No abstract

We investigated vibrational Feshbach resonances of multipole-bound states in the time-domain for the first time. State-specific autodetachment rate measurement in phenoxide (PhO-) dipole-bound state (DBS) [1,2] and 4-cyanophenoxide (4-CP-) quadrupole-bound state (QBS) [3] was performed in the cryogenically-cooled ion trap combined with the time-resolved photoelectron velocity map imaging technique. For the phenoxide DBS, the most prominent 11 ′1 (+519 cm −1) peak showed τ ~ 33.5 ps. The lifetime of the each vibrational peak is highly mode-dependent to give ~5 ps for the 18 ′1 (+632 cm −1) , and the multi-quanta excitation of the ν 11 mode gives ~ 11.9 ps for the 11 ′2 (+1036 cm −1) and ~9.2 ps for the 11 ′3 (+1556 cm −1). These mode-specific autodetachment rates are well-described by the Fermi's golden rule. Interestingly, the 11 ′1 18 ′1 combination mode showed bifurcation dynamics to the 11 0 18 1 and 11 1 18 0 neutral product with much faster autodetachment rate of τ ~ 1.4 ps. This result implies that the each bifurcated channel showed accelerated autodetachment dynamics compared to the detachment from the single-mode respectively. For the 4-CP-QBS, the lifetime of the most prominent 12 ′1 peak gives τ ~ 56.4 ps lifetime, which is ~1.68-fold slower than the similar 11 ′1 mode of phenoxide DBS. This may originate from the weaker interaction of the charge-quadrupole potential in long-range region compared to that of the charge-dipole interaction. The mode-dependency is also confirmed by measuring the lifetime on the other vibrational modes of QBS, which could be described by the wobbling of the quadrupole moment elliposoid and Fermi's golden rule. This research provides unprecedental insights into the interaction between the electronic and nuclear dynamics of the non-valence bound states.

A pump-probe experiment with the picosecond laser system was designed to reveal the vibrational mode-effect on the anion reaction dynamics. Vibrational excitation by IR pulse initiates the reaction of anions, such as autodetachment, electron-transfer, and isomerization. Visible pulse after IR pulse can detach the electron from the reaction product. Reaction rate, yield, energy, and isomer distribution can be obtained from the kinetic energy distribution of detached electron. Anion source, mass selection system, VMI photoelectron spectrometer, and high-intensity mid-IR [1] have been prepared for this IR-Vis pump-probe experiment. To overcome the low density of the ion packet and low absorption cross-section for the IR, we constructed the high-intensity (50~100 uJ) mid-IR optical setup for the picosecond laser system. Using the nonlinear property of the KTA crystal, we obtained mid-IR pulse (3000 ~ 5000 nm) from the difference frequency mixing between 791 nm and near-IR (950 ~ 1050 nm) pulses. 791 nm laser pulse was obtained from the 1 kHz picosecond amplifiers seeded by the Ti:sapphire oscillator. Near-IR pulse was generated from the optical parametric amplifier system pumped by 791nm pulse For the first target system, we planned the time-resolved photoelectron experiment for the nitromethane anion. Electrons can be autodetached by vibrational excitation [2] and we will measure autodetachment rate with our picosecond laser system. By comparing the autodetachment rate for each vibrational mode, we will be able to reveal the mechanism and vibrational mode effect on the autodetachment dynamics of ground state anion.

The new design of the spectroscopy apparatus for cryogenic anion photoelectron spectroscopy consists of velocity map imaging (VMI) and filament ionization source as described in Figure.1. Quadrupole mass filter delivers mass-selected ions into a copper quadrupole ion trap (QIT) that is cryogenically cooled by a closedcycle cryostat. Especially, copper QIT has a better ability to decrease the temperature of ions trapped compared to normal QIT made from stainless steel. [3] The improved cooling efficiency of copper QIT makes ions not to excite to vibrational state, so it enables us to observe vibration-controlled reaction dynamics. A cryogenically cooled ion packet is extracted from QIT and travels in the Wiley-McLaren time of flight (TOF) region. Both deflectors and einzel lenses fix the turned beam before entering the VMI region. Also, SIMION simulation provides proper electrostatic ion optics and predicts the ion pathway. Based on the simulation, we optimized this described design for increasing the number of focused ions in the interaction region. By using the above system, we obtained a mass spectrum confirming that the TOF system operates well to perform a further experiment.