A tuneable ultra-compact high-power, ultra-short pulsed, bright gamma-ray source based on bremsstrahlung radiation from laser-plasma accelerated electrons Novel measurements of electromagnetic radiation above 10 MeV are presented for ultra intense laser pulse interactions with solids. A bright, highly directional source of ␥ rays was observed directly behind the target. The ␥ rays were produced by bremsstrahlung radiation from energetic electrons generated during the interaction. They were measured using the photoneutron reaction ͓ 63 Cu(␥,n) 62 Cu͔ in copper. The resulting activity was measured by coincidence counting the positron annihilation ␥ rays which were produced from the decay of 62 Cu. New measurements of the bremsstrahlung radiation at 10 19 W cm Ϫ2 are also presented.

In the present Letter we report that a number of polyatomic molecules (M) when irradiated with short pulse lasers <90 fs at 750−790 nm and intensities up to 1015 W cm-2 produce multiply charged parent ions and do not fragment to any great degree. This surprising observation is found in both linear and ring structured molecules and is very similar to the behavior of inert atoms such as xenon under the same irradiation conditions. This is a very different behavior from irradiating with nanosecond pulses at 109 W cm-2 where low-mass fragments dominate the spectrum. For the hydrocarbon molecules presented in this work, there exists an envelope of 2+ ionized peaks, which corresponds to the parent and a number of (M − nH) satellites. This feature is characteristic of these molecules in the intensity region 1014-15 W cm-2 and is interpreted as evidence for tunneling or barrier suppression. Coulomb explosion leading to multiply charged atoms, which is evident for CS2, does not seem to be operating for the larger hydrocarbon molecules.

It is well-known that aromatic molecules, when irradiated by UV laser beams in the nanosecond pulse width regime, yield exclusive parent ions at laser intensities of 107 W cm-2 (soft ionization). As the laser intensities increase up to 109 W cm-2, however, extensive fragmentation takes place such that small mass fragments dominate the spectra at the expense of parent signature ions. The reason for this is that the dissociative lifetimes are shorter than the laser pulse width and ladder-switching fragmentation takes place. With the development of high power femtosecond lasers, these dissociative lifetimes can often be bypassed. Presently, at laser intensities up to the order of 1015 W cm-2 with pulse widths as short as 50 fs at near-infrared (IR) wavelengths (790 nm), soft ionization again takes place. This has been seen using a technique known as femtosecond laser mass spectrometry (FLMS). Under such conditions, stable multiply charged parent and adjacent satellite ions are observed and the fragmentation is minimal. In this paper, these effects are described for the medium-mass aromatic molecules benzene, monodeuterated benzene, toluene, and naphthalene. Other studies have shown that for diatomic and triatomic ions, the absorption of many photons produces transient highly ionized parent species which subsequently fragment on a time scale of femtoseconds, leading to multicharged atomic species. This so-called “Coulomb explosion” model has proved effective in describing the fragmentation of small molecules but seems a less attractive model to explain the results which are presented here, at the above-mentioned beam intensities. In contrast, the polyatomic molecules studied presently using IR FLMS display atomic-like characteristics.