Photon Dissociation Dynamics Of H₂ D₂ And HD Studied By XUV Laser And Velocity Map Imaging Method

Photodissociation refers to dissociation of molecules into fragments after they absorb photons.Among the various of experimental techniques developed recently to study photodissociation,the pump-and-probe method and the H-atom Rydberg tagging technique are two powerful tools.Using high resolution XUV/VUV laser the pump-and-probe method,and the H-atom Rydberg tagging technique,we studied the predissociation dynamics of H2,HD and D2 molecules,and obtained some important results.When the excitation energy is above 14.678 eV,the H2 molecule will dissociate:H2+XUV→H(1s)+H(2s)/H(2p).The predissociation states of and for H2 and for D2 have been studied with the XUV laser pump and UV laser probe method and the velocity map imaging technique.The fragment yield spectrum of H(2s/2p)D(2s/2p),the branching ratios of H(2s)/(H(2p)+H(2s)),D(2s)/(D(2p)+D(2s))and the angular distributions of the dissociation fragments were measured.We found that for the transition to the states the branching ratios are around 78%～85%.By a comparision between the measured angular distributions compared with the theoretical calculation,the main component of dissociating states are found to have symmetry.The predissociation of the state produce only H(2p)fragments.For the predissociation process,the measured branching ratios and angular distributions were in agreement with the theoretical predictions.Using pump-and-probe method,the branching ratios of H(2s)/(H(2p)+H(2s)),D(2s)/(D(2p)+D(2s))and the angular distributions from the direct dissociation of para-H2,HD,D2 molecules were measured as well.The observed branching ratio of H(2s)/(H(2s)+H(2p))and D(2s)/(D(2s)+D(2p))oscillatte with the change of the fragment translational energies.Taking H2 as an example,there are three states involved in the direct dissociation:the 3pσ state correlating to the channel H(2s)+H(1s),the and states correlating to channel H(2p)+H(1s).The oscillating behavior indicate the interferences of nuclear motions among these channels.Using XUV laser and H-atom Rydberg tagging time-of-flight technique,the angular-resolved and fully rotational-resolved Beutler-Fano profiles for the predissociation of H2 of the R(0)and R(1)transitions in have been measured.The results show that the Beutler-Fano profiles have different shapes for the fragments H(2s/2p)recoiling at parallel and perpendicular directions relative to the polarization direction of the XUV laser.Tuning the excitation photon energies from the resonance center to the lower energy in the Beutler-Fano profile,the measured fragment angular distributions change in a trend indicating more components of the direct dissociation states and in comparison with the predissociation state,in addition to this,the branching ratios H(2s)/(H(2s)+H(2p))decrease.