Effect of Laser Parameters on Ultrafast Hydrogen Migration in Methanol Studied by Coincidence Momentum Imaging

Abstract

The effect of intensity, duration, and polarization of ultrashort laser pulses (795 nm, 40-100 fs, and 0.15-1.5 × 10(15) W/cm(2)) on the hydrogen migration in methanol is systematically investigated using Coulomb explosion coincidence momentum imaging. The ratio of the ion yield obtained for the migration pathway CH(3)OH(2+) → CH(2)(+) + OH(2)(+) with respect to the sum of the yields obtained for the migration pathway and for the nonmigration pathway CH(3)OH(2+) → CH(3)(+) + OH(+) exhibits a small (10-20%) but clear dependence on laser pulse properties, that is, the ratio decreases as the laser peak intensity increases but increases when the pulse duration increases as well as when the laser polarization is changed from linear to circular.

Interaction of toluene with two-color asymmetric laser fields: Controlling the directional emission of molecular hydrogen fragments

“…This result is somewhat unexpected since the formation of molecular hydrogen ions and hydrogen migration channels are described in the related literature as processes competitive to multiple ionization. 3,6,26,29 In order to evaluate the relative contribution of hydrogen atoms originating from the CH 3 -or the phenyl (C 6 H 5 -) moieties we recorded the mass spectra of two deuterium labeled isotopes of toluene (C 6 D 5 CH 3 , C 6 D 5 CD 3 ) at similar irradiation conditions.…”

“…3 At the same time, several researchers have attempted a step further towards the control of hydrogen migration, following different experimental techniques. [4][5][6][7] For the case of acetylene isomerization, Wells et al demonstrated an enhancement of the migration efficiency by orders of magnitude by optimizing the laser pulse characteristics. The novel experimental technique employs an adaptive control scheme, where the feedback variable is the ratio of the CH + fragments (coming from the two-body dissociation of the doubly charged acetylene: [C 2 H 2 ] 2+ → CH + + CH + ) to CH 2 + cations (formed by the dissociation of doubly charged vilyldene: [C 2 H 2 ] 2+ → CH 2 + + C + ), which are recorded in each step by VMI (velocity map imaging).…”

“…It was shown that as the laser intensity is increased the ratio of the migration to non-migration pathways decreases, while the opposite trend is observed for longer durations. 6 Finally, Xie et al have shown that isomerization process taking place from doubly charged parent ions ([P 2+ ]) of polyatomic hydrocarbons can be manipulated by few cycle carrier envelope phase controlled pulses. 7 The controlling mechanism is based on fine tuning the ponderomotive energy of the rescattering electron released after single ionization, in order to excite different dissociative states in the [P 2+ ] ionic manifold.…”

Controlling a Molecule’s Fate

“…Excitation to higher ionic energy levels during the laser pulse, on the other hand, can be achieved by field-induced transitions [9][10][11][12] or by electron recollision [13][14][15]. In practice, a mixture of these excitation mechanisms may be at work and their relative importance may strongly depend on the laser pulse parameters, in particular, on intensity and pulse duration [16,17]. In order to extend light-field control of molecular fragmentation to larger and more complex polyatomic molecules where a variety of different dissociation pathways coexist, it is necessary to enhance our knowledge about the coupling between the fast field-induced electronic dynamics and the resulting slower nuclear motion, and about the dependence of these processes on the laser parameters.…”

“…Hydrocarbons with their intrinsically very fast intramolecular nuclear dynamics of the light hydrogen atoms or protons, which leads to a rich restructuring dynamics, are particularly interesting and have been the object of numerous experimental studies, both using coincidence detection of the molecular fragments [1,[6][7][8]16,17,[21][22][23][24][25] and without coincidence detection [9,10,26,27].…”