Studies On Photodissociation Dynamics Of Polyatomic Molecules By Velocity Map Imaging

This thesis mainly introduces studies on photodissociation dynamics of polyatomic molecules with the time-sliced ion imaging technique. The first one is the photodissociation dynamics of HNCO involved both on the singlet and triplet potential energy surfaces. The second topic is the photodissociation dynamics of CO2molecules at157nm. The last one is the indepth studies on photodissociation dynamics of CH3CHO at wavelengths in UV region. We obtained the insight of the reaction dynamics based on the translational energy distributions and angular distributions of the recorded products ion images.Photodissociation dynamics studies of HNCO for NH(a1â–³)+CO(X1Σ+) channelThe NH(a1â–³)+CO(X1Σ+) product channel for the photodissociation of HNCO at201nm was investigated using the sliced velocity map ion imaging technique with the detection of both CO(X1Σ+) and NH(a1â–³) products via (2+1) resonance enhanced multiphoton ionization (REMPI). The negative anisotropy parameter measured for the products indicates a direct dissociation process for the N-C bond cleavage in the S1state. Correlation between the NH(a1â–³) and CO rovibrational state distributions were determined from these images. The vibrational excitation of the correlated CO product is anti-correlated to the1NH(v=0|j) rotational excitation. But the vibrational energy of the correlated NH fragment increases as the rotational state of CO product increases initially and decreases afterwards. Two fragments show different variation. The vibrational energy of both fragments is anti-correlated and the rotational energy of1NH(v=O) fragment is also anti-correlated to the CO(v=1) rotational excitation. From the image of’NH fragment, a bimodal rotational distribution of CO(v=0) has been observed clearly. We speculated that it derived from two different dissociation pathways in the S1state and gave a possible explanation-as being related to the two different stable isomers (trans-and cis-HNCO).Photodissociation dynamics studies of HNCO for NH (X3Σ-)+CO (X1Σ+) channelWe studied the photodissociation of HNCO for the NH (X3Σ-)+CO (X1Σ+) product channel at230nm by the velocity map ion imaging technique with the detection of CO fragments via (2+1) REMPI. By analyzing the total translational energy of fragments we found that the vibrational energy of3NH fragment is inverted and peak at v=1. The different angular distribution and signal strength indicated that the3NH vibrational ground state and the first excited state should come from different dissociation pathways. For the3NH first excited state, the dissociation path should be S1-â†'S0â†'T1â†'3NH (v=1)+CO, the life of intermediate state is long, so the angular distribution of fragment is isotropic. However, the3NH vibrational ground state should come from S1-â†'S0â†'T1â†'3NH (v=0)+CO. The life of intermediate state is relatively short, so the angular distribution of product is anisotropic.Photodissociation dynamics studies of CO2at157nmPhotodissociation dynamics of CO2for the CO(1Σ+)+O(1D) channel at157nm has been investigated using time-sliced velocity map imaging technique. Both CO and O(1D) fragments were probed by (2+1) REMPI. The results of O(1D) image indicated that CO products were distributed in the vibrational ground state and the first excited state. The results of CO image indicated that the angular distribution of fragments changed as CO rotational energy changed. The lower rotationally excited CO fragments have higher anisotropy parameters than the highly rotationally excited CO products. An impact modal could be employed to illuminate this phenomenon. The anisotropy of highly rotationally excited CO fragments shows an anomalous variation. We speculated that it derived from the coupling between potential energy surfaces. Photodissociation dynamics studies of acetaldehydeThe photodissociation dynamics of acetaldehyde in the radical channel CH3+HCO has been reinvestigated via time-sliced velocity map imaging method in the photolysis wavelength range of275-321nm. The CH3fragments have been probed via (2+1) resonance-enhanced multiphoton ionization (REMPI). Images are measured for the CH3formed in the ground and excited states (V2=0and1) of the umbrella vibrational mode. For acetaldehyde dissociation on T1state after intersystem crossing from S1state, the products are formed with high translational energy release and low internal excitation. The rotational and vibrational energy of both fragments increases with increasing photodissociation energy. The triplet barrier height is estimated at3.881±0.006eV above the ground state of acetaldehyde.