Advanced Physics Practical Course: Experiments at the IOAP in the Winter Semester 2023/24

This modern project experiment P07 was set up stepwise in the period 2007-2010 in the Dopfer group. It offers basic knowledge in many different areas of experimental physics (molecular spectroscopy, optics, laser, resonators, vacuum, atmospheric and environmental physics). The technique of Cavity-Enhanced Absorption Spectroscopy (CEAS) is, along the with the related method of Cavity-Ringdown Spectroscopy (CRDS), the most sensitive approach for sensing trace gases. It relies on resonator-enhanced absorption in a cavity of highly reflective mirrors (R>0.9994), reaching effective absorption lengths of several km. In this experiment, CEAS is employed to measure the multiple forbidden electronic transition of molecular oxygen (O₂) at very high spectral resolution. To this end, we determine fundamental molecular constants of O₂ such as rotational constants and collision cross sections. Hence, this experiment provides deep insight into molecular physics using the example of O₂, the second most abundant molecule in our atmosphere. The measured transition does not only play an important role for the radiation balance of the earth atmosphere but may also be used to detect O₂ (and thus life as we know it) in the atmosphere of exoplanets with telescopes (astrobiology).

Spectroscopy & Molecular Physics

• molecular Hamilton operator (electronic, vibration, rotation, electron spin)
• eigenvalues (energies), matrix elements (intensities), selection rules, term symbols
• LCAO method
• line widths (natural, Doppler, pressure)
• Iodine as secondary wavelength standard
• molecular constants (bond length, rotational constant, collision cross section)
• CEAS and CRD methods

Optics

• resonators and etalon, dielectric mirror, telescope, optical grating
• wavelength calibration using Fabry-Perot interferometer
• polarisation effects and Faraday isolator
• setting up a complex laser beam path, alignment of optical elements

Laser Physics

• principle of a tuneable single-mode diode laser
• resonator modes of a He/Ne laser

Vacuum Physics

• generation and measuring of vacuum

Atmospheric & Climate Physics

• greenhouse effect
• detection of trace and greenhouse gases

Data Analysis

• data treatment (linearisation, background subtraction, curve fitting)
• error analysis, multidimensional fitting

Analysis & Protocol

• data storage and management
• laboratory book, documentation of analysis
• writing a protocol
• detailed error analysis
• critical analysis and evaluation of relevance of results

As part of the 2018 Long Night of Science, a working replica of the first beamline for two-stage imaging with electrons was developed based on Ernst Ruska's model to repeat his groundbreaking 1931 experiment [1]. The replica is now a permanent part of the advanced practical course and provides physics students with a unique approach to electron optics, as a dedicated transmission electron microscope (TEM) is built for imaging and diffraction as part of the FP experiment.

[1] M. Knoll and E. Ruska, Ann. Physik 12, 607 and 641 (1932)

Learning objectives and methods:

• Construction of an own electron microscope
• Generation and measurement of pre- and high vacuum
• Handling of high voltage
• Electron generation using thermal emitters
• Electromagnetic fields as electron lenses
• Imaging and diffraction at the polycrystal in the electron microscope

The experiment is supervised by Frederik Otto from AG Lehmann.

The question "How can a particle interfere with itself?" is always a hot topic among physicists. In this experiment you will not find out the answer, because there is none within the scope of our (limited) horizon of experience; nevertheless, you will experience that electrons sometimes behave as particles, sometimes as waves. Thereby the concept of coherence takes a wide space. In addition, the experiment will introduce you to the basics of off-axis electron holography and give you a taste of the usefulness of the Fourier transform. The experiment will take place on the FEI Titan 80-300 Berlin Holography Special electron microscope, which is one of the best electron microscopes in the world for electron holography.

Learning objectives and methods:

• Basic setup of an electron microscope; to get in the mood and familiarize yourself with the most important basics and operating elements, image gold clusters with atomic resolution.
• Topic of wave-particle dualism and interference as a wave property: observe Fresnel diffraction fringes on the biprism filament and the formation of interference fringes when a positive filament voltage U_F is applied.
• Coherence: observation of interference fringe contrast as a function of source distribution
• Recording of interference pattern as a function of the filament voltage U_F
• Off-axis electron holography on latex spheres and gold clusters, respectively, reconstruction of the electron hologram

The experiment is supervised by Dr. Tore Niermann from AG Lehmann.