Article pubs.acs.org/JPCA
Rotational Alignment of NO (A2Σ+) from Collisions with Ne Jeffrey D. Steill,† Jeffrey J. Kay,† Grant Paterson,‡ Thomas R. Sharples,‡ Jacek Kłos,§ Matthew L. Costen,*,‡ Kevin E. Strecker,† Kenneth G. McKendrick,‡ M. H. Alexander,*,§ and David W. Chandler*,† †
Sandia National Laboratories, Livermore, California 94550, United States School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K. § Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States ‡
ABSTRACT: We report the direct angle-resolved measurement of collision-induced alignment of short-lived electronically excited molecules using crossed atomic and molecular beams. Utilizing velocity-mapped ion imaging, we measure the alignment of NO in its first electronically excited state (A2Σ+) following single collisions with Ne atoms. We prepare A2Σ+ (v = 0, N = 0, j = 0.5) and by comparing images obtained using orthogonal linear probe laser polarizations, we experimentally determine the degree of alignment induced by collisional rotational excitation for the final rotational states N′ = 4, 5, 7, and 9. The experimental results are compared to theoretical predictions using both a simple classical hard-shell model and quantum scattering calculations on an ab initio potential energy surface (PES). The experimental results show overall trends in the scattering-angle dependent polarization sensitivity that are accounted for by the simple classical model, but structure in the scattering-angle dependence that is not. The quantum scattering calculations qualitatively reproduce this structure, and we demonstrate that the experimental measurements have the sensitivity to critique the best available potential surfaces. This sensitivity to the PES is in contrast to that predicted for ground-state NO(X) alignment.
1. INTRODUCTION Studies of the vector properties of gas-phase chemical processes have made a significant contribution to our understanding of bimolecular reactions,1−3 unimolecular dissociations4−6 and nonreactive collisions,7−9 the focus of this article. In particular, comparison of vector properties from experiment and rigorous dynamical calculations have been shown to be an excellent test of the accuracy of ab initio potential energy surfaces (PESs), and hence of the electronic structure methods used to determine these. The spatial characteristics of a collision process may be expressed in terms of vector correlations, average quantities describing the angular distributions of the velocities and angular momenta of the colliding species both before and after the collision. The angular distribution of the velocity of a scattered molecule with respect to the initial relative velocity of the scattering partners is the differential cross section (DCS), while anisotropy in the angular momentum distribution is referred to as angular momentum polarization.10 Measurements of these correlations generally follow one of two experimental strategies. One set of experiments focus on single collision conditions achieved through the use of crossed molecular beams of colliders within a high vacuum environment. The second involves the use of polarized light to create a sample of molecules with an anisotropic distribution of angular momenta within a bath of collider gas, the aim being to measure the rate of loss of this initially created angular momentum polarization © 2013 American Chemical Society
under multiple collisions at thermal energies. These techniques may be considered complementary, as crossed molecular beams typically probe collisions at significantly higher energies than the thermal bath studies, and hence sample shorter-range, more repulsive regions of the PES than the collisions at thermal energies, which conversely may be expected to yield more information about the long-range, attractive regions of the PES. The collisions of the diatomic radicals NO, OH, and CN with rare gas partners have been the focus of a sustained effort in which experimentally measured vector properties have been compared to the results of detailed theoretical treatments of these processes.11−51 A major motivation for this work has been the amenability of these systems to modeling using rigorous theoretical methods, and the consequent insight that may be gained into the influence on the collision dynamics of the complex open-shell electronic structure of these radicals. One of the major achievements of this extensive work has been the comparison of results across systems, which has allowed the different factors controlling the dynamics of these collisions, such as the effects of electrostatic interactions and the collision kinematics, to be disentangled. In particular, by comparing the collisions of the same molecule in different electronic states Special Issue: Stereodynamics Symposium Received: February 26, 2013 Revised: April 23, 2013 Published: April 23, 2013 8163
dx.doi.org/10.1021/jp402019s | J. Phys. Chem. A 2013, 117, 8163−8174
The Journal of Physical Chemistry A
Article
Figure 1. Nitric oxide photoexcitation and detection scheme and quantitative Newton diagram for the NO(A) + Ne scattering experiments. As shown in panel a, rotationally cold NO(X2Π) is initially pumped to the rotational ground state of the NO(A2Σ+) state, and after collisional excitation the rotational state and alignment of scattered NO(A2Σ+) is probed by a polarized laser-induced transition to NO(E2Σ+). In panel b, the lab-frame velocities of NO and Ne are represented by vNO and vNe, respectively, and the lab-frame center-of-mass velocity vector is represented by vcom. The relative velocity vector of NO and Ne is represented by vrel, and 0° corresponds to forward-scattering of NO. The outermost ring represents elastic scattering of the NO molecule; transfer of translational energy into NO(A) rotational energy results in the inelastic scattering rings represented by dotted lines.
developed in which NO(A2Σ+, v = 0, N = 0, j = 1/2) is generated from a beam of ground state NO molecules through laser excitation within the interaction region of a crossed molecular beam apparatus.16,17 Rotational state-selective resonance-enhanced multiphoton ionization (REMPI) detection of inelastically scattered molecules combined with velocity map ion imaging (VMI)52 has allowed the DCSs to be measured for a number of product rotational quantum states of NO(A) generated in collisions with both He and Ar. While quantum scattering (QS) calculations of the DCS on recently calculated PESs for these systems48 showed good agreement with experiment for collisions of NO with He, there were clear differences between experiment and theory apparent for the NO/Ar case, perhaps reflecting the difficulties associated with the accurate modeling of the PES of this system due to the Rydberg-like character of the NO(A) state. Here, this work is extended to the measurement of the angular momentum polarization of NO(A) molecules scattered in collisions with Ne. In contrast to the aforementioned collisional depolarization studies of NO + rare gas systems, where the behavior of the NO(X) and NO(A) molecules have been found to be broadly similar, this combined experimental and theoretical study reveals significant differences in the behavior of ground state and electronically excited NO which may be attributed to the very different topographies of their PESs.
with the same rare gas partner, the influence of changes to the PESs involved in the collisions and their interactions may be examined while the kinematics of the collision process remain unchanged. As an example, analysis of experiments measuring the depolarization rate constants for NO(X2Π1/2)42 and NO(A2Σ+)29,32,33 in collision with Ar, making use of theoretical predictions of the relative contributions to the depolarization rate from inelastic and elastic collisions, has demonstrated a similarity for the two systems. In both cases the scattering is dominated by inelastic contributions, but while the elastic contribution to the scattering remains relatively constant as a function of the initially excited rotational state of the molecule, the inelastic component shows a marked drop with increasing rotational excitation. Quasi-classical trajectory (QCT) calculations32 for the NO(A)/Ar system suggest that the shallow attractive well for this system provides a mechanism for inelastic depolarization for collisions of molecules with low N, but that this process is inefficient for high N, and it would appear that the shallow well on the NO(X2Π)/Ar potential has a similar effect, despite the different positions of the global minima for these two surfaces.44,48 The similarities in behavior for these two systems is in sharp contrast to the significantly larger elastic contribution to the depolarization rate observed for the OH(A2Σ+)/Ar system compared to that for OH(X2Π1/2)/Ar.30,38 The OH(A)/Ar potential is much more anisotropic than that for the ground state, possessing a deep potential well for approach toward the hydrogen atom that allows efficient elastic depolarization even at relatively high N, with QCT calculations providing strong evidence of the importance of trajectories in which the H atom is caught and swung around by the Ar atom during the collision.27 Largely because of the fact that optical pumping is a fundamental feature of the collisional depolarization experiments described above, these techniques have led the way in comparing the behavior of different electronic states of these systems and there have been few direct measurements of collision-induced vector correlations for electronically excited molecules.36 Very recently, however, experiments have been
2. METHODS Collisions of electronically excited NO (A2Σ+) molecules with Ne atoms were studied in a crossed-beam apparatus equipped with velocity map ion imaging (VMI) detection. The crossedbeam apparatus has been described previously.53 Two pulsed supersonic beams, one containing 5% NO in Ar and the other containing neat Ne, crossed at 90° in the center of an ultrahigh vacuum chamber. Three colinear laser beams propagated through a 400 μm diameter aperture and bisected the NO and Ne beams in the scattering plane. The photoexcitation and detection scheme has been described previously and is illustrated in panel (a) of Figure 1.16 Briefly, the first laser 8164
dx.doi.org/10.1021/jp402019s | J. Phys. Chem. A 2013, 117, 8163−8174
The Journal of Physical Chemistry A
Article
excited NO molecules from the v = 0, J = 0.5 level of the X2Π1/2 ground state to the v = 0, N = 0, J = 0.5 level of the A2Σ+ state via the Q1(0.5) or P1(1.5) transition at ∼226 nm, which prepared a population of NO (A2Σ+) molecules with no initial alignment. The molecules were then given 400 ns (approximately twice the radiative lifetime of the A2Σ+ state54) to collide, after which the products of rotationally inelastic collisions were probed using a [1 + 1′] REMPI detection scheme. In the first step of the detection scheme, a Nd:YAG-pumped dye laser (Rhodamine B dye, ∼600 nm, ∼0.03 cm−1 resolution) excited NO(A) molecules to the E2Σ+ state via the R(N′) branch of the E2Σ+ ← A2Σ+ (0,0) band. Because the bandwidth of the laser is greater than the spin-rotation splitting of the A2Σ+ state rotational levels considered here, molecules in both of the J = N ± 1/2 spin-rotation sublevels, which are each substantially populated by the collision, were excited simultaneously. However, the probe laser bandwidth is less than the Doppler width of the transitions, and the (E2Σ+ ← A2Σ+) probe laser wavelength was therefore repeatedly scanned over the transition Doppler profile during the course of data acquisition, to ensure unbiased detection of all velocity components of the scattering distribution. In the second step of the detection scheme, molecules in the E2Σ+ state were ionized by absorption of a 532 nm photon from a Nd:YAG laser pulse, delayed by 20 ns with respect to the ∼600 nm dye laser pulse. The resulting NO+ ions were velocity-mapped, perpendicular to the scattering plane, onto a position-sensitive imaging detector. The pump laser was of sufficient intensity to both saturate the A←X transition and to generate substantial ion signal from [1 + 1] REMPI via this state. This signal was minimized in the scattering image by utilizing the 400 ns delay of the scattered signal relative to the prompt ionization signal. The voltages applied to the detector microchannel plates were gated to collect only the ions produced in the detection step. A computer-controlled CCD camera acquired images of the detector. The data acquisition routine eliminated most unwanted background signals by subtracting an image acquired without the Ne molecular beam, from an image acquired with both the NO and Ne molecular beams present. Sensitivity to the rotational alignment of scattered NO(A) came from the linear polarization of the ∼600 nm probe light effecting the E2Σ+ ← A2Σ+ transition. The pulse energy (
