We report on an absolute frequency measurement of the hydrogen 1S-2S two-photon transition in a cold atomic beam with an accuracy of 1.8 parts in 10(14). Our experimental result of 2 466 061 413 187 103(46) Hz has been obtained by phase coherent comparison of the hydrogen transition frequency with an atomic cesium fountain clock. Both frequencies are linked with a comb of laser frequencies emitted by a mode locked laser.

We have measured the absolute optical frequency of the cesium D 1 line at 335 THz (895 nm). This frequency provides an important link for a new determination of the fine structure constant a. The D 1 line has been compared with the fourth harmonic of a methane stabilized He-Ne laser at 88.4 THz (3.39 mm). To measure the frequency mismatch of 18.39 THz between 4 3 88.4 THz 354 THz and the D 1 line a frequency comb spanning around 244 000 modes of a Kerr-lens mode-locked laser was used. We find 1 167 688 (81) kHz for the hyperfine splitting of the 6P 1͞2 state and 335 116 048 807 (41) kHz for the hyperfine centroid from which we derive a 21 137.035 992 4͑41͒.

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We have used the comb of optical frequencies emitted by a mode-locked laser as a ruler to measure differences of as much as 20 THz between laser frequencies. This is to our knowledge the largest gap measured with a frequency comb, with high potential for further improvements. To check the accuracy of this approach we show that the modes are distributed uniformly in frequency space within the experimental limit of 3.0 parts in 10(17) . By comparison with an optical frequency comb generator we have verified that the mode separation equals the pulse repetition rate within the experimental limit of 6.0 parts in 10(16).

We have measured the absolute frequency of the hydrogen 1S-2S two-photon resonance with an accuracy of 3.4 parts in 10 13 by comparing it with the 28th harmonic of a methane-stabilized 3.39 mm He-Ne laser. A frequency mismatch of 2.1 THz at the 7th harmonic is bridged with a phase-locked chain of five optical frequency interval dividers. From the measured frequency f 1S-2S 2 466 061 413 187.34͑84͒ kHz and published data of other authors we derive precise new values of the Rydberg constant, R` 10 973 731.568 639͑91͒ m 21 and of the Lamb shift of the 1S ground state, L 1S 8172.876͑29͒ MHz. These are now the most accurate values available. [S0031-9007(97)04182-3] PACS numbers: 31.30.Jv, 06.20.Jr, 21.10.FtFor almost three decades, the 1S-2S two-photon transition in atomic hydrogen with its natural linewidth of only 1.3 Hz has inspired advances in high resolution laser spectroscopy and optical frequency metrology [1]. This resonance has become a de facto optical frequency standard. More importantly, it is providing a cornerstone for the determination of fundamental physical constants and for stringent tests of quantum electrodynamic theory. In the future, it may unveil conceivable slow changes of fundamental constants or even differences between matter and antimatter.Here, we report on a new precise measurement of the absolute frequency of the 1S-2S interval which exceeds the accuracy of the best previous measurement [2] by almost 2 orders of magnitude. The 1S-2S resonance is observed by longitudinal Doppler-free two-photon spectroscopy of a cold atomic beam. The resonance frequency is compared with the frequency of a cesium atomic clock with the help of a phase-coherent laser frequency chain, using a transportable CH 4 -stabilized He-Ne laser at 3.39 mm as an intermediate reference.In this way, we have determined a 1S-2S interval of f 1S-2S 2 466 061 413 187.34͑84͒ kHz with an uncertainty of 3.4 parts in 10 13 , limited by the reproducibility of the He-Ne reference laser. This represents now the most accurate measurement of any optical frequency in the ultraviolet and visible region. Together with the results of other authors, in particular, the recent precision measurements of the 2S 1͞2 -8D 5͞2 transition frequency in hydrogen by the group of Biraben [3], we derive new and more precise values for both the Rydberg constant and the 1S Lamb shift. This Lamb shift provides now the best test of quantum electrodynamics for an atom.As in our earlier experiment [2] we are taking advantage of the near coincidence between the 1S-2S interval and the 28th harmonic of the frequency of a CH 4 -stabilized 3.39 mm He-Ne laser. However, a frequency mismatch of 2.

Measurement of exhaled nitric oxide is widely used in respiratory research and clinical practice, especially in patients with asthma. However, interpretation is often difficult, due to common interfering factors, and little is known about interactions between factors. We assessed the influences and interactions of factors such as smoking, respiratory tract infections and respiratory allergy concerning exhaled nitric oxide values, with the aim to derive a scheme for adjustment. We studied 897 subjects (514 females, 383 males; mean age+/-standard deviation 34.5+/-13.0 years) with and without respiratory allergy (allergic rhinitis and/or asthma), smoking and respiratory tract infection. Logarithmic nitric oxide levels were described by an additive model comprising respiratory allergy, smoking, respiratory tract infection, gender and height (p0.001 each), without significant interaction terms. Geometric mean was 17.5ppb in a healthy female non smoker of height 170cm, whereby respiratory allergy corresponded to a change by factor 1.50, smoking 0.63, infection 1.24, male gender 1.17, and each 10cm increase (decrease) in height to 1.11 (0.90). Factors were virtually identical when excluding asthma and using the category allergic rhinitis instead of respiratory allergy (n=863). Within each category formed by combinations of these different predictors, the range of residual variation was approximately constant. We conclude that the factors influencing exhaled nitric oxide, which we analyzed, act independently of each other. Thus, circumstances such as smoking and respiratory tract infection do not appear to affect the usefulness of exhaled nitric oxide, provided that appropriate factors for adjustment are applied.

We demonstrate a versatile new technique that provides a phase coherent link between optical frequencies and the radio frequency domain. The regularly spaced comb of modes of a mode-locked femtosecond laser is used as a precise ruler to measure a large frequency gap between two different multiples (harmonics or subharmonics) of a laser frequency. In this way, we have determined a new value of the hydrogen 1S-2S two-photon resonance, f(1S-2S) = 2 466 061 413 187.29(37) kHz, representing now the most accurate measurement of an optical frequency.