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The solar spectrum is a primary reference for the study of physical processes in stars and their variation during activity cycles. In Nov 2010 an experiment with a prototype of a Laser Frequency Comb (LFC) calibration system was performed with the HARPS spectrograph of the 3.6m ESO telescope at La Silla during which high signal-to-noise spectra of the Moon were obtained. We exploit those Echelle spectra to study the optical integrated solar spectrum . The DAOSPEC program is used to measure solar line positions through gaussian fitting in an automatic way. We first apply the LFC solar spectrum to characterize the CCDs of the HARPS spectrograph. The comparison of the LFC and Th-Ar calibrated spectra reveals S-type distortions on each order along the whole spectral range with an amplitude of +/-40 m/s. This confirms the pattern found by Wilken et al. (2010) on a single order and extends the detection of the distortions to the whole analyzed region revealing that the precise shape varies with wavelength. A new data reduction is implemented to deal with CCD pixel inequalities to obtain a wavelength corrected solar spectrum. By using this spectrum we provide a new LFC calibrated solar atlas with 400 line positions in the range of 476-530, and 175 lines in the 534-585 nm range. The new LFC atlas improves the accuracy of individual lines by a significant factor reaching a mean value of about 10 m/s. The LFC--based solar line wavelengths are essentially free of major instrumental effects and provide a reference for absolute solar line positions. We suggest that future LFC observations could be used to trace small radial velocity changes of the whole solar photospheric spectrum in connection with the solar cycle and for direct comparison with the predicted line positions of 3D radiative hydrodynamical models of the solar photosphere., Accept on the 15th of October 2013. 9 pages, 10 figures. ON-lINE data A&A 2013

Fragmentation of H2+ and D2+ in ion beams has been studied with short intense laser pulses (100 fs, I=5x10(13)-1x10(15) W/cm2) and by a high-resolution two-dimensional velocity imaging technique. In the Coulomb explosion channel, at intensities just above the threshold for this process, we observe a clear structure in the kinetic energy spectra not previously found or predicted. The peaks can be attributed to single vibrational levels. We interpret this observation as a dissociative allocation of the electron to a proton followed by enhanced ionization at a well-defined "critical" overstretched internuclear distance. When using longer pulses we observe three separate Coulomb explosion velocity groups corresponding to critical distances of about 8, 11, and 15 a.u.

We have doubled the frequency of a single mode distributed Bragg reflection (DBR) semiconductor laser. The laser is optically locked to an external enhancement ring resonator that contains an antireflection coated potassium niobate crystal. Unlike Fabry–Perot cavity lasers the DBR laser is found to operate in only one longitudinal mode and allows stable second harmonic generation. We present the results obtained with a singly and doubly resonant cavity. A total output of 1.2 mW tunable single mode radiation near 486 nm has been generated from a laser output power of 25 mW at 972 nm without the use of optical isolators or active feedback control.

The generation of high-order harmonics in strong laser fields has been studied for several years and considerable progress has been made in the understanding of the physics involved. In different laboratories around the world, scientists are now beginning to explore the use of the harmonic radiation in novel applications in the extreme ultraviolet spectral region (XUV). In some applications, the lack of information regarding the coherence properties of the harmonic radiation is a restricting factor. We here present experimental results from an interference experiment, illustrating the potentials of harmonic radiation in applications requiring temporally coherent radiation in the XUV region.

Hydrogen, the simplest of the stable atoms, provides unique opportunities for critical confrontations of spectroscopic experiment and quantum electrodynamic theory. Recent advances in the generation, stabilization, and measurement of optical frequencies are extending radiofrequency resolution and accuracy into the ultraviolet spectral region. Precise measurements and comparisons of the optical frequencies of sharp hydrogen two-photon transitions are yielding new values of fundamental constants, and they are permitting stringent new tests of basic physics laws.

Modern electronic signal processing techniques make it possible to use a conventional scanning Fabry-Perot interferometer for highly accurate measurement of (tunable) laser wavelengths. An experimental device is described in which the time intervals between resonances of an unknown wavelength are compared with those of a stabilized He-Ne reference laser by the use of highly accurate electronic timing devices. The accuracy is further enhanced through signal averaging over many scan periods. Accuracies approaching one part in 1O7 are expected to be readily achievable.

Highly resolved rotational spectra of two‐photon excited vibrational levels in the 1B2u state of C6H6 and C6D6 are recorded for linear and circular polarization of the exciting laser radiation, generated with a powerful narrow‐band dye laser system. The fitted theoretical spectra are in excellent agreement with the experimental results and provide molecular parameters (rotational constants, Coriolis constants, and structural parameters) of considerably improved accuracy.

The wavelength of a single-mode cw laser can be accurately measured with the help of a fringe-counting traveling Michelson interferometer, but unfortunately this method cannot easily be extended to pulsed lasers, such as the widely used nitrogen-pumped dye laser systems. Today, most laboratories seem to employ conventional grating spectrographs to measure the wavelength of pulsed lasers. The resolution of such instruments can actually be quite sufficient in view of the typical larger bandwidth of pulsed lasers. But because of mechanical imperfections of the grating drive, the available digital wavelength readout can only be used for rough estimates, and any absolute wavelength measurement with the help of spectral lamps and wavelength catalogs is at best tedious.

We present a new measurement of parity violation in atomic cesium. This is more precise than previous measurements of atomic parity violation and is approaching the precision of the best high-energy tests of the electroweak theory. This is a preliminary result and we expect considerable further improvement.

The advent of high monochromatic tunable laser sources has stimulated important advances in optical spectroscopy, as documented by several recent good reviews [1–3].

We will report on recent progress in the development and improvement of laser methods which can overcome the Doppler broadening of spectral lines of atoms and molecules.

Optical second-harmonic generation (SHG) can be an extremely sensitive tool for surface studies. The technique is capable of probing adsorbed molecules at various interfaces [1]. It is based on the idea that SHG is forbidden in a medium with inversion symmetry, but necessarily allowed at a surface. To see such a surface nonlinear optical effect, high laser intensity is often needed. Thus, in the experiments reported so far, pulsed lasers were used exclusively. From the consideration for practical applications, however, the technique would look much more attractive if the bulky pulsed laser can be replaced by a simple inexpensive CW diode laser. We demonstrate here at this conference that this is indeed possible. The work, described below, also constitutes the first experiment of surface SHG carried out with a CW laser.

Phase or frequency modulation (FM) spectroscopy is a powerful technique which offers the advantage of zero-background phase-sensitive heterodyne detection, and has been used in various types of spectroscopic investigation and in laser frequency locking. However, spurious amplitude modulation (AM) causes fluctuation in the baseline. This reduces the detection sensitivity and causes error in laser frequency locking. Here we have developed and demonstrated an active servo systeml to suppress the AM noise of a phase-modulated laser beam to achieve shot-noise limited detection in a linear absorption experiment.