The high order harmonic generation process from molecules can be used to probe the structure of the emitting medium with high spatial resolution [1, 2]. The measurements of molecular dynamics with such a probe is thus very appealing. Contrast is an important issue in pump probe spectroscopy: in general the probe only excites part of the molecules, so that the detected signal can be dominated by contributions from non-excited molecules. In this article we review two techniques that can be used to enhance the contrast of pump probe measurements: transient grating spectroscopy and polarization-resolved spectroscopy. We use these techniques to detect rotational wavepackets and demonstrate a dramatic improvement in the contrast of the measured signal. © EDP Sciences, 2009. ; peer reviewed: yes ; system details: This record was machine loaded using metadata from Scopus ; NRC Pub: yes