The detection of neutral complex high-mass particles is a key challenge in "quantum interferometry of everything", i.e. in the effort to develop means to prepare quantum states of particles as diverse as peptides, proteins, organic or metal clusters and more.
A soft ionization mechanism is required when matter-wave interferometry shall be combined with subsequent mass spectrometry. Compared to electron-impact, photo-ionization is known to induce much fewer fragments ("=soft-ionization") and to achieve a higher yield, since one can focus more photons per volume than electrons - which repell each other because of their charge.
While many metal and dielectric clusters exhibit ionization potentials below 6 eV, many biomolecules require at least 7.9 eV of photon energy - and most of them even rather energies in the range of 10-15 eV. The wavelength of 157.6 nm, derived from a F2 fluorine excimer laser, is the shortest wavelength that can be obtained from off-the shelf coherent light sources.
Among all amino acids, tryptophan is the only one that can be ionized by such low energy. The other two aromatic amino acids, phenylalanine and tyrosine only respond to 2-photon ionization, at about 260-290 nm. However, it is an open research question up to which mass and complexity biomolecules can still be photo-ionized. For polypeptides, such as gramicidin or amino acid clusters, such as Trp30 a positive answer could already be given.