Matthew L. Costen

Orientation and alignment depolarization in OH(X Π2)+Ar/He collisions

The depolarization of OH(X (2)Pi(3/2),v=0,J=1.5-6.5,e) rotational angular momentum (RAM) in collisions with He and Ar under thermal conditions (298 K) has been studied using two-color polarization spectroscopy (PS). Orientation or alignment of the OH RAM was achieved using circularly or linearly polarized pulsed excitation, respectively, on the off-diagonal OH A (2)Sigma(+)-X (2)Pi(1,0) band. The evolution of the ground-state OH(X) RAM polarization, exclusively, was probed using an independent, linearly polarized pulse tuned to the diagonal OH A (2)Sigma(+)-X (2)Pi(0,0) band. The PS signal decay rate constant k(PS) decreases with increasing rotational quantum number for OH(X)+Ar but does not vary monotonically for OH(X)+He. The measured k(PS) equals the sum k(RET)+k(Lambda)+k(dep), where k(RET), k(Lambda), and k(dep) are the rate constants for rotational energy transfer, Lambda-doublet changing collisions, and rotationally elastic depolarization (of orientation or alignment of the OH(X) angular momentum, as specified), respectively. Values of k(dep) can be extracted from the measured k(PS) with prior knowledge of k(RET) and k(Lambda). Because k(RET) and k(Lambda) were not previously available for collisions of Ar with OH(X, v=0), we performed exact, fully quantum-mechanical scattering calculations on a new potential energy surface (PES) presented here for the first time. The raw experimental results show that k(dep) is systematically markedly higher for alignment than for orientation for OH(X)+Ar but much more weakly so for OH(X)+He. Calculated k(RET) and k(Lambda) values at 298.15 K are consistent with a substantial contribution from k(dep) for OH(X)+Ar but not for OH(X)+He. This may point to the role of attractive forces in elastic depolarization. The experimental results provide a very sensitive test of the ability of the most recent ab initio OH(X)-He PES of Lee et al. [J. Chem. Phys. 113, 5736 (2000)] to reproduce k(RET)+k(Lambda) accurately.

Vector signatures of adiabatic and diabatic dynamics in the photodissociation of ICN

Nascent Doppler profiles of CN (X 2∑+) fragments from the A band photodissociation of room temperature ICN have been measured using high-resolution transient frequency modulated absorption spectroscopy. Results for dissociation at 222 nm, 248 nm, 266 nm, and 308 nm are presented. From the Doppler profiles of multiple CN states, we determine branching ratios of the coincident atomic iodine states, and bipolar moments characterizing the CN velocity and angular momentum anisotropy. The measurements provide sensitive tests of the strengths of optical coupling to the excited states contributing to the A band continuum, and the adiabatic and diabatic dynamics leading to the observed product states. Precise velocity measurements resolve differences in the average energy of the ICN molecules leading to selected fragment channels. We find a bond energy for ICN of 26 980±100 cm−1, somewhat higher than previous literature values.

Do vectors point the way to understanding energy transfer in molecular collisions

This tutorial review examines the proposition that vector properties reveal more about the underlying potential energy surfaces controlling the inelastic exchange of energy in intermolecular collisions than conventional scalar measurements. Exciting recent experimental progress is summarized in the form of six selected cases studies. The new information that has been extracted is compared with the predictions of complementary theory. Likely future prospects and promising avenues for further progress are discussed. The treatment should appeal to all those with interests in the forces governing intermolecular interactions, especially in gas-phase collisions.

Vector correlations in the reaction O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P)

The reaction O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P) has been studied using translationally aligned oxygen atoms formed from the 355 nm polarized photodissociation of NO2. The nascent CO product was detected by laser‐induced fluorescence (LIF) with sub‐Doppler resolution in order to extract the pair correlations between the reagent and product relative velocities k and k’ and the product rotational angular momentum J’. Previous theories interpreting the Doppler profiles of photodissociation products in terms of vector correlations have been extended to the case of bimolecular reactions. The system studied was seen to yield a close to isotropic distribution of product velocities k’ about the k direction, and a rotational alignment of J’ with k close to zero. The CO molecule departs with its rotational angular momentum vector J’ aligned preferentially perpendicular to the product relative velocity k’, hence exhibiting a negative k’, J’ correlation. Further insight has been gained on these results by quasiclassical ...

Measurement of orientation and alignment moment relaxation by polarization spectroscopy: Theory and experiment

A diagrammatic perturbation theory description of one-color polarization spectroscopy (PS) is developed which emphasizes the significance of orientation and alignment tensor moments of the rotational angular momentum, and their collisional evolution. The influences of Doppler motion, velocity-changing collisions, decay of population, orientation and alignment, and nuclear hyperfine depolarization on the calculated PS signal are discussed. Illustrative simulations are presented of the evolution of the PS signal as a function of pump-probe laser delay. These are generated by a Monte Carlo integration of the derived equations for the signal electric field over typical experimental pump and probe laser temporal profiles and velocity distributions for a commonly studied system, the OH A 2Sigma+ -X 2Pi (0,0) band. These predictions are compared with a preliminary set of results obtained in an experimental apparatus designed for one-color polarization spectroscopy using independent pump and probe lasers. Measurements are presented using linearly polarized pump light on the Q1(2.5) transition of the OH A 2Sigma+ -X 2Pi (0,0) band with He as the collision partner. The decay of the experimental PS pump-probe signal is discussed with reference to inelastic collisional population transfer rates in the literature. It is concluded that the collisional depolarization of rotational alignment is rapid, with a rate approximately twice that of population transfer. This is consistent with previous measurements in atmospheric pressure flames. PS is shown to be a viable novel spectroscopic method for determining rotational angular momentum orientation and alignment relaxation rates, which are valuable quantities because they are sensitive probes of the forces involved in inelastic collisions.

Relationship between bipolar moments and molecule-frame polarization parameters in Doppler photofragment spectroscopy

In terms of the molecular-frame polarization parameters aq(k)(p), an equation is derived that describes the shape of a photofragment Doppler profile as a function of the three angles Γ, Δ, and Φ that specify the photolysis and probe laser polarizations about the detection axis. This expression is specialized to linearly polarized photolysis and probe laser beams. For the particular value of the angle between the probe laser polarization and the detection axis, Δ=π/2, this equation can be reduced to the form of well-known laboratory-frame expressions that use the bipolar moment formalism introduced by Dixon. Comparison of these forms shows the equivalence of the two formalisms and gives the relationships between the bipolar moments βQK(k1k2) and the molecule-frame aq(k)(p) parameters. We show that linear combinations of the bipolar moments completely describe photofragment polarization in the molecular frame and possess distinct quantum mechanical significance. In particular, it is shown that the coherent ...

Orientation and alignment moments in two-color polarization spectroscopy

A theoretical analysis of two-color polarization spectroscopy (TCPS) is presented as an extension of a previous analysis of one-color PS. Three commonly used schemes in which pump and probe transitions share a common level are considered. Diagrammatic techniques are used to isolate the photon interaction sequences that can contribute to the signal. A perturbation-theory analysis expressing the signal in terms of spherical tensor moments is applied. The analysis emphasises the significance of orientation and alignment tensor moments of rotational angular momentum and their collisional evolution. The assumed context is transitions between single rotational states of gas-phase molecules that subsequently suffer discrete collisions. The time scale of the measurements is assumed to be long relative to the periods of molecular motion, as would typically be the case for signals excited by nanosecond-pulsed lasers from samples at moderate pressures. The Doppler motion of the probed species is included, as is an analytical solution to the integration over the Maxwell-Boltzmann distribution of velocities. The effects of nuclear hyperfine depolarization and velocity-changing collisions are discussed. It is shown that when pump- and probe-laser pulses are separated in time, TCPS creates and probes either orientation or alignment of rotational angular momentum in the common level shared by pump and probe transitions. Example simulations of one- and two-color polarization spectroscopies are included to demonstrate the resulting simplification of the measured signal using TCPS. TCPS is therefore a viable spectroscopic technique for the determination of rotational angular momentum orientation and alignment relaxation rates in molecular gases, of interest because they are sensitive probes of inelastic collisions.

Effect of collisions on one-color polarization spectroscopy of OH A 2Σ+–X 2Π

The effect of collisions on the magnitude of polarization spectroscopy (PS) signals from the OH radical on the A 2Σ+–X 2Π (0,0) band has been studied. OH was produced by the 266-nm photolysis of H2O2 and detected by one-color degenerate PS, using ≈5-ns pulses from a Nd:YAG pumped dye laser. Spectra with both circular and linear pump polarizations are reported, together with signal dependence on OH number density and pump pulse fluence. The relative line intensities in the spectra and measured square dependence on OH number density are consistent with a description of PS as a variant of four-wave mixing spectroscopy. The pump pulse fluence dependence is fitted well by literature saturation curves. The collisional dependence of the PS signal was investigated by adding increasing pressures of He, Ar, or N2 collider gases for fixed overlapping pump and probe pulses. The principal finding is the very rapid loss of the PS signal with increasing collider pressure. The resulting phenomenological rate constants ar...

Rotational angular momentum polarization: The influence of stray magnetic fields

We show that weak residual magnetic fields can significantly affect the preparation and measurement of molecular rotational angular momentum alignment in a typical gas-phase stereodynamics apparatus. Specifically, polarization spectroscopy, a third-order nonlinear spectroscopic technique, is used to prepare and probe the collisional and noncollisional losses of rotational angular momentum alignment of OH X (2)Pi. Residual magnetic fields of the order of the geomagnetic field are shown to have a significant effect on the prepared polarization on a submicrosecond timescale. This can be expected to be a significant effect for many gas-phase free radicals, such as those of interest in combustion, atmospheric chemistry, and the burgeoning field of cold molecules. We demonstrate a simple experimental remedy for this problem.

Collisional depolarisation of rotational angular momentum: influence of the potential energy surface on the collision dynamics?

We review recent progress in the loss or transfer of the polarisation of angular momentum in collisions of small free radicals with a thermal bath of closed-shell partners. Our primary theme is the connection between the observed behaviour and the nature of the underlying potential energy surfaces (PESs). We consider the systems NO(A2Σ+) + He and Ar; OH(A2Σ+) + He and Ar; OH(X2Π) + He, Ar and Xe; NO(X2Π) + Ar and CN(A2Π) + Ar, which vary both kinematically and in the strength of any attractive interaction. They are chosen because reliable theoretical PESs have been predicted; QS or QCT scattering calculations have been carried out; and they have been studied by recently developed experimental methods sensitive to polarisation. The efficiency of elastic depolarisation depends intimately on the competition with other, inelastic outcomes. It is generally found to be inefficient for systems dominated by impulsive forces, which promote instead changes of state. This is exacerbated by kinematic effects, in particular for the light He collision partner. Moderately attractive PESs support more efficient elastic depolarisation for low rotational levels, but this drops off rapidly with increasing rotation. Persistent elastic depolarisation across all rotational levels is a feature of deeply attractive, strongly anisotropic PESs. The 2Σ+-rare gas systems are characterised by a single controlling PES and are well described by a spin-spectator model. The 2Π-rare gas systems require two PESs, expressible as 2 A′ and 2 A′′ adiabatic surfaces or their diabatic sum and difference, V sum and V diff. Propensities rules reflecting the symmetries of these surfaces help to explain much of the detailed behaviour.