The Isotope Effect in Band Spectra, III. The Spectrum of Copper Iodide as Excited by Active Nitrogen

Excitation of spectra by active nitrogen.---Band spectra of CuF, CuCl, CuBr, and CuI have been obtained by the action of active nitrogen on the vapors of Cu${\mathrm{F}}_{2}$, CuCl and Cu${\mathrm{Cl}}_{2}$, Cu${\mathrm{Br}}_{2}$, and CuI, using the method of Rayleigh and Fowler, as previously applied by them to CuCl. Certain additional heads are present in the red, probably due to CuO formed as a result of slight oxygen impurity in the active nitrogen, and then excited by the latter. The halide bands are unusually sharp. Probably excited ${\mathrm{N}}_{2}$ molecules, in "impacts of the second kind," put halide molecules into the various excited electronic states necessary for the emission of visible band spectra. Many impacts, however, result in dissociation of the halide molecule, as the iodine are line $\ensuremath{\lambda}2062$ and over 80 lines of the copper arc spectrum were identified. The list of copper lines is exactly the same for CuCl as for CuI. Analogous results have been obtained with Pb${\mathrm{I}}_{2}$, Hg${\mathrm{I}}_{2}$, and Hg${\mathrm{Br}}_{2}$. Other reactions of active nitrogen are discussed.Emission of electronic band spectra by polar molecules.---It is suggested that the absence of electronic band spectra for the hydrogen, silver and alkali halides may be associated with the non-occurrence of higher valence compounds of type Na${\mathrm{Cl}}_{2}$, and that the emission of any one of the CuX band spectra follows the transfer of a ${\mathrm{Cu}}^{+}$ electron in the polar ${\mathrm{Cu}}^{+}$${\mathrm{X}}^{\ensuremath{-}}$ molecule from its normal state to one of a group of low lying excited states, whose existence can be correlated with the occurrence of the compounds Cu${\mathrm{X}}_{2}$, such easily excited electrons being absent in ions such as ${\mathrm{Na}}^{+}$. The above relation may be accounted for by supposing that polar molecules cannot carry electronic energy in excess of their heat of dissociation into atoms. By analogy with the observed absence of electronic band spectra for compounds of the NaCl type, the band spectra of the alkaline earth halides should not be due to compounds Me${\mathrm{X}}_{2}$, since the ${\mathrm{Me}}^{++}$ion contains no easily excited electron. The real emitter is probably MeX, which must contain a loosely bound valence electron like that in ${\mathrm{Me}}^{+}$ or in Na.Analysis of band spectrum of CuI, and confirmation of vibrational isotope effect.---The CuI bands, excited by active nitrogen, were photographed under moderate dispersion (see Plate I). The wave-lengths of over 260 heads between $\ensuremath{\lambda}5650$ and $\ensuremath{\lambda}3890$ were measured (Tables I-V). No other CuI bands are present between $\ensuremath{\lambda}1900$ and $\ensuremath{\lambda}7000$. Each ${\mathrm{Cu}}^{63}$I band is found to be accompanied, wherever resolution and intensity are sufficient, by a weaker ${\mathrm{Cu}}^{65}$I band, as expected. The ${\mathrm{Cu}}^{63}$I bands can all be arranged in five systems, the vibrational isotope effect serving as an almost indispensable key to this analysis. One system lies in the green; the other four, with their bands closely intermingled, lie in the blue and violet. Equations are given representing the positions of all the ${\mathrm{Cu}}^{63}$I heads within experimental error, in terms of the initial and final vibrational quantum numbers and their squares. From these equations, the corresponding theoretical equations for ${\mathrm{Cu}}^{65}$I are calculated. The observed ${\mathrm{Cu}}^{65}$I heads fall consistently within experimental error in the calculated positions. Near-equality of the coefficients (of both linear and quadratic terms) concerned with the final state of the molecule, shows that the five systems all correspond to a common final electronic state. This is almost certainly the normal state of the CuI molecule. Analogous relations probably hold for CuCl and CuBr, although the analysis is not yet completed. In all cases the vibration frequency is less and the interatomic distance greater for the excited states than for the normal state. The five excited states of the CuI molecule correspond to electronic energies equivalent to 2.44, 2.68, 2.70, 2.83, and 2.96 volts. An energy-level diagram for CuI is given, showing all the electronic and vibrational energy levels, and the observed transitions with their intensities. It is shown that the initial distribution of CuI molecules with respect to vibrational energy is of a thermal type and probably corresponds to the existing low temperature, thus differing markedly from the high-temperature non-thermal distribution characteristic of such compounds as BO and CN as excited by active nitrogen. Intensity distribution in the CuI bands. A marked tendency is noted here, as previously in the case of BO, for numerically large values of $\ensuremath{\Delta}n$ to be associated with large values of the initial vibrational quantum number ${n}^{\ensuremath{'}}$. This causes the maximum of intensity in band sequences (a sequence contains bands for all of which $\ensuremath{\Delta}n$ has the same value) to shift from the first to higher members of the sequence, as one proceeds to bands at increasing distances from the system-origin. Both the above effect and a preference for positive values of $\ensuremath{\Delta}n$ are explained in terms of Lenz's theory.