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.
