Knowledge Bank

Ion imaging methods have been used to study the dynamics of H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$/D$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ elimination from H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$/D$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ cations following photo-excitation to the A$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$A$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ state in the wavelength range 300$<{\lambda}<$360\; nm.\}\; \backslash underline\{\backslash textbf\{127\}\},\; 224307\; (2007).\}\; Ground\; (X$^{2}$B${1}$)stateparentionswereformedby2+1REMPIofH${2}$S/D${2}$S via the v=0 level of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$A$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$(...2b$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$$^{1}$4pb${2}$$^{1}$)Rydbergstate.ThisRydbergstatepredissociatessufficientlyslowlythattheREMPIspectrumshowsresolvedrovibronicstructure,therebyallowingfullquantumstateselectivityatthisstageofthecationpreparationprocess.AnalysisoftheS$^{+}$ ion images following one photon excitation of the resulting H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$$/$D$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ cations reveals that these fragments are formed in their ground ($\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$S) state, and that the H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$$/$D$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ co-fragments are formed in rotational states with either odd or even J rotational quantum number - depending on the chosen REMPI preparation wavelength. This striking specificity for forming ortho- or para-H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$$/$D$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ products can be traced to the state selectivity introduced in the REMPI preparation step. Two distinct fragmentation pathways for H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$$/$D$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$(A) cations are identified. One involves non-adiabatic (Renner-Teller) coupling to the X state at near linear configurations and subsequent (spin-orbit induced) coupling to the repulsive $\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$A$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ potential energy surface (PES) at smaller bond angles. This process operates throughout the photolysis wavelength range investigated and yields rotationally 'cool' and vibrationally 'cold' H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ products. The second shows a long wavelength threshold ${\lambda}{\sim}$335nm,andgraduallybecomesdominantasthephotolysiswavelengthisreduced.Thismechanisminvolvesvibronicallyfacilitatednon-adiabatictransferfromtheAtotheB$^{2}$B$_{2}$ state, followed by spin-orbit induced transfer to the $\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$A$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ PES; the resulting H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ products carry higher levels of rotational and vibrational excitation.