One-Color Molecular Photodissociation and Detection of Hydrogen Atoms

Abstract

The wavelength 205.14 nm is absorbed by many hydrogen-containing molecules, which then dissociate to form hydrogen atoms. These in turn can absorb two more 205.14 nm photons and reach the 3s or the 3d state. They can absorb a third photon and form hydrogen ions or decay to the ground state by stepwise fluorescence, first emitting the Balmer α line at 656.2 nm (3s or 3d→2p)and then the Lyman α line (2p→1s)at 121.6 nm. Thus the hydrogen atom kinetic energy can be probed in three different ways. This method broadens greatly the possibilities of investigating photodissociations leading to hydrogen atom products. It has the advantage of simplicity and the disadvantages of a single-color experiment. The method is tested with a molecule that has been extensively investigated, H2S, and then applied to three other molecules, formic acid (HCO2H), vinyl radical (C2H3), and allyl radical (C3H5). H2S has a perpendicular transition with a large release of kinetic energy. Studies of the latter molecules lead to the conclusion that formyl, carboxyl, vinyl, and allyl radicals absorb 205.1 nm light and release hydrogen atoms with a large fraction of the available energy. The dissociation pathways of formic acid are clarified.

Interacting Rydberg and valence states in radicals and molecules: experimental and theoretical studies

“…Neverthess, it was produced by both pyrolysis and photolysis [97,[144][145][146][147][148][149] and an excellent summary of previous work is given in [5]. Here we focus on the UV photodissociation from the 3s state, which was studied in the 230-265 nm range.…”

Signatures of Quantum-Tunneling Diffusion of Hydrogen Atoms on Water Ice at 10 K

“…The atoms desorbed into vacuum were selectively ionized by a 2 þ 1 REMPI laser at 1 mm above the ice surface, and further analyzed by a timeof-flight method. The REMPI employed the two-photon 2sð 2 S 1=2 Þ ← 1sð 2 S 1=2 Þ transition and also the weaker 3sð 2 S 1=2 Þ or 3dð 2 D 3=2;5=2 Þ ← 1sð 2 S 1=2 Þ transition to avoid signal saturation for the lower and higher fluxes, respectively [34,35]. The PSD-REMPI signals were recorded at three different REMPI wavelengths on a resonance peak as a function of delay time between the PSD and REMPI lasers to detect all the atoms with various kinetic energies.…”

Interference-free two-photon LIF imaging of atomic hydrogen in flames using picosecond excitation

“…In that work, very slight distortions of the measured LIF profiles were first visible for fluences in excess of 0.04, 0.04, 0.16, and 0.08 J cm À2 for U = 0.7, 1.0, 1.1, and 1.4 methane flames, respectively. Although there exist a large number of potential photolytic precursors to atomic hydrogen [24,25], we expect that only CH 3 [12,11] and H 2 O [6] are of significant concern in the present flames. Methyl radical is localized to the reactant side of the H-atom peak, and if significant, its photodissociation would affect the steep gradient in the LIF signal.…”