R. M. Lees

Laser-Stark spectrum of methyl alcohol at 890.761 GHz with the HCN laser

Abstract The laser-Stark spectrum of methanol has been studied up to Stark fields of about 55 kV/cm with the 890.761-GHz HCN laser, in both parallel and perpendicular polarizations. Extensive series of absorption lines have been assigned to the J = 6 to 15 members of the k = 6 ← 5 Q branch of the v = 0 E torsional state of CH3OH. Also, 21 M components of the Jk = 71 ← 62 A ± doublet in the v = 1 excited torsional state have been identified. Efforts to analyze the Q-branch spectra in terms of dipolar parameters and zero-field frequencies were hampered by very high statistical correlations in the least-squares fits. However, it proved necessary to permit the upper-state value of μa to exceed that of the lower state, by 0.0014 D, and to introduce J dependence of μa. Analysis of the Stark shifts of the excited torsional state transition yielded a μa of 0.93 D, about 0.04 D greater than the mean value for the ground state.

On the physical interpretation of torsion-rotation parameters in methanol and acetaldehyde: Comparison of global fit and ab initio results

Equilibrium structural constants and certain torsion–rotation interaction parameters have been determined for methanol and acetaldehyde from ab initio calculations using GAUSSIAN 94. The substantial molecular flexing which occurs in going from the bottom to the top of the torsional potential barrier can be quantitatively related to coefficients of torsion–rotation terms having a (1−cos 3γ) dependence on torsional angle γ. The barrier height, six equilibrium structural constants characterizing the bottom of the potential well, and six torsion–rotation constants are all compared to experimental parameters obtained from global fits to large microwave and far-infrared data sets for methanol and acetaldehyde. The rather encouraging agreement between the Gaussian and global fit results for methanol seems both to validate the accuracy of ab initio calculations of these parameters, and to demonstrate that the physical origin of these torsion–rotation interaction terms in methanol lies primarily in structural rela...

Torsion‐Vibration‐Rotation Interactions in Methanol. IV. Microwave Spectrum of CH3OH in the Excited CO Stretching State

The K=2← 1 E1 Q branch and the JK=10← 00 A‐E doublet have been observed for the first excited CO stretching mode in the microwave spectrum of normal methanol. A value for the excited state barrier to internal rotation of 397.9± 3 cm−1 is required to reproduce the Q‐branch origin frequency of 47 929.9 MHz, with reasonable limits on geometry changes assumed. This value is consistent with previous infrared results.

FOURIER SPECTRUM OF CH3OH BETWEEN 950 AND 1100 CM-1

Abstract The infrared spectrum ofCH 3 OH between 950 and HOOcm −1 has been measured by a high resolution Fourier transform spectrometer. This spectral region is of particular interest because of its overlapping with the CO 2 laser emissions used for exciting the CH 3 OH laser. A catalog of 3410 assigned lines is presented, as well as the Taylor development tables for evaluating the energies of the upper levels of the corresponding transitions.

On the equivalence of V6 to torsional flexing for molecules with threefold barriers to internal rotation

A comparison is made of the approaches of Lees and Baker and of Ewig and Harris to the Hamiltonian for internal rotation, both of which showed that the effects of a V6 term in a threefold torsional potential are identical to those of torsional flexing, i.e., to the distortion of molecular geometry due to torsion. The two approaches are proved to give identical results in the first order of perturbation. Earlier contrasting arguments that spectra could be explained by a V6 term but not by torsional flexing are examined and found to contain an incorrect averaging procedure in the perturbation treatment. Values of the torsional flexing term equivalent to published values of V6 are obtained to first order and are discussed. Although experimental information is not yet sufficiently precise or extensive to permit quantitative conclusions, it appears that torsional flexing could account for a significant part of the reported V6 in some cases.

Methanol and the optically pumped far-infrared laser

New results on the generation and spectroscopic analysis of optically pumped far-infrared (FIR) laser emission from CH/sub 3/OH have been obtained as part of a systematic study of methanol isotopomers as FIR laser sources utilizing the extended line coverage available from a recently developed high-resolution CO/sub 2/ laser of high efficiency. For normal CH/sub 3/OH, six new short-wavelength lines have been found using a 2 m long Fabry-Perot FIR laser cavity. Accurate heterodyne frequency measurements are reported for 14 CH/sub 3/OH FIR laser lines, nearly all above 100 cm/sup -1/, as well as accurate frequency offsets for most of the corresponding CO/sub 2/ pump lines. Spectroscopic assignments are presented for nine high-frequency FIR laser lines in four pump systems.

External cavity tunable diode laser NH3 spectra in the 1.5 μm region

Abstract NH 3 absorption spectra have been recorded from 6460 to 6522 cm −1 at room temperature and ∼5.5 Torr pressure with an external cavity tunable diode laser. The spectra served (a) as an initial test of a multipass optical assembly mounted in a large vacuum chamber designed for future slit nozzle experiments, (b) to further our understanding of the characteristics, stability and accuracy of our laser, and (c) to investigate the performance of our system by comparison with previous Fourier transform results. The system sensitivity was high and yielded an average observed line density of over 13 lines per cm −1 , about three times greater than reported before, with numerous weak absorptions clearly visible. The lines arise mainly from the perpendicular ( ν 1 + ν 3 ) NH-stretching combination band, plus possibly other combinations with excited ν 2 states. Transition wavenumbers, absorption coefficients, and line widths were determined. From extrapolation of trends in wavenumber, intensity and upper state energy for known transitions, 21 additional assignments are proposed for ( ν 1 + ν 3 ) band lines. The term values, l -doubling and a – s inversion splittings for the ( ν 1 + ν 3 ) and 2 ν 3 ( l =0) states in the 1.5 μm region are surveyed. Exploratory fitting was carried out to a linear effective Hamiltonian to assess the extent of off-diagonal interactions and perturbations.

Exploring molecular complexity with ALMA (EMoCA): Alkanethiols and alkanols in Sagittarius B2(N2)

Context. Over the past five decades, radio astronomy has shown that molecular complexity is a natural outcome of interstellar chemistry, in particular in star forming regions. However, the pathways that lead to the formation of complex molecules are not completely understood and the depth of chemical complexity has not been entirely revealed. In addition, the sulfur chemistry in the dense interstellar medium is not well understood. Aims. We want to know the relative abundances of alkanethiols and alkanols in the Galactic center source Sagittarius B2(N2), the northern hot molecular core in Sgr B2(N), whose relatively small line widths are favorable for studying the molecular complexity in space. Methods. We investigated spectroscopic parameter sets that were able to reproduce published laboratory rotational spectra of ethanethiol and studied effects that modify intensities in the predicted rotational spectrum of ethanol. We used the Atacama Large Millimeter Array (ALMA) in its Cycles 0 and 1 for a spectral line survey of Sagittarius B2(N) between 84 and 114.4 GHz. These data were analyzed by assuming local thermodynamic equilibrium (LTE) for each molecule. Our observations are supplemented by astrochemical modeling; a new network is used that includes reaction pathways for alkanethiols for the first time. Results. We detected methanol and ethanol in their parent 12 C species and their isotopologs with one 12 C atom substituted by 13 C; the latter were detected for the first time unambiguously in the case of ethanol. The 12 C/ 13 C ratio is ~25 for both molecules. In addition, we identified CH 3 18 OH with a 16 O/ 18 O ratio of ~180 and a 13 CH 3 OH/CH 3 18 OH ratio of ~7.3. Upper limits were derived for the next larger alkanols normal - and iso -propanol. We observed methanethiol, CH 3 SH, also known as methyl mercaptan, including torsionally excited transitions for the first time. We also identified transitions of ethanethiol (or ethyl mercaptan), though not enough to claim a secure detection in this source. The ratios CH 3 SH to C 2 H 5 SH and C 2 H 5 OH to C 2 H 5 SH are ≳21 and ≳125, respectively. In the process of our study, we noted severe discrepancies in the intensities of observed and predicted ethanol transitions and propose a change in the relative signs of the dipole moment components. In addition, we determined alternative sets of spectroscopic parameters for ethanethiol. The astrochemical models indicate that substantial quantities of both CH 3 SH and C 2 H 5 SH may be produced on the surfaces of dust grains, to be later released into the gas phase. The modeled ratio CH 3 SH/C 2 H 5 SH = 3.1 is lower than the observed value of ≳21; the model value appears to be affected most by the underprediction of CH 3 SH relative to CH 3 OH and C 2 H 5 OH, as judged by a very high CH 3 OH/CH 3 SH ratio. Conclusions. The column density ratios involving methanol, ethanol, and methanethiol in Sgr B2(N2) are similar to values reported for Orion KL, but those involving ethanethiol are significantly different and suggest that the detection of ethanethiol reported toward Orion KL is uncertain. Our chemical model presently does not permit the prediction of sufficiently accurate column densities of alkanethiols or their ratios among alkanethiols and alkanols. Therefore, additional observational results are required to establish the level of C 2 H 5 SH in the dense and warm interstellar medium with certainty.

Microwave and submillimeter-wave spectra of CH3OH

Abstract Frequency measurements and assignments have been made for CH 3 OH lines in the 15- to 400-GHz region. The a -type R -branch multiplets are reported up to J = 8 ← 7 for the v t =0 torsional ground state, and to J = 6 ← 5 for the v t =1 and v t =2 excited states. Several new Q branches are listed and many b -type P - and R -branch transitions have been identified over a wide range of J and k values.

A rotation–torsion–vibration treatment with three-dimensional internal coordinate approach and additional FTIR spectral assignments for the CH3-bending fundamentals of methanol

A theoretical model has been developed to account for certain features of both newly observed and previously reported CH3bending subbands between 1450 and 1570 cm � 1 in the high-resolution Fourier transform infrared spectrum of CH3OH [Can. J. Phys. 79 (2001) 435]. The features include (i) an apparent inversion of the rotationless E–A torsional splitting with respect to the ground state,i.e.,the A state located above the E state,(ii) a pronounced upward slope in the K-reduced torsion–vibration energy pattern for the subband origins,and (iii) unexpected A1=A2 inversion of the K¼ 2A and K¼ 3A J-rotational levels that led to ambiguity in identifying the vibrational mode as m4 ðA1Þ or m10 ðA2Þ. The model is an effective internal coordinate Hamiltonian constructed in G6 molecular symmetry with the CH3-bends coupled to each other and to torsion and including a- and c-type Coriolis coupling. With this model,33 out of 36 experimental upper-state K-term values for newly assigned m4; m5,and m10 subbands plus previous m4 subbands have together been fitted successfully,employing 9 adjustable parameters and 17 fixed parameters to give a standard deviation of 0.14 cm � 1 . The Pc Coriolis term appears to be the leading cause of the upward shift in the K-reduced energies. When J-dependence is introduced via a rotational Hamiltonian including b- and c-type Coriolis terms in addition to molecular asymmetry,the observed A1=A2 inversion of the K¼ 2A and 3A rotational levels can also be reproduced. Predictions using the fitted K-rotation–torsion–vibration Hamiltonian show an interesting Coriolis-induced crossover and mixing of the m5 and m10 torsion– vibration energy patterns. These predictions played a role in identifying two of the new m5 subbands in the crossing region,thereby helping to validate the model. 2003 Elsevier Science (USA). All rights reserved.