Knowledge Bank

We have reinvestigated the ultraviolet absorption spectrum of $I2$ vapor, beginning at low pressures ($\sim 0.01mm$) with the narrowly spaced Cordes bands ($1765-1950\backslash AA$) and ending rather abruptly at high pressures $(\sim 1atm)$ and temperatures $900\circ -1100\circ C$ with a pronounced absorption maximum at $3420\backslash AA$ (Skorko, 1933 and Warren, 1935). The very complex spectrum includes, however, several regions with a simple looking vibrational structure: at 1800-1840, 1900-1935, 2100-2200 and $2530-2560\backslash AA$, whereas the irregularly spaced bands at the long wave end at $3200-3430\backslash AA$ coincide with emission bands of the transition $D\to 1\Sigma g+$ (Elliott, 1940 and Venkateswarlu and Verma, 1957). We could set up first a band-head formula for the Cordes bands with $v\prime \prime =0,1,2$ for the ground state and $v\prime =365-270(\omega \prime =38-55cm-1)$ for the upper state. Our formula, if extended to higher $v\prime \prime$ and lower $v\prime$ (i.e. increasing $\omega prime$) values, accounts for all the prominent band-heads at 2100-2200 and $2530-2760\backslash AA$. At still higher $v\prime \prime$ up to 30 and lower $v\prime$ down to zero (with $\omega 0\prime \sim 103cm-1$) the formula can approximately be brought in coincidence with a vibrational formula given earlier for the transition $D\to 1\Sigma g+$ in emission (Wieland and Waser, 1952). Thus we can, in a more quantitative and extended way, confirm Mulliken's (1934) postulation of the pronounced ionic character of the excited state D. Our analysis is still far from being complete, especially in the long wave region. We are aiming at an improvement by using better spectrogramms taken with large gratings.