Wim G. Roeterdink

Femtosecond velocity map imaging of dissociative ionization dynamics in CF3IPresented at the Stereodynamics 2000 Conference on Dynamics and Stereodynamics of Chemical Reactions, El Escorial, Madrid, December 1–5, 2000.

The multi-photon dissociation dynamics of CF3I has been studied with femtosecond pump–probe spectroscopy and velocity map ion imaging. The CF3+ and I+ fragments produced in a time-delayed pump–probe excitation are detected with a two-dimensional ion imaging setup in a velocity map imaging configuration. The ion images for the selected ionic photofragments provide the velocity and angular distribution of the recoiling fragments. The experiments were performed with both parallel and perpendicular polarization geometry of the pump laser, at 264 nm, versus the probe laser at 396 nm. The velocity and angular distributions provide information on the multi-photon pathways and the potential energy surfaces involved. The CF3+ fragments are mainly formed by a two-photon pump excitation at 264 nm, via the one-photon resonant A band, to the 5pπ7sσ(2Π1/2) Rydberg state followed by a one-photon probe excitation to the CF3I+ parent states with subsequent dissociation. Analysis of the I+ data indicates that at most delay times the fragments are formed via a two-photon absorption at 264 nm to the Rydberg state, followed by a two-photon transition at 396 nm to the state of the parent ion. However, at delay times around 200–400 fs the kinetic energy distribution of the I+ fragments changes dramatically relative to 0 fs and 1000 fs. The origin of the very slow I+ fragments is probably a bound–free–bound excitation via the repulsive A band to a higher lying ion-pair state. It is shown that the ion imaging technique combined with femtosecond time-resolved spectroscopy provides a direct view of the complex dynamics and multi-photon pathways involved in the dissociative photodynamics of CF3I.

Charging-induced asymmetric spin distribution in an asymmetric (9,0) carbon nanotube

Asymmetry in the electronic structure of low-dimensional carbon nanomaterials is important for designing molecular devices for functions such as directional transport and magnetic switching. In this paper, we use density functional theory to achieve an asymmetric spin distribution in a typical (9,0) carbon nanotube (CNT) by capping the CNT with a fullerene hemisphere at one end and saturating the dangling bonds with hydrogen atoms at the other end. The asymmetric structure facilitates obvious asymmetry in the spin distribution along the tube axis direction, with the maximum difference between the ends reaching 1.6 e Å(-1). More interestingly, the heterogeneity of the spin distribution can be controlled by charging the system. Increasing or decreasing the charge by 2e can reduce the maximum difference in the linear spin density along the tube axis to approximately 0.68 e Å(-1) without changing the proportion of the total electron distribution. Further analyses of the electron density difference and the density of states reveal the loss and gain of charge and the participation of atomic orbitals at both ends. Our study characterizes the asymmetric spin distribution in a typical asymmetric carbon system and its correlation with charge at the atomic level. The results provide a strategy for controlling the spin distribution for functional molecular devices through a simple charge adjustment.

Laser induced alignment of state-selected CH3I

Hexapole state selection is used to prepare CH3I molecules in the |JKM〉 = |1±1∓1〉 state. The molecules are aligned in a strong 800 nm laser field, which is linearly polarised perpendicular to the weak static extraction field E of the time of flight setup. The molecules are subsequently ionised by a second time delayed probe laser pulse. It will be shown that in this geometry at high enough laser intensities the Newton sphere has sufficient symmetry to apply the inverse Abel transformation to reconstruct the three dimensional distribution from the projected ion image. The laser induced controllable alignment was found to have the upper and lower extreme values of 〈P2(cos θ)〉 = 0.7 for the aligned molecule and -0.1 for the anti-aligned molecule, coupled to 〈P4(cos θ)〉 between 0.3 and 0.0. The method to extract the alignment parameters 〈P2(cos θ)〉 and 〈P4(cos θ)〉 directly from the velocity map ion images will be discussed.

Rotational Dynamics of Quantum State-Selected Symmetric-Top Molecules in Nonresonant Femtosecond Laser Fields

Rotational dynamics of quantum state selected and unselected CH3I molecules in intense femtosecond laser fields has been studied. The orientation and alignment evolutions are derived from a pump-probe measurement and in good agreement with the numerical results from the time-dependent Schrödinger equation (TDSE) calculation. The different rotational transitions through nonresonant Raman process have been assigned from the Fourier analysis of the orientation and alignment revivals. These revivals are derived from a pump-probe measurement and in good agreement with the numerical results from the TDSE calculation. For the molecules in rotational state |1, ±1, ∓1⟩, the transitions can be assigned to ΔJ = ±1, ±2, while for thermally populated molecules, the transitions are ΔJ = ±2. Our results illustrate that the orientation and alignment revivals of the rotational quantum-state-selected molecules give a deep insight into the rotational excitation pathways for the transition of different rotational states of molecules in ultrafast laser fields.