Cold and ultracold NH-NH collisions in magnetic fields

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 ${}^{15}\mathrm{N}$H 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.

Figure 1

Lowest adiabatic potential curves for ${}^{15}\mathrm{N}$H-${}^{15}\mathrm{N}$H, calculated for $L_{\max }=4$ and $M=2$ at a magnetic field of 0.1 G. The adiabats are plotted as a function of $R$ in units of the Bohr radius $a_0$. The molecular eigenstates are labeled by $|M_{J_A},M_{J_B}\rangle$ and refer to the well-ordered states $|(N_A=0)J_A=1,M_{J_A}\rangle |(N_B=0)J_B=1,M_{J_B}\rangle$.

Figure 2

Elastic and inelastic $M_J$-changing cross sections for magnetically trapped ${}^{15}\mathrm{N}$H as a function of collision energy for various magnetic fields. The elastic cross sections are the same for all three magnetic field strengths.

Figure 3

Elastic and inelastic $M_J$-changing cross sections for magnetically trapped ${}^{15}\mathrm{N}$H as a function of magnetic field for various collision energies.

Figure 4

Inelastic $M_J$-changing cross sections for $M=2$ as a function of collision energy at 1 and 100 G. The different curves correspond to calculations with only the intermolecular magnetic dipolar coupling ($V_{\mathrm{magn}.\mathrm{dip}}$), the intramolecular spin-spin coupling ($V_{\mathrm{intra}\text{−}\mathrm{SS}}$), or the intramolecular spin-rotation coupling ($V_{\mathrm{intra}\text{−}\mathrm{NS}}$) included in $\mathrm{Ĥ}$.

Figure 5

Inelastic $M_J$-changing cross sections for $M=2$ as a function of magnetic field at ${10}^{-6}$ K and ${10}^{-3}$ K. The different curves are obtained from scattering calculations with only the intermolecular magnetic dipolar coupling ($V_{\mathrm{magn}.\mathrm{dip}}$), the intramolecular spin-spin coupling ($V_{\mathrm{intra}\text{−}\mathrm{SS}}$), or the intramolecular spin-rotation coupling ($V_{\mathrm{intra}\text{−}\mathrm{NS}}$) switched on.

Figure 6

State-to-state inelastic cross sections ($M=2$) for magnetically trapped ${}^{15}\mathrm{N}$H as a function of energy at 1 G. The final states are labeled by $|M_{J_A},M_{J_B}\rangle$.

Figure 7

Elastic and inelastic $M_J$-changing cross sections ($M=2$) for magnetically trapped ${}^{15}\mathrm{N}$H as a function of the scaling factor $\lambda$, calculated at a collision energy of ${10}^{-6}$ K and a magnetic field strength of 1 G.

Figure 8

Elastic and inelastic $M_J$-changing cross sections ($M=2$) as a function of the scaling factor $\lambda$, calculated for different basis sets at a collision energy of ${10}^{-6}$ K and a magnetic field strength of 1 G.