TRANSITION STRENGTHS IN THE VISIBLE ABSORPTION SPECTRUM OF I$_2$: ONE MORE PASS

Abstract

The absorption spectrum of I$_2$ is examined anew across the wavelength range 400-850 nm, where there is significant room-temperature absorption in the three overlapped electronic transitions. To better characterize the discrete absorption in the dominant $B-X$ system, spectra are recorded in the 520-640-nm region with high quantitative precision (0.0005 absorbance units) at moderate resolution (0.1 nm) and are analyzed by least-squares spectral simulation, yielding the $B-X$ electronic transition strength $\mu_e^{2}$ with unprecedented precision ($<2$ percent relative standard error) over most of the studied region. This treatment also yields directly new estimates of the continuous absorption, which support previous assessments of the $A-X$ transition but indicate that the $C- X$ transition is 20 percent weaker than thought. In companion studies, lower resolution (1 nm) spectra and multiple-temperature absorption data from the literature are analyzed as bound-free by quantum spectral simulation, to yield estimates of the small-$R$ potential curve extensions for all three excited states and their $R$-dependent transition moment functions. To increase the precision and range of description of the least-known $C$-state potential, the least-squares analysis is expanded to include quantum simulation of literature data for the $B-C$ predissociation. The result is a $C$-state potential obtained with a precision comparable to that achieved in many discrete spectroscopic studies, over the range where absorption and predissociation occur (2.5-2.9 A), and extending smoothly to its van der Waals well at 4.3 A. \vspace{1em} The discrete simulation method described here is applicable to any system where the spectrum can be reliably simulated, which must include treatment of the absorption and instrumental lineshapes. The I$_2$ $B-X$ results are directly applicable to the monitoring of I$_2$ in the atmosphere.