We demonstrate a technique that uses high-order harmonic generation in molecules to probe nuclear dynamics and structural rearrangement on a subfemtosecond time scale. The chirped nature of the electron wavepacket produced by laser ionization in a strong field gives rise to a similar chirp in the photons emitted upon electron-ion recombination. Use of this chirp in the emitted light allows information about nuclear dynamics to be gained with 100-attosecond temporal resolution, from excitation by an 8-femtosecond pulse, in a single laser shot. Measurements on molecular hydrogen and deuterium agreed well with calculations of ultrafast nuclear dynamics in the H2+ molecule, confirming the validity of the method. We then measured harmonic spectra from CH4 and CD4 to demonstrate a few-femtosecond time scale for the onset of proton rearrangement in methane upon ionization

No abstract

We have measured the energy absorption efficiency of high intensity, picosecond laser pulses in low density gases composed of large atomic clusters. We find that, though the average density of the resulting plasmas is low, the energy absorption can be very high ͑.95%͒, indicating that substantial laser energy is deposited per particle in the plasma. Ion energy measurements confirm that this efficient energy deposition results in plasmas with very high (multi-keV) ion temperatures.[S0031-9007(97)03016-0] PACS numbers: 52.50. Jm, 33.80.Eh, The production of high temperature plasmas with small scale, short pulse, high intensity lasers has been actively pursued during the last ten years. Of particular interest in these studies is the measurement of the energy absorption efficiency of high density plasmas created by intense irradiation of a solid target, and many groups have published work measuring solid target plasma absorption efficiency over a wide range of incident intensities and laser wavelengths on planar [1-5] and microstructured targets [6]. These studies have shown that the plasma typically absorbs a large fraction of the laser energy, between 10% and 80% of the incident energy, depending upon intensity and laser wavelength. Such experiments have shown that a large amount of energy can be deposited per unit area and that high temperatures ͑.100 eV͒ are achievable [1,7]. However, rapid heat conduction into the cold, solid substrate beneath the plasma will typically clamp the plasma temperature to a value of ,1000 eV [7-9]. Furthermore, most of the deposited energy is contained within the plasma electrons, which cool too rapidly by conduction and hydrodynamic expansion to transfer much of this laser energy to the cold ions.A gas of large atomic clusters ͑.1000 atoms͞cluster͒ presents a radically different environment for laser-plasma interaction dynamics [10,11]. In general, low density gases are expected to exhibit very low absorption efficiency ͑,1%͒ and the plasmas produced by intense irradiation will generally be quite cold (10-100 eV). The presence of clusters in a gas changes this situation dramatically [11]. Though the average density of a gas containing clusters is low, the local density within the cluster is near solid, and, consequently, will be subject to the rapid heating experienced by a solid target due to collisional inverse bremsstrahlung. Very bright x rays have been observed from gas target plasmas produced by intense femtosecond illumination of clusters [10], indicating that electron temperatures in these plasmas were quite high, far in excess of those expected from a gas composed only of single atoms [11]. These observations suggested that the clusters were very efficient in absorbing laser energy. In this Letter, we report the first laser energy absorption measurements of intense laser pulses in gaseous media containing atomic clusters. We find that cluster gases are at least as efficient as solid targets in absorbing short pulse laser energy, and that, furthermore, much of this energy is...

Exploiting the energetic interaction of intense femtosecond laser pulses with deuterium clusters, it is possible to create conditions in which nuclear fusion results from explosions of these clusters. We have conducted high-resolution neutron time-of-flight spectroscopy on these plasmas and show that they yield fast bursts of nearly monochromatic fusion neutrons with temporal duration as short as a few hundred picoseconds. Such a short, nearly pointlike source now opens up the unique possibility of using these bright neutron pulses, either as a pump or a probe, to conduct ultrafast studies with neutrons.

This paper provides an up-to-date review of the problems related to the generation, detection and mitigation of strong electromagnetic pulses created in the interaction of high-power, high-energy laser pulses with different types of solid targets. It includes new experimental data obtained independently at several international laboratories. The mechanisms of electromagnetic field generation are analyzed and considered as a function of the intensity and the spectral range of emissions they produce. The major emphasis is put on the GHz frequency domain, which is the most damaging for electronics and may have important applications. The physics of electromagnetic emissions in other spectral domains, in particular THz and MHz, is also discussed. The theoretical models and numerical simulations are compared with the results of experimental measurements, with special attention to the methodology of measurements and complementary diagnostics. Understanding the underlying physical processes is the basis for developing techniques to mitigate the electromagnetic threat and to harness electromagnetic emissions, which may have promising applications.