The photodissociation spectroscopy of MgCH 4 ϩ has been studied in a reflectron time-of-flight mass spectrometer. MgCH 4 ϩ molecular absorption bands are observed to the red of the Mg ϩ (3 2 P J ←3 2 S 1/2 ) 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 ϩ ͒, and two reactive fragmentation products ͑MgH ϩ and MgCH 3 ϩ ͒ are observed, all with similar action spectra. The product branching is independent of wavelength, Mg ϩ :MgCH 3 ϩ :MgH ϩ ϳ60:33:7. The absorption is assigned to the transition (1 2 E←1 2 A 1 ) in C 3v symmetry ͑with 3 coordination͒, followed by a geometrical relaxation of the complex toward states of 2 B 1 and 2 B 2 symmetry in C 2v geometry ͑with 2 coordination͒.Dissociation requires a nonadiabatic transition to the ground electronic surface. Analysis of broadening in the photofragment flight time profile shows the nonreactive Mg ϩ product angular distribution to be isotropic, with an average translational energy release which increases slightly from E t ϳ370Ϯ150 cm Ϫ1 at 332.5 nm to E t ϳ520Ϯ180 cm Ϫ1 at 315 nm. These values are less than 2% of the available energy and are well below statistical expectations. Analogous experiments on MgCD 4 ϩ show the kinetic energy release in the nonreactive channel to be significantly larger for the CD 4 case, ranging from E t ϳ540Ϯ180 cm Ϫ1 at 332.5 nm to E t ϳ830Ϯ200 cm Ϫ1 . These results clearly demonstrate that the dissociation is nonstatistical. Preliminary ab initio potential surface calculations suggest a possible dynamical mechanism to explain these unusual results.
The population distributions of the rotational quantum states of the @ascent MgH(u" =0 and 1) produced in the reaction of Mg(3s3p P& ) with H2 are bimoda1. With the use of the surprisal method, the two components are separated. The minor low-N component of the distribution in the u"=0 state is found to be larger than that in the v" = 1 state, whereas, the major high-E component of the distribution in the v"=0 state becomes roughly equivalent to that in the v"=1 state. The two parallel low-N and high-N processes are expected to correspond to two distinct types of reaction dynamics. One (minor) type produces MgH in lower rotational levels and preferentially v"=0, and the other (major) type produces MgH in higher rotational levels with comparable v" =0 and v" =1 populations. Possible dynamical models are discussed. PACS number(s): 82.30.b, 34.50.s, 35.20.i
Two ab initio methods have been employed to calculate the dynamical potential energy surfaces (PES’s) for the excited (B21 or A'1) and the ground (A11 or A'1) states in the Mg(3s3p1P1)–H2 reaction. The obtained PES’s information reveals that the production of MgH in the Σ+2 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 model has failed to comprehend. By far, different interpretations have been proposed especially for the low-N MgH product. The supply of a detailed PES’s information in this work helps to clarify the ambiguity. It is also conducive to an interpretation of the isotope and temperature effects on the product rotational distribution.
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 couple to a repulsive 3A′ surface, from which the collision complex will rapidly break apart into Li (2 2PJ) and H2. The probability for further surface hopping to the 2A′ or 1A′ surfaces is negligible, since the 3A′ and 2A′ surfaces are too far separated to allow for an efficient coupling. The Li (3 2P) state is expected to behave similarly. The observation also provides indirect evidence that the harpoon mechanism is not applicable to this system.
Articles you may be interested inA comparison of quantum and quasiclassical statistical models for reactions of electronically excited atoms with molecular hydrogen J. Chem. Phys. 129, 094305 (2008); 10.1063/1.2969812 Time-dependent wave packet calculation for state-to-state reaction of Cl+H 2 using the reactant-product decoupling approach Quantum mechanical calculation of product state distributions for the O ( 1 D)+ H 2 → OH+H reaction on the ground electronic state surface
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 S2−D2 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 S2-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 model applicable to the reactions between H2 and alkali atoms such as K, Rb, and Cs. Among them, K–H2 collisions appear to be the first case to possess a slight energy barrier. This finding of energy barrier may account for the discrepancy for the state reactivity towards H2 observed between K (or Rb) and Cs atoms.
We measured the temperature dependence of rotational population distribution of the nascent product MgH(2∑+) in the reaction of Mg(3s3p1P1) with H2. The results indicate that the reaction is dominated by an Mg‐insertive mechanism, consistent with the isotope effect reported previously. We also presented the vibrational population distribution, and thereby found that two parallel reaction pathways are responsible for the subject reaction following Mg‐H2 collision in a bent configuration. The major one produces MgH in higher rotational levels and comparable v″ = 0 and v″ = 1 populations, while the other minor one produces MgH in low rotational levels and preferentially v″ = 0. By means of a two‐dimensional potential energy sur‐face(PES) calculation, a deep insight into the reaction pathways has been gained. The resulting PES's information reveals the possibility of a nonadiabatic transition between the excited 1B2 PES and the ground PES. The bent intermediate MgH2 near the surface crossing starts trajectories either smoothly following the dissociation coordinate of Mg‐H distance or attractively falling down through a linear HMgH geometry before breaking apart. The former trajectory accounts for the minor reaction pathway to produce MgH, while the latter one responses to the major reaction pathway. The impact of isotope and temperature effects on MgH can also be readily explained with use of the calculated PES's.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.