Measuring H/sub 2//sup +/ vibrational wavepackets with sub-femtosecond precision

Summary form only given. A major effort is underway throughout the world to develop single attosecond optical pulses or trains of attosecond pulses based on the physics of high harmonic generation. During high harmonic generation, an electron is removed from the atom by a strong laser field. The electric field causes it to move a large distance from the atom, only to drive it back when the laser field reverses its direction. High-harmonics are produced during the electron-ion collision that can follow. The duration of the electron-ion collision, usually referred to as "re-collision", largely determines the duration of the attosecond photon pulse. The electron wave packet can also be used to probe the parent ion. Although only one electron can be involved, we adopt the language of electron beams to draw attention to the potential applications of re-collision electrons. We characterize the unusually large current density and the very brief duration seen by the ion with one-femtosecond precision. To do this, we ionize H/sub 2/ molecules, forming simultaneously two wave packets. One is a vibrational wave packet in H/sub 2//sup +/ and the other is the electron wave packet that we wish to study. By forming them together we can use the initial motion of the vibrational wave packet as a very fast clock (1/2-vibrational period /spl sim/ 7.5 fs) against which we measure the electron wave packet motion. We observe the re-collision, and measure the current density, by observing the kinetic energy of the protons released by the fragmentation of H/sub 2//sup +/ upon double ionization or collisional excitation. The kinetic energy resolution determines the time resolution of the "clock" to be /spl sim/ 1 fs.