Xianfeng Zheng

Investigation of laser-induced breakdown spectroscopy of a liquid jet

We investigate the feasibility of laser-induced breakdown spectroscopy for determination of heavy metal Pb in a Pb(NO3)2 aqueous solution by using a simple homemade vertical jet device and nanosecond laser pulses. Key experimental parameters that affect the analytical performance, such as delay of the time of observation, laser pulse energy, and liquid flow rate are optimized for the best limit of detection (LOD). The LOD was determined using Pb I emission at 405.781 nm, and after optimization, the 3σ LOD was found to be at the level of 60 ppm.

Measurement of photoionization cross sections of the excited states of titanium, cobalt, and nickel

Resonance-enhanced multiphoton ionization (REMPI) of Ti, Co, and Ni atoms has been investigated in the 285–320 nm region. We couple a laser-ablated metal target into a molecular beam to produce atoms. Ions produced from photoionization of the neutral atoms are monitored by a home-built time-of-flight mass spectrometer. Photoionization cross sections of the excited states of Ti, Co, and Ni were deduced from the dependence of the ion signal intensity on the laser intensity for photon energies close to the ionization threshold. The values obtained range from 0.2 to 6.0 Mb. No significant isotope dependence was found from measurements of the photoionization cross sections of T46i, T47i, T48i, N58i, and N60i.

State-dependent photoionization cross-sections of 3d transition metal atoms

Using the saturation method, we measured the absolute photoionization cross-sections of several excited states of titanium, vanadium, chromium, iron, and cobalt. These results are reported for the first time in this paper. The measured values range from 0.4 ± 0.1 Mb to 6.9 ± 2.0 Mb. The results show that the photoionization cross-section depends on the atomic state and not just on the electronic configuration.

Ultrafast Carrier Dynamics in CdSe/CdS/ZnS Quantum Dots

The intra- and inter-band relaxation dynamics of CdSe/CdS/ZnS core/shell/shell quantum dots are investigated with the aid of time-resolved nonlinear transmission spectra which are obtained using femtosecond pump-probe technique. By selectively exciting the core and shell carrier, the dynamics are studied in detail. Carrier relaxation is found faster in the conduction band of the CdS shell (about 130 fs) than that in the conduction band of the CdSe core (about 400 fs). From the experiments it is distinctly demonstrated the existence of the defect states in the interface between the CdSe core and the CdS shell, indicating that ultrafast spectroscopy might be a suitable tool in studying interface and surface morphology properties in nanosystems.

Measurement of Photoionization Cross Sections of the Excited States of Titanium

Resonance-enhanced multiphoton ionization of the titanium atoms has been investigated in the 293321 nm wavelength. We couple a laser-ablated metal target into a molecular beam to produce free atoms. Ions produced from photoionization of the neutral atoms are monitored by a home-built time-of-flight mass spectrometer. Photoionization cross sections of the excited states of Ti I were deduced from the dependence of the ion signal intensity on the laser intensity for photon energies close to the ionization threshold. The values obtained range from 0.2 Mb to 6.0 Mb. No significant isotope-dependence was found from measurements of the photoionization cross sections of 46Ti, 47Ti, and 48Ti.

Observation of a New 2 Excited Electronic State of SF by Resonance-Enhanced Multiphoton Ionization Spectroscopy

The (2+1) resonance-enhanced multiphoton ionization (REMPI) spectrum of SF has been obtained in the single-photon wavelength region of 307-321 nm. Five vibronic bands were observed and assigned to the two-photon transitions from the ground state to a 2Σ Rydberg state. The term value Te, vibrational frequency, and the rotational constant of the 2Σ Rydberg state were determined. Another 2P state was observed near 312 nm.

Photoionization cross section measurements of the excited states of cobalt in the near-threshold region

We present measurements of photoionization cross-sections of the excited states of cobalt using a two-color, two-step resonance ionization technique in conjunction with a molecular beam time of flight (TOF) mass spectrometer. The atoms were produced by the laser vaporization of a cobalt rod, coupled with a supersonic gas jet. The absolute photoionization cross-sections at threshold and near-threshold regions (0-1.2 eV) were measured, and the measured values ranged from 4.2±0.7 Mb to 10.5±1.8 Mb. The lifetimes of four odd parity energy levels are reported for the first time.