K. M. Evenson

Far infrared laser magnetic resonance of singlet methylene: Singlet–triplet perturbations, singlet–triplet transitions, and the singlet–triplet splittinga)

We have observed and assigned a number of far infrared laser magnetic resonance spectra of CH2 arising from rotational transitions within the lowest vibrational state of the a 1A1 electronic excited state and from transitions between such singlet levels and vibrationally excited levels of the X 3B1 electronic ground state. The singlet–singlet transitions are magnetically active, and the singlet–triplet transitions have electric dipole intensity because of the spin‐orbit mixing of singlet levels with vibrationally excited levels of the triplet state. By identifying four pairs of singlet and triplet levels that perturb each other we can accurately position the singlet and triplet state relative to each other and determine the single–triplet energy splitting. We determine that T0(a 1A1)=3165±20 cm−1 (9.05±0.06 kcal/mol; 0.392±0.003 eV), and Te(a 1A1)=2994±30 cm−1 (8.56±0.09 kcal/mol; 0.371±0.004 eV). A new ab initio calculation of the spin‐orbit matrix element between these two states has been of assista...

Accurate frequencies of molecular transitions used in laser stabilization: the 3.39‐μm transition in CH4 and the 9.33‐ and 10.18‐μm transitions in CO2

The frequencies of three lasers stabilized to molecular absorptions were measured with an infrared‐frequency synthesis chain extending upwards from the cesium frequency standard. The measured values are 29.442 483 315 (25) THz for the 10.18‐μm R(30) transition in CO2, 32.134 266 891 (24) THz for the 9.33‐μm R(10) transition in CO2, and 88.376 181 627 (50) THz for the 3.39‐μm P(7) transition in CH4. The frequency of methane, when multiplied by the measured wavelength reported in the following letter, yields 299 792 456.2(1.1) m/sec for the speed of light.

IMPROVED COUPLING TO INFRARED WHISKER DIODES BY USE OF ANTENNA THEORY

It is shown that the dependence of the output of a whisker diode on its orientation in the polarized beam of an infrared laser can be explained on the basis of simple long‐wire antenna theory. Outstanding improvements in coupling the diode to the radiation field can result when this fact is utilized in applications.

Tunable Far Infrared Spectroscopy

Tunable, cw, far‐infrared radiation has been generated by nonlinear mixing of radiation from two CO2 lasers in a metal‐insulator‐metal (MIM) diode. The FIR difference‐frequency power radiated from the MIM diode antenna to a calibrated indium antimonide bolometer. Two‐tenths of a microwatt of FIR power was generated by 250 mW from each of the CO2 lasers. The combination of lines from a waveguide CO2 laser, with its larger tuning range, with lines from CO2, N2O, and CO2 isotope lasers promises complete coverage of the entire far‐infrared band from 100 to 5000 GHz (3–200 cm−1) with stepwise‐tunable cw radiation. To demonstrate the usefulness of the technique, we observed the J=4–5 line of CO at 567 GHz.

Accurate rotational constants of CO, HCl, and HF: spectral standards for the 0.3-to 6-THz (10- TO 200-cm−1) region

Abstract Accurate high-resolution spectroscopic measurements require secondary standards which can serve as convenient calibration references in laboratory and field research. We have measured the frequencies of a series of rotational transitions between 0.3 and 6 THz for several stable and readily obtainable gases (CO, HCl, and HF) to an accuracy better than one part in 10 7 and present revised rotational constants for these molecules. The gases were selected (in part) due to their presence in the Earths atmosphere in significant amounts and thus are convenient for the frequency calibration of atmospheric spectra.

The rotational spectrum and hyperfine structure of the methylene radical CH2 studied by far-infrared laser magnetic resonance spectroscopy

Thirteen pure rotational transitions of CH2 in its X 3B1 ground vibronic state have been measured and assigned using the technique of far‐infrared laser magnetic resonance (LMR) spectroscopy. The energy levels thus determined led to the prediction and subsequent detection by microwave spectroscopy of a further rotational transition 404–313, at lower frequency (∼70 GHz). The analysis of these observations yields precise rotational constants as well as spin–spin, spin‐rotation, and hyperfine interaction parameters for gas phase CH2. Its rotational spectrum may enable interstellar CH2 to be detected by radio astronomy. Two rotaional transitions within the v1=1 excited vibrational state have also been identified in the LMR spectrum. Future observations of vibrationally excited CH2 may afford a means of determining the singlet–triplet splitting in methylene, and studies of CD2 and CHD will result in improved structural determinations.

Direct frequency measurement of the I 2 -stabilized He–Ne 473-THz (633-nm) laser

The absolute frequency of the 473-THz He-Ne laser (633 nm), stabilized on the g or i hyperfine component of the (127)I(2) 11-5 R(127) transition, was measured by comparing its frequency with a known frequency synthesized by summing the radiation from three lasers in a He-Ne plasma. The three lasers were (1) the 88-THz CH(4)-stabilized He-Ne laser (3.39 microm), (2) a 125-THz color-center laser (2.39 microm) with its frequency referenced to the R(II)(26) (13)C(18)O(2)laser, and (3) the 260-THz He-Ne laser (1.15 microm) referenced to an I(2)-stabilized dye laser at 520 THz (576 nm). The measured frequencies are 473 612 340.492 and 473 612 214.789 MHz for the g and i hyperfine components, respectively, with a total uncertainty of 1.6 parts in 10(10). The frequency of the i component adjusted to the operating conditions recommended by the Bureau International des Poids et Mesures is 473 612 214.830 +/- 0.074 MHz.

A review of frequency measurements of optically pumped lasers from 0.1 to 8 THz

We present a list of more than 800 far‐infrared laser lines emitted by optically pumped molecular lasers whose frequencies have been measured. For each line, frequency, wavelength, wave number, lasing molecule, CO2 pump line, and, if available, the assignment of the lasing transition, are given. The list is accompanied by a survey of the techniques of frequency measurement in the far infrared. Accuracies and limitations of the various techniques are also discussed.

Rate constants for the reactions of OH with CH4 and fluorine, chlorine, and bromine substituted methanes at 296 K

The absolute rate constants for the reactions of OH radicals with CH4 and fifteen fluorine, chlorine, and bromine substituted methane molecules have been measured using a discharge flow system and laser magnetic resonance detection of OH. Measurements were made at 296 K and at pressures between 107 and 1300 Pa. The results indicate that the reaction mechanism involves the abstraction of a hydrogen atom and formation of H2O and a methyl type radical product. Completely halogenated methane molecules are found to be relatively unreactive. Hydrogen containing molecules react with rate constants ranging from about 0.2 to 160×10−15 cm3/molecule⋅sec. The reactivity increases with decreasing carbon–hydrogen bond energies. Rough estimates are made of the Arrhenius parameters for the reactions.

HO2 detected by laser magnetic resonance

Far‐infrared absorption spectra of HO2 in the gas phase have been detected at six wavelengths of a water vapor laser magnetic resonance spectrometer. The identification of HO2 as the absorbing molecule is based on a partial analysis of the spectra and on a variety of different chemical methods used to produce the radical. Approximate values of rotational constants and spin doublet separations are derived from the spectra.