Jm Bayley

Molecular predissociation dynamics revealed through multiphoton ionisation spectroscopy. II. The C̄′1A′1 state of NH3 and ND3

Abstract Several vibronic levels of the C′ 1 A′ 1 state of NH 3 and ND 3 have been further investigated via three-photon absorption (four-photon ionisation) spectroscopy. Careful comparisons between the experimentally observed 3 + 1 multiphoton ionisation spectra and spectra predicted via use of the appropriate symmetric-top three-photon rotational linestrength theory reveal significant level-dependent differences for all C′-state vibronic levels with ν′ 2 ⩾ 3. Predissociation of the excited C′ state, brought about by non-adiabatic coupling to high levels of the lower-lying Ā 1 A″ ??? state is advanced as the likely cause of these discrepancies. Attempts to extend this study to an investigation of the intervening B t E″-state predissociation dynamics were handicapped by spectroscopic perturbations attributable to Coriolis interactions within the B state. Nevertheless it has proved possible to offer a consistent rationale for the observed trends in predissociation behaviour displayed not only by the various B-state vibronic levels but also by all documented higher electronic states of ammonia in terms of similar non-adiabatic coupling to levels of the A state.

Quantum state-selected photodissociation dynamics in H2O and D2O

Two photon excitation, tunable near 248·5 nm, has been used to dissociate H2O/D2O via the [Ctilde] 1 B 1 and [Btilde] 1 A 1 states. Rotationally resolved OH/OD(A 2Σ+) photofragment excitation spectra are reported, following excitation to predissociated levels of [Ctilde] 1 B 1. Rotational resolution of the OH/OD(A 2Σ+ → X 2Π) fluorescence, generated from individual J′ K a K c levels of [Ctilde] 1 B 1, allows full quantum state selection in both the entry and exit channels. The OH/OD(A 2Σ+) fragment is formed rotationally hot as a result of the large change in bond angle in going from [Xtilde] 1 A 1 (or [Ctilde] 1 B 1) to the linear dissociative [Btilde] 1 A 1 surface. Product alignment measurements allow assignment of the two photon continuum absorption to [Btilde] 1 A 1: a-axis rotation in [Ctilde] 1 B 1 destroys product alignment from these levels. Electronic branching from ⪷B 1 A 1 to A 1 B 1 (and/or [Xtilde] 1 A 1) during the dissociation forms ground state OH/OD(X 2Π). Relative branching ratios are o...

The rotational structure of three-photon resonances of polyatomic molecules

A general formulation of the selection rules and line strengths of three-photon excitation spectra is presented for molecules of arbitrary symmetry. For symmetric-top molecules the paper extends the discussion by Nieman in the context of rovibronic transitions of NH3. For asymmetric-top molecules it is shown that the rotational line strengths can be sensitive to quantum-mechanical interference between contributions from the various components of the three-photon vibronic transition tensor. This transition tensor is discussed within a perturbation-theory framework in terms of several models of intermediate-state participation.

Quantum-state-selected photodissociation of H2O(C̃1 B1)

Abstract Tunable, Line-narrowed KrF laser radiation has been used to populate individual rovibronic levels in the C 1 B 1 state of H 2 O, via two-photon absorption at pressures ≥5 mTorr. Predissociation into OH(A 2 v + ) ν . =0 , detected by scanning its rotationally resolved fluorescence, only occurs for levels with ( J a 2 ) ‡ 0. The homogeneous predissociation channel must generate alternative products. Thus the branching into the accessible electronic dissociation channels is influenced by rotation of the parent molecule. The rotational distribution in the OH(A) fragments peaks at lower values, N ′ pk = 14, than observed using single-photon excitation at 123.6 nm, where N ′ pk = 20.

Molecular predissociation dynamics revealed through multiphoton ionisation spectroscopy. III. New 1A2 and 1B1 rydberg states in H2S D2S

Abstract Two new Rydberg series in H2S and D2S habe been characterized as three-photon resonances in four-photon ionisation spectrometry. Members of the two series exhibit sufficient rotational structure to permit characterisation of their electronic symmetries as, respectively. A2 and B1. The first Rydberg series is identified with the (one-photon forbidden) excitations npb2 ← 2b1 (1A2 ← X 1A1) on the basis of the observed quantum defects. Geometry considerations indicate that second series, of 1B1 states, also arises as a result of electronic promotion from the highest occupied 2b1 orbital. The acceptor (a1) Rydberg orbitals possess substanial s character, but the polarisation dependence of the various 1B1- X 1A1 three-photon transition probabilities their hybrid I character, d (and quite possibly p) functions contribute also. The results provide further clear demonstration of the way in which multiphoton excitations, and MPI techniques in particular, can complement conventional one-photon absorption techniques. Members of both series are predissociated. Vibronic predissociation rates are found generally to decline with increasing n and to be slower in D2S than in H2S. The lowest (n = 4) member of the 1A2 series in both isotopic species appears immune from rovibronic predissociation but higher members show evidence of a ( J a2)-dependent rotationally-induced predissociation, the severity of which increases dramatically with n. This observation is explained in terms of electronic-rotational Coriolis coupling to a dissociated 1B2 state is presumed to be responsible for the observed ( J b2)-dependent heterogeneous predissociation of the 1B1 (n = 6) member in H2S. However, the dominant rotationally-induced predissociation mechanism that affects the counterpart in D2S scales with ( J a2). Wherever possible comparisons are drawn with the known spectroscopy and photophysics of the isovalent molecules H2O and D2O.

A new electronic state of water revealed by gas phase multiphoton ionization spectroscopy

A new electronic state of water has been identified by three photon absorption (four photon ionization) spectroscopy. Rotational analysis of the bands associated with this new D′ state in both H2O (ν0=84 434±2 cm−1) and D2O (ν0=84 646±2 cm−1) show it to be of B1 symmetry. The nature of the electronic promotion giving rise to this D′ state is considered.