High resolution photoionisation spectroscopy of vibrationally excited Ar·NO

Three-dimensional potential energy surfaces of ArNO (X̃ 2Π)

Until now, the potential energy surfaces (PESs) of the ArNO complex found in the literature were two-dimensional, with the NO interatomic distance being fixed. In this work, we present the first accurate three-dimensional ground state X̃ ²Π PESs (both A' and A″) of ArNO computed at the CCSD(T)/CBS level of theory. The equilibrium geometries and the well depths (D_e) are compared to several other electronic structure methods. We found that using the multireference method, MRCI-F12 makes the surfaces much shallower (by 25%) and the depth of the surfaces does not agree with experimental data. The explicitly correlated coupled-cluster method underestimates the well depth as well. Analytic representations for both A' and A″ surfaces were fit to 4380 ab initio points to within 2.71 cm^-1. A three-dimensional Numerov propagator method in Delves coordinates is used to compute the bound state spectrum up to J_tot = 6.5. The recommended dissociation energies are D₀ = 97.2 cm^-1 for the adiabatic ground state and D_e = 133.7 (128.1) cm^-1 for A' (A″).

Complex rovibrational dynamics of the Ar·NO+ complex

Rotational-vibrational states of the Ar·NO⁺ cationic complex are computed, below, above, and well above the complex's first dissociation energy, using variational nuclear motion and close-coupling scattering computations. The HSLH potential energy surface used in this study (J. Chem. Phys., 2011, 135, 044312) is characterized by a first dissociation energy of D₀ = 887.0 cm^-1 and supports 200 bound vibrational states. The bound-state vibrational energies and the corresponding wave functions allow the interpretation of the scarcely available experimental results about the intermonomer vibrational motion of the complex. A very large number of long-lived quasibound combination states of the three vibrational modes, exhibiting a very similar energy-level structure as that of the bound states, are found embedded in the continuum. Additional short-lived resonance states are also identified and their properties are analyzed.

The Ã-state dissociation continuum of NO-Ar and its near infrared spectrum

After preparing NO-Ar in a vibrational state correlating with the first overtone vibration in NO, we recorded its hot band UV spectrum by monitoring simultaneously the intensity in the NO(+) and the NO(+)-Ar ion channels. In this way, the bound as well as the continuous part of the electronic Ã←X̃ spectrum are observed directly. Below the dissociation threshold, the intensity is found exclusively in the NO(+)-Ar ion channel while above it is found in the NO fragment ion channel. We observe simultaneously intensity in both ion channels only for a very narrow frequency range near the dissociation threshold. Structures in the dissociation spectrum correlate well with the thresholds for production of NO(A) in different rotational states. At frequencies well above the dissociation threshold, NO-Ar is detected efficiently as a NO fragment. This fact has been exploited to record the near IR spectrum of NO-Ar with significantly increased sensitivity. The dissociation detected spectra are essentially identical to our previous constant photon energy sum (CONPHOENERS) scans [B. Wen, Y. Kim, H. Meyer, J. Kłos, and M. H. Alexander, J. Phys. Chem. A 112, 9483 (2008)]. Several hot band spectra have been remeasured with improved sensitivity enabling a comprehensive analysis yielding for the first time spectroscopic constants for levels associated with the potential surfaces of NO-Ar correlating with NO(v(NO) = 0 and 2). Since many NO-X complexes do not have a strong bound Ã-state spectrum, although they do have a Ã-state dissociation continuum, there is the possibility to record their near IR spectra by employing dissociation detection.

The dissociation of NO-Ar(A) from around threshold to 200 cm(-1) above threshold

We report an investigation of the dissociation of A state NO-Ar at energies from 23 cm(-1) below the dissociation energy to 200 cm(-1) above. The NO product rotational distributions show population in states that are not accessible with the energy available for excitation from the NO ground state. This effect is observed at photon energies from below the dissociation energy up to approximately 100 cm(-1) above it. Translational energy distributions, extracted from velocity map images of individual rotational levels of the NO product, reveal contributions from excitation of high energy NO-Ar X states at all the excess energies probed, although this diminishes with increasing photon energy and is quite small at 200 cm(-1), the highest energy studied. These translational energy distributions show that there are contributions arising from population in vibrational levels up to the X state dissociation energy. We propose that the reason such sparsely populated levels contribute to the observed dissociation is a considerable increase in the transition moment, via the Franck-Condon factor associated with these highly excited states, which arises because of the quite different geometries in the NO-Ar X and A states. This effect is likely to arise in other systems with similarly large geometry changes.

Joint experimental-theoretical investigation of the lower bound states of the NO(X2Pi)-Kr complex

We describe the first measurement of the near IR spectrum of the NO-Kr van der Waals complex. A variant of IR-REMPI double-resonance spectroscopy is employed in which the IR and UV lasers are scanned simultaneously in such a way that throughout the scan the sum of the two photon energies is kept constant, matching a UV resonance of the system. In the region of the first overtone vibration of the NO monomer, we observe several rotationally resolved bands for the NO-Kr complex. In addition to the origin band located at 3723.046 cm(-1), we observe excited as well as hot bands involving the excitation of one or two quanta of z-axis rotation. Another band is assigned to the excitation of one quantum of bending vibration. The experimental spectra are compared with results of bound-state calculations for a new set of potential energy surfaces calculated at the spin-restricted coupled cluster level. For the average vibration-rotation energies, there is excellent agreement between the theoretical results based on the coupled states (CS) approximation and the full close-coupling (CC) treatment. Finer details like the electrostatic splitting and the P-type doubling of the rotational levels are accounted for only within the CC formalism. The comparison of the CC results with the measured spectra confirms the high quality of the PESs. However, the high resolution of the experiments is sufficient to identify some inaccuracies in the difference between the potential energy surfaces of A' and A'' reflection symmetry.

Electronic spectroscopy of the 3d Rydberg states of NO–Rg (Rg=Ne,Ar,Kr,Xe) van der Waals complexes

We have employed (2+1) resonance-enhanced multiphoton ionization spectroscopy to record electronic absorption spectra of NO-Rg (Rg=Ne,Ar,Kr) van der Waals complexes. The nitric oxide molecule is the chromophore, and the excitation corresponds to an electron being promoted from the 2ppi* orbital to 3dsigma, 3dpi, and 3ddelta Rydberg states. We review the ordering of the 3dlambda states of NO and use this as a basis for discussing the 3d components in the NO-Rg complexes, in terms of the interactions between the Rydberg electron, the core, and the Rg atom. Predissociation of the H' 2Pi state occurs through the F2Delta state for NO-Ar and NO-Kr, and this will be considered. We shall also outline problems encountered when trying to record similar spectra for NO-Xe, related to the presence of atomic Xe resonances.