Rn Dixon

State‐selected photodissociation dynamics: Complete characterization of the OH fragment ejected by the HONO Ã state

Trans‐HONO is optically prepared in specific −N=O stretching vibrational levels (2n, n=1,2,3) of the A 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 in...

Primary product channels in the photodissociation of methane at 121.6 nm

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of CH4(CD4) at 121.6 nm. Contrary to the previous consensus view, we find simple C–H bond fission to be the dominant primary process following excitation at this wavelength. The resulting CH3 fragments are formed with very high levels of internal excitation: Some (∼25%) possess so much internal energy that they must undergo subsequent unimolecular decay. The present experiments do not provide a unique determination of the products of this secondary decay process, but statistical arguments presented herein suggest that they will be predominantly CH and H2 fragments. Similar considerations point to a significant role for the direct three body process yielding the same products H+H2+CH. This overall pattern of energy disposal can be rationalized by assuming that most of the initially prepared CH4(A 1T2) molecules undergo rapid internal conversion (promoted by the Jahn–Teller distortion of this excit...

Dissociation dynamics of H2O(D2O) following photoexcitation at the Lyman‐α wavelength (121.6 nm)

The technique of H(D) atom photofragment translation spectroscopy has been used to investigate the collision free photodissociation of jet cooled H2O(D2O) molecules following excitation to their B(1A1) excited state at 121.6 nm. The resolution of the total kinetic energy release spectrum obtained with this technique, allows assignment of the eigenvalues for the individual rotational quantum states and an estimation of the respective quantum state population distributions for the nascent OH(X 2Π) and OH(A 2Σ+) photofragments (and their deuterated analogs). This provides us the first experimental observations of high angular momentum states of OD(X). Analysis of the quantum state population distribution show both the ground (X 2Π) and electronically excited (A 2Σ+) OH(OD) fragments to be formed with little vibrational excitation but with highly inverted rotational distributions. Spectral simulation enables estimation of relative branching ratios for these two dissociation channels, and for the three‐body f...

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...

Predissociation dynamics of Ã-state ammonia probed by two-photon excitation spectroscopy

Abstract Electronically excited ND3(A¯) molecules have been prepared by laser two-photon excitation on theA¯1A″2—X¯1′1 transition and monitored via their resulting short-lived emission. The earlier observation of Douglas that ND3(A¯) molecules carrying one quantum of out-of-plane bending vibration ν′2 are least susceptible to predissociation, is confirmed. ND3(A¯) predissociation rates are found to be both vibronic and rovibronic level dependent. Both observations may be understood by considering the likely form of the potential energy surface for ND3(A¯) molecules in the region of the D2N—D dissociation coordinate. At short D2N—D separations this surface exhibits a barrier. The presence of a conical intersection (involving the ND3 ground state surface) further out along the dissociation coordinate has a crucial influence on the magnitude of this barrier. The envisaged form of theA¯-state potential energy surface also provides a qualitative rationale for all previous experimental findings concerning electronic branching ratios and energy disposal amongst the primary photofragments arising in the photodissociation ofA¯-state ammonia.

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.

The B̃ 1E‘ state of ammonia: Sub‐Doppler spectroscopy at vacuum ultraviolet energies

Spectra of the 2n0 progression in the B 1E‘–X 1A’1 transition in NH3 and ND3 have been recorded at an effective resolution down to 0.008 cm−1 at the VUV equivalent energy. Line shape analysis reveals a predissociation lifetime of 0.25(2) ns for ND3 and 6.1(7) ps for NH3 independent of rotational state and v2. Measurement of the magnetic sensitivity of low J lines gave an electronic g value. Rotational analyses of the 2n0 bands with n up to 6 for ND3 and 8 in NH3 are presented and indicate some small vibrational perturbations. The observed difference between the Coriolis coupling constant ζ derived from the rotational structure and the g value indicates a mild Jahn–Teller effect.

The influence of parent rotation on the dissociation dynamics of the à 1 A″2 state of ammonia

Model calculations are presented to account for the observed rotational dependence of the dissociation of NH3 from the lower vibrational levels of the A 1 A″2 state. Centrifugal modification of, an...

State‐selected photodissociation dynamics of HONO(Ã 1A‘): Characterization of the NO fragment

Trans‐HONO has been photodissociated through the 220 band of the A1A‘–X 1A’ system at 355 nm. The energy disposal in the NO fragment, and vector correlations between its motions, have been characterized using polarized one‐photon excited laser induced fluorescence. The rotational distribution, rotational alignment, degree of electron alignment, spin–orbit branching ratio, approximate vibrational distribution, and energy partitioning are determined. The rotational distributions in each of v(NO)=3,2 and 1 are highly inverted and approximately Gaussian in quantum number. The peak J decreases with increasing v to give a fairly narrow overall internal energy distribution. The disposal of the available energy into all NO and OH motions is 60% and 40%, respectively, and is also 40% into internal motions, mainly of NO, and 60% into translational recoil. The NO rotation shows a positive alignment. All the vector correlations are consistent with planar fragmentation dynamics, but with a finite dissociation lifeti...