Haichuan Liu

Photodissociation spectroscopy of Mg+–C6H5X (X=H, F, Cl, Br)

Photodissociation of a series of complexes Mg+–C6H5X (X=H, F, Cl, Br) has been studied. The formation of Mg+ was found to be the predominant dissociation pathway. We monitored the photodissociation product as a function of the excitation wavelength in a broad spectral region. Experiments on the complexes with different halide substitutions showed similar action spectra. This is explained by a similar structure shared by these complexes with Mg+ being above the benzene ring. In the photodissociation of Mg+–(C6H5F), MgF+ is also formed besides Mg+. The action spectra corresponding to these two channels were found to be different, and the MgF+ channel is believed to be from a different isomer. The experimental results are consistent with our quantum ab initio calculations.

Photodissociation spectroscopy of the complexes of Mg+ with di- and tri-ethylamine

We present the photodissociation spectroscopy of the complexes of Mg+ with di- and tri-ethylamine in the spectral range of 230–440 nm. Mass spectrometry of the two parent complexes exhibits the persistent product Mg+ from nonreactive quenching throughout the whole wavelength range. As for the reactive channels, C3H7•-elimination products are dominant when the complexes are excited to 3Px,y. Furthermore, photoexcitation of Mg+–NH(C2H5)2 to 3Px,y yields a minor MgH-elimination fragment. On the other hand, photodissociation of Mg+–N(C2H5)3 produces charge-transfer fragments N+(C2H5)3 and (C2H5)2N+CH2. The action spectra of the two complexes consist of two pronounced peaks on the red and blue side of the Mg+ 3 2P←3 2S atomic transition. The calculated absorption spectra of the two complexes using the optimized structures of their ground states agree nicely with the observed action spectra. Photofragment branching fractions of the products are shown to be nearly independent of the photon energy for the 3Px,y e...

Photodissociation studies of microsolvated metal cation complexes Mg+(NCCH3)n (n=1–4)

Acetonitrile-solvated Mg+ complexes Mg+(NCCH3)n were produced in a pick-up source. The Mg+(NCCH3)n clusters were subjected to photodissociation in a time-of-flight reflectron mass spectrometer. Except for a minor reactive product Mg+-NC at short wavelengths, only nonreactive (or evaporation) fragments were observed from photodissociation of Mg+(NCCH3)n (n=1–4). Action spectra of Mg+(NCCH3)n (n=1–4) were recorded in the spectral region of 230–560 nm, which provide insight into the structure and photodissociation dynamics of the complexes. For all the complexes we studied, the ligands are believed to be in the first solvation shell. Both the evaporation and reaction processes occur on the ground state surfaces of the complexes. By using the DFT/B3LYP and CIS methods, the most stable ground-state structures and the excitation spectra for Mg+(NCCH3)1–3 were obtained. The ab initio predictions appear to be consistent with the experimental results.

Comparative studies of the photoinduced reactions in the Mg+–SCNC2H5 and Mg+–NCSC2H5 complexes

The photoinduced reactions of the complexes Mg+-SCNC2H5 and Mg+-NCSC2H5 are studied comparatively in the spectral range of 230-440 nm. One-photon excitation of the complexes through the Mg+ chromophore (3 2P <-- 3 2S) gives rise to the evaporative fragment as well as the molecular activation and charge transfer products. The action spectra of the complexes consist of three broad peaks for Mg+-SCNC2H5 and two for Mg+-NCSC2H5, which accord with the structures obtained from quantum mechanics calculations. These calculations reveal two association isomers for Mg+-SCNC2H5: one is with Mg+ being linked to the S atom and the other to the N atom. The former is more stable than the latter by only 0.23 eV. Both of the isomers have been shown to exist in the complex source employed in our experiments. On the other hand, only one stable structure is found for the complex Mg+-NCSC2H5 characterized by the Mg+-N linkage. In general, the photofragments are dominated by Mg+ at lambda > 400 nm, which decreases with decreasing wavelength accompanied by the increase in other photoproducts. In addition, the branching ratios of Mg+ to other photoproducts are nearly constant in the short wavelength region but decrease with decreasing wavelength. The observed photoreactions have been reasonably explained.

Photo-induced intra-complex reactions in Mg+-2,2,2-trifluoroethanol

We induced the intra-complex reactions in Mg+-2,2,2-trifluoroethanol by photons in the spectral region of 240–410 nm. We observed the nonreactive channel product Mg+ and a number of reactive channel products throughout the whole excitation region. The reactive photoproducts originate from the scission of the C–O or C–F bond, as well as from the simultaneous rupture of both bonds. The action spectrum consists of two broad peaks between the atomic transition of Mg+(3 2P←3 2S). Ab initio calculations show a minimum-energy structure, in which Mg+ attaches to the O atom and one of the three F atoms of 2,2,2-trifluoroethanol, forming a five-membered ring. The calculated absorption spectrum corresponding to the minimum-energy structure agrees nicely with the experimental action spectrum. The branching fractions of the photoproducts are approximately constant in a given spectral range, but they change significantly across the spectral ranges. The constant branching fractions point to the ground state reactions fo...

Photo-induced reactions in mass-selected complexes Mg+(FCH3)n, n=1–4

Photo-induced reactions in the metal cation–molecule complexes Mg+(CH3F)n have been studied as a function of the number of solvent molecules. While a photoreaction of the singly solvated complex Mg+-FCH3 yields exclusively CH3+, excitation of larger complexes Mg+(FCH3)2–4 produces predominantly bare and solvated MgF+. Photo-induced evaporation of the larger complexes was also observed, although with much lower yields. Possible mechanisms are suggested to interpret the abrupt change in the photoreaction patterns with an increasing complex size. The action spectra of all the complexes are discussed based on the transitions 2P←2S centered on the Mg+ ion but perturbed by the presence of the FCH3 molecules. Quantum ab initio calculations were performed to obtain the structures and action spectra of the complexes, which are directly compared with the experimental results.

Photo-induced reactions in the ion-molecule complex Mg+-OCNC2H5

Ion–molecule complexes of magnesium cation with ethyl isocyanate were produced in a laser-ablation supersonic expansion nozzle source. Photo-induced reactions in the 1:1 complexes have been studied in the spectral range of 230–410 nm. Photodissociation mass spectrometry revealed the persistent product Mg+ from nonreactive quenching throughout the entire wavelength range. As for the reactive channels, the photoproducts, Mg+OCN and C2H5+, were produced only in the blue absorption band of the complex with low yields. The action spectrum of Mg+(OCNC2H5) consists of two pronounced peaks on the red and blue sides of the Mg+ 32P←32S atomic transition. The ground state geometry of Mg+–OCNC2H5 was fully optimized at B3LYP/6-31+G** level by using GAUSSIAN 98 package. The calculated absorption spectrum of the complex using the optimized structure of its ground state agrees well with the observed action spectrum. Photofragment branching fractions of the products are almost independent of the photolysis photon energy ...

Photoinduced reactions in the Mg+-NHn(CH3)(3-n) complex ions: Effect of the methyl substitution

Photoinduced reactions in Mg+–NH2CH3 and Mg+–NH(CH3)2 have been studied in the spectral range of 230–440 nm. Although the N–H bond activation channel was found to be prominent in the photodissociation of Mg+–NH3 [Yoshida, Okai, and Fuke, Chem. Phys. Lett. 347, 93 (2001)], it is very unfavorable as the ammonia is replaced by methylamines in the complex. Instead, C–H bond cleavage products are observed from Mg+–NH2CH3(CH2NH2+) and exclusively produced from Mg+–NH(CH3)2((CH3)HN+=CH2) after photoexcitation. For Mg+–NH2CH3, the C–N bond activation product Mg+NH2 and the charge transfer product CH3NH2+ are also abundant. The action spectra of the complexes consist of two pronounced peaks on the red and blue side of the Mg+ 3 2P←3 2S atomic transition. The calculated absorption spectra of the two complexes using the optimized structures of their ground states are in good agreement with the observed action spectra. On the basis of the branching fraction data and the calculated complex structures, the C–H bond act...

Novel cationic selenium-cluster nitride species [SenN]+(n = 1-11) formed by laser ablation of a Se target in the presence of N2.

Nitride cations of selenium clusters [SenN]+ (n = 1-11) were readily produced by laser ablation of a selenium disk that was surrounded by a trace amount of nitrogen seeded in helium and followed by supersonic expansion into a high vacuum. Even at high nitrogen partial pressures, the cluster mononitride cations were found to be essentially the only nitride products in the whole size range we studied. The exception was [Se3N2]+, which is known to be a stable five-membered ring with seven pi electrons. We propose that, in the laser-ablation plasma, the selenium clusters with n > 2 take on a chain conformation, and that the N species links the two ends of the selenium chains, thus forming stable mononitride cations of the cyclic selenium clusters. Their stability is supported by the results of ab initio calculations (at both B3LYP/ 6-31 + G* and MP2/6-31 + G* levels) and of mass-selected cluster-ion photodissociation experiments.