Showing papers by "Dino A. Jaroszynski published in 2002"

Femtosecond laser time-of-flight mass spectrometry of labile molecular analytes: laser-desorbed nitro-aromatic molecules.

Abstract: Femtosecond laser time-of-flight mass spectra of solid samples of trinitrobenzene (TNB), trinitrotoluene (TNT) and trinitrophenol (TNP) have been recorded. Desorption of the solid samples was enacted by the fourth harmonic output (266 nm) of a 5 ns Nd:YAG laser. Subsequent femtosecond post-ionisation of the plume of neutral molecules was achieved using 800 nm laser pulses of 80 fs duration. Mass spectra have been recorded for desorption laser intensities from 2-6 × 109 W cm-2 with ionisation laser intensities between 2 × 1014 and 6 × 1015 W cm-2. Femtosecond laser ionisation has been shown to be capable of generating precursor and characteristic high-mass fragment ions for labile nitro-aromatic molecules commonly used in high-explosive materials. This feature is critical in the future development of femtosecond laser-based analytical instruments that can be used for complex molecular identification and quantitative analysis of environmentally important labile molecules. Furthermore, a comparison of femtosecond post-ionisation mass spectra with standard 70 eV electron impact data has revealed similarities in the spectra and hence the fragmentation processes.

A High Mass Resolution Study of the Interaction of Aromatic and Nitro-Aromatic Molecules with Intense Laser Fields

Abstract: The technique of femtosecond laser mass spectrometry has been applied to benzene, nitrobenzene, toluene and nitrotoluene using pulses of 80 fs and I ) 800 nm (10 14 - 10 16 Wc m -2 ). The ultrafast laser pulses used were able to largely defeat the dissociation pathways associated with nanosecond ionization and produce a molecular ion for both the aromatics and the two photounstable nitro-aromatics. The high mass resolution (m/¢m ) 800) permitted, for the first time, the observation of various doubly charged species and allowed a study of the effect of the substituent NO2 group on the multiple ionization process. It was found that the femtosecond laser irradiation of benzene and toluene enabled the production of a doubly charged cation envelope in each case along with an additional doubly ionized contribution from certain lower mass fragments. Doubly ionized species were also observed for the nitro-aromatics including, most notably the loss of NO2 doubly charged ion ([M-NO2] 2+ ) although a doubly charged parent was not observed. In addition, an NO2 + ion was detected for both nitro-aromatics which was thought to be evidence of a “charge-separation” process involving a transient doubly charged molecular ion.

Coherent radiation sources based on laser plasma accelerators

Abstract: Laser‐driven plasma wakefield accelerators (LWFAs) based on table‐top terawatt lasers have the potential of producing high brightness ultra‐short electron bunches that are ideal for driving free‐electron lasers (FELs). These sources are excellent candidates for reaching the x‐ray spectral region. However, the creation of a compact radiation source based on this technology requires a number of difficult challenges to be met. Currently, LWFAs produce beams with excellent transverse emittance but very large energy spectra. To meet the requirement that the fractional energy spread should be less than the universal FEL gain parameter, ρ, the electron bunch injected into the accelerator must occupy a small region of phase space. We will discuss a new project that has recently been set up in the UK to develop LWFA technology and apply to the creation of a compact FEL. To meet the stringent injection requirements, 10 MeV ultra‐shot injection electron bunches, with durations a fraction of the plasma wake period, will be produced in a photoinjector. A fully ionized hydrogen filled capillary, with plasma densities up to 1019 cm−3, will have a dual function of acting as a preformed plasma waveguide for guiding the laser pulse while providing the medium for the LWFA. Table‐top terawatt Ti:sapphire lasers will be utilized as drive lasers. As a demonstration of the utility of the compact accelerator, electron bunches from the LWFA will be used to create coherent electromagnetic radiation in a FEL. Progress on the development of the plasma capillary channel and diagnostic systems based on terahertz time domain spectroscopic techniques are presented.

Free electron maser amplifier energy recovery experiments

Abstract: We present results from a reversed guide magnetic field Raman free electron maser (FEM) amplifier experiment at Strathclyde University. The FEM amplifier is designed for a near grazing intersection of the beam line and waveguide mode to maximise the frequency tuning range. The tuneability is further extended by adjustment of beam voltage and undulator field strength. The FEM amplifier has shown a gain of over 35 dB over a 30% frequency band. The frequency range of operation can be extended to 65% of centre frequency by adjustment of the cathode voltage. Saturated gain of 23 dB with an output power of 1 MW and efficiency of 5% has been measured. An experiment is currently under way to increase the overall efficiency of the device using a two stage depressed collector energy recovery system.

Electromagnetically induced transparency for two intense waves in plasma

Abstract: The coupled propagation of two electromagnetic waves in plasma is studied in order to find the conditions for induced transparency. This means unattenuated propagation of the waves through plasma which is overdense, and thus opaque, for one (or both) of them. This is made possible by a modulation of the refractive properties of the plasma, due to a relativistic increase in the electron mass, or to a variation in electron density caused by longitudinal plasma oscillations driven by the ponderomotive potential associated with the beat of the waves. Starting from a relativistic fluid description, we make an Ansatz containing two transverse monochromatic electromagnetic plane waves, and longitudinal plasma oscillations at the sum and difference of their frequencies. For weakly relativistic intensities we derive coupled dispersion relations, which take into account the polarization of the waves and the nonlinearities with respect to both their amplitudes. This serves to explore the conditions for induced transparency and the modes of propagation.

Collective Rayleigh scattering from dielectric particles

Abstract: An investigation is taking place into a new classical scattering phenomenon called 'collective Rayleigh scattering'. A collection of dielectric particles pumped by a laser radiation field may form a strong density grating on the scale of the radiation wavelength. The particles then coherently scatter the incident radiation. Current theoretical research is confined to collective Rayleigh scattering from particles small compared with the radiation wavelength, for which there are many possible applications in the field of nonlinear optics. However, by considering larger Mie particles, it can be seen that there are also potential applications in the areas of optical particle characterization and discrimination. This paper outlines the theoretical framework of CRS and the first observations from preliminary experiments utilizing a standing-wave gradient force trap.

Writing refractive index gratings in perspex and polymer optical fibres using femto-second laser irradiation

Abstract: Periodic refractive index structures have been written in un-doped PMMA using multiple pulses of 40fs duration from a 1 kHz Ti:sapphire femtosecond laser operating at the fundamental (800nm). A refractive index change (Δn) of 5±0.5×10–4 was observed before the onset of striations.Periodic refractive index structures have been written in un-doped PMMA using multiple pulses of 40fs duration from a 1 kHz Ti:sapphire femtosecond laser operating at the fundamental (800nm). A refractive index change (Δn) of 5±0.5×10–4 was observed before the onset of striations.

Spontaneous density grating formation in suspensions of dielectric nanoparticles

Abstract: Experimental evidence for nonlinear optical behaviour due to the spontaneous formation of wavelength-scale density modulations or gratings in suspensions of dielectric particles is presented. A collection of dielectric par- ticles pumped by a coherent radiation ®eld may simultaneously form a density grating on the scale of the radiation wavelength and a coherently backscattered radiation ®eld. The particle density grating is generated as a result of a periodic ponderomotive potential formed by the interference of the pump and back- scattered ®elds. The experiment used a water suspension of latex microspheres (radiuso56nm) pumped by a green CW laser (532nm, power 45W). A theoretical model of collective scattering of light from dielectric particles has been extended to include the eA ects of viscous and Brownian forces on the particles. This model predicts a small degree of particle bunching from which coherent backscattering of the pump occurs. The results of the theoretical model compare favourably with the experimental evidence. The relation between the results presented here and the phenomenon of Collective Rayleigh Scattering (CRS) is discussed.