Construction Of Infrared-Vacuum Ultraviolet Spectroscopic Apparatus And Its Application To The Study Of Neutral Clusters

The basic goal of cluster research is to reveal the structures,dynamics,and reaction mechanism at the molecular level,which is one of the most active and fruitful research frontiers nowadays.Currently the studies of clusters mainly focus on the anionic or cationic forms due to ease of size selection and detection,while neutral clusters have presented major experimental challenges as the absence of Coulomb charge makes it difficult for size selection and detection.The structures and dynamics of various neutral clusters have been investigated theoretically,while their experimental measurements remain elusive.In this thesis,we have built an infrared-vacuum ultraviolet(IR-VUV)spectroscopic apparatus and applied it to study a series of neutral atmospheric chemical clusters(i.e.,amines,ammonia,and water),which revealed the geometric structures,electronic properties,chemical bonding,and dynamics of these clusters.The present research would have important implications for fundamental understanding of aerosol nucleation mechanism.The main research contents and results are as summarized follows:(1)Our IR-VUV spectroscopic apparatus mainly includes cluster source,reflectron time-of-flight mass spectrometer,four-wave mixing device,and optical parametric oscillation/optical parametric amplification infrared laser system.The IR-VUV spectroscopic apparatus has been adapted to the beamline of Ultraviolet Free Electron Laser(VUV-FEL)delieved by Dalian Coherent Light Souce(DCLS).The mass resolution of reflection time-of-flight mass spectrometer is better than 20000(M/AM),and the resolution of infrared spectra is better than 4 cm-1.(2)IR-VUV spectroscopic studies of neutral methylamine clusters(i.e.,monomethylamine,dimethylamine and trimethylamine clusters)have been carried out using four-wave-mixing 118 nm light.The Fermi resonance phenomenon of methyl(CH3)in the range of 2800-3000 cm-1 was found.Theoretical calculations revealed that the Fermi resonance in CH3 groups is originated from the strong coupling between CH stretching fundamental and bending overtone within a CH3 group.The experimental IR-VUV spectra of neutral dimethylamine clusters show that NH stretch modes gradually redshift to 3200-3250 cm-1 with the increase of cluster size and no obvious Fermi resonance phenomenon occurs.(3)Based on the soft ionization of 118 nm laser generated by four-wave mixing,high-resolution infrared spectra of neutral binary mixed cluster of trimethylamine(TMA)and methanol(CH3OH)were measured.IR-VUV spectroscopic studies of TMA-CH3OH mixed cluster were used as a model to study the hydrogen bonding mechanism between amine and solvate clusters.Aided with the theoretical calculations,the blue-shifed OH stretch maybe due to the combination of H-bonded OH stretch and umbrella mode of N(CH3)3.We also found that with the increase of methanol molecules,the infrared spectrum of(TMA)(CH3OH)2 becomes much more complicated,revealing the rich hydrogen bonding features.(4)Based on the soft ionization of 118 nm laser generated by four-wave mixing,high-resolution infrared spectra of neutral ammonia dimer and trimer were measured.In particular,we found that three isomers may coexist in the(NH3)2 cluster.Theoretical calculations with accurate potential energy surface and multi-dimensional anharmonic algorithms indicated that the significant Fermi resonance is stemmed from a strong coupling between the fundamental frequency of N-H stretching vibration and the bending overtone.Well-resolved experimental spectra enable us to compare the performance of ab initio anharmonic algorithms at different levels.(5)Neutral water clusters of H2On(n=2-6)have been studied using near-threshold ionization by the tunable VUV-FEL(50-150 nm).The mass spectra of water clusters exhibit a significant size effect,revealing the high sensitivity and selectivity of VUV-FEL ionization.Infrared spectra of the smallest size water cluster,(H2O)2,has been studied as a model system to elucidate the rich hydrogen bonding features,providing an important benchmark for the investigation of hydrogen bonding mechanism and structural evolution of larger water clusters.