Weekyung Kang

State‐selective photofragment imaging of iodine atoms via photodissociation of CF3I at 277 nm

The photodissociation of CF3I cooled in a supersonic molecular beam has been investigated at 277 nm by state‐selective photofragment imaging. Fragmented iodine atoms of two spin–orbit states are state‐selectively ionized and projected onto a two‐dimensional position‐sensitive detector, to obtain their speed and angular distribution. The anisotropy parameter for an excited iodine atom I*(2P1/2), β(I*), is found to be 1.83 and is consistent with a dissociation lifetime in the order of 150–350 fs from rotational correlation function. Contrary to earlier reports, the parallel‐like distribution for the ground state iodine atom I(2P3/2) at this wavelength, shows a more favorable curve‐crossing dissociation path (68%) from 3Q0 to 1Q1 and a less favorable direct dissociation path (32%) from 3Q1. The recoil energy distribution of I is found to be broader than that of I* and is correlated with a variety of energy disposal channels by an e symmetry vibration at the crossing point. The results are compared with previ...

Photodissociation of alkyl iodides and CF3I at 304 nm: Relative populations of I(2P1/2) and I(2P3/2) and dynamics of curve crossing

Photodissociation of n‐alkyl iodides and CF3I has been studied using state‐selective ionization and pulsed‐field time‐of‐flight (TOF) mass spectrometry. The (2+1) resonance‐enhanced multiphoton ionization cross‐section ratio of I(2P1/2) and I(2P3/2) is measured from I2 photodissociation in the 304 nm region. Using this ratio, the relative populations of I(2P1/2) and I(2P3/2) by a pure parallel excitation to the 3Q0 state of n‐alkyl iodides and CF3I are obtained. This method can exclude the effects of clusters and spectrum overlap in different dissociation paths from branching ratio calculations. The product quantum yield of I(2P3/2) obtained from branching ratio reveals the 3Q0–1Q1 curve crossing probability—which increases from 0.70 for CH3I to 0.86 for n‐C4H9I (with recoil velocity decrease of fragments at the curve crossing point). The potential energies at the crossing point and the Landau–Zener parameters ζ, calculated from our modified model, are 385 kJ/mol and 1100 m/s for n‐alkyl iodides, and 391 ...

Photoelectron imaging spectroscopy for (2+1) resonance-enhanced multiphoton ionization of atomic iodine produced from A-band photolysis of CH3I

Photoelectron imaging spectroscopy has been applied to study on two-photon resonant third photon ionization of two states of atomic iodine (5p5 2P3/2 and 2P1/2) in the wavelength region 277–313 nm. The technique has yielded simultaneously both relative branching ratios to the four levels of I+(3P2, 3P0,1, 1D2 and 1S0) with 5p4 configuration and the angular distributions of outgoing photoelectrons. The product branching ratios reveal a strong propensity to populate particular levels in many cases. Several pathways have been documented for selective formation of I+(3P2), I+ (3P0,1) and I+(1D2) ions. In general, the final ion level distributions are dominated by the preservation of the ion core configuration of resonant excited state defined by the Jcl coupling scheme. Some deviations from this simple picture are discussed in terms of the configuration interaction of resonant states and the autoionization in the continuum. The photoelectron angular distributions are qualitatively similar, with a positive A2 ...

Molecular lens applied to benzene and carbon disulfide molecular beams

A molecular lens of the nonresonant dipole force formed by focusing a nanosecond IR laser pulse has been applied to benzene and CS2 molecular beams. Using the velocity map imaging technique for molecular ray tracing, characteristic molecular lens parameters including the focal length (f ), minimum beam width (W), and distance to the minimum beam width position (D) were determined. The laser intensity dependence of the observed lens parameters was in good agreement with theoretical predictions. W was independent of the laser peak intensity (I0), whereas f and D varied linearly with 1/I0. The differences in lens parameters between the molecular species were well correlated with the polarizability per mass values of the molecules. A high chromatographic resolution of Rs=0.84 was achieved between the images of benzene molecular beams undeflected and deflected by the lens. The possibilities for a new type of chromatography are discussed.

Separation of a benzene and nitric oxide mixture by a molecule prism

In molecule optics, a matter wave of molecules is manipulated by a molecule-optical component made out of external, typically radiative, fields. The molecule-optical index of refraction, n, for a nonresonant IR laser pulse focused onto a molecular beam can be obtained from the energy conservation and wave properties of molecules. Experimentally measured values of n for benzene and nitric oxide agreed well with the calculated values. Since n depends on the properties of molecules as well as those of the laser field, a molecule prism composed of the focused nonresonant laser field can separate a multi-component molecular beam into several components according to their molecule-optical refractive indices n. We obtained a chromatographic resolution of 0.62 for the spatial separation of a mixture beam of benzene and nitric oxide using a focused Nd:YAG laser pulse as a molecule prism.

Energy partitioning in photodissociation of methyl, ethyl and n-propyl iodides at 304 nm

Abstract The energy partitioning in photodissociation of n-alkyl iodides has been studied by state-selective photofragment translational spectroscopy combined with a pulsed molecular beam at 304 nm. Translational energies of photofragments are obtained by analyzing the time-of-flights of two product channel iodine atoms, i.e., I ( 2 P 3 2 ) and I ∗ ( 2 P 1 2 ) for CH3I, C2H5I and C3H7I. The average energy disposed into the internal mode (Eint) of alkyl radicals has shown an increasing tendency from 17% for the I∗ channel of CH3I to 53% for the I channel of n-C3H7I and is estimated to further increase to 76% for the I∗ channel of C4H9I. The methyl radical in CH3I photolysis has shown the vibrational excitation in its umbrella mode. The distribution peaks have appeared at ν=2 for I∗ and ν=4 for I product channels, respectively. The observed results are compared with a classical impulsive description dynamics. The energy partitioning is found to be very little dependent on the excitation wavelength for photodissociation all over the A-band region.

Reaction of O(3P) atoms with CF2 CXY (X, Y H, F, Cl, Br). Discharge flow-chemiluminescence imaging technique

Abstract The chemiluminescence imaging technique has been introduced to study kinetics in the discharge flow system. Concentration profile of O( 3 P) atom in the reaction with CF 2  CXY (X, Y  H, F, Cl, Br) has been monitored using NO 2 chemiluminescence imaging from the entire reaction zone by a charge-coupled device camera. The observed rate coefficients of O( 3 P) with CH 2  CH 2  CHF; CF 2  = CHCl; CF 2  CHBr CF 2  CFBr; and CF 2  CBr 2 at 298 ± 2 K are (6.81 ± 0.66), (4.05 ± 0.20), (3.21 ± 0.41), (2.16 ± 0.18), (3.39 ± 0.23) and (1.40 ± 0.27) × 10 −13 cm 3 molecule −1 s −1 , respectively. The present method gives reasonably good results and can therefore be considered as an acceptable new tool for kinetic studies.

Two-dimensional photoelectron imaging of state-selected iodine atom by (2+1) resonance-enhanced multiphoton ionization

Abstract The state-selective photoelectron imaging technique is applied to investigate the angular distribution of ejected electrons and the electronic state of the atomic ion produced by (2+1) resonance-enhanced multiphoton ionization of iodine atom in the photolysis of CH 3 I. Strong anisotropic distribution behaviors have been depicted on two electron images observed, one via the two-photon resonance 2 D 5 2 0 state from ground state iodine and, another via 4 D 5 2 0 from spin-orbit excited iodine. The configuration of core electrons is preserved in the ionization from the 2 D 5 2 0 intermediate state, while the ionization from the 4 D 1 2 0 state shows two distinct ionization paths, a preserved and a non-preserved path of core electrons.

Study on the Change in Photovoltage by Control of Cell Gap in Dye-Sensitized Solar Cells

Photo-electrochemical properties of dye-sensitized solar cells (DSSCs) were investigated by changing the gap between working and counter electrodes. The open-circuit voltage (V OC ) of DSSCs was significantly increased from 616 mV to 776 mV by about 26% with 1-methyl-3-butyl imidazolium iodide (BMII) based electrolyte and from 428 mV to 513 mV by 20% with lithium iodide (LiI) based electrolyte as the cell gap increased from 16 μm to 224 μm. From the electrochemical impedance spectroscopy, it was found that the resistance of the electrolyte was increased as the cell gap widened. This resulted in the reduction in the dark current associated with the V OC enhancement. The transient photovoltage spectroscopic measurement confirmed that the time constant for charge recombination between Ti0 2 and electrolyte became slower as the cell gap of the DSSC with LiI electrolyte increased, which could be an additional reason for the V OC enhancement. The optimal cell gap was determined to be around 31.3 μm for the BMII electrolyte system, and around 75.5 μm for the LiI electrolyte system in terms of the energy-conversion efficiency.