Optoacoustic laser Stark spectroscopy in the ν2 band of 14NH3

Laser Stark spectroscopy of the R(5, K) transitions in the ν₂ band of ammonia was carried out using coincidences with the 9-μm band CO₂ laser lines. We observed 22 Doppler-free resonances by using an optoacoustic detector and a Lamb-dip stabilized CO₂ laser. A simultaneous analysis of sa, aa, and ss lines yields zero-field transition frequencies with an absolute accuracy of 1 ～ 2 MHz.     

Design of an optoacoustic cell for laser-Stark spectroscopy

We report on the development of an optoacoustic cell that is suitable for high-resolution laser-Stark spectroscopy. It is a small volume non-resonant device that has been operated with strong electric fields, up to 36 kV/cm. A Doppler-free Stark spectrum of NH₃, obtained with a CO₂ laser, is given as an example of its performances.     

Doppler-free optoacoustic spectroscopy of O-18 methanol

The Doppler-free optoacoustic spectrum of the oxygen-18 species of methanol has been investigated with a CO₂ laser. Seventeen coincidences have been found between infrared absorptions of CH¹⁸₃OH and CO₂ laser lines. The strongest feature seen, pumped by the 10R(20) CO₂ laser line, has two plausible alternative identifications, determined from a parallel high-resolution spectroscopic study of the C-O stretching band. Further experimental tests are suggested to resolve the assignment, and predicted far-infrared laser frequencies are given for the two possibilities.     

Intensities of two-photon transitions in the 2ν2 band of NH3

We determine theoretically all the coincidences between the ¹⁴NH₃ν₂= 0−2 band and the spectrum of two CO₂ lasers; they can be induced either by a small frequency tuning of lasers or by a Stark tuning of molecular levels. For each two-photon transition we report the line intensity, the preferred laser lines and the necessary Stark field or laser tuning. The theoretical line intensities are compared to the results of an optoacoustic Doppler-free experiment.     

Optoacoustic detection of Doppler-free two-photon resonances in the v2 bands of NH3

A new approach to infrared two-photon spectroscopy is described. An optoacoustic cell operating at low pressure is used to detect Doppler-free transitions excited by a conventional and a waveguide CO₂ laser. Beams from the two lasers are passed through the sample in opposite directions and an electric field tunes the transitions into resonance by molecular Stark effect. A few examples are discussed to illustrate the features and the sensitivity of the method.     

New FIR laser lines from CD3OD optically pumped by a CO2 waveguide laser

In this work we report new FIR laser lines from CD₃OD optically pumped by a CO₂ waveguide laser. The wide tunability of this laser (290 MHz) makes it possible to pump absorption lines with large frequency offset relative to the CO₂ laser line center, which are not possible by using conventional CO₂ lasers. As a consequence 19 new laser lines have been discovered, ranging from 38.0 m to 455.2 m in wavelength. For all lines, precise frequency offset measurements between the CO₂ line center and the center of the absorber CD₃OD line were performed using the transferred Lamb-dip technique. We also present direct Doppler-free offset measurements of infrared absorption, obtained within the FIR laser cavity itself, using optoacoustic detection.Work supported by FAPESP, CNPq, FAP-Brasil and CNR-Italy     

Doppler-free optoacoustic spectroscopy

The first observation of Doppler-free optoacoustic spectroscopy is reported. As a first example the P (193) line of the 11-0 band of the B←X transition of ¹²⁷I₂ is used. The output of cw single mode dye laser is split into two equal intensity beams chopped at frequencies ω₁ and ω₂. The nonlinear compoment of the optoacoustic signal at the frequency (ω₁ + ω₂) is detected and Doppler-free resolution is obtained. Comparing the Doppler-free optoacoustic and fluorescence spectra of iodine measured under similar conditions, good agreement is found. Since optoacoustic and fluorescence methods complement each other, this opens up new possibilities for weakly or nonfluorescing molecules.     

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.     

Optoacoustic laser spectroscopy of excited vibrational molecular states

A new detection method for absorption from excited vibrational states is suggested, based on optoacoustic detection of weak absorption in a heated gas. Using this method CO₂ laser radiation (λ=9.6 μm) absorption was investigated from excited vibrational states of CO₂, BCl₃, and BF₃ molecules.     

Frequency Stabilization of Waveguide CO2 Laser by a Digital Technique

In this work we present a simple technique for laser frequency stabilization, based on Digital signal processing. The technique is used to stabilize a waveguide CO₂ laser of wide tunability by using three kinds of reference signals: the CO₂ laser ouptut power, an infrared absorption optoacoustic signal and the output power of a Far-Infrared optically pumped molecular laser.     

Doppler-free two-photon spectroscopy of the 2ν3 band of SF6

The assignments of most of the observed Doppler-free two-photon spectra of the 2ν₃ band of SF₆ near the P(14)–P(20) lines of a 10-μm CO₂ waveguide laser are reported. The rovibrational and anharmonic constants for this band were obtained, along with ground state constants determined from observed “forbidden” transitions.     

基于双共振吸收光谱法的14CO2探测

快速且准确地定量探测¹⁴CO₂在许多领域都有着重要应用. 超灵敏的光腔衰荡光谱技术是一种极具潜力的光学探测方法,但其存在灵敏度不足和易受其他CO₂同位素/杂质分子干扰的问题. 本文提出利用阶梯型双共振光谱法将¹⁴CO₂从基态激发至中间能级后,再用光腔衰荡光谱法去探测. 双共振的吸收过程可以有效提高探测的选择性. 本文分析了双共振吸收光谱的定量探测能力,模拟结果显示双共振光谱探测是无多普勒的,可以减少其他分子的吸收干扰,有望实现亚ppt水平的¹⁴CO₂高选择性探测.     

Measurement of linewidth of two-photon Doppler-free optoacoustic signal and influence of collision broadening

A simple method of measuring linewidths of Doppler-free optoacoustic signals is introduced, and the influence of collision broadening on the linewidth of such optoacoustic signals is analyzed in this paper.     

Continuous-wave CO2 laser spectroscopy of SF6, WF6, and UF6

The linear absorption of CO₂ laser radiation in SF₆, WF₆, and UF₆ has been measured by using optoacoustic detection techniques. Absolute absorption coefficients per Torr as low as 1 × 10^?7 cm^?1 Torr^?1 in a 2-cm active path length could be measured by taking advantage of calibration measurements performed with SF₆.     

Use of a beam expanding telescope in a grating-tuned waveguide CO2 laser

A Galilean telescope is used as a beam expander in a grating tuned CO₂ waveguide laser to improve line selection while minimizing cavity length to maximise the frequency tuning range. The laser has been used in optoacoustic measurements of saturation of absorption on the transition pumping the 891 m CH₂CF₂ laser.     

Experimental investigation of13CD3OH infrared transitions by means of optoacoustic spectroscopy

A systematic investigation of¹³CD₃OH infrared transitions around CO₂ laser line emissions is performed by means of optoacoustic detection. Using a waveguide CO₂ laser of 290 MHz tunability and, in some cases, an acoustooptic modulator to provide an extension of 180 MHz, 108 transitions are observed. Measurements of pump off-set and relative intensity have been performed. The data may be compared with those previously reported of high resolution Fourier Transform spectroscopy.Work supported by: FAPESP, CNPq, FAEP — Brasil CNR — ItaliaPermanent address: Depto de Ciências, FEIS-UNESP (15378) I.Solteira-SP, Brasil     

Method for observing Doppler-free hot-band transitions

A novel spectroscopic method for highly sensitive detection of hot-band transitions based on infrared-infrared double resonance in a molecular beam was successfully demonstrated. The intensity of the infrared transitions from a vibrationally excited state is enhanced by two orders of magnitude by resonant laser pumping. The hot-band transitions are identified in connection with the assigned transitions in the fundamental band. Doppler-free spectral lines are observed. The experiment was carried out on the 2v ₃←v ₃ transitions of CH₃F with CO₂ laser pumping and probing.     

Optoacoustic investigation of the mechanisms in the infrared optogalvanic effect

Simultaneous observations of the optogalvanic and optoacoustic effects were performed in CO₂, NH₃ and SF₆ discharges under irradiation by resonant infrared 10 μm laser radiation. The dependence of the galvanic and acoustic signals on the discharge current, and their time evolution following a switch of the laser radiation were investigated. The observations proved that the infrared optogalvanic effect occurs through two different mechanisms, a gas kinetic temperature dependence of the discharge parameters and a modification of the pion production through the vibrational molecular excitation.     

Periodic structure created by a CO2-laser pulse incident on an open water surface

Experiments on the effect of a CO₂-laser pulse on an open water surface are reported. They testify that the multilayer periodic structure of the resultant scattering region is due to the intensity modulation at the trailing edge of the pulse. The generation of optoacoustic pulses is studied at two different values of laser-pulse length. The shorter laser pulse is obtained through the trailing-edge absorption in a laser-spark plasma produced by focusing the radiation.     

Optoacoustic detection of multiple photon molecular absorption in a strong IR field

A new method based on the optoacoustic effect has been proposed to measure multiple photon absorption at vibrational molecular transitions in a strong IR laser field. Comparison measurements of the average absorbed energy have been done by this method for molecules with different dissociation limits in a strong CO₂-laser field, the field intensity changed therewith by four orders. For the poly-atomic molecules C₂H₄ and SF₆ having a comparatively low dissociation limit, the absorption increases monotonously as the power density of laser radiation P increases up to the dissociation limit. For three-atom molecules, such as D₂O and OCS, absorption saturation takes place with P ? 10 MW/cm².