In the wavelength range of 231-275 nm, we have studied the mass-resolved dissociation spectra of OCS＋ via B2∑＋←X2П3/2（000） and B2∑＋←X2П1/2（000, 001） transitions by preparing OCS＋ ions in the well-defined spin-orbit states. The spectroscopic constants of v1 （CS stretch）=828.9 （810.4） cm-1, u2 （bend）=491.3 cm-1 and v3（CO stretch）=1887.2 cm-1 for OCS＋（B2∑＋） are deduced. The observed dependence of the v2（bend） mode excitation of B2∑＋ on the spin-orbit splitting of X2П（Ω=1/2, 3/2） in the B2∑＋←-X2П transition can be attributed to the K coupling between the （000）2П1/2 and （010）2∑＋/2 vibronic levels of X2П state, which makes the B2∑＋（010）←X2П1/2（000） transition possible.
When the molecular ions XYZ＋ （XY2＋） are excited simultaneously from an electronic state E0 into two higher electronic states Ea and EZ with supervened dissociation or predisso- ciation, competition between the α and β excitation-dissociation channels occurs. A the- oretical model is provided to deal with the competition of the two excitation-dissociation channels with more than two kinds of ionic products for XYZ＋ （XY2＋）. Supposing that the photo-excitation rates of two states Eα and Eβ are much less than their dissociation or pre-dissociation rates, a theoretical equation can be deduced to fit the measured data, which reflects the dependence of the product branching ratios on the intensity ratios of two excitation lasers. From the fitted parameters the excitation cross section ratios are obtained. In experiment, we studied the competition between two excitation-dissociation channels of CO^2＋. By measuring the dependence of the product branching ratio on the intensity ratio of two dissociation lasers and fitting the experiment data with the theoretical equation, excitation cross section ratios were deduced.
The photodissociation dynamics of 2-bromobutane has been investigated at 264.77 and 264.86 nm by ion-velocity map imaging technique coupled with resonance-enhanced multi- photon ionization. The speed and angular distributions have been derived from the velocity map images of Br and Br^＊. The speed distributions of Br and Br^＊ atoms in the photodis- sociation of 2-bromobutane at -265 nm can be fitted using only one Gaussian function indicating that bromine fragments were produced via direct dissociation of C-Br bond. The contributions of the excited 3^Q0, 3Q1, and 1^Q1 states to the products （Br and Br^＊） were discussed. It is found that the nonadiabatic 1^Q1←3^Q0 transition plays an important role for Br photofragment in the dissociation of 2-C4HgBr at -265 nm. Relative quantum yield of 0.621 for Br（2P3/2） at -265 nm in the photodissociation of 2-bromobutane is derived. By comparing the photodissociation of 2-C4H9Br at -265 nm and that that at-234 nm, the anisotropy parameter β（Br） and β（Br^＊）, and relative quantum yield Ф（Br） decrease with increasing wavelength, the probability of curve crossing between 3 ^Q0 and 1^Q1 decreases with increasing laser wavelength.
The photodissociation dynamics of 2-bromobutane has been investigated at 233.62 and 233.95 nm by ion-velocity map imaging technique coupled with resonance-enhanced multiphoton ionization. The speed and angular distribution of Br and Br＊ fragments were determined from the map images. The two Gaussian components, shown in the speed dis- tributions of Br and Br＊ atoms, are suggested to attribute to the two independent reaction paths of photodissociation for 2-bromobutane at 233.62 and 233.95 nm. The high-energy component is related to the prompt dissociation along the C-Br stretching mode, and the low-energy component to the dissociation from the repulsive mode with bending and C-Br stretching combination. The contributions of the excited 3Q0, 3Q1, and 1Q1 states to the products （Br and Br＊） were discussed. Relative quantum yield of 0.924 for Br（2P3/2） at about 234 nm in the photodissociation of 2-bromobutane is derived.
