosphorescence.

Recombination of nitrogen atoms

This experiment on the recombination of nitrogen atoms involves as the first step the dissociation of nitrogen molecules in a microwave discharge. The following recombination of nitrogen atoms is studied by means of gas phase titration with nitric oxide and chemiluminescence in a flow tube. The collision of N atoms forms a chemically activated complex N2, which must be stabilized by a quick follow up collision with a third body to take away the excess energy. This is a classical reaction kinetics experiment. The derivation of the equation to extract the rate constant from the measured data involves the treatment of many competing effects.

Flash photolysis of iodine

The flash photolysis technique is the historical and in many commercial lasers the technical „precursor“ of laser photolysis. In the flash photolysis of iodine the third body effect on the stabilization of the recombination product is studied. To this end the rate constant is measured photometrically as a function of pressure. The iodine concentration-time profiles are displayed and digitally stored in an oscilloscope, which is up to now the state of the art technique.

Time resolved polarimetry

This experiment is a modern version of the experiments which mark the beginning of reaction kinetics experiments in the 19th century. The progress of the acid catalyzed sucrose inversion reaction is traced by polarimetry. This is an elegant technique to study the kinetics of suitable reaction systems.

Surface Science and Vacuum Techniques (M.Che.1308)

Scanning Probe Microscopy

Scanning force microscopy (SFM) and scanning tunneling microscopy (STM) are powerful methods to scrutinize surfaces. Not only atomic structures, but also information about sample parameters such as magnetism, piezo-electricity or stiffness can be examined. In several experiments we gain insight into this scanning technique. Starting with the optimization of the scanning parameters for scanning force microscopy, we show the difference between tapping-mode and contact measurements on different samples. For electrically conducting samples, the tunneling microscope is often the first choice to image the surface.

High vacuum

The study of solids and gases is often carried out under high vacuum conditions. This prevents bulk material and surfaces from being contaminated by ambient gases and allows samples to be handled at extremely high and low temperatures. Generating and measuring a good vacuum is challenging. In this experiment, we will learn how to use different pumps (turbo pump and backing pump) and the proper sequence of venting. We will determine unknown volumes and calibrate a Pirani vacuum gauge in a classical and efficient way using the Boyle-Mariotte law. Determination of molecular velocities by Knudsen effusion through a capillary tube also provides valuable information on how to design efficient recipients.

Mass spectrometry

At the beginning of the 20th century, it was confusing to recognize that chlorine gas had two mass signals in a mass spectrometer. With the help of such devices, there was the first direct indication of the isotopic property of elements and the indirect detection of the neutron. Commercially available residual gas analyzers (RGA), such as the one we are working with in this experiment, have a quadrupole mass spectrometer built into them. Using such an RGA, we will detect gas species and determine their concentration. We will learn to interpret the spectra of gas mixtures, such as ambient air and breath, isotope mixtures and more complex molecules. The fragmentation of butane isomers in lighter gas is another subject of investigation.

Temperature programmed desorption

The adhesion of gases depends in a complex way on the type of gas, the surface material and its atomic structure. Kinks and steps, which depend on the orientation of the crystal surface, play an important role in the desorption rate. In the experiments performed on gold and silver single crystals, we first have to prepare the surfaces to reveal their atomic arrangement. This is done by ion bombardment and subsequent control of the impurities by Auger spectroscopy. Finally, low-energy electron diffraction (LEED) is used to determine the orientation of the lattice planes on the clean sample. On the well-prepared samples, temperature programmed desorption (TPD) is applied to determine the binding energies and desorption kinetics (i.e., reaction order) of noble gases with different surface coverages. Therefore, a temperature ramp is run to heat the samples. The gas desorption thus occurring is measured with the aid of a residual gas analyzer.