and is centered around a peak corresponding to a substantially lower molecular weight moreover as is apparent from fig2 the effect is obtainable even with pre fractionated lubricant it was found that use of lubricant vapor rather than lubricant liquid by vaporizing lubricant liquid results in an in situ and on demand lubricant fractionation method thereby obviating the need for a separate pre fractionation process alternatively a pre fractionated lubricant can be employed with a somewhat different vapor phase molecular weight distribution comparing fig1 and 2 in addition to use with polymeric lubricant materials as hereinabove described the present invention enjoys utility with a variety of other types of lubricant materials including but not limited to monomeric oils and high vapor pressure solids an embodiment of the present invention is illustrated in fig3 and comprises an apparatus for vaporizing a lubricant and forming the vaporized lubricant into a collimated molecular beam for impinging on at least a selected portion of a magnetic recording medium such as a magnetic disc as illustrated apparatus 10 comprises an in line series of evacuable chambers in fluid communication through orifices formed in vertically extending walls which partition the apparatus into the various chambers source chamber 1 contains an amount of liquid lubricant 2 therein in equilibrium with lubricant vapor 3 a vertically extending wall portion 4 of source chamber 1 is formed with a first collimating.