Isotopically selective infrared multiphoton dissociation of vibrationally excited SiH4

We use mass-resolved 2+2 resonantly enhanced multiphoton ionization (REMPD of silicon atoms to detect isotopically selective infrared multiphoton dissociation (IRMPD) of both ground-state and vibrationally pre-excited SiH4. To demonstrate that the detection scheme provides a faithful monitor of the isotopic composition of the primary dissociation products. this approach was first tested on the products generated by both IRMPD and UV photodissociation of phenylsilane. It was then employed to evaluate the degree of isotopic enrichment achieved by IRMPD of around-state silane using an ammonia laser for dissociation. In a second type of experiment, silane molecules are pre-excited to the first overtone of the Si-H stretch, and then dissociated selectively with the ammonia laser. We find that pre-excitation increases the dissociation cross section by a factor of 230. Tuning the overtone pre-excitation laser while collecting the mass-resolved silicon ion signal generates an overtone excitation spectrum of SiH4 that is sorted by the silicon isotope. Certain combinations of overtone pre-excitation and ammonia laser dissociation frequencies lead to a high level of isotopic enrichment in the dissociation products: >99% in Si-28 or Si-21, and >96% in Si-30. We evaluate the practicality of an overtone-pre-excitation/IRMPD scheme for silicon isotope separation on a macroscopic scale.