Jrwei Ho - Academia.edu (original) (raw)

Papers by Jrwei Ho

The Journal of Physical Chemistry A, 2005

The photodissociation dynamics of the acetone S 2 (n, 3s) Rydberg state excited at 195 nm has bee... more The photodissociation dynamics of the acetone S 2 (n, 3s) Rydberg state excited at 195 nm has been studied by using femtosecond pump-probe mass-selected multiphoton ionization spectroscopy. For the first time, the temporal evolutions of the initial state, intermediates, and methyl products were simultaneously measured and analyzed for this reaction to elucidate the complex dynamics. Two mechanisms were considered: (1) the commonly accepted mechanism in which the primary dissociation occurs on the first triplet-state surface, and (2) the recently proposed mechanism in which the primary dissociation takes place on the first singlet-excitedstate surface. Our results and analyses supported the validity of the new mechanism. On the other hand, the conventional mechanism was found to be inadequate to describe the observed dynamics. The temporal evolution of methyl products arising from the secondary dissociation of hot acetyl intermediates exhibited a very complex behavior that can be ascribed to the combination of a nonuniform initial vibrational distribution and the competition between dissociation and slow intramolecular vibrational redistribution.

The Journal of Physical Chemistry B, 2015

The mechanism of the water-catalyzed excited-state protontransfer (ESPT) reaction for 7-azaindole... more The mechanism of the water-catalyzed excited-state protontransfer (ESPT) reaction for 7-azaindole (7AI) has long been investigated, but there are some controversial viewpoints. Recently, owing to the superiority of sensing biowaters in proteins by a 7AI analogue, 2,7-diazatryptophan, it is timely to reinvestigate water-catalyzed ESPT in 7AI and its analogues in an attempt to unify the mechanism. Herein, a series of 7AI analogues and their methylated derivatives were synthesized to carry out a systematic study on pK a , pK a *, and the associated fluorescence spectroscopy and dynamics. The results conclude that all 7AI derivatives undergo water-catalyzed ESPT in neutral water. However, for those derivatives with −H (7AI) and a electron-donating substituent at C(3), they follow water-catalyzed ESPT to form an excited N(7)−H proton-transfer tautomer, T*. T* is rapidly protonated to generate an excited cationic (TC*) species. TC* then undergoes a fast deactivation to the N(1)−H normal species in the ground state. Conversely, protonation in T* is prohibited for those derivatives with an electron-withdrawing group at the C(2) or C(3) or with the C(2) atom replaced by an electron-withdrawing nitrogen atom (N(2) in, e.g., 2,7-diazatryptophan), giving a prominent green T* emission. Additional support is given by the synthesis of the corresponding N(7)−CH 3 tautomer species, for which pK a * of the cationic form, that is, the N(7)−CH 3 N(1)−H + species, is measured to be much greater than 7.0 for those with electron-donating C(3) substituents, whereas it is lower than 7.0 upon anchoring electron-withdrawing groups. For 7AI, the previously missing T* emission is clearly resolved with a peak wavelength at 530 nm in the pH interval of 13.0−14.3 (H − 14.2).

The Journal of chemical physics, Jan 7, 2014

Photoionization-induced proton transfer (PT) in phenol-ammonia (PhOH-NH3) complex has been studie... more Photoionization-induced proton transfer (PT) in phenol-ammonia (PhOH-NH3) complex has been studied using ultrafast time-resolved ion photofragmentation spectroscopy. Neutral PhOH-NH3 complexes prepared in a free jet are photoionized by femtosecond [1+1] resonance-enhanced multiphoton ionization via the S1 state, and the subsequent dynamics occurring in the cations is probed by delayed pulses that result in ion fragmentation. The observed temporal evolutions of the photofragmentation spectra are consistent with an intracomplex PT reaction. The experiments revealed that PT in [PhOH-NH3](+) cation proceeds in two distinct steps: an initial impulsive wave-packet motion in ~70 fs followed by a slower relaxation of about 1 ps that stabilizes the system into the final PT configuration. These results indicate that for a barrierless PT system, even though the initial PT motions are impulsive and ultrafast, the reaction may take a much longer time scale to complete.

The Journal of Physical Chemistry A, 2011

Chemical Physics Letters, 2005

Abstract The deuterium isotope effect in the photodissociation of acetone S2 state was studied us... more Abstract The deuterium isotope effect in the photodissociation of acetone S2 state was studied using femtosecond pump–probe ionization spectroscopy. The transients obtained for both isotopomers can be well described by the same mechanism in which the primary dissociation occurs on the S1 surface. Substantial isotope effects were observed in every stage of the reaction. Our results indicted that upon full deuteration the initial-state decay from S2 to S1 slows down by a factor of three, the subsequent adiabatic dissociation on the S1 surface slows down by a factor of four, and the secondary dissociation slows down by a factor of ∼1.6.

Chemical Physics Letters, 2003

The photodissociation dynamics of the acetone S2(n,3s) Rydberg state has been studied by using fe... more The photodissociation dynamics of the acetone S2(n,3s) Rydberg state has been studied by using femtosecond pump–probe multiphoton ionization mass spectrometry. Acetone was excited at 195 nm and the temporal evolutions of the initial state and intermediates were monitored. We considered two mechanisms: (1) the conventionally accepted mechanism, in which the primary dissociation occurs on the first triplet-state surface; and (2) the newly proposed mechanism by the Zewail’s group, in which the primary dissociation takes place on the first singlet-state surface. The conventional mechanism was found to be inadequate to describe the observed dynamics. On the other hand, our results and analyses supported the validity of the new mechanism.

The Journal of Physical Chemistry A, 2008

Photodissociation of dimethyl sulfoxide at 200 nm has been studied using femtosecond time-resolve... more Photodissociation of dimethyl sulfoxide at 200 nm has been studied using femtosecond time-resolved spectroscopy. The temporal evolutions of the initial state, intermediates, and products (CH3 and SO) were measured by means of fs pump-probe mass-selected multiphoton ionization and laser-induced fluorescence. Femtosecond time-resolved photofragment translational spectroscopy was also employed to measure the CH3 product kinetic energy distributions as a function of reaction time. The ionization experiments revealed that there are at least three major CH3 product components, whereas the fluorescence experiments indicated that two SO product components are present. The combination of experimental and theoretical results suggested a complex multichannel mechanism involving both concerted and stepwise three-body dissociation pathways.

Journal of the American Chemical Society, 2007

Supporting Information I. Experimental details The experimental setup is similar to those describ... more Supporting Information I. Experimental details The experimental setup is similar to those described in our previous works. 1-3 The femtosecond (fs) laser system consists of a self-mode-locked Ti:sapphire laser (Spectra Physics, Tsunami), a 1 kHz chirped-pulse regenerative amplifier (CPA, Spectra Physics, Spitfire) and a five-pass optical parametric generator/amplifier (Light Conversion, TOPAS). The fs-laser pulses used in this work were obtained through harmonic generation and frequency mixing of the outputs from this laser system. The fs-LIF experiment for probing the SO product was carried out in a gas flow cell that is constantly pumped by an oil-free pump. DMSO vapor was slowly flowed into the cell through a needle valve to maintain a pressure of ~120 mtorr. DMSO was excited at ~200 nm and a second delayed pulse at ~240-260 nm was used to probe the temporal evolution of the free SO (X) fragments by monitoring the SO X→B laser-induced fluorescence (LIF) signal. Different SO (X) vibrational levels (v"=1, 2, 3) can be selectively monitored by tuning the probe lasers to the corresponding wavelengths. 4-6 The pump and probe beams were collinearly recombined via a dichroic mirror and focused through a f=50 cm lens into the gas cell. The laser-induced fluorescence of SO fragments was collected at right angle and detected by a photomultiplier tube. A bandpass filter centered at 330 nm and two long-pass filters (290 nm cutoff) were placed in the light collection system to further reduce the scattered light and to ensure that only photons in the 300-370 nm spectral region (SO B→X