Photoelectron-Photofragment Coincidence Spectroscopy With Ions Prepared in a Cryogenic Octopole Accumulation Trap: Collisional Excitation and Buffer Gas Cooling.

A cryogenic octopole accumulation trap (COAT) has been coupled to a photoelectron-photofragment coincidence (PPC) spectrometer allowing for improved control over anion vibrational excitation. The anions are heated and cooled via collisions with buffer gas <17 K. Shorter trapping times (500 µs) prevent thermalization and result in anions with high internal excitation while longer trapping times (80 ms) at cryogenic temperatures thermalize the ions to the temperature of the buffer gas. The capabilities of the COAT are demonstrated using PPC spectroscopy of O 3 - at 388 nm (Ehν = 3.20 eV). Cooling the precursor anions with COAT resulted in the elimination of the autodetachment of vibrationally excited O 2 - produced by the photodissociation O 3 - + hν → O + O 2 - (v ≥ 4). Under heating conditions, a lower limit temperature for the anions was determined to be 1,500 K through Franck-Condon simulations of the photodetachment spectrum of O 3 - , considering a significant fraction of the ions undergo photodissociation in competition with photodetachment. The ability to cool or heat ions by varying ion injection and trapping duration in COAT provides a new flexibility for studying the spectroscopy of cold ions as well as thermally activated processes.

Internal energy dependence of the photodissociation dynamics of O3- using cryogenic photoelectron-photofragment coincidence spectroscopy.

Photoelectron-photofragment coincidence (PPC) spectra of ozonide, O3-, were measured at 388 nm (Ehν = 3.20 eV) using a newly constructed cryogenic octopole accumulation trap coupled to a PPC spectrometer. The photoelectron spectra reveal three processes consisting of a stable photodetachment channel, and two distinct photodissociation pathways yielding (1) O2 + O- or (2) O + O2-. The first photodissociation pathway is observed in the PPC spectra by photodetachment of the O- product by a second photon, and produces electronically excited O2(1Δg). The O2- product of the second photodissociation pathway undergoes autodetachment for O2-(2Πg, v″ > 4), a process greatly enhanced by vibrational excitation of the precursor O3-. Cooling anions thermalized at 300 K to <17 K in a cryogenic octopole accumulation trap essentially turns off this autodetachment pathway. The product kinetic energy distribution in coincidence with the autodetached electrons from O2-(v″ = 4) exhibits resolved features consistent with bend (ν2), asymmetric stretch (ν3) and a stretching combination band (ν1 + ν3) in the intermediate electronic state, illustrating the insights that can be gained from kinematically complete measurements. These results are discussed in the context of the low-lying excited states of O3-.

Energetics and transition-state dynamics of the F + HOCH3 → HF + OCH3 reaction.

The F + HOCH3 → HF + OCH3 reaction is a system with 15 internal degrees of freedom that can provide a benchmark for the development of theory for increasingly complex chemical reactions. The dynamics of this reaction were studied by photoelectron-photofragment coincidence (PPC) spectroscopy carried out on the F-(HOCH3) anion, aided by a computational study of both the anion and neutral potential energy surfaces, with energies extrapolated to the CCSDT(Q)/CBS level of theory. Photodetachment at 4.80 eV accesses both the reactant and product channels for this reaction. In the product channel (HF + OCH3 + e-) of the neutral potential energy surface, vibrationally excited HF products in addition to the stable product-channel hydrogen-bonded complex (FH-OCH3) are observed in the PPC and photoelectron spectra. In addition, experimental evidence is observed for the reactant-channel van der Waals complex (F-HOCH3), in good agreement with the theoretical predictions. The relative stability of these long-lived complexes was probed by reducing the ion beam energy, increasing the product time-of-flight, indicating lifetimes on the microsecond timescale for the reactant- and product-channel complexes as well as providing evidence for long-lived vibrational Feshbach resonances associated with the HF(v > 0) + OCH3 product states. This system will provide a model for extending full-dimensionality quantum dynamics to larger numbers of degrees of freedom.

Photoelectron-photofragment coincidence studies of the tert-butoxide anion (CH3)3CO((-)), the carbanion isomer (CH3)2CH2COH((-)), and corresponding radicals.

A study of the photodetachment and dissociative photodetachment (DPD) of the C(4)H(9)O(-) isomers tert-butoxide, (CH(3))(3)CO(-), and the α-hydroxy carbanion (CH(3))(2)C(CH(2))OH(-) is reported. Photoelectron-photofragment coincidence spectroscopy was used to study these anions at 387, 537, and 600 nm. Supported by CBS-QB3 ab initio calculations, the product mass and translational energy distributions were found to be consistent with dissociation of either highly excited (CH3)(3)CO radicals or (CH(3))(2)C(CH2)OH alkylhydroxy radicals. Vibrationally resolved photoelectron spectra of stable radicals at 537 and 600 nm in conjunction with Franck-Condon simulations were used to assign the dominant channel to tert-butoxide ((CH(3)3)CO(-)) anions thermalized to a vibrational temperature of 550 K. DPD is assigned to highly vibrationally excited radicals produced by photodetachment of unrelaxed tert-butoxide products formed at an effective source temperature of 1400 K. The higher energy carbanion was found to be a minor channel and was not observed to dissociate. Calculated energetics for photodetachment and DPD of (CH(3))(3)CO(-) and (CH(3))(2)C(CH(2))OH(-) are discussed and compared with the experimental results.

Photoelectron-photofragment coincidence spectroscopy in a cryogenically cooled linear electrostatic ion beam trap.

A cryogenically cooled linear electrostatic ion beam trap for use in photoelectron-photofragment coincidence (PPC) spectroscopy is described. Using this instrument, anions created in cold, low-duty-cycle sources can be stored for many seconds in a ~20 K environment to cool radiatively, removing energetic uncertainties due to vibrationally excited precursor anions. This apparatus maintains a well-collimated beam necessary for high-resolution fragment imaging and the high experimental duty cycle needed for coincidence experiments. Ion oscillation is bunched and phase-locked to a modelocked laser, ensuring temporal overlap between ion bunches and laser pulses and that ions are intersected by the laser only when travelling in one direction. An electron detector is housed in the field-free center of the trap, allowing PPC experiments to be carried out on ions while they are stored and permitting efficient detection of 3-dimensional electron and neutral recoil trajectories. The effects of trapping parameters on the center-of-mass trajectories in the laser-ion interaction region are explored to optimize neutral particle resolution, and the impact of bunching on ion oscillation is established. Finally, an initial demonstration of radiative cooling is presented.

Communication: New insight into the barrier governing CO2 formation from OH + CO.

Despite its relative simplicity, the role of tunneling in the reaction OH + CO → H + CO(2) has eluded the quantitative predictive powers of theoretical reaction dynamics. In this study a one-dimensional effective barrier to the formation of H + CO(2) from the HOCO intermediate is directly extracted from dissociative photodetachment experiments on HOCO and DOCO. Comparison of this barrier to a computed minimum-energy barrier shows that tunneling deviates significantly from the calculated minimum-energy pathway, predicting product internal energy distributions that match those found in the experiment and tunneling lifetimes short enough to contribute significantly to the overall reaction. This barrier can be of direct use in kinetic and statistical models and aid in the further refinement of the potential energy surface and reaction dynamics calculations for this system.