Observation of correlated excitations in bimolecular collisions

Abstract

Although collisions between atoms and molecules are largely understood, collisions between two molecules have proven much harder to study. In both experiment and theory, our ability to determine quantum-state-resolved bimolecular cross-sections lags behind their atom-molecule counterparts by decades. For many bimolecular systems, even rules of thumb-much less intuitive understanding-of scattering cross sections are lacking. Here, we report the measurement of state-to-state differential cross sections on the collision of state-selected and velocity-controlled nitric oxide (NO) radicals and oxygen (O) molecules. Using velocity map imaging of the scattered NO radicals, the full product-pair correlations of rotational excitation that occurs in both collision partners from individual encounters are revealed. The correlated cross sections show surprisingly good agreement with quantum scattering calculations using ab initio NO-O potential energy surfaces. The observations show that the well-known energy-gap law that governs atom-molecule collisions does not generally apply to bimolecular excitation processes, and reveal a propensity rule for the vector correlation of product angular momenta.

Direct observation of product-pair correlations in rotationally inelastic collisions of ND3 with D2

“…In most of the images, up to three concentric rings can be seen which correspond to different rotational transitions of D 2 during the collision. The most intense ring, which coincides with the masked beam spot, 32 corresponds to the elastic D 2 channels, i.e., in these collisions the D 2 molecules remain in their initial quantum state. The three elastic j = 0 -0, j = 1 -1 and j = 2 -2 channels all have the same ring radius and are superimposed in the image.…”

“…1, and that has been described in detail before. 32,33 A molecular beam of ND 3 seeded in different carrier gases (Ar, Kr) at a typical pressure of 1 bar is formed using a Nijmegen Pulsed Valve (NPV), 51 and loaded into a 2.6 meter long Stark decelerator. After exiting the decelerator, the packet is scattered with a pulsed beam of neat D 2 at an intersection angle of 901.…”

“…We then directly compare the radial and angular distributions of the scattering intensity in both the experimental and simulated images. The details about the simulation and analysis methods are described elsewhere, 32,33 and will not be discussed here. The advantage of this procedure is that we do not have to rely on extensive fitting procedures to extract cross sections from our images, which can be prone to error and misinterpretation of the results.…”

Correlated energy transfer in rotationally and spin–orbit inelastic collisions of NO(X2Π1/2, j = 1/2f) with O2(X3Σg−)

“…As an extension of our previous experiment, 13 we study the dynamics at a higher collision energy of 480 cm À1 such that excitations of NO to higher rotational states can be probed. Furthermore, we study rotational excitation of NO within the X 2 P 1/2 (F 1 ) manifold, but we also study spin-orbit changing transitions to the X 2 P 3/2 (F 2 ) manifold that is located 123 cm À1 above the F 1 manifold.…”

“…It is, however, extremely challenging to experimentally resolve the individual circles, as the experimental blurring associated with, for instance, the collision energy spread and the crushing of spheres on the detector plane is in most cases much larger than the inherent spacing between the circles. [4][5][6][7][8][9][10][11][12] Recently, we reported the first observation of rotational product pair correlations in NO-O 2 inelastic collisions at a collision energy of 160 cm À1 , 13 by using a Stark decelerator to produce reagent beams of NO with very narrow velocity spread. 14 Here, we present a joint experimental and theoretical study of state-to-state resolved RET in the inelastic collisions of NO(X 2…”