Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userCold collisions in a molecular synchrotron
193
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We study collisions between neutral, deuterated ammonia molecules (ND$_3$) stored in a 50 cm diameter synchrotron and argon atoms in co-propagating supersonic beams. The advantages of using a synchrotron in collision studies are twofold: (i) By storing ammonia molecules many round-trips, the sensitivity to collisions is greatly enhanced; (ii) The collision partners move in the same direction as the stored molecules, resulting in low collision energies. We tune the collision energy in three different ways: by varying the velocity of the stored ammonia packets, by varying the temperature of the pulsed valve that releases the argon atoms, and by varying the timing between the supersonic argon beam and the stored ammonia packets. These give consistent results. We determine the relative, total, integrated cross-section for $mathrm{ND}_3+mathrm{Ar}$ collisions in the energy range of 40-140 cm$^{-1}$, with a resolution of 5-10 cm$^{-1}$ and an uncertainty of 7-15%. Our measurements are in good agreement with theoretical scattering calculations.
rate research
Read More
The relative orientation of colliding molecules plays a key role in determining the rates of chemical processes. Here we examine in detail a prototypical example: rotational quenching of HD in cold collisions with H2. We show that the rotational quenching rate from j=2 -> 0, in the v=1 vibrational level, can be maximized by aligning the HD along the collision axis and can be minimized by aligning the HD at the so called magic angle. This follows from quite general helicity considerations and suggests that quenching rates for other similar systems can also be controlled in this manner.
We develop a rigorous quantum mechanical theory for collisions of polyatomic molecular radicals with S-state atoms in the presence of an external magnetic field. The theory is based on a fully uncoupled space-fixed basis set representation of the multichannel scattering wavefunction. Explicit expressions are presented for the matrix elements of the scattering Hamiltonian for spin-1/2 and spin-1 polyatomic molecular radicals interacting with structureless targets. The theory is applied to calculate the cross sections and thermal rate constants for spin relaxation in low-temperature collisions of the prototypical organic molecule methylene [CH2(X)] with He atoms. To this end, two highly accurate three-dimensional potential energy surfaces (PESs) of the He-CH2(X) complex are developed using the state-of-the-art CCSD(T) method and large basis sets. Both PESs exhibit shallow minima and are weakly anisotropic. Our calculations show that spin relaxation in collisions of CH2, CHD, and CD2 molecules with He atoms occurs at a much slower rate than elastic scattering over a large range of temperatures (1 uK -- 1 K) and magnetic fields (0.01 - 1 T), suggesting excellent prospects for cryogenic helium buffer-gas cooling of ground-state ortho-CH2(X) molecules in a magnetic trap. Furthermore, we find that ortho-CH2 undergoes collision-induced spin relaxation much more slowly than para-CH2, which indicates that magnetic trapping can be used to separate nuclear spin isomers of open-shell polyatomic molecules.
Elastic and spin-changing inelastic collision cross sections are presented for cold and ultracold magnetically trapped NH. The cross sections are obtained from coupled-channel scattering calculations as a function of energy and magnetic field. We specifically investigate the influence of the intramolecular spin-spin, spin-rotation, and intermolecular magnetic dipole coupling on the collision dynamics. It is shown that $^{15}$NH is a very suitable candidate for evaporative cooling experiments. The dominant trap-loss mechanism in the ultracold regime originates from the intermolecular dipolar coupling term. At higher energies and fields, intramolecular spin-spin coupling becomes increasingly important. Our qualitative results and conclusions are fairly independent of the exact form of the potential and of the size of the channel basis set.
Rates of
We present elastic and inelastic spin-changing cross sections for cold and ultracold NH($X,^3Sigma^-$) + NH($X,^3Sigma^-$) collisions, obtained from full quantum scattering calculations on an accurate textit{ab initio} quintet potential-energy surface. Although we consider only collisions in zero field, we focus on the cross sections relevant for magnetic trapping experiments. It is shown that evaporative cooling of both fermionic $^{14}$NH and bosonic $^{15}$NH is likely to be successful for hyperfine states that allow for s-wave collisions. The calculated cross sections are very sensitive to the details of the interaction potential, due to the presence of (quasi-)bound state resonances. The remaining inaccuracy of the textit{ab initio} potential-energy surface therefore gives rise to an uncertainty in the numerical cross-section values. However, based on a sampling of the uncertainty range of the textit{ab initio} calculations, we conclude that the exact potential is likely to be such that the elastic-to-inelastic cross-section ratio is sufficiently large to achieve efficient evaporative cooling. This likelihood is only weakly dependent on the size of the channel basis set used in the scattering calculations.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
