Dean-Kuo Liu

Photodissociation spectroscopy of MgCH+4

The photodissociation spectroscopy of MgCH+4 has been studied in a reflectron time‐of‐flight mass spectrometer. MgCH+4 molecular absorption bands are observed to the red of the Mg+(3 2PJ←3 2S1/2) atomic ion resonance lines. The photofragmentation action spectrum consists of a broad structureless continuum ranging from 310 nm to 342 nm, and peaking near 325 nm. In this spectral region, both the nonreactive (Mg+), and two reactive fragmentation products (MgH+ and MgCH+3) are observed, all with similar action spectra. The product branching is independent of wavelength, Mg+:MgCH+3:MgH+∼60:33:7. The absorption is assigned to the transition (1 2E←1 2A1) in C3v symmetry (with η3 coordination), followed by a geometrical relaxation of the complex toward states of 2B1 and 2B2 symmetry in C2v geometry (with η2 coordination). Dissociation requires a nonadiabatic transition to the ground electronic surface. Analysis of broadening in the photofragment flight time profile shows the nonreactive Mg+ product angular distri...

Reaction pathway, energy barrier, and rotational state distribution for Li (2 2PJ)+H2→LiH (X 1Σ+)+H

By using a pump-probe technique, we have observed the nascent rotational population distribution of LiH (v=0) in the Li (2 2PJ) with a H2 reaction, which is endothermic by 1680 cm−1. The LiH (v=0) distribution yields a single rotational temperature at ∼770 K, but the population in the v=1 level is not detectable. According to the potential energy surface (PES) calculations, the insertion mechanism in (near) C2v collision geometry is favored. The Li (2 2PJ)–H2 collision is initially along the 2A′ surface in the entrance channel and then diabatically couples to the ground 1A′ surface, from which the products are formed. From the temperature dependence measurement, the activation energy is evaluated to be 1280±46 cm−1, indicating that the energy required for the occurrence of the reaction is approximately the endothermicity. As Li is excited to higher states (3 2S or 3 2P), we cannot detect any LiH product. From a theoretical point of view, the 4A′ surface, correlating with the Li 3 2S state, may feasibly co...

Rotational population distribution of KH (v=0, 1, 2, and 3) in the reaction of K(5 2PJ, 6 2PJ, and 7 2PJ) with H2: Reaction mechanism and product energy disposal

Using a pump–probe method, we have systematically studied the rotational distribution of KH (v=0–3) produced in the reaction of K (5P, 6P, and 7P) with H2. The resulting rotational states fit roughly a statistical distribution at the system temperature, while the vibrational populations are characterized by a Boltzmann vibrational temperature of 1800, 3000, and 3100 K for the 5p, 6P, and 7P states, respectively. These results provide evidence that the reaction follows a collinear collisional geometry. This work has successfully probed KH from the K(5P) reaction, and confirms that a nonadiabatical transition via formation of an ion‐pair K+H−2 intermediate should account for the reaction pathway. The available energy dissipation was measured to be (68±4)%, (26±2)%, and (6±3)% into the translation, vibration, and rotation of the KH product, respectively. The energy conversion into vibrational degree of freedom generally increases with the principal quantum number, indicating that the electron‐jump distance e...

State-specific reaction and product energy disposal of electronically excited potassium with hydrogen molecule

Using a pump–probe technique, we have systematically studied the state-selected effect on the K–H2 reaction, showing that the reactivity follows the trend of D<P<S. As long as the system is energetically allowed for reaction, the potential energy is not the key parameter, but the atomic orbital symmetry determines such a state selectivity. The observation of KH(v=0–3) rotational population in the reaction of K(6s,7s) corresponds to a statistical thermal distribution at 610±20 K. In contrast, the vibration is highly excited, yielding a Boltzmann vibrational temperature of 2946±110 and 3150±200 K. These results provide evidence that the attacking K atom approaches along a collinear geometry, and KH is produced via an ion-pair K+H2− intermediate as a likely pathway. The fraction of product energy partitioning yields 70%, 26%, and 4% for translation, vibration, and rotation. The individual energy disposal into vibration increases with the excitation energy of K. The fact indicates that the electron jumping di...

REACTION DYNAMICS OF MG(3S3P 1P1) WITH CH4 : ELUCIDATION OF REACTION PATHWAYS FOR THE MGH PRODUCT BY THE MEASUREMENT OF TEMPERATURE DEPENDENCE AND THE CALCULATION OF AB INITIO POTENTIAL ENERGY SURFACES

Using a pump–probe method, we have obtained the nascent bimodal rotational distribution of MgH (v″=0 and 1) products formed in the reaction of Mg(3s3p 1P1) with CH4. The low‐N component of the distribution in the v″=0 state is much larger than that in the v″=1 state, whereas the high‐N component in the v″=0 state is roughly equivalent to that in the v″=1 state. The MgH (v″=0) rotational distributions at three temperatures, 770, 830, and 880 K, were measured. The bimodal distribution does not change with temperature within a small experimental error. The findings suggest that the bimodal nature results from the same process, supporting a mechanism of Mg insertion into the C–H bond, irrespective of the geometry of the entrance approach. The result is consistent with that of Kleiber et al. using the far‐wing scattering technique, and is supported by Chaquin et al.’s theoretical calculations. We also calculated two‐dimensional potential energy surfaces for the excited and ground states of the reaction system....

Ab initio calculation for potential energy surfaces relevant to the microscopic reaction pathways for Mg(3s3p1P1)+H2→MgH(2Σ+)+H

Two ab initio methods have been employed to calculate the dynamical potential energy surfaces (PES’s) for the excited (1B2 or 1A′) and the ground (1A1 or 1A′) states in the Mg(3s3p1P1)–H2 reaction. The obtained PES’s information reveals that the production of MgH in the 2Σ+ state, as Mg(1P1) approaches H2 in a bent configuration, involves a nonadiabatic transition. The MgH2 intermediate around the surface crossing then elicits two distinct reaction pathways. In the first one, the bent intermediate, affected by a strong anisotropy of the interaction potential, decomposes via a linear HMgH geometry. The resulting MgH is anticipated to populate in the quantum states of rotational and vibrational excitation. In contrast, the second pathway produces MgH in the low rotational and vibrational states, as a result of the intermediate decomposition along the stretching coordinate of the Mg–H elongation. These two tracks may account for the previous experimental findings for the MgH distribution, which the impulsive...

Temperature effect on the deactivation of electronically excited potassium by hydrogen molecule

Time-resolved fluorescences from varied K excited states are monitored as a function of H2 pressure. According to a three-level model, the rate coefficients of collisional deactivation for the K 6 2S, 7 2S, and 8 2S states at 473 K have been determined to be 4.94±0.15, 5.30±0.15, and 5.44±0.15×10−9 cm3 molecule−1 s−1. In addition, the collision transfer of 2S–2D transition may be derived to be 5.03±0.21, 4.68±0.30, and 4.89±0.36×10−9 cm3 molecule−1 s−1, showing dominance of the 2S-state deactivation processes owing to the effect of near-resonance energy transfer. As the temperature is varied, the activation energies for the collisions of K(6 2S), K(7 2S), and K(8 2S) atoms with H2, respectively, may be estimated to be 5.38±0.33, 4.39±0.16, and 3.23±0.19 kJ/mol. The first two values are roughly consistent with the theoretical calculations of 3.1 and 0.9 kJ/mol in C∞v symmetry predicted by Rossi and Pascale. The obtained energy barriers are small enough to allow for occurrence of the harpoon mechanism, a mo...

Nascent rotational distribution and energy disposal of the CaH product in the reaction of Ca(4s4p 1P1)+H2→CaH(X 2∑+)+H

We have obtained for the first time the nascent rotational population distributions of CaH(v=0 and 1) in the reaction of Ca(4s4p 1P1) with H2. The rotational, vibrational, and translational fractions of product energy disposal may accordingly be evaluated to be 0.19±0.02, 0.33±0.02, and 0.48±0.02, respectively. Compared to the prior prediction, the low rotational and high vibrational fractions obtained suggest that the reaction mechanism should favor the collinear approach.

Reaction dynamics of Mg(3 1P1, 4 1S0) with H2: insertion versus harpoon mechanism

Abstract We have obtained nascent rotational distributions of MgH ( ν = 0 and 1) in the reaction of Mg(4 1 S 0 ) with H 2 . The resultant bimodal features of the MgH distributions are similar to those obtained from Mg(3 1 P 1 ). The spectral analysis and potential energy surfaces calculation suggest that the Mg(4 1 S 0 ) atom proceeds via a harpoon-type reaction pathway, in contrast to a direct insertion followed by Mg(3 1 P 1 ). The Mg(4 1 S 0 ) system is closely associated with the Mg(3 1 P 1 )−H 2 reaction coordinate, through evolution of a series of surface crossings along the ion-pair coordinate.

Reaction dynamics of Mg(4 1S0, 3 1D2) with H2: Harpoon-type mechanism for highly excited states

Using a pump–probe technique, the reactions of Mg (4 1 S 0 and 3 1 D 2 ) with H 2 have been measured to yield similar rotational distributions of MgH (v=0 and 1) as that obtained for the reaction of the Mg (3 1 P 1 ) state with H 2 . A series of measurements is conducted to clarify that the reactions are initiated directly by these higher states, rather than occurring from the lower 3 1 P 1 state following radiative and collisional relaxation. The reactivity of the Mg 4 1 S 0 state with H 2 is found to be comparable to that of the 3 1 P 1 state, but about three times larger than that of the 3 1 D 2 state. The Mg (4 1 S 0 , 3 1 D 2 )– H 2 reactions proceed via a harpoon-type process, and are closely associated with the Mg (3 1 P 1 )– H 2 reaction coordinate through evolution of a series of surface crossings. To support our suggestion that the harpoon mechanism is involved, the cross sections of collisional deactivation by H 2 for various excited states are measured. The ratios of cross sections observed for the 3 1 P 1 , 4 1 S 0 , and 5 1 S 0 state, equal to 1:2.85:4.3, are consistent with the calculated prediction of 1:2.62:4.24. The calculated cross sections are based on a simple hard sphere model with effective radii evaluated differently. Here, the effective radii for the higher states are determined from the crossing of ionic and covalent curves, while the Mg (3 1 P 1 )– H 2 radius is estimated from the nonadiabatic crossing between the reactive 1 1 B 2 state and the ground state. Consistency between observation and prediction confirms that the harpoon mechanism proposed in this work is plausible.