Removing electrons from more than one orbital: direct and indirect pathways to excited states of molecular cations

Abstract

Abstract. We use velocity map imaging of photoelectrons in coincidence with molecular cations to determine which ionic states are populated via strong field ionisation, and whether the ionisation to excited ionic states proceeds indirectly via the ground ionic state or directly from the neutral. We carry out measurements on a series of molecules that have different energy gaps between the ground ionic state and dissociative excited states. We measure both direct and indirect ionisation to excited states of the molecular cations, and find that the energy gap between non-dissociative and dissociative states plays an important role in determining the amount of excited state ionisation. Direct ionisation to dissociative states is generally comparable to ionization to the ground state for gap energies less than the photon energy, but is suppressed for gap energies larger than the photon energy.

Ionic dynamics underlying strong-field dissociative molecular ionization

“…with multiple low lying electronic states in the cation, but the former shows much more indirect ionization than the latter under similar experimental conditions [12].…”

“…When there is exactly one electron and one ion detected, it's considered a valid coincidence event. A detailed assessment of how false coincidences affect the measurement is provided in an earlier publication [12]. All electrons recorded have a partner ion, and they are grouped together to form the photoelectron spectra associated with the appropriate ion species.…”

“…In a recent work [12], we demonstrated that there are two pathways to populate an excited ionic state -(1) Direct ionization, in which the neutral molecule is directly ionized to an excited ionic state, corresponding to the removal of an inner orbital electron; (2) Indirect ionization (also known as post-ionization excitation), in which the molecule is first ionized to the ground (or a low-lying) ionic state and then excited by the field to a high-lying state.…”

“…While most work considers ionization to the ground state of the molecular cation, there have also been several observations of ionization to excited ionic states in SFI [5][6][7][8][9][10][11][12][13][14][15]. In this work, we consider two mechanisms underlying ionization to excited states, and compare their relative importance in different parameter regimes.…”

Time-resolved measurement of internal conversion dynamics in strong-field molecular ionization

“…The experimental results are interpreted with calculations of strong-field ionization which include vibrational dynamics on the neutral excited states as well as nonadiabatic couplings [24,30]. Dynamics in the molecular cation have been studied in earlier publications [31,32], but are not relevant to the current study since the photoelectron spectrum is not affected by dynamics in the cation following ionization [33].…”