Photodissociation Dynamics Of Cluster Ions

Gas-phase molecular ions and cluster ions are intermediate species of many important ion-molecule reactions,and studying their photodissociation dynamics at the quantum state and wavefunction levels is of great theoretical importance for understanding the ion-molecule microscopic reaction mechanisms and state-to-state reaction dynamics,ultimately guiding to solve key scientific problems in atmospheric chemistry,interstellar chemistry,and combustion processes.The main content of this PhD dissertation is the development of a cryogenic cylindrical ion trap velocity map imaging(CIT-VMI)spectrometer and its application in the state-resolved photodissociation dynamics of cluster ions.1.Development of a cryogenic ion trap velocity map imaging spectrometerA cryogenic cylindrical ion trap velocity map imaging(CIT-VMI)spectrometer has been developed to study the state-resolved photodissociation dynamics of molecular ions and cluster ions.A homemade cold cylindrical ion trap is used to cool down the internal temperatures of mass-selected ions and reduce the velocity spread and spatial size of ion packets.The photofragment images are detected using the high-resolution direct current slice velocity map imaging technique.High-resolution detections of recoiling momentum of both neutral and ion photofragments are realized by using a well-designed ion acceleration and velocity focusing electric field.The performances of the CIT-VMI setup are experimentally tested as:the number of trapped ions can reach 10000;the vibrational and rotational temperatures of the trapped ions can be effectively cooled down to?12 K;the velocity spread of the ions extracted from the trap can be reduced to be approximately±25 m/s,both radially and axially;the VMI velocity resolutions of ?v/v?1.5%(for Ar+)and ?v/v?4.6%(for Ar)have been derived from the photodissociation of cold Ar2+ ions.2.Imaging the three-body dissociation dynamics of Ar3+Multi-body dissociation dynamics play an important role in the high-energy physical and chemical processes.The three-body dissociation dynamics of Ar3+in the visible and ultraviolet region have been investigated using the CIT-VMI apparatus.Both of the Ar neutral and Ar+ionic fragments are measured in coincidence,and the corresponding translational energy distributions and angular distributions are derived.According to the momentum correlations of the products,we conclude that the three-body dissociation of Ar3+ follows a fast concerted dissociation mechanism of a quasi-linear configuration.At the visible excitation wavelengths,the concerted dissociation involves the excited ?(1/2)g state in combination with excitation of the symmetric stretching vibration and bending vibration modes and produces Ar+(2P3/2)product.At the ultraviolet excitation wavelengths,the concerted dissociation involves the excited?(1/2)g state,as well as three vibrational modes,the symmetric and antisymmetric stretching modes,and the bending modes,yielding the Ar+(2P1/2)ionic product.The experimental results reveal the mechanism of the nonadiabatic coupling effect among electronic and vibrational states during a fast dissociation process.3.Feshbach resonance mediated photodissociation of the weakly bound complex[CO2-CO2+]Reactive resonances or dynamical resonances are transiently trapped quantum states along the reaction coordinate in the transition-state region of a chemical reaction.The resonance states can substantially affect the reaction rate and product quantum state distributions.The photodissociation dynamics of(CO2)2+in the 430-765 nm region have been investigated using the CIT-VMI apparatus.Experimental measurements of the translational energy release of photofragmented CO2+ions show that,in addition to the fast velocity component product resulting from the direct dissociation of the repulsive excited state,there is,in addition,a velocity component with almost zero translational energy in the studied wavelength range.This component corresponds to a dissociation channel for which the product recoiling velocity is almost zero and independent of the excitation energy.In combination with the potential energy surfaces of(CO2)2+ions based on theoretical calculations,this channel originates from the Feshbach resonance of internal rotation excitation.These resonance states involve the excitation of internal rotations of the[CO2-CO2+]complex,which play the role of storing the exceeding photon energy above the dissociation limit as the internal rotations,making the products highly rotationally excited.4.Probing the charge-transfer potential energy surfaces by photodissociation of[Ar-N2]+Chemical reaction pathways and product state correlations of gas-phase ion-molecule reactions are governed by the involving nonadiabatic potential energy surfaces(PESs).The photodissociation dynamics of the charge-transfer complex[Ar-N2]+have been studied in the 2.15-2.67 eV excitation energies range using the CIT-VMI apparatus.The[Ar-N2]+complex is the intermediate of the Ar++N2-+Ar+N2+ charge transfer reaction.Ion images of Ar+ and N2+ are detected at five selected excitation energies,and total translational energy release spectra and vibrational state distributions of products from different channels are derived.High-resolution recoiling velocity images of photofragmented N2 and N2+from different dissociation channels exhibit a vibrational state-specific correlation,revealing the nonadiabatic charge-transfer mechanisms upon the photodissociation of[Ar-N2]+.The state-resolved product branching ratios have yielded an accurate determination of the resonant charge-transfer probabilities.