The recent development of novel extreme ultraviolet (XUV) coherent light sources bears great potential for a better understanding of the structure and dynamics of matter 1,2 . Promising routes are advanced coherent control and nonlinear spectroscopy schemes in the XUV energy range, yielding unprecedented spatial and temporal resolution 3,4 . However, their implementation has been hampered by the experimental challenge of generating XUV pulse sequences with precisely controlled timing and phase properties. In particular, direct control and manipulation of the phase of individual pulses within a XUV pulse sequence opens exciting new possibilities for coherent control and multidimensional spectroscopy 4 , but has

A novel technique to sensitively and selectively isolate multiple-quantum coherences in a femtosecond pump-probe setup is presented. Detecting incoherent observables and imparting lock-in amplification, even weak signals of highly dilute samples can be acquired. Applying this method, efficient isolation of one-and two-photon transitions in a rubidium-doped helium droplet beam experiment is demonstrated and collective resonances up to fourth order are observed in a potassium vapor for the first time. Our approach provides new perspectives for coherent experiments in the deep UV and novel multidimensional spectroscopy schemes, in particular when selective detection of particles in dilute gas-phase targets is possible.Multiphoton processes play an important role in many fields of science. Light conversion processes such as second harmonic generation or optical parametric amplification are routinely performed in many labs. Parametric downconversion[1] is the key technique to generate entangled photon pairs used in quantum cryptography applications or to study entanglement properties in various systems [2]. High harmonic generation has pioneered the development of state-of-the-art coherent light sources in the XUV spectral range having attosecond pulse duration which allow the real-time observation of electron dynamics [3,4]. Likewise, multiphoton absorption in a tight laser focus is used in nonlinear microscopy yielding 3D images of biological tissues with high spatial resolution and great penetration depth [5,6]. Furthermore, the energy conversion process in singlet fission incorporates a multiphoton process and has recently drawn great interest due to its potential application in solar light harvesting [7,8].The unique identification and efficient detection of multiphoton processes is however often challenging. Monitoring multiphoton absorptions for transitions with small cross sections as a function of intensity is cumbersome and identification by emission spectra is frequently compromised by multistep relaxation pathways or predominant dark relaxation channels. Coherent timeresolved spectroscopy offers a different approach to identify multiphoton processes. Here, the phase evolution of non-stationary states induced upon optical excitation is monitored. This allows not only unambiguous identification of one-and multiphoton processes but also provides high time resolution.The phase of superposition states induced by multiphoton transitions -commonly termed multiple-quantum coherences (MQCs) -evolves at a much higher frequency than for one-quantum coherences (1QCs) induced by onephoton transitions. In multidimensional spectroscopy, MQC signals have allowed to characterize the influence of electron correlations in molecular systems [9], probing the anharmonicity of molecular potentials [10] or unraveling the role of high-lying electronic states in ultrafast pho-toinduced processes [11]. Furthermore, many-body interactions in a weakly-interacting atomic gas [12] and in semiconductor nanstructures [13][14][15] have bee...

Long-range interparticle interactions are revealed in extremely dilute thermal atomic ensembles using highly sensitive nonlinear femtosecond spectroscopy.

Two-dimensional electronic spectroscopy (2DES) is one of the most powerful spectroscopic techniques with unique sensitivity to couplings, coherence properties and real-time dynamics of a quantum system. While successfully applied to a variety of condensed phase samples, high precision experiments on isolated systems in the gas phase have been so far precluded by insufficient sensitivity. However, such experiments are essential for a precise understanding of fundamental mechanisms and to avoid misinterpretations. Here, we solve this issue by extending 2DES to isolated nanosystems in the gas phase prepared by helium nanodroplet isolation in a molecular beam-type experiment. This approach uniquely provides high flexibility in synthesizing tailored, quantum state-selected model systems of single and many-body character. In a model study of weakly-bound Rb2 and Rb3 molecules we demonstrate the method’s unique capacity to elucidate interactions and dynamics in tailored quantum systems, thereby also bridging the gap to experiments in ultracold quantum science.

The relaxation dynamics of superexcited superfluid He nanodroplets is fully unravelled by means of extreme-ultraviolet (XUV) femtosecond electron and ion spectroscopy complemented by time-dependent density functional theory (TDDFT). Three main...