J. S. Pilgrim

Photodissociation spectroscopy of Mg+–H2O and Mg+–D2O

Mg+–H2O ion–molecule complexes are produced in a pulsed supersonic nozzle cluster source. These complexes are mass selected and studied with laser photodissociation spectroscopy in a reflectron time‐of‐flight mass spectrometer system. An electronic transition assigned as 2B2←X 2A1 is observed with an origin at 28 396 cm−1. The spectrum has a prominent progression in the metal‐H2O stretching mode with a frequency (ω’e) of 518.0 cm−1. An extrapolation of this progression fixes the excited state dissociation energy (D’0) at 15 787 cm−1. The corresponding ground state value (D‘0) is 8514 cm−1 (24.3 kcal/mol). The solvated bending mode, and symmetric and asymmetric stretching modes of water are also active in the complex, as are the magnesium bending modes. A second electronic transition assigned as 2B1←X 2A1 is observed with an origin at 30 267 cm−1 and a metal stretch frequency for Mg+–H2O of 488.5 cm−1 (ΔG1/2). Spectra of both excited states are also observed for Mg+–D2O. Partially resolved rotational struc...

Photodissociation spectroscopy of the Mg+-CO2 complex and its isotopic analogs

Mg+–CO2 ion–molecule cluster complexes are produced by laser vaporization in a pulsed nozzle cluster source. The vibronic spectroscopy in these complexes is studied with mass‐selected photodissociation spectroscopy in a reflectron time‐of‐flight mass spectrometer. Two excited electronic states are observed (2) 2Σ+ and 2Π. The 2Π state has a vibrational progression in the metal–CO2 stretching mode (ωe’=381.8 cm−1). The complexes are linear (Mg+–OCO) and are bound by the charge–quadrupole interaction. The dissociation energy (D0‘) is 14.7 kcal/mol. Corresponding spectra are measured for each of the 24, 25, and 26 isotopes of magnesium. These results are compared to theoretical predictions made by Bauschlicher and co‐workers.

Spectroscopy of weakly-bound magnesium ion complexes

Abstract Weakly bound complexes of the form Mg+-L (L = CO2 , H2O, N2, Ar, etc.) are prepared in a pulsed nozzle/laser vapourization cluster source and studied in the molecular beam environment. The ion complexes are jet cooled and mass selected in a specially designed reflectron time-of-flight mass spectrometer for their study. The mass-selected ions are excited with a tunable dye laser, and the products, if any, from photodissociation are mass analysed and detected as a function of the excitation laser wavelength. This photodissociation spectroscopy experiment reveals the decomposition channels of excited complexes and their absorption spectra. Photodissociation channels vary from simple metal ion-ligand bond breaking, to metal-to-ligand charge transfer, to metal insertion/elimination reactions in the excited state. In reactive systems, the spectra are broad and featureless. However, in systems with simple metal—ligand dissociation, vibrational and partial rotational resolution is obtained in the spectra...

Rotationally resolved photodissociation spectroscopy of Mg+–Ar

The metal ion‐complex 24Mg+–Ar has been prepared in a pulsed nozzle/laser vaporization source, mass selected with a reflection time‐of‐flight mass spectrometer and studied with photodissociation spectroscopy at high resolution. The (5,0) band of the A 2Πr←X 2Σ+ transition has been rotationally analyzed and the rotational constants, B″=0.1409(7) cm−1 and B′=0.1836(8) cm−1, and spin–orbit constant, A′=73.94(2) cm−1, have been determined. The bond distances in the ground and excited states of the complex (r0″=2.88 A, r5′=2.52 A) compare well with the values predicted by theory, and they confirm the suspected nature of the electrostatic bonding in this system.

Electronic spectroscopy of the Mg+–N2 complex: Evidence for photoinduced activation of N2

The ion–molecule complex, Mg+–N2 is formed in a supersonic expansion and studied with mass‐selected photodissociation spectroscopy. The lowest energy bands observed in the electronic excitation spectrum are redshifted more than 12 000 cm−1 from the Mg+ (2P←2S) atomic transition at 280 nm. The red‐shift, resulting from differential bonding in the ground and excited states of the complex, is much larger than the shifts observed in previously studied Mg+–ligand complexes. Resolved vibronic structure is observed extending for more than 5000 cm−1. The observation of spin–orbit multiplet structure indicates that the complex is linear and that the electronic transition is 2Π←X 2Σ+. The spin–orbit splitting of 46 cm−1 is significantly less than that observed for other Mg+–L complexes. Vibronic intervals of about 1000 and 500 cm−1 are assigned respectively to a stretching mode and to double quanta in a bending mode. The study of isotopically substituted complexes indicates that the best assignment for the stretch ...

Photoionization electronic spectroscopy of AlOH

Abstract An electronic spectrum is observed for AlOH formed in a laser vaporization pulsed molecular beam source. The spectrum is detected near 250 nm by resonant two-photon ionization spectroscopy. Two electronic states are observed, spaced by only 1674 cm −1 . Vibrational progressions are observed in both states corresponding to the excited state AlOH stretching mode (ω′ e = 825 cm −1 ) and the AlOH bending mode (ω′ e = 654 cm −1 ). The spectrum is consistent with a quasi-linear ground state and a more strongly bent excited state, as predicted by theory.

Photodissociation spectroscopy of Mg+H2O

Mg+H2O ion—molecule complexes are produced in a pulsed supersonic nozzle cluster source. These weakly bound complexes are mass selected and studied with laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer system. An electronic transition assigned as 2B2→X2A1 is observed with an origin at 28399 cm−1 (vac). The spectrum has a prominent progression in the metal—H2O stretching mode with a frequency (ω′e) of 517.1 cm−1. An extrapolation of this progression fixes the excited state dissociation energy (D′0) at 16008 cm−1. The corresponding ground state value (D″0) is 8732 cm−1 (25.0 kcal/mol). The solvated bending mode and asymmetric stretching mode of water are also active in the complex. A second electronic transition assigned as 2B1←X 2A1 is observed with an origin at 30386 cm−1 and a metal stretch frequency of 483.4 cm−1. This study was guided by ab initio calculations by Bauschlicher and co-workers, which provide accurate predictions of the electronic transition energies, vibrational constants and dissociation energies.

Photoionization spectroscopy of ionic metal dimers: LiCu and LiAg

Electronic spectra are reported for the heteronuclear metal dimers LiCu and LiAg, with resonant one-color two-photon ionization (R2PI). The dimers are produced in a pulsed supersonic molecular beam by laser vaporization of either a copper or silver rod coated with a thin film of vacuum deposited lithium metal. A total of twelve excited electronic states for LiCu and seven for LiAg are observed. Analysis of the vibrational progressions yields ground and excited state vibrational frequencies and dissociation energies for both LiCu and LiAg. In addition, selected vibronic bands are rotationally resolved. This data, together with that obtained by Morse and co-workers for LiCu [J. Chem. Phys. (to be published)], gives bond lengths for LiCu and LiAg (r0″=2.26 and 2.41 A, respectively). The bond lengths for LiCu and LiAg are significantly shorter than expected by comparison to the homonuclear diatomics Li2 and Cu2 or Ag2. Dissociation energies in the heteronuclear dimers are also much greater than the mean of th...

A convenient modification to the Newport pulsed molecular‐beam valve

We describe a modification to Newport Corporation’s BV‐100 double solenoid pulsed molecular‐beam valve that replaces the Viton tip seal with an O‐ring‐based plug. This alteration produces the same characteristic short and cold gas pulses as the originally described beam valve without the difficulties associated with replacing the seal.

Photochemical cleavage of metal—carbon nanocrystals and their reconstruction into met—cars clusters

Abstract Titanium and zirconium metal—carbon clusters are produced by laser vaporization in a pulsed nozzle source and detected with time-of-flight mass spectrometry. In addition to the now-familiar “met-cars” stoichiometry (M8C12), larger magic number clusters are produced with near 1:1 metal—carbon ratios. The special stoichiometries observed correspond to face-centered cubic crystal fragments, with a strong preference for fragments with symmetrical x,y,z dimensions. Mass-selected photodissociation experiments are used to investigate the structural patterns and stabilities of these systems. Photodissociation of the larger “nanocrystal” clusters leads to cleavage along crystal planes, producing smaller crystals also having highly symmetric dimensions. Photoexcitation of all these crystallites, in particular the 3 × 3 × 3 species, also leads to surface reconstruction, forming the M8C12 met-cars cluster and/or the M8C13 cluster, the latter of which is assigned to a met—cars cage with an endohedral carbon atom.