Parity-dependent rotational rainbows in D2-NO and He-NO differential collision cross sections.

The (j′, - ′, ′) dependent differential collision cross sections of D2 with fully state selected (j=12, =12, =-1) NO have been determined at a collision energy of about 550 cm-1. The collisionally excited NO molecules are detected by (1+ 1′) resonance enhanced multiphoton ionization combined using velocity-mapped ion-imaging. The results are compared to He-NO scattering results and tend to be more forward scattered for the same final rotational state. Both for collisions of the atomic He and the molecular D2 with NO, scattering into pairs of rotational states with the same value of n= j′ - ′ 2 yields the same angular dependence of the cross section. This "parity propensity rule" remains present both for spin-orbit conserving and spin-orbit changing transitions. The maxima in the differential cross sections-that reflect rotational rainbows-have been extracted from the D2 -NO and the He-NO differential cross sections. These maxima are found to be distinct for odd and even parity pair number n. Rainbow positions of parity changing transitions (n is odd) occur at larger scattering angles than those of parity conserving transitions (n is even). Parity conserving transitions exhibit-from a classical point of view-a larger effective eccentricity of the shell. No rainbow doubling due to collisions onto either the N-end or the O-end was observed. From a classical point of view the presence of a double rainbow is expected. Rotational excitation of the D2 molecules has not been observed. © 2006 American Institute of Physics

Multielectron effects and nonadiabatic electronic dynamics in above threshold ionization and high-harmonic generation

We explore the effects of multiple participating final ion states during strong field ionization (SFI) and high-harmonic generation (HHG) using a two-electron two-center reduced-dimensionality model. In particular, we propose to use above threshold ionization (ATI) photoelectron spectra to identify ionic states that are active in SFI, and demonstrate the feasibility of our proposal to track multiple ionic states in a dissociation scenario. In addition to offering clear signatures of multiple participating ionic states, our method is shown to be sensitive to sub-cycle nonadiabatic laser-driven couplings between the ionic states. Finally, we calculate the high-harmonic emission and show how the multiple ionic states identified in the ATI spectra can manifest themselves in the high-harmonic spectra.Peer reviewed: YesNRC publication: Ye

The quasi-quantum treatment of rotationally inelastic scattering from a hard shell potential: its derivation and practical use

The QQT is a quasi-quantum mechanical treatment of the collision between molecules. Instead of a partial wave expansion approach, it uses a kind of Feynman path integral method that exploits the path length differences originating from the different orientations of an anisotropic molecule. As a result, the QQT provides valuable physical insight while requiring very little computational effort. The current paper gives a systematic derivation of the QQT and explains its underlying principles. The expression for the scattering amplitude is shown to be self-consistent, without any normalisation factors, when the rotational energy level spacing is negligible. The constant curvature approximation that is presented makes the QQT conceptually even more simple, and its effect on the calculated differential cross-sections (DCSs) turns out to be small. As examples we present QQT calculations of the DCSs for Ne-CO(1) and He-NO(2), at collision energies of, respectively, 511 cm-1 and 514 cm-1. The anisotropy of the hard shell potential energy surface for Ne-CO in terms of the incoming de Broglie wavelength is about twice as large as for He-NO. This leads to state-to-state DCSs that have up to three maxima of comparable amplitude, instead of only one large maximum as is found for He-NO. The QQT results for these two applications are compared with results from close coupling calculations

The Multielectron Ionization Dynamics Underlying Attosecond Strong-Field Spectroscopies

Subcycle strong-field ionization (SFI) underlies many emerging spectroscopic probes of atomic or molecular attosecond electronic dynamics. Extending methods such as attosecond high harmonic generation spectroscopy to complex polyatomic molecules requires an understanding of multielectronic excitations, already hinted at by theoretical modeling of experiments on atoms, diatomics, and triatomics. Here, we present a direct method which, independent of theory, experimentally probes the participation of multiple electronic continua in the SFI dynamics of polyatomic molecules. We use saturated (n-butane) and unsaturated (1,3-butadiene) linear hydrocarbons to show how subcycle SFI of polyatomics can be directly resolved into its distinct electronic-continuum channels by above-threshold ionization photoelectron spectroscopy. Our approach makes use of photoelectron-photofragment coincidences, suiting broad classes of polyatomic molecules

Molecular Movies from Molecular Frame Photoelectron Angular Distribution (MF-PAD) Measurements

We discuss recent and on-going experiments, where molecular frame photoelectron angular distributions (MFPADs) of high kinetic energy photoelectrons are measured in order to determine the time evolution of molecular structures in the course of a photochemical event. These experiments include, on the one hand, measurements where single XUV/X-ray photons, obtained from a free electron laser (FEL) or by means of high-harmonic generation (HHG), are used to eject a high energy photoelectron, and, on the other hand, measurements where a large number of mid-infrared photons are absorbed in the course of strong-field ionization. In the former case, first results indicate a manifestation of the both the electronic orbital and the molecular structure in the angle-resolved photoelectron distributions, while in the latter case novel holographic structures are measured that suggest that both the molecular structure and ultrafast electronic rearrangement processes can be studied with a time-resolution that reaches down into the attosecond and few-femtosecond domain

Optimization of laser field-free orientation of a state-selected NO molecular sample

International audienceWe present a theoretical investigation of the impulsive orientation that can be induced by exposing a sample of state-selected NO molecules to the combination of a dc field and a short laser pulse. A strong degree of laserfield-free alignment and orientation is already achievable at moderate intensities and can be further improved by tailoring the temporal profile of the laser pulse. The alignment and orientation are only limited by the maximum value of the applied dc field and the pulse energy in the femtosecond laser pulse. Using an evolutionary algorithm coupled to a non-perturbative calculation of the timedependent Schrödinger equation, the solutions that are obtained suggest that hcos i = 0.964 is experimentally achievable

Impulsive orientation and alignment of quantum-state-selected NO molecules

Manipulation of the molecular-axis distribution is an important ingredient in experiments aimed at understanding and controlling molecular processes(1-6). Samples of aligned or oriented molecules can be obtained following the interaction with an intense laser field(7-9), enabling experiments in the molecular rather than the laboratory frame(10-12). However, the degree of impulsive molecular orientation and alignment that can be achieved using a single laser field is limited(13) and crucially depends on the initial states, which are thermally populated. Here we report the successful demonstration of a new technique for laser-field-free orientation and alignment of molecules that combines an electrostatic field, non-resonant femtosecond laser excitation(14) and the preparation of state-selected molecules using a hexapole(2). As a unique quantum-mechanical wavepacket is formed, a large degree of orientation and alignment is observed both during and after the femtosecond laser pulse, which is even further increased (to < cos theta > = -0.74 and < cos(2)theta > = 0.82, respectively) by tailoring the shape of the femtosecond laser pulse. This work should enable new applications such as the study of reaction dynamics or collision experiments in the molecular frame, and orbital tomography(11) of heteronuclear molecules.No Full Tex