Utilization of stimulated Raman excitation and coherent anti-stokes Raman scattering in studies of bond- and mode-selective chemistry

The state resolved photodissociation of rovibrational states of the fundamental symmetric stretch of water, H₂O (1,0,0), and of the O-H or O-D stretch vibrations of HOD was studied using laser induced fluorescence and Doppler polarization spectroscopy. The control that the initially selected state exerts over the product state distribution, vector correlations and bond selectivity was demonstrated. The specific energy deposition via the lowest level of vibrational excitation leads to the enhancement of bond breaking and to structured rotational state distributions of the OH fragment which depends on the prepared rotational state of the parent. The correlations are close to the maximum attainable values expected for an idealized orientation where the transition dipole moment of the parent is parallel to the fragment angular momentum and perpendicular to its velocity. The photodissociation of HOD in which the O-H stretch is initially excited, leads to enhanced bond breaking and selective production of OD + H, while when the O-D is excited no enhancement is obtained. These results show that experiments which prepare the parent molecule in a particular state before a second laser dissociates it, provide a powerful means for bond- and mode-selective chemistry and molecular dynamics studies.