Molecular interferometry and nanostructuring

Abstract

Matter wave interference experiments are an excellent tool for the examination of quantum mechanics. The matter-wave duality, complementarity and uncertainty are only some of the features of quantum mechanics which can be investigated by performing such experiments. A review on matter wave interference experiments, their applications and the fundamental questions which they raise can be found in [7].<br />Talbot-Lau interferometry of hot, massive and internally complex molecules is a suitable tool to study the transition from the quantum regime to the classical. Changing the molecule itself (and therefore its mass, its symmetry or other properties), or changing parameters like the temperature [108] or the external pressure [46, 47], can give a close look at this transition. New experimental techniques must be developed in order to do interference experiments with objects even more complex and massive.<br />This diploma work focuses on a new detection scheme for the Talbot-Lau interferometer, where molecular interference patterns are directly recorded onto a surface. This surface can then be imaged and analyzed using various microscopic techniques, in this work especially scanning tunnelling and Fluorescence microscopy. This scheme has several advantages. In earlier experiments [13, 14, 79, 105] the molecules had to be ionized for detection, but surface probe methods allow to image the neutral particles themselves. Also, instead of probing the density distribution of the molecular beam behind the diffraction grating, every molecule is individually localized on the detection surface. The larger the interfering objects, the easier this should be. Thus, this method is intrinsically scalable to higher molecular masses. Another advantage of the surface detection scheme is the high achievable detection efficiency. With our microscopic techniques, we can detect each molecule of interest with a probability close to one, provided the sticking coefficient of the detection surface is close to one. Finally, because a Talbot-Lau interferometer makes it possible to deposit a pattern of molecules onto the detection surface with a period that is only a fraction of the period of the diffraction grating [28], it may also be used as a lithographic tool for patterns with periods below 50nm.<br />Within this thesis, various experimental tools have been developed to combine Talbot-Lau matter wave interferometry with surface microscopy. As the alignment of the interferometer is crucial to the success of the experiments, an intense effort was required to find an alignment procedure capable of providing the required precision. The distance between the two gratings and the distance between the diffraction grating and the surface can now be controlled to a precision of a few um. The relative pitch and yaw of the two gratings can be aligned to below 200 urad, and the alignment of these angles can be veried in situ. Before every experiment it can therefore be checked, whether the alignment has changed or not, as for example could happen during the baking procedure, which is necessary to obtain genuine UHV conditions.<br />The vacuum chamber has been built to guarantee a base pressure below 5x10^(-10) mbar in the deposition chamber during the interference experiment.<br />For the scanning tunneling microscope (STM) we implemented a reliable method for preparing sharp scanning tips and a process that now allows us to routinely obtain reconstructed Si (111) 7x7 surfaces over large areas, as required for the surface immobilization of the molecules. Given the large number of deposited single particles, the assessment of molecular deposits can be rather complex. In reply to this challenge a software was developed to characterize the rich STM recordings.<br />We also tested various potential optical detection methods, which shall finally lead to an in situ, real-time detection scheme for deposited molecular nanopatterns.