Probing photoelectron angular distributions in molecules with polarization-controlled two-color above-threshold ionization

Abstract

We present polarization-controlled multiphoton two-color above-threshold ionization (TCATI) of molecules. The intensity modulations of valence photoelectron intensities of molecules arising from varying the relative orientation of the linear polarization vectors of femtosecond infrared (IR) and vacuum-ultraviolet (VUV) radiation in TCATI of the highest occupied molecular orbitals of H 2 O, O 2 , and N 2 are reported. The results on the molecular systems are compared to the 3p photoionization of atomic Ar, which serves as a reference system. Modeling the large differences of the modulation amplitudes within the soft-photon approximation enables us to extract the one-photon-ionization anisotropy parameter β 2 . Accounting only for the first sideband due to two-photon TCATI by one VUV and one IR photon we find satisfactory agreement between experiment and simulation for H 2 O and O 2 . However, the model fails for N 2 and possible reasons are discussed. We discuss that the described approach may represent an alternative way of determining photoelectron angular distributions from valence shells of molecules and indicate future directions for modeling TCATI of molecules. The occurrence of sidebands in atomic photoemission with femtosecond vacuum-ultraviolet (VUV) pulses in the presence of an infrared (IR) dressing field was already discovered more than 20 years ago for the laser-assisted Auger decay (LAAD) [1] and for two-color above-threshold ionization (TCATI) [2]. Sidebands occur when the photon density of the dressing field is high enough to enable simultaneous absorption or emission of one or more IR photons by the photoelectron arising from ionization with the VUV pulse [3,4]. They are expressed in the spectra as satellite lines shifted in kinetic energy with respect to the main line from single-photon VUV ionization by the energy of one or more IR photons. As the sideband intensity represents a cross-correlation signal of the femtosecond IR and VUV pulses, two-photon TCATI on gaseous as well as solid samples has ever since been used to determine the temporal resolution in pump-probe photoelectron spectroscopy experiments with laboratory VUV and soft x-ray sources and free-electron lasers (FELs) [5][6][7][8][9][10][11][12][13][14][15][16][17].In TCATI in atoms it was discovered that the sideband intensity depends on the relative orientations of the linear polarization vectors of IR and VUV radiation [18,19]. This effect was then used in polarization-controlled TCATI to extract the angular distribution and the symmetry of the outgoing TCATI electron waves in He as a function of the dressing photon field density [20]. The polarization dependence in TCATI can be understood in terms of polarization-dependent coupling of the intermediate states with the continua, and the angular distribution is in general described by two asymmetry parameters, β 2 and β 4 [21,22] An atomic model connecting sideband polarization sensitivity with the one-photon-ionization anisotropy parameter β 2 , which characterizes the photoelectro...

Different approximations in the polarization control of two-color water molecule ionization

“…The same behavior was reported for laser-assisted photoionization in the case of water molecules, considering of only one harmonic selected from an XUV pulse train in the presence of an IR field [17]. Provided that the energy of the assistant IR laser is substantially smaller than the photoelectron energy, these experiments might be described by means of a simple first-order atomic model developed within the framework of the soft-photon approximation (SPA) [16][17][18]. However, the SPA fails for low and near-threshold photoelectron energies, mainly because of the plane wave (PW) representation of the continuum states.…”

“…In order to quantify the degree of polarization control of the total cross-section achieved using the two-photon two-color ionization process, it is convenient to define the sideband intensity modulation as [17]…”

Two-color above-threshold ionization of atoms and ions in XUV Bessel beams and intense laser light

“…are often referred to as partial amplitudes. As seen from expression (23), the angular distribution of the photoelectrons is now determined by a convolution of the sideband amplitudes F with the Fourier coefficients of the vortex Bessel beam a κm (k ⊥ ) from Eq. ( 4) and a phase factor that just contains the impact vector b.…”

Angle-resolved photoelectron energy spectrum from the high-order above-threshold ionization process in IR+XUV two-color laser fields

“…Furthermore, it was found that a sideband structure is presented in the photoelectron spectrum as well as the Auger electron spectrum [37][38][39], which can be in favor of the in-vestigation of the properties of pulses reversely. Also, these sidebands have been applied in investigating the molecular orbitals of O 2 , H 2 O and N 2 [40]. Additionally, the ATI spectrum presents a steplike structure when the XUV photon energy is much higher than the atomic ionization potential [41,42] and the dependence of ATI spectrum on polarization direction of two-color laser fields is identified [43].…”