P. Y. Cheng

Reflectron time‐of‐flight mass spectrometer for laser photodissociation

We describe a new reflectron time‐of‐flight mass spectrometer configuration for laser photodissociation of mass‐selected ions and the initial performance characteristics observed for this instrument. Ions are produced by laser photoionization within the acceleration region of the instrument or by laser vaporization in an external pulsed‐nozzle cluster ion source. Mass selection is accomplished with pulsed deflection plates at the end of an initial drift section. Laser photodissociation of selected ions takes place at the turning point in the ion trajectory in the reflectron. The transit time through a second drift section defines the fragment ion masses. Optimized operating conditions and the role of mass discrimination in this instrument are discussed.

Vibronic spectroscopy and dynamics in the jet-cooled silver trimer

Abstract Silver atom clusters are produced in a supersonic molecular beam with pulsed-nozzle laser vaporization. Mass-resolved two-photon photoionization produces an electronic spectrum for the trimer (Ag 3 ) with an origin at 27010±10 cm −1 (3.35 eV). Vibronic structure extending over 1500 cm −1 involves cleanly resolved bands at low energy merging to a quasi-continuum at higher energy. Vibronic analyses are considered for a triangular structure with or without Jahn-Teller distortion in the excited state. No simple analytic expression successfully fits the level structure. However, the spectrum is qualitatively consistent with dynamic Jahn-Teller interaction for a hindered pseudorotor with an excited state barrier of about 200 cm − . Vibrational frequencies are 161.1 cm −1 (stretch) and 96.3 cm −1 (bend).

Laser photoionization and spectroscopy of gas phase silver clusters

Silver clusters containing up to 40–50 atoms are produced by laser vaporization in a pulsed-nozzle molecular beam source and studied with laser photoionization mass spectroscopy. A variety of Nd:YAG pumped dye laser and UV excimer laser wavelengths are used to achieve ionization. Ionization dynamics are studied by varying the laser wavelength and fluence. Bracketing experiments under single-photon ionization conditions are used to estimate ionization potentials as a function of cluster size. An even-odd ionization potential alternation is observed with odd-numbered clusters (N=3, 5, 7 ...) having lower ionization potentials than adjacent even-numbered species. Shell closings at clusters containing 2, 8 20 and 40 electrons are observed consistent with a one-electron shell model picture of cluster electronic structure. Resonance-enhanced ionization produces a vibrationally resolved spectrum for the trimer, Ag3, yielding an electronic state assignment and excited state vibrational frequencies. Fragmentation in dimer ionization via theE state at 249 nm establishes the dissociation energy of Ag2+ to be <2.1 eV.

Electronic spectroscopy of silver dimer rare gas complexes

Vibrationally resolved electronic spectra are reported for the metal dimer‐rare gas complexes Ag2–Ar and Ag2–Kr. These spectra are obtained using resonant two‐photon photoionization in the energy region near the Ag2 B←X electronic transition (280–285 nm). Both complexes exhibit extensive activity in three vibrational modes, making it possible to determine vibrational constants, anharmonicities, and cross‐mode couplings. An unusual cancellation of factors results in the Kr complex (ω’e =72.6 cm−1) having nearly the same metal‐rare gas stretching frequency as the Ar complex (ωe=73.9 cm−1). Progressions extending over a significant range of the excited state potential surfaces make it possible to derive the excited state dissociation energies (D’0=755 and 1205 cm−1 for Ar and Kr, respectively). Combination with the red‐shifted electronic state origins yields the corresponding ground state dissociation energies (D■0=275 and 394 cm−1 for Ar and Kr, respectively). Potential energy surfaces are investigated for ...

On the assignment of Jahn–Teller effects in the ultraviolet absorption spectrum of Ag3

A Jahn–Teller linear‐plus‐quadratic Hamiltonian is shown to account for most of the observed band positions and intensities in the absorption and emission spectra of Ag3. Coupling parameters obtained for a simultaneous fit to absorption and emission results are k=1.93, g=0.25 for the E’ ground state and k=0.19, g=0.02 for the E‘ excited state. At higher vibrational energies, simple Jahn–Teller calculations predict fewer bands than observed. Calculations including spin–orbit coupling with larger linear coupling partially quenched by the Ham effect offer a reasonable explanation for this higher energy structure. Splittings and intensity sharing at lower vibrational energies, however, demand higher‐order Jahn–Teller coupling, indicating the need for more extensive calculation simultaneously incorporating quadratic Jahn–Teller effects, spin–orbit coupling, and perhaps anharmonicity.

Vibronic spectroscopy of Ag2-Ar

Abstract The weakly bound complex Ag 2 -Ar is produced using laser vaporization of solid silver in a pulsed supersonic molecular beam. Resonant two-photon photoionization yields a vibrationally-resolved electronic spectrum with an origin at 35329 cm −1 . This origin is red-shifted 480 cm −1 from that of the uncomplexed silver dimer. Progressions are observed for the solvated Ag 2 stretch (156 cm −1 ), the van der Waals stretch (74 cm −1 ) and the bending mode (28 cm −1 ). Excited state (755 cm −1 ) and ground state (275 cm −1 ) binding energies are obtained.

Resonance-enhanced photodissociation spectroscopy of mass-selected metal cluster cations

Abstract Vibrationally resolved electronic spectra are reported for the diatomic cations Te + 2 and Bi + 2 using the technique of resonance-enhanced photodissociation spectroscopy(REPD). These ions are generated by laser vaporization in a pulsed nozzle source, jet-cooled, mass-selected with a reflectron time-of-flight spectrometer, and dissociated on resonance with a tunable dye laser. Electronic excitation spectra are recorded by monitoring the appearance of the fragment atomic cation as the dissociation laser is tuned through vibronic resonances. Analysis of these new spectra yields vibrational constants for the ground and excited state ions. Potential applications of this dissociation spectroscopy method for polyatomic clusters are discussed.

Resonant two‐photon ionization electronic spectroscopy of the silver trimer

Silver metal clusters are produced by excimer lasers vaporization (308 nm) in a pulsed supersonic nozzle cluster source. The triatomic species is selectively ionized in a resonant two‐photon absorption process via an excited electronic state with an origin at 26971±10 cm−1 (3.35 eV). Vibronic structure extending over 1500 cm−1 involves cleanly resolved bands at low energy merging to a quasi‐continuum at higher energy. Vibronic analysis are considered for a triangular structure with or without Jahn–Teller interaction in the excited state. No simple analytic expression successfully fits the level structure. However, the spectrum is qualitatively consistent with a 2E‘ excited state with intermediate Jahn–Teller interaction.