409In our earlier comm~nication~~ we extrapolated our results obtained with 193 nm for Nb,CN/Nb,Br for x 1 2 to give a monomer ratio NblCN/NblBr of approximately 15. That extrapolation may still be valid. The 193 nm might be a wavelength at which @(NbxCN)+/@'(Nb,Br)+ = 1; thus the relative peak intensity ratio reflects the reaction cross section. Because the I P S of NblBr and NblCN are above 6.42 eV, one is not able to prove whether or not a good correlation can be made between the crossed-beam experiments and the cluster studies like the one presented. With the availability of tunable light sources in the wavelength region below 190 nm, one could try to softly ionize the niobium monomer products, hoping that the ionization cross sections of both products are not too different at that (these) wavelength(s). The 157-nm ionization laser is able to ionize all the Nb,CN and Nb,Br product clusters by one-photon absorption. However, its one-photon energy is -3 eV higher than the average IP of the large clusters. This might leave their cluster ions electronically excited after ionization. The conversion of electronic excitation into vibration energy leads to internally hot cluster ions which can be cooled off by evaporating small neutral or ionic clusters. The unexpected (NblCN)+/ (NblBr)+ value of less than unity could be explained by considering the following evaporation mechanism:(1)If other factors are equal, the ratio R could be determined by @]b/@2b. For the Nb,P+ fragment (where P can be either C N or Br), the branching ratio of NbP+/NbPl+ depends on the inverse ratio of the IP's of the two neutral species (Le., IPNb,,/IPNbp). Niobium bromide, with the bromine atom having three pairs of nonbonding electrons, is expected to have a lower ionization potential than NblCN. Due to back-donation from C N to the metal atoms, a slight positive charge can be created on the bonded niobium atom. This would increase the IP of the Nb,CN product. as observed. This conclusion is based on the assumption that the ionization cross sections of NbCN and NbBr are comparable. If this is not true, then the difference in their ionization cross sections could account for the observed product mass intensity ratio.
Possible ConclusionsIn this paper, we have studied the reaction of BrCN with gaseous niobium clusters. Impulsive type collisions are expected to dominate in the reaction between BrCN and metallic atoms. This gives rise to the high value of the stereochemical selectivity ratio of MCN to MBr.39*40 As the number of atoms in the cluster increases, it is expected that the nature of the collisions changes to the "sticky" type. This leads to a decrease in the selectivity ratio.Of the four laser wavelengths used in this study, two are found to give results in agreement with this expectation. The other two, one with higher photon energy (1 57 nm) and the other with lower photon energy (218 nm), gave results that are not in agreement with our chemical intuition. This discrepancy again emphasizes the importance of the laser-cluster interac...