Can-Hua Zhou

Photolysis of o-nitrobenzaldehyde in the gas phase: a new OH* formation channel.

Photolysis of gaseous o-nitrobenzaldehyde (o-NBA) with selected different excitation wavelengths (355-400 nm) is investigated, and the nascent OH radical is detected by the single-photon laser-induced fluorescence (LIF) technique. The relative quantum yield and rotational excitation of OH formation are found to be dependent on the excitation energy. The distributions of rotational, spin-orbit, and Lambda-doublet states are obtained at 355-400 nm by analyzing the experimental data. The OH radicals are found to be vibrationally cold at all photolysis wavelengths. The spin-orbit and Lambda-doublet states have nonstatistical distributions. To understand the dissociative process involved in the OH-generating channel, DFT calculations are performed. Based on both experimental and theoretical results, possible photolysis channels of o-NBA leading to the OH fragment are proposed and discussed.

H2 stimulated Raman scattering in a multi-pass cell

Stimulated Raman Scattering (SRS) is an effective means of laser wavelength conversion. Hydrogen is an excellent Raman medium for its high stimulated Raman gain coefficient and good flowability which can rapidly dissipate the heat generated by SRS process. In this paper we reported the H2 SRS in multiple-pass cell pumped by the fundamental frequency output of a Q-switched Nd: YAG laser. Two concave reflection mirrors (with 1000 mm curvature radius and 50 mm diameter) were used in our experiment, both mirrors with a hole near the edge and were positioned to form co-center cavity, therefore the laser could repeatedly pass and refocus in the Raman cell to achieve a high SRS conversion efficiency and reduce SRS threshold for pump laser. By changing the pass number (1～17) of optical path in the Raman cell and the pump power(0～2.5MW), the Stokes conversion efficiency is optimized. Experimental results indicated that the Raman threshold was 0.178MW and the highest photon conversion efficiency was 50 %.

Detection of OH Radical in the Photodissociation of p-Aminobenzoic Acid at 266 nm

Photodissociation of p-aminobenzoic acid at 266 nm was investigated by probing the nascent OH photoproduct employing the laser-induced fluorescence technique. It was found that the nascent OH radical was vibrationally cold and its rotational state distribution conformed to be a Boltzmann behavior, characterized by a rotational temperature of 1040 110 K. The rotational energy of OH was determined to be 8.78 0.84 kJ/mol. Between the two spin-orbit states of OH, 23/2 and 21/2, the former was found to be preferentially populated. The distribution of the (A) state for the -doublet was dominant. Finally, a probable mechanism for the formation of OH produced from the photodissociation of p-aminobenzoic acid is discussed.

The amplification of stimulated Raman scattering in H2 pumped by a Q-switched Nd:YAG laser

The experimental study of the amplification of stimulated Raman scattering (SRS) in high purity H2 gas was demonstrated employing a Q-switched Nd:YAG laser at 1064 nm as the pump source. A part of the 1064 nm pump light (20% in energy) was focused into the first H2 gas cell to generate the backward first Raman Stokes light (BS1), which is taken as the Raman seed light. The BS1 seed light combined to the residual pump light were focused into the second H2 gas cell to get the amplification of the S1 1900 nm infrared Raman light. In this study, the maximum quantum conversion efficiency of the S1 light was estimated to be 76%. Under the condition of the same pump energy, especially for the low pump energy (lower than 40 mJ), the quantum conversion efficiency of the S1 light with the Raman seed light was significantly increased comparing to the single focus geometry (without the Raman seed light).

Small signal gain measurement of liquid oxygen under different wavelength laser pump

Oxygen molecules existed in pairs under liquid condition, the radiation from vibrational ground state of 1 Δ state to the first vibrational excited state of 3 ∑ state was electronic dipole moment transition allowed, and a photon with wavelength of 1580 nm was emitted. In our experiment, dye laser with wavelength of 581 nm, 634 nm, 764 nm was used to excite liquid oxygen to different excited states, while a tunable OPO was used as the seeder laser, and the small signal gain was measured to be 0.23 cm-1, 0.3 cm-1 and 0.076 cm-1 respectively. The small signal gain (pump by photon of 634 nm) was significantly higher than that of common solid state lasers and chemical lasers. When the fundamental output of a Q-switched Nd:YAG laser was used as the pump source, the corresponding small signal gain was 0.12 cm-1. The profiles of small signal gain form 1579.2 nm to 1580.8 nm were also presented. These results were consistent with theoretical calculation. The high positive gain indicated that the liquid oxygen was a potential medium for high energy laser. A comprehensive parameter optimization was still necessary in order to improve the mall signal gain.