Ultrafast X-ray physics

Time-Resolved ESCA: a Novel Probe for Chemical Dynamics

Electron spectroscopy for chemical analysis (ESCA) is a well established tool for quantitative studies of the composition and the chemical environment of molecular systems. Recent developments in the generation and utilization of ultrashort X-ray pulses now add the dimension of time to this technique and will expand the possibilities of femtochemistry in terms of chemical selectivity, quality of information, and temporal resolution. The properties and capabilities of various X-ray pulse sources are discussed, along with their prospects for dynamical studies. Examples of time-resolved electron spectroscopy are presented in the femtosecond (1 fs = 10−15 s) as well as the attosecond (1 as = 10−18 s) regime, the latter marking the current ultimate limit for the time-resolution in pump-probe experiments.

Reshaping of attosecond x-ray pulses in thin crystals

A method for reshaping and control of the duration of attosecond x-ray pulses in thin crystals is proposed. The finite width of the reflection and transmission curves around the Bragg angle allows one to engineer Fabry-Perot-type filters for the generation of a large variety of attosecond pulse shapes. The method considered here can be used to manipulate attosecond pulses produced by high-harmonic generation and also for shorter wavelengths for attosecond pulses from x-ray free-electron lasers. X-ray pulses with controllable amplitude and phase may find useful applications in the newly emerging area of attosecond time-resolved spectroscopy.

Generation and propagation of a picosecond acoustic pulse at a buried interface: time-resolved x-ray diffraction measurements

We report on the propagation of coherent acoustic wave packets in (001) surface oriented Al0.3Ga0.7As/GaAs heterostructure, generated through localized femtosecond photoexcitation of the GaAs. Transient structural changes in both the substrate and film are measured with picosecond time-resolved x-ray diffraction. The data indicate an elastic response consisting of unipolar compression pulses of a few hundred picosecond duration traveling along [001] and [001] directions that are produced by predominately impulsive stress. The transmission and reflection of the strain pulses are in agreement with an acoustic mismatch model of the heterostructure and free-space interfaces.