The Isotope Effect in Band Spectra, IV: the Spectrum of Silicon Nitride

Abstract

Silicon nitride bands.—(1) A system of bands shaded toward the red, lying between 3800 and 5300A was obtained by Jevons in 1913 by the introduction of Si${\mathrm{Cl}}_4$ vapor into active nitrogen. His measurements have been repeated and extended to all possible heads. It is important that no oxygen be present. From a sharp intense head each band decreases rapidly in intensity to zero at the null line, then increases to a maximum at about $m=13\mathrm{}$ in the negative branch, in which the lines get far enough apart to be resolved. With the aid of the quantum theory, Jevons' arrangement was revised, and correct initial ($n^{\prime }$) and final ($n^{\prime \prime }$) vibrational quantum numbers assigned. The null lines of the ${\mathrm{Si}}^{28}$N bands correspond to the wave-number equation: ${\nu }^{\circ }=24234.2+1016.30\mathrm{} n^{\prime }-17.77\mathrm{}{n}^{\prime 2}+0.410\mathrm{}{n}^{\prime 4}-0.00487\mathrm{}{n}^{\prime }-1145.00\mathrm{}{n}^{\prime \prime }+6.570\mathrm{}{n}^{\prime \prime 2}$. Partial analysis of the band structure indicates that the lines of any band are given by the equation: $\nu ={\nu }^{\circ }+B^{\prime }+2\mathrm{}{B}^{\prime }m+\mathrm{C}{m}^2$ where $B^{\prime }=0.728-0.097\mathrm{}{n}^{\prime }$ and $\mathrm{C}=-0.0121-0.0097\mathrm{}{n}^{\prime }+0.0053\mathrm{}{n}^{\prime \prime }$; from this the internuclear distance for the vibrationless SiN molecule is 1.56×${10}^{-8\mathrm{}}$ cm. Additional weak heads are present whose positions agree very closely with those calculated for the isotopes ${\mathrm{Si}}^{29}$N and ${\mathrm{Si}}^{30}$N, isotope 29 being slightly the stronger. This agreement definitely excludes SiO or any other compound than SiN as the emitter of the bands, confirming the chemical evidence. The agreement is also definite, but not conclusive, evidence against the existence of half-quantum numbers for SiN, a result opposite to that for BO. Plate I shows most of the ${\mathrm{Si}}^{28}$N bands together with some isotope heads. The intensity distribution in each band corresponds to thermal equilibrium at about 80°C. In respect to the initial vibrational quantum number, the distribution is non-thermal and corresponds to a high effective temperature as in other cases such as BO and CN. The intensity distribution with respect to $\Delta n$ is similar to that of the violet CN bands. The sequence $\Delta n=0\mathrm{}$ is by far the strongest, other sequences of diminishing intensity being observed as far as -3 and +3 or +4. (2) New system of bands, extensive but weak, probably also due to SiN, is described. They all have double heads ($\Delta \nu =27\mathrm{}$), are shaded toward the red and extend from the ultraviolet through the green. An equation for the heads is given. The intensity distribution is characterized by numerically large values of $\Delta n$ and by the absence of the band (0→0) and some neighboring bands.