$Ortho\mathrm{\text{−}}Para$ ${\mathrm{H}}_2$ Conversion by Proton Exchange at Low Temperature: An Accurate Quantum Mechanical Study

We report extensive, accurate fully quantum, time-independent calculations of cross sections at low collision energies, and rate coefficients at low temperatures for the ${\mathrm{H}}^{+}+{\mathrm{H}}_2(v=0,j)\to {\mathrm{H}}^{+}+{\mathrm{H}}_2(v=0,j^{\prime })$ reaction. Different transitions are considered, especially the ortho-para conversion ($j=1\to j^{\prime }=0$) which is of key importance in astrophysics. This conversion process appears to be very efficient and dominant at low temperature, with a rate coefficient of $4.15\times {10}^{-10}{\mathrm{cm}}^3{\mathrm{molecule}}^{-1}{\mathrm{s}}^{-1}$ at 10 K. The quantum mechanical results are also compared with statistical quantum predictions and the reaction is found to be statistical in the low temperature regime ($T<100\mathrm{K}$).

Figure 1

TIQM (solid line) and SQM (dashed line) integral cross sections as a function of the collision energy for ${\mathrm{H}}^{+}+{\mathrm{H}}_2(v=0,j=1)\to {\mathrm{H}}^{+}+{\mathrm{H}}_2(v=0,j^{\prime })$ (a) and ${\mathrm{H}}^{+}+{\mathrm{H}}_2(v=0,j=0)\to {\mathrm{H}}^{+}+{\mathrm{H}}_2(v=0,j^{\prime })$ (b).

Figure 2

TIQM (solid line), SQM (dashed line), and SM (dotted line) rate coefficients (in ${\mathrm{cm}}^3{\mathrm{molecule}}^{-1}{\mathrm{s}}^{-1}$) as a function of the temperature for ${\mathrm{H}}^{+}+{\mathrm{H}}_2(v=0,j=1)\to {\mathrm{H}}^{+}+{\mathrm{H}}_2(v=0,j^{\prime })$ (a) and ${\mathrm{H}}^{+}+{\mathrm{H}}_2(v=0,j=0)\to {\mathrm{H}}^{+}+{\mathrm{H}}_2(v=0,j^{\prime })$ (b).