Anthony J. McCaffery

Rotational transfer, an angular momentum model

We have re‐examined critical experiments on collision induced rotational transfer (RT) and conclude that the probability of RT is controlled by the factors that control the probability of angular momentum (AM) change. The probability of energy change seems less important in this respect. In the light of this we suggest a model for RT in which the probability of AM change is calculated directly and present a formalism for this purpose. We demonstrate that such a calculation leads to an exponential‐like fall of RT probabilities with transferred AM, a consequence of the radial dependence of the repulsive part of the intermolecular potential. Thus in this AM model, the exponential gap law has a simple physical origin. The AM model we describe may be used as the basis of an inversion routine through which it is possible to convert RT data into a probability density of the repulsive anisotropy. Through this model therefore it is possible to relate experimental RT data directly to the forces that are responsible...

A fitting law for rotational transfer rates: An angular momentum model with predictive power

We have formulated a law for state‐to‐state rotational transfer (RT) in diatomic molecules based on the angular momentum (AM) theory proposed by McCaffery et al. [J. Chem. Phys. 98, 4586 (1993)]. In this, the probability of angular momentum change in the rotor is calculated by assuming the dominant process to be the conversion of linear to angular momentum at the repulsive wall of the intermolecular potential. The result is a very simple expression containing three variable parameters, each of which has physical significance in the context of the model. Fits to known RT data are very good and suggest strongly that linear to angular momentum change is indeed the controlling process in RT. The parameters of the fit are sufficiently available to give the model predictive power. Using this formulation, RT probabilities may be calculated for an unknown system with little more than the atomic masses, bond length, and velocity distribution. We feel that this represents an important step in the development of a s...

Energy and angular momentum transfer in atom-diatom collisions from polarized laser fluorescence

We report a study of collisional reorientation of diatomic lithium molecules by rare gas atoms using high resolution circularly polarized laser fluorescence. As in the case of I2 we find that elastic collisions are very inefficient at reorienting Li2 molecules and a selection rule ΔM′ J = 0 appears to be in operation. Rotationally inelastic collisions, particularly those with argon, cause some degree of reorientation and are the result of relatively long range intermolecular interactions. Vibrational transfer features are more strongly depolarized. Optical pumping studies of oriented 1Σ g Li2 molecules have yielded a cross section for inelastic transfer of 35 ± 15 A2.

Persistence of MJ following vibrational and rotational energy transfer in molecular iodine excited by a single mode tunable dye laser

Abstract Excitation of iodine fluorescence with a single mode tunable dye laser allows extensive observation of features from vibrationally and rotationally transferred states populated through collisions. We have measured the circular polarisation of these features and observe a very high degree of polarisation following both vibrational and rotational energy transfer: thus transitions from states following Δ J ′ = 60 and Δν′ = 1, Δ J ′ = 60 are still highly polarized. This implies strong conservation of M J throughout energetic inelastic collisions.

Variation of the polarization ratio for rotationally inelastic collisions with laser selected velocity

We extend the technique of velocity selection by narrow linewidth laser excitation by measuring the polarization of emission following energy transfer as a function of selected velocity. We apply this to rotationally inelastic collisions in A1ΣuLi2–Xe and find that for most transitions, there is a noticeable decrease in the circular polarization ratio at the center of the Doppler profile. We speculate as to whether the reason for this is dynamical, geometrical, or a combination of the two. We are able to deconvolute cross sections for the transfer of orientation as a function of relative speed which can then be compared with cross sections for the transfer of population.

Complete determination of the state multipoles of rotationally resolved polarized fluorescence using a single experimental geometry

When laser radiation is used to prepare single rovibronic levels in molecules, the excited state Mj distribution is invariably polarized. In many such experiments the polarization of the excited state is ignored, which is an inadequate basis for accurate work as much valuable detail is lost. A better approach is a completely polarization resolved experiment in which the preparation, dynamics, and detection of the excited state polarization components (the state multipoles JJρKQ) are fully described. A treatment of polarized excitation in terms of the state multipoles JJρKQ is presented and consideration of excited state symmetry indicates that a common experimental geometry for linearly and circularly polarized excitation is feasible. A complete determination of the state multipoles (K=0,1,2) is shown to be possible within a single experimental geometry. It is shown that neglect of polarization phenomena can lead to ambiguities in the interpretation of some experiments.

Energy and angular momentum transfer in homonuclear diatomic molecules from polarized laser fluorescence

Polarized fluorescence studies of elastic and inelastic collisions between iodine molecules indicate that these interactions show characteristics of both strong and weak collisions. Very large changes in rotational angular momentum are accompanied by a strong memory of the original orientation. The polarization of the resonance doublets from rotational states J′ = 2 to J′ = 21 of ν′ = 16 have been measured and compared with values calculated by either neglecting or including the effects of nuclear spin. Only the latter model provides an accurate description, particularly for low J′ values, and it is clear that nuclear spin plays an important role in determining the polarization behaviour of molecular iodine in the 3Π ou + excited state. When this effect is properly included there is no depolarization which may be attributed to collisions. This, together with earlier evidence that resonance features are not depolarized at very high foreign gas pressures, leads to the proposal that in homonuclear diatomics,...

On the measurement of molecular anisotropies using laser techniques. I. Polarized laser fluorescence

The tensor density matrix formalism is used to derive expressions for the circular and linear polarization of laser‐induced fluorescence from molecules which have an anisotropic distribution in the spatial orientation of their ground state angular momentum components. The generalized anisotropic distribution is expressed as a series of state multipolar moments and it is shown that the excited state multipolar moments created therefrom by the absorption of laser radiation may be quite complex even in the absence of perturbations which cause cross relaxation. Under these circumstances, polarized laser fluorescence does not give an unambiguous measure of the ground state multipolar moments and in succeeding papers we discuss methods which do yield these quantities without ambiguity.

Laser fluorescence studies of molecular iodine

We have studied quantitatively the circular polarization of rotationally resolved features in the laser-induced fluorescence from I2 excited by the 5145 A argon laser line, and its variation with various experimental parameters such as light intensity, iodine and foreign gas pressure. Optical pumping of the ν″ = 0, J″ = 13 level of 1Σ g + has been unambiguously demonstrated and the variation of C with gas pressure has been analysed to yield cross sections for elastic and inelastic collisions which relax the oriented ground state. Experiments on the dipolar array of oriented 3Π0u + molecules at very high pressures show that the circular polarization ratio does not change under conditions of many gas kinetic collisions per lifetime. This indicates that a selection rule ΔM = 0 operates on elastic collisions of I2* with other atoms and molecules, which appears to be rigorously adhered to.

Electronic structure of ferricenium ion from absorption, MCD, and ESR studies

The absorption and MCD spectra of the ferricenium ion have been measured at 290 and 9 °K. The data have been used, together with 20 °K ESR spectra to obtain quantitative information about the ground state of this molecule. The ground state emerges as a E‴(2Eg) state into which is mixed the E″(2Eg) state via a low symmetry perturbation. Evaluation of this from ESR and MCD data yield the values δ = 1043 cm−1 at 20 °K and δ = 263 cm−1 at 290 °K. The MCD strongly confirms the assignment of the electronic absorption band at 16 200 cm−1 as 2E2g → 2E1u.