A threshold-based approach to calorimetry in helium droplets: measurement of binding energies of water clusters

Infrared spectroscopy of C3-(H2O)n and C3-(D2O)n complexes in helium droplets

The C₃ molecule is an important species with implications in combustion and astrochemistry, and much of the interest in this molecule is related to its interactions with other species found in these environments. We have utilized helium droplet beam techniques along with a recently developed carbon cluster evaporation source to assemble C₃-(H₂O)_n and C₃-(D₂O)_n complexes with n = 1-2 and to record their rovibrational spectra. We observe only a single isomer of the n = 1 complex, in agreement with theoretical predictions as well as data from earlier matrix isolation studies. The spectra of the n = 1 complex are consistent with the ab initio structure, which involves a nearly linear arrangement of CCC-HO atoms in the complex. The C₃-H₂O spectrum we obtain exhibits slight differences from the analogous C₃-D₂O spectrum, which we assign to a difference in linewidth between the two spectra. We have also examined the n = 2 species and obtained a structure that appears to be distinct from those observed in matrix isolation studies and, to our knowledge, has not been previously observed.

Note: A simple detection method for helium droplet spectroscopy experiments

Helium droplet methods are currently established as a premier experimental technique for the production and spectroscopic study of novel clusters and complexes. Unfortunately, some of the essential equipment required to perform the experiments, such as the detector used to monitor photon-induced depletion of the helium droplet beam, can be relatively large, complex, and expensive. Most often this detector is a quadrupole mass spectrometer (QMS). In this report, we describe the development and evaluation of an extremely simple, straightforward, small, and inexpensive droplet beam detector for use in helium droplet spectroscopy experiments and compare its performance to that of a QMS by recording the infrared spectra of helium droplets doped with either ¹³CO₂ or CD₄.

Infrared spectroscopy of Mg-CO2 and Al-CO2 complexes in helium nanodroplets

The catalytic reduction of CO2 to produce hydrocarbon fuels is a topic that has gained significant attention. Development of efficient catalysts is a key enabler to such approaches, and metal-based catalysts have shown promise towards this goal. The development of a fundamental understanding of the interactions between CO2 molecules and metal atoms is expected to offer insight into the chemistry that occurs at the active site of such catalysts. In the current study, we utilize helium droplet methods to assemble complexes composed of a CO2 molecule and a Mg or Al atom. High-resolution infrared (IR) spectroscopy and optically selected mass spectrometry are used to probe the structure and binding of the complexes, and the experimental observations are compared with theoretical results determined from ab initio calculations. In both the Mg-CO2 and Al-CO2 systems, two IR bands are obtained: one assigned to a linear isomer and the other assigned to a T-shaped isomer. In the case of the Mg-CO2 complexes, the vibrational frequencies and rotational constants associated with the two isomers are in good agreement with theoretical values. In the case of the Al-CO2 complexes, the vibrational frequencies agree with theoretical predictions; however, the bands from both structural isomers exhibit significant homogeneous broadening sufficient to completely obscure the rotational structure of the bands. The broadening is consistent with an upper state lifetime of 2.7 ps for the linear isomer and 1.8 ps for the T-shaped isomer. The short lifetime is tentatively attributed to a prompt photo-induced chemical reaction between the CO2 molecule and the Al atom comprising the complex.

Helium droplet calorimetry of strongly bound species: carbon clusters from C₂ to C₁₂

Helium droplet beam methods are a versatile technique that can be used to assemble a wide variety of atomic and molecular clusters. In recent years, methods have been developed to utilize helium droplets as nano-calorimeters to measure the binding energies of weakly bound complexes assembled within the droplet. In the current investigation we extend the helium droplet calorimetry approach to the study of a very strongly bound system: carbon clusters which are bound by several eV per atom. We utilize laser heating of bulk carbon samples to dope the helium droplets with evaporated carbon species. Depending on the laser target, the vaporization plume is found to consist primarily of C3 alone or C2 and C3. These species are sequentially captured by the droplet and assembled into larger carbon clusters in a stepwise manner. The assembled C(n) clusters are detected via mass spectrometry of the doped droplets and the droplet sizes required to detect the various carbon clusters observed are used to estimate the reaction energies of the associated assembly pathways. The helium droplet data qualitatively reflect the trends in assembly energetics, but at first glance appear to yield energies that differ dramatically from theoretical values. Statistical modeling of the helium droplet calorimetry experiment reconciles the differences quantitatively. Our modeling also generates a calibration curve that relates the assembly/reaction energy and threshold mean droplet size over a range of energies from van der Waals interactions to chemical bonding, enabling helium droplet calorimetry methods to be applied quantitatively to a large number of systems.