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

Abstract

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.

Rotational and vibrational state distributions of NaH in the reactions of Na(4S2,3D2,and6S2) with H2: Insertion versus harpoon-type mechanisms

“…13,14,16,29 The radiation sources contained two tunable dye lasers, which were pumped individually by a Nd:YAG laser operating at 10 Hz with a pulse duration of 5 -8 ns. The first dye laser at wavelength was used to prepare the Na atom in the 4 2 S, 3 2 D, and 6 2 S states at / 2 = 388.5, 342.8, and 274.9 nm by twophoton absorption, using the dyes LDS 765, LDS 698, and rhodamine 575, respectively.…”

“…The K͑n 2 S and n 2 P͒ plus H 2 reactions are anticipated to follow harpoon-type mechanisms via an ion-pair K + H 2 − intermediate as a likely pathway. 13,14,16 The ionization energy of K atom ͑4.34 eV͒ is smaller than that of the Na atom ͑5.14 eV͒, and thus the reaction for an excited K atom with even lower ionization energy may take place feasibly through electron transfer from K to its colliding partner at long-range distance.…”

“…K, Rb, and Cs elements with larger sizes are grouped together, while Li and Na are in the other class. The harpoon model essentially holds for the chemical reactions of high-lying K, 7,[11][12][13][14][15][16] Rb, 17,18 and Cs states with H 2 . 1, 8,[19][20][21][22][23][24] The resultant products of KH, RbH, and CsH share the following common features.…”

Significant non-adiabatic effects of the K(4s2S) + H2 reaction

“…Their results indicate that the combination of steric and energic effects determine the major quenching pathways for the reaction between alkali metal atoms and the H 2 molecule. In 2000, the rate coefficients for the deactivation processes of the K(6 2 S, 7 2 S, 8 2 S) + H 2 reaction were measured by Hsiao et al [8]. Their results indicate that the effects of near-resonance energy transfer dominate the deactivation processes of the 2 Sstate.…”

“…In recent decades, the reactions between the K atom and the H 2 molecule have been extensively studied in both experiments [1][2][3][4][5][6][7][8][9][10] and theory [11,12]. The K atom is always excited to the excited state in an experiment using a laser technique because the K + H 2 reaction is highly endothermic in the ground state.…”