The Vulcan Nd : glass laser at the Central Laser Facility is a Petawatt (10 15 W) interaction facility available to the UK and international user community. The facility came online to users in 2002 and considerable experience has been gained operating the Vulcan facility in this mode. The facility is designed to deliver irradiance on target of 10 21 W cm −2 for a wide-ranging experimental programme in fundamental physics and advanced applications. This includes the interaction of super-high-intensity light with matter, fast ignition fusion research, photon induced nuclear reactions, electron and ion acceleration by light waves and the exploration of the exotic world of plasma physics dominated by relativity.

The photodissociation of nitrobenzene and the nitrotoluene isomers at 375 nm, induced by a 90 femtosecond laser, is analyzed and compared with the fragmentation by a 10 nanosecond laser at the same wavelength. The molecular ionization is attributed to a nonresonant multiphoton process, and the observed fragmentation can be explained predominantly by an above ionization mechanism (ladder climbing). The mass spectra of the three nitrotoluene isomers show differences which can be used for analytical purposes. The molecular rearrangement taking place prior to the dissociation is also discussed. For nitrobenzene, it is suggested that most of the dissociation occurs from the nitrobenzene structure rather than that of phenyl nitrite. In the case of o-nitrotoluene, it seems that the hydrogen transfer from the -CH 3 to the NO 2 group (ortho effect) is favored in ionic states, while the rearrangement to a nitrite structure is possible in the excited electronic states.

The photochemistry of nitromethane has been studied extensively for many years. Although it is generally agreed that the principal photodissociative process is cleavage of the C−N bond to yield the methyl radical and nitrogen dioxide, there is some evidence of minor competing dissociation channels. A number of different groups have used lasers of different wavelengths, but the results of these studies vary considerably and no clear picture of the minor dissociative channels has yet emerged. The use of femtosecond (fs) duration laser pulses for photoionization of molecules is currently an area of considerable interest, since the process can lead to the efficient production of intact molecular ions. It was felt that femtosecond laser mass spectrometry (FLMS) could provide added information on the dissociation pathways of nitromethane. Laser pulses of 90 fs time duration at wavelengths of 375 and 750 nm, coupled to a time-of-flight mass spectrometer, have been used in this study, and contrary to photoexcitation using nanosecond (ns) pulses, a large parent ion, 61 (CH3NO2 +), is detected together with strong peaks at m/e = 15 (CH3 +), 30 (NO+), 46 (NO2 +) as well as a number of other minor peaks. This fragmentation pattern can be explained by a predominantly ID (ionization followed by dissociation) route.

Resonance-enhanced multiphoton ionization spectrometry (REMPI), using time-of-flight mass spectrometers and tuned lasers, has proved an important ultra-sensitive analytical technique. Nevertheless, conventional nanosecond REMPI suffers from a number of shortcomings: most importantly, REMPI often fails through rapidly (pre)dissociating states. In the case of thermally labile molecules, which include the nitro-molecules, either no or very small parent or high/mass fragment ion peaks exist, making the interpretation of the mass spectra ambiguous at best and often impossible. Femtosecond laser mass spectrometry (FLMS) can often 'defeat' these dissociative states, resulting in large parent or high-mass fragment ion peaks which make the interpretation less ambiguous. In the present paper, nanosecond and femtosecond multiphoton ionization and fragmentation are compared using time-of-flight mass spectrometry for NOz gas and a number of different nitro-molecules: nitromethane, nitrobenzene, rn-nitrotoluene, dinitrotoluene and trinitrotoluene.

We have used interferometric techniques to show unambiguously that the fragment ion energies produced in the multielectron dissociative ionization (MEDI) of iodine vary with laser pulse rise time. A field ionization, Coulomb explosion model is found to reproduce MEDI data on I 2 remarkably well. The lack of features associated with post-dissociative ionization in the MEDI spectra of Cl 2 is explained in terms of low dissociation rates for the singly (and possibly doubly) charged parent ions. These results remove the need for the much discussed, but highly speculative, process of 'laser-induced stabilization'.

A new concept 1 kHz repetition rate 1 ps time resolved resonance Raman (TR 3 ) apparatus based on optical parametric amplifiers (OPAs) has been developed. The two purpose built OPAs are pumped simultaneously by a titanium sapphire regenerative amplifier. Each OPA provides several microjoules of energy per pulse, is tunable from 470 to 2200 nm and the spectral bandwidth of the probe beam is maintained at about 20 cm −1 across this tuning range by spectrally filtering the light pulse during the OPA process. Further wavelength coverage down to ultraviolet wavelengths is achieved by using frequency mixing of the OPA outputs with the regenerative amplifier. The potential this unique system has for obtaining the vibrational spectra of photoexcited transients is demonstrated by presenting some preliminary results obtained from several TR 3 spectroscopy experiments.

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We report on what is believed to be the first large-aperture and high-energy optical parametric chirped pulse amplification system. The system, based on a three-stage amplifier, shows 25% pump-to-signal conversion efficiency and amplification of the full 70 nm width of the seed spectrum. Pulse compression to 84 fs achieved after amplification indicates a potential of 300 TW pulse power for 35 J amplified pulse energy.