Two-colour time-resolved spectroscopy of helium using high-order harmonics

The radiative lifetime of the IsZp'P state of helium is measured in a two-colour ionization experiment with a 5% accuracy. The state is excited by the 13th harmonic of a tunable 80 ps laser and ionized by a synchronous uItraviolet laser, with a variable time delay. This experiment demonstrates that the high harmonics generated in a jet of me gas exposed to an intense laser field provide a tunable short-pulse xw s o w e ideally suited for pumplpmbe type of sNdies and, in pxticular, short lifetime measurements Pump/probe techniques using visible or ultraviolet lasers have become an essential tool in atomic, molecular and solid-state physics. One of the most spectacular applications is the so-called 'femtochemistry', pioneered by Zewail and co-workers (1988). where femtosecond laser pulses are used to investigate a photodissociation process in real time. The extension of these techniques to processes where one of the photons, the pump or the probe is in the extreme ultraviolet (Xuv) region presents several difficulties. Traditional laser-based techniques, such as multi-step excitation (Baumert et al 1991) or frequency mixing (Cromwell et al 1992, Eikema el al 1993) are limited in spectral range (typically above 60 nm) and/or time resolution. Synchrotron radiation light sources (Nenner et d 1994). continuously tunable in the xuv, can be synchronized with pulsed lasers, as shown recently by Lacoursihre et ul (1994), but not easily, and moreover are limited in temporal resolution to a few tens of picoseconds at best. A domain where pumplprobe time-resolved methods with the pump in the xuv range can be very useful in atomic specboscopy for the accurate measurements of the radiative lifetimes of high-lying selectively-excited atomic or ionic states. Time-resolved fluorescence techniques (Svanberg 1991, &me et ol 1994) cannot be employed easily in the XW range due to the poor collection efficiency. However, by ionizing the excited atom or ions with a second short-pulse excitation delayed in time and detecting the ions produced, a high collection efficiency as well as a high temporal resolution can be obtained. In the present work, we show that the high harmonics emitted when an intense laser is focused into a rare gas can be an ideal xw source for pump/probe types of experiment, and in particular for the measurement of short (subnanosecond) lifetimes. High-order harmonics 8 Resent address: The Blackett Labaratory. Imperial College. Prince Consort Road. London SW7 ZBZ, UK. 0953-(075/95/030053+06$19.50