Understanding and controlling the dynamic evolution of electrons in matter is among the most fundamental goals of attosecond science. While the most exotic behaviors can be found in complex systems, fast electron dynamics can be studied at the fundamental level in atomic systems, using moderately intense (≲103 W/cm2) lasers to control the electronic structure in proof-of-principle experiments. Here, we probe the transient changes in the absorption of an isolated attosecond extreme ultraviolet (XUV) pulse by helium atoms in the presence of a delayed, few-cycle near infrared (NIR) laser pulse, which uncovers absorption structures corresponding to laser-induced “virtual” intermediate states in the two-color two-photon (XUV+NIR) and three-photon (XUV+NIR+NIR) absorption process. These previously unobserved absorption structures are modulated on half-cycle (~1.3 fs) and quarter-cycle (~0.6 fs) timescales, resulting from quantum optical interference in the laser-driven atom.

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Photoisomerization dynamics of a light-driven molecular rotary motor, 9-(2-methyl-2,3-dihydro-1H-cyclopenta[a]naphthalen-1-ylidene)-9H-fluorene, is investigated with trajectory surface-hopping dynamics at the semiempirical OM2/MRCI level. The rapid population decay of the S excited state for the M isomer is observed, with two different decay time scales (500 fs and 1.0 ps). By weighting the contributions of fast and slow decay trajectories, the averaged lifetime of the S excited state is about 710 fs. The calculated quantum yield of the M-to-P photoisomerization of this molecular rotary motor is about 59.9%. After the S → S excitation, the dynamical process of electronic decay is followed by twisting about the central C═C double bond and the motion of pyramidalization at the carbon atom of the stator-axle linkage. Although two S/S minimum-energy conical intersections are obtained at the OM2/MRCI level, only one conical intersection is found to be responsible for the nonadiabatic dynamics. The existence of "dark state" in the molecular rotary motor is confirmed through the simulated time-resolved fluorescence emission spectrum. Both quenching and red shift of fluorescence emission spectrum observed by Conyard et al. [ Conyard, J.; Addison, K.; Heisler, I. A.; Cnossen, A.; Browne, W. R.; Feringa, B. L.; Meech, S. R. Nat. Chem. 2012 , 4 , 547 - 551 ; Conyard, J.; Conssen, A.; Browne, W. R.; Feringa, B. L.; Meech, S. R. J. Am. Chem. Soc. 2014 , 136 , 9692 - 9700 ] are well understood. We find that this "dark state" in the molecular rotary motor is not a new electronic state, but the "dark region" with low oscillator strength on the initial S state.

Solid state astrochemical reaction pathways have the potential to link the formation of small nitrogen-bearing species, like NH3 and HNCO, and prebiotic molecules, specifically amino acids. To date, the chemical origin of such small nitrogen containing species is still not well understood, despite the fact that ammonia is an abundant constituent of interstellar ices toward young stellar objects and quiescent molecular clouds. This is mainly because of the lack of dedicated laboratory studies. The aim of the present work is to experimentally investigate the formation routes of NH3 and HNCO through non-energetic surface reactions in interstellar ice analogues under fully controlled laboratory conditions and at astrochemically relevant temperatures. This study focuses on the formation of NH3 and HNCO in CO-rich (non-polar) interstellar ices that simulate the CO freeze-out stage in dark interstellar cloud regions, well before thermal and energetic processing start to become relevant. We demonstrate and discuss the surface formation of solid HNCO through the interaction of CO molecules with NH radicals -one of the intermediates in the formation of solid NH3 upon sequential hydrogenation of N atoms. The importance of HNCO for astrobiology is discussed.