Magnetic resonance on single molecules in an external magnetic field. The Zeeman effect of a single electron spin and determination of the orientation of individual molecules

The interaction of a single electron spin with an external magnetic field is presented. The broadening and shift in the electron spin resonance lineshape is discussed on the basis of classical ensemble theories. Due to a novel frequency stabilization scheme we were able to perform time consuming investigations at different orientations of the external magnetic field with respect to the molecule. Two different orientations of the molecules in the sample have been found which coincide in their relative orientation with two sites of matrix molecules. The orientational disorder of the molecules belonging to one of these sites is below 8°.