We discuss theoretically and demonstrate experimentally the robustness of the adiabatic sum frequency conversion method. This technique, borrowed from an analogous scheme of robust population transfer in atomic physics and nuclear magnetic resonance, enables the achievement of nearly full frequency conversion in a sum frequency generation process for a bandwidth up to two orders of magnitude wider than in conventional conversion schemes. We show that this scheme is robust to variations in the parameters of both the nonlinear crystal and of the incoming light. These include the crystal temperature, the frequency of the incoming field, the pump intensity, the crystal length and the angle of incidence. Also, we show that this extremely broad bandwidth can be tuned to higher or lower central wavelengths by changing either the pump frequency or the crystal temperature. The detailed study of the properties of this converter is done using the Landau-Zener theory dealing with the adiabatic transitions in two level systems.
Functional group dependence is observed in the dissociative electron attachment (DEA) to various organic molecules in which the DEA features seen in the precursor molecules of the groups are retained in the bigger molecules. This functional group dependence is seen to lead to site-selective fragmentation of these molecules at the hydrogen sites. The results are explained in terms of the formation of core-excited Feshbach resonances. The results point to a simple way of controlling chemical reactions as well as interpreting the DEA data from bigger biological molecules.
We observe energy-dependent angle-resolved diffraction patterns in protons from strong-field dissociation of the molecular hydrogen ion H + 2 . The interference is a characteristic of dissociation around a laser-induced conical intersection (LICI), which is a point of contact between two surfaces in the dressed 2-dimensional Born-Oppenheimer potential energy landscape of a diatomic molecule in a strong laser field. The interference magnitude and angular period depend strongly on the energy difference between the initial state and the LICI, consistent with coherent diffraction around a cone-shaped potential barrier whose width and thickness depend on the relative energy of the initial state and the cone apex. These findings are supported by numerical solutions of the time-dependent Schrödinger equation for similar experimental conditions.The Born-Oppenheimer approximation (BOA) represents intramolecular dynamics as the motion of nuclear wave packets on potential energy surfaces (PES) of electronic eigenvalues embedded in the space of nuclear geometries. A molecule on a single PES remains there so long as the adiabatic condition is obeyed, i.e. so long as nuclear kinetic energies are small compared to electronic state separations. This assumption must break down, however, if two or more PESs approach each other [1][2][3]. When this happens non-adiabatic couplings between the nearly-degenerate surfaces become important. The true eigenvalues can then be calculated by diagonalizing the Hamiltonian in the reduced space of the near degeneracy.According to simple geometrical arguments, molecules with at least two dimensions of internal nuclear motion (i.e. three or more atoms) must have some points where two or more PESs become degenerate, a condition known as a conical intersection (CI) [4]. These CIs play a key role in the relaxation dynamics of most polyatomic molecules including important biochemical processes such as the photostability of DNA [5], and the preliminary process of vision [6]. The dimension of the CI manifold is two less than the internal nuclear geometry. Thus for the simplest case of a tri-atomic molecule, the CI manifold has dimensionality of 3N − 8 = 1. The topological nature of CIs allows the nonadiabatic couplings to diverge and thus display related phenomena such as a geometric or Berry's phase [4,7] in wavepackets that circumnavigate the CI.Naturally occurring CIs cannot exist for a free diatomic molecule because the internuclear separation vector R is the only internal nuclear degree of freedom, and this is insufficient to fulfil the crossing condition. The nonadiabatic terms in the full Hamiltonian cause the BOA states to repel according to the so-called "no-crossing" rule. A diatomic molecule in a strong laser field, however, has a second degree of freedom defined by the laser polarization ε. When viewed in a Floquet basis of laserdressed electronic states, a molecule coupled by this field can exhibit a point of degeneracy called a light-induced conical intersection (LICI) in the 2−dimension...
Momentum-imaging apparatus for the study of dissociative electron attachment dynamics Rev. Sci. Instrum. 84, 033104 (2013); 10.1063/1.4794093 Velocity map photoelectron-photoion coincidence imaging on a single detector Rev. Sci. Instrum. 83, 093103 (2012); 10.1063/1.4749843 Dissociative electron attachment resonances in ammonia: A velocity slice imaging based study Positive/negative ion velocity mapping apparatus for electron-molecule reactions Rev. Sci. Instrum. 83, 013108 (2012);A velocity slice imaging method is developed for measuring the angular distribution of fragment negative ions arising from dissociative electron attachment ͑DEA͒ to molecules. A low energy pulsed electron gun, a pulsed field ion extraction, and a two-dimensional position sensitive detector consisting of microchannel plates and a wedge-and-strip anode are used for this purpose. Detection and storage of each ion separately for its position and flight time allows analysis of the data offline for any given time slice, without resorting to pulsing the detector bias. The performance of the system is evaluated by measuring the angular distribution of O − from O 2 and comparing it with existing data obtained using conventional technique. The capability of this technique in obtaining forward and backward angular distribution data is shown to have helped in resolving one of the existing problems in the electron scattering on O 2 .
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