  As a primary material frequently used in industry, chlorine is relatively
easy to obtain and available even in large quantities. Despite its high
toxicity, molecular chlorine is readily available since it is an essential
educt in the chemical industry. Over the past decades, numerous accidents
involving injured and dead victims have occurred. Furthermore, it was already
misused as a warfare agent at the beginning of the last century with still
reported attacks. Early detection, localization, and monitoring of sources and
cloud movements are essential for protecting stationary facilities, mobile
operations, and the public. In contrast to most chemical hazardous materials,
where it is possible to detect them by vibrational spectroscopic methods
(e.\,g., passive hyper-spectral absorption technologies in the infrared),
halogens are inactive to infrared absorption. Raman-based technologies rely on
changes in the polarizability of the molecule and provide
vibrational-spectroscopic access to such diatomic molecules and therefore close
the gap in infrared detection capabilities. Here we present a straightforward
approach for a standoff Raman detector in a backscattering configuration. This
paper uses a simplified model to discuss optimum excitation wavelengths in
achievable detection ranges. We validate the model by spontaneous (vibrational)
Raman spectroscopic measurements between 20 and 60~m standoff distance. We also
briefly discuss detection performances and technical and physical aspects as
prospects of system design.
