The predissociation dynamics of B Rydberg state of methyl iodide has been studied with femtosecond two-color pump-probe time-of-flight spectra at pump pulse of 400nm and probe pulse 800 nm. The dominant product channels are the CH3I+ and CH3+ formation. The time-dependent signals for CH3I+ and CH3+ ions are obtained. Both of the signal curves can be fitted by biexponential decays with time constants of O 1 and O 2, O 1 was assigned to the lifetimes of high Rydberg states, which can be accessed by absorbing three 400 nm pump pulses and O 2 reflects the dynamics of B Rydberg state, which is accessed with two pump pulses. The lifetime of B Rydberg state is determined to be about 1.57 ps, which is incredibly consistent with the previous studies. The results were interpreted as a multiphoton dissociative ionization processes.

Velocity imaging technique combined with (2 + 1) resonance-enhanced multiphoton ionization (REMPI) has been used to detect the Br fragment in photodissociation of o-, m-, and p-bromofluorobenzene at 266 nm. The branching ratio of ground state Br(2P3/2) is found to be larger than 96%. Its translational energy distributions suggest that the Br fragments are generated via two dissociation channels for all the molecules. The fast route, which is missing in p-bromofluorobenzene detected previously by femtosecond laser spectroscopy, giving rise to an anisotropy parameter of 0.50-0.65, is attributed to a direct dissociation from a repulsive triplet T1(A' ') or T1(B1) state. The slow one with anisotropy parameter close to zero is proposed to stem from excitation of the lowest excited singlet (pi,pi*)state followed by predissociation along a repulsive triplet (pi,sigma*) state localized on the C-Br bond. For the minor product of spin-orbit excited state Br(2P1/2), the dissociating features are similar to those found in Br(2P3/2). Our kinetic and anisotropic features of decomposition obtained in m- and p-bromofluorobenzene are opposed to those by photofragment translational spectroscopy. Discrepancy between different methods is discussed in detail.

We report the vibronic and cation spectra of four rotamers of m-methoxystyrene, recorded by using the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques. The excitation energies of the S1← S0 electronic transition are found to be 32 767, 32 907, 33 222, and 33 281 cm(-1), and the corresponding adiabatic ionization energies are 65 391, 64 977, 65 114, and 64 525 cm(-1) for these isomeric species. Most of the observed active vibrations in the electronically excited S1 and cationic ground D0 states involve in-plane ring deformation and substituent-sensitive bending motions. It is found that the relative orientation of the methoxyl with respect to the vinyl group does not influence the vibrational frequencies of the ring-substituent bending modes. The two dimensional potential energy surface calculations support our experimental finding that the isomerization is restricted in the S1 and D0 states.

No abstract