6. Descripción de la experiencia
6.5 Descripción de la experiencia a analizar: practica educativa del periodo 2018-2
directed reactivity
In order to achieve efficient laser-driven, charge-directed reactivity [71], the timescale of the laser control over the electronic motion should match the timescale of the nuclear motion. For example, the dissociation time for the molecular hydrogen ion via BS may be estimated as approximately half of a vibrational period, thus requiring laser control over about 12 fs [31]. At the same time CEP-control of the electron motion requires the number of contributing laser cycles to be small. Longer wavelengths λ allow pulses with optical periods T = λ/c (where c is the speed of light) which are significantly longer in time as compared to the pulses (with the same numbers of cycles) in the near-IR. Consequently, few-cycle laser pulses in the mid-IR fulfill both criteria and therefore can be better suited for CEP-control of charge-directed reactivity.
In addition, mid-IR laser fields allow studying atomic and molecular systems deep in the tunneling regime. For exampleγ = 0.54 for the D2 experiments described below, while
γ = 1.1–1.2 in experiments on D2 and HD with near-IR laser pulses that were described above. Here γ is the Keldysh parameter [36]: γ =
q
Ip
2Up, where Ip is ionization potential
of an atom or a molecule andUp is the ponderomotive potential. Up represents the average
energy of the electron’s quiver motion in the electric field:
Up(eV) =
e2E02
4meω2
= 9.33×10−14 I [W cm−2] λ2 [µm] (3.14)
where me and e are the mass and charge of the electron, E0 is the field amplitude, ω the angular frequency and λ the wavelength of the electric field. γ much less than one corre- sponds to the tunneling regime, γ much greater than one corresponds to the multiphoton regime. Considering a decreasing photon energy at longer wavelengths, nonlinear field- effects in the multiphoton regime would require a higher number of involved photons. The tunneling ionization determines the time window at which the nuclear dynamics starts. A smaller γ parameter might help to confine the ionization time [91] and contribute to the higher temporal control of the dynamics.
3.6 Subcycle-controlled, charge-directed reactivity employing mid-IR
few-cycle fields 33
Figure 3.11: Relevant potential energy surfaces of D2, D+2 and D2+2 obtained by ab-initio calculations described in the text. The higher-lying excited electronic states of D+2 above the A-state are labeled alphabetically accordingly to their symmetry. The red arrow indi- cates tunnel ionization of D2, the blue arrow BS and the green arrows recollision induced excitation (RCE) and ionization (RCI). From [28].
in intense few-cycle CEP-stable mid-IR (2.1µm) laser fields and compared the results with TDSE calculations. A detailed description of the laser setup can be found in chapter 2 and Ref. [54]. The laser pulses of approximately 25 fs duration were focused to an intensity of (6.2±1.5)×1013 W cm−2 and D+ fragments resulting from the dissociative ionization of D2 were measured via VMI.
The multiple pathways for the investigated dissociative ionization of D2 are summarized in Fig. 3.11. D+2 is produced from D2 by tunnel ionization (red arrow) whereby a nuclear wave packet is launched on the potential of the X2Σ+g state. Several subsequent processes leading to the dissociation of the molecular ion can be identified: i) BS (blue arrow), ii) RCE, and iii) recollisional ionization (RCI) (green arrows). In our study fragments resulting from both BS and RCE channels can be distinguished and controlled simultaneously [28].
Fig. 3.12a shows a cut for pz=0 through the 3D momentum distribution of the D+ions.
The polarization of the laser is vertical. The corresponding D+ kinetic energy spectrum, integrated over the full solid angle, is displayed in Fig. 3.12b (red line). The spectrum reveals 4 regions, which are also indicated in Fig. 3.12a by white dashed circles. The most intense contribution in the spectrum between 0 to 1 eV is assigned to BS. Three additional
34 isotopes
Figure 3.12: (a) Inverted CEP-averaged D+ momentum distribution (py vs. px at pz=0),
the signal was left-right as well as up-down symmetrized, a logarithmic color scale was used. The laser is polarized along the py axis. The dashed circles separate the four contributions
discussed in the text. (b) Angle-integrated kinetic energy spectrum of D+ ions obtained from the experimental data (red curve) and calculated spectra for the BS dissociation involving only the X and A-states of D+2 (green curve) and also the 11 higher states of D+2 shown in Fig. 3.11 (blue curve). The theoretical results for the dissociation via recollision induced excitation (RCE) and ionization (RCI) are shown as solid and dashed black lines, respectively. All spectra are normalized by their maximum values. Adapted from [28].
3.6 Subcycle-controlled, charge-directed reactivity employing mid-IR
few-cycle fields 35
contributions in the ranges 1–4 eV, 4–9 eV and 9–13 eV are discussed in detail below together with theoretical analysis of the control.
The directional D+ ion emission as a function of phaseϕand momentumpis analyzed by the angle-integrated asymmetry parameterA(p, ϕ) withα=10◦similar to Eq. 3.11 and is shown in Fig. 3.13a. The experimental phase offset was calibrated achieving best agreement between experimental and calculated CEP-dependent asymmetry oscillations for the BS channel. A high degree of asymmetry (with an amplitude of approximately 0.2) is found for the BS channel (energies below 1 eV). The observation of such a strong asymmetry in the BS channel is very remarkable when compared to earlier results obtained in the near-IR [23]. A second asymmetry contribution in the energy range 1–4 eV has a weaker (max. 0.1) amplitude.