An instrument combining an electrospray ionization source and a velocity-map imaging spectrometer for studying delayed electron emission of polyanions

Thermionic Emission of Negative Ions of Molecules and Small Clusters as a Probe of Low-Energy Attachment

We have been studying the thermionic emission of negatively charged molecules and small clusters for more than a decade. The kinetic energy released distribution (KERD) of mass-selected negative ions has been measured with a velocity map imaging spectrometer. A comparison of the experimental KERD to detailed balance models provided information on the reverse process, namely, the electron attachment to the parent. The electron attachment to neutral systems (reverse process of the electron emission from anions) is usually described in a simplified way as a single electron capture in the framework of the classical Langevin model. Our measurements show that this approach is insufficient and that, in addition to the capture step, an intramolecular vibrational redistribution (IVR) step should be included. As far as multiply charged anions are concerned, the electron attachment to anions (reverse process of the electron emission from dianions) is strongly affected by the repulsive Coulomb barrier (RCB). Previous studies assumed a pure over-the-barrier process, which is in disagreement with our study. Indeed, electron emission is measured below the RCB, revealing significant thermal tunneling. In the present review, we summarize these works on singly and doubly charged anions in an attempt to present a unified view of the involved processes. It is worth noting that the detailed measurements of KERDs in the very low kinetic energy region (typically around 0.1 eV) have been made possible thanks to electron imaging methods, without which all of this work could never have been done, with time-resolution capabilities allowing the disentangling of direct and delayed electron emission.

On-the-fly investigation of XUV excited large molecular ions using a high harmonic generation light source

We present experiments where extreme ultraviolet femtosecond light pulses are used to photoexcite large molecular ions at high internal energy. This is done by combining an electrospray ionization source and a mass spectrometer with a pulsed light source based on high harmonic generation. This allows one to study the interaction between high energy photons and mass selected ions in conditions that are accessible on large-scale facilities. We show that even without an ion trapping device, systems as large as a protein can be studied. We observe light induced dissociative ionization and proton migration in model systems such as reserpine, insulin and cytochrome c. These results offer new perspectives to perform time-resolved experiments with ultrashort pulses at the heart of the emerging field of attosecond chemistry.

Kinetic energy released in the vibrational autodetachment of sulfur hexafluoride anion

The kinetic energy release distribution (KERD) in the vibrational autodetachment (VAD) from sulfur hexafluoride anion SF₆ ^- has been measured in a velocity map imaging spectrometer for delays in the range of a few tens of microseconds. The experimental KERD is analyzed within the framework of the detailed-balance: first using the standard Langevin model and subsequently using a more refined and realistic model based on the experimental attachment cross section. A discussion on the processes involved in the attachment and the VAD is presented based on an empirical fit of the attachment cross section. The lifetime derived from the model is in good agreement with the experimental time window, strengthening this theoretical approach for this model system.

A detailed-balance model for thermionic emission from polyanions: The case of fullerene dianions

A detailed-balance model for thermionic emission from polyanions has been developed and applied to fullerene dianions. The specificity of this delayed decay process is electron tunneling through the repulsive Coulomb barrier (RCB). An analytical expression of the RCB is derived from electrostatic modeling of the fullerene cage. The reverse process, namely, electron attachment to the singly charged anion, is described by a hard sphere cross section weighted by the Wentzel-Kramers-Brillouin tunneling probability. This simple expression leads to a very good agreement with a measured time-resolved kinetic energy distribution of C₈₄^2-. Electron binding energy is reduced when the fullerene cage size decreases, leading to an almost zero one for C₇₀^2- and a negative one for C₆₀^2-. Extension of the model to these systems of interest is discussed, and model outputs are compared with the experimental data from the literature.

An experimental setup to study delayed electron emission upon photoexcitation of trapped polyatomic anions

A Velocity Map Imaging (VMI) spectrometer has been designed and integrated with an electrostatic ion beam trap to study delayed electron emission from trapped polyatomic anions upon photodetachment. The VMI spectrometer is small in size and can record a wide range of photoelectron energies, with variable magnification. Delayed electron emission can be recorded in our experimental setup for any time duration after the photoexcitation of the polyatomic anions. Experiments were carried out with trapped O^- and C₅^- ions to demonstrate the capability of the spectrometer. Delayed electron emissions from C₅^- as well as prompt photoelectrons from O^- were detected by the VMI spectrometer upon photoexcitation. The design and performance of the spectrometer are presented in detail.