New scheme for measuring the angular momentum spatial anisotropy of vibrationally excited H2 via the I 1Πg state

We report the spectroscopic detection of vibrationally excited molecular hydrogen using 2+1 resonantly enhanced multiphoton ionization (REMPI) via the I (1)Pi(g) (v' = 0) - X (1)Sigma(g)(+) (v" = 3) band ca. 198 nm. Vibrationally excited H-2 was produced by passing room-temperature 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)Sigma(g)(+) - X (1)Sigma(g)(+) 2+1 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)Sigma(g)(+) (v' = 2, 3, 4), GK (1)Sigma(g)(+) (v' = 1), and J (1)Delta(g) (v' = 0). We propose the I-X transition as a suitable candidate for the determination of the rotational anisotropy of vibrationally excited ground-state H-2 molecules. We support this contention with a calculation of the line strength moments and sensitivities to the second- (quadrupolar) and fourth-rank (hexade-capolar) moments of the rotational angular momentum distributions, which is compared against the well-established Q-branch members of the EF (1)Sigma(g)(+) - X (1)Sigma(g)(+) two-photon transition.