Two-Pulse Field-Free Orientation Reveals Anisotropy of Molecular Shape Resonance

Abstract

We report the observation of macroscopic field-free orientation, i.e. more than 73 % of CO molecules pointing in the same direction. This is achieved through an all-optical scheme operating at high particle densities (> 10 17 cm −3 ) that combines a one-color (ω) and a two-color (ω + 2ω) non-resonant femtosecond laser pulses. We show that the achieved orientation solely relies on the hyperpolarizability interaction as opposed to an ionization-depletion mechanism thus opening a wide range of applications. The achieved strong orientation enables us to reveal the molecularframe anisotropies of the photorecombination amplitudes and phases caused by a shape resonance. The resonance appears as a local maximum in the even-harmonic emission around 28 eV. In contrast, the odd-harmonic emission is suppressed in this spectral region through the combined effects of an asymmetric photorecombination phase and a sub-cycle Stark effect, generic for polar molecules, that we experimentally identify.Techniques for fixing molecules in space are invaluable tools for a broad range of experiments in ultrafast science [1,2]. The availability of transiently aligned molecular samples has particularly advanced strong-field and attosecond spectroscopies, providing new insights into the electronic structure of molecules and its temporal evolution [3][4][5][6][7]. High-harmonic spectroscopy (HHS) provides a new access to the rich structures of photoionization continua, such as Cooper minima [8][9][10][11] and shape resonances [11][12][13]. The investigation of the inherent structural and dynamical anisotropies of polar molecules has however been prevented by the difficulty of orienting molecules. Interesting phenomena tied to polar molecules include the predicted recombination-site dependence of structural minima [14,15] and attosecond charge migration [16][17][18] triggered by strong-field ionization. Here, we demonstrate a protocol for molecular orientation which achieves macroscopic field-free orientation and exploit this progress to probe the anisotropy of photorecombination dipole moments at a molecular shape resonance.Successful approaches to laser-induced molecular orientation include the combination of an electrostatic field with a rapidly turned-off laser field [19], alignment in combination with quantum-state selection and a weak dc-field [20][21][22][23], adiabatic [24] and impulsive two-color orientation [25][26][27]. All of these techniques are subject to substantial limitations: The presence of electric fields may alter the electronic structure of the molecule, its photo-induced dynamics or the subsequent probing process. The low particle densities available after quantumstate selection make the application of such techniques to high-harmonic and attosecond spectroscopies challenging to impossible. The two-color scheme [22,25], recently applied to HHS [26,27], relies on an ionization-depletion mechanism [28] and thus ties the achievable degree of orientation to the ionization fraction of the sample. This fact does not only limi...

Observation of the field-free orientation of a symmetric-top molecule by terahertz laser pulses at high temperature

“…Note that a larger degree of orientation can be achieved with the two-color mechanisms through ionization depletion [53]. In this second regime, molecular orientation has been recently addressed experimentally for linear molecules using only a THz field [54] or its combination with a laser field [55], and in [56,57] for an excitation process with two-color laser fields.…”

Bicircular High-Harmonic Spectroscopy Reveals Dynamical Symmetries of Atoms and Molecules

“…High-harmonic generation from a linearly polarized driver is sensitive to inversion symmetry, the breaking of which leads to emission of even harmonics [15,16] that characterize the time-dependent electronic asymmetry of the studied sample [14,17]. In this Letter, we generalize the sensitivity of HHS to rotational symmetries of atoms and molecules and their time-dependent breaking by introducing bicircular HHS (BHHS), driven by a circularly polarized fundamental field of frequency ω and its counterrotating second harmonic 2ω.…”

Probing and extracting the structure of vibratingSF6molecules with inner-shell photoelectrons

“…Such a scheme has been proposed before; for example, in Refs. [47][48][49], where it is used to enhance molecular alignment and orientation. Furthermore, it should be possible to extend the method to the case when the target is prepared in electronically excited states, in particular dissociating or predissociating states, in which the nuclear wave packet experiences significant dynamical changes.…”