Note: Energy calibration of a femtosecond photoelectron imaging detector with correction for the ponderomotive shift of atomic ionization energies

Abstract

Femtosecond photoelectron imaging spectroscopy is a powerful technique for following state-resolved molecular transformations in complex coupled potential energy landscapes. To avoid unwanted nonlinear side-effects, the employed laser pulse energies are usually reduced to minimal values. However, the energy calibration of the photoelectron imaging detector is ideally performed using multi-photon above-threshold ionization of suitable atomic species, for which rather high laser intensities are required. In this work, we show that the calibration spectra of xenon obtained with high laser pulse energies cannot be directly used for the evaluation of molecular photoelectron spectra recorded using low-energy laser pulses. The reason is the intensity-dependent AC Stark shift of the atomic ionization energies to larger values, which in turn leads to a corresponding decrease of the photoelectron kinetic energies. We present a simple procedure to quantify this so-called ponderomotive shift and calculate the theoretically expected un-shifted photoelectron energies.

Photoelectron imaging of resonance-enhanced multiphoton ionization and above-threshold ionization of ammonia molecules in a strong 800-nm laser pulse*

“…The laser intensity was calibrated by measuring the energy shift of the non-resonant ATI peaks of Xe; for further details, please see Refs. [25] and. [26] In our measurement, the laser polarization was parallel to the detector of velocity map imaging (VMI).…”

Probing time delay of strong-field resonant above-threshold ionization*

“…In the earlier studies, it has been demonstrated that intensity is a useful knob to shift the position of ATI peak in the energy domain due to the pondermotive energy shift. [44,45] Therefore, the Coulomb effect can be compared between ATI peaks with very close energies by changing laser intensity slightly. We first show how the ATI peaks are shifted in the energy domain by varying the laser intensity from 2.6 × 10 13 W/cm 2 to 8.4 × 10 13 W/cm 2 in Fig.…”

Sub-optical-cycle electron dynamics of NO molecules: the effect of strong laser field and Coulomb field

“…For extracting the electron momentum distribution, each measured image is separated into a left and a right part along the polarization and the Abel inversion is performed by using the Basex method [46]. The intensity calibration for both w and 2 w lasers is given by measuring the Stark shift of the electron energy spectra of xenon [47,48].…”