Phase singularities and coherence vortices in linear optical systems

It is demonstrated for a time-invariant linear optical system that there exists a definite connection between the optical vortices (phase singularities of the field amplitude) which appear when it is illuminated by spatially coherent light and the coherence vortices (phase singularities of the field correlation function) which appear when it is illuminated by partially coherent light. Optical vortices are shown to evolve into coherence vortices when the state of coherence of the field is decreased. Examples of the connection are given. Furthermore, the generic behavior of coherence vortices in linear optical systems is described.     

Time-resolved luminescence and singlet oxygen formation under illumination of hypericin in acetone

We present time- and spectral-resolved phosphorescence study of hypericin as well as evolution of singlet oxygen formation and elimination under the illumination of hypericin in acetone solution. The obtained time-resolved hypericin phosphorescence can be satisfactorily fitted by using a two-exponential decay curve. The value of shorter component is about 0.29 μs and is independent of the hypericin concentration in the studied range (2–200 μM). The rise time of singlet oxygen production matches this value perfectly. It confirms that singlet oxygen formation represents the significant channel of hypericin triplet state deactivation under aerobic conditions at room temperature. The total phosphorescence intensity of hypericin is linearly proportional to the hypericin concentration within the studied concentration range, but the singlet oxygen phosphorescence intensity exhibits saturation behavior. This observation is a result of at least two effects: quenching of singlet oxygen by hypericin, as well as quenching of singlet oxygen by singlet oxygen.     

Electrochemical processes in electrospray ionization mass spectrometry

Editorial Comment Last month we presented, as a Special Feature, a set of five articles that constituted a Commentary on the fundamentals and mechanism of electrospray ionization (ESI). These articles produced some lively discussion among the authors on the role of electrochemistry in ESI. Six authors participated in a detailed exchange of views on this topic, the final results of which constitute this month's Special Feature. We particularly hope that younger scientists will find value in this month's Special Feature, not only for the science that it teaches but also what it reveals about the processes by which scientific conclusions are drawn. To a degree, the contributions part the curtains on these processes and show science in action. We sincerely thank the contributors to this discussion. The give and take of intellectual debate is not always easy, and to a remarkable extent this set of authors has maintained good humor and friendships, even when disagreeing strongly on substance. Graham Cooks and Richard Caprioli Copyright 2000 John Wiley & Sons, Ltd.     

Derivatization for electrospray ionization-mass spectrometry. 4. Alkenes and alkynes

A survey of derivatization strategies and prospective derivatization reactions for conversion of simple alkenes and alkynes to 'electrospray-active' species is presented. General synthetic strategies are discussed and illustrative examples of prospective derivatives prepared from model compounds are presented along with their electrospray ionization (ES) mass spectra. The identified derivatives of these neutral, nonpolar analytes are either ionic or are ionizable in solution through Bronsted acid/base chemistry, by Lewis acid/base chemistry, or by chemical or electrochemical electron-transfer chemistry. Once ionized, the derivatives are expected to be amenable to detection by electrospray ionization-mass spectrometry. Derivatives are identified for positive and negative ion analysis of both alkenes and alkynes. Copyright 2000 John Wiley & Sons, Ltd.     

Time-resolved Luminescence and Singlet Oxygen Formation After Illumination of the Hypericin–Low-density Lipoprotein Complex

Time-resolved fluorescence and phosphorescence study of hypericin (Hyp) in complex with low-density lipoproteins (LDL) as well as the evolution of singlet oxygen formation and annihilation after illumination of Hyp/LDL complexes at room temperature are presented in this work. The observed shortening of the fluorescence lifetime of Hyp at high Hyp/LDL molar ratios (>25:1) proves the self-quenching of the excited singlet state of monomeric Hyp at these concentration ratios. The very short lifetime (∼0.5 ns) of Hyp fluorescence at very high Hyp/LDL ratios (>150:1) suggests that at high local Hyp concentration inside LDL molecules fast and ultrafast nonradiative decay processes from excited singlet state of Hyp become more important. Contrary to the lifetime of the singlet excited state, the lifetime (its shorter component) of Hyp phosphorescence is not dependent on Hyp/LDL ratio in the studied concentration range. The amount of singlet oxygen produced as well as the integral intensity of Hyp phosphorescence after illumination of Hyp/LDL complexes resemble the dependence of the concentration of molecules of Hyp in monomeric state on Hyp/LDL until a concentration ratio of 60:1. This fact confirms that only monomeric Hyp is able to produce the excited triplet state of Hyp, which in aerobic conditions leads to singlet oxygen production. The value of singlet oxygen lifetime (∼8 μs) after its formation from the excited triplet state of Hyp in LDL proves that molecules of singlet oxygen remain for a certain period of time inside LDL particles and are not immediately released to the aqueous surrounding. That Hyp exists in the complex with LDL in the monodeprotonated state is also demonstrated.     

Measurement of atmospheric turbulence strength by vortex beam

The behavior of spatial correlation singularities suggests a possible method for measuring atmospheric turbulence strength with a vortex beam. The refractive index structure constant can be obtained by measuring the radius of a ring dislocation of a vortex beam which has passed through atmospheric turbulence. An approximate analytic expression for the radius of a ring dislocation as a function of has been derived, and its accuracy is verified by numerical examples.     

Sources of arbitrary states of coherence that generate completely coherent fields outside the source

A new theorem concerning nonradiating stochastic sources is derived. The theorem is used to describe sources that can have a variety of different states of coherence and yet each of them generating a field that is completely spatially coherent outside the source domain. The bearing of this result on a basic unsolved problem is spectroscopy of partially coherent sources is noted.     

Rotational frequency shifts for electromagnetic fields of arbitrary states of coherence and polarization

The rotational frequency shift is studied for fields of arbitrary states of coherence and polarization. It is shown that the power spectrum of the field in the rotating frame is influenced both by the degree of polarization and the degree of coherence. Examples for some model field classes are given.     

Phase and coherence singularities generated by the interference of partially coherent fields

A condition for the complete destructive interference of partially coherent fields emerging from pinholes in an opaque screen is derived, with the assumption of symmetry in both their geometric positions and coherence properties. We use this condition to theoretically investigate the simultaneous production of phase singularities of the optical field and of the spectral degree of coherence. We find that in cases where the number of point sources is even, a new type of mixed field/correlation singularity is observed.