The Photodissociation Dynamics Of H₂O

Water is a common constituent of the earth's atmosphere and in the interstellar space.Water photodissociation is an important process in these environments.In addition,water molecules in different electronically excited states have different dissociation dynamics,thus,it serves as a prototype for understanding these processes in unimolecular dissociation.In the last few years,photodissociation of water molecules has been extensively studied in experiments and theory.However,due to the lack of suitable VUV light source,photodissociation dynamics of H2O at ?<120 nm has not been investigated so far.Fortunately,the Dalian Coherent Light Source(DCLS)was successfully built in early 2017,which brought us an opportunity to study the photodissociation dynamics of small molecules in the VUV region.In this thesis,combining the H-atom Rydberg "tagging" time-of-flight(HRTOF)technique with free electron laser(FEL),photodissociation dynamics of water molecules in the VUV region was studied systematically,and a lot of dynamical phenomena were observed.The peak around 122 nm in the absorption spectrum of H2O belongs to the D1A1 state.The photodissociation dynamics of H2O and D2O in the D1A1 state were studied successfully at 122.12 nm and 121.95 nm respectively,suggesting that the dissociation dynamics on the D state can be generally described by using the B state dissociation dynamics.The branching ratios of OH(A)/OH(X)and OD(A)/OD(X)have also been determined,1.0/3.0 for H2O at 122.12 nm and 1.0/2.2 for D2O at 121.95 nm,which are reasonably consistent with the values predicted by the previous theory.The peaks at 117.5 nm and 115.2 nm belong to the vibrational bands of D1A1 state,and were assigned to the(100)and(110)band respectively.The vibrationally excited OH(X)super rotors were observed in the photodissociation of H2O for the first time.Accoding to all of the experimental results,the production of vibrationally excited OH(X)super rotors is a unique phenomenon in the photodissociation of water molecule at 115.2 nm.Furthermore,a striking enhancement in the yield of vibrationally-excited OH super rotors was detected when exciting the bending vibration of the water molecule.The photodissociation of H2O has been investigated at VUV wavelengths between 112.6 nm and 113.0 nm.The TKER spectra at 112.70,112.78 and 112.89 nm were obtained.The TKER distributions obtained at these wavelengths are essentially similar,showing insensitivity to the initial excitation of the E1B1 state rotational levels.The experimental results indicate that the extremely highly vibrationally excited OH(X)is generated from water photolysis at ??112.8 nm.A kinetic analysis involving the available solar flux,the whole absorption spectra of H2O and atmospheric water mixing ratios demonstrates that the water photochemistry could be an important source for the observed OH airglow in the upper atmosphere of the Earth,and may need to be added into the related atmospheric models.The E'1B2 state of H2O locating in the region of 112.0-112.5 nm,exists many partially resolved rotational lines.The dissociation pictures at these rotational lines are extremely different,indicating the dynamical pictures induced by rotational excitation of parent molecules.However,there is no way to scan the action spectrum,and the location of these rotational lines are hard to be predicted theoretically due to the extremely high excitation energy,preventing us to reveal the dissociation mechanism clearly.At ?<110 nm,there are a series of Rydberg states,three body dissociation channel is the dominant channel in these states,but there are also some sharp stuctures above the three body dissociation profile.The electronically excited OH super rotors have been observed in the photodissociation of H2O at 96.4 nm.According to the theoretical calculations,the lifetimes of the OH(A)super rotor decreases dramatically with the increase of the rotational quantum number,the lifetimes of the OH(A,v=0,N=36?40)super rotors are around 370 ps?57 ps(a few times of molecular rotation periods).By analyzing the experimental data in 92-110 nm,it is found that the three-body dissociation channels are the dominant dissociation channels in this region,and almost three-quarters of water molecules undergo three-body dissociation.The relative abundance of water in the Earth's primitive atmosphere with its exposure to intense VUV radiation suggests that three-body photodissociation of H2O should be a major source for oxygen generation as well as oxidation in the Earth's prebiotic atmosphere,which may have significant implications for the atmospheric evolution in water-rich terrestrial planets in general.