Gas phase reactions are ubiquitous in the daily life, from combustion engines to planetary atmospheres, and have a profound impact on the environment and thus on humans. These reactions are driven by reactive intermediates, called radicals. Detecting and identifying all radicals and stable products formed in a reaction is necessary to understand the complex mechanisms involved, in order to predict the behaviour of complex systems. Collaboration between four different laboratories funded by the Agence Nationale de la Recherche (ANR) has produced a unique experimental setup that has recently been installed at the DESIRS beamline, capable of multiplex species detection and identification.
Simultaneous real-time detection of all species present in a complex mixture at any given time has long been the Holy Grail of reactivity studies. In this ideal case, it would be possible to create a detailed model with perfect predicting capabilities to know the internal state of the system at any given time, and to anticipate its response to external changes.
Among all the existing methods considered for this matter, mass spectrometry is commonly regarded as the most universal detection method because all species can be ionized with high yield. However, laboratory based ionization sources such as lasers, discharge lamps or electron impact have undesirable side effects that are either excessive fragmentation (high ionization energy), or the inability to ionize every species (low ionization energy). Therefore, in the past decade several research groups turned towards tuneable VUV synchrotron radiation based photoionization mass spectrometry, also known as PIMS, to complete laboratory based data. The tuneability meant that near-threshold ionization conditions, also called soft ionization, could be achieved to avoid fragmentation. By scanning the photon energy, the shape of the photoion efficiency (PIE) curves also gave important structural information, so that isomer identification could be reached in some favourable cases.
In another step towards universal detection, a consortium called SYNCHROKIN between four different laboratories—PC2A, Lille; ISM, Bordeaux; CRF, Sandia; DESIRS, Gif-sur-Yvette—has been formed and recently funded by the ANR to apply synchrotron-based double imaging photoelectron/photoion coincidence (i2PEPICO) techniques to the study of gas phase reactivity. PEPICO schemes have been applied to fundamental ionization processes for several decades but the addition of powerful imaging capabilities is more recent. Contrary to simply recording the ion current, the photoelectron energy and angular information can be correlated to any particular species in the mass spectrum, providing an electronic fingerprint far more sensitive to structure than the PIE curves.
Two types of reactors have been chosen to study gas-phase reactivity and radicals: a continuous reactor with an injector based flow-tube for slow reactions (msec-sec range), and a photolysis cell reactor for fast reactions (µsec-msec range). Combined to the tuneability, high flux and resolution of the DESIRS beamline and the state-of-the-art i2PEPICO spectrometer DELICIOUS3 which incorporates imaging techniques for a high level of multiplex detection, we are as close as it is nowadays possible to universal product detection. Moreover, for many of the radicals, their photoelectron spectrum (or fingerprint) is not known due to the difficulty of producing these reactive species. In these cases, the flow-tube has been used to form the radicals by H-addition or abstraction to then measure the photoelectron spectra, which also yields important structural information when compared to ab initio methods.
The setup, shown in Figure 1, has been applied to the spectroscopic study of various elusive radical species, which have been formed in the flow-tube reactor by H-abstraction, with the OH/OD example given in Figure 2. It has also been applied to the identification of products from reactions of atmospheric interest.A long list of external projects already demands the use of this experiment for radical and reactivity studies.
Figure 1 : 3D schematics of the injector/reactor system inside the SAPHIRS molecular beam end station at the DESIRS beamline. Radicals are formed directly via microwave discharge, or by H abstraction (see inset) and then react in the flow-tube with a stable molecule, or with another radical. The reaction time is measured through the reaction distance d because the constant laminar flow speed is known. A similar setup is used for the photo-initiated reactions (not shown here) except that radicals are created by pulsed laser photolysis, with the reaction time measured with respect to the laser pulse.
Figure 2 :
(a) Mass spectrum of the composition of the flow-tube reactor obtained at a photon energy of 18 eV. The powerful ion imaging techniques separate the species on the reactor (black curve) from those that constitute the background of vacuum chambers (red curve). OH/OD were produced with a MW discharge of F2 and subsequent mixing of the resulting F atom with H2O/D2O. All the species can be identified by their electronic fingerprint and have been labelled. The photoelectron spectra of some of the species are plotted in panel (b). In particular, that of the OH radical was obtained for the first time without contributions from any contaminant, due to the coincidence scheme. This fingerprint can be used to study the structure of the radicals by comparison with theoretical models, but also as a powerful means for species identification in complex mixtures, as explained in the text above. Similar results were obtained for its deuterated counterpart.