When a VUV photon is absorbed by a molecule in the gas phase, one of the main relaxation processes is photoionization, i.e. electron ejection, leaving the molecular cation with a given internal energy (electronic, vibrational and rotational). In many cases, the cation is formed in a dissociative state leading to its fragmentation. The photon energy deposited is therefore shared between the electron kinetic energy, the fragments translational energy and the internal energy of the fragments.
In order to disentangle this energy sharing and to understand the physical (photoionization and fragmentation dynamics) and physico-chemical (ionization potentials, appearance energy of the fragments, enthalpies of formation) properties of such photon-induced processes, it is necessary to use different electron/ion coincidence schemes (PEPICO), so that for instance a state-to-state chemistry could be unraveled: the detection of a threshold photoelectron (TPE) selects a given ionic state, whose fragmentation channels are studied by Mass Spectrometry (MS). Reciprocally, the photoionization process leading to a given ion should be studied by photoelectron spectroscopy (PES) on mass-selected compounds.
We recently designed a new versatile imaging electron/ion spectrometer, called DELICIOUS2, to be inserted in our permanent molecular beam end-station SAPHIRS, which satisfies a strong scientific demand in the fields of physics and chemistry. Based upon the use of a Velocity Map Imaging (VMI) spectrometer coupled with a MS-TOF spectrometer, DELICIOUS2 can be used to perform:
(i) Angle-Resolved PES on mass-selected samples (AR-PEPICO): for the study of clusters, for instance, as well as to cancel out any spurious contribution to a PES from the residual gas or an impure jet. The electron energy resolution is of the order of 5% with kinetic energy ranging from 0 to 17 eV, associated with a typical mass resolution above 150.
Threshold electron/ion coincidences (TPEPICO): necessary to determine precise ionization potentials, appearance energies, etc. The measured electron resolution is less than one meV.
Mass-selected ion imaging: to study the dynamics of ion fragmentation (angular information as well as kinetic energy release).
The continuous operation of the extraction fields and the independence from the electron’s time of flight are well suited to the quasi-continuous multi-bunch mode of the synchrotron radiation. This, combined with the high transmission of both the electron and ion detection, allows a high coincidence counting rate (of the order of 10 kHz) and facilitates the subtraction of false coincidences.
The performance characteristics of DELICIOUS 2, the very simple way of handling the coincidence events and its versatility (for example,both AR-PEPICO and TPEPICO can be performed on the very same sample) are very much appreciated by both the external and in-house user community. DELICIOUS2 is now installed quasi-permanently in SAPHIRS and is used during about 50% of the available beamtime on DESIRS.
A full account can be found in: « A Versatile Electron-Ion Coincidence Spectrometer For Photoelectron Momentum Imaging and Threshold Spectroscopy on mass-selected ions using Synchrotron Radiation », G. Garcia, H. Soldi-Lose and L. Nahon, Rev. Sci. Instrum. 80, 023102 (2009).
Gustavo Garcia (firstname.lastname@example.org)
Laurent Nahon (email@example.com)
Figure 1 : AR-PEPICO summary for a mixture of Ar and Xe photoionized at 16 eV. Upper left: Raw electron image (13000 electron /sec) showing the two spin-orbit ionic states corresponding to the 2p-1 (Ar) and 5p-1 (Xe) channels, the polarization of light is given by the white arrows; Lower left: MS-TOF spectrum recorded in coincidence (start = arrival of the electron, stop = arrival of the ion); Upper right: raw electron image obtained by post-processing the initial composite image by selecting the events in coincidence with the Ar+ ion peak, together with the inverted electron image and the corresponding PES; Lower right: same for the Xe case.
Figure 2 : TPES of Ar in the 2p-1 spin-orbit autoionization region obtained with a photon bandwith of 0.27 meV. Left-hand side: Rydberg states diagram of Ar; Upper right: TPES obtained with DELICIOUS2 showing an electron resolution of 0.75 meV, only the TPE lines are visible; Lower right: Ar+ ion yield showing the ns’ and nd’ autoionization resonances producing fast electrons.