T r a n s-HONO is optically prepared in specific −N=O stretching vibrational levels (2n, n=1,2,3) of the à state at 369, 355, and 342 nm. The ejected OH fragment is completely characterized by Doppler and polarization laser excitation spectroscopy. In this manner the OH translational energy, angular distribution, rotational alignment, and internal state distribution (vibration, rotation, spin-orbit and Λ-doubling components) are probed through the OH A–X system. The OH fragment is found to be translationally hot (∼0.5 eV) with a nearly sin2 θ angular distribution about the electric vector of the photolysis laser. The corresponding line shapes are Doppler split. However, the line shapes and widths do not noticeably depend on either fragment rotation or parent vibration. The internal motion of the OH fragment is vibrationally and rotationally cold; the spin-orbit components and the Λ doublets are not in equilibrium. The OH fragment is aligned and its π lobe lies preferentially in the plane of rotation. With increasing rotational excitation, these effects become more pronounced. This information allows us to construct a detailed photodissociation mechanism. The fragmentation is prompt and the trajectories of the recoiling fragments lie close to the initial HONO plane. The impulse associated with the central O–N bond fission contributes predominantly to OH translation while the rotation appears to arise from the zero-point motion of the parent in-plane bending and torsional vibrations. The OH energy content is found to be quite insensitive to the parent ν2 vibration, suggesting that the à state surface is rather ‘‘flat’’ along the −N=O stretch compared to the steep fragmentation coordinate.

It is proposed that the two Λ-doublet levels of linear molecules with nonzero electronic orbital angular momentum be labeled Λ(A′) and Λ(A″), e.g., Π(A′) and Π(A″) for Π states, etc., according to the following prescription: All series of levels in which the electronic wave function at high  J is symmetric with reflection of the spatial coordinates of the electrons in the plane of rotation will be designated Λ(A′) for all values of J, and all those for which the electronic wave function is antisymmetric with respect to reflection will be denoted Λ(A″). It is emphasized that this notation is meant to supplement, and not replace, the accepted spectroscopic e/f labeling and the parity quantum number. The utility of the Λ(A′)/Λ(A″) notation is that it is of most relevance in the mechanistic interpretation of reactive or photodissociative processes involving open-shell molecules.

Low-lying vibrational levels (v=2 and 3) of the outer well of the double-minimum E, F 1Σ+g state of molecular hydrogen are experimentally observed for the first time permitting a rotational analysis. The E, F state is excited by two-photon absorption using tunable (∼195 nm) Raman-shifted radiation of a frequency-doubled dye laser. The nonlinear absorption is detected by monitoring the subsequent photoionization of the E, F state. Homogeneous perturbations are observed between the vE=1 and vF=2 levels of the inner and outer wells, respectively. The molecular parameters for these levels (derived by a deperturbation analysis) as well as those for the unperturbed vF=3 level are compared with recent ab initio calculations and small deviations are noted. The rotational intensity distribution of the E–X (0,0) band mirrors the ground state rotational population, whereas the corresponding distributions of the E–X (1,0) and (2,0) bands as well as the F-X (4,0) band do not follow this pattern. This is interpreted in terms of vibronic coupling between the inner and outer wells of the E, F state. A comparison between the experimental rotationless intensities and those calculated from ab initio ground and excited state vibrational wave functions suggests that photoionization from the outer well is more effective than from the inner well.

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Sensitive detection of gaseous toxic compounds of environmental concern by cavity ring-down spectroscopy (CRDS) is demonstrated. In particular, CRDS is applied to the detection of nitrogen dioxide and four chlorinated aromatic volatile organic compounds. Detection limits in this feasibility study are in the parts-per-million range, but experimental improvements will enhance the sensitivity to the parts-per-billion range or better. For chlorinated aromatics, the sensitivity is found to be independent of the degree/site of chlorination. In this respect, it is superior to other laser-based methods such as laser-induced fluorescence and resonance-enhanced multiphoton ionization that are quite strongly influenced by excited-state nonradiative decay induced by the presence of chlorine substituent(s). In addition, since CRDS is self-calibrating, fairly simple to implement and perfectly general, it promises to be a universal environmental toxic gas detector.