Velocity Map Imaging of NO (A) from Photodissociation of the Ar-NO Complex

Abstract

We have photodissociated the Ar-NO complex from 4 to 337 cm^-1 above the energy required to form NO (A ²∑⁺). In the experiment, the NO (A) product was nonresonantly ionized, and velocity map ion images were recorded at a resolution sufficient to identify individual rotational states. From the data, we determined the minimum energy required to form the NO (A) product to be 44294.3 ± 2.2 cm^-1. We also determined the ground state dissociation energy for Ar-NO to be 95.4 ± 2.2 cm^-1 and the excited-state dissociation energy to be 52 ± 2 cm¹. The ground-state dissociation energy agrees with experimental results and a recent three-dimensional ab initio potential energy surface; however, the excited-state dissociation energy is larger than predicted by theoretical surfaces. We also determined the NO (A) rotational state distribution, which was bimodal due to the rotational rainbow effect and showed significantly less contribution from hotbands compared with previous experiments. Our data also allowed for measurement of the angular anisotropy parameter, β, over the range of NO (A) rotational states. We observed that for the lowest rotational levels, β changes from ∼ -0.3 at low excitation energy to ∼+0.4 at high excitation energy, which is consistent with previous work. Furthermore, we report β values for the highest rotational states formed at each excitation energy. Our experiment demonstrated that for a given excitation energy, the high rotational states consistently exhibited a more negative β compared with the low rotational levels. Qualitatively, this behavior can be attributed to excitation to regions of the excited state resembling a skewed T-shape that then dissociate to high NO (A) rotational levels with β ≤ 0.

1

Left frames show the velocity map ion images of NO obtained from photodissociation of the Ar-NO complex at 44316 cm^–1 photon energy (A) and 44630 cm^–1 photon energy (B). The arrow in frame A shows the orientation of the photolysis and probe laser polarizations used for all imaging experiments. The frame to the right of each image gives the corresponding CM translational energy distribution (points) along with the fit (red curve) discussed in the text.

2

Photon energy versus square of the maximum observed NO radius from the ion images (filled circles). The blue line gives a linear fit of the data with a y-intercept of 44293 cm^–1, which corresponds to E _appearance.

3

Frames A and C are the anisotropy parameter, β, values for each NO (A) rotational level, determined from the ion images given in Figure . Frames B and D are the NO (A) rotational state distributions determined by fitting the P(E _T) in Figure .

4

Observed rotational quantum number versus $\sqrt{E_{\mathrm{avail}}}$ . The blue and red points correspond to the rotational quantum numbers for the peaks in the distributions with the blue squares corresponding to the higher n peak and the red circles corresponding to the lower peak.

5

Anisotropy parameter, β, versus available energy for the n = 0 level of NO (A).