A review of optically pumped far-infrared laser lines from methanol isotopes

The technique of optical pumping in polar molecules is the most efficient for Far-Infrared (FIR) laser generation, providing also a versatile and powerful tool for molecular spectroscopy in this spectral region. Methanol (CH₃OH) and its isotopic varieties are the best media for optically pumped FIR laser, with over thousand lines observed, and the most widely used for investigations and applications. In this sense, it is important organize and make available catalogues of FIR laser lines as complete as possible. Since the last critical reviews of 1984 [1] on methanol and its isotopic varieties [2,3,4], over hundred papers have been published dealing with hundreds of new FIR laser lines. In 1992 a review of FIR laser lines from CH₃OH was presented [5]. In this communication we extend this work to the other methanol isotopes, namely CH₃OD, CD₃OH, CD₃OD,¹³CH₃OH,¹³CD₃OH,¹³CD₃OD, CH₃ ¹⁸OH, CH₂DOH, CHD₂OH and CH₂DOD.Work supported by FAPESP, CNPq, FAEP-Brasil, and CNR-Italia     

Confirmation of far infrared laser assignments from CH3-deformation and CH3-rocking states of optically pumped methanol

High-resolution Fourier transform spectroscopy has been applied to confirm previously proposed assignments for nine far infrared (FIR) laser lines from the CH₃-deformation state of CH₃OH and one line from the CH₃-rocking state. Accurate frequencies are deduced for the observed and other predicted FIR laser transitions. FIR torsional branch frequencies in the ground state which were used in the confirmation are presented. Comments are also made on the OH-bending mode of CH₃OH.     

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.     

A high power CH3OH FIR laser system with long-term stability

A CH₃OH FIR system has been developed for plasma diagnostics. A CO₂ laser devoted to pump the CH₃OH molecule has been studied. Different internal diameters of FIR cavity, input coupling holes and focusing lenses have been experimented.It has been found that the system formed by three subsystems-pump laser, guiding channel and FIR cavity-behaves like an unique system. A long term stability has been reached with a proper system design.Work supported by ENEA contract No. C/451 (Dipart. Fusione, Associazione Euratom-Enea, CRE Frascati, C.P. 65, 00044 Frascati, Italy).Student     

Far infrared laser lines produced by methanol and its isotopic species: A review

Methanol (CH₃OH) is considered today one of the most important active media for the generation of laser radiation in the far-infrared (FIR) spectral region. Together with ten of its other isotopic species, it is responsible for the major part of the laser lines generated by the optical pumping technique. Due to the extreme importance of those molecules as laser generators, we understood that there was a necessity of a comprehensive work which would include as much pratical information as possible about each line.Chang et al⁽¹⁾ early recognized methanol as a source of strong FIR laser lines. Since then, more than 100 papers were published containing information about new laser emission. Recently, Moruzzi et al⁽¹¹⁴⁾ presented a review of 575 lines produced by¹²CH₃OH. In the present paper, we have extended the review to the various isotopic modifications of this molecule (namely¹³CH₃OH, CD₃OH,¹³CD₃OH, CD₃OD,¹³CD₃OD, CH₃OD, CH ₃ ¹⁸ OH, CH₂DOH, CH₂DOD and CHD₂OH), a total of nearly 2000 lines with wavelengths ranging from 19µm to 3030µm.     

Single line high efficiency tunable fir generation by resonant four-wave mixing in CH3 pumped by a multiatmosphere tunable CO2 laser

By pumping CH₃F with a high pressure tunable TE-CO₂ laser, the resonant four-wave mixing process (RFWM) generates a very efficient tunable single line FIR emission at the Raman frequency. This result is strictly related to the spectroscopic structure of the CH₃F molecule. By means of this process, a tunable FIR emission on a 0.1 cm^–1 bandwidth 150 kW (8 mJ) single line, is obtained which can be used for many FIR multiphoton applications.     

The Spectrum of Ground-State Reversed Transition of CH3OH Far-Infrared Laser

Using the CH₃OH molecular energy levels data base management program, we deduced that when a CH₃OH laser pumped by CO₂-9P(16) line, there should be a FIR laser line of 918m wavelength, which corresponding to a transition in ground-state reversed three-level energy system. For the first time, the spectra of ground-state reversed three-level system transitions were calculated by solving the density matrix equations, and the spectrum characteristics of 918m line were studied. Experimentally, the CH₃OH FIR laser line of 918m wavelength was obtained by pumping with a TEA-CO₂ laser. The experimental results were in good agreement with the theoretical calculations.     

Methanol laser lines from torsionally excited co stretch states, and from OH-bend, CH3-rock, and CH3-deformation states

Using a quasi-CW CO₂ oscillator-amplifier combination with peak power 300 Watt, we have generated FIR laser emission in weak absorption bands of CH₃OH. 40 new lines are reported, and their wavelengths are measured with a relative accuracy of 5×10^–5. A total of 72 lines are assigned. 34 of these involve torsional n=1, 2, and 3 states of the CO stretch and the vibrational ground state. The remaining lines are associated with the CH₃-rock, OH-bend, and CH₃-deformation modes. The latter are located 1460 cm^–1 above the ground state, and are pumped by simultaneous vibrational excitation and torsional deexcitation.     

Assignments of optically pumped CD3OH laser lines

Six FIR laser lines from CD₃OH pumped by the 10R(36) and the 10R(18) CO₂ laser lines are assigned to specific rotational energy levels in the excited C–0 stretch state. It is found that their upper laser levels are shifted by a Fermi resonance between the C–0 stretch vibration and the third and forth harmonics of the torsional mode. The Fermi resonance shifts are +0.332 cm^–1 and +2.251 cm^–1 for the upper laser levels pumped by the 10R(36) and the 10R(18) CO₂ laser lines, respectively. Calculated frequencies of the pump and the laser transitions agree with those of the pump CO₂ laser lines and the observed FIR laser lines within estimated accuracy.     

13CH3OH and13CD3OH optically pumped fir laser: New large offset emission and optoacoustic spectroscopy

27 new, large offset, FIR laser lines from¹³CH₃OH and one from¹³CD₃OH have been discovered by pumping with a high tunability waveguide CW CO₂ laser. Optoacoustic measurements of isotopic methyl alcohol have also been performed and the pump offsets of the new and of previously known lines have been measured and checked. Frequency tunability by Stark effect has been observed for 6 strong lines. Some assignments are discussed.     

Operating parameter optimization of optically pumped CH3OH far-infrared laser

By solving the density matrix equations of a quantum system, the output power intensity of an optically pumped CH₃OH FIR laser (CH₃OH-OPFIRL) was calculated by means of iteration method, and the spectral characteristics were got. Base on the calculation, optimization of operating parameters including operating gas pressure, pumped power and output coupling coefficient of the CH₃OH cavity laser were systematically studied. Experimental, a series of FIR emissions of the CH₃OH cavity laser pumped by TEA-CO₂ laser with 9P(16) line were measured. The experimental results were in good agreement with theoretical calculations. The project was supported by the Special Research Foundation of Doctorate Station in University of P.R.C.     

Measurement and assignment of new FIR laser lines in12CH3OH and13CH3OH

We have found twelve new FIR laser lines in¹²CH₃OH and thirty three in¹³CH₃OH. Both molecules were pumped by a regular cw CO₂ laser. We have also assigned 20¹³CH₃OH laser lines to specific rotational energy levels in the excited C-O stretchhindered rotation combined states     

FIR Laser Lines from CH3OD: A Review

The methanol isotopic species CH₃OD has also proved to be an efficient and powerful medium to generate radiation in the far infrared (FIR) region. After the critical review of 1994, six papers have been published dealing with new FIR laser lines from this molecule. As a consequence of the use of wide tunability waveguide CO₂ lasers as well as a new pulsed CO₂ laser operating at hot and sequential bands, as of optical pumping sources, the total number of the FIR laser lines increased from 122 in 1994 to 227 today. In this communication we present an updated and complete catalogue of FIR laser lines generated from CH₃OD. Information on wavelength, offset, relative polarization, intensity, and optimum operation pressure is generally available.     

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.     

Assignments of fir laser emissions from CH3OH optically pumped by an intra-cavity etalon tuned TEA-CO2 laser

The author reports that TEA-CO₂ with an intra-cavity etalon is a useful pumping source which can deliver widely tunable beam (tunable range 1 GHz) with accurate oscillation frequencies and with high power compared to a tunable wave guide laser.The source is applied to the excitations of CH₃OH and the FIR emissions from it are well assigned for there absorption transitions and FIR emissions. This proves that the source display not only usefulness for development of new FIR emissions but also for molecular spectroscopy.     

Optically pumped cw CH2DOH fir laser: New lines and frequency measurements

We have measured the output powers and relative polarizations of 66 cw FIR laser lines from CH₂DOH (including 50 not previously reported), which were optically pumped by a CO₂ laser. The frequencies of 43 of these lines were measured relative to stabilized CO₂ lasers.     

Study of miniature optically pumped NH3 fir cavity laser

A miniature NH₃ molecule FIR cavity laser of 15cm in length pumped by a TEA CO₂ laser was built. The spectral characteristics of the cavity laser and the optimum operating parameters were studied.Supported by the National Natural Sciences Foundation of China     

Experimental study on miniature pulsed CH3OH far-infrared laser

Using a grating tuned TEA-CO₂ laser with 9P(16) line to pump a 10cm or 20cm long miniature Fabry-Perot cavity CH₃OH laser, pulsed far infrared(FIR) lines of 570.6um and 918um wavelengths were observed, the latter is a new line. Operation performance and spectral characteristics of the miniature CH₃OH laser were studied. The project was supported by the Special Research Foundation of Doctorate Station in University of P.R.C.     

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 37.5m 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.