Todd Anderson

The analysis of the rotational spectrum of methanol to microwave accuracy

Abstract A method for the analysis and characterization of a large number of rotational-torsional transition frequencies of methanol (CH 3 OH) to microwave accuracy is presented. It is based on our earlier work which used a direct diagonalization of an internal axis method Hamiltonian. In the work reported here the analyses of the A and E symmetry species were separated and careful attention was paid to the selection of cross terms between rotational and torsional operators to be included in the Hamiltonian. In order to facilitate comparison, the previous data set, which contained 470 rotational transitions, was also used for this new work. In our original analysis the root-mean-square (rms) deviation was 1.2 MHz. This same data set was subsequently reanalyzed by Nakagawa, Tsunekawa, and Kojima with an effective Hamiltonian based on a Watson-like transformation with a larger number of adjustable parameters. Their approach reduced the rms deviation to ∼0.6 MHz, but substantial model error remained. In the work reported here rms deviations of 0.065 MHz and 0.062 MHz for the A and E symmetry states, respectively, have been achieved. These deviations approximate experimental error, indicating that the model error has now been reduced from ∼1 MHz to essentially zero. Of perhaps more importance, our new approach has significantly enhanced the predictive powers of the model and provides a means for an accurate characterization of the rotational-torsional spectrum of methanol.

Additional measurements and a refined analysis of the millimeter- and submillimeter-wave spectrum of methanol

A total of 414 previously unobserved millimeter- and submillimeter-wave transitions of (C-12)H3OH through J = 10 and v(t) = 2 are assigned. These data and previous measured spectral transitions are combined and fitted by a modified internal axis method (IAM) treatment in which the A and E substates are treated separately and a wide variety of new rotational-torsional interaction terms are employed. The modified IAM treatment reduces the overall rms deviation of the fits to the level of experimental accuracy. The resultant spectroscopic constants allow for the accurate prediction of 569 additional transitions through J = 12 or less. 25 refs.

Millimeter- and submillimeter-wave spectrum of highly excited states of water

To facilitate studies of water in the interstellar medium and late-type stars, the frequencies of 30 new millimeter- and submillimeter-wave transitions of H2O-16 have been measured, which lie between 100 GHz and 600 GHz. This represents almost a doubling of the number of water lines that have been observed in the laboratory in this spectral region at high resolution. All of the newly observed lines are highly excited, lying between 2400 and 4200/cm above the ground level. Some of these have large excitation energies because of their high rotational states and others because they lie in excited vibrational states. These lines are potentially of substantial astrophysical significance because they are related to the study of interstellar masers and because their high excitation eliminates the atmospheric self-absorption associated with the more well-known water lines.

An extension of the high-resolution millimeter- and submillimeter-wave spectrum of methanol to high angular momentum quantum numbers

The high-resolution laboratory millimeter- and submillimeter-wave spectra of C-12H(3)OH and C-13H(3)OH have been extended to include transitions involving significantly higher angular momentum quantum numbers than studied previously. For C-12H(3)OH, the data set now includes 549 A torsional substate transitions and 524 E torsional substate transitions through J is not greater than 24, exclusive of blends. For C-13H(3)OH the data set now includes 453 A torsional substate transitions and 440 E torsional substate transitions through J is not greater than 24, exclusive of blends. The extended internal axis method Hamiltonian has been used to analyze the transitions to experimental accuracy. The molecular constants determined by this approach have been used to predict accurately the frequencies of many transitions through J = 25 not measured in the laboratory.

The millimeter and submillimeter spectrum of CF

The application of a recently described technique for producing significantly enhanced concentrations of molecular ions for spectroscopic study to the detection and measurement of the millimeter and submillimeter wave spectrum of CF(+) is reported. The experimental procedure is discussed, and the measured absorption frequencies are shown and compared with those calculated from spectral constants. These constants are given together with those from the infrared spectrum by Kawaguchi and Hirota (1985).

The millimeter- and submillimeter-wave spectrum of (C-13)H3OH revisited

A total of 245 previously unobserved millimeter- and submillimeter-wave transitions of (C-13)H{sub 3}OH for J not above 10 were assigned on the basis of measurements with a spectrometer which uses 40-60 GHz klystrons as fundamental radiation sources. These transitions were combined with the previously assigned 596 transitions and were analyzed using a modified internal-axis-method model which treats the symmetric (A) and degenerate (E) substates separately. As a result of the expansion of the (C-13)H{sub 3}OH data set to cover the same range as the data set of a previous (C-12)H{sub 3}OH analysis by Anderson et al. (1990), rigid criteria of self-consistency could be applied for the selection and the value of the constants between the isotopic species. The spectral constants obtained were used to make accurate predictions of 562 additional transitions of (C-13)H3OH for v(t) = 0, 1, 2 and rotational quantum number J equal to or less than 12. 18 refs.

The laboratory millimeter- and submillimeter-wave spectrum of C-13 methanol

A comprehensive millimeter- and submillimeter-wave study of the rotational spectrum of (C-13)H3OH is presented. Approximately 300 new transitions of this species were measured at frequencies up to 650 GHz, and the transitions were analyzed along with previously measured spectra using the internal axis method of Herbst et al. (1984). The resulting spectral constants allow prediction of an additional large number of transitions which should allow astronomers to identify many strong rotational lines of C-13 methanol throughout much of the submillimeter region.

Microwave generation from picosecond demodulation sources

A mode‐locked picosecond laser has been used to prebunch electron beams at a photocathode. These electrons are subsequently accelerated through coupling structures and microwaves radiated. The relation between the microwave output and the properties of the picosecond pulse train and the electron beam output coupling process are reported and related to theory. Among the important attributes of these devices are their ability to generate almost arbitrary microwave waveforms, to operate in the 100–200 kV region with very simple power supplies and excellent spectral purity, and to generate substantially more microwave power than contained in the optical drive.

The laboratory millimeter- and submillimeter-wave spectrum of the first two excited torsional states of (C-13)H3OH

The rotational-torsional spectrum of (C-13)H/sub 3/OH is presented in both the symmetric and degenerate substates of the first two excited torsional states. Of the new transitions reported, 122 lines are from the v(t) = 1 torsional state and 110 lines are from the v(t) = 2 torsional state, and all are confined to the rotational quantum number J = 8 or less. The data are combined with previously reported millimeter and submillimeter data for the v(t) = 0 torsional state and 44 previously measured excited torsional state lines to form a global data set of 596 transitions, which is analyzed and fitted using an extended internal axis method. The fit results in an overall rms deviation of 1.98 MHz. The spectral constants generated by the fit are used to predict an additional 123 lines of (C-13)H/sub 3/OH in the v(t) = 1,2 excited torsional states with frequencies up to 612 GHz and rotational quantum number J = 11 or less. 24 references.