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Register a new userRotor in a Cage: Infrared Spectroscopy of an Endohedral Hydrogen-Fullerene Complex
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We report the observation of quantized translational and rotational motion of molecular hydrogen inside the cages of C60. Narrow infrared absorption lines at the temperature of 6K correspond to vibrational excitations in combination with translational and rotational excitations and show well resolved splittings due to the coupling between translational and rotational modes of the endohedral H2 molecule. A theoretical model shows that H2 inside C60 is a three-dimensional quantum rotor moving in a nearly spherical potential. The theory provides both the frequencies and the intensities of the observed infrared transitions. Good agreement with the experimental results is obtained by fitting a small number of empirical parameters to describe the confining potential, as well as the ortho to para ratio.
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Infrared absorption spectroscopy study of endohedral water molecule in a solid mixture of H$_2$O@C$_{60}$ and C$_{60}$ was carried out at liquid helium temperature. From the evolution of the spectra during the ortho-para conversion process, the spectral lines were identified as para- and ortho-water transitions. Eight vibrational transitions with rotational side peaks were observed in the mid-infrared: $omega_1$, $omega_2$, $omega_3$, $2omega_1$, $2omega_2$, $omega_1 +omega_3$, $omega_2 +omega_3$, and $2omega_2+omega_3$. The vibrational frequencies $omega_2$ and 2$omega_2$ are lower by 1.6% and the rest by 2.4%, as compared to free water/. A model consisting of a rovibrational Hamiltonian with the dipole and quadrupole moments of water interacting with the crystal field was used to fit the infrared absorption spectra. The electric quadrupole interaction with the crystal field lifts the degeneracy of the rotational levels. The finite amplitudes of the pure $v_1$ and $v_2$ vibrational transitions are consistent with the interaction of the water molecule dipole moment with a lattice-induced electric field. The permanent dipole moment of encapsulated water/ is found to be $0.5pm 0.1$ D as determined from the far-infrared rotational line intensities. The translational mode of the quantized center of mass motion of water/ in the molecular cage of C$_{60}$ was observed at 110cm$^{-1}$ (13.6meV).
We measure the electron spin resonance spectrum of the endohedral fullerene molecule $^{15}mathrm{N@C}_{60}$ at pressures ranging from atmospheric pressure to 0.25 GPa, and find that the hyperfine coupling increases linearly with pressure. We present a model based on van der Waals interactions, which accounts for this increase via compression of the fullerene cage and consequent admixture of orbitals with a larger hyperfine coupling. Combining this model with theoretical estimates of the bulk modulus, we predict the pressure shift and compare it to our experimental results, finding fair agreement given the spread in estimates of the bulk modulus. The spin resonance linewidth is also found to depend on pressure. This is explained by considering the pressure-dependent viscosity of the solvent, which modifies the effect of dipolar coupling between spins within fullerene clusters.
Cavity-enhanced frequency comb spectroscopy for molecule detection in the mid-infrared powerfully combines high resolution, high sensitivity, and broad spectral coverage. However, this technique, and essentially all spectroscopic methods, is limited in application to relatively small, simple molecules. Here we integrate comb spectroscopy with continuous, cold samples of molecules produced via buffer gas cooling, thus enabling the study of significantly more complex molecules. We report simultaneous gains in resolution, sensitivity, and bandwidth and demonstrate this combined capability with the first rotationally resolved direct absorption spectra in the CH stretch region of several complex molecules. These include nitromethane (CH$_3$NO$_2$), a model system that presents challenging questions to the understanding of large amplitude vibrational motion, as well as several large organic molecules with fundamental spectroscopic and astrochemical relevance, including naphthalene (C$_{10}$H$_8$), adamantane (C$_{10}$H$_{16}$), and hexamethylenetetramine (C$_{6}$N$_4$H$_{12}$). This general spectroscopic tool has the potential to significantly impact the field of molecular spectroscopy, simultaneously improving efficiency, spectral resolution, and specificity by orders of magnitude. This realization could open up new molecular species and new kinetics for precise investigations, including the study of complex molecules, weakly bound clusters, and cold chemistry.
We present a variational MonteCarlo (VMC) and lattice regularized diffusion MonteCarlo (LRDMC) study of the binding energy and dispersion curve of the water dimer. As a variation ansatz we use the JAGP wave function, an implementation of the resonating valence bond (RVB) idea. Actually one the aim of the present work is to investigate how the bonding of two water molecules, as a prototype of the hydrogen-bonded complexes, could be described within an JAGP approach. Using a pseudopotential for the inert core of the Oxygen, with a full optimization of the variational parameters, we obtain at the VMC level a binding energy of -4.5(0.1) Kcal/mol, while LRDMC calculations gives -4.9(0.1) Kcal/mol (experiment 5 Kcal/Mol). The calculated dispersion curve reproduces both at the VMC and LRDMC level the miminum position and the curvature.The quality of the WF gives us the possibility to dissect the binding energy in different contributions by appropriately switching off determinantal and Jastrow terms in the JAGP: we estimate the dynamical contribution to the binding energy to be of the order of 1.4(0.2) Kcal/Mol whereas the covalent contribution about 1.0(0.2) Kcal/Mol. JAGP reveales thus a promising WF for describing systems where both dispersive and covalent forces play an important role.
The endohedral fullerene $^{15}mathrm{N@C}_{60}$ has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clocks projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.
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