Photodissociation Dynamics Of CS₂⁺ And H₂S⁺ By DC Slice Imaging

The dissertation mainly presents the photodissociation dynamics of the pure CS₂⁺ and H 2S⁺ molecular ions investigated by the DC slicing ion imaging method. The photo-excitation dissociation was mediated by numerous vibronic levels of one specific electronic state. The translational energy releases spectra and angle distributions of the fragments were obtained, and further the dissociation mechanisms of the mode selected molecular ions were discussed.[1+1] photodissociation of CS₂⁺ via the vibrationally mediated （A| ^）2∏u state: The ionic parent CS₂⁺ was prepared in its （X|^{） 2}∏g ground state through a [3 + 1] resonance enhanced multiphoton ionization （REMPI） process at 483.14 nm with the aid of the 4 pσ 3∏u Rydberg state of the neutral CS 2 molecule.The ground state of CS ₂⁺（ （X|^{） 2}∏g） cation was excited to different （A|^{） 2}∏u,3/2,1/2 （υ 1 ,υ₂,0） vibronic levels over a wavelength range of 445– 480 nm, followed by a predissociation process leading to the first dissociation channel S⁺ （4 S ）+ CS（X¹∑⁺） by absorbing one more photon. The translational energy release （TER） spectra of the S S pair were derived from the velocity map images. The energy partitionings and vibrational populations were obtained simultaneously.The C S（X ¹∑⁺） fragments were found to form in a mode-selective manner. The mean value of the rotational quantum number J in the （A|^） [3/ 2]（0,0,0） case is about 25, and it is derived that the rotational temperature is about 1500 K; while the other CS₂⁺ （ （A|^） （υ 1 ,υ₂,0））cases have a maximum proportion J at 35-40 range, corresponding to the rotational temperature being about 3500K.In terms of the ability in reducing + to zero （i.e., to approach isotropicity to a full extent）, the vibrational excitation of CS₂⁺ （ （A|^） [ 3/ 2]（υ1 ,υ2,0）） arising from the v₁（symmetric stretch） mode takes preference over that from the V 2（bend） mode. 77% and 75% of the excess energy goes to the internal excitation of CS fragments in the A^[ 3/2]（ （A|^） [3/ 2]（0,0,0）of 72%） and （A|^） [1/ 2]dissociation processes, respectively, implying that indirect （or metastable） rather than direct fragmentation takes place.One UV photo-excitation dynamics of CS₂⁺ :With the same CS₂⁺ preparation, S ⁺ fragments ion images using a single UV photo-excitation were obtained that reaches the same energy region as in the [1 + 1] photo-excitation scheme via the （A| ^）2∏u state. Both the [3/2] and [1/2] components of the molecular cation CS₂⁺ （ （X| ）²∏g,3/2,1/ 2）were excited at one specific resonant wavelength. The dissociation results are listed below.（1） Contribution variation from the CS₂⁺[1/2] parent cations. The amounts of the fragments from CS₂⁺[1/2] vary largely at different resonant wavelengths.（2） Fragments rotational distribution from the [3/2] and [1/2] components differs. The fragments from [3/2] components have a mean value of the rotational quantum number J at about 35-40 range, corresponding to the rotational temperature being about 3500K; while these from the [1/2] components have a maximum proportion J at 20, corresponding to about 1000K.（3） Dual peaks rotational distribution from the [3/2] components at the specific 242.144 nm wavelength. The additional peaks lie at J ²5-30.The dissociation mechanism was proposed as the （X| ）²∏g state interacts vibronically with the high vibrational levels of the （X| ）²∏g state leading to the S⁺ （⁴ S ）+ CS（X¹∑⁺） channel. The new dissociation path at 242.144 nm was ascribed to the perturbation by other involved state/states in the explored region. [1+1] photodissociation of CS₂⁺ via the vibrationally mediated （B|^{） 2}∏u state:CS 2（ （X|^{） 2}∏g） was dissociated by a [1 + 1] photo-excitation mediated via the vibrationally selected B? state over a wavelength range of 267 - 283 nm. At these wavelengths the （C| ）²∑_g⁺ and （D| ）²∑_g⁺ states are reached, followed by numerous S⁺ and CS⁺ dissociation channels. The lowest S⁺ channel S⁺ （ 4 S ） + CS（ X 1∑⁺）is missing.The S⁺ channels specified as three distinct regions （Regions A, B and C） were shown with vibrationally resolved structures, in contrast to the less-resolved structures being presented in the CS⁺ channels. The average TER of the fragmental ions were obtained, and the S⁺ / CS⁺ branching ratios with mode specificity were measured. The amount of fragments via the CS⁺ + S channels in this region is a bit larger than that via the S⁺ channels. The bending mode excitation leads to the significant increase of the S⁺ channels.Two types of dissociation mechanisms are proposed. One mechanism is the direct coupling of the （C|^） and （D|^） states with the repulsive satellite states leading to the fast photo-fragmentation. The other mechanism is the internal conversion of the C? and D? states to the （B|^） state, followed by the slow fragmentation occurred via the coupling with the repulsive satellite states. H 2S⁺ photodissociation mediated via the （A|^） 2 A1 （V ₁ , V₂ ,0）K state:The molecular cation H 2S⁺ was obtained by a [2+1] REMPI progress at 302.56 nm, and then was excited via different （A|^） 2 A1 （V 1 , V₂ ,0）K levels with the 2 2（A|^） A1 B1 transition over 314-327.5 nm, leading to the S + （ 4 S u ） + H 2（ （X| ）²∏g） channel. The internal distribution of H 2（ （X| ）²∏g） was obtained. Dissociation via different vibronic levels resulted in a difference in rotational distribution. The ortho-/para- selected H 2S⁺ prepared at 314 nm has no obvious influence to the dissociation mechanism. The dissociation mechanism in the energy region tends to be the （A|^） 2 A1 state couples directly to the 4 A 2state through the spin-orbit interaction, leading to the products.