Phosphorus Cation Reactions in the Interstellar Space.
 
 
Although the interstellar chemistry of the abundant first row elements C, N, and O owes its richness in part to the large abundance of these elements, and in part to their volatility, the chemistry of the second row elements (Na, Mg, Si , P, S, and Cl) is much less undertsood. In general, if molecules containing second row elements were formed with efficiencies comparable to those containing first row elements, then second row molecular species would be easily detectable in interstellar medium. The scarcity of second row compounds presumably stems from either a large depletion within dense molecular clouds, or from peculiarities of the gas phase chemistry of these elements, which do not favor molecule formation at low interstellar density and temperatures.

Among the second row elements phosphorus have attracted a great interest in the last decade. Part of it was due the detection in 1987 and 1990, respectively, of the first phosphorus containing molecules, the PN and the PC dimers. Early experimental work , based in ion-cyclotron resonance techniques, showed that the ion-molecule chemistry of P is significantly different from that of N under interstellar conditions. Unlike NHn+ ions, PHn+ ions react endothermically with H2, so that PHn compounds (n=1-3) are very unlikely. Species containing the P-O bond were predicted to dominate, bacause P+ and PH+ react readily with water molecules, which are highly abundant. Therefore, PO was predicted to be the most abundant P containing species, whereas, species conatining P-Na and P-C bonds were predicted to be rare because they require the reaction of P+ and PH+ with NH3 and CH4, which are much less abundant.

However, the failure to detect PO, and the concomitant successful detection of PN and PC, was inconsistent with this proposed model. To reconcile the ion-molecule experimental data with these detections, other processes have been invoked, such as grain disruption or temperature effects. Thus, it was stated that the ion-molecule reactions of the second row elements should be endothermic, and thus the modelling for the chemistry of these species should be guided by a bit higher temperature thermo-equilibrium chemical modelling.

Further experimental work using the selected ion flow tube (SIFT) on ion-molecule reactions of PHn+ ions (n=0-4) with several neutrals confirmed the propensity of P+ to form bond with O, N, C and, S atoms. The large majority of the species thus synthesized were found unreactive towards H2 and CO, the most abundant interstellar molecules. The P containing ions could also dissociatively recombine with electrons to produce their corresponding neutrals. These data is supportive of a very rich ionic chemisty of P+ , and seems to be in contrast with respect to low fractional abundance of phosphorus containing molecules in the interstellar clouds. However, it was also wisely noticed that the dissociative recombination of HPO+ and HPS+, which results in the reactions of P+ with H2O and H2S respectively, could results in total fragmentation to atoms, whereas this is less probable for HPN+, whose dissociative recombination leads to PN+. Therefore, this model accounts for the failure to detect PO and the successful detection of PN.

In this contect we have studied the reactivity of P+( 3P, 1D) towards the first and second row hydrides plus acetylne, methylphospihe and methanol, in order to learn more about the ion-molecule chemistry of phosphorus and help building an overall dynamical model that could shed light on the molecular formation processes in interstellar media. Please refer to the Publications section for more information.