Subfemtosecond Tracing of Molecular Dynamics during Strong-Field Interaction

Abstract

We introduce and experimentally demonstrate a method, where the two intrinsic time scales of a molecule, the slow nuclear motion and the fast electronic motion, are simultaneously measured in a photo-electron photo-ion coincidence experiment. In our experiment, elliptically polarized, 750 nm, 4.5 fs laser pulses were focused to an intensity of 9×10 14 W/cm 2 onto H2. Using coincidence imaging, we directly observe the nuclear wavepacket evolving on the 1sσg state of H + 2 during its first roundtrip with attosecond temporal and picometer spatial resolution. The demonstrated method should enable insight into the first few femtoseconds of the vibronic dynamics of ionization-induced unimolecular reactions of larger molecules. t −∞ E(t )dt . Thus, measurement of the electron emission angle in the laboratory frame determines the ionization time t i within one cycle. In addition, the magnitude of the emitted electron's momentum vector |p e | is proportional to the instantaneous field strength and thereby provides a measure about the ionization time arXiv:1907.05347v1 [physics.chem-ph]

Clocking Enhanced Ionization of Hydrogen Molecules with Rotational Wave Packets

“…However, since the nuclear dynamics of H 2 and its ion proceeds rapidly on a few-femtosecond timescale, it has been a challenge to disentangle the precise timings of the different pathways. Using electron and ion momentum imaging, the bound state wave packet of the H 2 þ molecular ion was recently resolved within the first few femtoseconds [16]. The time delay between abovethreshold dissociation (ATD) and bond softening was determined using polarization-skewed laser pulses [17].…”

Experimental Separation of Subcycle Ionization Bursts in Strong-Field Double Ionization of H2

“…The observation of the subcycle bursts and the unambiguous assignment of the two detected electrons to the first and second ionization burst was accomplished by combining our technique developed for atoms [14] with our recently published method for the reconstruction of molecular dynamics during laser interaction [16].…”

“…1(b) and 1(c). While the kinetic energy released, KER ¼ 1=R, with R as the internuclear distance at which the Coulomb explosion is initiated, provides a precise measure for the nuclear motion in between the two ionization steps [16,24,25], the momenta of the two emitted electrons p e1;e2 provide information on the emission times of the two electrons within a laser cycle. This is because in elliptically polarized light the ionization phase within a laser cycle ωt i is approximately mapped onto the emission angle of the photoelectron by the laser field via p ei ¼ −Aðt i Þ, i ¼ f1; 2g [26,27], cf.…”

“…The laser vector potential AðtÞ is connected to the laser electric field by AðtÞ ¼ − R t −∞ Eðt 0 Þdt 0 . Thus, measurement of the electron emission angle in the laboratory frame determines the ionization time t i within one cycle [14,16,30,31].…”

“…1(f). Hence, selection of a certain range in KER allows selecting a well-defined range of Δt values [16].…”