A new millimeter-wave observation of the weakly bound CO–N2 complexby L.A. Surin, A. Potapov, H.S.P. Müller, S. Schlemmer

Journal of Molecular Spectroscopy

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Millimeter-wave spectroscopy van der Waals complex

CO–N2 complex

Internal rotor siti pec f 37 following a recent infrared study by Rezaei et al. (2013). The observation of new subbands fixes with higher precision not only these upper K = 0 and K = 2 but also lower K = 1(f) levels, not linked with other een N2 atmos irect re these infrared works [1,2], the microwave observations [3–5] as well as millimeter-wave study [4] have been made. The rotational spectra [3–5] revealed hyperfine structure due to the presence of two 14N nuclei providing additional information on the average orientation of the N2 moiety in the complex. These first studies were later continued in the infrared [6] and millimeter-wave complex. Further, e two equ inct groups ara-N2. Or with resultant nuclear spin I = 0 or 2, has even values of jN2 paraN2, with I = 1, has odd values of jN2.

The first observed CO–orthoN2 spectra involved only th tion less jN2 = 0 states (the jN2 = 2 state was detected only in the latter infrared study [9]), therefore they resembled those of CO–rare gas complexes (e.g. Ne–CO or Ar–CO). Rotational levels occurred in ‘stacks’ with increasing values of total rotational angular momentum, J, and well-defined values of K, its projection on the intermolecular axis. Now three such stacks are known for

CO–orthoN2 in the ground (vCO = 0) state: K = 0 (jCO, jN2 = 0, 0) and ⇑ Corresponding author at: I. Physikalisches Institut, Universität zu Köln,

Zülpicher Str. 77, 50937 Cologne, Germany.

Journal of Molecular Spectroscopy 307 (2015) 54–58

Contents lists availab

Journal of Molecul .eregion of the CO stretching vibration by Kawashima and Nishizawa [1] using a pulsed molecular beam and by Xu and McKellar [2] using a continuous slit-jet nozzle expansion, both combined with a diode laser spectrometer. With the help of the prediction from though this rotation is somewhat hindered in the due to symmetry and nuclear spin statistics of th atoms in N2 all levels are separated into two dist sponding to complexes formed from ortho- or phttp://dx.doi.org/10.1016/j.jms.2014.12.016 0022-2852/ 2014 Elsevier Inc. All rights reserved.ivalent corretho-N2, , while e rota-atmosphere. The CO–N2 system is a van der Waals complex, and in general, its bound states are sensitive to the interaction potential. Thus high-resolution spectroscopy of CO–N2 is an important tool for probing of intermolecular forces.

The experimental studies of the CO–N2 complex are already quite extensive. First spectra were recorded in the 4.7 lm infrared picture of the CO–N2 energy levels in both the ground and excited

CO vibrational states, vCO = 0 and 1.

As in the case of many other weakly-bound complexes (for example, CO–H2 [10,11]), it is useful to label the energy levels of

CO–N2 using the free rotor numbers corresponding to rotational angular momenta of the constituent monomers, jCO and jN2, evenQuadrupole coupling

OROTRON 1. Introduction

Intermolecular interactions betw constituent, and CO, a significant natural interest because of their dstacks in earlier rotational studies. For the more abundant form, CO–orthoN2, five new P-branch rotational transitions of the K = 0–0 ‘‘CO bending’’ subband are reported, thus extending previous measurements.

Nuclear quadrupole hyperfine structure due to the presence of two equivalent 14N nuclei was partly resolved and analyzed to give additional information about the angular orientation of the N2 molecule in the complex.  2014 Elsevier Inc. All rights reserved. , the main atmospheric pheric pollutant, is of levance to the Earth’s [7,8] regions extending the previous tentative assignments and providing further details on the CO–N2 complex. Recently Rezaei et al. reported a broad-band (2135–2165 cm1) infrared spectrum of CO–N2 obtained using a tunable quantum cascade laser [9]. They also summarized all previous studies giving a fairly extensiveKeywords: connecting the (jCO, jN2) = (1,1) and (jCO, jN2) = (0,1) internal rotor states. The upper K = 0 and K = 2 ‘‘stacks’’ of rotational levels were probed for the first time here by millimeter-wave spectroscopyA new millimeter-wave observation of th complex

L.A. Surin a,b,⇑, A. Potapov a, H.S.P. Müller a, S. Schlem a I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Cologne, German b Institute of Spectroscopy, Russian Academy of Sciences, 142190 Troitsk, Moscow, Russi a r t i c l e i n f o

Article history:

Received 1 December 2014

In revised form 9 December 2014

Available online 17 December 2014 a b s t r a c t

New millimeter-wave tran intracavity OROTRON jet s form, CO–paraN2, a total o journal homepage: wwwweakly bound CO–N2 er a ons of the CO–N2 van der Waals complex have been observed using the trometer in the frequency range of 103–159 GHz. For the less abundant rotational transitions were assigned to three K = 0–0, 0–1, 2–1 subbands le at ScienceDirect ar Spectroscopy l sevier .com/locate / jms

K = 0, 1 (jCO, jN2 = 1, 0). In the excited (vCO = 1) state, the three analogous stacks with K = 0 and 1 are known, together with additional three stacks with K = 0, 1, 2 (jCO, jN2 = 2, 0) and two stacks with

K = 0, 1 (jCO, jN2 = 1, 2). The spectra of CO–paraN2 are more complicated because of the presence of N2 angular momentum (jN2 = 1) even at lowest energy. Stacks with K = 0, 1 (jCO, jN2 = 0, 1) and

K = 0, 1, 2 (jCO, jN2 = 1, 1) are known for vCO = 0, and analogous stacks together with two additional stacks with K = 2 and 3 (jCO, jN2 = 2, 1) are known for vCO = 1. The origins of observed rotational levels stacks of CO–orthoN2 and CO–paraN2 are depicted in Fig. 2 and Fig. 3 of Ref. 9 respectively.

All known stacks of the CO–N2 complex in the ground vCO = 0 state have been well characterized now by microwave and millimeter-wave spectroscopy with exception of two recently observed [9] K = 0 and 2 stacks (jCO, jN2 = 1, 1) of the CO–paraN2 spin modification. In this paper we present new survey spectroscopic investigations of CO–N2 with the OROTRON spectrometer in the frequency range of 103–159 GHz. It resulted, first, in the observations of the K = 0–0, K = 0–1 and K = 2–1 subbands of