Photodissociation Study of Ethyl Bromide in the Ultraviolet Range by the Ion-Velocity Imaging Technique

Photolysis dynamics of m- and o-fluorophenol: Substitution effects on tunneling mechanism

The photolysis dynamics of m-fluorophenol (m-FPhOH) and o-fluorophenol (o-FPhOH) have been investigated with time-resolved velocity map imaging (TR-VMI) and time-resolved ion-yield (TR-IY) techniques. Following excitation to the origin of S₁ (ππ∗) state of m- and o-FPhOH, H atoms elimination mediated by tunneling through the potential barrier under the S₁ (ππ∗)/S₂ (πσ∗) conical intersection (CI) has been observed as a Gaussian feature signal centered at a total kinetic energy release (TKER) of ∼6000 cm^-1 for both molecules. The quantum tunneling mechanism has been identified as the main decay pathway of S₁ state for m-FPhOH, and the tunneling lifetime of 2.1 ns has been obtained from the TR-VMI measurements of H fragments. This tunneling mechanism is further confirmed by the studies on the selective O-H deuterated species, m-FPhOD, and consolidated by our theoretical calculations. However, the photolysis dynamics is quite different for the photoexcited o-FPhOH. The much lower yield of the H atoms originating from tunneling hinders the extraction of a reliable tunneling lifetime for o-FPhOH. Our theoretical calculations exhibit a broader and higher potential barrier exists beneath the S₁/S₂ CI of o-FPhOH, which increase the difficulty for tunneling. Furthermore, the special existence of intramolecular hydrogen bond in o-FPhOH is probably also the key factor that affects the tunneling rate, which would restrict the O-H stretch motion.

Electronic spectroscopy of ethyl bromide probed by VUV photoabsorption and quantum chemical calculations

A comprehensive study of the electronic absorption spectra of C₂H₅Br and C₂D₅Br using synchrotron radiation photoabsorption spectroscopy and TDDFT calculations.

Ultrafast Photodissociation Dynamics of Highly Excited Iodobenzene on the C Band

The photodissociation dynamics of highly excited iodobenzene from the C band absorption has been studied by femtosecond time-resolved ion yields techniques. Detailed photodissociation routes are discussed with the aid of high-level, spin-orbit resolved ab initio calculations of 1D potential energy curves. Upon 200 nm excitation within the C band, iodobenzene molecules on 7B₂ and 7B₁ states decay to 7A₁ and 8B₂ states through internal conversion in 75 fs, with electronic energy converted into high vibrational energy of 7A₁ and 8B₂ states. Subsequently, 7A₁ and 8B₂ states decay through internal vibrational energy redistribution in 540 fs, accompanied by the excited C-I mode and the resulting cleavage of the C-I bond. The overall time for the reaction starting from the phenyl-type modes and ending in final C-I fragmentation for I(²P_3/2) production is 1.2 ps.

Multiphoton dissociative ionization of tert-pentyl bromide near 265 nm

We report on the photodissociation dynamics of tert-pentyl bromide near 265 nm investigated by time-sliced velocity map imaging. The speed and angular distributions have been analyzed for both the ground-state Br((2)P(3∕2)) atom (denoted Br) and the spin-orbit excited-state Br((2)P(1∕2)) atom (denoted Br*). The speed distributions of Br and Br* atoms are all found to consist of three Gaussian components, which correlate to three independent dissociation pathways on the excited potential energy surfaces: (1) the high translational energy (E(T)) component from the prompt dissociation along the C-Br stretching mode, (2) the middle E(T) component from the repulsive mode along the C-Br stretching coupled with some bending motions, and (3) the low E(T) component from the repulsive mode along the C-Br stretching coupled with more bending motions. More interestingly, we have also observed the tert-C(5)H(11)(+) ions in 263-267 nm. The near-zero kinetic energy distributions extracted from the three tert-C(5)H(11)(+) images near 265 nm show the typical characteristics that are attributable to multiphoton dissociative ionization, suggesting the existence of a neutral superexcited state of the parent tert-pentyl bromide molecule. The contribution of bromine atoms formed in this dissociative ionization channel adds in the total relative distribution of low E(T) component in the Br*(Br) formation channel, which reasonably explains the abnormal distributions observed in between the middle and low E(T) components in the Br*(Br) formation channel.

Comparative Study of Multielectron Ionization of Alkyl Halides Induced by Picosecond Laser Irradiation

The interaction of C2H5X, 1-C3H7X, 1-C4H9X, where X = I, Br, Cl, with strong (1 x 10(13)-1.2 x 10(14) W/cm2) 35 ps laser pulses at 1064 nm is studied by means of time-of-flight mass spectrometry. The multielectron ionization following the C-X bond elongation has been verified for the studied molecules. By combination of the intensity dependence of the ion yields, the estimated kinetic energies of the released fragment ions, and their angular distributions, we have identified the different dissociation channels of the transient multiply charged parent ions. From the dependence on the laser intensity of the ratio of the doubly charged halogen ions to the singly charged ones, it is concluded that the molecular coupling with the laser field increases with the molecular size.

Photodissociation study of 1,3-dibromopropane at 234 nm via an ion velocity imaging technique

The photodissociation of 1,3-dibromopropane has been studied at 234 nm using a 2D photofragment ion velocity imaging technique coupled with a [2 + 1] resonance-enhanced multiphoton ionization scheme. The velocity distributions for the Br (2P1/2) (denoted Br) and Br (2P3/2) (denoted Br) fragments are determined, and each can be fitted by a narrow single-peaked Gaussian curve, suggesting that bromine fragments are generated as a result of direct dissociation via repulsive potential energy surfaces. The recoil anisotropies were measured to be beta = 0.80 for Br and 1.31 for Br, and the product relative quantum yields at 234 nm is Phi234 nm(Br) = 0.21.