S.P. Belov

The Rotational Spectrum of SH and SD

With the Cologne terahertz spectrometer, the lower J rotational spectrum of the SD radical has been detected for the first time in the laboratory, in analogy with our previous recent detection of the SH radical. The radicals were produced by discharging H2S (D2S) buffered with He and H2 (D2). The spectra were analyzed employing Hunds case (a) coupling scheme. The SH radical has an inverted 2Π state, as does OH, so that the 2Π3/2 state is the energetically lower one. For both SH and SD, the rotational and centrifugal distortion constants, the spin orbit, the spin rotation, the Λ-doublet, and the hyperfine interaction constants were determined by combining the newly acquired experimental data with those available in the literature. The rotational constant B0 for SD is 146885.297 (26) MHz, while our latest value for SH is 283587.62 (12) MHz (the values in parentheses denote the calculated uncertainty obtained by a least-squares fit). The SH radical and its deuterated isotopomer may be detectable in interstellar space because of the high cosmic abundance of elemental sulfur and the high abundance of HS. Since the 2Π1/2 electronic state lies nearly 400 cm–1 above the 2Π3/2 state, the rotational transitions in the 2Π1/2 state should only be detectable in hot interstellar sources, and they are potentially good indicators of star formation.

Transitions between Hund's coupling cases for the X 2Π state of NO

The submillimetre-wave spectra of 14N16O, 15N16O, and 14N18O in both the X 2Π1/2 and X 2Π3/2 states have been measured in the frequency region 773 GHz to 1.1 THz and assigned. Transitions involving J values between 6.5 and 10.5 have been recorded for the first time. The higher J values display an uncoupling of the electronic spin S and the angular momentum L from the rotor axis. The S-uncoupling leads to an intermediate coupling condition between Hunds case (a) and (b), whereas the L-uncoupling causes a coupling scheme intermediate between Hunds case (a) and (d). An improved set of molecular parameters has been obtained for these isotopic species, including magnetic dipole and electric quadrupole constants.

Ro-inversional Spectrum of Ammonia

The ground state J = 2 ←1 , K = 0 and K = 1 ro-inversional spectrum of 14NH3 and 15NH3 at 1.2 THz has been measured with an accuracy of 20 kHz using the Cologne terahertz spectrometer. The measured frequencies for the K = 0 components are: 14NH3 (J, K) = a(2, 0) - s( 1,0): 1 214 852.942(20) MHz, 15NH3 (J, K) = a(2, 0 ) - s( 1,0) : 1 210 889.556(20) MHz. In addition we have determined from saturation dip measurements of the J = 1 - 0 transition the spin-rotation constant CN = 6.7(3) kHz and the unsplit line center frequency: 14NH3 (J, K) = s( 1, 0) - a(0, 0): 572 498.163( 10) MHz. The new results are in excellent agreement with existing high resolution Fourier transform data. The terahertz line frequencies are of considerably higher accuracy than the FT-data. by about two orders of magnitude. They will serve as future calibration lines. The ro-inversional transitions are of astrophysical interest

A Quasi-Optical Multiplier for Terahertz Spectroscopy

Abstract The rotational K-structure (K = 0, ... 6) of the a-type transition (J = 65 - 64) of the symmetric top molecule methylcyanide, CH3CN, has been recorded at 1.2 THz employing a newly designed quasi-optical frequency multiplier. A planar Schottky diode is used as non-linear element for producing the harmonic power and is mounted in the focus of an off-axis antenna across a short single ridged waveguide. The 2nd harmonic of the fundamental frequency is filtered with adichroic plate and is focused onto the InSb bolometer after passing an absorption cell. The multiplier is driven by a high power backward wave oscillator with a fundamental frequency near 600 GHz.

Sub-Doppler Measurements and Terahertz Rotational Spectrum of 12C18O

Abstract The five lowest rotational transitions of 12C180 (J =2 ← 1 to J = 6 ← 5) have been measured by saturation dip spectroscopy with an experimental accuracy of 1 to 1.5 kHz, employing phase-stabilized backward-wave oscillators. The five J rotational transitions cover the frequency range between 219 and 658 GHz. In addition, we have measured in the Doppler limited mode the rotational transitions J = 1 ← 0, J = 7 ← 6 and J = 8 ← 7. The accuracy achieved for the individual frequencies ranges between 5 and 20 kHz. Moreover the three rotational transitions J = 16 ← 15 to J = 18 ← 17 in the frequency region 1.7-2.0 THz were measured with an accuracy of 15 to 30 kHz by using the Cologne side band spectrometer for terahertz applications COSSTA.

Sub-Doppler Spectroscopy of Ammonia Near 570 GHz

Abstract We have employed the Cologne terahertz spectrometer [ 1,2] to observe Lamb dip and crossover phenomena in rotational spectra at 572 GHz. Because of the large power output of the backward wave oscillators used in the system, no special means are required to induce this saturation effect; the coincidentally reflected radiation from the bolometer present in our quasi-optical absorption cell is sufficient to saturate the υz = 0 sub-group. This effect has allowed the observation of the three previously unresolved hyperfine components of the l4NH3 A ← S, JK = 10 ← 00 transition, and was also observed for the analogous 15NH3 transition in natural abundance (i. e. 0.366 %). These measurements indicate a possible improvement of molecular transition frequencies in the submillimeter region.