Photodissociation of $CF_3Br$ at 234 nm
The photodissociation dynamics of $CF_3Br$ in the A-band has been investigated utilizing a two-dimensional photofragment ion-imaging technique coupled with a state-selective resonance-enhanced multiphoton ionization scheme. The total translational energy distributions for the $Br(^2P_{3/2})$ and $Br^*(^2P_{1/2})$ channels are well characterized by Gaussian functions with average translational energies of 183 and 151 kJ/mol, respectively. The recoil anisotropies were measured to be β = 0.66 for Br and 1.83 for $Br^*$. It was found that $Br^*$ production is preferred, with a relative quantum yield of 0.80. The reduction in the recoil anisotropy for Br results from nonadiabatic coupling between the $^1Q_1$ and $^3Q_0$ states. The fraction of molecules that dissociate via a distorted pathway induced by symmetry reduction from $C_{3v}$ to $C_s$ is estimated to be 0.11.
Photodissociation dynamics of $C_2H_4BrCl$ near 234 nm
The $Br(^2P_j; j =1/2,3/2)$ atom formation of $C_2H_4BrCl$ was studied 234 nm. A two-dimensional photofragment ion-image technique coupled with a [2+1] resonant multiphoton ionization scheme was utilized to obtain speed and angular distributions of the nascent $Br(^2P_{3/2})$ and $Br^*(^2P_{1/2})$ atoms. The recoil anisotropy parameters for the Br and $Br^*$ production channels were measured to be β = 0.49 for Br and 1.55 for $Br^*$. The relative quantum yield for Br was found to be Φ (Br) + 0.67 ± 0.05, which favor Br. The probabilities of nonadiabatic transition from 3A´ to 4A´ surface and from 3A´ to 4A´ surface were found to be $P_{up} = 0.47$ and $P_{down} ＼leg 0.91$, respectively. The nonadiabatic transition was discussed in the regime of $C_s$ symmetry.
Photodissociation dynamics of $C_6H_5Br$ and $C_6F_5Br$ near 234 nm
Photodissociation dynamics of $C_6H_5Br$ and $C_6F_5Br$ have been investigated at 234 nm using two-dimensional velocity mapping techniques. Trimodal translational energy distributions of $Br(^2P_j; j = 1/2,3/2)$ have been observed after photolysis of $C_6F_5Br$. Low-velocity components with a Boltzmann shapes are produced via the internal conversion between the initially pumped $^1(＼pi,＼pi^*)$ state and vibrationally excited ground state. Middle-velocity components fitted with a Gaussian function are originated from the intersystem crossing between the bound $^1(＼pi,＼pi^*)$ state and the dissociative $^3(＼pi,＼pi^*)$ state. High-velocity components are formed via the curve-crossing from $^1(＼pi,＼pi^*)$ state to $^1(n,＼sigma)$ and/or $^1(＼pi,＼sigma^*)$ states. Bimodal translational distributions of Br and $Br^*$ have been observed after photodissociation of $C_6F_5Br$ which is consisted of one Boltzmann and Gaussian distributions. The effects of fluorine atom substitutions on the photodecomposition of C-Br bond on bromobenzene derivative are discussed.
Membrane inlet-based portable time-of-flight mass spectrometer for environmental analysis
A miniaturized time-of-flight mass spectrometer (TOFMS) with an electron impact ionization source and a sheet membrane introduction (MI) has been developed. The advantages and features of the mass spectrometer include high sensitivity, simple structure, low cost, compact volume with field portable capability, and ease of operation. A mass resolution of 500 at 142 amu has been obtained with a 30 cm instrument. Upon optimized conditions the detection limits for the volatile organic compounds (VOCs) studied were 0.3-10 ppm by volume concentration range and limit dynamic ranges extends beyond three orders of magnitude. The response time for the various VOCs measured using a silicone membrane of 125 μm thickness were in the range of 25-45 s, which provides the sample analysis time of less than one minute. The results indicate that the MI-TOFMS will be capable of use in a wide range of field applications, particularly for environmental monitoring.
Track-membrane mediated introduction of solvated cluster ions
into the TOF mass spectrometer
Glycerol-wetted track membranes (polyethylene terephthalate) were used to interface a low-vacuum facility $(~10^{-3} Torr)$ with an ambient pressure liquid analytes. High field charge extraction conditions were routinely maintained between the liquid samples and a grid collector. The latter was positioned just near to the vacuum-facing side of such membranes. Upon establishing a steady-state charge extraction regime, the collector currents were monitored and recorded at various solute concentration levels. The collector currents, which depend on solute concentration, were found to agree with recent theoretical treatments of such processes. Both positively- and negatively-charged species from organic solutions were routinely extracted. Ion injection for the low- and the high-mobility species has favored the diffusion-limited and the evaporation-limited schemes, respectively. Variable concentrations of 1-phyrenoylmethylpyridinium bromide as well as naphthylacetic and anthracenecarboxylic acids in glycerol were used.
