No abstract

No abstract

We have measured an upper limit of 17 J/cm2 for the threshold fluence for collision-free CO2 laser IRMPD of silane by detecting the SiH2 product in its electronic and vibrational ground state. The threshold drops to ∼4 J/cm2 when silane molecules are irradiated by all lines from a pulsed NH3 laser. This value further drops to less than 2 J/cm2 when the molecules are pre-excited to the first Si−H stretch overtone. Using the NH3 laser on the 868 cm-1 line, the dissociation yield of pre-excited SiH4 reaches saturation at a laser fluence of 6 J/cm2. The substantial difference in the fluence threshold and optimal dissociation frequency of the ground and excited molecules allows us to perform highly selective dissociation of pre-excited silane in the presence of a large excess of ground-state molecules.

We present the design of a highly efficient, pulsed ammonia laser which is optically pumped by a commercial TEA CO2 laser and is line tunable in the spectral range of 780–930 cm−1. The power, spectral, and temporal characteristics of the laser have been investigated for different configurations of the optical cavity. Using a stable resonator, the laser operates with an efficiency of 24% (940 mJ) in a multiline configuration and 16.5% on a single line. Changing the resonator to an unstable configuration produces a diffraction limited output on a single line with an efficiency of 13%. These output characteristics make this laser suitable for experiments in the field of infrared multiphoton dissociation.

We use mass-resolved 2+2 resonantly enhanced multiphoton ionization (REMPI) 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 ground-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 28Si or 29Si, and >96% in 30Si. We evaluate the practicality of an overtone-pre-excitation/IRMPD scheme for silicon isotope separation on a macroscopic scale.