IR-UV double resonance scheme is shown on schematic radial potentials for CN (B 2Σ+, X 2Σ+) + Ne. The IR laser prepares CN-Ne complexes with two quanta of CN stretch and possible intermolecular excitation. The UV laser subsequently promotes vibrationally excited CN-Ne to the excited electronic state correlating with CN B 2Σ+ (v = 0) + Ne, giving rise to a laser-induced fluorescence signal. Upon photolysis at 193 nm, CN-Ne X (νCN = 2) can also be observed directly in the jet expansion (without IR excitation) and is probed on B (νCN = 0)-X (νCN = 2) transitions, denoted as UV′.
UV transitions observed for CN-Ne complexes in the B (νCN = 0)-X (νCN = 2) region following IR excitation at 4061.7 cm−1, which prepares the X (νCN, n K ) = (2,11) state. The UV transitions terminate on various n K hindered rotor states (with ΔE spacings) in the excited B (νCN = 0) electronic state as indicated by the energy level diagram and spectral band labels.
Rotationally structured CN-Ne bands in the CN overtone region observed by IR-UV double resonance spectroscopy. The n K hindered rotor states (and ΔE spacings) associated with the IR transitions are indicated in the energy level diagram and spectroscopic labels. The 00-00 transition (gray) is not observed in IR spectra because of Δn = ±1 selection rules, but is deduced from electronic transitions (see text and Table I).
Experimental and simulated rotational band contours for the (νCN, n K ) = (2,11) ← (0,00) (upper panel) and (2,10) ← (0,00) (lower panel) transitions. The red/blue traces are experimental spectra from IR-UV double resonance measurements, while the black traces are simulations with rotational temperature of 10 K and laser linewidth of 0.1 cm−1. The simulations are based on fits to P- and R-branch lines only. Rotational assignments are shown as ticks. The dashed gray line illustrates the contour for the Q-branch anticipated using the same (perturbed) rotational constant for the (2,11) upper state. The experimental Q-branch structure is well represented (red) using the deperturbed rotational constant derived for the (2,11) upper state. The simulated line intensities are based on Hönl-London factors.
Potential anisotropy for CN X 2Σ+ (νCN = 2) + Ne as a function of intermolecular angle θ. Upper panel: The solid line indicates the effective angular potential, represented by Eq. (5), while the dotted and dashed lines give the individual contributions from the smaller V 10 and dominant V 20 terms derived from the experimental hindered rotor spacings. The gray shaded regions indicate the experimental uncertainties associated with V 10 and V 20. Lower panel: The ab initio data points are illustrated by red crosses. The black trace is a fit to the ab initio data using the Legendre expansion with an additional isotropic component.
Potential anisotropy for CN B 2Σ+ (νCN = 0) + Ne as a function of θ. Upper panel: The solid line indicates the effective angular potential, represented by Eq. (5), while the dotted and dashed lines represent the dominant V 10 and much smaller V 20 terms deduced from the experimental hindered rotor spacings in the excited electronic state. Lower panel: The ab initio data points are indicated by red crosses. The black trace is a fit to the ab initio data using the Legendre expansion with an additional isotropic component.
CASSCF(9,8)//MRCI+Q/CBS ab initio potentials (cm−1) for the CN X 2Σ+ + Ne and CN B 2Σ+ + Ne electronic states. Points were generated every 0.25 Å in the radial coordinate between 3 and 4.5 Å and then at 6 Å. Radial data were produced every 20° between θ = 0° and 180°. The potential energy surfaces show distinctly different minimum energy geometries. Lower panel: CN X 2Σ+ + Ne displays a potential minimum at θ = 80° with an R e of ∼3.5 Å and a D e of 41 cm−1. Upper panel: CN B 2Σ+ + Ne exhibits a potential minimum for linear CN-Ne (θ = 0°) with a significantly longer R e of ∼3.75 Å and a D e of 44 cm−1.
Hindered rotor probability distributions for the CN-Ne (0,00), CN-Ne (0,10), and CN-Ne (0,11) states (red) correlating with CN X 2Σ+ + Ne compared with the free rotor probability distributions (blue). The hindered rotor wavefunctions are generated from the body-fixed diagonalization of the experimental angular potential (Fig. 5).
Experimental CN-Ne band origins (cm−1) with corresponding CN stretch and hindered rotor assignments (νCN, n K ) for IR and UV transitions in IR-UV double resonance spectra and one-photon B-X electronic transitions. Uncertainties in the band positions are consistent with the IR and UV laser bandwidths of 0.10 cm−1.
Spectroscopic constants (cm−1) for CN-Ne derived from infrared overtone spectra.
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