Hydrogen scrambling in H 3 + generation from ethane induced by ultrashort intense laser fields

Two-body Coulomb explosion processes of ethane (CH3CH3) and its isotopomers (CD3CD3 and CH3CD3) induced by an intense laser field are investigated by a coincidence momentum imaging method. From the yield ratios of H3 , H2D , HD2 , and D3 + ejected from CH3CD3 + induced by an ultrashort-pulsed intense laser field, nearly statistical randomization of H and D atoms called hydrogen atom scrambling was


Introduction
When hydrocarbon molecules are exposed to an intense laser field, a variety of dynamical processes originating from ultrafast motion of hydrogen atoms proceed such as ultrafast hydrogen migration and ejection of H 3 + , as has been shown in our recent studies on methanol [1], ethane [2][3][4], and other hydrocarbon molecules [2].In the present study, in order to clarify how hydrogen atoms and/or protons migrate within an ethane molecule and to what extent the hydrogen atoms and protons exchange their positions among each other, we measured coincidence momentum imaging maps of fragment ions produced from ethane exposed to an intense laser field.

Experimental setup
Ultrashort laser pulses ( = 800 nm, I = 1.0×10 14 W/cm 2 , t = 120 fs, 5 kHz) were introduced into an ultrahigh vacuum chamber (7×10 −11 Torr) and focused with a quartz lens (f = 150 mm) at the point where the laser pulses crossed at right angles with a molecular beam of ethane (CH 3 CH 3 ), ethane-d 6 (CD 3 CD 3 ), or ethane-1,1,1-d 3 (CH 3 CD 3 ).The ions generated at the laser focal spot were projected by four equally spaced parallel-plate electrodes in the velocity mapping configuration onto a position-sensitive detector with delay-line readout.The three-dimensional momentum vectors of the fragment ions were determined by their position and arrival time on the detector plane.The laser polarization direction was set to be parallel to the detector plane.The false coincidence events EPJ Web of Conferences  .The angle θ is the angle between the laser polarization vector and the momentum vector of the fragement ions.originating from two or more parent molecular ions were discriminated from the true coincidence events by imposing the momentum conservation conditions.Furthermore, the probabilities of accidental momentum conservation were estimated from the momentum distributions of the fragment ions, and were taken into account in the data analysis.

Result & Discussion
The filled triangles in Figure 1(a) show the angular distribution of the momentum vector of CH 3 + ions generated through CH 3 CD 3 2+ → CH 3 + + CD 3 + with respect to the laser polarization vector.The CH 3 + ions are ejected preferentially to the direction of the laser polarization vector ( = 0 degree), showing that the CH 3 CD 3 2+ is produced preferentially when the C-C bond is parallel to the laser polarization vector, and that the C-C bond is immediately broken before the overall rotation of CH 3 CD 3 2+ proceeds.On the other hand, the filled circles in Figure 1(a) show the angular distribution of the momentum vector of H , the angular distribution, plotted with the open squares in Figure 1(a), is almost isotropic, but exhibits weak preference along the laser polarization direction.The filled circles and the open squares in Figure 1(b) show the kinetic energy release (KER) of H 3 + + C 2 H 3 + and H 2 + + C 2 H 4 + pathways, respectively.No significant differences can be found in these KER distributions.However, the peak position of the H 3 + pathway is higher than that of the H 2 + pathway by 0.4 eV.The corresponding angular distributions and KER distributions obtained for CD 3 CD 3 and CH 3 CD 3 are similar to those obtained for CH 3 CH 3 .

Fig. 1 .
Fig. 1.(a) Angular distributions and (b) KER distributions of the fragment ions generated from the Coulomb explosions of CH 3 CD 3 2+ and C 2 H 6

Table 1 .
The χ values for the decomposition pathways of doubly charged ethane.