Cluster spectroscopy using high-intensity pulses from vacuum UV free electron lasers

Free electron lasers (FELs), recently developed sources for producing intense femtosecond pulses of vacuum ultraviolet (VUV) radiation, offer new ways of studying the interaction of radiation with matter. In this paper, a first series of experiments on the laser excitation of atomic cluster beams are reviewed, which showed that intense VUV pulses interact with clusters very differently than their optical or near infrared (IR) counterparts do. First experimental results on cluster beam spectroscopy performed with VUV FEL pulses are examined. FEL operation principles and performance parameters are briefly discussed. How intense ultrashort IR and VUV pulses excite clusters is analyzed, interaction parameters between radiation and clusters are discussed, and the formation and heating mechanisms of cluster plasma are explored. What is similar and fundamentally different between the ways IR and VUV radiations excite clusters is pointed out. Other applications of VUV and soft X-ray FEL radiation are reviewed as well.