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J. Phys. Chem. A 2010, 114, 4798–4804
Intermolecular Potential Energy Surface between Ne and NO (2Πr)† Yoshihiro Sumiyoshi and Yasuki Endo* Department of Basic Science, Graduate School of Arts and Sciences, The UniVersity of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan ReceiVed: September 30, 2009; ReVised Manuscript ReceiVed: NoVember 30, 2009
Rotational and rovibrational transitions with parity doublings and hyperfine structures of the Ne-NO complex in the electronic ground state have been observed by Fourier-transform microwave spectroscopy, and they are analyzed by employing a free-rotor model with a standard deviation of the least-squares fit to be 12 kHz. A 2-dimensional intermolecular potential energy surface for the Ne-NO complex has been determined from the present high-resolution spectroscopic data with the aid of high-level ab initio calculations. I. Introduction Investigations on the rotational and vibrational energy transfer processes of the NO radical in the ground electronic state, X2Πr, caused by inelastic collisions with para-H2 or rare gas atoms, especially He, are very important in interstellar chemistry because energy distributions of NO in the interstellar clouds, which is one of the fundamental diatomic radicals in interstellar space, are subjected to inelastic collision processes.1 A large number of experimental and theoretical studies have thus been reported to investigate intermolecular potential energy surfaces (IPESs) between various rare gas atoms and NO.2 Although extensive efforts have been made for the Ar-NO and He-NO systems,2,3 there have been fewer studies to determine the intermolecular potential energy surface for the Ne-NO system so far. Experimental information on the IPES of Ne-NO in the ground electronic state has been obtained by a molecular beamscattering experiment by Thuis et al., in which the minimum distance of 3.11 Å between Ne and NO with a well depth of about 50 cm-1 was reported.4 Meyer and co-workers have applied (2 + 1) resonance enhanced multiphoton ionization (REMPI) spectroscopy to observe electronic transitions to excited states correlating to several low-lying Rydberg states of NO, where the depth of 45 cm-1 has been derived for the ground state.5 Rotationally resolved infrared spectra of the Ne-NO complex associated with the first overtone transition of the NO moiety using IR-REMPI double resonance spectroscopy have been reported by the same research group.6 They reported several combination bands, including different intermolecular bend-stretch vibrations, and a shallower depth, De ) 35 cm-1, has been derived in combination with ab initio calculations.7 They also performed a molecular beam-scattering experiment and measured state-resolved integral and differential cross sections for the Ne-NO system.8 Ayles et al. tried to observe electronic transitions to the excited state correlating to A2Σ+ of NO with (1 + 1) REMPI spectroscopy for various rare gas complexes, Rg-NO (Rg: Ne, Ar, Kr, and Xe), for which, however, no spectrum for Ne-NO was observed, in contrast to successful observations of other rare gas complexes.9 By comparing ab initio wave functions for bound states of the A2Σ+ state10 and that of the zero-point vibrational state of the ground †
Part of the special section “30th Free Radical Symposium”. * Corresponding author. Phone: +81-3-5454-6748. Fax: +81-3-54546721. E-mail:
[email protected].
electronic state,9 the failure of the observation was ascribed to the fact that the Franck-Condon factors are almost zero for the Ne-NO complex. Ab initio calculations for the ground electronic state of the Ne-NO complex have been performed by Alexander et al.7 in a joint study with IR-REMPI double resonance spectroscopy.6 They carried out the calculations with the restricted coupledcluster singles and doubles with perturbative inclusion of triple excitations [RCCSD(T)]11 using the augmented correlationconsistent valence quadruple-ζ basis of Dunning and co-workers, aug-cc-pVQZ.12,13 The calculated IPES was used to derive the bound states of the complex, and good agreement with the experimental results of ref 6 was reported. The zero-pointcorrected dissociation energy, D0, of the complex was calculated to be 29.4 cm-1. We have demonstrated that high-resolution spectroscopic data for open-shell complexes with resolved fine and hyperfine structures play significant roles to determine their IPESs in a series of our studies on open-shell atom-diatom complexes, Ar-OH14 and Ar-SH,15-18 in which a free-rotor model explicitly considering the van der Waals (vdW) stretching motion was applied. Recently, we showed that the same approach worked successfully for the Ar-NO system, consisting of a heavier diatomic radical than the Ar-OH and Ar-SH systems, in determining its IPES. In the study, all the microwave (MW) data19-21 were utilized to determine the IPES, providing the D0 value, 87.6 cm-1,21 which was in good agreement with the previously reported experimental value, 87.8 ( 0.3 cm-1.22 In the present study, transitions with ∆P ) 0 (pure rotational transitions) and ∆P ) +1 (rovibrational transitions) for the Ne-NO complex have been observed with clearly resolved hyperfine structures by Fourier-transform microwave spectroscopy. The observed transition frequencies were analyzed using the free-rotor model with a reasonable standard deviation of the least-squares fit, 12 kHz. A determined 2-dimensional IPES in the ground electronic state is reported. II. Experiment Pure rotational and rovibrational transitions have been observed using a Fourier-transform microwave (FTMW) spectrometer with 4-40 GHz frequency coverage. Details of the experimental setup have been given in our previous paper.17 The Ne-NO complex was produced by expanding a premixed gas (0.3% of NO diluted in Ne) into a vacuum chamber at a
10.1021/jp909389q 2010 American Chemical Society Published on Web 12/28/2009
Potential Energy Surface of Ne-NO
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4799 TABLE 1: FTMW Data of Ne-NOa
Figure 1. Example of the observed spectrum of Ne-NO with J ) 0.5 r 0.5, P ) 0.5 r -0.5, F ) 1.5(+) r 1.5(-) obtained by 500 shots of accumulation. The signs in the parentheses stand for total parity.
Figure 2. Rovibrational energy level diagrams of Ne-NO by the fitted potential, where relative energy values from the lowest level are shown. Quantum numbers J are given on the left side for P ) -0.5 and the right side for P ) 0.5 of the energy levels, with total parity in parentheses. The hyperfine splittings for each J level are too small to be shown in the figure.
stagnation pressure of about 10 atm. A total of 56 lines for the rovibrational transitions with ∆P ) +1 (P ) -0.5 to 0.5) and the pure rotational transitions with ∆P ) 0 (with P ) -0.5 and 0.5), including P-type doublings and hyperfine structures, have been observed. Figure 1 shows an example of the spectrum obtained by 500 shots of accumulation. Since the electric dipole moment of NO is quite small,
