Investigation of 13CH3OH methanol: new laser lines and assignments

We have reinvestigated ¹³CH₃OH as a source of far-infrared (FIR) laser emission using a CO₂ laser as a pumping source. Thirty new FIR laser lines in the range 36.5 μm to 202.6 μm were observed and characterized. Five of them have wavelengths between 36.5 and 75 μm and have sufficient intensity to be used in LMR spectroscopy. Using Fourier-transform spectroscopic data in the infrared (IR) and FIR regions we have determined the assignment for 10 FIR laser transitions and predict nine frequencies for laser lines which have yet to be observed. Received: 17 July 2000 / Published online: 6 December 2000     

Discovery and frequency measurement of short-wavelength far-infrared laser emissions from optically pumped <Superscript>13</Superscript>CD<Subscript>3</Subscript>OH and CHD<Subscript>2</Subscript>OH

A three-laser heterodyne system was used to measure the frequencies of twelve previously observed far-infrared laser emissions from the partially deuterated methanol isotopologues ¹³CD₃OH and CHD₂OH. Two laser emissions, a 53.773 μm line from ¹³CD₃OH and a 74.939 μm line from CHD₂OH, have also been discovered and frequency measured. The CO₂ pump laser offset frequency was measured with respect to its center frequency for twenty-four FIR laser emissions from CH₃OH, ¹³CD₃OH and CHD₂OH. PACS 07.57.Hm; 42.55.Lt; 42.62.Eh     

Discovery and frequency measurement of far-infrared laser emissions generated by optically pumped CH2DOH

A recently improved three-laser heterodyne system was used to frequency measure ten previously observed optically pumped far-infrared (FIR) laser emissions from the partially deuterated methanol isotopologue CH₂DOH. Also, a 64.0 μm FIR emission generated by the 9P32 line of the carbon dioxide (CO₂) laser was discovered and frequency measured. These newly measured frequencies have fractional uncertainties on the order of ±2×10^-7 and correspond to laser wavelengths ranging from 42.6 to 152.7 μm. The offset frequency for the CO₂ pump laser was measured for twenty-two CH₂DOH FIR laser emissions. PACS 07.57.Hm; 42.55Lt; 42.62.Eh     

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.     

New short-wavelength laser emissions from partially deuterated methanol isotopes

The partially deuterated isotopes of methanol, CH₂DOH and CHD₂OH, have been reinvestigated as sources of far-infrared (FIR) laser emissions using an optically pumped molecular laser (OPML) system recently designed for wavelengths below 150 μm. With this system, 10 new FIR laser emissions from these isotopes ranging from 32.8 to 174.6 μm have been discovered. This includes the shortest known OPML emission from CHD₂OH, at 32.8 μm. These lines are reported with their operating pressure, polarizations relative to the CO₂ pump laser and wavelengths, measured to ±0.5 μm. In addition, polarizations for three previously observed FIR laser lines from CHD₂OH were measured for the first time. This paper is dedicated to the memory of Dr. K.M. Evenson, a pioneer in the field for his role in the development of optically pumped molecular lasers and their use in laser frequency measurements and the laser magnetic resonance technique. His scientific expertise, guidance, mentoring and friendship will be greatly missed. Received: 27 March 2002 / Published online: 8 May 2002     

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.     

Laser frequency measurements and spectroscopic assignments of optically pumped 13CH3OH

A three-laser heterodyne system was used to measure the frequencies of twelve optically pumped laser emissions from ¹³CH₃OH in the far-infrared (FIR) region. These emissions, ranging from 54 to 142 μm, are reported with fractional uncertainties up to ±2×10^-7 along with their polarization relative to the CO₂ pump. Using the 9P32 and 10R14 CO₂ lines, complete spectroscopic assignments for two laser systems were confirmed. Received: 31 May 2001 / Published online: 19 September 2001     

New large offset far-infrared laser lines from CD3OH

The CD₃OH molecule has been investigated for new far-infrared laser lines by optically pumping with a cw waveguide CO₂ laser. The increased tunability (300 MHz) with respect to a conventional CO₂ laser permits to pump many new CD₃OH lines. As a consequence 108 new laser lines have been discovered, ranging from 42.9 to 1155 m in wavelength. On some lines the effect of an electric Stark field has been investigated demonstrating a laser frequency tuning. The total number of known FIR laser lines from CD₃OH is increased to about 340 making this molecule the most prolific together with CH₃OH.     

Stabilization of a CO2 pump laser using external CH3OH stark-cell modulation

In order to stabilize the 118.8 μm CH₃OH laser output, frequency stabilization of the CW 9P(36) CO₂ laser pump is carried out using external CH₃OH Stark-cell modulation without internal frequency modulation. The CO₂ laser output power around the CH₃OH absorption center is stabilized to within ±1.0% for 60 min. The frequency stability of the CO₂ laser is estimated to be within ±1.4 MHz for 60 min, assuming the output fluctuation to be caused by this frequency fluctuation. As a result of the CO₂ pump laser stabilization, an output power stability of about ± 1.0% for 30 min is obtained for the 118.8 μm CH₃OH laser.     

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.     

Short-wavelength far-infrared laser cavity yielding new laser emissions in CD3OH

A significantly improved far-infrared laser has been used to generate optically pumped laser emissions from 26 to 150 μm for CD₃OH. Using an X–V-pumping geometry, several new laser emissions have been found for CD₃OH. In addition, an increase in power, by factors from 10 to 1000, for many of the previously known shorter-wavelength laser lines, below 100 μm, has been observed. Frequency measurements for several lines have also been performed and have been reported to a fractional uncertainty up to ±2×10^-7, permitting the spectroscopic assignment of the laser transition. One of the frequency-measured lines, 44.256 μm observed using the 10R34 pump, has confirmed the assignment of the previously reported FIR emission (n,K;J)=(1,7;20)?(0,8;20)A in the ground vibrational state. Received: 26 October 2000 / Published online: 7 February 2001     

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     

Frequency measurement of optically pumped FIR laser emissions from the CH3OD methanol isotope

A three-laser heterodyne system has been used to measure the frequencies of seven previously observed optically pumped FIR laser emissions generated by the CH₃OD methanol isotope. The frequencies, measured for the first time, are reported with fractional uncertainties up to ±2×10^-7 for laser emissions ranging in wavelength from 80.1 to 183.3 m. A previously measured FIR laser emission from CH₃OD, observed at 57.2 m, was also re-measured in this work. PACS 07.57.Hm; 42.55Lt; 42.62.Eh     

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.     

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 m.     

Frequencies of cw FIR laser lines for use in laser magnetic resonance spectroscopy

A transversely pumped far-infrared laser cavity has been used to discover two optically pumped laser lines: the 357.7 μm line from H₂C=CF₂ and the 242.3 μm line from CH₃OH. Using heterodyne techniques, the frequencies of fourteen far-infrared laser emissions have been measured, six for the first time. PACS  07.57.Hm; 42.55.Lt; 42.62.Eh     

CH<Subscript>3</Subscript>OH optically pumped by a <Superscript>13</Superscript>CO<Subscript>2</Subscript> laser: new laser lines and assignments

We report 12 new THz (far-infrared) laser lines from methanol (CH₃OH), ranging from 58.1 μm (5.2 THz) to 624.6 μm (0.5 THz). A ¹³CO₂ laser of wide tunability (110 MHz) has been used for optical pumping, allowing access to previously unexplored spectral regions. Optoacoustic absorption spectra were used as a guide to search for new THz laser lines, which have been characterized in wavelength, polarization, offset, relative intensity, and optimum operation pressure. For 20 laser lines previously observed, we have measured the absorption offset with respect to the ¹³CO₂ laser line center. PACS 33.20.Ea; 33.20.Vq; 33.80.-b     

Optically Pumped Far-Infrared Laser Lines of Methanol Isotopomers: 12CD3OH, 12CH3OD, and 12CH2DOH

Twenty-seven new FIR, far-infrared, laser lines from the isotopomers of methanol: ¹²CD₃OH, ¹²CH₃OD, and ¹²CH₂DOH, were obtained by optically pumping the molecules with an efficient cw CO₂ laser. The CO₂ laser provided pumping from regular, sequence, and hot-band CO₂ laser transitions. The 2 m long far-infrared cavity was a metal-dielectric waveguide closed by two, flat end mirrors. Several short-wavelength (below 100 m) lines were observed. The frequencies of 28 laser lines observed in this cavity (including new lines and already known lines) were measured with a fractional uncertainty limited by the fractional resetability of the far-infrared laser cavity, of 2 parts in 10⁷.     

IR absorption measurements and new FIR emission lines in CH3OH by use of a frequency-tunable CW waveguide CO2 laser

The IR absorption in CH₃OH in the vicinity of CO₂ laser lines has been measured quantitatively by use of a 300 MHz tunable waveguide CO₂ laser with output powers of about 3 W. Information on frequency offsets from the CO₂ line centers, small signal and saturated absorption coefficients of FIR laser pump transitions is obtained. Some stronger pump transitions with frequency offsets larger than 50 MHz gave rise to the observation of 8 new FIR emission lines with wavelengths from 43 to 125 μm.     

Space-borne remote sensing of CO2, CH4, and N2O by integrated path differential absorption lidar: a sensitivity analysis

CO₂, CH₄, and N₂O are recognised as the most important greenhouse gases, the concentrations of which increase rapidly through human activities. Space-borne integrated path differential absorption lidar allows global observations at day and night over land and water surfaces in all climates. In this study we investigate potential sources of measurement errors and compare them with the scientific requirements. Our simulations reveal that moderate-size instruments in terms of telescope aperture (0.5–1.5 m) and laser average power (0.4–4 W) potentially have a low random error of the greenhouse gas column which is 0.2% for CO₂ and 0.4% for CH₄ for soundings at 1.6 μm, 0.4% for CO₂ at 2.1 μm, 0.6% for CH₄ at 2.3 μm, and 0.3% for N₂O at 3.9 μm. Coherent detection instruments are generally limited by speckle noise, while direct detection instruments suffer from high detector noise using current technology. The wavelength selection in the vicinity of the absorption line is critical as it controls the height region of highest sensitivity, the temperature cross-sensitivity, and the demands on frequency stability. For CO₂, an error budget of 0.08% is derived from our analysis of the sources of systematic errors. Among them, the frequency stability of ± 0.3 MHz for the laser transmitter and spectral purity of 99.9% in conjunction with a narrow-band spectral filter of 1 GHz (FWHM) are identified to be challenging instrument requirements for a direct detection CO₂ system operating at 1.6 μm. PACS 42.68.Wt; 95.75.Qr