Photodissociation of NO2 in the (2)B-2(2) state: A slice imaging study and reinterpretation of previous results

The photodissociation dynamics of nitrogen dioxide have been probed above the second dissociation limit at photolysis wavelengths close to 226 nm. The O(3PJ)+NO(2) product channel has been examined using direct current slice velocity map imaging of the O(3PJ) and NO(2) fragments. Mass-resolved resonantly enhanced multiphoton ionization spectroscopy and velocity map imaging have been used to probe directly the rovibrational population distributions of the NO fragments. We also examine possible interference from the dissociation of N2O4 by investigating the effect of the sample temperature on the O(3PJ) fragment energy distributions. The O(3PJ)+NO(2) dissociation channel has been found to favor the production of vibrationally cold, highly rotationally excited NO(2) products with all three oxygen spin-orbit components. Other minor dissociation channels which produce O(3PJ) atoms have also been identified. We discuss the significance of these dissociation channels and present a reinterpretation of previous studies of NO2 dissociation on excitation to the (2)2B2 state

Time-dependent photoionization of azulene: Competition between ionization and relaxation in highly excited states

Pump-probe photoionization has been used to map the relaxation processes taking place from highly vibrationally excited levels of the S2 state of azulene, populated directly or via internal conversion from the S4 state. Photoelectron spectra obtained by 1+2’ two-color time-resolved photoelectron imaging are invariant (apart from in intensity) to the pump-probe time delay and to pump wavelength. This reveals a photoionization process which is driven by an unstable electronic state (e.g. doubly excited state) lying below the ionization potential. This state is postulated to be populated by a probe transition from S2 and to rapidly relax via an Auger like process onto highly vibrationally excited Rydberg states. This accounts for the time invariance of the photoelectron spectrum. The intensity of the photoelectron spectrum is proportional to the population in S2. An exponential energy gap law is used to describe the internal conversion rate from S2 to S0. The vibronic coupling strength is found to be larger than 60±5 μeV

Optical biosensor techniques for monitoring organic pollutants in the aquatic environment

The principal contribution of Southampton University to the BIOPTICAS project is in the realization of planar optical waveguide probes to determine the optical properties of attached sensing films. Three types of device are being investigated: surface plasmon resonance (SPR), directional coupler and chemiluminescence sensors. Techniques have been established for the deposition of compatible electrodes for electrochemical modulation of sensing reactions as an integral part of devices, and equipment has been set up for the fabrication of waveguides in glass substrates by field-assisted ion-exchange. The modelling and design stages for the devices are now close to completion, and we have begun the fabrication and evaluation of preliminary designs and verification of models. Interaction with partners has resulted in the establishment of standardised sensor chip formals and plans for comparative evaluations of the sensors developed in the project, using standardised sensing reactions are well in hand

Forces on a Rayleigh particle in the cover region of a planar waveguide

We report on the optimization of a waveguide structure for the maximization of the radiation forces exerted on a Rayleigh particle in the cover region. The two main radiation forces involved are the transverse gradient force which attracts a particle into the waveguide and the combined scattering and dissipative forces which drive the particle forward along the channel. The dependence of these forces on parameters including the incident wavelength, the surrounding medium embedding the particles, and the polarizability of the particles is discussed. Both dielectric and metallic gold spheres of radius 10 nm are considered in the model. Special emphasis is devoted to the maximization of the transverse gradient force due to the optical intensity gradient at the waveguide surface, and the wavelength dependence of the polarizability of gold nanoparticles

Waveguide surface plasmon resonance biosensor for the aqueous environment

We report the fabrication and performance of gold coated waveguide surface plasmon resonance biosensors. Biotin-avidin binding reactions at the sensor surface were observed. The output power of the sensor showed a decrease of 32% on binding a dual layer of biotin-avidin

Integrated optical Mach-Zehnder interferometer as simazine immunoprobe

Immunoassay has become a versatile tool in several fields of analytical chemistry. We describe the characterization and the application of different integrated optical channel waveguide Mach-Zehnder interferometers (MZIs) as label-free immunoprobes. The performance of the classical MZI is compared with that of a modified structure which incorporates a 3x3 coupler. Characterization of the devices demonstrates a dramatic improvement gained by using the 3x3 coupler. Two main advantages are achieved by the modified device. First, the possibility of referencing the output signal allows the elimination of signal fluctuations due to coupling and light-source instabilities. An increase of the signal-to-noise ratio by a factor of up to 10 is achieved. Secondly, the phase shift between the three outputs allows unambiguous detection with optimum sensitivity. For the detection of the herbicide simazine, the functional properties of the transducer surface are optimized by an appropriate chemical modification. Using this improved device, a simazine immunoassay has been carried out with a test midpoint of 0.3 ppb and a detection limit of approximately 0.1 ppb. The excellent performance, established manufacturing techniques and the potential for simplification and parallelization make the device attractive for further development

Study of luminol electrochemiluminescence with a planar optical waveguide for peroxide sensor application

The work presented in this paper is aiming at the development of a highly sensitive, specific, cheap and widely applicable new sensor based on the combination of optical and electrochemical techniques. In addition to the analytically valuable information of light intensity generated, the light transient resulting from a double potential step experiment contains kinetic information for both the electrochemical step as well as for the successive diffusion and chemical steps in the reaction layer. The comparison of transients due to short range waveguide-evanescent field coupling as shown in Fig. 2 and those obtained by measuring light over the full depth of the diffusion layer in Fig. 3 can be used to obtain such information

Quantisation of Monopoles with Non-abelian Magnetic Charge

Magnetic monopoles in Yang-Mills-Higgs theory with a non-abelian unbroken gauge group are classified by holomorphic charges in addition to the topological charges familiar from the abelian case. As a result the moduli spaces of monopoles of given topological charge are stratified according to the holomorphic charges. Here the physical consequences of the stratification are explored in the case where the gauge group SU(3) is broken to U(2). The description due to A. Dancer of the moduli space of charge two monopoles is reviewed and interpreted physically in terms of non-abelian magnetic dipole moments. Semi-classical quantisation leads to dyonic states which are labelled by a magnetic charge and a representation of the subgroup of U(2) which leaves the magnetic charge invariant (centraliser subgroup). A key result of this paper is that these states fall into representations of the semi-direct product U(2) \semidir R^4. The combination rules (Clebsch-Gordan coefficients) of dyonic states can thus be deduced. Electric-magnetic duality properties of the theory are discussed in the light of our results, and supersymmetric dyonic BPS states which fill the SL(2,Z)-orbit of the basic massive W-bosons are found.Comment: 57 pages, harvmac, amssym, two eps figures, minor mistakes and typos corrected, references added; to appear in Nucl. Phys.

Integrated optical directional coupler sensor for pesticide analysis

Integrated optical transducers for the measurement of interactions between biological molecules and the specific detection of chemical and biochemical species are the subject of growing interest. Targeted applications include environmental monitoring, industrial process control and medical diagnostics. Integrated optical devices are capable of delivering the high detection sensitivity achievable through optical techniques in a compact format, and offer the potential for the detection of several analytes simultaneously through the fabrication of multiple transducers on a single chip. Here we describe the use of a new type of integrated optical sensor applied to the detection of low concentrations of the pesticide atrazine in aqueous solution. The transducer is based on a planar waveguide directional coupler structure fabricated by Ag+-Na+ ion-exchange in a low-index glass substrate. This sensor has the advantage of differential outputs, which gives improved signal-to-noise characteristics and offers the potential for the simultaneous measurement of the real and imaginary parts of the refractive indices of bulk or thin-film analytes

Nonlinear atom optics and bright gap soliton generation in finite optical lattices

We theoretically investigate the transmission dynamics of coherent matter wave pulses across finite optical lattices in both the linear and the nonlinear regimes. The shape and the intensity of the transmitted pulse are found to strongly depend on the parameters of the incident pulse, in particular its velocity and density: a clear physical picture for the main features observed in the numerical simulations is given in terms of the atomic band dispersion in the periodic potential of the optical lattice. Signatures of nonlinear effects due the atom-atom interaction are discussed in detail, such as atom optical limiting and atom optical bistability. For positive scattering lengths, matter waves propagating close to the top of the valence band are shown to be subject to modulational instability. A new scheme for the experimental generation of narrow bright gap solitons from a wide Bose-Einstein condensate is proposed: the modulational instability is seeded in a controlled way starting from the strongly modulated density profile of a standing matter wave and the solitonic nature of the generated pulses is checked from their shape and their collisional properties