W.M.G. Ubachs

A source of 2 terawatt, 2.7 cycle laser pulses based on noncollinear optical parametric chirped pulse amplification

We demonstrate a noncollinear optical parametric chirped pulse amplifier system that produces 7.6 fs pulses with a peak power of 2 terawatt at 30 Hz repetition rate. Using an ultra-broadband Ti:Sapphire seed oscillator and grating-based stretching and compression combined with an LCD phase-shaper, we amplify a 310 nm wide spectrum with a total gain of 3x10(7), and compress it within 5% of its Fourier limit. The total integrated parametric fluorescence is kept below 0.2%, leading to a pre-pulse contrast of 2 x10(-8) on picosecond timescales.

Determination of the ionization and dissociation energies of the hydrogen molecule.

The transition wave number from the EF (1)Sigma(g)(+)(v = 0, N = 1) energy level of ortho-H(2) to the 54p1(1)(0) Rydberg state below the X(+) (2)Sigma(g)(+)(v(+) = 0, N(+) = 1) ground state of ortho-H(2)(+) has been measured to be 25,209.99756 +/- (0.00022)(statistical) +/- (0.00007)(systematic) cm(-1). Combining this result with previous experimental and theoretical results for other energy level intervals, the ionization and dissociation energies of the hydrogen molecule have been determined to be 124,417.49113(37) and 36,118.06962(37) cm(-1), respectively, which represents a precision improvement over previous experimental and theoretical results by more than one order of magnitude. The new value of the ionization energy can be regarded as the most precise and accurate experimental result of this quantity, whereas the dissociation energy is a hybrid experimental-theoretical determination.

Extreme Ultraviolet Frequency Comb Metrology

The remarkable precision of frequency-comb (FC) lasers is transferred to the extreme ultraviolet (XUV, wavelengths shorter than 100 nm), a frequency region previously not accessible to these devices. A frequency comb at XUV wavelengths near 51 nm is generated by amplification and coherent up-conversion of a pair of pulses originating from a near-infrared femtosecond FC laser. The phase coherence of the source in the XUV is demonstrated using helium atoms as a ruler and phase detector. Signals in the form of stable Ramsey-like fringes with high contrast are observed when the FC laser is scanned over P states of helium, from which the absolute transition frequency in the XUV can be extracted. This procedure yields a (4)He ionization energy at h×5 945 204 212(6)  MHz, improved by nearly an order of magnitude in accuracy, thus challenging QED calculations of this two-electron system.

A Stringent Limit on a Drifting Proton-to-Electron Mass Ratio from Alcohol in the Early Universe

Varying Constant? Searches for time-varying fundamental constants provide a means to look beyond the standard model of particle physics. Bagdonaite et al. (p. 46, published online 13 December) set an improved limit on the possible timevariation of the proton-to-electron mass ratio by comparing the frequencies of methanol transitions observed in a galaxy at a look-back time of 7 billion years with those measured in the laboratory. The values agree within 10−7, consistent with no variation over cosmic time. The proton-to-electron mass ratio inferred from methanol lines in a distant galaxy is in accord with the laboratory value. The standard model of physics is built on the fundamental constants of nature, but it does not provide an explanation for their values, nor require their constancy over space and time. Here we set a limit on a possible cosmological variation of the proton-to-electron mass ratio μ by comparing transitions in methanol observed in the early universe with those measured in the laboratory. From radio-astronomical observations of PKS1830-211, we deduced a constraint of ∆μ/μ = (0.0 ± 1.0) × 10−7 at redshift z = 0.89, corresponding to a look-back time of 7 billion years. This is consistent with a null result.

Experimental verification of Rayleigh scattering cross sections

The cavity-ringdown technique is applied to measure Rayleigh extinctions of Ar, N(2), and SF(6) in the 560-650-nm region at 294 K. It is shown that experimental and calculated Rayleigh scattering cross sections agree within an experimental uncertainty of 1% (for SF(6), 3%).

A high-velocity narrow absorption line outflow in the quasar J212329.46 − 005052.9

We report on the discovery of a high-velocity narrow absorption line outflow in the redshift 2.3 quasar J212329.46-005052.9.Five distinct outflow systems are detected with velocity shifts from 9710 to 14,050 km s 1 and Civ ��1548,1551 line widths of FWHM � 62 to 164 km s 1 . This outflow is remarkable for having high speeds and a degree of ionization similar to broad absorption line (BAL) flows, but line widths roughly 100 times narrower than BALs and no apparent X-ray absorption. This is also, to our knowledge, the highest-velocity narrow absorption line system confirmed to be in a quasar outflow by all three indicators of line variability, smooth super-thermal line profiles and doublet ratios that require partial covering of the quasar continuum source. All five systems have stronger absorption in Ovi ��1032,1038 than Civ with no lower ionization metal lines detected. Their line variabilities also appear coordinated, with each system showing larger changes in Civ than Ovi and line strength variations accompanied by nearly commensurate changes in the absorber covering fractions. The metallicity is approximately twice solar. These data require five distinct outflow structures with similar kinematics, physical conditions and characteristic sizes of order 0.01-0.02 pc (based on partial covering). The coordinated line variations, occurring on time scales 60.63 yr (quasar frame), are best explained by global changes in the outflow ionization caused by changes in the quasar’s ionizing flux. An upper limit on the acceleration, .3 km s 1 yr 1 , is consistent with blobs of gas that are gravitationally unbound and coasting freely &5 pc from the central black hole. Additional constraints from the variability time indicate that the full range of plausible distances is 5 . R . 1100 pc. However, if these small absorbing structures were created in the inner flow, they should be near the �5 pc minimum radius because they can travel just a few pc before dissipating (without external confinement). An apparent double line-lock in Civ suggests that the flow was radiatively accelerated and its present trajectory is within �16 o of the radial (line-ofsight) direction. The absence of strong X-ray absorption shows that radiative shielding in the far-UV and X-rays is not needed to maintain moderate BAL-like ionizations and therefore, apparently, it is not needed to facilitate the radiative acceleration to high speeds. We argue that the ionization is moderated, instead, by high gas densities in small outflow sub-structures. Finally, we estimate that the kinetic energy yield from this outflow is at least two orders of magnitude too low to be important for feedback to the host galaxy’s evolution.

Fundamental Vibration of Molecular Hydrogen

The fundamental ground tone vibration of H(2), HD, and D(2) is determined to an accuracy of 2×10(-4) cm(-1) from Doppler-free laser spectroscopy in the collisionless environment of a molecular beam. This rotationless vibrational splitting is derived from the combination difference between electronic excitation from the X(1)Σ(g)(+), v=0, and v=1 levels to a common EF(1)Σ(g)(+), v=0 level. Agreement within 1σ between the experimental result and a full ab initio calculation provides a stringent test of quantum electrodynamics in a chemically bound system.

Efficient temporal compression of coherent nanosecond pulses in a compact SBS generator-amplifier setup

A pulse compressor based on stimulated Brillouin scattering (SBS) in liquids is experimentally and theoretically investigated. It allows for the compression of Fourier-transform limited nanosecond pulses of several hundreds of millijoules of energy with both high conversion efficiency and a high temporal compression factor. The two-cell generator-amplifier arrangement is of a compact design not requiring external attenuation of the generator cell input energy. Pulses from an injection-seeded, frequency-doubled Nd:YAG laser of 300-mJ energy were compressed by a factor variable between 6 and 21 at up to 75% reflectivity. Deviation from unity SBS reflectivity is predominantly determined by optical component losses. The generation of 270-ps pulses with high beam quality was achieved in liquid methanol. These powerful pulses of variable duration are difficult to produce with common laser systems and are highly suited for the generation of high-harmonics in gases.

Keck Telescope Constraint on Cosmological Variation of the Proton-to-Electron Mass Ratio

Molecular transitions recently discovered at redshift z(abs) = 2.059 towards the bright background quasar J2123-0050 are analysed to limit cosmological variation in the proton-to-electron mass ratio, mu equivalent to m(p)/m(e). Observed with the Keck telescope, the optical echelle spectrum has the highest resolving power and largest number (86) of H-2 transitions in such analyses so far. Also, (seven) HD transitions are used for the first time to constrain mu-variation. These factors, and an analysis employing the fewest possible free parameters, strongly constrain mus relative deviation from the current laboratory value: delta mu/mu = (+5.6 +/- 5.5(stat) +/- 2.9(sys)) x 10-6, indicating an insignificantly larger mu in the absorber. This is the first Keck result to complement recent null constraints from three systems at z(abs) > 2.5 observed with the Very Large Telescope. The main possible systematic errors stem from wavelength calibration uncertainties. In particular, distortions in the wavelength solution on echelle order scales are estimated to contribute approximately half the total systematic error component, but our estimate is model dependent and may therefore under or overestimate the real effect, if present. To assist future mu-variation analyses of this kind, and other astrophysical studies of H-2 in general, we provide a compilation of the most precise laboratory wavelengths and calculated parameters important for absorption-line work with H-2 transitions redwards of the hydrogen Lyman limit.

Determination of the ionization and dissociation energies of the deuterium molecule (D2)

The transition wave numbers from selected rovibrational levels of the EF (1)Sigma(g) (+)(v=0) state to selected np Rydberg states of ortho- and para-D(2) located below the adiabatic ionization threshold have been measured at a precision better than 10(-3) cm(-1). Adding these wave numbers to the previously determined transition wave numbers from the X (1)Sigma(g) (+)(v=0, N=0,1) states to the EF (1)Sigma(g) (+)(v=0, N=0,1) states of D(2) and to the binding energies of the Rydberg states calculated by multichannel quantum defect theory, the ionization energies of ortho- and para-D(2) are determined to be 124 745.394 07(58) cm(-1) and 124 715.003 77(75) cm(-1), respectively. After re-evaluation of the dissociation energy of D(2) (+) and using the known ionization energy of D, the dissociation energy of D(2) is determined to be 36 748.362 86(68) cm(-1). This result is more precise than previous experimental results by more than one order of magnitude and is in excellent agreement with the most recent theoretical value 36 748.3633(9) cm(-1) [K. Piszczatowski, G. Łach, M. Przybytek et al., J. Chem. Theory Comput. 5, 3039 (2009)]. The ortho-para separation of D(2), i.e., the energy difference between the N=0 and N=1 rotational levels of the X (1)Sigma(g) (+)(v=0) ground state, has been determined to be 59.781 30(95) cm(-1).