Two-photon photodissociation dynamics of H2O via the D̃ electronic state

Abstract

Photodissociation dynamics of H(2)O via the D state by two-photon absorption have been investigated using the H-atom Rydberg tagging time-of-flight technique. The action spectrum of the D<--X transition band has been measured. The predissociation lifetime of the D state is determined to be about 13.5 fs. The quantum state-resolved OH product translational energy distributions and angular distributions have also been measured. By carefully simulating these distributions, quantum state distributions of the OH product as well as the state-resolved angular anisotropy parameters were determined. The most important pathway of the H(2)O dissociation via the D state leads to the highly rotationally excited OH(X,v=0) products. Vibrationally excited OH(X) products (up to v=10) and electronically excited OH(A,v=0,1,2) have also been observed. The OH(A)/OH(X) branching ratios are determined to be 17.9% at 244.540 nm (2omega(1)=81,761.4 cm(-1)) and 19.9% at 244.392 nm (2omega(2)=81,811 cm(-1)), which are considerably smaller than the value predicted by the theory. These discrepancies are attributed to the nonadiabatic coupling effect between the B and D surfaces at the bent geometry.

Quantum state-selected photodissociation dynamics of H2O: Two-photon dissociation via the C̃ electronic state

“…2͒, which are similar to that observed from the H 2 O two-photon photodissociation via the D state. 29 However, these two peaks are not observed from the H 2 O one-photon photodissociation process. 24 From the translational energy distributions, the difference of the threshold energy between the two peaks can be determined to be ϳ15 000 cm −1 .…”

State-to-state photodissociation dynamics of triatomic molecules: H2O in the B band

“…For example, Yang and co-workers have recently employed the high resolution H-Rydberg tagging technique 3 to measure product state-resolved differential cross sections (DCSs) for the photodissociation of H 2 O. [4][5][6][7][8][9] While providing the most detailed information about dynamics, state-to-state DCSs in polyatomic photodissociation have seldom been calculated quantum mechanically until very recently, 10 despite the existence of the photodissociation theory for more than 30 years. 11 The water molecule absorbs a photon in the vacuum ultraviolet region 12 starting with its first (A) structureless band near 190 nm.…”

Energy Expenditure for Water Molecule Ionization by Electron Impact in Weakly Ionized Plasma

“…The H 2 O molecule has the ground electron state 1 1 (see Table 1), and its dissociation energy equals (H − OH) = 5.0992 eV [16]. Excitation of the electron levels 3 1 , 1 1 , 3 1 , and 1 1 results in the dissociation of a water molecule by several channels [37] into H and OH( 2 Π ) with excited rotationalvibrational levels [38].…”