Biomedical subjects
Publications and source records attributed to W Ubachs.
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.
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.
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.
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.
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.
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.
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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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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A high precision dense grid of reference lines in the hyperfine structure of the B-X system of molecular iodine (127I2) is presented. A simple parametrization has been derived, predicting the hyperfine line positions of the "t" components for vibrational bands (13-1) up to (18-1) and rotational quantum numbers J = (9-140). The analysis in this paper is based on Doppler-free saturation spectroscopy spectra used for calibration of more than 100 new components in the hyperfine structure of 127I2. The data presented contains a prediction (with 2 MHz accuracy) for the positions of 1584 "t" components in the rotational structure of iodine, covering the wavelength interval 571-596 nm. Copyright 1998 Academic Press. Copyright 1998Academic Press
The b1Sigma+g-X3Sigma-g (1,0) band of 16O17O, 16O18O, 18O2, 17O18O, and 17O2 isotopomers was investigated employing the technique of cavity-ring-down spectroscopy. More than 400 transition frequencies of magnetic dipole lines were determined with a typical uncertainty of 0.01 cm-1. This work results in new or improved accurate molecular constants for the excited b1Sigma+g, v = 1 state of all isotopomers and for the X3Sigma-g, v = 0 ground state of 17O2. Copyright 1998 Academic Press.
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