High-order harmonic generation in large molecules and atomic clusters

Summary form only given. The generation of coherent, soft X-ray radiation through the production of high order harmonics in a gas has been well studied. As a result, much of the effort in studying this phenomena has turned to optimizing the harmonics characteristics and yields. One potential approach to enhancing harmonic yields is to use a gas composed of molecules as opposed to a gas of monomer atoms, exploiting a potentially higher polarizability in molecules. Initial studies of harmonic generation with diatomic molecules indicate that the high harmonics behave in a manner similar to harmonics produced in monatomic gases with a similar ionization potential. Recent studies of the soft X-rays produced from the intense irradiation of large atomic clusters (100-10,000 atoms per cluster) suggested that these clusters, which form in the adiabatic expansion of a gas jet into vacuum, may be a unique medium for high-order harmonic generation. We have studied the characteristics of the harmonic generation from these clusters and found that, unlike small molecules, clusters can dramatically affect the behavior of the high-order harmonics. We have also studied the harmonic yields in clusters of a variety of species. Studies of the harmonic yield as a function of gas jet backing pressure suggest that the harmonic yield may be enhanced with the onset of clustering. The position of the harmonic cut-off is also seen to shift to shorter wavelengths with the onset of clustering in the gas. We discuss the differences between monomer and cluster media.