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Register a new userExperimental Determination of the Interaction Potential between a Helium Atom and the Interior Surface of a C60 Fullerene Molecule
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Authors
G. R. Bacanu
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The interactions between atoms and molecules may be described by a potential energy function of the nuclear coordinates. Non-bonded interactions are dominated by repulsive forces at short range and attractive dispersion forces at long range. Experimental data on the detailed interaction potentials for non-bonded interatomic and intermolecular forces is scarce. Here we use terahertz spectroscopy and inelastic neutron scattering to determine the potential energy function for the non-bonded interaction between single He atoms and encapsulating C60 fullerene cages, in the helium endofullerenes 3He and 4He, synthesised by molecular surgery techniques. The experimentally derived potential is compared to estimates from quantum chemistry calculations, and from sums of empirical two-body potentials.
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It is demonstrated that the forbidden lines, which must be present in the SERS, TERS and SEIRA spectra of molecules with sufficiently high symmetry, associated with a strong quadrupole light-molecule interaction, are absent in the fullerene C60. This result is an experimental manifestation of an electrodynamical forbiddance of the strong quadrupole light-molecule interaction, which must be not only in molecules with cubic symmetry groups, but in the fullerene C60 also.
It is demonstrated that in fullerene C70 which can be considered as a deformed fullerene C60 in some sense there is a withdrawal of an electrodynamical forbiddance of a strong quadrupole light-molecule interaction which is realized in the fullerene C60. This situation occurs because of the reduction of symmetry of C60 from the icosahedral symmetry group Yh to the group D5h. The withdrawal results in appearance of the lines in the SERS spectra of C70 which are forbidden in usual Raman scattering and are allowed in infrared absorption while such lines are forbidden in the SERS spectrum of the fullerene C60 due to the electrodynamical forbiddance. The measured SERS spectra of C70 demonstrates existence of such lines that strongly confirms our ideas about the dipole quadrupole SERS mechanism.
Encapsulation of a single water molecule in fullerene-C60 via chemical surgery provides a unique opportunity to study the distinct rotational dynamics of the water spin isomers at cryogenic temperatures. Here, we employ single-cycle terahertz (THz) pulses to coherently excite the low-frequency rotational motion of ortho- and para-water, encapsulated in fullerene-C60. The THz pulse slightly orients the water electric dipole moments along the field polarization leading to the subsequent emission of electromagnetic waves, which we resolve via the field-free electro-optic sampling technique. At temperatures above ~100 K, the rotation of water in its cage is overdamped and no emission is resolved. At lower temperatures, the water rotation gains a long coherence decay time, allowing observation of the coherent emission for 10-15 ps after the initial excitation. We observe the real-time change of the emission pattern after cooling to 4 K, corresponding to the conversion of a mixture of ortho-water to para-water over the course of 10 hours.
Using a hydrogen molecule as a test system we demonstrate how to compute the effective potential according to the formalism of the new density functional theory (DFT), in which the basic variable is the set of spherically averaged densities instead of the total density, used in the traditional DFT. The effective potential together the external potential, nuclear Coulomb potential, can be substituted in the Schrodinger like differential equation to obtain the spherically averaged electron density of the system. In the new method instead of one three-dimensional low symmetry equation one has to solve as many spherically symmetric equations as there are atoms in the system.
Conventional information processors freely convert information between different physical carriers to process, store, or transmit information. It seems plausible that quantum information will also be held by different physical carriers in applications such as tests of fundamental physics, quantum-enhanced sensors, and quantum information processing. Quantum-controlled molecules in particular could transduce quantum information across a wide range of quantum-bit (qubit) frequencies, from a few kHz for transitions within the same rotational manifold, a few GHz for hyperfine transitions, up to a few THz for rotational transitions, to hundreds of THz for fundamental and overtone vibrational and electronic transitions, possibly all within the same molecule. Here, we report the first demonstration of entanglement between states of the rotation of a $rm^{40}CaH^+$ molecular ion and internal states of a $rm^{40}Ca^+$ atomic ion. The qubit addressed in the molecule has a frequency of either 13.4 kHz or 855 GHz, highlighting the versatility of molecular qubits. This work demonstrates how molecules can transduce quantum information between qubits with different frequencies to enable hybrid quantum systems. We anticipate that quantum control and measurement of molecules as demonstrated here will create opportunities for quantum information science, quantum sensors, fundamental and applied physics, and controlled quantum chemistry.
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