Singlet versus triplet dynamics of β-carotene studied by quantum control spectroscopy

Biomolecules very often present complex energy deactivation networks with overlapping electronic absorption bands, making their study a difficult task. This can be especially true in transient absorption spectroscopy when signals from bleach, excited state absorption and stimulated emission contribute to the signal. However, quantum control spectroscopy can be used to discriminate specific electronic states of interest by applying specifically designed laser pulses. Recently, we have shown the control of energy flow in bacterial light-harvesting using shaped pump pulses in the visible and the selective population of pathways in carotenoids using an additional depletion pulse in the transient absorption technique. Here, we apply a closed-loop optimization approach to β-carotene using a spatial light modulator to decipher the energy flow network after a multiphoton excitation with a shaped ultrashort pulse in the near-IR. After excitation, two overlapping bands were detected and identified as the S₁ state and the first triplet state T₁. Using the transient absorption signal at a specific probe delay as feedback, the triplet signal could be optimized over the singlet contribution.     

Comparative study of the specific heat of superconducting and non-superconducting YBa2Cu3O7−x

We have measured the specific heat, the electrical resistance and the magnetic susceptibility of YBa₂Cu₃O_7–x in a superconducting (sc) and in a non-superconducting (nsc) version. The latter was obtained by inducing a small loss of oxygen. In the sc sample we find a jump of the specific heat with C=3.6 J mole^–1 K^–1. Below the maximum nearT _c the specific heat of the sc version drops too fast for an electronic effect alone: there is a cross-over from excess to deficiency already at 0.9T _c . The specific heat of the nsc version shows a change of slope in the temperature range between 70 and 80 K, which indicates the existence of a second specific heat anomaly, which apparently exists independently of that due to the onset of superconductivity and explains at least partially the premature crossover.Alexander von Humboldt fellow. On leave in absence from Centro Atomico Bariloche—CNEA Argentina     

Bestimmung von leichten Elementen in Silicium und Selen durch Ionenaktivierungsanalyse

Zusammenfassung Elemente mit niedriger Kernladungszahl lassen sich durch Aktivierung mit thermischen Neutronen entweder überhaupt nicht oder nur mit sehr geringer Empfindlichkeit bestimmen.In den letzten Jahren gewinnt daher die Bestrahlung mit Ionen (Protonen, Deuteronen, Tritium-, ³He- und ⁴He-Ionen) zur Aktivierungsanalyse der leichten Elemente zunehmend an Bedeutung. Zwischen Ionen- und Neutronenaktivierungen bestehen prinzipielle Unterschiede, die diskutiert werden.Aufgrund ihrer relativ geringen Reichweite in Materie eignen sich Ionen besonders zur Untersuchung von Oberflächen sowie dünnen Aufdampfschichten und Korrosionsschichten. Da die Aktivierungstiefe durch entsprechende Wahl der Versuchsparameter festgelegt werden kann, laßt sich auch die Tiefenverteilung von Fremdstoffen im Matrixmaterial untersuchen.Zur Bestimmung von Sauerstoff, Stickstoff und Kohlenstoff werden Protonen- und Deuteronenreaktionen herangezogen; aber auch Elemente wie Bor und Aluminium lassen sich dureh Ionenaktivierung erfassen. Bei einer Einschußenergie der Ionen von 3,0 MeV, einer Flußdichte von 5 A/cm² und einer Bestrahlungszeit, die der Halbwertszeit des zu aktivierenden Elementes entspricht, liegt die Bestimmungsempfindlichkeit für die genannten Elemente im Bereich von 1–100 ppb.Über die Bestimmung von Kohlenstoff in Silicium und Sauerstoff in aufgedampften Selenschichten wird berichtet.

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Determination of light elements in silicon and selenium by ion-activation analysis

Elements with low atomic number either cannot be determined at all by activation with thermal neutrons or only with very poor detection limit.In recent years, irradiation with ions (protons, deuterons, tritium ions, ³He- and ⁴He-ions) for activation analysis of the light elements has therefore gained increasing importance. There are basic differences between ion and neutron activation, which are discussed.Because of their relatively small range in matter, ions are particularly suited for analyzing surfaces as well as thin deposited films and corrosion layers. Since the activation depth can be established in advance by appropriate selection of the analysis parameters, the distribution of foreign matter in the matrix material can also be analyzed.Proton and deuteron reactions are employed to determine the presence of oxygen, nitrogen and carbon, but other elements such as boron and aluminium can also be detected by ion activation. The detection limit attained in analyzing the elements mentioned is within the range of 1 to 100 ppb with an ion bombarding energy of 3.0 MeV, a flux density of 5 A/cm² and an irradiation period which corresponds to the half-life of the element to be activated.A report is given on the determination of carbon in silicon and oxygen in thin-deposited selenium films and the results are discussed.

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Vortrag bei der Tagung Analytische Probleme der Reindarstellung von Halbleitern, Wiesbaden, 24.–25. 9. 1968.

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Herrn Prof. Dr. W. Gebauhr danken wir für viele anregende Diskussionen. Für die Mithilfe bei der Bestrahlung und Auswertung sind wir Frau K. Appler, Frl. E. Lüders und Herrn H. Pfann zu Dank verpflichtet.     

Transport properties of LaCu2Si2 and CeCu2Si2 between 1.5K and 300K

We report measurements of the electrical resistivity, the thermal conductivityk and the thermoelectric powerS between 1.5K and 300K on the anomalous CeCu₂Si₂ compound and on LaCu₂Si₂ as reference compound. For LaCu₂Si₂ the temperature dependences of andS are in accord with those found in otherd band metals. For CeCu₂Si₂ the observed resistivity (220 µ cm at 200K) leads to a very short electronic mean free path which is of the order of the Ce-Ce spacing. Correspondingly,k is almost identical with the phonon contributionk _p . Below 20K, resistivity and thermoelectric power strongly suggest Fermi liquid behavior with a degeneracy temperature between 20K and 40K. Above 200K, both andS decrease proportionally to –ln(T/1 K).Work performed within the research program of the Sonderforschungsbereich 125 Aachen/Jülich/KölnPart of this work will be presented at the Int. Conf. on Rare Earths in the Metallic State, St. Pierre de Chartreuse, Sept. 1978     

Electrical and thermal conductivity of CePd3, YPd3, GdPd3 and some dilute alloys of CePd3 with Y and Gd

Resistivity and thermal conductivity of the intermediate valence compound CePd₃ between 1.3 and 300 K are compared with those of the nonmagnetic and magnetic reference compounds YPd₃ and GdPd₃ and of alloys of the type Ce₁–xRE _x Pd₃ with RE=Y, Gd and 0.01<x<0.5. The analysis reveals the existence of a maximum metallic resistivity of 300 cm for CePd₃. The intrinsic resistivity of CePd₃ rises proportional toT ² up to 100 K, with a coefficientB about 10⁷ times larger than e.g. in copper. We interpret this with indirect electron-electron scattering mediated through valence transitions.     

Supramolecular templating in kirromycin biosynthesis: the acyltransferase KirCII loads ethylmalonyl-CoA extender onto a specific ACP of the trans-AT PKS

In the biosynthesis of complex polyketides, acyltransferase domains (ATs) are key determinants of structural diversity. Their specificity and position in polyketide synthases (PKSs) usually controls the location and structure of building blocks in polyketides. Many bioactive polyketides, however, are generated by trans-AT PKSs lacking internal AT domains. They were previously believed to use mainly malonyl-specific free-standing ATs. Here, we report a mechanism of structural diversification, in which the trans-AT KirCII regiospecifically incorporates the unusual extender unit ethylmalonyl-CoA in kirromycin polyketide biosynthesis.