Study On The Absorption Spectra And Dissociation Dynamics Of Small Gas Molecules In Vacuum Ultraviolet Wavelength Range

This dissertation mainly presents the experimental studies on the absorption spectrum of nitrous oxides and acetylene and the LIF spectrum of CO dimer in vacuum ultraviolet (VUV) range by using two-photon resonant four-wave difference frequency mixing (2PR-4WDM) technique in rare gases (Xe/Kr)。Through analysis of the spectrum, we understand the dissociation process of N₂O and C₂H₂ deeply and discuss the construction of CO dimer in electronic excited state.1. The absorption spectrum of N₂O molecule in the wavelength range of 143.6 nm 146.9 nm has been measured under the jet-cooled condition by using the high-resolution vacuum ultraviolet radiation generated by resonance-enhanced difference-frequency mixing, which corresponds to the C¹Î â†X¹Î£⁺ transition. Three vibrational bands have been clearly observed with a frequency interval of 500 cm^-1 and superimposed on a background of a wide absorption band. Quantum chemical calculations indicate that the C¹Î state of N₂O is typically dissociative, no barrier exists on the potential energy surface along N-O elongation direction, while the state is bounding along the N-N bond stretching or bending motion. Therefore, the observed vibrational progression is assigned as a Feschbach resonance of dissociation barrier. From the Fourier transformation analysis of the absorption spectra of the C¹Î state of N₂O, a recurrence period of the unstable periodic orbit near the transition state has been derived to be 71 fs. The corresponding vibrational frequency of this motion is 546.4 cm^-1, which is consistent with the observed frequency interval and close to frequency of bending of electronic states. Thus this unstable periodic orbit is mostly contributed by the bending motion of the C¹Î state of N₂O coupled with dissociation, and the N-N vibrational stretch does not participate in its formation. Therefore, the behavior of dissociating N₂O on potential energy surface of the C¹Î  state is predicted. The quantum wave packets initiated in the Franck-Condon region randomly dissipate, most part of them go along the N-O elongation direction to dissociate, while a few wave packets oscillate along the unstable periodic orbit of bending motion and go back to their initial position prior to fast dissociate. Thus, coupling between the oscillation along the unstable periodic orbit and fast dissociation contributes the vibrational peaks above a background of a wide absorption band in the observed absorption spectrum.2. The absorption spectra of acetylene molecules have been measured under jet-cooled conditions in the wavelength range of 142.8nm and 152.3nm, with a tunable and highly resolved vacuum ultraviolet (VUV) laser generated by two-photon resonant four wave difference frequency mixing processes (FWDM). According to sufficient vibrational and rotational cooling effect of the molecule and the higher resolved VUV laser, the observed absorption spectra in our work exhibit more simple spectral features than the previous works. The major three vibrational bands have been assigned as a C-C symmetry stretching vibrational progress (v₂=0²) of the (C|⁾¹Î _u state of acetylene. The shoulder peak at 148.3 nm is assigned as the first overtone band of the asymmetric bending modeν₄ of the (C|⁾¹Î _u state of acetylene. Additionally, the two components, 4o₂(μ₁Π_u) and 4o²(κ¹Î _u), are observed in present absorption spectra, due to Renner-Teller effect and transition selection rule. All bands origins and bandwidths are obtained subsequently. As a result, bandwidths are broadened and lifetimes decrease gradually with the excitation energy increasing. Then, the dissociation dynamics of acetylene in the wavelength of 142.8 and 152.3 nm is discussed as well.3. The laser induced fluorescence spectrum of CO molecular has been measured under the jet-cooled condition by using the high-resolution vacuum ultraviolet radiation generated by resonance-enhanced difference-frequency mixing, which corresponds to the A1Π←X¹Î£⁺ transition. New vibrational bands have been found at the red side of each CO molecular vibrational band with the vibrational quantum number n=2, 3, 4, 5. The new bands may belong to the CO cluster: (CO)_n . Through the analysis of the peaks area integration ratio between the CO cluster and the CO monomer, the n value of (CO)_n shows about 2, and the CO cluster is CO dimer of a great possibility. Taking the recent research conclusion of the CO dimer electronic ground state by other group, we get the rotational constant A, B, C of the CO dimer ground state through Gauss static calculation, and use them as the parameters known to fit the experimental spectrum. The rotational temperature can be decided by fitting the fluorescence spectrum of CO monomer. The new bands belong to vibrational quantum number of 2, 4, 5 have been fitted and the rotational constants of the CO dimer excited state have been obtained. The A value has a bigger change compared with the B and C. We know the configuration of the CO dimer excited state primarily through analysis the rotational constants obtained.