Waveform control of the electron localization in hydrogen deuteride

deuteride

Following the experimental demonstration of CEP-dependent charge-directed reactivity in D₂ [10], further experiments were conducted on isotopes of molecular hydrogen. Here, experiments on electron localization in hydrogen deuteride (HD) [11] are described, which were part of the work performed within this thesis. Hydrogen deuteride is a prototype system that contains a single remaining electron after the ionization step and is a het- eroatomic molecule, where the charged dissociation fragments H+ and D+ could be easily distinguished by their TOF spectra. The dissociation scheme for HD is shown in Eq. 3.12:

HD−→hν HD+ −→ H + D +

H++ D

(3.12)

The experimental conditions for the presented study are very similar to the previously discussed experiments with D2 [10]. CEP-stabilized laser pulses with a central wavelength of 760 nm and an intensity of about 1014 W cm−2 were applied to a hydrogen deuteride target. Momentum distributions of D+ and H+ fragments, products of the dissociative ionization of HD, reveal reaction pathways analogues to the case of D2. Fig. 3.5a shows the CEP-averaged momentum distribution of H+ ions. The distribution is left-right sym- metrized with respect to the vertical polarization of the laser. Fig. 3.5b illustrates an angle-integrated energy spectrum. Note that H+ and D+ momenta are found to be ap- proximately equal, thus the kinetic energiesW =p2/(2m) of the species differ by about a factor of their mass ratios, i.e. W_H+/W_D+ 'm_D+/m_H+ = 2.

We chose to analyze the ion emission within a restricted angular range of [0, α] with

α=20◦ where the angle-dependent asymmetry A(W, θ, ϕCEP) is maximal. This angle-

integrated asymmetry A(W, ϕCEP) for H+ ions is displayed in Fig. 3.5c. The asymmetry

map was energy integrated to obtain the data points shown in (d), red and blue curves are offset by approximately π/2. In general, the proton signal tends to show more noise,

3.3 Experimental observation of electron localization in molecular hydrogen

and its isotopes in few-cycle near-IR laser fields 25

Figure 3.5: Asymmetry data obtained for H+ fragments from the dissociative ionization of HD. (a) Momentum distribution of H+ after inversion. (b) Spectra (linear scale) integrated over the full angle. (c) Asymmetry map obtained for an angular integration of 40◦ (2α). (d) Asymmetry within integrated energy ranges of 1.3–2.3 eV (blue dots) and 3–8 eV (red dots) fitted by cosine functions.

because of an additional H+ contribution from ionization of H2O that was present as back- ground in the experimental chamber. Apart from this additional noise, the D+ and H+ ion spectra exhibit comparable asymmetry features. The asymmetry of H+ fragments dis- tinguishes the different dissociation pathways even more clearly than the spectra lone: the asymmetry from the RCE channel appears between 3 and 9 eV and the asymmetry from the BS channel appears between 1 and 3 eV.

The HD measurements were accompanied by reference measurements of ATI in xenon, allowing to assign absolute phases for the H+and D+scans. Taking into account the angle- integrated asymmetryA(W, ϕCEP) with α= 20◦, the absolute phaseϕCEP was set to zero

at a position where the cutoff electron emission from ATI of xenon reaches its maximum in the upward direction [52]. Note that measurements of the electron emission from ATI of xenon are often considered as a standard reference [18, 19, 24, 78, 79, 80]. The absolute phase calibration used here is based on classical arguments. For a better calibration of the absolute phase we have compared the ATI spectra in xenon to results from the recently developed quantitative rescattering (QRS) theory [51, 81, 82, 83].

Interestingly, accordingly to the findings by Tong and Lin [68] and by D. Geppertet al.

26 isotopes not necessarily coincide with phase values of ϕCEP = nϕ where n is an integer number.

The resulting asymmetry appears as a sum over contributions of all peaks of the laser field with sufficient amplitude to contribute to the ionization of the target. This might result in differences when the experimental results are compared to theoretical results considering just one peak of the laser contributing to the ionization.

3.3.5

Angle-dependent asymmetry by the example of D

and HD

The angular distributions of the D+and H+fragments from the dissociative ionization of D2 and HD, respectively, are further explored in Fig. 3.6. The angle-dependent asymmetries obtained via Eq. 3.10 as functions of kinetic energy W, fragment emission angle θ and phase ϕCEP were fitted to aϕCEP-dependent cosine function of the following form:

A(W, θ, ϕCEP) =A0(W, θ) cos(ϕCEP + ∆ϕ(W, θ)). (3.13)

This was done in order to determine the amplitude of the asymmetry A0(W, θ). Although CEP-dependent observables do not necessarily have to follow a cosine (or sine) function, the fits shown in Fig. 3.5 give an indication that the asymmetries follow to a good ap- proximation a cosine (or sine)-like behavior. This finding is in agreement with molecular studies described in this thesis and other studies [17, 18, 52, 84], where CEP-dependent asymmetries were explored.

Taking a closer look at Fig. 3.6, it becomes clear that for fragment emission angles above 45–50◦ the asymmetry vanishes. In general, the angle- and energy-dependent asym- metry amplitudes obtained for D+ fragments from the dissociative ionization of D2 and H+ fragments from HD look very similar. The BS channel of H+ fragments from HD (1.3– 2.3 eV) is more visible than an analogous low energy contribution for D+ from D2. Note that the H+ data were binned in momentum (∆p) and angular (∆θ) intervals to improve the signal-to-noise ratio. Also, a lower number of iterations in the iterative inversion proce- dure was used to reduce the noise. As a result, the asymmetry amplitude for H+ions from HD looks smoothed, but the contribution from the BS channel appears more visible. The angle- and energy-dependent asymmetry amplitude obtained for D+ fragments from HD (not shown) exhibits a similar structure. A butterfly shape of the asymmetry amplitudes further indicates different mechanisms for the generation of the asymmetry at low and high energies, identified above as BS and RCE channels.