# On the visible continuum and lines in the Interstellar Extinction Curve

## TL;DR

This paper uses molecular modeling to explore how amorphous interstellar dust structures produce the visible extinction curve and diffuse interstellar bands, linking theoretical spectra to observations.

## Contribution

It demonstrates that distortions in crystalline structures lead to amorphous states with weak and strong electronic transitions, explaining the observed continuum and spectral features.

## Key findings

- Weak continuum arises from large amorphous structures' weak transitions.
- Strong transitions can emerge as discrete features when clustered.
- Simulated spectra show parallels with observed interstellar extinction and bands.

## Abstract

This work purports to help understand the InterStellar Extinction Curve in and near the visible range. In this range, crystalline materials are known to be transparent, so amorphous dust is needed. Molecular modeling experiments are used to compute the electronic spectra of various, relatively large, carbon and silicate structures. Hardly any transition shows up beyond 0.4 mum when the structure is in its ground state (the lowest, most stable state, usually crystalline). This is no longer the case as soon as the structure is distorted in any way. Examples of simulated distortions (or ``defects'') are: angular or linear bond alteration, insertion of free radicals near the main structure, dangling bonds; their cumulative effects lead to the amorphous state. It is shown that, in this state, a structure bears a majority of weak transitions and a minority of strong ones. As the structure grows in size, the former ultimately form a weak continuum already detected experimentally, in the visible, on amorphous carbons and silicates. The stronger transitions will manage to emerge above the continuum, especially when they bunch together by chance near the same wavelength. Parallels are drawn between several properties of the computed continua and transitions and the observed continuum and Diffuse Interstellar Bands.