Proton-enhanced nuclear induction spectroscopy (PENIS), also sometimes called Cross Polarisation (CP), is a nuclear magnetic resonance technique invented by Michael Gibby and Alexander Pines while they were graduate students in the lab of Professor John S. Waugh at the Massachusetts Institute of Technology. It was one of the first of Pines' experiments transferring spin orientation from one atomic nucleus to another, which has been one of the running themes throughout his career as a leading pioneer in the applications of NMR to the study of non-liquid samples. The PENIS technique was patented in 1972.
In the PENIS technique, the natural polarization of an abundant spin (1H, the "proton" which begins the name of the technique) is exploited to increase the polarization of a rare spin (such as 13C) by irradiating the sample with radio waves at the frequency which corresponds to the difference between the rotation frequencies of the two different spins.
Besides its utility for boosting signals from dilute spins, transferring spin-polarization can also be used by surface-scientists to selectively enhance the spin-polarization of molecules on a sample's surface over the spins in the bulk by transferring spin-polarization from a gas to the surface.
Due to the suggestive nature of its acronym (PENIS), the proposed name for this pulse sequence was never widely accepted. This method is now more commonly known as cross-polarization and is an integral part of most solid-state NMR experiments involving spin-1/2 nuclei.