New Scheme for Measuring the Angular Momentum Spatial Anisotropy of Vibrationally Excited H2 via the I 1Πg State

Abstract

H 2 Molecule / Vibrationally Excited / 2+1 REMPI / Rotational AnisotropyWe report the spectroscopic detection of vibrationally excited molecular hydrogen using 2ϩ1 resonantly enhanced multiphoton ionization (REMPI) via the I 1 Π g (v′ ϭ 0) Ϫ X 1 Σ ϩ g (v″ ϭ 3) band ca. 198 nm. Vibrationally excited H 2 was produced by passing roomtemperature hydrogen over a hot ion gauge filament in a high-vacuum chamber. The internal energy distributions were characterized spectroscopically by use of the EF 1 Σ ϩ g ϪX 1 Σ ϩ g 2ϩ1 REMPI detection scheme. We have identified band origins for the S, Q, R, and P rotational branches of the I-X (0,3) band, as well as isolated lines corresponding to two-photon transitions into other nearby H 2 gerade states, including EF 1 Σ ϩ g (v′ ϭ 2, 3, 4), GK 1 Σ ϩ g (v′ ϭ 1), and J 1 ∆g (v′ ϭ 0). We propose the I-X transition as a suitable candidate for the determination of the rotational anisotropy of vibrationally excited ground-state H2 molecules. We support this contention with a calculation of the line strength moments and sensitivities to the second-(quadrupolar) and fourth-rank (hexadecapolar) moments of the rotational angular momentum distributions, which is compared against the well-established Q-branch members of the EF 1 Σ ϩ g ϪX 1 Σ ϩ g two-photon transition.

Preparation of oriented and aligned H2 and HD by stimulated Raman pumping

“…For the H 2 molecule and its isotopomers, the O-and S-branch line sensitivity factors for the E , F -X system are typically larger by an order of magnitude or more than those for the Q-branch lines. 20 Q-branch lines are consequently of little utility for angular momentum polarization studies, although they have already proven to be useful for measuring rotational-state population distributions. [20][21][22][23][24][25] It remains as a challenge whether or not the two-photon E , F -X band system can be used to detect polarized molecular hydrogen produced in chemical reactions under singlecollision conditions.…”

Line strength factors for E,F1Σ+g(v′ = 0, J′ = J′′) – X1Σ+g (v′′, J′′) (2 + 1) REMPI transitions in molecular hydrogen

“…This hot filament promotes recombination reactions that populate excited states of molecular hydrogen, and the excited products can be rotationally but not vibrationally relaxed by collisions before detection. This technique has been shown previously to produce high concentrations of vibrationally excited hydrogen that is rotationally thermalized to slightly above room temperature (15)(16)(17)(18).…”

Electron-volt neutron spectroscopy: beyond fundamental systems

“…Other situations of practical interest where H 2 is a key player include its generation in photocatalytic water-splitting reactions [39], as well as its widespread use in a wide range of industrial chemical processes [40]. On the experimental front, the study of H 2 is not a trivial task given the absence of dipoleallowed optical transitions in the ground state and the need of photon sources in the UV and VUV to access electronic excitations [41,42]. In the above context, we illustrate how DINS has been used with success in the study of H 2 uptake by carbon-based intercalation compounds.…”