Recoil Inversion in the Photodissociation of Carbonyl Sulfide near 234 nm

We report the observation of recoil inversion of the CO ($v=0$, $J_{\mathrm{CO}}=66$) state in the UV dissociation of lab-frame oriented carbonyl sulfide (OCS). This state is ejected in the opposite direction with respect to all other ($>30$) states and in absence of any OCS rotation, thus resulting in spatial filtering of this particular high-$J$ rovibrational state. This inversion is caused by resonances occurring in shallow local minima of the molecular potential, which bring the sulfur closer to the oxygen than the carbon atom, and is a striking example where such subtleties severely modify the photofragment trajectories. The resonant behavior is observed only in the photofragment trajectories and not in their population, showing that stereodynamic measurements from oriented molecules offer an indispensable probe for exploring energy landscapes.

Figure 1

(a) Sliced image of ionized sulfur atoms produced by the photodissociation of nonoriented OCS molecules at $\sim 42700{\mathrm{cm}}^{-1}$. (b) Values of the ${\beta }_2$ coefficient for $60\le J_{\mathrm{CO}}\le 66$ states, extracted from (a). (c) Same as (a) for the photodissociation of oriented OCS molecules at $\sim 42700{\mathrm{cm}}^{-1}$. The CO fragments move antiparallelly to the detected $S$-atom cofragments. (d) Values of ${\beta }_1$ coefficient for the states with $60\le J_{\mathrm{CO}}\le 66$ extracted from (c).

Figure 2

The $J_{\mathrm{CO}}=66$ state. (a) Dependence of the ${\beta }_1$ parameter on the photolysis energy, for oriented OCS parent molecules. (Inset) Sliced ion images for three different photolysis energies. (b) Dependence of the ${\beta }_2$ parameter on the photolysis energy, for nonoriented OCS parent molecules. (Inset) Sliced ion images for three different photolysis energies. (c) Total intensity as a function of the photolysis laser energy. The red dashed line shows the total absorption cross section [40, 41].

Figure 3

Contours of the adiabatic PES for the (a),(b) $A$ (brown) and (c),(d) $X$ (cyan) electronic states projected in the $(R,\gamma )$ plane ($r=2.2$ bohr), with representative periodic orbits and quantum resonance states (see the text). (a) For the $A$ PES, a resonant quantum state is shown at $42100{\mathrm{cm}}^{-1}$ and POs covering $[38000–49000]{\mathrm{cm}}^{-1}$. (b) Same as (a) for the resonant quantum state at $42700{\mathrm{cm}}^{-1}$. (c) For the $X$ PES, POs covering $[33000–49000]{\mathrm{cm}}^{-1}$ (red) and $(35000–48000){\mathrm{cm}}^{-1}$ (green), and a resonant quantum state at $33715{\mathrm{cm}}^{-1}$ alongside the periodic orbits. (d) Same as (c) for a resonant quantum state at $34430{\mathrm{cm}}^{-1}$.

Figure 4

Angular integration of the ion image shown in Fig. 1, depicting the relative intensity of the various $J_{\mathrm{CO}}$ states (the black solid line), along with the nonadiabatic (blue dots) populations. The red dashed line is the result of single adiabatic calculations in the $A$ state.

Figure 5

(a) Time evolution of the overlap integral squared between the excited wave packet and the resonance wave function in the $A$ state shown in Fig. 3. (b) The overlap integral of the evolving excitation wave packet in the $A$ and $X$ states with the product states $J_{\mathrm{CO}}=63–68$ wave functions in the nonadiabatic calculations.