Study On Spectra And Dynamics Of Transient Diatomic Molecules By Laser Spectroscopy With High Sensitivity

Transient molecules include radical molecule, molecular ion, excited molecule and Rederberg molecule, etc. These species are of the characteristics of short lifetime and high chemical reactivity, and they widely present in interstellar space, process of chemical reaction, combustion and explosion; therefore, these species have been attracting large attention from physicists, chemists, astrophysicists and spectroscopists. Spectroscopy is one of the most direct and powerful methods to uncover the mysterious structure of atoms and molecules, which help researchers to know microscopic particles's formation, internal interactions and dynamics.In laboratory, the concentration of generating transient molecules is only one part per million of the stable or even less, so the spectroscopy of high sensitivity has to be needed to detect the transient's spectra. The system of Optical Heterodyne-Velocity/Concentration Modulation Spectroscopy (OH-VMS/CMS) established at the State Key Laboratory of Precision Spectroscopy in ECNU has reached a detectable absorption magnitude of 10-9(1 s integrated). This spectroscopic technique is recognized as one of the best methods for investigation of transient molecules. Based on this technique, the respective spectra of discharging pure helium (He) gas and He with CS2 were studied in the region of 12 000-13 300 cm-1The spectra of the c3∑g+-a3∑u+and b3∏g6-a3∑u+systems for He2 were observed in the discharging pure He gas employing OH-CMS. Fifty-nine lines of He2 were assigned to the (1, 0), (3,1), (4,2) and (2,0) bands of the c3∑g+-a3∑u+system, and more precise molecular constants were determined using the non-linear least-squares fitting program. Because He2 acts as disturbers in investigation of other transient species that are generated by discharging carrier gas (He) with the sample, this work is helpful for further study. The predissociated state of He2b3∏g (n=9) was studied on the basis of the observing broadened spectral lines. The relationship between rotation levels and their linewidth was obtained and the mechanism of predissociation was discussed. According to Femi-Golden Rule, the lines broadening due to predissociation were calculated, which is basically in agreement with the experimental.CS+and CS are produced in the discharge of the mixer of CS2 and helium. The spectrum of the (2,1) band in the A2∏-X2∑+ system of CS+ was studied by OH-VMS and by the aid of OH-CMS. The interference from the neutral CS can be eliminated by comparing the spectra simultaneously recorded by OH-VMS and OH-CMS. The precise molecular parameters were derived through fitting the identified lines to the standard 2∏-2∑+ Hamiltonian matrix by the nonlinear least-squares fitting program. The triplet band (d3△-a3∏) of CS was recorded by OH-CMS. The overtone band (12,0) in the a3∏2-a3∏0 system and A-doubling in the a3∏1 state were first observed, and about 350 lines were recognized. Based on the obtained precise molecular constants, the perturbations of d3△(v=6) were quantitatively analyzed in detail. The deperturbation analysis of d3△(v=6) is quite difficult due to complicated energy structures of lower excited states of CS. Besides direct perturbation, the second order perturbations that break the perturbation selection rules appear in observation. Based on the first-order non-degenerate perturbation theory, the perturbation mechanisms of second order perturbations were reasonably explained.In addition, the peak-seeking servo-controlled technology was developed to bring the external optical cavity into resonance with the scanning tunable laser. When the cavity was locked, the relative power fluctuation of cavity output was lower than 2% and the cavity was capable to follow the laser scanning during an uninterrupted range of 2 GHz. The servo system can automatically adjust (less than 120 MHz) to re-lock the cavity to the laser for next 2 GHz range. This technical preparation will be used in the cavity enhanced spectroscopy.