Ultrafast photoelectron spectroscopy of solutions: space-charge effect

Abstract

The method of time-resolved XUV photoelectron spectroscopy is applied in a pump-probe experiment on a liquid micro-jet. We investigate how the XUV energy spectra of photoelectrons are influenced by the space charge created due to ionization of the liquid medium by the pump laser pulse. XUV light from high-order harmonic generation is used to probe the electron population of the valence shell of iron hexacyanide in water. By exposing the sample to a short UV pump pulse of 266 nm wavelength and ∼55 fs duration, we observe an energy shift of the spectral component associated with XUV ionization from the Fe 3d(t 2g ) orbital as well as a shift of the water spectrum. Depending on the sequence of the pump and probe pulses, the arising energy shift of photoelectrons acquires a positive or negative value. It exhibits a sharp positive peak at small time delays, which facilitates to determine the temporal overlap between pump and probe pulses. The negative spectral shift is due to positive charge accumulated in the liquid medium during ionization. Its dissipation is found to occur on a (sub)nanosecond time scale and has a biexponential character. A simple meanfield model is provided to interpret the observations. A comparison between the intensity dependencies of the spectral shift and the UV ionization yield shows that the space-charge effect can be significantly reduced when the pump intensity is attenuated below the saturation level of water ionization. For the given experimental conditions, the saturation intensity lies at 6 10 10 W cm −2 .

Charge-transfer and impulsive electronic-to-vibrational energy conversion in ferricyanide: ultrafast photoelectron and transient infrared studies

“…25 At short time delays, PES often allows for a more immediate interpretation than transient absorption spectra as there is no need to disentangle overlapping contributions from excited state absorption, ground state bleach and stimulated emission. Nevertheless, a few ps after photo excitation, effects such as space charge 26 and solvent heating 27 can be difficult to separate from the transient signal. Transient IR spectroscopy provides a unique probe of the symmetry of the complex and the ligand vibrations are very sensitive to the oxidation state of a central metal atom.…”

Ultrafast kinetics of linkage isomerism in Na2[Fe(CN)5NO] aqueous solution revealed by time-resolved photoelectron spectroscopy

“…The energy shift was caused by the space-charge effect induced by the pump beam in the liquid sample. This shift was shown to be dependent on the time delay between the pump and probe pulses 60 . Therefore, each spectrum of a given series needed to be corrected separately.…”

Multiscale Photo-Based In-Situ and Operando Spectroscopies in Time and Energy Landscapes

“…The group headed by Dr. Kiyan as a part of Prof. Aziz's institute has recently completed its laser lab in which ultrashort XUV probe pulses are generated via upconverting the laser frequency in the process of high-order harmonic generation (HHG). [15,16] Combining this tabletop technique with the liquid microjet, they are able to achieve femtosecond temporal resolution to investigate electronic and structural dynamics of molecular complexes in solutions and at interfaces (see Figure 5). [17][18][19]…”