quantum transitions deep inside atoms
After an electron is knocked out of an inner shell of an atomic electron system, a “replacement electron” from an external shell undergoes a transition to fill the vacancy. Processes of this type, where the electron gets closer to the nucleus by some billionths of a centimetre, may be instrumental in the development of handy x-ray lasers. The time it takes for the transition to be completed is of key importance. It can now be directly measured with the attosecond transient recorder
, by creating the vacancy instantly with an attosecond xuv/x-ray pulse (illustrated with blue ray) and capturing the emission of the “second ejected electron” (secondary electron) with the controlled electric field of a few-cycle laser pulse, which creates a streak image of the time history of emission with attosecond resolution. Because this emission lasts as long as the inner-shell electron transition under scrutiny, the measurement of its time history reveals how the electron “jumps” from an external to an internal orbit in real time. From the attosecond transient recorder measurement this jump is reconstructed in slow-motion replay, see Fig. 1. Making this inner-atomic process accessible to human perception requires a dilatation of time by ~ 1,000,000,000,000,000. The first real-time observation of electron motion deep in the interior of atoms was highlighted among the most important advances in science in 2002 by the Editors of Science and Nature magazines.
, by creating the vacancy instantly with an attosecond xuv/x-ray pulse (illustrated with blue ray) and capturing the emission of the “second ejected electron” (secondary electron) with the controlled electric field of a few-cycle laser pulse, which creates a streak image of the time history of emission with attosecond resolution. Because this emission lasts as long as the inner-shell electron transition under scrutiny, the measurement of its time history reveals how the electron “jumps” from an external to an internal orbit in real time. From the attosecond transient recorder measurement this jump is reconstructed in slow-motion replay, see Fig. 1. Making this inner-atomic process accessible to human perception requires a dilatation of time by ~ 1,000,000,000,000,000. The first real-time observation of electron motion deep in the interior of atoms was highlighted among the most important advances in science in 2002 by the Editors of Science and Nature magazines.
More information:
The first look into an atomic inner-shell process has been reported by:
Time-resolved atomic inner-shell spectroscopy, Nature 419, 803 (2002) 
See also:
the commentary of L. F. DiMauro:
Atomic photography, Nature 419, 789 (2002)
the story of Kenneth Chang:
Smile, Electron! Fast 'Camera' Captures Action Around Atom
The New York Times (29 October, 2002)
The topic has also been featured on the cover of several magazines, including Nature and Physics Today and highlighted among the
most important advances in all areas of science in 2002 by the magazines Science and Nature:
Fast moves, The Editors of Science, Science 298, 2299 (2002) The fast show, The Editors of Nature, Nature 420, 737 (2002)
Fig. 1. A series of electron energy spectra acquired with the attosecond transient recorder, as a function of delay between the attosecond xuv pulse knocking an electron from an inner (M-) shell of a krypton atom free and the few-femtosecond, few-cycle near-infrared laser pulse. Because the emission this time lasts longer than the oscillation period of the laser pulse, the streak image of the emitted electrons does not exhibit the oscillatory behaviour displayed in the previous Section rather, coincidence of the secondary-electron emission with the laser field is manifested in an additional peak in the energy distribution of electrons (highlighted in red and blown up by a factor of 20). This distribution can be measured for different time delays of the “photographing” laser pulse with respect to the attosecond pulse starting the process. From the frames recorded at different delays the intra-atomic electron transition can be reconstructed in slow-motion replay. The appearance and disappearance of the laser-induced peak highlighted in red indicate the appearance and disappearance of the inner-shell vacancy in real time. The first real time observation of an electron’s quantum transition deep inside an atom shown here has yielded a "lifetime" of about 8 femtosecond (= 8000 attoseconds) for the M-shell vacancy of krypton before it was filled by an outer electron.
, as a function of delay between the attosecond xuv pulse knocking an electron from an inner (M-) shell of a krypton atom free and the few-femtosecond, few-cycle near-infrared laser pulse. Because the emission this time lasts longer than the oscillation period of the laser pulse, the streak image of the emitted electrons does not exhibit the oscillatory behaviour displayed in the previous Section rather, coincidence of the secondary-electron emission with the laser field is manifested in an additional peak in the energy distribution of electrons (highlighted in red and blown up by a factor of 20). This distribution can be measured for different time delays of the “photographing” laser pulse with respect to the attosecond pulse starting the process. From the frames recorded at different delays the intra-atomic electron transition can be reconstructed in slow-motion replay. The appearance and disappearance of the laser-induced peak highlighted in red indicate the appearance and disappearance of the inner-shell vacancy in real time. The first real time observation of an electron’s quantum transition deep inside an atom shown here has yielded a "lifetime" of about 8 femtosecond (= 8000 attoseconds) for the M-shell vacancy of krypton before it was filled by an outer electron.Fig. 1. A series of electron energy spectra acquired with the attosecond transient recorder, as a function of delay between the attosecond xuv pulse knocking an electron from an inner (M-) shell of a krypton atom free and the few-femtosecond, few-cycle near-infrared laser pulse. Because the emission this time lasts longer than the oscillation period of the laser pulse, the streak image of the emitted electrons does not exhibit the oscillatory behaviour displayed in the previous Section rather, coincidence of the secondary-electron emission with the laser field is manifested in an additional peak in the energy distribution of electrons (highlighted in red and blown up by a factor of 20). This distribution can be measured for different time delays of the “photographing” laser pulse with respect to the attosecond pulse starting the process. From the frames recorded at different delays the intra-atomic electron transition can be reconstructed in slow-motion replay. The appearance and disappearance of the laser-induced peak highlighted in red indicate the appearance and disappearance of the inner-shell vacancy in real time. The first real time observation of an electron’s quantum transition deep inside an atom shown here has yielded a "lifetime" of about 8 femtosecond (= 8000 attoseconds) for the M-shell vacancy of krypton before it was filled by an outer electron.
, as a function of delay between the attosecond xuv pulse knocking an electron from an inner (M-) shell of a krypton atom free and the few-femtosecond, few-cycle near-infrared laser pulse. Because the emission this time lasts longer than the oscillation period of the laser pulse, the streak image of the emitted electrons does not exhibit the oscillatory behaviour displayed in the previous Section rather, coincidence of the secondary-electron emission with the laser field is manifested in an additional peak in the energy distribution of electrons (highlighted in red and blown up by a factor of 20). This distribution can be measured for different time delays of the “photographing” laser pulse with respect to the attosecond pulse starting the process. From the frames recorded at different delays the intra-atomic electron transition can be reconstructed in slow-motion replay. The appearance and disappearance of the laser-induced peak highlighted in red indicate the appearance and disappearance of the inner-shell vacancy in real time. The first real time observation of an electron’s quantum transition deep inside an atom shown here has yielded a "lifetime" of about 8 femtosecond (= 8000 attoseconds) for the M-shell vacancy of krypton before it was filled by an outer electron.