Experimental investigation of ultrafast internal conversion in aniline and 1,4-diazabicyclo[2.2.2]octane (DABCO)

Pump-probe time-resolved photoelectron spectroscopy is employed to investigate ultrafast non-adiabatic dynamics in aniline, deuterated aniline (aniline-D7) and 1,4- diazabicyclo[2.2.2]octane (DABCO). Aniline molecules are photoexcited over a wide range of wavelengths between 269 and 236 nm. Our observations reveal direct population of the S2(π3s/πσ∗) state. The photoelectron energy and angular distributions obtained in our experiments show an interesting bifurcation of the Rydberg population to two non-radiative decay channels. One of these involves ultrafast relaxation from the Rydberg component of the S2(π3s/πσ∗) state to the S1(ππ∗) state, from which the population relaxes back to the electronic ground state on a much longer timescale. The other channel appears to involve motion along the πσ∗dissociative potential energy surface. At higher excitation energies, the dominant excitation is to the S3(ππ∗) state, which undergoes extremely efficient electronic relaxation back to the ground state. Aniline-D7 is photoexcited with 260 nm wavelength light. The photodynamics of aniline-D7 is similar to those observed in aniline. Comparison of the dynamics of aniline and aniline-D7confirm that the relaxation of the π3s component of the S2(π3s/πσ∗) state to the S1(ππ∗) state is a main relaxation pathway. Photodynamics of DABCO molecules were also studied over a wide range of wavelengths between 251 and 234 nm. The ultrafast internal conversion in DABCO between the S2 3px,y(+) and S1 3s(+) Rydberg states follows a biexponential decay. It was found that initial randomly oriented molecules are partially aligned after absorption of linearly polarised light and, therefore, DABCO exhibits a preferential direction of excitation

Influence of main beam parameters stability on beam size measurements at PITZ

The photo injector test facility in Zeuthen (PITZ) is used to develop and characterize electron sources which produce a nominal bunch charge of 1 nC with the lowest possible transverse emittance. Measurements of the beam size provide us with knowledge on important beam characteristics. For example the measurement of the emittance is based on beam size measurements. Therefore, the control of uncertainties of beam size measurements is very important to validate the experimental results. The statistical uncertainty of transverse beam size measurements due to the beam energy and bunch charge jitters is considered in this paper.Стенд випробувань фотоінжекторів PITZ використовується для розробки та вивчення характеристик джерел електронів, що випромінюють пучки з номінальним зарядом в 1 нКл та якомога меншим емітансом. Одним з базових вимірів є вимір поперечного розміру електронного пучка. Наприклад вимірювання емітансу базується на вимірах розміру пучка. Таким чином врахування невизначеності вимірів розміру пучка є важливим для визначення вірогідності експериментальних результатів. В даній роботі розглянута статистична невизначеність виміру поперечного розміру електронного пучка в PITZ.Стенд испытания фотоинжекторов PITZ служит для разработки и характеризации источников электронов, которые способны производить пучки с номинальным зарядом в 1 нКл и малым поперечным эмиттансом. Одним из базисных измерений при характеризации фотоинжектора является измерение поперечного размера электронного пучка. В частности, измерение поперечного эмиттанса основано на измерениях размера пучка. Знание погрешности в измерении размера пучка является очень важным условием для определения достоверности экспериментальных результатов. В данной работе рассмотрена статистическая неопределенность в измерении поперечного размера электронного пучка, вызванная флуктуациями энергии и заряда пучка

Wide-Field Multiphoton Imaging Through Scattering Media Without Correction

Funding: This work is supported by the UK Engineering and Physical Sciences Research Council for funding through grants EP/P030017/1 and EP/M000869/1, and has received funding from the European Union’s Horizon 2020 Programme through the project Advanced BiomEdical OPTICAL Imaging and Data Analysis (BE-OPTICAL) under grant agreement no. 675512, The Cunningham Trust and The RS MacDonald Charitable Trust. KD acknowledges the financial support of Elizabeth Killick and Susan Gurney.Optical approaches to fluorescent, spectroscopic, and morphological imaging have made exceptional advances in the last decade. Super-resolution imaging and wide-field multiphoton imaging are now underpinning major advances across the biomedical sciences. While the advances have been startling, the key unmet challenge to date in all forms of optical imaging is to penetrate deeper. A number of schemes implement aberration correction or the use of complex photonics to address this need. In contrast, we approach this challenge by implementing a scheme that requires no a priori information about the medium nor its properties. Exploiting temporal focusing and single-pixel detection in our innovative scheme, we obtain wide-field two-photon images through various turbid media including a scattering phantom and tissue reaching a depth of up to seven scattering mean free path lengths. Our results show that it competes favorably with standard point-scanning two-photon imaging, with up to a fivefold improvement in signal-to-background ratio while showing significantly lower photobleaching.Publisher PDFPeer reviewe

The SCAPA LWFA beamline

The Scottish Centre for the Application of Plasma based Accelerators situated at the University of Strathclyde in Glasgow, UK, is coming online. It comprises three radiation shielded concrete bunkers housing a total of seven beamlines and interaction chambers, each driven by one of a pair of high power Ti sapphire laser systems a 350 TW and a 40 TW

Characterisation of Laser Wakefield Acceleration Efficiency with Octave Spanning Near-IR Spectrum Measurements

We report on high efficiency energy transfer in a GeV-class laser wakefield accelerator. Both the transfer of energy from the laser to the plasma wakefield, and from the plasma to the accelerated electron beam were diagnosed experimentally by simultaneous measurement of the deceleration of laser photons and the accelerated electrons as a function of acceleration length. The extraction efficiency, which we define as the ratio of the energy gained by the electron beam to the energy lost by the self-guided laser mode, was maximised at $27\pm2$ % by tuning of the plasma density, plasma length and incident laser pulse compression. At higher densities, the laser was observed to fully redshift over an entire octave, from 800~nm to 1600~nm.Comment: 7 pages, 5 figure

Characterisation of a laser plasma accelerator x-ray source size using a Kirkpatrick-Baez microscope

Laser plasma accelerators are highly versatile and are sources of both radiation and particle beams, with unique properties. The Scottish Centre for Application based Plasma Accelerators (SCAPA) 40 TW and 350 TW laser at the University of Strathclyde has been used to produce both soft and hard x-rays using a laser wakefield accelerator (LWFA). The inherent characteristics of these femtosecond duration pulsed x-rays make them ideal for probing matter and ultrafast imaging applications. To support the development of applications of laser plasma accelerators at the SCAPA facility an adjustable Kirkpatrick-Baez x-ray microscope has been designed to focus 50 eV - 10 KeV x-rays. It is now possible to produce high quality at silicon wafers substrates that can be used for x-ray optics. Platinum-coated (40 nm) silicon wafers have been used in the KB instrument to image the LWFA x-ray source. We simulate the source distribution as part of an investigation to determine the x-ray source size and therefore its transverse coherence and ultimately the peak brilliance. The OASYS SHAODOW-OUI raytracing and wave propagation code has been used to simulate the imaging setup and determine instrument resolution

Laser-wakefield accelerators for high-resolution X-ray imaging of complex microstructures

Laser-wakefield accelerators (LWFAs) are high acceleration-gradient plasma-based particle accelerators capable of producing ultra-relativistic electron beams. Within the strong focusing fields of the wakefield, accelerated electrons undergo betatron oscillations, emitting a bright pulse of X-rays with a micrometer-scale source size that may be used for imaging applications. Non-destructive X-ray phase contrast imaging and tomography of heterogeneous materials can provide insight into their processing, structure, and performance. To demonstrate the imaging capability of X-rays from an LWFA, we have examined an irregular eutectic in the aluminum-silicon (Al-Si) system. The lamellar spacing of the Al-Si eutectic microstructure is on the order of a few micrometers, thus requiring high spatial resolution. We present comparisons between the sharpness and spatial resolution in phase contrast images of this eutectic alloy obtained via X-ray phase contrast imaging at the Swiss Light Source (SLS) synchrotron and X-ray projection microscopy via an LWFA source. An upper bound on the resolving power of 2.7 ± 0.3 µm of the LWFA source in this experiment was measured. These results indicate that betatron X-rays from LWFA can provide an alternative to conventional synchrotron sources for high resolution imaging of eutectics and, more broadly, complex microstructures

Generation of electron high energy beams with a ring-like structure by a dual stage laser wakefield accelerator

The laser wake-field accelerator (LWFA) traditionally produces high brightness, quasi-monoenergetic electron beams with Gaussian-like spatial and angular distributions. In the present work we investigate the generation of ultra-relativistic beams with ring-like structures in the blowout regime of the LWFA using a dual stage accelerator. A density down-ramp triggers injection after the first stage and is used to produce ring-like electron spectra in the 300 – 600 MeV energy range. These well defined, annular beams are observed simultaneously with the on-axis, high energy electron beams, with a divergence of a few milliradians. The rings have quasi-monoenergetic energy spectra with an RMS spread estimated to be less than 5%. Particle-in-cell simulations confirm that off-axis injection provides the electrons with the initial transverse momentum necessary to undertake distinct betatron oscillations within the plasma bubble during their acceleration process