A laser augmented reaction: SF6+ SiH4→S*2+ SiF4+ HF + H2· retention of isotopic selectivity during detonation

A single unfocused pulse of a free running CO
2
laser, area ∼ 8 cm
²
, initiates an explosive reaction between SF
6
and SiH
4
. This occurs at a minimum energy of 4 J [full width at half maximum (FWHM)
\sim 1.5 /mu
s] of which about one half is absorbed in an 8 cm long cell; total pressure 12 torr; 0.65 <
p
(SiH
4
)/
p
(SF
6
) < 1.8. The spectral and temporal distributions of the emitted chemiluminescence depend sensitively on the fuel to oxidizer ratio, and on the pulse energy; we investigated the range 4 → 20 J. The principal emission is due to S
2
(
B^{3}\Sigma-_{u} \rightarrow X^{3}\Sigma-_{g}
). Transitions
\upsilon' (0-4) \rightarrow \upsilon" (2-15)
were recorded. In the
³
\Sigma-_{u}
state, vibrational temperatures range from 3000-13000 K. The luminosity peaks sharply at (SiH
4
)/(SF
6
) = 1.0 ± 0.05. On each side of the maximum of the emission versus composition curve [at (SiH
4
)/(SF
6
) ≈ 0.95 and 1.22, for a 12 J pulse] the residual SF
6
(0.2-0.5 percent of initial amount) is enriched in
³⁴
SF
6
; the observed fractionation factors at these two compositions are 8 ± 2. The separation between the two sharply peaked optimum compositions appears to increase with increasing pulse energy. Preliminary results with other fuels suggest that the concurrent absorption of CO
2
laser radiation by the fuel, as well as a highly exothermic reaction, are pre-requisite for fine tuning of composition, injected power, and total pressure for optimum isotope fractionation.