Kuo-mei Chen

Reactivities of spin-orbit states in Al(2P1 2,3 2) + O2(X3Σg−) → AlO(X 2Σ+) + O(3P0,1,2). A fluorescence imaging study

A crossed beam chemical reaction, Al2P1 2,3 2) + O2(rmX 3Σg−) → AlO(X 2Σ) + O2P0,1,2) was investigated by fluorescence imaging techniques. Multiplex detection of speeds and quantum states of both reactants and products has been implemented. The two spin-orbit states of Al atoms display distinctive reactivities towards O2 molecules. An electrostatic interaction model was employed to elucidate thier dynamic behavior. A barrierless, attractive potential energy surface in the entrance valley of the allowed channel can be inferred from experimental results and model calculations.

Direct determination of number density—angle—velocity distributions in the center-of-mass frame of scattering processes by gated two-dimensional imaging techniques

Abstract Number density—angle—velocity distributions of scattered products from photofragmentation, inelastic and reactive collisions in the center-of-mass frame can be directly measured by utilizing laser-induced-fluorescing illumination and gated two-dimensional imaging techniques. The physical principle behind this direct method and its experimental realization are elucidated. An illustrative example from the photofragmentation process is analyzed accordingly.

Imaging photofragments in velocity space by laser sheet illumination techniques. Photodissociation of ICN

Abstract Combining laser sheet illumination and two-dimensional imaging techniques, photogragmentation of ICN at 248 nm was studied in detail. The three-dimensional distribution of number density, angle and velocity of state-selected CN photofragments was successfully determined by slicing an expanding, spherical shell from the photodissociation center sequentially. Studies of photodissociation processes and elementary chemical reactions by these experimental techniques are expected to provide new insights into their dynamic properties, as evidenced in the present experiment on photofragmentation of ICN.

HELICITY OF ORIENTATION PARAMETERS OF PHOTOFRAGMENTS IN FLUORESCENCE-IMAGING EXPERIMENTS

A detection scheme in fluorescence-imaging experiments is presented for determining helicity of orientation parameters of photofragments prepared by circularly polarized photolysis lasers. In a framework of density matrix theory, explicit fluorescence intensity formulas are derived for detection of right- and left-circularly polarized fluorescence photons in various transition sequences. A pattern recognition from fluorescence images of photofragments in a state of definite helicity with respect to their recoil directions has been established, where differences between left- and right-circularly polarized fluorescence image patterns should be taken. Information on the coherence among various magnetic sublevels of an angular momentum state can be obtained by monitoring fluorescence images in a ΔJ=−1 transition (P branch in absorption).

Time-resolved intensity patterns of photofragments by laser sheet illumination and two-dimensional imaging techniques

Abstract Time-resolved intensity patterns of state-selected photofragments in velocity space can be probed by the laser sheet illumination and two-dimensional imaging techniques. A physical model is developed to correlate the photodissociation dynamics and image patterns from this direct method. Slicing an expanding, spherical shell of state-selected photofragments by the pulsed laser sheet at a displaced position and various delay times relative to the photolysis event, we simulate numerically the resultant image frames in velocity space. To accommodate realistic experimental conditions, intensity patterns which take into account contributions from a Gaussian sphere and an assembly of photolysis centers on an arc are presented.

Vibrational analyses of the 2B2-2A1 fluorescence spectra of NO2 under dye laser excitations

Abstract The parent states of the irregular 2 B 2 vibronic origins of NO 2 in the visible spectral region can be identified through the analysis of their dispersed fluorescence spectra. For a practical deconvolution scheme, vibrational levels of the 2 B 2 excited states are assigned by matching experimentally inferred band intensities of NO 2 LIF spectra against theoretically calculated two-dimensional Franck-Condon factors. Preliminary results on the vibrational analysis of the 2 B 2 excited states of NO 2 under cw dye laser excitations are reported.

Characteristics of excimer laser-generated pulsed Na beams

Abstract Characteristics of excimer laser-generated pulsed Na beams are studied by time-resolved emission spectroscopy and LIF techniques. Fast speed Rydberg state Na atoms, slow speed Na * 2 and Na atoms are identified experimentally. Their formation mechanisms are attributed to the inverse bremsstrahlung and the photothermal processes. Rydberg state metal atoms with kinetic energies in the range of a few to more than 100 eV can be generated by this laser vaporization technique.

Rotational line strengths of the v2-active two-photon transitions of the methyl radical

To extract information on the rotational population distributions of the methyl radical from photodissociation by the 2+1 resonance-enhanced multiphoton ionization technique, its rotational line strength formulas of the two-photon transitions have been reexamined. Symmetry-adapted rovibronic-nuclear spin wave functions of CH3 and CD3 in the |X 2A2″〉 and |np 2A2″〉 electronic states were utilized in the derivation. Transformation properties of the rovibronic and nuclear spin basis functions under the permutation-inversion group D3h(M) have been employed to construct the total wave functions which follow the appropriate statistics of CH3 and CD3, respectively. Explicit expressions of the two-photon rotational line strengths of the v2-active vibronic bands of the methyl radical were reported.

Laser induced fluorescence Doppler profiles of photofragments in the presence of v–J correlation: A density matrix formalism

1+1 LIF Doppler profile functions of photofragments in the presence of v–J correlation are derived in a formalism of density matrix. Radon transforms which are imposed by the resonance condition of Doppler spectroscopy have been implemented exactly. Independent parameters which characterize the angular momentum polarizations of photofragments in a specific state on an expanding Newton sphere have been examined carefully to compare the present treatment with the bipolar moment formalism of Dixon [R. N. Dixon, J. Chem. Phys. 85, 1866 (1986)]. To extract information on speed distributions of photofragments from a combined, isotropic Doppler profile, the proper procedure to take the linear combination of experimental profiles from various detection geometries and rotational branches of transitions has been presented. For future analyses of Doppler spectroscopic measurements of photofragmentation processes, it is recommended that the Doppler profile function in the present framework should be utilized.

Characterization of luminous beams from laser ablation by imaging techniques

Abstract Atomic and molecular species in excited electronic states from laser ablation processes are best studied by two-dimensional imaging techniques. For a prototypical, cylindrically symmetric image pattern, an inverse Abel transform is executed to reconstruct the three-dimensional number density distribution of luminous beams. Quantitative information on the average velocity and the spatial profile of laser-ablated species can be extracted from their spectrally and time-resolved image frames.