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Abstract

Over the past decades, there has been a growing interest in properties of molecules with pronounced quantum properties found at very low temperatures. This dissertation focuses on spectroscopic studies of molecules in extremely cold conditions, with two main topics: Search for molecular superfluidity, a characteristic of fluids with zero viscosity that appears at ultra-low temperatures, and the intriguing applications of cold spectroscopy to investigate chiral molecules and their interactions. Superfluidity, a distinctive phase of matter found at very low temperature, was first observed in liquid helium in 1938. However, it remains unconfirmed in molecules despite predictions in 1972. The first part of this dissertation presents a comprehensive exploration for superfluid signature in hydrogen molecules. To identify the superfluidity in molecular hydrogen, we initially examined the properties of cold cluster beams having about 10⁵− 10⁷ H₂ molecules. Using laser-induced fluorescence spectroscopy, we characterized the thermodynamic states of the clusters. Our findings suggested the fluidity, potentially superfluidity, in the clusters formed by gas condensation, which had not been explored before. To further explore the superfluidity of hydrogen from a microscopic point of view, we conducted spectroscopic studies on smaller H₂ clusters with < 100 molecules created inside helium droplets at 0.4 K. We probed the rotational motion of methane inside these hydrogen clusters using high-resolution infrared spectroscopy. Detailed analysis revealed that at least 60% of the parahydrogen clusters exhibit quantum bosonic exchanges, signatures of microscopic superfluidity. The second part of this dissertation explores cold molecule spectroscopy in helium droplets at 0.4 K, focusing on chiral molecules. Our goals include identifying spectral differences in homochiral and heterochiral dimers and characterizing structural variances in diffusion-controlled and energy-controlled clusters. We observed stable methyl lactate dimers formed through diffusion-controlled processes. This research has broad implications for condensed matter physics, theory development, biochemistry, and astrochemistry. Exploring molecules in ultra-cold environment may yield new perspectives and deeper insights into various fields of science, possibly leading to novel discoveries and technological advancements.