Jia-lin Chang

Rydberg states of propyne at 6.8–10.5 eV studied by two-photon resonant ionization spectroscopy and theoretical calculation

The vibronic spectra of jet-cooled propyne at 6.8–10.5 eV have been observed using 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The ns (n=4–13), np (n=3–4), and 3dz2 Rydberg states of propyne have been identified, of which seven are newly discovered. The symmetries of the excited vibronic states have been determined directly from polarization-ratio experiments applying linearly and circularly polarized lasers. Under a C3V group, the observed s Rydberg series are of E symmetry and the p Rydberg states belong to A1 or E. Clear doublet splittings in the ns Rydberg states (n=4–9) are observed for the first time. The splittings, 306 cm−1 at 4 s, decrease with increasing n. The doublets of A′ and A″ symmetries, identified from the polarization-ratio measurement, are that due to CS molecular geometry, rather than C3V, for the ns Rydberg states. The term values for the ns Rydberg series (n=6–13) converge to an adiabatic ionization energy of 83 625±2 cm−1 with a quantum defect of δ=0.95. Com...

Ab initio calculations of low-lying electronic states of vinyl chloride

The equilibrium geometries, vibrational frequencies, excitation energies, and oscillator strengths of vinyl chloride in the ground and five lowest-lying excited singlet states have been calculated using MP2, CIS, CASSCF, and MRCI methods with the 6-311++G** basis set. The geometries and vibrational frequencies of the ground and excited states are utilized to compute Franck–Condon factors. Calculated vibronic spectra for the transitions from the ground state to these five excited states are in agreement with experiment at 52 500–60 000 cm−1, with major contributions from the A(1 1A″)←X(1 1A′) and C(2 1A′)←X(1 1A′) transitions. In this study, two spin-forbidden transitions of b(1 3A″)←X(1 1A′) and c(2 3A″)←X(1 1A′) are calculated to locate in 45 000–54 000 cm−1, and could be responsible for the observed one-photon absorption spectrum due to an intensity borrowing caused by the spin–orbit coupling of the Cl atom. Based on calculation, we speculate that upon the excitation of vinyl chloride at 193 nm t...

Rydberg states of the allyl radical observed by two-photon resonant ionization spectroscopy

The vibronic spectrum of the allyl radical (CH2CHCH2) at 6–8 eV has been observed using 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The allyl radicals were produced in the nozzle of a supersonic jet expansion by the pyrolysis of allyl iodide. Five new Rydberg states, a 3d and four ns states (n=4,6–8), are observed for the first time. Prominent vibrational progressions with the gross spacing of ∼430 cm−1 are observed in the s Rydberg series. The totally symmetric ∠CCC-bending vibration in the Rydberg states is responsible for the observed vibrational progressions. Facilitated with the ab initio calculation of Franck–Condon factors, other observed vibrational states in the s Rydberg series have been identified. The term values for the ns Rydberg series converge to an adiabatic ionization energy of 65 638±18 cm−1 with a quantum defect of 0.966±0.005.

High-lying Rydberg states and ionization energy of vinyl chloride studied by two-photon resonant ionization spectroscopy

Abstract High-lying Rydberg states of jet-cooled vinyl chloride at 9.3–10 eV are investigated using 2+1 resonance-enhanced multiphoton ionization spectroscopy. Four Rydberg series are observed and tentatively assigned as due to the transitions of π→ n s, n pσ, n pπ, and n f Rydberg orbitals associated with quantum defects of 0.94, 0.89, 0.48, and 0.02, respectively. All of the four series converge to the same ionization energy limit corresponding to the ground state of vinyl chloride cation. The adiabatic ionization energy of vinyl chloride is determined to be 80 720±6 cm −1 , in excellent agreement with previously reported values.

Two-photon vibronic spectra of vinyl chloride at 7.3–10 eV

The vibronic spectra of vinyl chloride at 7.3–10 eV have been studied using 2+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The geometries and vibrational frequencies of vinyl chloride and its cation calculated with ab initio methods are utilized to compute the Franck–Condon factors. The transitions to the Rydberg states of π→ns, npσ, npπ, nd, nf, and nCl→3s are responsible for the observed REMPI spectra of vinyl chloride. The complex vibrational structures for the observed transitions are successfully identified with the aid of calculated Franck–Condon factors.

Studies on the multiphoton ionization spectrum of the jet-cooled acetyl radical at 6.2–7.6 eV

Abstract Acetyl radical was produced in a molecular beam by pyrolysis of acetone with a flash pyrolysis nozzle and was cooled by supersonic expansion. A continuous multiphoton ionization (MPI) spectrum of the jet-cooled acetyl radical at 6.2–7.6 eV was first observed in the experiment. The spectrum from 6.2 eV to ionization threshold has resulted from 2+1 resonance-enhanced MPI (REMPI) through its high-lying electronic states. These states are shown to be dissociative and metastable. The adiabatic ionization potential of the acetyl radical was redetermined to be 7.01±0.02 eV from the derivative of MPI spectrum near ionization threshold directly. This work demonstrates that MPI technique can be easily used to monitor the produced acetyl radical in molecular beams.

Two-photon vibronic spectroscopy of allene at 7.0–10.5 eV: experiment and theory

2 + 1 resonance-enhanced multiphoton ionization (REMPI) spectra of allene at 7.0–10.5 eV have been observed. The excited vibronic symmetry has been determined from polarization-ratio measurements. Based on the vibronic energies and peak intensities calculated using ab initio MO and time-dependent density functional theory, the very congested REMPI spectra have been assigned as due to π* ← π, 3p ← π, 4s ← π, 4p ← π, and 4d ← π transitions. Vibrational progressions related to the CH2 twisting (ν4 ∼770 cm−1) have been observed for several excited electronic states. Calculated Franck–Condon factors also confirm that CH2 twisting is the most active mode in the vibronic spectra of allene. In this study, theoretical calculations of two-photon intensities and polarization ratios have been made through the ab initio computed one-photon transition dipole moments to various electronic states as intermediates. As a starting point to interpret the complicated vibronic spectrum of allene, the theoretical approach, without vibronic couplings, has been applied to predict the peak positions, spectral intensities, and polarization ratios of Rydberg states, and qualitatively shows a considerable agreement with experimental observations.

Determination of velocity distribution, angular distribution, and vector correlation of photofragments using sub-Doppler fluorescence-imaging method

With sub-Doppler resolution, the fluorescence-imaging techniques can be modified to determine velocity distribution, angular distribution, and vector correlation of state-selected photofragments, even in an uncollimated molecular beam. This new method is proposed as “sub-Doppler fluorescence-imaging” in which two experimental schemes are developed. The dependence of fluorescence intensities, at any selected velocity and recoil angle in the scattering plane, with respect to the variation of polarization vectors of the probe laser and emitted fluorescence is derived using density matrix formalism. The intensity patterns of photofragments with v–J and μ–v–J correlations are simulated. The laser ablation of B atoms at 248 nm demonstrates the feasibility of this method. Two-dimensional velocity distribution of the laser-ablated B(2P1/2,3/20) atoms is measured and the ablation mechanism is discussed.

A New Sub-Doppler Fluorescence Imaging Method in Studying Laser Ablation of B Atoms at 248 nm

A new sub-Doppler fluorescence imaging method has been applied to study the laser ablation of B atoms at 248 nm with a power density of ∼1.7 × 108 W/cm2. Two-dimensional velocity distributions of the laserablated B( P°1/2,3/2) atoms are measured. The angular distributions of the ablated B atoms are velocitydependent; the higher the speed of the B atom is, the more centralized the distribution of the forward peaking will be, indicating that the ablation plume undergoes an unsteady adiabatic expansion. The speed distributions of the B atoms are well fitted to the shifted Maxwellian functions and are found to be bimodal, including a fast component with a temperature of 1.8 × 104 K topping out at 5.8 eV and a slow component of 3.1 × 103 K reaching a maximum at 2.8 eV. While a plasma reaction is responsible for the fast component, the slow one results from photochemical processes.