Determination of the bond length and binding energy of the helium dimer bydiffraction from a transmission grating

A molecular beam consisting of small helium clusters is diffracted from a 1 00 nm period transmission grating. The relative dimer intensities have been measured out to the 7th order and are used to determine the reduction of t he effective slit width resulting from the finite size of the dimer. From a theoretical analysis of the data which also takes into account the van der Waals interaction with the grating bars, the bond length (mean internuclea r distance) and the binding energy are found to be [r] = 52 +/- 4 Angstrom and [E-b] = 1 1 + 0.3/ - 0.2 mK.