Diffraction of large molecules

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Our experiment for diffraction of molecules can demonstrate the quantum wave nature of massive and complex molecules. The key components in this experiment are the source, a diffraction element and the detector.

The Source

We need to launch individual molecules in a high vacuum chamber, first. We do that with laser evaporation. For that purpose we focus a laser beam (waist of 1.5 µm) onto a glass plate coated with molecules in the mass range of 500-600 atomic mass units (amu), e.g. Phthalocyanine. The heated molecules evaporate with velocities between 100 m/s and 200 m/s. Such a thermal source emits molecules in all directions. For our diffraction experiments we use two collimation slits to select a tiny molecular ray within an acceptance angle of approximately 10 µrad. This angle needs to be narrower than the quantum diffraction angle at the following grating.

The diffractive element

To verify the delocalized quantum nature of every individual molecule, we have mounted a thin membrane with two (double slit) or several (grating) slits. As soon as there are at least two fundamentally indistinguishable possibilities for a particle to get fly to the same spot on the detector we expect to see a characteristic interference pattern to appear on the screen.

Our gratings have been cut into a 10 nm thick membrane of silicon nitride. The group of Prof. Ori Cheshnovsky at Tel Aviv University used a focused ion beam to cut the slits with a width of 50 nm and a period of 100 nm.

The Detector

At the end of the vacuum chamber the molecules hit a quartz plate where they are immobilized. A red laser beam excites them to emit fluorescence light which is imaged by a microscope objective onto a sensitive CCD-camera. Molecules are much smaller than the wavelength of light. Two nearby particles can therefore not be optically resolved. If, however, they are separated by more than 500 nm or if they arrive with sufficient time lag, we can determine their position to better than 10 nm (!) since the center of a function can be better determined than its width – if only the signal-to-noise ratio is sufficiently high.

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Experimental Challenge: Overview

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