  Transmission spectroscopy is a powerful technique for probing exoplanetary
atmospheres. A successful ground-based observational method uses a differential
technique based on high-dispersion spectroscopy, but that only preserves narrow
features in transmission spectra. Here we use the chromatic Rossiter-McLaughlin
(RM) effect to measure the Rayleigh-scattering slope in the transmission
spectrum of HD 189733b with the aim to show that it can be effectively used to
measure broadband transmission features. The amplitude of the RM effects
depends on the effective size of the planet, and in the case of an atmospheric
contribution therefore depends on the observed wavelength. We analysed archival
HARPS data of three transits of HD 189733b, covering a wavelength range of 400
to 700 nm. We measured the slope in the transmission spectrum of HD 189733b at
a $2.5\sigma$ significance. Assuming it is due to Rayleigh scattering and not
caused by stellar activity, it would correspond to an atmospheric temperature,
as set by the scale height, of $T = 2300 \pm 900 \mathrm{K}$, well in line with
previously obtained results. This shows that ground-based high-dispersion
spectral observations can be used to probe broad-band features in the
transmission spectra of extrasolar planets, by using the chromatic RM effect.
This method will be particularly interesting in conjunction with the new
echelle spectrograph ESPRESSO, which currently is under construction for ESOs
Very Large Telescope and will provide a gain in signal-to-noise ratio of about
a factor 4 compared to HARPS. This will be of great value because of the
limited and uncertain future of the Hubble Space Telescope and because the
future James Webb Space Telescope will not cover this wavelength regime.
