Secion of Microwave spectroscopy was established in 2003 on the base of the Microwave spectroscopy group from the Laboratory of High Resolution Molecular Spectroscopy and Analytical Spectroscopy. The head of the Section is Prof. B.S.Dumesh.
The main scientific activity of the Section is a development of highly sensitive equipment for/and investigation of spectra of molecules and molecular complexes in millimeter-wave region. The first intracavity microwave spectrometer based on Orotron was developed and constructed in collaboration with Institute of Metrology for Time and Space. Orotron generator itself is a free non-relativistic electron laser, which is highly stable and tunable source in wide frequency range. For the absorption measurements the investigated gas is placed inside Orotron resonator. The spectrometer can operate with different types of modulation technique (source frequency modulation, Stark modulation, optical modulation) and uses electronic detection principle. Present spectrometer operates with Doppler spectral resolution in frequency range 100-150 GHz (2-3 mm) and has a record for this spectral range limit of detection of the absorption coefficient of γmin=Çõ10-10 cm-1 [1-2]. This spectrometer allowed unique measurements of the forbidden spectra of spherical-top type molecules, highly excited inversion transitions of ammonia and extended analytical possibilities of microwave spectroscopy [3-5].
The modification of this spectrometer aimed at the investigations of molecules and molecular complexes in supersonic jet was designed in collaboration with the Institute of Metrology for Time and Space and the University of Cologne . The sensitivity achieved is about two orders of magnitude better than that of common models [6-7]. With this spectrometer we systematically studied the spectra of weakly-bond Van der Waals complexes R-CO, where R is H2 [8], D2 [9], He [10], CO [11,12], N2 [13,14], and Ne [15] (see also [16]). The three first complexes are most probably expected to exist in the interstellar media in quantities sufficient for sure detection. Our laboratory results will help to study the complexes in space.
The bond energy of these complexes is so weak, that the monomers can move almost free within them. We have already got an indication of practically free rotation motion of two-atomic molecules in complexes. Studying the tunneling motion in CO dimer yielded a very nontrivial result: unusual dependence of tunneling splitting value on the rotational quantum numbers and the isotopic mass.
Our joined team tries also to extend the operational range of the spectrometer. Special Orotron for the 3-4 mm frequency range was constructed in 2003. Recently, it was designed a broad-band Orotron for sub-mm frequency range [17].
Fig. 1. Doppler-free line of two-photon absorption at J=12←10 transition in OCS molecule [18].
Our other activity is the improvement of the methods of nonlinear spectroscopy with sub-Doppler resolution (under Doppler limit). We first measured directly the two-photon Doppler-free absorption in rotational molecular spectra. A two-photon absorption line J=12←10 in OCS molecule is shown in Figure 1. The line-width is about ten times narrower than the Doppler limit [18].
In addition to mentioned above, the NMR spectroscopy technique is improved in the Section. There was designed a continuous-mode NMR spectrometer of the decimeter-wave region based on a “split-ring coil” resonator with automatic frequency control aimed at the resonance measurements of magnetic ions in magnetically ordered matters. The main advantage of this spectrometer is ability to measure the NMR in magnetically ordered materials in strong magnetic fields. With a spectrometer of this type, which is built up in IPP RAS with our assistance, a unique cycle of investigations in quasi 1D antiferromagnets with triangle magnetic order was done [19].
References
1 B.S.Dumesh, V.P.Kostromin, F.S.Rusin and L.A.Surin. Highly sensitive millimtre-wave spectrometer based on an orotron. Measurement Science and Technology 3 , p.873-878, 1992.
2 B.S.Dumesh and L.A.Surin. Two highly sensitive microwave cavity spectrometers. Rev.Sci.Instrum. 67 , p.3458-3464, (1996)
3 B.S.Dumesh, V.D.Gorbatenkov, L.A,Surin. Observation of the micro-wave spectra of silane 28 SiH 4 in the vibration dyad of n 2 / n 4 . Optika I Spectrosc. v.83 , 384-387, (1997) ( in Russian ).
4 B.S. Dumesh., V.A.Panfilov and L.A.Surin. High J Pure Inyersion of ND 3 in the n 2 = 1 State., J. Molecular Spectroscopy, v. 194, 142-144, (1999).
5 B.S.Dumesh, V.D.Gorbatenkov, V.G.Koloshnikov et al. Application of highly sensitive millimeter-wave cavity spectrometer based on orotron for gas analysis. Spectrochimica Acta, PtÀ, v.53, 835-843, (1997).
6 Novel Intracavity Jet Millimeter Wave Spectrometer: Detection of b-Type Rotational Transitions of Ne-CO., J. Molecular Spectroscopy, v. 192, 243-246, (1998).
7 L.A.Surin, B.S.Dumesh et al. Millimeter Wave Intracavity Jet OROTRON Spectrometer for Investigation of van der Waals complexes. Rev.Sci.Instrum . 72, 2535-2542, (2001)
8 Pak I. , Surin L., Dumesh B. . Discovery of the rotational spectrum of the weakly bound complex CO-H 2 ., Chem. Phys. Letters , v. 304, 145-149, (1999).
9 L. Surin, B. Dumesh, G.Winnewisser, I. Pak. The weakly bound complex CO-orthoD 2 : detection of millimeter-wave transitions. J. Chem. Phys . 113. 9351-9352, (2000)
10 L.A. Surin, D Roth, I. Pak, B. Dumesh et. al. Detection of the millimeter wave spectra of the weakly bound complexes 3 He-CO and 4 He-CO. J. Chem. Phys. 112, 4064-4068 (2000).
11 D. Roth, I. Pak, L. Surin, B. Dumesh, G.Winnewisser. The CO dimer millimeter wave spectrum: detection of tunneling transitions. J. Chem. Phys .113. 3034-3038 (2000)
12 A.R.W. McKellar, L.A. Surin et al. Millimeter wave spectra of the CO dimer: Three new states and further evidence of distinct isomers J. Mol. Spectrosc . v. 214, p. 87-93, (2002), L.A. Surin, D.N. Fourzikov et al. The CO Dimer: New Light on a Mysterious Molecule . J. Mol. Spectrosc . v. 222, p. 93-101, (2003).
13 Yunjie Xu, W. Jager, L. A. Surin et al. Microwave and millimeter wave study of Ortho-N2 states of CO–N2 J. Chem. Phys . v. 111, 10476-10483 (1999)
14 L.Surin, B.Dumesh et al. The millimeter wave observation of the bending vibration of the CO– ortho N 2 complex. J. Molecular Structure v. 612, p. 207-211, (2002).
15 D. Roth, I. Pak, L. Surin, B. Dumesh, G.Winnewisser. Millimeter wave spectroscopy of Ne-CO. Z.Naturforsch . 55a, No 9, 754-758, (2000)
16 B.S.Dumesh. Micro-wave spectroscopy of the molecular complexes R-CO (R=Ne, He, N2, H2), interesting for astrophysics, UFN 169 , 351-353 (1999) ( in Russian ).
17 V.L. Bratman, B.S. Dumesh, A.E. Fedotov, et al. Broadband orotron operation at millimeter and submillimeter waves. Int. J. Infrared and Millimeter Waves , , v.23, No.11, pp. 1595-1601, (2002)
18 L. Surin, B. Dumesh, F. Rusin, G.Winnewisser , I. Pak Doppler-free two-photon millimeter wave transitions in OCS and CHF 3 Phys. Rev. Lett. 86, 2002-2005, (2001)
19 B.S.Dumesh. The influence of quantum fluctuations on the magnetic properties of quasi-1D triangle antiferromagnets. UFN 170 , 403-418 (2000) ( in Russian ).