Vibronic analysis of the A→X̃ laserinduced fluorescence of jetcooled methoxy (CH3O) radical J. Chem. Phys. 99, 9465 (1993); 10.1063/1.465481 Investigation of the gasphase B-X̃ electronic spectra of CH-Ar by laserinduced fluorescence J. Chem. Phys. 99, 91 (1993); 10.1063/1.465708The lowlying bending vibrational levels of the CCH (X2Σ+) radical studied by laserinduced fluorescenceThe spectroscopy of the B 2 A '-X 2 A ' system of the formyl radical has been studied by laserinduced fluorescence. HCO was generated by photolysis of acetaldehyde, and a tunable laser operated near 245 nm excited eight bands ofB-X. The (0,0,2)-(0,0,0) band has been rotationally analyzed, yielding A ' = 14.46 cm -I and (B' + C')/2 = 1.13 cm -I for this slightly asymmetric top; asymmetry splitting and spin doubling are observed. The intense branches have 1U( = 0 but there also are weaker perpendicular components with the transition moment near the b axis. Vibronic transition energies agree with those from matrix absorption but with a 130 cm -I blueshift. Resolved fluorescence spectra to X levels as high as 15 000 cm -I furnish vibrational constants for the ground state.

We apply the techniques of resonance enhanced multiphoton ionization (REMPI) and time-of-flight photoelectron spectroscopy (TOF-PES) to TiO molecules cooled in a pulsed nozzle expansion to obtain vibronic spectra of gas phase TiO+. The adiabatic first ionization energy is refined to I1(TiO)=54 999±52 cm−1=6.819±0.006 eV, which yields D0(Ti+–0) =159.9±2.2 kcal/mol. For the X 2Δ state of TiO+, we resolve spin–orbit pairs of vibrational levels for v=0–14, yielding ωe=1045±7 cm−1 and ωexe =4±1 cm−1. The spin–orbit splitting ΔEso =210±6 cm−1 permits confirmation of the state symmetry by comparison with the known spin–orbit splittings of the X 3Δ state of TiO. We also observe a new excited B 2∑+ state at T0=11 227±17 cm−1 with ωe =1020±9 cm−1 and ωexe =6±2 cm−1. This state is distinct from the A 2∑+ state (average frequency 860±60 cm−1) previously observed by Dyke and co-workers. From components of certain PESs apparently due to one or more metastable states of TiO, we infer the existence of a previously unobserved state of neutral TiO at T0=2980 cm−1, possibly the 3∑− state. Finally, we discuss the electronic structure and vibrational frequencies of TiO, TiO+, and other third row metal oxides from both molecular orbital and ligand field points of view in order to understand the ordering of electronic states and certain trends in vibrational frequencies. The molecular orbital model readily explains why nominally isoelectronic neutral and cationic metal oxides, such as TiO+ and ScO, are electronically quite dissimilar.

We have combined the techniques of multiphoton ionization (MPI), time-of-flight mass spectrometry, and creation of cold neutral free radicals by photolysis of precursors upstream in a free-jet expansion to study the-MPI optical spectrum of the allyl radical. We resolve 19 vibronic bands in the nominal two-photon resonant, 3s 2AI -X ZA2, four-photon ionization spectrum from 488 to 513 nm. The shapes and widths of the vibronic bands are remarkably sensitive to the ionizing laser flux. The spectrum includes both hot and cold bands. Comparison with previous gas-phase and matrix data and with ab initio calculations of harmonic frequencies permits assignment and identification of three fundamental vibrational levels in both the ground and excited states: the CCC bend, the symmetric CH2 twist, and the antisymmetric CH2 twist. Using the same technique, we have detected the ethyl radical by MPI for the first time. The three-photon ionization yield is small and the spectrum is apparently structureless in the wavelength region 398.5-409.5 nm.

No abstract

Collisional removal rate constants for the OH radical in v=12 of the ground electronic state are measured for the colliders CO2, O2, N2, H2, He, and Ar. OH molecules, generated in v=8 by the reaction of hydrogen atoms with ozone, are excited to v=12 by direct overtone excitation with pulsed infrared laser light. The temporal evolution of the v=12 radicals is probed as a function of collider gas pressure by a time-delayed pulsed ultraviolet probe laser. The probe laser is used to excite the molecules via the B 2Σ+–X 2Πi(0,12) electronic transition, and the resulting B 2Σ+–A 2Σ+ fluorescence is detected. We measure rate constants for CO2:(5.6±1.5)×10−11; O2:(1.6±0.2)×10−11; He:(3.6±0.6)×10−12; H2:(3.0±0.8)×10−12; Ar:(2.6±0.5)×10−12; N2:(2.5±0.7)×10−12 (all in units of cm3 s−1). These rate constants are over fifty times faster in all cases than the vibrational relaxation rate constants for the lower levels (v=1 and v=2) of the ground state.

Resonant two-photon ionization of gas phase CU 2 in a cold molecular beam in conjunction with time-of-flight photoelectron spectroscopy provides new vibronic state spectroscopic infonnation for the dimer cation Cu 2 + . One color ionization via the 0-0, 1-0, and 2a-O bands of Smalley's System V neutral CU2 resonant states (J ..... X transition) accesses Cut states in the range 0-1.4 eV. The electron kinetic energy measurements slightly refine the first adiabatic ionization energy ofCu 2 to II (Cu 2 ) = 7.899 ± 0.007 eV. We observe two electronic states of CU2+ which we assign as X 2~t and an excited 2n spin-orbit pair of sublevels with origins at TOen3/2) = 1.143 ± 0.002 eVand T O en 1 /2) = 1.256 ± 0.002 eV. The absence of spin -orbit splitting identifies the ground state 2~ symmetry; the spin-orbit splitting of 898 ± 8 cm-1 identifies the excited states as 2n. Within X 2~g+ we observe a remarkably long vibrational progression, perhaps extending from v = 0-80. The vibrational intervals detennine the constants We = 188 ± 4 cm-I and w.xe = 0.75 ± 0.09 cm-I . The 2n vibrational intervals detennine We = 244 ± 6 cm -I. The adiabatic bond dissociation energy of ground state Cut is Do(Cu+ -Cu) = 1.84 ± 0.08 eV. The intensity pattern oftheX2~g+ vibrational bands exhibits multiple peaks whose positions and amplitudes are sensitive to the resonant J state vibrational level. For 0-0 excitation, we observe reproducible band intensity alternation. We present preliminary mass spectral and photoelectron data indicating that the cause of the highly non-Franck-Condon band intensities is excitation oflong lived, dissociative autoionization states which undergo extensive nuclear motion on the time scale of electron ejection. We propose an autoionization mechanism that includes a description of the CU 2 J state and explains the observed phenomena invoking only one electron transition.3854 Sappey. Harrington. and Weisshaar: Spectroscopy of CU 2 Pi = (J-lJ26/J-li) 1/2, where J-l126 = 31.46 amu is the reduced

Planar laser-induced fluorescence (PLIF) is used to monitor Cu atoms and Cu2 produced by excimer laser ablation of a copper target (308 nm, ≳10 J/cm2, 1–3 GW/cm2) expanding rapidly into helium background gas at pressures ranging from 10 to 100 Torr. The Cu2 results from gas phase condensation of the copper atoms ablated from the target in the regions of highest Cu atom density as expected, but the maximum Cu2 laser-induced fluorescence (LIF) signal occurs significantly after the maximum of the Cu signal. Rotationally resolved excitation scans of Cu2 utilizing the A–X (0,0) transition indicate that the Cu2 has reached equilibrium with the 300 K background gas. An extensive search for Cu3 via LIF failed, indicating that Cu3 is present only in very low ‘‘steady state’’ number density in the plume. This data is explained qualitatively by a simple kinetic model. In addition to the kinetic information, it is clear from the PLIF images that viscous eddy formation becomes more pronounced as the backing gas pressure increases; however, we see no evidence of turbulence in the plume even at the highest backing gas pressure studied. The PLIF technique allows us to observe the onset of condensation directly as well as to obtain information about the expansion dynamics of the plume not easily obtainable by other means.

No abstract