.
\nObservations of the ionization of argon atoms under extremely
\n bright and energetic illumination could prove a boon to research
\n.
\nUnder extremely intense illumination materials may exhibit so-called
\n nonlinear optical properties such as ceasing to absorb light beyond
\na certain brightness, or becoming highly ionized. Yasumasa Hikosaka,
\nMitsuru Nagasono and colleagues at RIKEN and several other
\nJapanese research institutes have now described the details of
\nthis ionization process by using very short bursts of bright laser
\nlight. Their finding is relevant to a broad range of pure and applied
\n research, including x-ray imaging of biological molecules, ultrafast
\n optical switches, fusion and astrophysics.
\n.
\nThe researchers focused on the behavior of argon atoms, which is
\neasy to handle and well-characterized, under illumination by laser
\nlight about one hundred trillion times brighter than the noonday
\nsun, and containing about seven times more energy per photon
\nthan the bluest light visible to the human eye. Previous work by
\nother researchers showed that such intense, energetic light removes
\nmultiple electrons from target atoms, resulting in highly charged
\nions. While the mechanism of the ionization process was partially
\nunderstood from observations of the yields and momenta of these
\nions, important details were missing.
\n.
\nHikosaka, Nagasono and colleagues chose to observe the electrons
\nemitted during the ionization process (Fig. 1), instead of the ions
\nthemselves. Not only do these electrons carry unique information
\nabout the ionization process, but they can be measured after each
\n ultra-short laser pulse. Since the laser spectrum and power are
\n constantly fluctuating, the fine details of the ionization process
\n are averaged or \u2018smeared\u2019 during a continuous measurement.
\nA shot-by-shot measurement, however, can account for laser
\nfluctuations.
\n.
\nThe experiment showed that the dominant ionization pathway of
\nthe argon atoms has two steps: first, a single laser photon is absorbed
\nto create singly-ionized argon, and then two more photons are
\nabsorbed to create doubly-ionized argon. The researchers also
\nfound that the intermediate argon ion states had energy levels,
\nor energy resonances, that induced this pathway.
\n.
\nThe research leverages the recent development of free electron
\nlasers, which are uniquely capable of producing very bright, energetic
\nand short pulses of radiation. The work also illustrates that energy
\nresonances are key to multi-photon, multiple ionization processes,
\na finding that is likely to be relevant to a variety of research programs.
\nHikosaka says that the research team will continue to focus on the
\n basic science, as well as applications: \u201cOur goal is to develop and
\n leverage a deep understanding of the mechanism and dynamics
\n of non-linear processes in order to manipulate or control these
\nprocesses and their final products.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"
. Observations of the ionization of argon atoms under extremely bright and energetic illumination could prove a boon to research . Under extremely intense illumination materials may exhibit so-called nonlinear optical properties such as ceasing to absorb light beyond a certain brightness, or becoming highly ionized. Yasumasa Hikosaka, Mitsuru Nagasono and colleagues at RIKEN and … <\/p>\n