Takamasa Momose

Motivated by recent detections of several aromatic nitriles in Taurus Molecular Cloud 1, we report laboratory and theoretical investigations of the vibrational spectroscopy and photochemistry of singly and doubly cyanosubstituted benzene in solid para-hydrogen matrices. We compare the photochemistry of cyanobenzene (benzonitrile) and three dicyanobenzene isomers initiated by excitations at 193 nm. In addition, we report the photochemistry of deuterated cyanobenzene (d ₅ -cyanobenzene), enabling us to determine the major products produced during the cyanobenzene photodissociation. The major products observed in the photolysis of all the nitriles are HCN and HNC, which are likely produced by hydrogen abstraction from para-hydrogen by the CN radical. This indicates that the major photodissociation channel involves cleavage of the bond between the ring and the nitrile group, forming the phenyl (or cyanophenyl) radical + CN. We observe secondary photoproducts similar to those found during benzene photolysis. Our findings may aid the interpretation of recent JWST mid-infrared observations of aromatics in photodissociation regions.

Precision measurements of the fundamental properties of trapped antihydrogen in the (ALPHA) experiment at CERN require an accurate determination of the antihdyrogen energy distribution. We introduce a new diagnostic technique, termed Spatio-Temporal Annihilation Mapping (STAM), which measures this distribution by exploiting the dynamics of antihydrogen in adiabatically time-varying magnetic fields. By analyzing the spatial and temporal patterns of annihilations recorded during adiabatic release from magnetic confinement and comparing them with detailed simulations, we infer the underlying energy distributions. STAM enables the simultaneous measurement of axial and transverse energy components, achieving a relative precision of approximately 10% over a mean energy range of ∼ 10–100 mK in the ALPHA trap. The technique is applied to both uncooled and laser-cooled antihydrogen populations, with results benchmarked against established laser-based diagnostics. While laser-based methods provide high-resolution measurements of the transverse energy and limited sensitivity to the axial component, an accurate determination of the axial energy remains challenging. In addition to complementing existing techniques, STAM is expected to access energy regimes that are currently inaccessible to laser-based diagnostics due to intrinsic limitations of those methods. These capabilities make STAM broadly applicable to future high-precision spectroscopic and gravitational studies with antihydrogen. Furthermore, STAM provides a novel probe of off-axis magnetic field structures, addressing a critical gap in current magnetometry techniques within the ALPHA experiment.

Antihydrogen, the bound state of a positron and an antiproton, is the only pure anti-atomic system ever studied. It is produced exclusively in the laboratory, as it has never been observed in nature. This unique system is of great interest for searching for tentative differences between matter and antimatter. Antihydrogen has been routinely trapped since 2010 and accumulated since 2017, enabling, for example, the first precision spectroscopic study of the anti-atom in 2018 and the first observation of the influence of gravity in 2023. Here we report an eight-fold increase in the trapping rate of antihydrogen, enabled by sympathetic cooling of positrons with laser-cooled beryllium ions. With beryllium sympathetic cooling, we now accumulate over 15000 antihydrogen atoms in under seven hours. This technique transforms our ability to study systematic and sidereal effects in existing experiments while paving the way for studies that would otherwise remain out of reach.

Polycyclic aromatic hydrocarbons, particularly acenes, are gaining attention as candidates for organic semiconductors. TIPS-pentacene, a functionalized acene with triisopropylsilyl (TIPS) side groups, demonstrates enhanced physical stability, solubility, and superior charge transport properties due to improved molecular packing. This study presents a high-resolution laser-induced fluorescence study comparing TIPS-pentacene and pentacene isolated in helium nanodroplets and attached to solid rare-gas clusters (neon and argon). Our findings reveal distinct differences in the vibronic structures of these molecules, with TIPS-pentacene displaying pronounced vibrational progressions of low frequency vibrational modes of the molecular side groups. The results offer insights into matrix effects and advance our understanding of TIPS-pentacene’s vibronic structure, recently reported to contribute via coherent vibrational wave packets to ultrafast singlet fission processes.

Molecular hydrogen (H ₂ ) has long been predicted to exhibit superfluidity—a state of zero viscosity—at extremely low temperatures. However, its existence remains under debate despite several experimental reports. In this study, we investigated the infrared transitions of methane embedded in clusters of parahydrogen molecules at 0.4 K using high-resolution helium nanodroplet spectroscopy. Our results revealed fully quantized rotational states of methane with minimal interference from surrounding H ₂ molecules, enabling precise determination of the rotational constant for each hydrogen cluster. The cluster-size dependence of the determined rotational constant aligns with behavior predicted by path-integral Monte Carlo simulations, indicating that more than 60% of the hydrogen molecules in the clusters participate in quantum bosonic exchanges, a characteristic feature of superfluidity. This work provides strong experimental evidence for the existence of a superfluid phase of molecular hydrogen at 0.4 K, representing a major step forward in understanding quantum behaviors in molecular systems.

Nuclear spin angular momentum in molecules with identical nuclei is conserved in many physical and chemical processes, but its role in chemical reactions involving atomic rearrangements remains underexplored. In this study, we investigate the nuclear spin selection rules in the reaction between methylene (CH₂) and hydrogen (H₂) using high-resolution infrared spectroscopy in quantum solid parahydrogen. Our results show that in the triplet methylene (³CH₂) reaction, the nuclear spin distribution of the products matches perfectly with the expected nuclear spin selection rules for the stepwise reaction mechanism. In contrast, in the excited singlet methylene (¹CH₂) reaction, the nuclear spin states of the methane product deviate from those predicted with a simple direct insertion mechanism. This deviation likely results from hydrogen atom dissociation due to the excess energy of the excited methane product or from a competitive process between relaxation to triplet methylene and stabilization of the methane product. Notably, our findings confirm that nuclear spin conservation is maintained even in reactions involving intermediates with unpaired electron spins such as methylene. This demonstrates the importance of detecting nuclear spin states as a powerful tool for revealing the details of reaction mechanisms, particularly in processes related to hydrocarbon combustion and planetary atmospheric chemistry.