Giovanni Moruzzi

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.

FAR INFRARED-LASER LINES AND ASSIGNMENTS OF CH3OH - A REVIEW

Methyl alcohol is the most important lasing molecule in the Far-Infrared (FIR) spectral region, and the most widely used for investigation and for applications. Since the last critical review of 1984, over seventy papers have been published dealing with the FIR laser lines and the infrared spectroscopy of CH3OH. In 1984 we could list about 330 FIR laser lines, 98 of which were measured in frequency and 105 assigned. Since then more than 70 papers were published increasing the number of the known laser lines to 575 (103 measured in frequency). Also the FIR and IR spectroscopy was largely improved thanks to the analysis of high resolution FT spectra, and the number of the correctly assigned laser lines has been increased to 224. The wavenumbers of the assigned lines can now be predicted with an accuracy of about 0.001 cm−1 or better, thus approaching the accuracy of the experimental frequency measurements.

The spectrum of CH3OH between 100 and 200 cm−1: Torsional and “forbidden” transitions

Abstract In the framework of a systematic investigation of the infrared and far infrared spectrum of CH3OH by high-resolution Fourier transform spectroscopy, we present a catalog of 6000 absorption lines between 100 and 200 cm−1 (which means practically all the measured lines in this region which are above a reasonable peakfinder threshold), and a list of 955 recently assigned lines below 100 cm−1, which had not been included in a previous paper. The Taylor expansion coefficients for evaluating the energies of the levels involved in the transitions are also given. Most of the lines occurring in the 100–200 cm−1 region involve torsionally excited states. Another interesting feature of this region is the presence of a forbidden Q branch (Δn = 1 and ΔK = 0 simultaneously). Two further forbidden Q branches, occurring at higher wavenumbers, are also presented.

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.

High resolution spectrum of Ch3OH between 8 and 100cm−1

Abstract The FIR spectrum of CH3OH between 8–100 cm−1 has been measured by a high resolution Fourier transform spectrometer. A computer best fit program, based on the Taylor series expansion of the energy levels, has been used for the line assignments. The region between 40–100 cm−1 is presented for the first time. The region between 8–40 cm−1, covered in a previous work, is revisited in the light of the new measurements at higher frequencies, and new assignments are given. The available microwave and radio-frequency assignments have been inserted into the fit program. A catalogue of 6725 assigned MW and FIR lines below 101.8 cm−1 is presented.

The spectrum of CH3OH between 200 and 350 cm−1: Torsional transitions and evidence for state mixings

Abstract In the framework of our systematic investigation of the infrared and far infrared spectrum of CH3OH by high-resolution Fourier transform spectroscopy, we present a catalog of 8800 absorption lines between 200 and 352 cm−1. The Taylor expansion coefficients for evaluating the energies of the levels involved in the transitions are also given. All of the lines occurring in the 200–352 cm−1 region correspond to transitions between torsionally excited states. Some forbidden lines (Δn ≠ 0; ΔK = 0), indicating state mixings, have been found.

Acoustooptic extension of the frequency tunability of CW CO2 lasers: New FIR lasers emissions from CH3OH and13CH3OH

We have increased the frequency tunability of our CW waveguide CO2 lasers by means of an acoustooptic amplitude modulator, operating at the fixed frequency of 90 MHz. The up-shifted, or down-shifted, laser optical sideband can be generated independently by adjusting the orientation of the modulator. The efficiency is larger than 50%. The frequency tunability of the CO2 laser around each laser line is thus increased by 180 MHz. To demonstrate the possibilities of this method, a source composed of the above modulator and of a CW, 300 MHz tunable waveguide CO2 laser has been used for the search of new large offset FIR laser lines from optically pumped CH3OH and13CH3OH molecules. As a result 15 and 10 new large offset laser lines were discovered respectively. New assignments of some laser lines are also proposed. We have also measured the Stark effect, the offset, and the polarization of other already known lines. In particular a Stark effect frequency tuning of about 1 GHz is demonstrated for a laser line at 208.399 μm.

Applications of high resolution spectroscopy to the identification of far-infrared laser emission from optically pumped13CH3OH

High-resolution infrared (IR) and far infrared (FIR) Fourier transform absorption spectra have been employed to investigate assignments of FIR laser lines reported from optically-pumped13CH3OH. The spectroscopic measurements are used in conjunction with the reported IR pump and FIR laser frequencies to form closed combination loops for several systems, serving to confirm the assignments and in some cases to improve the accuracy of the FIR laser frequencies. Frequency predictions from combination differences are also presented for a number of potential new FIR laser lines.

FIR Stark spectroscopy on optically pumped CH3OH laser

An extensive theoretical and experimental analysis of the absorption and emission spectrum of a CH3OH FIR-laser excited by a conventional CO2 laser is presented. Particular interest is devoted to the Stark shifts of the pump and lasing lines and to the electric field dependence of the Fir-laser output of the various lines. The offsets with respect to the exciting radiation and the Stark shifts of the IR absorption (pump) lines are measured by means of the transferred Lamb dip technique. The theoretical behaviours of the Stark patterns are calculated for several choices of the quantum numbers and selection rules involved in the transitions. A large variety of experimental results are reported and compared to theory. Non-linear Stark shifts have been observed for the 37.5μm FIR laser line and for the IR-pump transitions excited by the 9-P(38) and 10-R(38) CO2 laser Lines. Line assignments are proposed and new FIR laser lines are reported.

INTRACAVITY TRIPLE RESONANCE SPECTROSCOPY IN THE CH3OH FAR-INFRARED LASER

Abstract Triple resonance experiments have been performed on optically pumped CH3OH lasers in order to investigate the K doubling for A symmetry both in the ground and in the first excited v5 vibrational state. RF triple resonance signals are observed as a change in the power output both of already known FIR laser lines (excited by the 10R(38) CO2 laser line) and of two newly discovered FIR laser lines excited, with a large offset, by the CO2 10R(42) emitted by a waveguide laser. Triple resonance has not been observed for two additional new laser lines pumped, also with a large offset, by 10R(20), thus disproving a possible A state assignment. The K doubling has been measured for five states, v = 0, K = 4, J = 25 and 26, and v = 1, K = 4, J = 23, 24, and 25, respectively. The usual formula for the K doubling has been extended in order to improve the fit of the experimental data. The fir laser lines assignments have also been tested by using data from high-resolution FT spectra.