Attosecond time delay spectroscopy of the hydrogen molecule

Attosecond science has extended experimental studies of atomic and molecular photoionization into a new dimension. In atoms, a time delay between absorption of an attosecond XUV pulse and subsequent emission of a photoelectron has been measured [1, 2, 3]. In molecules, electron localization and attosecond control have been demonstrated in pump-probe photoionization experiments [4, 5]. Experimental time-delay studies in molecules are yet to be performed either by using the attosecond streaking [1] or by the interferometric sideband oscillation technique [2]. In the meantime, molecules offer a very rich and complex photoionization picture in which the cross-section and angular distribution of photoelectrons depend sensitively on the molecular orientation relative to the polarization axis of VUV radiation. Thus the attosecond time delay studies can be very beneficial to molecular photoionization by defining the phase of the ionization amplitude and thus achieving the complete photoionization experiment [6]. In the simplest case of a homonuclear diatomic molecule, its orientation is defined by the mutual angle θN of the molecular and polarization axes. Photoionization cross-section and angular distribution of photoelectrons in H2 and D2 depend strongly on this angle varying between the limits of Σ (θN = 0) and Π (θN = 90◦) orientations. In single photoionization, the amplitude and cross-section of the Σ orientated H2 display a deep minimum [7] which can be attributed to the two-center electron interference [8]. In double photoionization (DPI), the angular correlation pattern in two-electron continuum shows strong variation with the angle θN . Because of the Coulomb explosion of the doubly ionized H2 molecule, its orientation at the moment of ionization can be measured experimentally [9, 10]. In principle, neutral polarizable molecules can also be aligned by a strong laser field [11]. However, reports of single photoionization of aligned molecules are not known to the authors. In this Letter, we offer an alternative strategy of at-