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Biomedical subjects

W Ubachs

Publications and source records attributed to W Ubachs.

At least 19 recordsLinked to original sources
PubMed2006

Indication of a cosmological variation of the proton-electron mass ratio based on laboratory measurement and reanalysis of H2 spectra.

Based on highly accurate laboratory measurements of Lyman bands of H2 and an updated representation of the structure of the ground X 1sigma(g)+ and excited B 1sigma(u)+ and C 1pi(u) states, a new set of sensitivity coefficients K(i) is derived for all lines in the H2 spectrum, representing the dependence of their transition wavelengths on a possible variation of the proton-electron mass ratio mu = m(p)/m(e). Included are local perturbation effects between B and C levels and adiabatic corrections. The new wavelengths and K(i) factors are used to compare with a recent set of highly accurate H2 spectral lines observed in the Q 0347-383 and Q 0405-443 quasars, yielding a fractional change in the mass ratio of deltamu/mu = (2.4 +/- 0.6) x 10(-5) for a weighted fit and deltamu/mu = (2.0 +/- 0.6) x 10(-5) for an unweighted fit. This result indicates, at a 3.5sigma confidence level, that mu could have decreased in the past 12 Gyr.

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PubMed2006

Spectral redshift in harmonic generation from plasma dynamics in the laser focus.

High-precision spectral measurements on the 9th harmonic generated in xenon gas are compared with calculations of the plasma dynamics resulting from multiphoton ionization in the laser focus. For the regime of 300 ps pulses and above-saturation intensities a novel mechanism producing redshifts in the harmonics is uncovered and explained. Ions play a double role: the nonlinear susceptibility of the ions is decisive for the harmonic intensity, while their mutual repulsion and the associated increase of the index of refraction is identified as the cause of the redshift.

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PubMed2006

Cavity ring-down spectroscopy for detection in liquid chromatography: extension to tunable sources and ultraviolet wavelengths.

In earlier studies, it was demonstrated that the sensitivity of absorbance detection in liquid chromatography (LC) can be improved significantly by using cavity ring-down spectroscopy (CRDS). Thus far, CRDS experiments have been performed using visible laser light at fixed standard wavelengths, such as 532 nm. However, since by far most compounds of analytical interest absorb in the ultraviolet (UV), it is of utmost importance to develop UV-CRDS. In this study, as a first step towards the deep-UV region, LC separations with CRDS detection (using a previously described liquid-only cavity flow cell) at 457 and 355 nm are reported for standard mixtures of dyes and nitro-polyaromatic hydrocarbons (nitro-PAHs), respectively. For the measurements in the blue range a home-built optical parametric oscillator (OPO) system, tunable between 425 and 478 nm, was used, achieving a baseline noise of 2.7 x 10(-6) A.U. at 457 nm, improving upon the sensitivity of conventional absorbance detection (typically around 10(-4) A.U.). An enhancement of the sensitivity can be seen at 355 nm as well, but the improvement of the baseline noise (1.3 x 10(-5) A.U.) is much less pronounced. The sensitivity at 355 nm is limited by the quality of the UV-CRDS mirrors that are currently available: whereas the ring-down times as obtained at 457 nm are around 70-80 ns for the eluent, they are only 20-25 ns at 355 nm. Critical laser characteristics for LC-CRDS measurements, such as pulse length and mode structure, are given and prospects for going to shorter wavelengths are discussed.

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PubMed2005

Control of diabatic versus adiabatic field dissociation in a heavy Rydberg system.

A novel phenomenon is observed in the dynamics of laser-prepared coherent wave packets, bound by the Coulombic 1/r potential of an ion-pair system. After exciting weakly bound (approximately 3 meV) H(+)(-) wave packets in a Stark field, and permitting them to evolve in time, control of field dissociation via adiabatic and diabatic routes is demonstrated by applying delayed pulsed-electric fields, involving a zero-field crossing. Control manifests itself through the production of ions from each pathway at a different instant in time. This phenomenon is applied to map the oscillatory behavior of an angular momentum wave packet in a heavy Rydberg system. The characteristic frequencies of the observed Stark oscillations verify predicted mass-scaling laws for heavy Rydberg systems.

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PubMed2005

Optical observation of the 3s sigma gF3Pi u Rydberg state of N2.

Using ultrahigh-resolution 1 XUV+1 UV two-photon ionization laser spectroscopy, the F (3)Pi(u)<--X (1)Sigma(g) (+)(0,0) transition of N(2) has been optically observed for the first time, and the 3s sigma(g)F (3)Pi(u)(upsilon=0) Rydberg level fully characterized with rotational resolution. The experimental spectroscopic parameters and predissociation level widths suggest strong interactions between the F state and the 3p pi(u)G (3)Pi(u) Rydberg and C(') (3)Pi(u) valence states, analogous to those well known in the case of the isoconfigurational (1)Pi(u) states.

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PubMed2005

Production of narrowband tunable extreme-ultraviolet radiation by noncollinear resonance-enhanced four-wave mixing.

Fourier-transform-limited extreme-ultraviolet (XUV) radiation (bandwidth approximately < 300 MHz) tunable around 91 nm is produced by use of two-photon resonance-enhanced four-wave mixing on the Kr resonance at 94 093 cm(-1). Noncollinear phase matching ensures the generation of an XUV sum frequency 2 omega1 + omega2 that can be filtered from auxiliary laser beams and harmonics by an adjustable slit. Application of the generated XUV light is demonstrated in spectroscopic investigations of highly excited states in H2 and N2.

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PubMed2005

Isotopic variation of experimental lifetimes for the lowest 1 Pi u states of N2.

Lifetimes of several (1)Pi(u) states of the three natural isotopomers of molecular nitrogen, (14)N(2), (14)N(15)N, and (15)N(2), are determined via linewidth measurements in the frequency domain. Extreme ultraviolet (XUV)+UV two-photon ionization spectra of the b (1)Pi(u)(v=0-1,5-7) and c(3) (1)Pi(u)(v=0) states of (14)N(2), b (1)Pi(u)(v=0-1,5-6) and c(3) (1)Pi(u)(v=0) states of (14)N(15)N, and b (1)Pi(u)(v=0-7), c(3) (1)Pi(u)(v=0), and o (1)Pi(u)(v=0) states of (15)N(2) are recorded at ultrahigh resolution, using a narrow band tunable XUV-laser source. Lifetimes are derived from the linewidths of single rotationally resolved spectral lines after deconvolution of the instrument function. The observed lifetimes depend on the vibrational quantum number and are found to be strongly isotope dependent.

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PubMed2005

Miniaturized cavity ring-down detection in a liquid flow cell.

A novel method for applying cavity ring-down spectroscopy in the liquid phase, compatible with LC analyses, is presented. The core of the setup is a home-built cavity ring-down flow cell (cell volume 12 microL) that is constructed using a silicon rubber spacer, which is clamped leak-tight between two high-reflectivity mirrors. The mirrors are in direct contact with the liquid flow, which provides for a small path length and short ring-down times. Inside the cavity there are no windows, reflection losses, or Brewster angles to be considered. Due to the small size of the presented cavity geometry, the setup can be implemented in conventional-size LC apparatuses. With a flow injection setup, a detection limit of 2.5 nM was obtained for Crystal Violet in ethanol, and the linear dynamic range of the system is at least 2 orders of magnitude. The method has the potential to become a powerful alternative for commercial LC UV/visible absorbance detectors.

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PubMed2004

Deep-ultraviolet cavity ringdown spectroscopy.

The sensitive optical detection technique of cavity ringdown spectroscopy is extended to the wavelength range 197-204 nm. A novel design narrowband Fourier-transform-limited laser is used, and the technique is applied to gas-phase extinction measurements in CO2, SF6, and O2. Further demonstration of the system capabilities is given in high-resolution recordings of the Schumann-Runge (0, 0), (1,0), and (2, 0) bands in O2.

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PubMed2004

Lifetime and predissociation yield of 14N2 b 1Piu(v=1).

The lifetime of the b 1Piu(v=1) state in 14N2 has been determined experimentally using a laser-based pump-probe scheme and an exceptionally long lifetime of 2.61 ns was found. Semiempirical close-coupling calculations of the radiative lifetime, which include Rydberg-valence interactions in the singlet manifold, are consistent with this large value, giving a value of 3.61 ns and suggesting a predissociation yield of approximately 28% for this level of the b state.

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PubMed2004

Highly accurate H2 Lyman and Werner band laboratory measurements and an improved constraint on a cosmological variation of the proton-to-electron mass ratio.

Transition wavelengths on a large set of H2 Lyman and Werner band spectral lines have been obtained at an accuracy of 5 x 10(-8), using a narrow band tunable extreme ultraviolet laser. The data are used to determine a constraint on a possible cosmological variation of the proton-to-electron mass ratio (mu=Mp/me) from a comparison with highly redshifted spectral data of quasistellar objects, yielding a fractional change in the mass ratio of Deltamu/mu=-0.5+/-3.6 x 10(-5) (2sigma), which would correspond to a temporal change of d/dt(Deltamu/mu)=-0.4+/-3.0 x 10(-15) per year (2sigma) if a linear cosmological expansion model is assumed.

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PubMed2003

High-order harmonic generation yielding tunable extreme-ultraviolet radiation of high spectral purity.

Production of extreme-ultraviolet radiation by high-order harmonic generation is demonstrated to yield unprecedented spectral purity of lambda/Delta lambda=2.5 x 10(5) at wavelengths covering the entire range 40-100 nm. Tunability and sub-cm(-1) bandwidth of the harmonics are demonstrated in recordings of the He (1s4p) and Ar (3p(5)3d') resonance lines at 52.2 and 86.6 nm. Frequency shift of the harmonics due to chirp-induced phenomena are investigated and found to be small, resulting in a frequency accuracy of about 5 x 10(-7) in the domain of extreme-ultraviolet radiation.

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PubMed2001

Observation of coherent wave packets in a heavy Rydberg system.

Coherent dynamical behavior is observed in the heavy Rydberg system H+H-. Because of the large mass, time scales of the wave-packet evolution are orders of magnitude slower than for a Rydberg electron. In the presence of a weak electric field, wave packets made up of about 1000 Stark states are excited by near-Fourier transform limited nanosecond laser pulses. Pulsed-field dissociation reveals coherent time evolution on a microsecond time scale, observed as oscillations with frequencies explained by a linear Stark model applied to the H+H- system.

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PubMed2001

Double-well states of ungerade symmetry in H2: first observation and comparison with Ab initio calculations.

The observation of a new class of long-lived outer well states of ungerade symmetry (B"B1Sigma+u) in molecular hydrogen, lying above the ionization threshold, is reported. Rovibrational levels within a potential extended over internuclear separations of R = 7-25 a.u. are experimentally investigated in a triple resonance scheme. Good agreement ( <0.5 cm(-1)) with updated ab initio calculations is found for vibrational levels up to v = 26, demonstrating that such calculations can now be extended to this energetic range above ionization, as long as interaction with the Rydberg manifolds is shielded by a barrier. The dynamical behavior (predissociation and autoionization) of this class of " u" symmetry states is remarkably different from similar outer well states of " g" symmetry; this phenomenon can be understood from the structure of doubly excited electronic states.

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PubMed2001

Cavity-Ring-Down Spectroscopy on Water Vapor in the Range 555-604 nm.

The method of pulsed cavity-ring-down spectroscopy was employed to record the water vapor absorption spectrum in the wavelength range 555-604 nm. The spectrum consists of 1830 lines, calibrated against the iodine standard with an accuracy of 0.01 cm(-1); 800 of these lines are not obtained in the HITRAN 96 database, while 243 are not included in the newly recorded Fourier transform spectrum of the Reims group. Of the set of hitherto unobserved lines, 111 could be given an assignment in terms of rovibrational quantum numbers from a comparison with first principles calculations. Copyright 2001 Academic Press.

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PubMed2000

A Dense Grid of Reference Iodine Lines for Optical Frequency Calibration in the Range 595-655 nm.

A dense grid of reference lines in the hyperfine structure of the B-X transitions of molecular iodine ((127)I(2)) is presented. Frequencies of 481 "t"-hyperfine components in 15 vibrational bands covering the wavelength range 595-655 nm were determined with an absolute accuracy of 1.0 MHz (1varsigma uncertainty). Spectra were obtained by Doppler-free saturation spectroscopy using a cw ring-dye laser of 1-MHz bandwidth. Center-of-gravity frequencies of rovibronic lines, determined by correcting for the calculated hyperfine shift, are included separately in a least-squares parametrization for each band. From the obtained molecular constants a prediction is made for all t-hyperfine components of the 15 bands covering 595-655 nm. Hence a dense grid of reference lines of 1 MHz (1varsigma) absolute accuracy is created, with a line in each interval of 1 cm(-1) in the entire range. This spectral atlas of about 3400 lines may provide a useful calibration tool in many laboratories. Copyright 2000 Academic Press.

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