Polychromatic guide star: feasibility study

International audienceAdaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. It turns out that the sky coverage is disastrously low in particular in the visible wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereinafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return-of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because $APEX 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial sky coverage for the tilt. The only one providing us with a full sky coverage is the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D; program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play. We finally shortly described the effort in Europe to develop the LGS

Polychromatic guide star: feasibility study

International audienceAdaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. It turns out that the sky coverage is disastrously low in particular in the visible wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereinafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return-of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because $APEX 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial sky coverage for the tilt. The only one providing us with a full sky coverage is the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D; program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play. We finally shortly described the effort in Europe to develop the LGS

Investigation of the Competition between Electron and Energy Transfer in the Quenching of Aromatic Ketones in the Triplet State using Picosecond Transient Grating Spectroscopy

The competition between electron transfer (ET) and triplet energy transfer (TT) in the quenching of benzophenone, xanthone, and anthraquinone in the triplet state by molecules with both a sufficiently small oxidation potential and low triplet state was investigated in the picosecond to microsecond time scales. In the longer time scale, the product distribution depends strongly on the relative exergonicity of ET and TT processes, the yield of the lower energy product being at least four times larger than that of the other product. Picosecond transient grating measurements reveal that if TT is more exergonic than ET, the TT product is predominantly formed by two sequential ET reactions, i.e., by spin-allowed back ET within the triplet geminate ion pair formed upon ET quenching. However, if ET is more exergonic than TT, no conversion from the TT product to the ET product could be detected. In this case, the product distribution in the microsecond time scale seems to reflect the competition between the two processes. When both processes are exergonic, ET appeared to be always faster than TT. This is in agreement with the severe orbital overlap requirement for TT via the Dexter exchange mechanism

Picosecond Time Resolved Dispersion Spectroscopy Using the Interference between Population and Thermal Phase Gratings

A new method is presented for measuring directly the dispersion spectrum of short‐lived transients, based on heterodyne transient grating spectroscopy. The light diffracted from a preformed thermal phase grating is used as a local oscillator, in phase with the light diffracted from the population phase grating. Time‐resolved dispersion spectra measured upon photoinduced electron transfer reaction between 9,10‐dicyano‐anthracene and anisole in acetonitrile are presented. These spectra and the corresponding absorption spectra obtained by Kramers‐Kronig transformation are compared with those deduced from the homodyne transient grating spectra. Potential applications of this technique are discussed

Direct Investigation of the Dynamics of Charge Recombination Following the Fluorescence Quenching of 9,10-Dicyanoanthracene by Various Electron Donors in Acetonitrile

The dynamics of the intermediate generated upon diffusional electron transfer (ET) quenching of 9,10-dicyanoanthracene by electron donors of varying oxidation potential in acetonitrile has been investigated using several transient grating techniques. With most of the donor/acceptor pairs studied, the transient grating spectrum cannot be differentiated from those of the free ions. Exciplex fluorescence, with the same lifetime as that of the ion pair, is observed with all donors. To extract from the measured kinetics the rate constant of exciplex dissociation,, and of back ET,, within these exciplexes, three different schemes have been considered. The best agreement is obtained by assuming that charge recombination predominantly takes place within the exciplex. The obtained values are substantially different from the BET rate constants deduced indirectly from the free-ion yields and with a donor-independent rate constant of separation. For each class of donors, exhibits a logarithmic free energy dependence with a slope of about −2 eV-1. Moreover, is not constant but increases continuously with diminishing donor's oxidation potential

Picosecond transient Grating Spectroscopy: the Nature of the Diffracted Spectrum

A ps transient grating setup using white light continuum for probing is presented. Measurements on an aromatic molecule in solution have been carried out with this system. The diffracted spectrum is analyzed using Kogelnik's coupled wave theory. At short time delay after excitation, the diffracted spectrum is strongly dominated by absorption and in this case transient grating spectroscopy is equivalent but more sensitive to transient absorption spectroscopy. If some of the excitation energy is dissipated as heat, the diffracted spectrum is essentially the same as the dispersion spectrum of the transient species at time delays approaching half the acoustic period. The performances of this technique and of transient absorption spectroscopy are compared

PASS-2: quantitative photometric measurements of the polychromatic laser guide star

International audienceWe present results from measurements of the return flux from a polychromatic sodium laser guide star produced in Pierrelatte, France during the PASS-2 experiment. In the experiment, photometry of light at 330, 569, 589, and 589.6 nm emitted by mesospheric sodium under two-color laser excitation (569 and 589 nm) was performed. The variation of oscillator and laser configurations as well as simultaneous measurements of the atmospheric coherence length and the mesospheric sodium density permit a comparison of the results with atomic physics models. Using the results, we can determine the setup that produces the maximum return flux from the polychromatic laser guide star. The knowledge gained will be used to aid the ELP- OA project, which has as its goal the design, testing, and implementation of an adaptive optics system that uses a polychromatic laser guide star for wave front tilt measurements

ELPOA: toward the tilt measurement from a polychromatic laser guide star

International audienceAdaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source, which is located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength of the observation, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. Several papers have addressed the problem of the sky coverage as a function of these parameters (see e.g.: Le Louarn et al). It turns out that the sky coverage is disastrously low in particular in the short (visible) wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (which is not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return- of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because approximately equals 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial or total sky coverage for the tilt, such as the dual adaptive optics concept, the elongation perspective method, or the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D; program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play

ELP-OA: measuring the wavefront tilt without a natural guide star

International audienceWe describe the current status of the ELP-OA project in which we try to demonstrate in practice that it is possible to measure the tilt of a wave front using only a polychromatic laser guide star and no natural guide star. The first phase of ELP-OA, consisting of feasibility experiments, has recently been completed successfully. This paper provides an overview over the results of this first phase and over the continuation of the ELP-OA project