Signatures of Molecular Orbital Structure in Lateral Electron Momentum Distributions from Strong-Field Ionization

Abstract

Strong-field ionization of aligned diatomic and polyatomic molecules such as O 2 , N 2 , C 2 H 4 , and others in circularly polarized laser fields is investigated theoretically. By calculating the emission-angle-resolved lateral width of the momentum distribution perpendicular to the polarization plane, we show that nodal planes in molecular orbitals are directly imprinted on the angular dependence of the width. We demonstrate that orbital symmetries can be distinguished with the information obtained by observing the lateral width in addition to the angular distributions. DOI: 10.1103/PhysRevLett.114.103004 PACS numbers: 33.80.Rv, 33.20.Xx Progress in strong-field science has led to the possibility to image dynamics in atoms and molecules on the attosecond time scale and angstrom spatial scale [1][2][3]. Laser-based methods to image molecular structure include diffraction of recolliding electrons [4][5][6], Coulomb explosion imaging [7,8], and orbital tomography [9][10][11][12][13][14]. Information about the orbital structure of aligned molecules is also contained in the dependence of the ionization rate on the angle between the laser polarization direction and the molecular axis [15,16] with particular care needed for polar molecules [17]. Rich information is obtained by measuring three-dimensional photoelectron momentum distributions (PMDs) of aligned molecules. This is possible by combining laser-induced alignment of molecules [18,19] and velocity-map imaging [20,21]. Alternatively, PMDs can be measured in a reaction microscope [22], where information on the molecular orientation is obtained by electron-ion coincidence. For molecular imaging, circularly polarized light effectively delivers a 360°scan of the aligned molecule [23][24][25], which is an advantage over linear polarization. In the classical trajectory model without Coulomb effects, electrons measured at an emission angle ϕ originate from ionization at the angle ϕ AE 90°with the sign depending on the rotation direction of the field. Another advantage of circular polarization is the absence of electron recollisions that could obscure the measured distribution. Nodal structures have been measured successfully for various molecules using many-cycle circularly polarized pulses [26][27][28][29]. Theoretical prediction of the PMDs is complicated by the nonperturbative nature of the ionization process in the commonly used near-infrared fields. Only for very small molecules such as H 2 þ [30] or in proof-of-principle calculations with model orbitals [31], the solution of the timedependent Schrödinger equation (TDSE) is an option. In contrast, the strong-field approximation (SFA) can be used to calculate PMDs of arbitrary molecules and often yields accurate results [32][33][34]. In polar molecules, an extended SFA including the Stark shift is beneficial [35][36][37].To the authors' knowledge, none of the published experimental work on molecular imaging has made use of the lateral photoelectron momentum distribution (LPMD), i.e., the distribution of the m...

Molecular high-order-harmonic generation due to the recollision mechanism by a circularly polarized laser pulse

“…For instance, this HHG process offers an ideal framework for the study of strong-field dynamics [37]. Moreover, as the CPLP has already shown its great power in the application of (one-shot) imaging or structure detection of molecules [39][40][41][42], the HHG by CPLP can be potentially utilized for the imaging and detection of molecules with an all-optical method.…”

“…It is found that the higher efficiency is mainly contributed in the recombination step and is at least partly due to the higher recollision probability for molecules. Since this HHG process involves the unique recollision mechanism under a circularly polarized laser field, it will stimulate new potential applications, such as offering an ideal framework for study of strong-field dynamics [37] and (one-shot) imaging or structure detection of molecules with all optical methods [39][40][41][42].…”

Journeys from quantum optics to quantum technology

“…Some of these activities can still be traced back to origins in the work done in the group of Peter Knight during the years 2001/2002. For example, the imaging of molecular structure still plays a major role in current work, see [120].…”

Longitudinal photoelectron momentum shifts induced by absorbing a single XUV photon in diatomic molecules

“…In these studies, the dipole approximation are widely accepted when a Ti:Sapphire laser pulse with an intensity below 10 16 W/cm 2 is introduced [4]. Within the dipole approximation, the photoelectron momentum distribution along the laser propagation direction has a symmetric distribution centered at zero (see Ref [5][6][7] for example).…”