Theory of selective excitation in Stimulated Raman Scattering

A semiclassical model is used to investigate the possibility of selectively exciting one of two closely spaced, uncoupled Raman transitions. The duration of the intense pump pulse that creates the Raman coherence is shorter than the vibrational period of a molecule (impulsive regime of interaction). Pulse shapes are found that provide either enhancement or suppression of particular vibrational excitations.Comment: RevTeX4,10 pages, 5 figures, submitted to Phys.Rev.

Momentum state engineering and control in Bose-Einstein condensates

We demonstrate theoretically the use of genetic learning algorithms to coherently control the dynamics of a Bose-Einstein condensate. We consider specifically the situation of a condensate in an optical lattice formed by two counterpropagating laser beams. The frequency detuning between the lasers acts as a control parameter that can be used to precisely manipulate the condensate even in the presence of a significant mean-field energy. We illustrate this procedure in the coherent acceleration of a condensate and in the preparation of a superposition of prescribed relative phase.Comment: 9 pages incl. 6 PostScript figures (.eps), LaTeX using RevTeX, submitted to Phys. Rev. A, incl. small modifications, some references adde

Ultrafast relaxation dynamics of uracil probed via strong field dissociative ionization

We study the ultrafast relaxation dynamics of uracil excited to the first bright \u3c0\u3c0* state (S2) by an ultrafast laser pulse in the deep ultraviolet (central wavelength \u3bb0 = 260 nm). With a unique combination of strong field dissociative ionization measurements, state of the art strong field ionization calculations, and high level ab initio calculations of excited neutral and ionic states at critical points along the neutral potentials, we are able to gain a detailed picture of the relaxation dynamics of the molecule, which resolves earlier disagreements regarding measurements and calculations of the relaxation. \ua9 2013 American Chemical Society.Peer reviewed: YesNRC publication: Ye

Neutral-ionic state correlations in strong-field molecular ionization

We study correlations between neutral and ionic states in strong-field molecular ionization. We compare predictions based on Dyson orbital norms and quasistatic semiclassical tunneling theories (Keldysh and molecular orbital Ammosov-Delone-Krainov) with more detailed calculations of strong-field ionization which take into account (i)the Coulomb interaction between the outgoing continuum electron wave packet and the remaining bound electrons and (ii)electron-core interactions that cause distortions of the electronic continuum states during the ionization event. Our results highlight the prominence of electronic rearrangement effects in strong-field ionization with intense ultrafast laser pulses, where the outgoing continuum electron can cause electronic transitions in the parent ion. Calculations and measurements for excited uracil molecules reveal the breakdown of Keldysh-weighted Dyson norm predictions for ionization to different states of the molecular cation in the strong-field regime. \ua9 2012 American Physical Society.Peer reviewed: YesNRC publication: Ye

Dyson norms in XUV and strong-field ionization of polyatomics: Cytosine and uracil

The extreme-ultraviolet (XUV) and strong-field valence ionization of cytosine and uracil is considered. We examine some simple estimates of the relative yields of the cation states populated following ionization and compare these to the results of a recently developed ab initio-type numerical model designed to compute strong-field ionization of molecules, the so-called time-dependent resolution in ionic states (TD-RIS) method. In analogy with one-photon XUV ionization, where the photoionization matrix elements can be related to the Dyson orbitals, we construct estimates for the yield of strong-field ionization (SFI) to different cation states based on the Dyson orbital norms and the Keldysh tunneling ionization rate. In the case of XUV ionization, the Dyson norms are shown to be good predictors of the relative cation yields when compared with the TD-RIS yields. The Dyson- and Keldysh-based models underestimate the yield to excited cation states in the case of SFI. The increased yield to the excited cation states in the TD-RIS results is attributed to the inclusion of multielectron effects and continuum structure not present in the simple models. The molecular Ammosov-Delone-Krainov (MO-ADK) method of calculating SFI is also considered. This later method is seen to agree more closely with the Dyson- and Keldysh-based estimates as it also fails to capture the multielectron effects and continuum structure included in the TD-RIS approach. \ua92012 American Physical Society.Peer reviewed: YesNRC publication: Ye

Analysis of the Toolkit method for the time-dependent Schrödinger equation

0907.2200 and 1004.5233International audienceThe goal of this paper is to provide an analysis of the ''toolkit'' method used in the numerical approximation of the time-dependent Schrödinger equation. The ''toolkit'' method is based on precomputation of elementary propagators and was seen to be very efficient in the optimal control framework. Our analysis shows that this method provides better results than the second order Strang operator splitting. In addition, we present two improvements of the method in the limit of low and large intensity control fields

Excitation, Fragmentation and Control of Large Finite Systems: C60\mathrm{C_{60}} in Moderately Strong Laser Fields

Recent progress in the understanding of the primary excitation mechanisms of the C$_{60}$ fullerene in intense laser pulses is reported. By analyzing mass spectra as a function of pulse duration, laser intensity and time delay between pump- and probe pulse insight into fundamental photoinduced processes such as ionization and fragmentation is obtained. Using ultrashort sub-10fs pulses excitation times are addressed which lie well below the characteristic time scales for electron–electron and electron–phonon coupling. The measured saturation intensities of multiply charged parent ions indicate that for higher charge states the well known C$_{60}$ giant plasmon resonance is involved in creating ions and a significant amount of large fragments through a non-adiabatic multi-electron dynamics. To enhance the formation of large fragments femtosecond laser pulses tailored with closed-loop, optimal control feedback were used. A characteristic pulse sequence excites oscillations in C$_{60}$ with large amplitude by coherent heating of nuclear motion. Again, the experimental findings can be understood by a laser-induced multi-electron excitation via the electronically excited resonance followed by efficient coupling to the radial symmetric breathing vibration of C$_{60}$