Coherent time-resolved spectroscopy of dilute gas phase samples and detection of multiple-quantum coherences

Molecular beam experiments are routinely performed in Physics and Chemistry. However, due to the low target densities, coherent fs spectroscopy in such setups has remained a challenging task. In this context we are investigating a phase modulation pump-probe technique established by the Marcus group [1]. The combination of continuous acousto-optical phase modulation with lock-in detection greatly improves the signal-to-noise ratio in this scheme. Quantum beat signals are detected in the rotating frame which significantly reduces the demands on phase stability in the optical setup and lock-in demodulation efficiently isolates and amplifies even weak signals in the presence of large background contributions.
We have applied this method to a Rb-doped He droplet beam yielding a drastic increase in signal quality when compared to the standard pump-probe technique. By incorporating mass-resolved photoion detection, fully resolved vibrational spectra of Rb*He exciplexes are obtained, revealing a much higher resolution than was previously achieved with other methods [2]. Based on the phase modulation scheme, we have also established a technique to sensitively and selectively detect multiple-quantum coherences [3]. We use this technique to observe collective resonances in a dilute potassium vapor up to the fourth order. Our technique can be integrated in a 2D spectroscopy setup and provides new perspectives for studying many-body effects. We further suggest this method being a promising approach to facilitate coherent spectroscopy in the XUV spectral range.

[1] P. F. Tekavec, T. R. Dyke, and A. H. Marcus, J. Chem. Phys. 125, 194303 (2006).
[2] L. Bruder, M. Mudrich, and F. Stienkemeier, Phys. Chem. Chem. Phys. 17, 23877 (2015).
[3] L. Bruder, M. Binz, and F. Stienkemeier, Phys. Rev. A (in press)