Volume 215, Issue 12 - 12

We have studied the formation mechanisms of the S + and SF + and also CF + ions resulting from the multiple-photon IR + UV excitation of the SF 6 and CF 2 HCl molecules, respectively. It is demonstrated that in SF 6 exist at least two ion formation channels. The first is the UV multiphoton fragmentation and ionization of the SF 5 radicals formed in the course of the IR MP dissociation of SF 6 . The second starts with the UV excitation of the highly vibrationally excited SF 6 molecules themselves. As for the IR + UV irradiation of CF 2 HCl, the experiments performed allow us to conclude that the CF + ions observed are produced in the course of the UV multiphoton excitation of the CF 2 radicals, followed by their fragmentation and ionization, the radicals themselves being formed as a result of the IR MPD of CF 2 HCl. This effect has been used to study the unimolecular decay kinetics of CF 2 HCl, as well as the kinetics of the isotopic selectivity of the IR MPD process under molecular beam conditions.

Laser is an example of how new markets are created by breakthroughs in basic research. The time development of important research contributions and of product ideas is described, as well as the personal background of A. L. Schawlow, one of the pioneers in both areas. A  Learning Curve of Research is postulated, in analogy to the well known learning curve of production. It is characterized by the accumulated knowledge of the scientific leaders in the retrospectively defined field, and by the uncertainty of the first product concept, both as a function of time. In case of the LASER, basic research knowledge accumulated rather slowly over a period of 34 years, followed by 10 years of extremely fast growth, during which an outsider had little chance to become one of the pioneers. This science-based breakthrough demonstrates, that major research-based innovations take several decades, and that they are sufficiently random to be evolutionary rather than open to goal-setting and planning. Fritz Schäfer ist einer der Pioniere des Lasergebiets. Er hat mit seinen frühen Entdeckungen zu Laserfunktion in organischen Farbstoffen neue Türen geöffnet, und über viele Jahrzehnte hinweg originelle Beiträge zur Anwendung des Lasers als Instrument der Forschung geleistet. Darüber hinaus wurde er durch sein ausgeprägtes Interesse für die praktische Umsetzung wissenschaftlicher Erkenntnisse in industrielle Tätigkeit zum Geburtshelfer bei der Gründung der Firma Lambda Physik , und zum Initiator des Laserlaboratoriums Göttingen e.V. Zu dessen 10jährigem Jubiläum (14.5.1997) reflektierte der Autor in einem Festvortrag am Beispiel des Lasers darüber, wie schnell ein auf Durchbrüchen der Grundlagenforschung basierender Innovationssprung realisierbar ist. Zu Ehren des 70. Geburtstags von Fritz Schäfer wurde diese Fragestellung zur vorliegenden, ihm gewidmeten Veröffentlichung ausgearbeitet.

Singlet methylene radical ( 1 CH 2 ) concentrations were measured for the first time in the pyrolysis of methane (CH 4 ) and ethane (C 2 H 6 ) behind shock waves. The very sensitive frequency modulation (FM) spectroscopy, already established for sensitve detection of amino radicals (NH 2 ) [1,2], was used for that purpose. Applying computer simulations using a complex reaction mechanism the experimental 1 CH 2 signals were fitted and rate coefficients of different reaction channels were obtained. For the reaction channel (1 a ) CH 4  + M → CH 3  + H + M an extented Arrhenius expression of k 1 a  = 6.5 ·10 18  · ( T /298) −1.70  · exp[−366 kJ mol −1 / RT ] cm 3  mol −1  s −1  (± 50%), and futhermore rate coefficients for (7)  3 CH 2  + CH 3 → H + C 2 H 4  with  k 7  = 3.2 · 10 13  cm 3  mol −1  s −1  (± 40%), (11)  3 CH 2  + H → CH + H 2  with  k 11  = 7.9 · 10 13  cm 3  mol −1  s −1  (± 40%), and for the intersystem crossing via (6)  1 CH 2  + M →  3 CH 2  + M with  k 6  = 1.40 ·10 10  · ( T /K) 0.9 cm 3  mol −1  s −1  (± 40%) were determined. The experimental conditions ranged from 1900 to 4000 K with corresponding pressures between 0.23 to 0.54 bar. The 1 CH 2 absorption coefficients were extracted from the numerical evaluation of the experiments as function of temperature. The following (fitted) expression α( T ) = 5.70 · 10 5  · exp(1.064 · 10 −3 (1690− T /K)) cm 2 /mol reflects well the experimental results obtained for the b˜ 1 B 1 (0, 14, 0)4 04  ← ã 1 A 1 (0, 0, 0)4 14 transition at 590.707 nm probed in this work.

For a long time the capability to perform measurements with femtosecond time-resolution belonged exclusively to the domain of optics. However, in the last few years laser-driven X-ray sources have been developed which enable femtosecond time-resolution to be extended to the X-ray regime.

A polarization multiplexing method (based on Sagnac interferometer’s principle) was used in combination with a table-top wide aperture discharge pumped excimer amplifier to amplify femtosecond UV pulses. It is possible to optimize the operational conditions of the device within a wide range of input energies by changing the optical setup, while keeping the signal-to-noise ratio (short pulse – amplified spontaneous emission (ASE) ratio) of the pulses on a reasonably low value. The amplifier was tested for two different front ends. With ∼40 μJ input energy ∼50 mJ output energy was obtained in a 3-pass arrangement, and applying a seed pulse carrying an energy of around 2–3 mJ a maximum output energy of 100 mJ was reached in a single subpicosecond beam at 248 nm.

The instantaneous transient Kerr lens in a transparent material offers a simple and efficient method to measure the spectrogram of a white light continuum. Even if the kernel of the spectrogram is not known very well, the instantaneous frequency may be extracted with high accuracy. It is shown that the knowledge of the phase modulation of a white light continuum simplifies the design of the prism compressors in a NOPA system. It also makes cross-correlation measurements at different frequencies in white light pump probe experiments obsolete.

A miniature cryostat for optical emission measurements is described which uses a standard storage dewar as the liquid helium bath. Excitation and emission light is guided to and from the sample by optical fibres and spherical mirrors.

With focus on commercial aspects the paper reviews the historical development and various technological achievements in the field of excimer lasers since their discovery in 1970. The first commercial excimer laser model in the world, the EMG 500 from Lambda Physik was operating at maximum repetition rates of only 20 Hz, whereas today multi kHz excimer lasers as the NovaLine industrial lasers are available. In 2000, Lambda Physik introduced the most powerful commercial excimer laser, the Lambda Steel with 300 W output power having extremely high stability. In the more than 30 years of excimer lasers their properties have been studied in a multitude of research laboratories at universities, institutes, national laboratories, and industry. In the attempts to reach parameters in the UV never seen before, energies ranging from mJ to kJ, pulse durations from sub μs down to fs, spectral bandwidths from fractions of 1 nm down to less then 0.1 pm, and beam divergence from 10 mrad down to 10 μrad could be demonstrated at the various excimer lasers wavelengts. Over the years various technological achievements like the NovaTube laser chamber technology, the NovaPowerSwitch pulsed power modules and the HaloSafe fluorine and chlorine generators could be made which make the use of excimer lasers simple and user friendly. Recently, the development of DuraTube technology gave a strong push towards the development of high power 157 nm laser technology for microlithography and laser based micro-machining of “difficult” materials as fused silica or teflon. The paper describes important steps and quantum leaps in developing and improving excimer lasers. Since a large part of excimer laser development was driven by application requirements some applications developed over the years are touched as well – including early photochemistry and dye laser pumping in F. P. Schäfers laboratories. Finally, currently ongoing and for the future expected technological developments are discussed which will strengthen the position of excimer lasers as user friendly work horses in many industrial and research areas.

Using frequency modulation (FM) spectroscopy singlet methylene radicals have been detected for the first time behind shock waves. The thermal decomposition of ketene served as source for metylene radicals at temperatures from 1905 to 2780 K and pressures around 450 mbar. For the unimolecular decomposition reaction, (1) CH 2 CO + M → CH 2  + CO + M, the rate constants obtained are: k 1  = (9.5 ± 5.7) · 10 15  · exp[(−244 ± 25) kJ mol −1 / RT ]    cm 3  mol −1  s −1 . As a first study of a methylene reaction at high temperatures by diretly tracing methylene the reaction of methylene with hydrogen, (8 + 9) 1,3 CH 2  + H 2  → CH 3  + H, was investigated at temperatures from 1930 to 2455 K and pressures around 500 mbar. For the total rate constant of the singlet and triplet methylene reaction a temperature independent value was obtained: log( k 8+9 /(cm 3  mol −1  s −1 )) = 13.89 ± 0.26. A comparison with low temperature literature data and the systematics of activation energies of triplet methylene reactions allowed a consistent description of singlet and triplet contributions and of the forward and reverse reaction.

In this communication, we apply a recently presented nonperturbative molecular orbital approach (Th. Koslowski, J. Chem. Phys. 113 (2000) 10703) to the energetics of small polaron formation and hopping in tungsten bronzes in the regime of small excess electron concentrations. The electronic structure of the system is described by a tight-binding Hamiltonian, including a nonretarded reaction field to account for the dielectric polarizabilty of the solid. The resulting model is solved self-consistently, leading to a localized excess electron charge distribution. The potential energy curve for polaron hopping is presented and analyzed in terms of a two-site donor-acceptor model.