Far infrared laser frequencies of CH3OD and N2H4

The frequencies of 26 laser lines with wavelengths between 57 and 534 m have been measured in the optically pumped laser gases CH₃OD and N₂H₄. A pair of stabilized cw 12_{CO 2} lasers was used as a frequency standard for the heterodyne frequency measurements. Seven of the 26 lines are new.     

Frequency measurements of far infrared cw lasing lines in optically pumped CHCl2F

The wavelengths, frequencies, and relative polarizations of 15 FIR cw lasing lines obtained by optically pumping CHCl₂F with a cw CO₂ laser have been measured. The lines are in the wavelength range from 340.3 to 905.4 m and were pumped by P- and R-branch laser lines in the 9 m band of CO₂.Supported in part by a joint grant with the U. S. National Science Foundation grant # INT80-19014 and the Brazilian Consellio Nacional de Pesquisas (CNPq)Contribution of the U. S. Government, not subject to copyright.     

Soil matrix tracer contamination and canopy recycling did not impair 13CO2 plant–soil pulse labelling experiments

When conducting ¹³CO₂ plant–soil pulse labelling experiments, tracer material might cause unwanted side effects which potentially affect δ¹³C measurements of soil respiration (δ¹³C_SR) and the subsequent data interpretation. First, when the soil matrix is not isolated from the atmosphere, contamination of the soil matrix with tracer material occurs leading to a physical back-diffusion from soil pores. Second, when using canopy chambers continuously, ¹³CO₂ is permanently re-introduced into the atmosphere due to leaf respiration which then aids re-assimilation of tracer material by the canopy. Accordingly, two climate chamber experiments on European beech saplings (Fagus sylvatica L.) were conducted to evaluate the influence of soil matrix ¹³CO₂ contamination and canopy recycling on soil ¹³CO₂ efflux during ¹³CO₂ plant–soil pulse labelling experiments. For this purpose, a combined soil/canopy chamber system was developed which separates soil and canopy compartments in order to (a) prevent diffusion of ¹³C tracer into the soil chamber during a ¹³CO₂ canopy pulse labelling and (b) study stable isotope processes in soil and canopy individually and independently. In combination with laser spectrometry measuring CO₂ isotopologue mixing ratios at a rate of 1 Hz, we were able to measure δ¹³C in canopy and soil at very high temporal resolution. For the soil matrix contamination experiment, ¹³CO₂ was applied to bare soil, canopy only or, simultaneously, to soil and canopy of the beech trees. The obtained δ¹³C_SR fluxes from the different treatments were then compared with respect to label re-appearance, first peak time and magnitude. By determining the δ¹³C_SR decay of physical ¹³CO₂ back-diffusion from bare soils (contamination), it was possible to separate biological and physical components in δ¹³C_SR of a combined flux of both. A second pulse labelling experiment, with chambers permanently enclosing the canopy, revealed that ¹³CO₂ recycling at canopy level had no effect on δ¹³C_SR dynamics.     

On submillimeter laser lines from optically pumped12CH3OH

Several new submillimeter laser lines have been observed in a CH₃OH submillimeter laser, optically pumped by CO₂—laser radiation in a low power pulsed mode.Assignments have been suggested for a number of the lines. A few lines have wavelengths close to atmospheric water vapor absorption peaks, and can be observed only when the air path from the submillimeter resonator exit mirror to the detector is kept short or flushed with dry nitrogen.     

New FIR laser lines from optically pumped C2H3F, C2H3Cl, C2H3Br, C2H3CN, C2H5F, CH2CF2, HCOOH and CH3Br

Twenty-seven new cw far infrared laser lines with wavelengths between 137 and 988m have been observed from optically pumping C₂H₃F, C₂H₃Cl, C₂H₃Br, C₂H₅F, C₂H₃CN, CH₂CF₂, HCOOH and CH₃Br with a CO₂ laser. The wavelengths of these FIR laser lines were determined together with their optimum pressures and relative intensities.     

Optoacoustic Laser Stark spectroscopy and FIR laser action on CH3Br using a waveguide CO2 laser

The optoacoustic spectrum of CH₃Br around 10 m band lines of a tunable cw waveguide CO₂ laser is investigated. Several new infrared absorptions are observed and most of the correspond ing molecular transitions are assigned. Far infra red laser action is reported by pumping with the same CO₂ laser: pump offsets are given using the Transferred Lambs dip (TLD) technique. A new FIR laser emission is obtained and assigned. An optoacoustic Laser Stark spectroscopy technique is used to investigate off resonance infrared tran sitions.     

Submillimeter laser action of CW optically pumped CF2Cl2 (Fluorocarbon 12)

Eleven new CW far infrared (FIR) laser lines have been observed in the 600 m–1200 m range from the CF₂Cl₂ (Fluorocarbon 12) molecule optically pumped by a CO₂ laser. A 510^–4–10^–3 accuracy is achieved in the measurement of the FIR wavelengths.The frequency offset between the CO₂ pump center and the absorption line centers are measured using the transferred Lamb dip technique. Owing to a recent spectroscopic study of the CF₂ ³⁵Cl₂ molecule three lines may be assigned with great confidence as rotational transitions in thev ₆ vibrational band 923 cm^–1 of this main isotope.     

Wavelengths for new CH3OH laser lines

Using a grating-tuned CO₂-TEA laser to pump CH₃OH in a metallic-wave-guide resonator, submillimeter laser emissions have been produced by pumping with 36 lines in the 9.4m CO₂ band and with 23 lines in the 10.6m band. Several dozen new SMM laser lines were observed. Wavelengths accurate to within 0.1% are given for 12 previously unmeasured lines between 34m and 225m.     

High power FIR NH3 laser using a folded resonator

A compact FIR laser cavity having a couple of folding mirrors was designed. By inserting this FIR cavity into the pump TE CO₂ laser resonator, we observed 19 FIR NH₃ laser lines of which 4 new lines were included. The maximum output power was over 500 W at 90 m.     

Theoretical prediction on the wavelength-temperature shift in pulsed TE CO2 lasers

The wavelength-temperature shift observed in pulsed TE CO₂ lasers is discussed theoretically by means of Six-temperature model rate equations for tunable TE CO₂ lasers. Numerical calculations of the temperature-wavelength shift in a pulsed TE CO₂ laser with a simple plano-concave stable resonator, whether excited by conventional low-inductance fast-discharge scheme or by a long-pulse Pulser/sustainer discharge scheme, show that the laser output wavelengths are within the 10P branch as the ambient temperature varies from 228 to 338 K, but will change as the ambient temperature varies. The laser output wavelengths will move to the transition lines with longer wavelengths in the 10P branch as the ambient temperature increases and vice versa. The calculated results also illustrate that near the ambient temperature of 310 K, the laser is more likely to operate on multi-transition lines. Considering this wavelength-temperature shift, the chilling device adopted in high-power high repetition rate TE CO₂ lasers is important in maintaining a stable laser output spectra as well as a stable laser output power. The numerical results also suggest that a frequency agile resonator is highly recommended if stable laser output spectra are required in TE CO₂ lasers.     

Compact, high power FIR NH3 laser pumped in a three mirror CO2 laser cavity

A compact CO₂–NH₃ FIR laser system where an NH₃ laser cavity was inserted in pump, three mirror CO₂ laser cavity was designed. The total length of this system was about 1.8 m. Output energy of about 1 mJ (10 KW pea power) was obtained at the 152 m and 90 m lines in NH₃. Power conversion efficiency of 1.7 percent was obtained at the 90 m line.     

New CW FIR laser lines obtained in ammonia pumped by a CO2 laser, using the Stark tuning method

We report fifty seven CW FIR emissions observed in NH₃, by resonant pumping with a CO₂ laser. Exact coincidences between IR absorption lines of the gas and emission lines of the CO₂ laser have been carried out by Stark tuning. IR frequency shifts, up to 30 GHz, have allowed the pumping of forty three NH₃ transitions.These FIR emissions correspond to thirty one different wavelengths in the 50–400 m range; eighteen ones of them are new emitted wavelengths by pumping with the CO₂ laser.     

Influences of CO2 laser annealing on mechanical and laser-induced damage properties of ZrO2 thin films

Zirconium dioxide (ZrO₂) thin films were deposited on BK7 glass substrates by the electron beam evaporation method. A continuous wave CO₂ laser was used to anneal the ZrO₂ thin films to investigate whether beneficial changes could be produced. After annealing at different laser scanning speeds by CO₂ laser, weak absorption of the coatings was measured by the surface thermal lensing (STL) technique, and then laser-induced damage threshold (LIDT) was also determined. It was found that the weak absorption decreased first, while the laser scanning speed is below some value, then increased. The LIDT of the ZrO₂ coatings decreased greatly when the laser scanning speeds were below some value. A Nomarski microscope was employed to map the damage morphology, and it was found that the damage behavior was defect-initiated both for annealed and as-deposited samples. The influences of post-deposition CO₂ laser annealing on the structural and mechanical properties of the films have also been investigated by X-ray diffraction and ZYGO interferometer. It was found that the microstructure of the ZrO₂ films did not change. The residual stress in ZrO₂ films showed a tendency from tensile to compressive after CO₂ laser annealing, and the variation quantity of the residual stress increased with decreasing laser scanning speed. The residual stress may be mitigated to some extent at proper treatment parameters.     

Tunable te-CO2 laser-pumped formaldehyde far-infrared lasers: Offsets, assignment, and superfluorescence

Sixty and three absorption transitions in D₂CO and H₂CO, respectively, have produced a number of far infrared laser lines when they are pumped by an etalon-tuned TE-CO₂ laser. Almost all the absorption transitions pumped previously by a free runing TE-CO₂ laser have been efficiently pumped by the etalon tuned CO₂ laser and found to have offset within ±500 MHz from the line-center of the relevant CO₂ pump lines. 22 (1) absorption and 63 (4) emission lines of D₂CO (H₂CO) are assigned. Some of these lines have generated superfluorescence. In paticular, the D₂CO 319-m line pumped by CO₂-9P(32) delivered an output energy of approximately one half that of the well-known D₂O 66-m. It is shown that a large electric dipole moment and an appreciable amount of fractional population in the lower level of the pump transition of this line are responsible to the superfluorescence.     

New FIR Laser Lines and Frequency Measurements of CD3OD

The isotopomer of methanol, CD₃OD, was optically pumped by a cw CO₂ laser resulting in 53 new far-infrared (FIR) laser lines. The wavelengths are in the range 42.9 m to 189.9 m with the majority below 100 m. Ten different CO₂ lines were used for the first time to pump CD₃OD yielding new FIR laser lines. Some of the CO₂ pump lines belong to the hot-bands and sequence-bands of CO₂ in the 10 m region. The frequency was measured for 40 FIR laser lines in the range 1.5 THz to 6.9 THz, 32 of which were new laser lines.     

New far infrared laser lines obtained by optically pumping13CD3OD

We have obtained laser action on 34 far infrared lines for the first time in fully deuterated methyl alcohol with the¹³C isotope (¹³CD₃OD), and we have measured the frequency of 13 lines. The molecule was pumped by a cw CO₂ laser. We have measured the wavelength, the relative polarization, the relative intensity of most lines, the frequency, and the CO₂ pump frequency offset of the strongest lines. The new lines are distributed in the wavelength region from 75.27 m to 464.7 m.Supported in part by a joint grant with the U.S. National Science Foundation grant # INT 80-19014 and the Brazilian Conselho Nacional de Pesquisas (CNPq).     

重金属铜胁迫下玉米的光谱特征及监测研究

农作物重金属污染监测是当今高光谱遥感研究的重要内容之一,旨在设计一种新的窄带植被指数,以实现不同培育期的两种玉米品种的重金属铜胁迫监测。研究设计了不同浓度的铜污染实验,采用SVCHR-1024I型高性能地物光谱仪测量不同浓度铜离子（Cu2+）胁迫下玉米叶片的光谱反射率,并同步获取了玉米叶片中Cu2+含量数据。首先,对玉米叶片原始光谱数据进行一阶差分处理,并计算一阶差分反射率与叶片中Cu2+含量的相关系数（r）,筛选对铜胁迫敏感的波段。计算结果显示,489~497,632和677 nm波长附近的一阶差分反射率与叶片中Cu2+含量显著相关,可将其视为敏感波段。其次,根据以上3个敏感波段,建立基于一阶差分反射率的铜胁迫植被指数（dVI）。对所有可能的dVIs和Cu2+含量进行一元回归分析,并采用决定系数（R2）和均方根误差（RMSE）对回归结果进行评估,以筛选最佳指数。最后,采用不同生长年份的玉米实验数据对敏感波段的稳定性及dVI的适用性进行了验证评估;同时,通过与归一化植被指数（NDVI）、红边叶绿素指数（CI_red-edge）、红边位置（REP）、光化学反射指数(PRI)等常规重金属胁迫植被指数进行应用比较,证明dVI更具有优越性。结果表明：一阶差分处理后,在450~500,630~680和677 nm波长处的叶片反射率与Cu2+含量的相关系数明显增大。基于一阶差分反射率的特征波段具有稳定性,对于不同生长年份的玉米叶片数据,特征波段的波长位置不变。一元回归分析结果表明,结合497,632和677 nm波长的一阶差分反射率的指数与Cu2+含量具有显著的相关性,对于不同生长年份的2种玉米品种数据集,R2都高达0.75以上。另外,与常规植被指数比较结果表明,该研究所提出的dVI具有更好的鲁棒性及有效性,可为冠层尺度的重金属胁迫监测提供理论基础。     

HCOOH high-resolution spectroscopy in the 9.18 μm region

We report on highly accurate absolute frequency measurement against a femtosecond frequency comb of six saturated absorption lines of formic acid (HCOOH) with an accuracy of 1 kHz. We also report the frequency measurement of 17 other lines with an accuracy of 2 kHz. Those lines are in quasi coincidence with the 9R(36) to 9R(42) CO₂ laser emission lines and are probed either by a CO₂ or a widely tunable quantum cascade laser phase locked to a master CO₂ laser. The stability of HCOOH stabilized lasers is characterized by a fractional Allan deviation of 3.1 × 10⁻¹² τ^−1/2. They give suitable frequency references for Doppler-free two-photon spectroscopy.     

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.     

Optically-Pumped Far-Infrared Laser Lines of Methanol Isotopomers: 12CH3OH, 12CH3OD, and 12CH2DOH

Twenty-seven new 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.