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In some cases there are hidden correlations in a highly fluctuating signal, but these are lost in a conventional averaging procedure. Covariance mapping allows these correlations to be revealed unambiguously. As an example of the applicability of this technique, the dynamics of fragmentation of molecules ionized by an intense picosecond laser are analyzed.

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With the development of high power, subpicosecond lasers, it has become possible to study the process of multielectron dissociative ionization of small molecules at intensities in excess of 1014 W cm-2. At such intensities the conventional multiphoton approach is too complex and the field ionization model becomes more attractive. This article reviews the experimental results obtained to date and indicates how certain aspects of the process, namely the mode of multiple ionization, the thresholds for multiple ionization, the charge symmetry of the fragment ion production and their peaked angular distributions, can be explained in terms of a field ionization, Coulomb explosion model. However, a more sophisticated approach is undoubtedly required to explain the lack of variation of fragment ion kinetic energies with laser risetime, the variation of kinetic energies with laser wavelength and the molecule specificity of the process.

Bond hardening of H 1 2 has been observed in the intensity range of 100 200 TW͞cm 2 using 792 nm laser pulses. This effect can be understood in terms of a light-induced potential well created at twice the normal (free) equilibrium internuclear distance by an adiabatic mixing of 1-and 3-photon resonances. The trapped population dissociates into H 1 and H when the potential well becomes convex on the trailing edge of the pulse. The dynamics of the nuclear wave packet was manipulated by chirping the pulse duration from 45 to 500 fs and observing a reduction of the kinetic energy release from 0.3 to 0.0 eV. This energy shift is interpreted as a dynamic Raman effect within the laser bandwidth. resonances were inferred from rather noisy data in the early 1990's. They were received with great interest, as at that time the stabilization of the molecular bond was a candidate for a universal mechanism explaining the invariance of ion kinetic energies with changes of intensity and pulse duration [5,6]. Later, it was established that this invariance is a signature of rapid, sequential ionization at the critical internuclear distance [7][8][9]. Since then there has been a surprising lack of clearcut confirmation of the bond hardening effect. With more recent work casting doubt on the existence of light-induced bound states [10,11], the idea of bond hardening has become again only a remote theoretical possibility. Against this trend of scepticism, we present an experimental observation of bond hardening in H 1 2 . In these experiments, chirped pulses from a Ti:sapphire laser were amplified to 10 mJ in energy and compressed to about 50 fs duration at a repetition rate of 10 Hz (see [12] for a recent review of high power ultrafast lasers). The pulse bandwidth had an almost perfect Gaussian shape centered at 792 nm and an extent of 22 nm, full width at half maximum (FWHM). The pulse length was varied by scanning the separation of the two compressor gratings and introducing some uncompensated chirp. The linearly polarized beam, 5 mm in diameter, was focused in an ultrahigh vacuum chamber using an f͞4 parabolic mirror to give a peak intensity of the order of 10 14 W͞cm 2 . Hydrogen gas was introduced into the vacuum chamber via simple effusion, raising the ambient pressure to about 10 26 torr. At this pressure and intensity no space charge effects were observed. Following the process of multiphoton ionization of H 2 , an external electric field directed "forward" and "backward" fragment ions into a vertical, 13-cm-long drift tube. Ions were detected by microchannel plates with a 10-mm circular restriction in front to improve the energy and angular resolutions. The ion time-of-flight (TOF) spectrum and pulse energy were recorded at each laser shot by a digital oscilloscope and stored in a computer. Figure 1 shows ion TOF spectra recorded at several pulse lengths. For each grating separation the peak intensity was adjusted to about 150 TW͞cm 2 , i.e., below saturation of any ion channel [13]. At each laser pulse the ion signal and t...