Ricardo Lopez-Delgado

Focusing all the synchrotron radiation (2π radians) from an electron storage ring on a single point without time distortion

Abstract The geometrical-optical analysis of synchrotron radiation shows that it is possible to define a mirror able to focus all the emitted light by an electron storage ring on a single point lying anywhere in space, keeping unchanged the typical time patterns of the radiation. Because of the electron ultrarelativistic conditions, the electron “orbital path” is assimilated to an “underground bent photon path”. We prove then that the focusing condition is simply given by establishing the invariance of the photon path: “optical photon path” + “underground bent photon path” = constant. Some selected numerical examples show that it is possible to collect huge amounts of radiation producing extremely high excitation densities in a given small volume.

Lifetime and time-resolved fluorescence measurements with aco synchrotron radiation☆

Abstract For the first time storage ring synchrotron radiation has been used as a repetitive source for time spectroscopy. Very fast fluorescence decays (shorter than 1 nsec) have been measured with an accuracy of the order of 10%, by the Single Photon Counting technique. Also fast protolytic reactions occuring in the excited state of 2-naphthol and fluorescein have been investigated by the Delayed Coincidence technique (time-resolved fluorescence). Storage ring synchrotron radiation characteristics are compared with those of the nanosecond flash lamps, conventionally used for this kind of measurements.

Time-resolved excitation spectra of xenon vapor in the 1500 Å region

Abstract Time-integrated and time-resolved excitation spectra, as well as fluorescence lifetimes, have been measured for xenon vapor as a funtion of pressure (for pure xenon as well as with different collision partners: krypton and helium), monitoring the 1700 A second continuum afterglow. Three very different decay components have been observed: (a) A short component with lifetimes of the order of 2 ns, which is attributed to emission from the upper vibrational levels of the O + u ( 1 Σ + u ) state of Xe* 2 . (b) A long component with τ ≈ 60 ns, corresponding to emission from thermally relaxed vibrational levels of the 1 u ,O − u ( 3 Σ + u ) states of Xe* 2 . No intervention of any Xe atomic excited species is invoked to explain the excitation and deactivation processes of Xe 2 molecules for these two components of the fluorescence. (c) A very long component with lifetimes in the 150–500 ns range, having a broad − 1000 cm −1 - excitation band, centered at ≈ 1470 A and showing a striking screening and self-absorption effect as well as a strong effect when the partial pressure of a collision partner (Kr, He) is increased. The mechanism of this last excitation is not yet very well understood.

Comments on the application of synchrotron radiation to time-resolved spectrofluorometry

Abstract Since the first experiments on lifetime measurements and time-resolved spectroscopy using Synchrotron Radiation (SR) were performed during the fall of 1972 with ACO (the Orsay electron-positron collision ring in those times - now an electron storage ring, SR fully dedicated) by A. Tramer and myself, much progress has been accomplished in the field, mainly on moving the excitation wavelength towards the VUV spectral region and also on the understanding of optico-geometrical, timing and statistical properties of SR. In the present paper these properties are commented in view of the spectrofluorometric experiment. Classical methods to measure photon-time correlation functions are briefly reviewed in order to show how SR can be advantageously coupled with all of them as a “standard excitation function λ-invariant”. The work is finished by making some guess for a near future.

On xenon molecule excited state lifetimes

Abstract Time-resolved excitation spectra of xenon vapor in the 150 nm region are analysed in terms of four main fluorescence lifetimes corresponding to decays of four stable excited electronic states of the Xe dimer. The two shortest decay times, ≈ 2 ns and ≈ 60 ns, are assigned to the direct radiative relaxation of the two lowest excited ungerade states, ( 1 Σ + u )0 + u and ( 3 Σ + u ))1 u respectively. The two longest decay times, ≈ 150 ns and ≈ 500 ns, must correspond to the overall depopulation rates of the two lowest excited gerade states, ( 3 Σ + g )1g and ( 1 Σ + g )0 + g , decaying into the gerade ground state by cascading down through the intermediate ungerade states.